Hydrodesulfurization and Hydrodenitrogenation

1.27 Hydrodesulfurization and Hydrodenitrogenation R A Sa´nchez-Delgado, Brooklyn College, NY, USA ª 2007 Elsevier Ltd. All rights reserved. 1.27.1

Hydrodesulfurization and Hydrodenitrogenation

1.27.1.1 The Organometallic Modeling Approach 1.27.2

760 762

Organometallic Models of the Hydrodesulfurization Reaction

763

1.27.2.1 Coordination and Activation of Thiophenes in Metal Complexes

763

1.27.2.1.1 1.27.2.1.2 1.27.2.1.3 1.27.2.1.4 1.27.2.1.5 1.27.2.1.6 1.27.2.1.7 1.27.2.1.8

1-S-bonded metal thiophene complexes 2-(CTC)-bonded metal thiophene complexes 3(S,CTC)-bonded metal thiophene complexes 4-Bonded metal thiophene complexes 5-Bonded metal thiophene complexes 6-Bonded metal benzothiophene and dibenzothiophene complexes Metal 1-S-bonded thiophene complexes as adsorbents for the removal of dibenzothiophenes Conclusion

765 766 767 767 768 769 769 770

1.27.2.2 Metal Complex–catalyzed Homogeneous Hydrogenation of HDS-relevant Aromatic Compounds 1.27.2.2.1 1.27.2.2.2 1.27.2.2.3

770

Thiophene hydrogenation Benzothiophene hydrogenation Aqueous-biphasic and solid-supported catalysts for benzothiophene hydrogenation as a pretreatment for HDS

770 771 772

1.27.2.3 C–S Bond Activation, Hydrogenolysis, and Desulfurization of Thiophenes by Metal Complexes 1.27.2.3.1 1.27.2.3.2 1.27.2.3.3

773

Stoichiometric ring opening, hydrogenolysis, and desulfurization of thiophenes Catalytic hydrogenolysis and hydrodesulfurization of thiophenes Conclusion

1.27.2.4 HDS-relevant H2 and H2S Reactions with Metal Complexes 1.27.2.4.1 1.27.2.4.2

1.27.3

Hydrogen activation on complexes containing sulfido or disulfido ligands Reactions of H2S with metal complexes

Organometallic Models of the Hydrodenitrogenation Reaction

1.27.3.1 Binding Modes of N-Heterocycles in Transition Metal Complexes 1.27.3.1.1 1.27.3.1.2

Complexes of pyrrole, indole, carbazole, and related ligands Complexes with pyridine, quinoline, and related ligands

1.27.3.2 Reactions of N-heterocycles in Transition Metal Complexes Related to HDN 1.27.3.2.1 1.27.3.2.2

Hydrogenation of N-heteroaromatic compounds Metal-mediated C–N bond-activation reactions relevant to HDN

773 782 784

784 784 785

787 787 787 790

792 792 794

1.27.3.3 Conclusion

795

1.27.4

796

Concluding Remarks

References

796

759

760

Hydrodesulfurization and Hydrodenitrogenation

1.27.1 Hydrodesulfurization and Hydrodenitrogenation Hydrodesulfurization (HDS) is the reaction of organosulfur compounds with hydrogen over a catalyst leading to the extrusion of sulfur as H2S with concomitant production of the corresponding hydrocarbon (Equation (1)). Hydrodenitrogenation (HDN) refers to the analogous removal of nitrogen from organonitrogen compounds to produce ammonia and hydrocarbons (Equation (2)). These are the means by which sulfur and nitrogen are removed from petroleum feedstocks in refineries; HDS and HDN are the most important and most thoroughly studied reactions of the complex ‘‘hydrotreating’’ process involved in fuel production.1–3

[R–S]

+

H2

[R–N] +

H2

cat.

cat.

[R–H]

+

H2S

ð1Þ

[R–H]

+

NH3

ð2Þ

Petroleum is a complex mixture of hydrocarbons containing varying amounts of heteroatoms. The most abundant is invariably sulfur, present in concentrations up to 5 wt.% in a variety of compounds; thiols, sulfides, and disulfides can be removed with relative ease, but thiophenes, benzothiophenes, and specially dibenzothiophenes (Figure 1) are more refractory as a result of their increased aromatic character. Of particular interest are 4,6-dialkyldibenzothiophenes, since they are the most difficult to degrade. Removal of such sulfur through ‘‘deep desulfurization’’ is a most demanding task for which a satisfactory solution has not yet been found. The nitrogen content of crudes is lower than the sulfur content, typically around 0.1 wt.%, although it may reach levels as high as 1 wt.%. Nitrogen is present predominantly in the heavier and cracked fractions, and the highly refractory molecules relevant to HDN are those derived from pyridine (py) and from pyrrole (Pyr) with one or several short-chain alkyl substituents, as well as higher polycyclic homologs (see Figure 1). Organo- S- and N-compounds generate sulfur and nitrogen oxides (SOx and NOx) upon combustion; if released into the atmosphere, they are responsible for acid rain and other polluting effects. Therefore, environmental legislation imposes severe restrictions on the amounts of sulfur and nitrogen allowed in transportation fuels.1–4b Currently available technologies cannot fully meet these specifications, and therefore there is an urgent need for improved methods to produce cleaner fuels, specifically for the development of more efficient catalysts. ‘‘Promoted Mo catalysts’’ commonly employed in industry are composed of Co–Mo sulfides supported on g-alumina; other useful combinations include Ni–Mo and Ni–W. Co–Mo catalysts are excellent for HDS and less active for HDN, which is better performed over Ni–Mo or Ni–W. Operating conditions for HDS range between 300–450  C and 10–250 atm H2 (1 atm ¼ 101.325 kPa). Many other metal sulfides are active in HDS catalysis, and maximum activities are invariably found for Ru, Os, Rh, Ir; this is important in relation to organometallic HDS modeling, since a good proportion of such chemistry is concerned with Ru, Rh, and Ir complexes. Due to the higher CTN bond strength and the smaller atomic radius of N, the removal of nitrogen is more difficult to achieve. Strongly hydrogenating catalysts perform the best, the most commonly used being NiMo/Al2O3.

Rn

Rn

Rn

S

S

S

Rn

Rn

NH

Rn

Rn

Rn

N Figure 1 Important components of petroleum fractions.

Rn N

Rn NH

NH

Rn

Rn

Rn

Rn

Rn N

Hydrodesulfurization and Hydrodenitrogenation

A complicated set of reactions takes place during HDS and HDN, and large amount of work has been devoted to the study of their kinetics and mechanisms. The best-understood reaction within hydrotreating is no doubt HDS, particularly of thiophene (T), the most extensively used model compound. Mechanistic knowledge for benzothiophene (BT) and dibenzothiophene (DBT) has also advanced considerably, as have the mechanisms of HDN. Even for the simplest substrates HDS and HDN networks are complex, involving a number of elementary steps. An extensive literature on heterogeneous reaction mechanisms is available,1–3 and there is a general agreement on some key fundamental steps that need to be taken into account in any mechanistic proposal, namely:    

the generation and the nature of the active sites, the dissociative adsorption of hydrogen on the surface of the catalyst, the chemisorption of the organosulfur or organonitrogen compound on the catalytic sites, and the reactions of the adsorbates: hydrogenation of unsaturated bonds and hydrogenolysis of C–S or C–N bonds.

Some examples of the most widely accepted reaction networks for important HDS and HDN model compounds are presented in Schemes 1 and 2, respectively.

S

+

S

+ S

S

S

nH2

Butenes + butane + H2S

+ S

S

S +

+H2S

nH2 S

S

S

+ S

S

S

S +

nH2 + H2S

Scheme 1 General mechanisms of HDS of thiophene, benzothiophene, and dibenzothiophene on solid catalysts.

761

762

Hydrodesulfurization and Hydrodenitrogenation

+H2

+3H2

+2H2

+NH 3 +H2

+H2 NH2

N H

N

N H

N +3H2

+3H2

+H2

+H2

+3H2

+NH3

+3H2

+3H2

+2H2

N H

NH2

N H

N

+H2

+NH 3

NH2 +H2

+H2 H2N

N H

+NH3

+3H 2

+3H2

+H2

+H2 N H

+3H2

H2N +NH3

Scheme 2 General mechanisms of HDN of pyridine, quinoline, and carbazole on solid catalysts.

1.27.1.1 The Organometallic Modeling Approach Even though the chemistry and engineering of HDS and HDN have been in continuous development and industrial application for decades, new environmental constraints have produced a resurgence of the field, particularly an intensified search for novel catalysts capable of meeting present and future standards for cleaner fuels. Despite impressive practical achievements over the last 40–50 years, new discoveries have been delayed by the lack of a better understanding of some key issues, notably the nature of HDS–HDN active sites on metal sulfide catalysts and the details of the elementary reactions implicated in the catalytic schemes. Within this context, organometallic chemistry has become an additional powerful tool for understanding the HDS4–6,6a,6b and HDN4,4a,4b,7,7a reactions. The possibility of preparing metal complexes of model substrates such as the thiophenes or the pyridines, stable enough to be characterized by NMR spectroscopy and X-ray diffraction, but at the same time sufficiently reactive to allow the detailed study of their reaction mechanisms, has provided an interesting possibility for connecting organometallic and surface chemistry in relation to a problem of fundamental interest and environmental and industrial importance. Prior to 1985, very few metal complexes of the thiophenes were known and virtually no mention of them could be found in the HDS literature. Today, a large number and variety of such compounds are known4–6,6a,6b and they have become of obliged reference in the heterogeneous HDS field. On the other hand, metal derivatives of N-donor ligands such as pyridines or amines have been extensively studied over the years but their relevance to HDN has only recently been recognized and reviewed.4,4a,4b,7,7a Some early proposals for the modes of adsorption of thiophenes, pyridines, and pyrroles on metal sulfides have now been probed by comparison with the structures of well-characterized metal complexes; this has allowed the identification of the most reasonable alternatives for reactive surface intermediates and of new possibilities not previously considered. Theoretical studies on well-defined complexes have also contributed to a clear and consistent picture of the bonding modes of organosulfur and organonitrogen compounds to catalytic metal centers. Thus, when results from molecular chemistry are combined with the information available from surface techniques and heterogeneous catalysis, the chemisorption of these compounds appears as a

Hydrodesulfurization and Hydrodenitrogenation

well-understood phenomenon. This is no doubt one of the most important achievements of the organometallic modeling approach, as described in detail below. A number of possible reaction schemes have been derived for heterogeneous HDS and HDN catalysis, based on sound experimental evidence and/or extensive calculations.1–3 Because of the intrinsic complexity of the problem, some of key mechanistic points have remained speculative. By studying analogous reactions on discrete wellcharacterized transition metal complexes in solution, a better distinction of the most sensible reaction pathways from the less likely ones has become possible, especially in the case of HDS modeling. The new knowledge thus obtained can be extrapolated – no doubt with caution – to surface reactions. Perhaps more importantly, some patterns have emerged connecting some characteristics of the metal centers with the available or preferred modes of bonding of thiophenes, as well as with the specific reactions that such activated substrates may reasonably follow in each of these bonding situations. Similar considerations apply in the case of HDN-related substrates, but the advances in this direction are more modest. This chapter describes the major achievements of the organometallic modeling approach and attempts to build some conceptual bridges between molecular and surface chemistry in relation to HDS and HDN catalysis, following the outline of a previous monograph.4 It is intended here to summarize the most important aspects of this chemistry published up to mid-2005.

1.27.2 Organometallic Models of the Hydrodesulfurization Reaction 1.27.2.1 Coordination and Activation of Thiophenes in Metal Complexes In order to understand HDS reactions, it is important to define the ways in which thiophenes are bonded to metal centers on catalytic surfaces. A number of modes in which T interacts with surfaces have been proposed, the most important ones being the ‘‘one-point adsorption’’, that is, a strong interaction between the S atom and a ‘‘vacancy’’ on the surface, and the ‘‘multi-point adsorption’’ involving the S atom plus one or both of the CTC bonds in a delocalized -bonding.1–3 While it is difficult to experimentally obtain detailed information on the bonding of thiophenes to surface sites, several coordination modes of thiophenes have been authenticated in metal complexes (Figure 2). Metal–T complexes have been widely studied by NMR spectroscopy, and a good number of X-ray structures are available, providing a sound basis for understanding the chemisorption of such entities by analogies between the bonding modes in complexes and the proposed chemisorption modes on solid catalysts; extensive calculations have also contributed to a better understanding of the electronic structures of metal thiophene complexes. Perhaps more importantly, some patterns have emerged concerning the reactivity associated with thiophenes in each particular bonding mode; this allows some interesting parallels to be drawn with the ways in which thiophenes are thought to be degraded on the active sites of heterogeneous HDS catalysts. In this section, the syntheses and structures of the most important types of metal complexes of the thiophenes will be described. Table 1 provides a fairly comprehensive list of known complexes of the thiophenes.

S S M

S M

M

η 3(S,C=C)

η 2(C≡C)

η 1-S S

S M

M

η4

η4

S M

M

η5 Figure 2 Bonding modes of thiophenes in metal complexes.

η6

S

763

764

Hydrodesulfurization and Hydrodenitrogenation

Table 1 Metal complexes of thiophenes Complex

Characterized by

References

NMR NMR þ X-rays NMR þ X-rays NMR NMR þ X-rays NMR NMR þ X-rays NMR NMR þ X-rays NMR þ X-rays NMR þ X-rays NMR

8 8 9 8 10 8 8 8 8 11 12 13

1

 S-bonded complexes Cr(CO)5(Th) (Th ¼ 2,5-Me2T, BT, 3-MeBT) Cr(CO)5(DBT) Mo(CO)3[2,5(Ph2PCH2CH2)2T] Mo(CO)5(DBT) [Me2Si(C5Me4)Mo(DBT)] W(CO)5(Th) (Th ¼ 2,5-Me2T, BT) W(CO)5(DBT) CpMn(CO)2(Th) (Th ¼ 2,5-Me2T, BT) CpMn(CO)2(DBT) Cp* Re(CO)2(T) Cp9Re(CO)2(BTh) (BTh ¼ BT, 2-MeBT, 3-MeBT) [CpRe(NO)(PPh3)(Th)]BF4 (Th ¼ T, 2-MeT, 2,5-Me2T, BT, 2-MeBT, 3-MeBT) Re2(CO)9(BTh) (BTh ¼ BT, 2-MeBT, 3-MeBT, 3,5-Me2BT) CpFe(CO)2(T)]BF4 [CpFe(CO)2(2,5Me2T)]PF6 [CpFe(CO)2(BT)]BF4 [CpFe(CO)2(DBT)]BF4 [C5H4CH2–2-C4H3S)Ru(PPh3)2]BF4 RuCl2(Ph2P–DBT)2 RuCl2(Ph2P–DBT)2(CO) Ru(H)2(H2)(PCy3)2(DBT) [Cp9Ru(CO)n(PPh3)m(Th)]BF4 (Cp9 ¼ Cp, Cp* ; n ¼ 1,2; m ¼ 1,0; Th ¼ T, 2,5Me2T, 3-MeT, BT, Me4T, DBT, 4-MeDBT, 4,6-Me2DBT, 2,8-Me2DBT) {Co(CO)2[2,5(Ph2PCH2CH2)2T]}BPh4 {Rh(CO)[2,5(Ph2PCH2CH2)2T]}BPh4 [Ir(H)2(PPh3)2(Th)2]PF6 (Th ¼ T, THT, BT, DHBT, DBT) Cp* IrCl2(DBT)

NMR NMR NMR NMR þ X-rays NMR þ X-rays NMR þ X-rays NMR þ X-rays NMR NMR þ X-rays

14 15 16 15 15 17 18 18 19 20

NMR NMR þ X-rays NMR þ X-rays NMR þ X-rays

9 9 21 22

2(CTC)-bonded complexes TpW(NO)(PMe3)(Th) (Th ¼ T, 2-MeT, 2,5-Me2T) [Os(NH3)5(Th)]2þ (Th ¼ 2-MeT, 3-MeT, 2,5Me2T, 2-MeOT, BT) Cp* Re(CO)2(BT)

NMR þ X-rays NMR NMR

23 24 12

3(S,CTC)-bonded complexes {(triphos)Ir[3(S,CTC)–BT]}þ (triphos ¼ MeC(CH2PPh2)3)

NMR

25

NMR NMR þ X-rays NMR þ /or X-rays NMR þ X-rays NMR þ X-rays NMR þ X-rays NMR NMR þ X-rays NMR NMR þ X-rays

26 27 28

NMR þ X-rays NMR NMR NMR NMR

33, 34 35–37 38 39, 40 41–43

NMR NMR

44 45–48

4

 -Bonded complexes Cp* Ir(Th) (Th ¼ T, 2-MeT, 3-MeT, BT, DBT) Cp* Ir(2,5-Me2T) Cp* Ir(2,5-Me2T?A) (A ¼ BH3, CH3þ, CS2, Ru(6-C6H6)Cl2, Fe(CO)4, Co4(CO)11, Ru3(CO)11, Re2(CO)9) Cp* Rh(Me4T) Cp* Rh[Me4T?Fe(CO)4] Cp* Rh(Me4T?O) (6-C6Me6)Ru(Th) (Th ¼ T, 2-MeT, 2,5-Me2T, Me4T) (6-C6Me6)Ru[4-T?Mo(CO)5] (5-Me4T)Ru(4-Me4T) (5-Me4T)Ru(4-Me4T?Fe(CO)4) 5-Bonded complexes Cr(CO)3(T) Cr(CO)3(Th) (Th ¼ T, 2-MeT, 3-MeT, 2,5-Me2T, Me4T, . . .) Mo(PMe3)3(T) [Mn(CO)3(Th)](OTf) (Th ¼ T, 2-MeT, 2,5-Me2T, Me4T) Cp9Fe(Th) (Cp9 ¼ Cp; Th ¼ T, 2-MeT, 3-MeT, 2,5-Me2T, Me4T; Cp9 ¼ EtCp; Th ¼ 2,5-Me2T, Me4T) [Fe(Me4T)2](PF6)2 [Cp9Ru(Th)]X (Cp9 ¼ Cp; X ¼ BF4; Th ¼ T, 2-MeT, 3-MeT, 2,5-Me2T, 2,3,5Me3T, Me4T; Cp9 ¼ Cp* ; X ¼ PF6; Th ¼ T, 3-MeT, 2,5-Me2T)

29 29 30 31 31, 32 31, 32 31, 32

(Continued)

Hydrodesulfurization and Hydrodenitrogenation

Table 1 (Continued) Complex

Characterized by

References

[Ru(Th)2](X)2 ((Th)2 ¼ (Me4T)2, X ¼ BF4; Th2 ¼ T, 2-MeT, Me4T, X ¼ OTf [Ru(Th)2](BF4)2 [Ru(Me4T)(p-cymene)](BF4)2 [Ru(Me4T)Cl2]2 Ru(Me4T)Cl2(PR3) Ru(Me4T)Cl2(H2NTol) [(Me4T)Ru(Cl)]3S(BF4) [(Me4T)Ru(L)3](OTf)2 (L ¼ H2O, NH3) [Rh(T)(PPh3)2]PF6 [Rh(Th)(diene)]PF6 (Th ¼ 2,5-Me2T Me4T; diene ¼ COD, NBD) [Cp* Rh(Th)][PF6]2 (Th ¼ T, Me4T) [Cp* Ir(Th)][X]2 (Th ¼ T, 2-MeT, 2,5-Me2T, Me4T; X ¼ PF6, BF4)

NMR NMR þ X-rays NMR NMR þ X-rays NMR NMR NMR þ X-rays NMR þ X-rays NMR þ X-rays NMR NMR NMR

49 50 51 49, 50 49 49 49 50 52 51 52, 53 49, 51, 53

6-Arene-bonded complexes Cr(CO)3(Th) (Th ¼ BT, DBT, BNT) [Mn(CO)3(Th)]BF4 (Th ¼ 7-MeBT, 7-EtBT) [CpFe(Th)]PF6 (Th ¼ BT, DBT) trans-[(CpFe)2(DBT)2](PF6)2 [CpRu(Th)]PF6 (Th ¼ BT, DBT) trans-[(CpRu)2(DBT)2](PF6)2 [(C6Me6)Ru(BT)2](BF4)2 Co4(CO)9(DBT) Cp* M(Th) (M ¼ Rh, Ir; Th ¼ BT, 2-MeBT, 3-MeBT, 2,3-Me2BT)

NMR NMR NMR NMR NMR NMR þ X-rays NMR NMR NMR

54 54 54, 55 55 54, 56 57 54 58 53

1.27.2.1.1 1

1-S-bonded metal thiophene complexes

 -S-bonded thiophenes are neutral 2e ligands. Some of the earliest proposals for the adsorption of T involved vertical bonding to the surface through the S atom only, and thus the characterization of 1-S complexes has shed light on the ‘‘one-point adsorption’’ on solid catalysts. In all the X-ray structures available, the S atom is pyramidal, corresponding to approximate sp3-hybridization, but the ring itself is planar and perturbed only by a slight lengthening of the C–S bonds. The T ring is invariably tilted away from a perpendicular arrangement at angles ranging from 16 in [Me2Si(C5Me4)Mo(DBT)]10 to 61 in [CpRu(CO)(PPh3)(2-MeT)]BF4,20,20a,20b in agreement with spectroscopic studies of the adsorption of T on clean surfaces, which also concluded that it is tilted away from perpendicularity.4,4a,4b This is now accepted as a general phenomenon both in metal complexes and on surfaces, and it represents an important advance in connection with the early view of the vertical adsorption. Detailed calculations on the bonding in 1-S complexes of thiophenes show that the interaction is predominantly a ligand-to-metal donation of electron density from orbitals concentrated on the sulfur lone pairs, in agreement with a small effect of coordination on the C–S bonds or on the ligand as a whole.61,61a The ‘‘-acceptor’’ ability of T becomes important as the electron density on the metal increases, and this results in the weakening of the C–S bonds through backdonation into an antibonding * -orbital; this provides a reasonable pathway for the activation of T. Indeed, electron-rich metal fragments promote C–S bond scission of thiophenes. Tilting of the ring away from perpendicular binding avoids a repulsive interaction between filled ligand -orbitals and an occupied d-metal orbital. The IR spectral features of 1-S-bonded thiophene in complexes are very similar to those of thiophene adsorbed on sulfided Mo/Al2O3, and a detailed analysis of such spectra concludes that the S-only bonding causes C–S bond weakening, and therefore the most reasonable pathway for thiophene activation in HDS is 1-S-bonding followed by C–S bond scission.60 Examples of S-bonded T complexes are available for Mn, Re, Cr, Mo, W, Fe, Ru, Co, Rh, and Ir; their syntheses are usually straightforward, involving addition of the thiophene to an unsaturated metal precursor or displacement of a labile ligand. The M–S bond is generally weak, the stability increasing along the trend thiophenes < benzothiophenes < dibenzothiophenes. This has limited the study of the transformations of 1-bonded thiophenes, since ligand-exchange processes dominate the behavior of such compounds in solution. Some exceptions to this behavior are worth noting: Cp* (CO)2Re(1S–T) reacts with Fe2(CO)9 to yield the thiophene-bridged bimetallic derivative Cp* (CO)2Re(-T)Fe(CO)3 1 in which the T ligand remains 1-S-bonded to Re but also binds in an 4-fashion to the Fe(CO)3 fragment.62 Bases such as OH or Et3N induce the activation of the C–H bond  to S in [Cp* (NO)(PPh3)Re(1-S–T)]þ to yield the corresponding 2-thienyl derivative.13,13a The equilibrium constants for

765

766

Hydrodesulfurization and Hydrodenitrogenation

thiophene exchange in a series of Ru complexes [Cp(CO)(L)Ru(1S–Th)]þ (L ¼ CO, PPh3; Th ¼ T, 2-MeT, 3-MeT, 2,5-Me2T, Me4–T, BT, DBT), measured by NMR, show that increased methylation on the thiophene results in stronger binding to the metal and that steric effects are important for the stability of the complexes. 1-S–BT and 1-S–DBT complexes are also more stable than their T analogs, and tetrahydrothiophene (THT) binds 106 times more strongly than T. This is in parallel with trends reported for the adsorption and desulfurization of thiophenes on Co–Mo/Al2O3 catalysts.5–5d Also important in connection with HDS mechanisms, the S-bonded complex Cp* (1-S–T)Rh(PMe3) is considered the key precursor toward insertion of the metal into the C–S bond from experimental and theoretical arguments.63,63a

CO CO

Re S

Fe(CO)3

1

Complex [Ir(H)2(PPh3)2(T)2]PF6 2 and the analogs containing BT, DBT, THT, DHBT were obtained through the mild hydrogenation of [Ir(COD)(PPh3)2]PF6 in the presence of the appropriate organosulfur ligand and characterized by X-ray diffraction for T, THT, DHBT.21 In addition, RuH2(2-H2)2(PCy3)2 reacts with DBT to yield RuH2(2-H2)(1-S–DBT)(PCy3)2 3.19 These complexes demonstrate that a single metal center can activate a hydrogen molecule and two thiophenes. PPh3 S H

+

Ir S H PPh3

H H2

2

PCy3 H Ru S PCy3

3

T complexes of Mo or W, the metals commonly used in heterogeneous catalysts, are scarce. Theoretical studies predict that M(CO)5(1-S–T) (Mo64, W65) should be stable, and the W derivative has been studied spectroscopically by following the kinetics of substitution of cyclohexane by T in W(CO)5(cyclohexane).65 Extremely labile Mo(CO)5(1-S–DBT) and its 2,5-Me2T 4 and BT analogs were obtained by photolysis of Mo(CO)6 in the presence of the appropriate thiophene and characterized by NMR.8 More stable Cr and W analogs M(CO)5(1-S–Th) (M ¼ Cr, Mo; Th ¼ 2,5-Me2T, BT and DBT) were obtained in a similar manner and characterized spectroscopically and crystallographically. The only crystal structure available of an Mo complex containing an 1-S-bonded thiophenic ligand is that of ansa-Cp20Mo(1-S–DBT) 5, prepared by photolysis of Cp20Mo(H)2 in the presence of DBT.10 This structure displays the usual features of 1-S-bonded derivatives but with the smallest tilt angle  observed so far (16 ); this is most likely due to steric interactions between DBT and the methyl substituents on the Cp rings. The reaction can be reversed by the treatment of the DBT adduct with H2 at 80  C and 1 atm. No examples of 1-S-bonded T complexes of Co or Ni, the other important metals in industrial HDS catalysts, have been reported. CO S Mo CO CO CO CO

4

1.27.2.1.2 2

Me2Si

Mo S

5

2-(CTC)-bonded metal thiophene complexes

Stable  -(CTC) complexes with thiophenes acting as olefin-like 2e ligands are very scarce. A series of complexes TpW(NO)(PMe3)(2-(CTC)–Th) (Th ¼ T, 2-MeT, 2,5-Me2T) was obtained by reduction of TpW(NO)(PMe3)Br in the presence of the S-donor ligands; these compounds are particularly interesting in that they are rare examples of thiophene complexes of the HDS-useful metal W. The X-ray structure of the 2,5-Me2T derivative is the only example for an 2(CTC)-bonded T complex. These compounds also displayed a rich reactivity, mainly deriving from the enhanced basicity of the coordinated thiophenes, which promoted, for instance, the facile protonation at C2 and the mild hydrogenation of the uncoordinated CTC bond by Pd/C.23 Reduction of [Os(NH3)5(OTf)]2þ in the presence of Th yields {Os(NH3)5[2(CTC)–Th]}2þ (Th ¼ 2-MeT, 3-MeT, 2,5-Me2T, 2-MeOT, and BT), characterized spectroscopically.24,24a These complexes readily add

Hydrodesulfurization and Hydrodenitrogenation

electrophiles to the S atom, and the resulting adducts react with nucleophiles (H, CN, OAc, py, PrNH2, N3, PPh3, PhO, PhS) to yield the corresponding ring-opened 2-4-(alkylthio)-1,3-butadiene complexes. In contrast, protonation of {Os(NH3)5[2(CTC)–Th]}2þ with triflic acid proceeds via addition to the exo-side of the ring to give an 2-2H–thiophenium product {Os(NH3)5[2(CTC)–Th?H]}2þ.66 The complexes Cp9Re(CO)2[2(CTC)–BT] (Cp9 ¼ Cp, Cp* ) slowly interconvert with their 1-S isomers in solution; the 2(CTC) form is favored by the presence of the more electron-donating Cp* ligand. Introducing steric congestion and a stronger donor ability of the sulfur atom, as in 2-Me– and 3-MeBT, leads to the formation of the S-bonded isomers only. If the BT ligand is previously 6-coordinated to Cr(CO)3, reaction with Cp(CO)2Re(THF) leads exclusively to the 2(CTC) isomer of the Re moiety in the bimetallic product 6, as a result of the greater -acceptor ability of the CTC bond in the Cr complex. On the other hand, when the 1–2 mixtures of Cp9Re(CO)2[2(CTC)–BT] are allowed to react with electrophiles, only the S atom coordinates to the incoming group, for example, Me3Oþ produces Cp9Re(CO)2[2(CTC)–BT?Me] while W(CO)5 yields the S-adduct 7 exclusively.12 Similarly, reaction of Cp9Re(CO)2(BT) with Cp9Re(CO)2(THF) gives a product containing the BT ligandbound 1-S to one Cp9Re(CO)2 group and 2(CTC) to the other. In the reaction between Cp* Re(CO)2(BT) and CpRe(CO)2(THF), the only product isolated contains the BT ligand-bound 1-S to the CpRe(CO)2 group and 2(CTC) to Cp* Re(CO)2, consistent with the more electron-rich metal fragment binding to the -accepting olefin, while the less electron-rich one accepts electron density from the sulfur donor.

CO CO

CO Re CO S

6

1.27.2.1.3

Cr CO

Re S

CO (OC)5W

CO

7

3(S,CTC)-bonded metal thiophene complexes

Only one example is available of a complex containing the 4e combination of 1-S þ 2(CTC) bonding at a single metal center, viz, {(triphos)Ir[3(S,CTC)–BT]}þ 8 (triphos ¼ MeC(CH2PPh2)3).25 This bonding mode coincides with the structure suggested in early ‘‘multi-point adsorption’’ proposals in heterogeneous catalysis. On refluxing in THF, further reaction takes place to break one C–S bond, which demonstrates that this type of coordination can be important for activating thiophenes toward ring opening. Density functional theory (DFT) calculations indicate that this 3(S,CTC) bonding is more stable than the 1-S or 4-coordination modes of thiophene in [(T)Ir(PH3)3]þ model complexes, and it is also more adequate for promoting C–S bond scission by d 8-ML3 14e fragments.67 P P + Ir P

S

8

1.27.2.1.4

4-Bonded metal thiophene complexes

The diene-type 4-bonding of Th ligands to transition metals (Th ¼ T and its methylated derivatives) is infrequent. Ru, Rh, and Ir complexes containing 4-bonded thiophenes with Cp or arene co-ligands have been synthesized by chemical or electrochemical reduction of the corresponding 18e 5-Ru, -Rh, and -Ir precursors;26–32 upon addition of two extra electrons to, for example, Cp* Ir(5-Th), the -thiophene ligand transforms into a 4e donor to avoid oversaturation at the metal. For instance, reduction of Cp* Rh(5-Me4T) with Cp2Co leads to Cp* Rh(4-Me4T); one ring of [Ru(5-Me4T)2]2þ is reduced in an analogous manner to yield a mixed 5–4 species [(5-Me4T)Rh(4-Me4T)]2þ. Complexes of the higher homologs are exemplified by Cp* Ir(4-BT) and Cp* Ir(4-DBT), characterized by in situ NMR spectroscopy. The crystal structures of 4-Th metal complexes invariably show the T ring highly distorted with the four carbon atoms coordinated to the metal, and the sulfur bending out of the plane. The C–S distances in 4-Th derivatives are significantly longer than those measured in free thiophenes, indicating a significant perturbation of the ligand. The 4-Th bonding is imposed by the electron count at the metal center, even if there are

767

768

Hydrodesulfurization and Hydrodenitrogenation

no important spatial restrictions to accommodate the ligand in a flat 5-bonding fashion. This type of bonding has been discussed at length;61,61a it resembles that of simple dienes but additionally it incorporates a substantial antibonding metal–sulfur interaction; thus, breaking one C–S bond may be viewed as a way of relieving the repulsive M–S interaction. Ring opening of thiophenes in Cp* Ir(4-Th) is catalyzed by basic alumina or by triethylamine, or promoted by UV light.26 Reaction of Cp* Ir(4-Th) with H2 also promotes ring opening. 4-Coordination renders the organosulfur molecules highly reactive; a common reaction of 4-T metal complexes is the formation of adducts with Lewis acids binding to the S atom. For instance, Cp* Ir(4-Me2T) reacts with M(CO)nL6n (M ¼ Cr, Mo, W) complexes to yield a series of derivatives Cp* Ir(4-Me2T)?M(CO)5, where the thiophene is 1-S-bonded to M. The 4-complex also isomerizes in solution to the ring-opened form, which can also bind to M(CO)n fragments in a variety of ways.68 Protonation of (6-C6Me6)Ru(4-T) gives a transient cationic thioallylic intermediate 9, which evolves by C–S bond scission into a butadienethiolate ligand in 10 (Equation (3)).69

+ Ru

H+

+ Ru

+ Ru

ð3Þ S H

S S

H

H

9

1.27.2.1.5

H

10

5-Bonded metal thiophene complexes

Complexes containing 5-bonded thiophenes, where the ring formally donates six electrons, are the most numerous and stable of the transition metal–thiophene derivatives (see Table 1). Examples are available for Cr, Mn, Re, Fe, Ru, Rh, and Ir. Curiously, Cr(CO)3(5-T), the first -thiophene metal complex reported in 1958,33 is still the only case of a group 6 metal -bonded to T; its X-ray structure was solved in 1965 albeit with a strong rotational disorder for the T ligand.34 Early CNDO calculations predicted the stability of Mo(CO)3(5-T), but attempts to synthesize it have failed, most likely due to its extreme lability in solution.64 The first example of an Mo complex containing an 5-bonded thiophene ligand, Mo(PMe3)3(5-T), has only recently become available from the reaction of Mo(PMe3)6 with T at room temperature.38 No examples of 5-T derivatives of W or of the promoter metals Co or Ni have been reported to date, and therefore many important synthetic challenges remain in this area. The bonding in 5-thiophene metal complexes resembles that of the Cp analogs, except that T is a poorer electron donor but a better acceptor than Cp; in addition, the presence of the larger S atom in T causes a slight tilting of the ring relative to a perfectly horizontal disposition (and in some cases a ring slip); consequently, thiophenes are less strongly bound to metals than Cp ligands.61,61a An interesting exception to the common case of d 6-complexes is [(5-T)Rh(PPh3)2]PF6 11, a rare example of a five-coordinated d 8-5-T complex prepared by hydrogenation of [(COD)Rh(PPh3)2]PF6 in the presence of excess T. This compound provided the first non-disordered X-ray structure of a -bonded metal–T complex.52 The ring in this case is not only tilted as usual, but also slightly bent in what may be viewed as a unique case of an intermediate structure between a ‘‘normal’’ 5- and an 4-mode, in which the sulfur tends to be located away from the metal, but nevertheless remains coordinated to Rh in order to attain an 18e configuration. The energy level diagram of the PH3 analog61,61a nicely accounts for these interesting features. S Rh PPh3

PPh3

11 Methyl-substituted thiophenes bind more strongly to transition metals than the unsubstituted ligand, as illustrated by ligand-exchange reactions of CpRu(5-Th) complexes. The equilibrium constant measured by NMR increases by a factor of about 6 for each methyl group added on the thiophene;47 benzene, BT, and DBT also displace T. These observations can be related to the inhibition effect of benzene and other aromatic hydrocarbons on HDS of BT over solid catalysts, ascribed to a stronger binding of arenes on metal sulfide surfaces as compared to common sulfur

Hydrodesulfurization and Hydrodenitrogenation

heterocycles. The -bonded T ring, particularly in cationic complexes, is activated as expected from electron donation to the metal center. For instance, H–D exchange is observed at the H(2,5) positions of T in CpRu(5-Th) complexes in CD3OD in the presence of OD, while H(3,4) exchange takes place much more slowly, in agreement with an increased acidity of coordinated T; this is an interesting parallel to the trends observed for H–D exchange in T when D2 is passed over conventional HDS catalysts.46 Another appealing feature in relation to HDS mechanisms is the activation of -bonded thiophenes in cationic complexes toward nucleophilic attack by hydrides to yield reactive intermediates such as 12 and 13, which ultimately lead to hydrogenation of the CTC bonds39 or to C–S bond breaking.70 Neutral 5-thiophene complexes do not suffer attack by nucleophiles but can be deprotonated with strong bases to yield 2-thienyl products.

CO

CO

CO

Mn

Ru S

S H

12

H

H

H

13

In summary, 5-coordination does not produce a great alteration in the charge distribution on the T ring, and nucleophilic attack is observed only in positively charged complexes. It thus seems that it is the overall charge on the metal derivative rather than changes in the electronic structure of the bound thiophenes that leads to activation toward nucleophiles. It is clear from organometallic HDS models that most of the reactions of metal–T complexes with hydrogen leading to ring saturation or C–S bond breaking are associated with other types of thiophene bonding, especially 1-S and 2-(CTC). Thus, 5-adsorption on surface sites, frequently invoked in the heterogeneous literature, is probably a peripheral situation, rather than a crucial phenomenon directly related to the actual HDS reactions.

1.27.2.1.6

6-Bonded metal benzothiophene and dibenzothiophene complexes

6

 -Coordination of arenes to 12e metal centers is well documented, as is the consequent activation of such aromatic fragments for further reactions.71,71a A number of complexes containing BT and DBT -coordinated through the benzene ring are known, indicating a strong preference of these two ligands for this type of arene bonding. Examples are available for Cr, Mn, Fe, Ru, Co, Rh, and Ir, as shown in Table 1. Such structures are not of general relevance as HDS models, since they produce activation predominantly of the coordinated benzene ring toward nucleophilic attack.72,72a However, other aspects of the chemistry of 6-bonded metal BT and DBT complexes are important in connection with HDS. In particular, 4-Me– and 4,6-Me2DBT are desulfurized through prior hydrogenation of one or both of the arene rings,1–3 and therefore the study of 6-DBT complexes could be of use in developing new catalysts for reducing the arene moieties in this type of molecule. In addition, 6-BT or DBT complexes such as [Mn(CO)3(6-BT)]þ are activated toward C–S bond scission by a second metal center,54,54a,54b as discussed in Section 1.27.2.3.1.(iii). A related interesting class of 6-bonded complexes involves thiametallacycles derived from ring opening of thiophenes acting as 6e donors.68

1.27.2.1.7

Metal 1-S-bonded thiophene complexes as adsorbents for the removal of dibenzothiophenes

1-S-coordinated thiophenes are usually labile ligands in metal complexes, and this has been applied to model an extraction process of dibenzothiophenes, in which a mixture is desulfurized by removal of the intact sulfur compound and regeneration of the adsorbing complex. [Ru(NH3)5(H2O)]þ reacts with DBT or 4,6-Me2DBT in DMF/H2O solution to yield the corresponding [Ru(NH3)5(1-S-DBT)]þ derivatives. By increasing the water concentration, the reverse reaction can be induced to liberate the DBT and regenerate the aqua complex (Equation (4)). This has been used to desulfurize a model mixture containing toluene, hexanes, and 500 ppm of DBT. One extraction cycle removes 50% of DBT and five cycles bring the content down to 25 ppm; 4,6-Me2DBT could also be removed but with lower efficiency. The complexes [CpRu(CO)2]BF4 and [CpFe(CO)2(2-2-methylpropene)]BF4 perform similarly in solution or when supported on mesoporous silica, achieving 99% removal of DBT and 72% of 4,6-Me2DBT.73,73a [Ru(NH3)5 (H2O)]+

+

DBT

[Ru(NH3)5(η1-S-DBT)]+

+

H2O

ð4Þ

769

770

Hydrodesulfurization and Hydrodenitrogenation

1.27.2.1.8

Conclusion

The organometallic chemistry of thiophenes is now well developed; synthetic methods are available for a variety of metals and thiophenes, and the general structural trends are well established. The various bonding modes of thiophene ligands in metal complexes have been studied in detail, and a deep understanding of the interactions involved and the factors affecting them have been reached. Mo and W thiophene complexes have been reported but remain scarce and examples of Co and Ni derivatives are still lacking, so this remains as an important synthetic challenge. It is possible to draw some parallels between the coordination modes of thiophenes in complexes and the ways in which such molecules are adsorbed on surfaces. Each type of binding seems to be associated with a particular kind of reaction of the activated substrate in connection with HDS schemes.

1.27.2.2 Metal Complex–catalyzed Homogeneous Hydrogenation of HDS-relevant Aromatic Compounds Hydrogenation reactions play a major role in hydrotreating. The general, HDS mechanisms shown in Scheme 1 include the partial or complete saturation of a thiophene ring and/or a benzene ring. Hydrogenation may precede C–S bond scission, but ring opening of the heterocycle may also take place prior to hydrogenation.1–3 Despite the impressive advances of homogeneous hydrogenation over recent decades, hydrogenation of simple arenes remains rather undeveloped in contrast with heterogeneous systems widely employed in industry and in research laboratories. Few organometallic complexes have been reported to catalyze the reduction of benzene and related molecules, and recent reviews indicate that most of them are not truly homogeneous but instead the hydrogenation ability is due to decomposition of the complexes into active metal nanoparticles or colloids, with the exception of d 0-Nb and -Ta hydride catalysts that appear to be truly homogeneous but are not related to HDS chemistry.74,75 On the other hand, the homogeneous hydrogenation of polynuclear aromatic hydrocarbons may be achieved with a number of complexes such as RuCl2(PPh3)3,76 Ru(H)2(H2)(PPh3)3,77,77a (6-C6Me6)2Ru2(-H)2(-Cl)]Cl2,78 and [Rh(MeOH)2(diphos)]þ.79 Here we will concentrate on the metal complex-catalyzed hydrogenation of HDS-related substrates, particularly of S-aromatic hydrocarbons, for which the number of catalysts and the level of understanding is growing; mechanistic details are related whenever possible to hydrogenation within HDS mechanistic networks. Other reviews are available on this subject.4,4a,4b,80,80a,80b

1.27.2.2.1

Thiophene hydrogenation

Examples of homogeneous hydrogenation of T or substituted analogs are very scarce. Co2(CO)8 slowly reduces T under forcing conditions,81 but no mechanistic details are available, and the formation of active metal particles cannot be ruled out in this case. The first example of a well-defined system for the reduction of T to THT was [Ir(H)2 (1S-T)2(PPh3)2]PF6 3,82 which operates at atmospheric pressure and 80  C, according to the mechanism depicted in Scheme 3. Some salient features of this cycle are as follows. (i) Activation of T and of H2 on a single metal site, which has been considered in mechanistic models for heterogeneous reactions on Co–Mo–S sites. (ii) The regio- and stereospecific intramolecular transfer of hydrides to the 1-S/2-bound T leading to an 3-(S,CTC) thioallyl intermediate 14, similar to one proposed in heterogeneous catalysis. (iii) 2,3-Dihydrothiophene (2,3-DHT) is the actual

H H

P + S Ir S P

–T

P + Ir

H H

P

P + H Ir

P S

S

14

2 +2THT +H2, –THT H H

P + S

Ir

P

S

P

P + H Ir H S

P +H2

P + Ir S

15 Scheme 3 The mechanism of homogeneous hydrogenation of thiophene by [IrH2(PPh3)2(1S-T)2]þ.

Hydrodesulfurization and Hydrodenitrogenation

+T, – H2

P H H

Ru P

P H2

P

H2

17

H Ru S

18 +H2, –THT

Scheme 4 Catalytic hydrogenation of thiophene by RuH2(H2)2(PCy3)2.

intermediate toward ring saturation, in agreement with some proposals for heterogeneous T hydrogenation. (iv) Although the reaction is formally catalytic, turnover numbers are very low due to catalyst poisoning by strong binding of the product to the metal in [Ir(H)2(1-S–THT)2(PPh3)2]PF6 15. Methyl-substituted thiophenes react in an analogous manner as long as there is at least one unsubstituted carbon atom next to sulfur, but 2,5-Me2T was unreactive.4,4a,4b In a related set of stoichiometric reactions, T in (5-S–T)Mn(CO)3þ was partially reduced by external hydride attack to yield (3,1-S–thioallyl)Mn(CO)3 12, followed by protonation to produce 16 with 2,3-DHT bonded to Mn through the sulfur atom and the remaining CTC bond (Equation (5)).12

CO

Mn

CO

CO

CO CO H+

CO

CO Mn

HCl

CO

Mn

Cl

ð5Þ S

S

12

CO

16

RuH2(2-H2)2(PCy3)2 17 reacts with T to yield the thioallyl derivative [RuH(4-(S,C)T–H)(PCy3)2] 18, which upon further reaction with H2 liberates THT and regenerates 17. This forms the basis of the catalytic cycle shown in Scheme 4, and complex 17 is indeed the only efficient catalyst known for the mild homogeneous hydrogenation of T to THT (up to 50 turnovers in 17 h at 80  C, 3 atm). 2-MeT is also catalytically converted into 2-MeTHT by 17 at rates about one order of magnitude slower than for T, while BT is hydrogenated to DHB at rates comparable to those observed for T.19

1.27.2.2.2

Benzothiophene hydrogenation

Hydrogenation of BT to DHBT takes place much more readily than reduction of T, since the C(2)TC(3) bond in this molecule behaves essentially as an olefin. Ru, Os, Rh, and Ir complexes efficiently catalyze the reduction of BT to DHBT under moderate reaction conditions. Examples of catalyst precursors include RuCl2(PPh3)3,76,83 RuHCl(CO)(PPh3)3,83 RuCl3?3H2O þ TPPTMS,84,84a,84b [Ru(triphos)(MeCN)3][BF4]2,85 RuH2(2-H2)(PCy3)2,19 OsHCl(CO)(PPh3)3,83 RhCl(PPh3)3,75,83 [Cp* Rh(NCMe)3][BF4]2,86 [M(COD)(PPh3)2]PF6 (M ¼ Rh, Ir),83,87,88 Rh(COD)(sulphos),89,89a and [Ru(sulphos)(MeCN)3]SO3CF3.80,89,89a The BT hydrogenation cycle for [Cp* Rh(NCMe)3]2þ,90 as well as the kinetics and mechanisms for [M(COD)(PPh3)2]PF6 (M ¼ Rh, Ir),87,88 and for [(triphos)Ru(NCMe)3][BF4]2 [(triphos ¼ MeC(CH2PPh2)3] (the fastest homogeneous BT hydrogenation catalyst known)85 have been elucidated in detail and previously reviewed.4,4a,4b,80,80a,80b The elementary steps involved are in clear parallel with olefin hydrogenation mechanisms, and the 1-S , 2-S interconversion is generally accepted as the step leading to activation of the CTC bond of BT, required to enter the catalytic cycle. Scheme 5 depicts the generally accepted mechanisms for this reaction involving metal monohydride and dihydride complexes. Some interesting key points common to the well-understood catalysts are as follows. (i) All effective BT hydrogenation catalysts are based on ‘‘HDS promoter metals’’ (Ru, Os, Rh, Ir), and they are specific for the hydrogenation of the S-containing ring, while saturation of the benzene ring has never been detected. (ii) In all the mechanisms elucidated in solution, coordination of BT occurs in an 1-S or an 2(CTC) fashion prior to hydrogen transfer. Although the olefin-type coordination has not been directly observed under catalytic conditions, the occurrence of the 1-S $ 2(CTC) equilibrium in metal BT complexes has been well established,11 and this is now generally accepted

771

772

Hydrodesulfurization and Hydrodenitrogenation

H [ML n] H

[ML n]

H

H

S

[ML n] or S

H

[MLn]

S

S

–DHBT +BT

[ML n] H

[MLn]

S

–DHBT +BT

H

H

[ML n]

S

S +H2

[MLn] S

H +H2

[MLn] S

Scheme 5 General mechanisms of the homogeneous hydrogenation of benzothiophene.

in BT hydrogenation cycles. (iii) Calculations87,91 indicate that the C(2) atom of BT is more negatively charged than C(3), and thus it should be more susceptible to electrophilic attack by the metal; therefore, a 2-benzothienyl intermediate is more likely to be involved in the catalytic cycle upon hydride migration from the metal to BT.

1.27.2.2.3

Aqueous-biphasic and solid-supported catalysts for benzothiophene hydrogenation as a pretreatment for HDS

Apart from the mechanistic considerations discussed above, metal complex-catalyzed hydrogenation of organosulfur compounds has potential for practical applications in connection with HDS of refined fuels or refinery cuts. A series of patents by INTEVEP, the Venezuelan petroleum research company,84,84a,84b describes the use of water-soluble catalysts for the biphasic reduction of benzothiophenes. The catalysts are generated in situ by reaction of RuCl3?3H2O with m-monosulfonated or trisulfonated triphenylphosphine (TPPMS, TPPTS) in the presence of a basic co-catalyst such as aniline or quinoline; the resulting mixtures hydrogenate BT at reasonably fast rates under 30 atm H2 at 120  C. This partial reduction of BT has been used for several consecutive cycles as a pretreatment for naphtha, which was subsequently subjected to mild HDS over conventional catalysts, thereby improving sulfur removal without compromising the benzene rings, which contribute to a high octane number. These systems operate through mechanisms involving similar features to those described in the preceding section, with, for example, RuHCl(PPh3)2(aniline)2 as the active species entering the catalytic cycle. Other water-soluble catalysts have been developed80,80a,80b by using polydentate ligands such as NaO3S(C6H4)CH2C(CH2PPh2)3 (Na sulphos, the sulfonated analog of triphos) and NaO3S(C6H4)CH2)2C(CH2PPh2)2 (Na2DPPDS). The complexes [(sulphos)Rh(COD)], [(sulphos)Ru(NCMe)3]-SO3CF3, and Na[{Ru(sulphos)}2(-Cl)3] catalyze BT hydrogenation under mild reaction conditions in aqueous biphasic media or when supported on silica. The solid-supported catalyst [(sulphos)Ru(NCMe)3]SO3CF3/SiO2 (Ru(II)/SiO2) reduces BT at rates superior to those observed for homogeneous or liquid-biphasic systems, with excellent recyclability even when used in a BT-doped naphtha. Other heterocycles are also reduced. Similar materials containing Ru(0)/SiO2 derived from Ru3(CO)12 did not hydrogenate BT.80,80a,80b Further research into easily recoverable hydrogenation catalysts would be welcome, as a pretreatment for HDS of difficult thiophenic substrates. A particularly appealing possibility for practical applications in deep desulfurization would be the development of a two-stage process incorporating a highly active catalyst (liquid-biphasic or supported metal complex) for the hydrogenation of one or both of the benzene rings in, for example, 4,6-Me2DBT, followed by mild HDS of the saturated product on conventional Co–Mo–S or similar catalysts, as shown in Scheme 6. Arene hydrogenation has been achieved with bimetallic materials combining molecular and nanostructured catalysts derived from Rh complexes on Pd/SiO292,92a–92c or [(sulphos)Rh(COD)]/Pd/SiO2,80,80a,80b but no attempts have been disclosed of applications of this chemistry to deep HDS.

Hydrodesulfurization and Hydrodenitrogenation

immobilzed metal complex

S

Co –Mo–S

HDS

S

Scheme 6 A possible two-stage process for deep HDS.

1.27.2.3 C–S Bond Activation, Hydrogenolysis, and Desulfurization of Thiophenes by Metal Complexes C–S bond breaking is the reaction responsible for desulfurization. Thus, understanding of the pathways through which such reactions take place is a key element in HDS modeling. A great proportion of the work has dealt with C–S bond rupture under mild conditions, which is induced by complexes of different metals and varied structures. The mechanisms of C–S activation are understood, and some homogeneous and supported catalysts for the hydrogenolysis and hydrodesulfurization of thiophenes have become available. The extensive mechanistic knowledge gained from organometallic models4–6,6a,6b represents a significant contribution to the understanding of the reaction networks in heterogeneous catalysts.

1.27.2.3.1

Stoichiometric ring opening, hydrogenolysis, and desulfurization of thiophenes

C–S bond energies of thiophenes are comparable to C–H bond energies. C–H bond activation is established for a variety of metal complexes through well-documented mechanisms; although less extensively developed, activation of C–S bonds by metal complexes has experienced an impressive growth in the last few years. Ring opening of thiophenes through oxidative addition of the C–S bond has been achieved via a concerted process or through reactions involving intermediates containing the thiophene molecule coordinated to the metal. The ring-opened 2-(C,S) products have been characterized by use of NMR spectroscopy and X-ray crystallography, and theoretical studies have complemented the experimental data. The thiametallacycles may be described as containing a thiabutadiene fragment with localized CTC bonds as in structure 19 or as a delocalized thiametallabenzene ring as in 20.

M Ln

S

M Ln

19

S

20

1.27.2.3.1.(i) C–S bond activation by complexes of cyclopentadienyl and related ligands Pioneering mechanistic studies on C–S bond activation were performed with [Cp* Rh(PMe3)], generated by thermolysis of [Cp* Rh(PMe3)(H)(Ph)] or by photolysis of [Cp* Rh(PMe3)(H)2].63,63a Upon thermal reaction with T, BT, or DBT, the unsaturated Rh fragment yielded the corresponding thiametallacycles Cp* Rh(PMe3)[2-(S,C)–Th] (Th ¼ T, BT, DBT) (Equation (6)). The low-temperature photochemical experiment yielded the same thiametallacycles mixed with the corresponding 2-thienyl hydride complexes Cp* Rh(PMe3)(H)(1-C–Th) resulting from C–H activation; the latter slowly converted into the thermodynamically stable ring-opened product, demonstrating that C–S and C–H bond activation pose similar energy requirements; ring opening is preceded by 1-S-coordination of T to Rh. Similar reactions with MexT, BT, and DBT invariably lead to activation of the unsubstituted C–S bond.93

Rh Me3P

Ph

H

Me3P

Rh

S

Rh Me3P S

ð6Þ

In a similar set of reactions, 1-S-coordination of thiophene and benzothiophene followed by ring opening has been observed for (PMe3)3Ir(Th)Cl,94 while the thermal and photochemical reactions of T with Tp* Rh(PMe3)(C2H4) afford mixtures of the corresponding C–H and C–S activation products, with the former being the predominant species.95 In a rare example of Co chemistry, Cp* Co(C2H4)2 activates the C–S bond of T, BT, and DBT, presumably through undetected 1-S-bonded intermediates, to produce 21.96 The fragment [ansa-Cp20Mo] [Cp20 ¼ Me2Si(C5Me4)2] generated by photolysis of [ansa-Cp20Mo(H)2] readily inserts into the C–S bond of T and

773

774

Hydrodesulfurization and Hydrodenitrogenation

BT to produce the corresponding ring-opened products, as exemplified for T in 22.10 No intermediates containing intact T or BT could be observed, although they are probably involved in this reaction, as suggested by the formation of the stable 1-S-adduct Cp20Mo(1-S–DBT) 5. The S-bonded derivative does not open the DBT ring but its reaction (80  C) with T leads to the ring-opened T-derived product. These C–S bond-breaking reactions induced by Mo complexes are very important in connection with modeling MoS2-based catalysts. Other interesting Cp systems used in HDS-related chemistry include Cp* Ir(4-T), which isomerizes to the ring-opened isomer upon contact with basic alumina or triethylamine, or by UV irradiation.26 [Cp* M(5-Me2T)]þ (M ¼ Rh, Ir) undergo a complex series of reactions with aqueous base leading to a variety of mononuclear and polynuclear products containing ring-opened ligands resulting from nucleophilic attack on the T ring and related reactions.97 [CpRu(5-T)]þ is attacked by H or other nucleophiles promoting C–S activation.5–5d [W(NPh){o-(Me3SiN)2C6H4}(C5H5N)2] opens the rings of T, 2-MeT, and BT at 65  C in toluene;98 this represents a rare example of C–S activation induced by W, one of the most frequently used metals in heterogeneous HDS.

S Co

Co Cp *

C p*

Me 2Si

21

Mo

S

22

1.27.2.3.1.(ii) C–S bond activation by complexes with phosphine ligands The fragments [(triphos)Ir]þ or [(triphos)MH] (M ¼ Rh, Ir), thermally generated from [(triphos)Ir(4-C6H6)]þ, [(triphos)Ir(H)2(C2H5)], or [(triphos)Rh(H)3] (triphos ¼ MeC(CH2PPh2)3) display a very rich HDS-related chemistry.4–6,6a,6b The unsaturated Rh and Ir triphos fragments readily add the C–S bonds of T,99 BT,25 DBT,100 and some of their methylated derivatives,101 leading to the corresponding thiametallacycles. The propensity of these Rh(I) and Ir(I) species to activate the C–S bond is related to the rigidity of the tripodal ligand. In particular, [(triphos)Ir(4-benzene)]BF4 reacts with thiophene to give the ring-opened product [(triphos)Ir(2C,S-T)]þ, which has provided a complete stepwise HDS model reaction by sequential addition of thiophene, H, and Hþ, as shown in Scheme 7.99 This illustrates the fact that electron-rich systems promote ring opening of thiophene and that addition of hydrogen can take place heterolytically via addition of hydride followed by protonation to produce 1,3-butadiene þ H2S, the primary products of thiophene HDS, as invoked in some heterogeneous mechanisms. This is not catalytic but it is an excellent model in which various elementary steps involved in the degradation of thiophene could be authenticated, in clear parallel with surface chemistry. An analogous set of reactions has been described for BT with the important difference that a stable intermediate 8 containing a unique 3(S,CTC)-bonded BT was isolated. Above 40  C, this intermediate converts into the ring-opened isomer [(triphos)Ir(2C,S-BT)]þ, characterized by X-ray diffraction.25 The main reactions of the BT-derived metallacycle with H2 or with H/Hþ couples, summarized in Scheme 8, lead to the formation of ethylbenzenethiolate, thereby modeling the hydrogenolysis of BT but not its complete HDS.25 The extensive [(triphos)Ir] model chemistry shows that all the necessary steps for HDS of T or for hydrogenolysis of BT can be achieved without previously saturating the heterocyclic ring, in agreement with one of the commonly invoked

H2 S + P P Ir+ S P

H– –50 °C

P P Ir P H S

25 °C

21

P

HCl

P Ir P S

PhSH CO

HBF4

P P

Ir

P PhS S Scheme 7 Stepwise HDS of thiophene on Ir-triphos complexes.

P P Ir+ P CO S

+ C 4H7SH + P Cl P Ir P Cl Cl

Hydrodesulfurization and Hydrodenitrogenation

P + P Ir P

P P Ir+ S P

80 °C S

P P Ir S P H

H– –50 °C

8

25 °C

+

2

P P P

Ir H

S S

P P Ir + P

H

P Ir P P

H2

P P Ir P S

HBF4 S

Scheme 8 Stepwise hydrogenolysis of benzothiophene on Ir-triphos complexes.

‘‘desulfurization route’’ in HDS catalysis. Nevertheless, hydrogenolysis of the saturated cyclic thioether 2,3-DHBT also proceeds by use of [(triphos)Ir(H)2]þ, in the presence of a strong base, to produce ethylbenzenethiol plus [(triphos)Ir(H)3]. This demonstrates that even though presaturation of the thiophenic rings is not required for desulfurization, the C–S bonds of saturated cyclic thioethers can be broken by (triphos)Ir(III), geometrically very similar to but less electron rich than, the Ir(I) fragments that promote ring opening of thiophenes. ReH7(PPh3)2, in conjunction with 3,3-dimethyl-1-butene as a hydrogen acceptor, promotes hydrogenolysis of T through a stable thioallyl intermediate 23, which upon photolysis affords the corresponding thiol. In contrast, if 23 is treated thermally, hydrogenation to THT is observed.102 HH L

L Re S H

23 A set of reactions, which provides a nice model for heterogeneous Mo catalysts, involves C–S bond activation of T by Mo(PMe3)6 under very mild conditions to yield the butadienethiolate complex 24. The formation of this species involves C–S bond activation and attack by a hydrogen atom from one PMe3 ligand. Protonation of 24 regenerates PMe3 and causes a change in conformation of the butadienethiolate ligand in 25 (see Equation (7)). The X-ray structures of these two derivatives are the only ones available of butadienethiolate–metal complexes, a class of compounds often alluded to in organometallic HDS modeling. Interestingly, Mo(PMe3)3(5-T) is formed as a minor product of this reaction but it does not interconvert with 24.38 S Mo H 2C

S

PMe3

P PMe3 Me2

24

Me3P

Mo

PMe3

ð7Þ

PMe3

25

The HDS-related chemistry of Pt phosphine complexes has been extensively developed.103,103a–103f [PtLn] (L ¼ PEt3, PMe3; L2 ¼ Ph2PCH2CH2PPh2, dppe) reversibly insert into the C–S bond of T, BT, and DBT and their methylated derivatives to yield ring-opened products [PtL2(2-C,S–Th)]. The thiaplatinacycle derived from BT is about 10 times more stable than those derived from T or DBT, which are, in turn, similar to each other. Reaction of the PEt3 thiametallacycle derived from DBT with Et3SiH results in complete HDS to yield biphenyl plus [Pt(PEt3)2(H)(SH)] (Scheme 9); upon exposure to HCl(g) the latter generates H2S plus Pt(PEt3)2Cl2 to complete the HDS scheme. Similar reactions were observed for the ring-opened products of T and BT, with relative rates BT > DBT >> T; this shows again that electron-rich metal centers are prone to oxidatively add C–S bonds.

775

776

Hydrodesulfurization and Hydrodenitrogenation

+ Pt(PEt3) 2(H)(SH)

Et3SiH Et3P Et3P

+HCl –H2S

Pt S HCl

+ Pt(PEt3) 2Cl2 SH

Scheme 9 Desulfurization and hydrogenolysis of dibenzothiophene on Pt-PEt3 complexes.

In contrast to the reactions with hydrides, the interaction of the DBT and BT metallacycles with HCl produces the corresponding free thiols plus PtCl2(PEt3)2, (Scheme 9). Thus, the hydridic or protonic character of the hydrogen atoms transferred to the ring-opened intermediates directs the reaction toward the Pt–C bond-breaking process followed by a second C–S bond rupture, or by a Pt–S bond scission, respectively. Methylated T, BT, and DBT also provide the corresponding thiaplatinacycles. Reaction of Pt(PEt3)4 with 4,6-Me2DBT led to C–H activation only; nevertheless, the ring-opened product of 4,6-Me2DBT (DMDBT) was obtained indirectly by reaction of [PtCl2(PEt3)2] with the substrate and Na metal under hydrogen at low pressure. If the PMe3 and dppe PtL2 analogs are used, complete HDS of DBT and 4-MeDBT to biphenyl and 3-Me–biphenyl, respectively, takes place under 20 atm H2 at 100  C. Replacement of one PEt3 ligand by a phosphite leads to new series of phosphine/phosphite or bis(phosphite) complexes; the differences in ability to promote HDS were accounted for in terms of donor–acceptor properties of the ligands involved in each case.104 Thiophene substituted with groups other than alkyls (e.g., –Cl, –NO2, –OMe, –OAc) also form thiaplatinacycles, which are notable in that they promote catalytic desulfurization. These substituted thiophenes also produced the first thiapalladacycles by reaction with the corresponding Pd–phosphine complexes, as well as Ni analogs.105 3,6-Dimethylthieno[3,2-b]thiophene also forms a thiaplatinacycle by activation of a C(vinyl)–S bond upon reaction with [Pt(PEt3)4]; 2,29-bithiophene and 1-methyl-2-(2-thienyl)pyrrole react similarly but also promote C–H activation.106 The mild HDS of DBT by complexes of Ni, a widely used promoter metal in industrial catalysts, has also been reported.107,107a–107c As shown in Scheme 10, [(dippe)NiH]2 [dippe ¼ (i-Pr2PCH2)2] readily reacts with DBT to release H2 and insert into the C–S bond, yielding the thiametallacycle [(dippe)Ni(2-C,S–DBT)] 26, which evolves

Pr i2 P

H

Ni

Pr i2 P

P Pr i2

Ni S P Pri2

Ni H

P Pri2

Pr i2 P

26

Pr i2 P Ni P Pr i2

27

Pr i2 P Ni

S

Pr i2 P

Pr i2 P

P Pr i2

P Pr i2

Ni

P Pr i2

28

Scheme 10 Stepwise HDS of thiophene on Ir-triphos complexes.

Ni

S

Hydrodesulfurization and Hydrodenitrogenation

into [(dippe)Ni(2,29-biphenyl)] 27 and a transient mononuclear intermediate (dippe)2NiTS, which spontaneously dimerizes to [(dippe)2Ni2(-S)] 28. The chemistry of 28 has been further explored as a model of reactions of NiS2, widely used as a promoter in HDS.108 Complex 12 reacts with H2 at atmospheric pressure and room temperature to liberate biphenyl and regenerate the dimeric hydride, demonstrating that desulfurization can take place without participation of H2 or Hþ/H couples. Hydrogen seems to be required for this system only in the subsequent release of the desulfurized product, biphenyl, from 26. Analogous reactions of T and BT with [(dippe)NiH]2 also allowed the isolation of C–S insertion products. On the other hand, 4-MeDBT produces a metallacycle from insertion into the C–S bond distal to the methyl group, which upon interaction with an excess of the Ni hydride under H2 at 1 atm and 90  C yields the HDS product 3-methylbiphenyl. No nickel insertion products could be identified in similar reactions with 4,6-Me2DBT, but the reaction of the dimeric hydride with an excess of Me2DBT under 1 atm H2 at 90  C did lead to the formation of the hydrodesulfurized product 3,39-dimethylbiphenyl plus monomeric Ni(dippe)2 as the only identifiable metal-containing species. In contrast, the reaction of the Pt analog [(dippe)PtH]2 with 4,6-Me2DBT afforded [(dippe)Pt(2-C,S– DMDBT)] 29. This compound reacted further with an excess of the Pt hydride at 160  C to yield the HDS product 3,39-dimethylbiphenyl (Equation (8)) and presumably a sulfido-bridged dimer [(dippe)2Pt2(-S)], which decomposed under the reaction conditions. Analogous complexes originating from insertion of DBT and 4-MeDBT were also characterized for the Pt system, thus providing a nice family of complexes resulting from C–S activation of the dibenzothiophenes with varying degrees of methylation.106

P P

Pt S

+

[(dippe)PtH]2

+

“[(dippe)2 Pt2(μ -S)]”

ð8Þ

29

1.27.2.3.1.(iii) Reactions of thiophenes on polynuclear complexes Multi-site reactions are important in heterogeneous catalysis, and it is likely that more than one metal center is needed to effect sulfur removal from organic molecules. Therefore, binuclear or polynuclear metal complexes are of interest in modeling HDS reactions in solution.4–6,6a,6b,109 Thiophenes interact with Fe3(CO)12 in refluxing benzene to produce thiaferroles 30 through metal insertion into the less hindered C–S bond of the thiophene.110,110a,110b The thiaferroles are subsequently desulfurized to the corresponding ferroles 31 in refluxing benzene. Interaction of 30 with H2 yields ethylbenzene and related thiols; the fate of the extracted sulfur is not clear. Dibenzothiophenes failed to undergo C–S bond activation under similar conditions. Ru3(CO)12 reacts with thiophenes and BT to give products analogous to 30, together with some C–H activation products. In the case of 2-MeT, complex 32 was isolated. The X-ray structure displays the separated S atom capping an Ru triangle and the butadiene linked through two M–C -bonds to one Ru atom in the triangle and 4 to a fourth ‘‘spiked’’ Ru center, in a remarkable ‘‘snapshot’’ of the desulfurization process as it takes place.111,111a Substituted BTs as well as DBT react with Ru3(CO)12 to yield related products, the most remarkable one being 33, which upon reaction with H2 releases biphenyl.112

(CO) 3 Fe (OC)3Fe

S H

30

(CO)3 Fe Fe (CO)3

31

OC OC Ru OC

OC CO O C Ru

S

CO Ru Ru CO OC Ru CO OC CO OC CO

32

Ru OC CO CO

33

(CpRu)3(-H)3(3-H)2 breaks both C–S bonds of BT and DBT to yield ethylbenzene and biphenyl, respectively. In the case of BT, the reaction proceeds through an intermediate thiaruthenacyclohexadiene complex 34 formed through insertion of one Ru atom across the vinylic C–S bond. This intermediate evolves into a 3-alkylidene-3sulfido-bis(2-hydrido) cluster 35, which was further hydrogenated in THF at 80  C and 7.2 atm H2 for 1 week to produce ethylbenzene and (Cp* Ru)3(-H)3(3-S). DBT is also cleaved by (Cp* Ru)3(-H)3(3-H)2 in toluene at

777

778

Hydrodesulfurization and Hydrodenitrogenation

110  C over 8 days to yield biphenyl, plus the same 3-sulfido complex.113 C–S bond-breaking reactions of saturated cyclic thioethers are favored when the sulfur atom bridges two metal centers [54].114 For instance, thermolysis of Os3(CO)10(THT)2 yields an intermediate 36 containing a butenethiolate ligand formed by ring opening of THT via an Os3(CO)10(2-THT).

S H Ru *Cp H

C

Cp*

Ru

H Ru Cp*

*Cp Ru H

34

S

Ru Cp* H Ru Cp* S

(CO)2 Os

Os(CO) 4 Os(CO)3

H

35

36

Re2(CO)10 reacts photochemically with T to yield complex 37 and with BT to yield 38; several methyl-substituted BT analogs react in a similar way, via 1-S-bonded intermediates. Under H2, photolysis of Re2(CO)10 in the presence of 3,5-Me2BT leads to a ring-opened partially hydrogenated ligand in 39. Reaction of 38 with PMe3 affords further ring-opened products 40. Also, the non-photochemical reaction of Re2(CO)10 with BT, under a hydrogen atmosphere, produces an interesting cluster 41 containing BT molecules that have undergone hydrogenation and hydrogenolysis; the reaction mechanism involves prior formation of H4Re3(CO)12, which is the species actually interacting with BT.115,115a,115b Me

Me

Re(CO)4 S SH Re(CO) 4

Re (CO)3

S (OC)3Re

(OC)3 Re

37

Re(CO)4 H

38

39

Re(CO) 3(PMe 3) S

S (OC)3Re Re(CO)4(PMe 3)

40

H S

Re(CO)3 H Re (CO)3

41

The reaction of [Cp* IrH3]2 with T in the presence of tert-butylethylene as a hydrogen acceptor yields a product 42, containing the separated sulfur atom and the butadiene fragment bridging the same two metal atoms, another remarkable ‘‘snapshot’’ of a desulfurization process taking place. The coordinated butadiene is readily displaced by CO to form [Cp* Ir(CO)]2(-S); hydrogenation of 42 leads to butane, but the fate of the sulfur or of the metal complex were not clearly established. 2-MeT reacted similarly, but 2,5-Me2T, BT and DBT failed to react. [Cp* IrHCl]2 also reacted with T or BT at 90  C under H2 to give thiolate-bridged products [Cp* IrCl]2[(-H)(-SBun)] and [Cp* IrCl]2[(-H)(-S)(C6H4)Et], respectively, arising from hydrogenolysis of the thiophenic substrates; both of them could be completely desulfurized to butane or ethylbenzene by further reaction with hydrogen.116,116a,116b The sulfur-bridged dimer Cl3W(-THT)3WCl3 undergoes C–S bond cleavage upon attack by H to yield Cl3W(-THT)2(-SBun)WCl3 43 [55], in a rare case of homogeneous C–S bond scission by tungsten. Apparently, the acute W–S–W angle (ca. 62.5 ) arising from a very short W–W bond contributes to weaken the C–S bonds.117

Hydrodesulfurization and Hydrodenitrogenation

S

*Cp

Ir S

Cl Cl W Cl

Ir Cp*

42

S S

Cl W Cl Cl

43

Industrial HDS catalysts are composed of at least two metals, and therefore heterobimetallic complexes are better models for HDS-related reactions. Fe3(CO)12 reacts with the 4-activated thiophene in Cp* Rh(4-T) via a stable intermediate 44, containing the T ligand 4-bonded to Cp* Rh and 1 to Fe(CO)4, to yield a desulfurized bimetallic product 45 containing a ferrole unit -bonded to Rh; both complexes were characterized by X-ray diffraction.29 Similarly, Cp* Ir(2-C,S–Th) (Th ¼ 2,5-Me2T) reacts with iron carbonyls through cleavage of both C–S bonds to produce compound 46, containing the separated sulfur atom and organic fragment bonded to the metal framework.5,5a–5d Fe(CO)4 S

OC OC Fe OC Rh

Rh Cp*

*Cp Ir (OC)4Fe

Cp*

44

45

S Fe(CO)2

46

Another case of bimetallic cooperative C–S bond activation involves the reaction the triphos–Rh BT-derived metallacycle with W(CO)5 to yield the heterobimetallic sulfur-bridged species 47, which upon thermolysis under H2 (30 atm) induced HDS of BT to ethylbenzene, plus [(triphos)RhH(CO)] together with an insoluble ‘‘W–S material’’ (Equation (9)). This shows that the S-bridged Rh–W couple switches the reactivity from hydrogenolysis to hydrodesulfurization.118 P P Rh P S (OC)5 W

H2

PH P Rh H P S W

H2 >70 °C

P P Rh CO + P H

+“WSx”

ð9Þ

47 It is tempting to speculate whether the role of ‘‘promoter’’ HDS metals (Co, Ni) is the C–S activation of the thiophenes whereas the ‘‘catalytic’’ metal (group 6, Mo, W) would be responsible for the transfer of hydrogen to the activated substrate. This is a very likely pathway to be envisaged on metal sulfide surfaces where sulfides or thiolates bonded to two or more metal atoms are plentiful. However, this type of interaction is obviously unavailable in single-site complexes, and this could explain why ring opening is frequent but complete desulfurization is rare on mononuclear derivatives. Another type of activation involves attack of a metal carbonyl fragment to a thiophene previously coordinated to another metal to yield bimetallic products, which display a rich reactivity, frequently involving C–S bond scission. Examples of this chemistry involve the reductive cleavage of [Mn(CO)3(5-Me2T)]þ with a second Mn unit to produce 48, which is subsequently hydrogenated to 49 as in Equation (10).119 +

Mn(CO) 3 H2 S

S Mn(CO)4

48

(OC) 3Mn

H

49

Mn(CO)3

ð10Þ

779

780

Hydrodesulfurization and Hydrodenitrogenation

Similarly, Cp* Ir(4-Me2T) and its metallacyclic isomer undergo a series of interesting reactions with a variety of metal carbonyl fragments to yield heterobimetallic derivatives such as 50–52 (M ¼ Cr, Mo, W), which further react to yield products of relevance to HDS modeling such as 53–55.20,20a,20b,68 The analogous complex containing the thiairidacycle -bonded to FeCpþ is attacked by H at the carbocyclic fragment, as in Equation (11); several other nucleophiles react similarly to yield related derivatives.121 M(CO) 5 S M(CO) 3 S

Ir

Ir Cp*

M(CO) 5

Ir S

Cp

50

*Cp

51

52

(CO)3 Mo

S Ir W(CO)4 C *Cp O

C O Ir

53

Ru

S

Cp′

Ru OC

Cp*

54

CO

55

Cp Fe

+FeCp

Ir S

Cp

+

ð11Þ

Et3BH – Ir

*Cp

HS

Cp*

An interesting alternative makes use of the ‘‘remote activation’’ of the thiophene ring of BT and DBT produced by previous 6-binding to a carbocyclic ring, to induce C–S bond activation by a second metal center under mild conditions, as exemplified in Equation (12) for an Mn/Pt combination. A variety of metal-containing fragments can produce this type of activation through 6-bonding, and the second metal inserting into the C–S bond need not be identical to the one -bonding the arene ring. For instance, Pt(PPh3)2, which does not react with free BT, readily inserts into the C(vinyl)–S bond of (6-BT)MLn complexes, the ease of insertion following the order MLn ¼ Ru(C6Me6)2þ, Mn(CO)3þ > FeCpþ, RuCpþ  Cr(CO)3. The thiophene rings of alkylated BTs and DBTs previously activated by 6-bonding to Mn(CO)3þ are also cleaved by Pt(PPh3)2(C2H4). Reductive cleavage by use of cobaltocene under CO also leads to ring-opened bimetallic products through initial activation of the C(aryl)–S bond, followed by isomerization to a more stable configuration (Equation (13)). The latter reaction has been studied in detail, showing that the course of the reaction is markedly influenced by the substituents on BT.54,54a,54b,122 A theoretical analysis of some of the reactions of thiophenes involving more than one metal center is available.123 +

(OC)3 Mn

(OC) 3Mn + +

ð12Þ

Pt(PPh 3)2 S

S (OC)3Mn+

e–, CO S

(OC)3Mn+

Pt(PPh3) 2

(OC)3Mn+ S Mn (CO) 4

ð13Þ S

Mn(CO)4

The closest models available for real HDS catalysts involve Co–Mo molecular clusters. The ‘‘butterfly’’-shaped [Cp92Mo2Co2S3(CO)4] (Cp9 ¼ MeCp) reacts with T in solution under hydrogen (150  C, 15 atm) to produce the free C4 hydrocarbons corresponding to HDS and a new cubane cluster [Cp92Mo2Co2S4(CO)2] in which the extruded sulfur

Hydrodesulfurization and Hydrodenitrogenation

has been incorporated (Equation (14)). This cubane cluster is converted back to the original one by reaction with CO yielding COS, thus completing a cyclic reaction; CO inhibits the forward desulfurization reaction and therefore the process cannot be rendered catalytic. The exact mechanism of thiophene HDS by use of these clusters is not clearly understood.124 Nevertheless, extensive related mechanistic studies on the desulfurization of thiols and sulfides using the same cluster Cp92Mo2Co2S3(CO)4 allowed the main reaction pathways for desulfurization of these substrates to be understood, according to the overall mechanism represented in Scheme 11.125 ′ –

–

ð14Þ ′

′

′

If this mechanism is combined with the reaction networks that have been worked out for the ring opening and hydrogenolysis of thiophenes by electron-rich late transition metal complexes, a very complete mechanistic model for heterogeneous HDS emerges. In addition, it is important to note that in this model, there are several low-energy steps related to breaking and forming C–S bonds, and that the proton is highly mobile. These features are also typical of Co–Mo–S HDS catalysts, which are very dynamic metal sulfide surfaces acting as reservoirs of protons and electrons. A further point of interest that emerges from Scheme 11 is the concept of ‘‘latent vacancies and sulfur mobility’’. The Co atom, which is the primary site of attack of thiols, in this case, is electronically saturated and the empty site required for the incoming thiol is formed only when needed through an energetically little demanding rearrangement of a quadruply bridging sulfide (bonded to 2Co þ 2Mo atoms) into a triply bridging mode (bonding 1Co þ 2Mo atoms). This is consistent with the Co–S bond (promoter metal) being weaker than the Mo–S bond. At the end of the HDS reaction, the cobalt loses the ‘‘extra ligand’’ and the quadruple bridge can be easily reconstructed. Within this framework, there is no need to invoke physical ‘‘vacancies’’, but a resting state of the cluster possessing latent vacancies. Although the distinction is rather subtle, the idea of ‘‘latent’’ or ‘‘potential’’ vacancies in a ‘‘resting state’’ rather than actual physically empty sites around electron-rich areas of space seems like an adequate and useful evolution of the classical concept of ‘‘anionic vacancies’’ in HDS catalysts. Modeling substrate adsorption on Ni–Mo–S catalysts, the Ni atom in the unsaturated cubane-like cluster [Me– Cp3Mo3S4Ni]þ binds HDS-relevant molecules such as Me2S, Et2S, But2S, THT, as well as HDN-related compounds such as pyridine and quinoline. No derivatives of thiophenes could be observed.126 RSH S

S Co S

Mo Mo

Co S

rds

S

R

H S

Co

S

.

. S Mo Co Mo

Co

S

H R S Co Mo Mo

H S

S

S Co S

R

Co Mo Co S S Mo

Mo

H S

S

Co S

H R S Co Mo S

R S Co Mo

Mo

S

–RH S Co S

S S CH2 –2CO Mo Co Mo

S

Co S

S Mo Co S Mo

Scheme 11 Desulfurization of thiols on Co–Mo–S clusters. Reproduced from ‘Organometallic modeling of the hydrodesulfurization and hydrodenitrogenation, by R. A. Sa´nchez-Delgado; in Catalysis by Metal Complexes; B. R. James and P. W. N. M. van Leeuwen, Eds.; Kluwer Academic Publishers: Dordrecht, Vol. 24, p. 126 (2002). With kind permission of Springer Science and Business Media.

781

782

Hydrodesulfurization and Hydrodenitrogenation

1.27.2.3.2

Catalytic hydrogenolysis and hydrodesulfurization of thiophenes

A few well-defined molecular catalysts for the hydrogenolysis and HDS of thiophenic substrates to their corresponding thiols in solution are available.4,4a,4b,6,6a,6b The electron-rich fragments [(triphos)RhH] and [(triphos)RuH] are efficient homogeneous hydrogenolysis catalysts; reactions are typically performed at 160  C and 30 atm H2, in the presence of strong bases such as NaOH or KOtBu, conditions that are well tolerated by the very stable triphos derivatives. The main reaction pathways have been elucidated by kinetic studies, high pressure in situ NMR measurements, and the isolation and independent syntheses of key intermediates, together with the extensive knowledge previously gained from stoichiometric modeling studies involving analogous (triphos)Ir complexes. The most important features of the catalytic cycles are exemplified in Scheme 12 for the hydrogenolysis of BT. It is interesting to point out that the S atom of the ring-opened intermediate readily binds to W(CO)5 and the resulting heterobimetallic intermediate 47 reacts subsequently with H2 achieving desulfurization, albeit in a non-catalytic way. While the anionic Ru(0) complex [(triphos)RuH] is active and selective for the conversion of BT into 2-ethylthiophenol,101 the corresponding cationic Ru(II) species [(triphos)RuH]þ, with the same geometry but two electrons less, very efficiently saturates the thiophene ring of BT to form DHBT catalytically, but it is unable to break the C–S bonds.85 This is a remarkable switch in selectivity brought about by a 2e difference between two otherwise very similar catalysts; this is consistent with electron-rich 16e fragments being particularly adapted for 1-S-bonding and C–S bond scission of thiophenes, while electron-poorer more strongly electrophilic centers prefer the 2(CTC) bonding leading to hydrogenation. The [(triphos)Ru]x systems demonstrate that the selectivity of a metal site can be fine-tuned by a simple 2e variation while maintaining essentially the same geometrical characteristics around the active site. A Co–Mo–S surface is electron rich and it allows electrons to move freely, mainly through the sulfur bridges, which are easily formed, broken, and reconstructed. Other low-energy 2e redox pathways are available in metal sulfide surfaces, such as the interconversion of surface S22, S2, SH, etc., under HDS conditions; therefore, these organometallic systems provide important clues on how a single-surface geometric configuration containing few metal atoms and several forms of sulfur ligands can give rise to different types of reactivity. The hydrogenolysis of T, DBT, and dinaphthothiophene (DNT) can also be performed with the Rh-based catalyst.127 This work has been extended to aqueous biphasic systems for BT hydrogenolysis, by use of Na–sulphos in combination with Rh, to generate active catalysts that are easily recovered through simple decantation. The Rh–sulphos derivative has also been grafted onto polystyrene/divinylbenzene affording effective and recyclable catalysts for BT hydrogenolysis under conditions analogous to the homogeneous reactions. This polymer-supported catalyst also promoted the complete HDS of BT to ethylbenzene, albeit with lower efficiency. Both Rh(I) and Ru(II) were attached to silica through strong hydrogen bonds, but these systems were not effective as they do not tolerate strong bases. A variation involving a triphos analog covalently linked to silica did result in active Ru and Rh heterogenized catalysts.80,80a,80b The first homogeneous catalytic HDS reaction reported involved the conversion of DBT to biphenyl plus H2S, together with some hydrogenolysis products by use of [(triphos)IrH], generated by thermolysis of [(triphos)IrH2(Et)] in THF solution. The reaction proceeds slowly at 170  C. The catalytic cycle (Scheme 13) was established from the isolation of most of the intermediates involved and the independent study of the individual reactions implicated.100

H–M +BT

H

H M S

H M S

+BT –SH

Et Et

+H2 M S

H–M S

Scheme 12 The mechanism of homogeneous catalytic hydrogenolysis of benzothiophene.

Hydrodesulfurization and Hydrodenitrogenation

H Ir H H

+H2 –H2S, –Ph–Ph, –Ph–Ph–SH

+H2 –DBT

S Ir

Ir S H

+DBT –HS–Ph–Ph +H 2 H Ir H S

Scheme 13 Homogeneous catalytic HDS and hydrogenolysis of dibenzothiophene on Ir-triphos complexes.

The most active catalysts for the homogeneous HDS of a series of substituted thiophenes (2-MeOT, 3-MeOT, 3-AcT) to the corresponding hydrocarbons make use of thiaplatinacycles derived from the reactions of [Pt(PEt3)3] with the corresponding thiophene; up to 81 turnovers were achieved at 100  C and ca. 20 atm of H2 in THF in the presence of mercury as a ‘‘sulfur trap’’.105 In addition, the first Ni-based homogeneous HDS catalysts have been recently discovered; [(dippe)NiH]2 and Ni(PEt3)4 promote the desulfurization of DBT, 4-MeDBT, and 4,6-Me2DBT under mild conditions (refluxing toluene), by means of a cross-coupling reaction with Grignard reagents, according to the mechanism depicted in Scheme 14. For instance, DBT is converted mainly into Me–Ph–Ph–Me when MeMgBr is used in conjunction with the Ni complexes; curiously, when PriMgCl is used instead, the major product is Ph–Ph, implying that the alkyl group attacks the Ni center and undergoes a -H elimination to generate a transient Ni hydride, which is the species actually performing a straightforward HDS reaction. These results are extremely interesting, since they involve one of the commonly used metals in industrial

Pr i2 P

+DBT -R-Ph-Ph-R

Ni S P Pr i2 +RMgX +DBT –H2

Pr i2 P P Pr i2

Ni R R

Pr i2 P

Pr i2 P

H

Ni H

P Pr i2

Ni P Pr i2

Pr i2 P Ni R P XMgS Pr i2

+RMgX –MgX2, +MgS Pr i2 P XMgS Ni P Pr i2

R

Scheme 14 Homogeneous catalytic desulfurization of dibenzothiophene on Ni complexes.

783

784

Hydrodesulfurization and Hydrodenitrogenation

HDS and some of the most difficult substrates that remain in the heavier fractions during hydrocracking. Further work on these and related systems is expected to follow.

1.27.2.3.3

Conclusion

A variety of metal complexes activate the C–S bonds of thiophenic substrates, including the highly refractory Me2DBT. Although a good proportion of this chemistry has been performed with metals known to be active in HDS as sulfides but not used in practice (Ru, Rh, Ir), examples involving typical components of commercial heterogeneous catalysts (Mo, W, Co, Ni) are now available and further examples of such chemistry will probably emerge. Electron-rich metal fragments are particularly adapted for 1-S-bonding to thiophenes, and this seems to be the best form of activation toward C–S bond activation. Organometallic modeling suggests that in real catalysts, electron-rich surface metal centers of strained geometry with one coordinative unsaturation are the most likely active sites. In contrast, electron-poorer centers are better suited for hydrogenation reactions of CTC bonds of S-heterocycles, for example, benzothiophenes. Subtle electronic changes without major geometrical rearrangements can lead to drastic differences in reactivity, and it is interesting to extrapolate this idea to metal sulfide surfaces, where redox processes are facile and electron mobility is high. In contrast to heterogeneous systems, breaking the C–S bonds of thiols or thiolates is more difficult than breaking the first C–S bond of a thiophenic molecule in metal complexes, and complete desulfurization involves sulfido-bridged dinuclear species or clusters, or external ‘‘sulfur traps’’ like Hg or Mg. Efficient catalysts for the hydrogenolysis and hydrodesulfurization of thiophenes, including 4,6-Me2DBT, continue to be discovered, and immobilization of homogeneous systems on solid supports offers much promise for practical developments. Further exciting results in the area of multimetallic complexes in HDS modeling as well as on new catalytic systems are to be expected.

1.27.2.4 HDS-relevant H2 and H2S Reactions with Metal Complexes A key feature in HDS is the ability of metal sulfides to activate H2. Experimental and theoretical evidence indicate that on Co–Mo–S catalysts hydrogen activation may occur on the metal or on any one of the various forms of surface sulfur atoms (MTS, M–S–M, M(S2), M2(-S2)). Hydrogen atoms activated on a metal–sulfide surface can behave as protons (in –SH sites) or as hydrides (on the metal centers, M–H); also, bridging H atoms in M–H–S, M–H–M, or S–H–S groups display intermediate acidity. Adsorbed H atoms are highly mobile and migrate throughout the catalyst surface via low-energy pathways, notably redox processes that easily interconvert Hþ and H units, as well as M–S, S–S, and S–H bond breaking and forming. In addition, the adsorption and reactions of H2S at the surface of a catalyst are of great importance in connection with the formation and deactivation of active sites, and with the actual desulfurization processes.1–3 The chemistry of metal complexes with sulfur ligands129,129a provides interesting models for the interaction of H2 with metal sulfide surfaces, but the relation of such chemistry to HDS catalysis has been little addressed in organometallic modeling studies. An interesting option is the heterolytic activation of H2 with an S-donor ligand acting as the base required to remove the proton. Well-defined metal complexes of sulfide or disulfide ligands are relatively scarce but a number of reactions of H2 with metal complexes of simple sulfur ligands are available and they may be related to H2 activation on metal sulfide catalysts. In addition, the chemistry of metal complexes containing H2S and –SH ligands provides information relevant to HDS.4,4a,4b Such compounds are rather uncommon and usually unstable; important examples of metal–sulfur derivatives and their reactions are described in this section.

1.27.2.4.1

Hydrogen activation on complexes containing sulfido or disulfido ligands

The reactions of hydrogen with dinuclear cyclopentadienyl Mo and Re complexes containing bridging sulfido and disulfido ligands have been extensively studied.130,130a–130e In a clear parallel to MoS2 surfaces, [(Cp9Mo)2(-S2) (-S)2] (Cp9 ¼ methylated cyclopentadienyls) take up H2 at room temperature and atmospheric pressure to yield the corresponding bis(hydrosulfido) derivatives [(Cp9Mo)2(-SH)2(-S)2] (Equation (15)). Hydrogen addition takes place exclusively at the S22 site; it is not reversible and it does not proceed further to yield H2S, as is the case on solid catalysts. The bis(hydrosulfido) complex catalyzes H/D exchange under H2/D2 at room temperature and the homogeneous hydrogenation of elemental sulfur to H2S at 75  C and 1–3 atm H2. Thiols exchange readily with the –SH ligands of the Mo dimer to liberate H2S and form the corresponding thiolato derivatives (Equation (16)); this is a good molecular analog of a reaction commonly thought to intervene in HDS over solid sulfide catalysts.

Hydrodesulfurization and Hydrodenitrogenation

H

H S

S

S MoCp′

Cp′Mo S

+ H2

25 °C

S

R S MoCp′ + 2RSH

Cp′Mo S

S

S

S

S

S

ð15Þ MoCp′

Cp′Mo

H

H

S

R S

ð16Þ MoCp′ + 2H2S

Cp′Mo S

S

Only the bridging disulfido ligand reacts homolytically with hydrogen, in agreement with some heterogeneous mechanisms that invoke the dissociation of H2 on surface S22 groups. Nevertheless, in the related complex [(CpMo)2(-SR)(-S)(-S2CH2)], a bridging S2 group heterolytically activates H2 assisted by an external base. A related rhenium complex [Cp9Re(-S2)2]2Cl2 containing two bridging S22 units also reacts with H2 under mild conditions to yield H2S and a trinuclear rhenium cluster [(Cp9Re)3S4Hx]nþ.131 This hydrogenolysis of bridging sulfides is an important mechanistic model for the well-accepted pathway in the formation of anionic vacancies on metal sulfides. Hydrogen activation also takes place on [(triphos)Rh(-S)2Rh(triphos)]2þ (triphos ¼ MeC(CH2PPh2)3) by heterolytic splitting mediated by the metal and the sulfido ligand to yield [(triphos)Rh(H) (-SH)2Rh(H)(triphos)]2þ. In this case, 2-H2 bonding precedes heterolytic activation with the bridging sulfide acting as the base required to capture the proton.132,132a This reaction is reminiscent of a proposal frequently encountered in the heterogeneous literature, which has been supported by solid-state NMR studies of hydrogenated RuS2. Similarly, Ir2S2(PPh3)4 reacts with hydrogen by sequential homolytic and heterolytic H2 splitting to yield Ir2H2(-H)(-SH)(-S)(PPh3)4.133 In summary, the reactions of dimeric complexes containing -S2 and -S22 ligands with H2 are interesting models for some key steps believed to happen on MoS2 and related catalysts. Both homolytic and heterolytic activation pathways have been authenticated in these complexes and it is easy to extrapolate such mechanisms to surfaces. Hopefully, this chemistry will continue to be developed, thus providing further insight into the role of surface sulfur species in HDS mechanisms. Besides hydrogen activation on bridging S22 groups, mechanistic proposals for HDS on metal sulfides contemplate either the homolytic splitting of H2 on a monometallic M(S22) site to yield M(SH)2, or the heterolytic activation by a terminal MTS group leading to M(H)(SH) species. These possibilities are nicely modeled by reaction of Cp* 2Ti(2-C2H4) with sulfur to yield Cp* 2(py)TiTS, which further reacts with sulfur or with ethylene sulfide to produce Cp* 2Ti(2-S2).134,134a The sulfido derivative reversibly reacts with H2 at atmospheric pressure to produce the corresponding hydrido–hydrosulfido species Cp* 2Ti(H)(SH), through heterolytic splitting of a coordinated dihydrogen molecule with the S atom acting as the base. In turn, Cp* 2Ti(SH)2 is gradually produced upon interaction of Cp* 2Ti(S2) with H2 at 70  C. Both products were characterized by NMR spectroscopy, but Cp* 2Ti(H)(SH) was unstable under those reaction conditions, readily eliminating H2 and reverting back to Cp* 2(py)TiTS. An alternative route to Cp* 2Ti(H)(SH) involves treatment of Cp* 2Ti(2S2) with H2 in the presence of PPh3 or PMe3 as a sulfur scavenger. This remarkable set of reactions represents another excellent model for H2 activation on metal sulfides; hopefully, other examples will become available in the future.

1.27.2.4.2

Reactions of H2S with metal complexes

A limited number of complexes containing coordinated H2S have been prepared, and very few have been structurally characterized.4,4a,4b,129,129a The synthetic methods used involve addition to coordinatively unsaturated compounds, displacement of labile ligands, or protonation of hydrosulfido ligands. Group 6 derivatives129,129a,135,135a,135b such as (CO)5M(SH2) (M ¼ Cr, W) or Cp(CO)3M(SH2) (M ¼ Mo, W) are good models for H2S adsorption on MoS2 or WS2 surfaces although in the complexes the oxidation state of the metal is lower than in metal sulfides. For group 7, Cp(CO)2Mn(SH2), (PPh3)(CO)4Mn(SH2), [(CO)5Re(SH2)]BF4, and [(triphos)(CO)2Re(SH2)]OTf are

785

786

Hydrodesulfurization and Hydrodenitrogenation

known.129,129a,136,136a,136b Ru derivatives are the most numerous and stable, including [(NH3)5Ru(SH2)](BF4)2, [(NH3)4(ISN)Ru(SH2)](BF4)2 (ISN ¼ isonicotinamide), [(LS4)(PPh3)Ru(SH2)] (LS4 ¼ SC6H4S(CH2CH2SC6-H4S), [(P–N)(PR3)RuX2(SH2)] (P–N ¼ Ph2PC6H4NMe2; R ¼ Ph, p-tol; X ¼ Cl, Br); [Cp9(PPh3)2Ru(SH2)]OTf (Cp9 ¼ Cp, C5H3S–CH2–C5H4); [Ru2(CO)5(-etipdp)2(SH2)](SbF6)2 (etipdp ¼ N-ethyl(tetraisoppropoxy)diphosphazane); and [Ru(3,3-C10H16)Cl2(SH2)].4,4a,129,129a,137,137a–137f For the heavier metals, [IrH2(SH2)2(PPh3)2] BF4138 and [Pt(PPh3)2(SH2)]139 are worth mentioning. H2S complexes are easily oxidized and their stabilization is usually accomplished by use of electron-rich metal centers in combination with bulky ligands. Other Lewis bases readily displace H2S, which is also activated toward deprotonation by strong bases. Examples of oxidative addition of H2S to give hydrido–hydrosulfido species are known; in some cases the oxidative addition proceeds further with elimination of dihydrogen and the consequent formation of a stable sulfido complex. For instance, Cp* 2Zr(CO)2 reacts with 2 equiv. H2S to yield Cp* Zr(SH)2, but if H2S is the limiting reagent, the product of the reaction is [Cp* Zr(SH)]2(-S). Also, the reaction of Cp* 2Zr(CO)2 with elemental sulfur in the presence of pyridine yields the terminal sulfido complex Cp* 2Zr(S)(py), which undergoes oxidative addition of H2S to produce the bis(hydrosulfido) complex.129,129a Reaction of Cp* 2TaH3 with excess sulfur yields [Cp* 2Ta(H)(2S2)], and with H2S produces [Cp* 2Ta(H)(S)].140,140a Mo and W complexes of formula M(PMe3)4(2-CH2PMe2)H (M ¼ Mo, W) readily react with H2S to yield trans-M(PMe3)4(S)2; in the case of tungsten, a bis(hydrido)bis(hydrosulfido) intermediate can be isolated, which readily dehydrogenates in solution to yield W(PMe3)4(S)2.141,141a The most numerous and thoroughly studied metal hydrosulfide complexes are those of Mo and they have often been used as models for HDS-related reactions and intermediates.129,129a For instance, the tris(dithiocarbamate) derivative 56 containing a hydrosulfido ligand on an Mo(IV) center in a ‘‘sulfur-only’’ environment is prepared by S-atom transfer from [Cp4Fe4S6] to [(CO)2Mo(S2CNEt2)2]. The structure of 56 closely resembles some of the geometries proposed for the active sites in MoS2 or WS2 catalysts.142

Et2N

SH S S W S S S S

NEt2

NEt2

56 ReS4 reacts with H2S in the presence of PMe3 to yield ReH(SH2)(PMe3)4, according to Equation (17). As suggested by the stoichiometry of this reaction, the Re(H)(H2S) complex is a catalyst for the reaction of H2S with PMe3 to yield H2 þ SPMe3, and also for H–D exchange between H2 and D2 and between H2S and D2.143 ReS4– + 3H2S + 8PMe3

ReH(SH)2(PMe3)4 + H2 + 4S=PMe3

ð17Þ

The complex [(PP3)Fe(SH)]BPh4, where PP3 ¼ P(CH2CH2PPh2)3, was obtained by bubbling H2S through an ethanolic solution of [Fe(OH2)6][BF4]2 in the presence of the PP3 ligand.144 Alternatively, this compound can be prepared from [(PP3)Fe(H)(H2)](BPh4) or [(PP3)Fe(H)(N2)](BPh4) by interaction with H2S.129,129a Several examples of oxidative addition of H2S to Ru complexes, leading to hydrosulfido derivatives, are known. They include the formation of Ru(SH)2(CO)2(PPh3)2 from Ru(CO)2(PPh3)3 and the transformation of cis/trans-Ru(H)2(dpm)2 (dpm ¼ Ph2PCH2PPh2) into trans-RuH(SH)(dpm)2 and cis/trans-Ru(SH)2(dpm)2 by reaction with H2S.129,129a,145 Dimeric Pd complexes [(X)Pd(-dppm)2Pd(X)] (X ¼ Cl, Br) promote the decomposition of H2S into H2 þ coordinated S. Kinetic and spectroscopic data led to the mechanism depicted in Scheme 15. H2S adds reversibly across the Pd–Pd bond to yield [(X)(H)Pd(-dppm)2Pd(X)(SH)], which then undergoes elimination of H2 by a concerted deprotonation of the –SH group and protonation of the Pd–H bond to produce the bridging sulfido derivative [(X)Pd(-S)(-dppm)2Pd(X)]. Removal of the bridging S atom from through oxidation by H2O2 or m-chloroperbenzoic acid yields [(X)Pd(-SO2)(-dppm)2Pd(X)], which reversibly loses SO2 to regenerate the starting complex. In this way, the process becomes catalytic for the conversion of H2S into H2 þ SO2. In a variation of this H2S decomposition catalysis, an excess of the diphosphine can be used to abstract the bridging sulfide as dppm(S).145

Hydrodesulfurization and Hydrodenitrogenation

Ph2P X Pd Ph2P

PPh2 Pd X

Ph2P + H2S

PPh2

PPh2 SH Pd

Pd X X Ph2P PPh2 –H2

–SO2 Ph2P O2 PPh2 S Pd Pd X X PPh2 Ph2P

H

+H2O2, –H2

Ph2P

PPh2 S

Pd Pd X X PPh2 Ph2P

Scheme 15 Homogeneous catalytic conversion of H2S into H2 þ SO2 on Pd complexes.

1.27.3 Organometallic Models of the Hydrodenitrogenation Reaction As mentioned in Section 1, HDN model studies using organometallic complexes are not as numerous as for HDS.4,4a,4b,7,7a The substrates most commonly used in HDN modeling (see Figure 1) may be divided into two main classes: the less basic pyrrole (Pyr) and indoles (In), in which the nitrogen lone pair is not available for interaction with electrophiles; and the strongly basic pyridine (py) and quinoline (Q), in which the N atom is accessible for bonding to metal ions. In both cases, it is also important to consider their higher homologs, their alkyl-substituted derivatives, and their partially or completely hydrogenated products. HDN modeling includes the structural characterization and bonding features of metal complexes of such N-donor ligands, as well as the hydrogenation N-heteroaromatics and the metal-mediated C–N bond activation.

1.27.3.1 Binding Modes of N-Heterocycles in Transition Metal Complexes Both the basicity of the nitrogen atom and the presence of carbocyclic rings in addition to the N-containing ring are important in determining the binding modes of N-heterocycles to single or multiple metal sites. In addition, the steric and the electronic effects induced by substituents in the vicinity of the N atom play a key role in the stability and the chemistry of each type of complex. The most important possible bonding modes of HDN-related organonitrogen molecules are summarized in Figure 3. Table 2 contains a representative list of metal complexes of interest in connection with HDN; details for compounds reported previous to 1995 can be found in earlier reviews,4,4a,4b,7,7a,146,147 and the more recently synthesized relevant derivatives will be referred to in the following sections.

1.27.3.1.1

Complexes of pyrrole, indole, carbazole, and related ligands

No examples of complexes containing 1-N-bonded pyrrole or carbazole (Cbz) as ligands are known in accord with the low basicity of the N atom in these molecules. On the other hand, some examples of 1-N-bonded indole species have been reported, but they are better described as 1-N–3H–indolenine species, for example, PdCl2[1(N)–3H–In9]2 (In9 ¼ 2-MeIn and 2,5-Me2In), Ir(CO)(PPh3)2[1(N)-3H–Me2In], CpRe(NO)(PPh3)[1(N)– 3H–In9]2. In addition, an osmium carbonyl cluster containing an orthometallated form of this tautomer of indole is known.4,4a,4b,7,7a Metal derivatives containing olefin-like ligands, in either one of the tautomeric forms, 2(CTC)–1H–Pyr or 2  (CTC)–2H–Pyr, have been characterized for [Os(NH3)5(L)]2þ complexes.66 The 4-mode has only been observed in Cp* Ir[4-1–But–2,3,4,5-(CO2Me)4Pyr], whose structure displays the pyrrole ligand bonded through the two CTC bonds, while the nitrogen atom is bent away from the ring to a non-bonding distance from Ir, analogous to 4-thiophene structures. 5-Bonded metal complexes of pyrroles are known for Cr, Mn, Fe, Co, Ru, Rh, Re, and Ir (see Table 2).4,4a,4b,7,7a,146,147 Pyrrolyl ligands are more frequent throughout the periodic table, and depending on the degree of electronic unsaturation of the metal fragment, binding through the N atom only, or through the entire -ring is observed.

787

788

Hydrodesulfurization and Hydrodenitrogenation

R

.. N

H (R) N

N

M N M

η 4-Pyr

N

M N

H

N

M

η

η

(N)-3H-In

η

-In

-Inyl

N H

η 1(N)-indoline

M

M

M

M

η 1(N)

η6

η 1(C)

η 2(C,C)

η 2(N,C)

η

(N)

N

M

M

M

η (N,C) 2

M M

μ 2,η

2

(N,Cα )

M M

N M

M M

μ 3,η 2(N,C)

M M

μ 2,η 1(N)

η 2(N,C)-pyridyl

M

M

η (πC)

η 1(N)-indolinyl

N

N N 6

η 6-indoline

N

H

M

1

N M

N N

N

N M

η 5-Pyl

M

η 1(C)-Inyl

η 6-Inyl

N

N

N

η 1(N)-Inyl

M

η 3,η 2-Pyl

N H

M

M 5

η 1C-Pyl

H M N

N–

M 6

η 1N-Pyl

η 5-Pyr

N M

M

η 2(C,C)-2H-Pyr

M N

N

M

M

M

η 2(C,C)-1H-Pyr

1

R N

R N

μ 2,η 2(N,C)

N M

M M

μ 2,η 2(N,C)

Figure 3 Bonding modes of N-heterocycles in metal complexes.

The 5-bonded pyrrolyl complexes transform to 1-bonded species by addition of extra ligands, as in the case of CpFe(5-Pyl), which reacts with CO or RNC to yield CpFe(1-Pyl)L2. Oxidative addition of C–H bonds leading to 1(C)pyrrolyl(hydride) metal complexes has also been described.4,4a,4b,7,7a The indolyl anion can bind to mononuclear complexes using the nitrogen atom only 1-N, the entire heterocyclic ring 5 or the entire carbocyclic ring 6, thus making it a very versatile ligand; the dominant coordination mode for indoles is 6, analogous to what is observed for BT, whereas for indolyl-type ligands 5 is preferred. It does not seem that such bonding is directly related to HDN mechanisms on solid catalysts, but some of these species easily interconvert as, for instance, in the base-assisted 6-In ! 5-indolyl shift that takes place on Cp* M (M ¼ Rh, Ir) or the acid-promoted 1(N)–indolyl ! 1(N)– indolenine conversion on CpRh(PPh3)(NO). Electron-rich metal fragments react with pyrroles or indoles yielding either the kinetic C–H or the thermodynamic N–H insertion products. C–N bond activation, on the other hand, is not observed in contrast with C–S bond breaking, which is readily promoted by electron-rich metal centers, as described above. Despite the fact that carbazole is a very good model compound of the most highly refractory HDN substrates, its coordination chemistry has not been investigated in detail, and to our knowledge, there are no examples of metal complexes containing an intact carbazole ligand. Thus, N–H activation leading to metal hydrido derivatives containing the carbazoyl ligand bound through the nitrogen atom is observed in the reaction of Cp* Rh(PMe)3(H)(Ph) with carbazole to yield Cp* Rh(PMe)3(H)(1Ncarbazoyl) together with the kinetic C–H activation isomer.148 Mo(PMe3)6 reacts with pyrrole to yield MoH(PMe3)3(5-Pyl), the only Mo–pyrrolyl complex known, and with indole to give MoH(PMe3)4(1N–Inyl) 57; the latter rearranges with loss of a phosphine as in Equation (18) into MoH(PMe3)3(5-Inyl) 58, which can revert back to the 1-N form by phosphine addition, or thermally convert into MoH(PMe3)3(6-Inyl) 59, a rare example of an 6bonded indolyl anion. The latter complex may be regarded as zwitterionic with a formal positive charge on the Mo and a formal negative charge on the N. 59 does not convert back to 58 thermally, but this reverse reaction is brought

Hydrodesulfurization and Hydrodenitrogenation

Table 2 HDN-relevant metal complexes of N-donor ligands Pyrrole complexes [Os(NH3)5(2(CTC)-Pyr9)][OTf]2 (Pyr9 ¼ Pyr, 1-MePyr, 2,5-Me2Pyr, 1,2,5-Me3Pyr, 5-EtPyr, 1,4-Me2Pyr) Cp* Ir[4-1tBu-2,3,4,5(CO2Me)4Pyr] Cr(CO)3(5-1-MePyr) [Mn(CO)3(5-Pyr)]þ [CpFe(5-1-MePyr9)]þ (Pyr9 ¼ 1,2,5-Me3Pyr, 2,3,4,5Me4Pyr, Me5Pyr) [Fe(5-1-MePyr)2]2þ [CpCo(5-Me5Pyr9)]2þ [(p-cymene)Ru(5-Pyr9)][OTf]2 Pyr9 ¼ 2,3,4,5-Me4Pyr, Me5Pyr [(p-cymene)Ru(5-Me5Pyr)][OTf]2 (PPh3)2Re(H)2(5-Me5Pyr) [Cp* Rh(5-Me5Pyr9)]2þ (PPh3)2Re(H)2(5-Me5Pyr) [Cp* Ir(5-2,3,4,5-Me4Pyr9)]2þ

Pyrrolyl complexes MoH(PMe3)4(5-Pyl) CpFe(1-Pyl)L2 (L ¼ CO, RNC, R2NPF2) Fe(dmpe)2H(1-Pyl) Ru(dmpe)2H(1-Pyl) Ni(R)(PMe3)2(1-Pyl9) (R ¼ alkyl; Pyl9 ¼ Pyl, 2,5-Me2Pyl) IrHCl(PMe3)3(1-Pyl) Mn(CO)3(5-Pyl) [CpFe(5-Pyl9)]þ Pyl9 ¼ Pyl, 2,3,4,5-Me4Pyl, 2,5-tBu2Pyl (Carborane)Co(5-Pyl9) (Pyl9 ¼ 2,5-Me2Pyl, 2,3,4,5Me4Pyl) Cp* Ru(5-Pyl) Ru(5-Pyl)2 (PR3)2RuCl2(5-Pyl) [(p-cymene)Ru(5-2,3,4,5-Me4Pyl)][OTf] [(p-cymene)Os(5-2,3,4,5-Me4Pyr)][OTf] Cp* Rh(H)(Pyl) MH(PEt3)2(1N-Pyl) (M ¼ Ni, Pd, Pt)

Indole complexes PdCl2[1(N)-3H-In9]2 (In9 ¼ 2-MeIn and 2,5-Me2In) {Ir(CO)(PPh3)2[1(N)-3H–Me2In]}þ CpRe(NO)(PPh3)[1(N)-3X–In9]2 (X ¼ H, Me; In9 ¼ In, 3-MeIn, 3-EtIn) Cr(CO)3(6-In) [Mn(CO)3(6-In)]þ [Cp9Ru(6-X-In]þ [(p-cymene)Ru(6-In9)][OTf]2 In9 ¼ In, 1-MeIn, 2MeIn, 2,3-Me2In [Cp* M(6-In)]2þ (M ¼ Rh, Ir)

Indolyl complexes [(p-cymene)Ru(1-Inyl)(NCMe)2][OTf]2 CpRe(NO)(PPh3)(1-Inyl9) (L ¼ CO, RNC, R2NPF2) Ir(CO)(PPh3)2[1(N)-2,3-Me2Inyl] Mn(CO)3(5-2-MeInyl) [Cp* M(5-Inyl)]þ (M ¼ Rh, Ir) [(p-cymene)Ru(6-In9)][OTf] (In9 ¼ In, 1-MeIn, 2MeIn, 2,3-Me2In) MH(PEt3)2(1N–Inyl) (M ¼ Ni, Pd, Pt)

Indoline complexes MoH(PMe3)4(1-indoline) MoH(PMe3)(5-indoline) MoH(PMe3)(6-indoline) [(cymene)Ru(1-Indoline)(NCMe)2][OTf]2 PdCl2(PPh3)(1-indoline) Cr(CO)3(6-1-MeIndoline) [(p-cymene)Ru(6-indoline)][OTf]2 (In9 ¼ In, 1-MeIn, 2MeIn, 2,3-Me2In)

Unusual pyridine and quinoline complexes 2,1(N)-complexes Mo2O2[S2P(OPri)2]2(-O)(-S)[,1(N)–Py] 2(C,C)-complexes TpRe(CO)(MeIm)(2(C,C)-2,6-lutidine) [Os(NH3)5(2(C,C)-2,6-Me2Py)][OTf]2 2(C,N)-pyridine and pyridyl complexes [2(C,N)-Py]Nb(silox)3 [2(C,N)-Py]Ta(silox)3 [2(C,N)-2,4.6-t-Bu3Py]Ta(OAr)2Cl (Ar ¼ 2,6-iPrPh) [2(C,N)-Q9]Ta(OAr)nClm (Q9 ¼ Q, 6-MeQ; Ar ¼ 2,6-iPrPh; n ¼ 2, 3; m ¼ 1, 0) Cp* Lu(2(N,C)–NC5H4) Cp* Sc(2(N,C)–NC5H4). MoH(PMe3)4(2(C,N)Py) 6-complexes Mo(PMe3)3(6N–Q) Mo(PMe3)3(6C–Q) [(Cp* Ru(6-Py)][OTf] Cp* Rh(6-2,4,6-Me3Py)

about photochemically. Acridine produces only the 6-bonded species.149 Reaction of RuH2(H2)2(PCy3)2 with pyrrole leads to RuH(PCy3)2(5-Pyr), which can be subsequently protonated with HBF4 to yield [RuH2(PCy3)2(5-Pyr)]BF4.150 Zerovalent M(PEt3)3 (M ¼ Ni, Pd, Pt) complexes that are very active for C–S bond-breaking reactions activate the N–H bond of pyrrole exclusively to produce the N-bonded derivative M(PEt3)2(H)(1NPyl); indole and carbazole react in the same way.151

789

790

Hydrodesulfurization and Hydrodenitrogenation

PMe3

Me 3P

N–

N –PMe3

H Mo N Me3P PMe 3

H Me3P

57

80 °C

Mo PMe 3 PMe 3

hν

58

H Me3P

Mo+ PMe 3 PMe 3

ð18Þ

59

The interactions of dinuclear and polynuclear complexes with pyrroles and indoles have attracted attention as analogs of surface interactions. Clusters containing intact pyrrole or indole ligands are not known, as the reactions invariably involve N–H and/or C–H bond activation to yield derivatives containing bridging pyrrolyl or indolyl ligands. Nucleophilic attack of Pyr at the disulfide bond of [Cp9Mo(-S)2(S2CH2)]22þ yields [(Cp9Mo)2(S2CH2) [(-S)(-S-pyrrolyl)]þ through heterolytic scission of the S–S bond by the heterocycle to give -pyrrolylthiolate and -SH dimeric products (Equation (19)). 1-MePyr, 1,2,5-Me3Pyr, and 1-MeIn behave similarly, although the regiochemistry of electrophilic addition may vary depending on the number and on the position of the substituent.152 This reaction is interesting in that it shows the interaction of pyrrole with a framework resembling a catalytic MoS2 surface. S S Cp′Mo 2+ S MoCp′ S S S + Cp′Mo MoCp′ S S

N

R

H S S S S + + MoCp′ MoCp′ Cp′Mo + Cp′Mo S S S S

N R

ð19Þ

[Cp* Ir(H2)(-H)]2, which cleaves C–S bonds of thiophenes, also promotes the selective C–H bond cleavage of N-methylpyrrole under comparable experimental conditions, leading to Cp* Ir(H){2,1C,2(C,C)Pyr}(-H)2IrCp* .153 This is consistent with the higher energy barrier to C–N insertion as compared to C–S or C–H activation. Other examples of metal clusters containing pyrrole-derived ligands include the zwitterionic Ru3(-H)(3, 3-C4H3NMe)(CO)9 60, Ru3(-H)2(3,3-C4H2NMe)(CO)9 61, Os3(-H)2(3,3-C4H2NMe)(CO)9 62, Os3(-H) (,1-C4H2NMe)(CO)9 63.154 H + Me N H H (OC)3 Ru – (OC)3 Ru

60

Ru(CO)3 H

H H (OC) 3Ru

H N

H

Me

Ru(CO)3 H Ru (CO)3

61

H (OC) 3Os

N

+ N Me

Me

H

Os(CO)3 Os H (CO)3

62

(OC)4Os

Os(CO)3 – H Os (CO) 3

63

Both the 6 and 1(N) coordination modes of indoline, as well as their interconversion, have been reported for Re, Ru, and Pd complexes.4,4a,4b,7,7a This type of ligand is of particular relevance since N-aromatics need to be hydrogenated before nitrogen removal can take place in HDN catalysis. As an example, [(p-cymene)Ru(1indoline)(CH3CN)2)](OTf)2 converts into [(p-cymene)Ru(6-indoline)](OTf)2 upon gentle heating in CH2Cl2, and can be deprotonated by use of, for example, triethylamine to give the corresponding (1-indolinyl) complex.155

1.27.3.1.2

Complexes with pyridine, quinoline, and related ligands

The pyridines and quinolines and their higher homologs are more basic ligands than five-membered heterocycles, and although the coordination chemistry of such molecules is considerably more developed, comparatively little organometallic chemistry is available. The most common coordination mode for py is by far through the nitrogen atom, which uses its lone pair for interaction with Lewis acceptors. Unlike thiophene, which binds to metals in a bent fashion, the lone pair in the nitrogen atom of py is located in the ring plane, and thus the M–N vector in 1(N)–py complexes is also in the ring plane. Indeed, pyridine is one of the most frequently encountered classical ligands in coordination chemistry, and there are examples of 1-N–py complexes for virtually every transition metal in more than one oxidation state;156 therefore, no attempt will be made to include such numerous compounds in Table 2, where only some unusual coordination modes are exemplified, which may be of relevance in connection

Hydrodesulfurization and Hydrodenitrogenation

with HDN.4,4a,4b,7,7a The 6-mode of py is fairly common, and therefore, many studies directed at modeling HDN reactions have centered on the 1(N)- and 6-modes and the factors controlling their interconversion and/or prevalence. 1(C)–Py derivatives are obtained by replacement of a proton by a metal fragment, usually via C–H bond activation by electron-rich metal fragments. The organometallic chemistry of quinoline (Q) does not differ significantly from that of Py except that the presence of the carbocyclic ring allows a further bonding mode 6-C through the arene moeity. Extensive studies on Cp–Ru(II) and –Rh(III) complexes of N-heterocycles show that in the absence of any steric constraints, the 1(N) mode prevails over the 6, particularly with the more electrophilic Rh(III) center. For example, [Cp* Rh(CH3CN)3]2þ reacts with 2-Mepy and 2,6-Me2py yielding [Cp* Rh(1(N)-py9)(CH3CN)2]2þ, while the trisubstituted 2,4,6-Me3py prefers to form the 6-adduct [Cp* Rh(6-py9)]2þ. In the case of [CpRu(CH3CN)3]þ, in which the Ru center is more electron rich than Rh(III), a greater propensity of Ru(II) to stabilize the 6-mode is observed and the 1(N) ! 6 interconversion of 2-methylpyridine and 2,4-dimethylpyridine was followed by NMR spectroscopy. With unsubstituted py, [CpRu(1(N)-Py)3]þ is obtained; however, when Cp* is employed, [Cp* Ru(1(N)–Py)3]þ is the kinetic product that thermally converts into the corresponding 6-derivative.4,4a,4b,7,7a As for py, the most common coordination mode of Q in mononuclear complexes is the 1(N), often in equilibrium with the 6-arene form, as in the 1(N) ! 6 rearrangement detected by NMR in the reaction of Q with [CpRu(CH3CN)3]þ (Equation (20)). If Cp* is used instead of Cp, the 6-complex is rapidly formed, while the corresponding 1(N) adduct is not observed. Reaction of RuH2(H2)2(PCy3)2 with py leads to RuH2(H2)(1-py)(PCy3)2 with an extremely labile py ligand; in an analogous reaction, acridine binds in the rare 4-mode to two CTC bonds in the product RuH2(4acridine)(PCy3)2.150 The complexes [IrH2(PPh3)2(1-N–L)2]PF6 (L ¼ py, iQ, and pip, piperidine) were synthesized by hydrogenation of [Ir(COD)(PPh3)2]PF6 in the presence of the appropriate N-donor ligand. These complexes undergo reactions with small molecules leading to, for example, the carbonyl derivative [IrH2(PPh3)2(CO)(1-N– pip)]PF6 characterized by X-ray diffraction;157 examples of metal–piperidine complexes and their crystal structures are very rare, and they are of particular relevance since piperidine is the initial product of the reaction of pyridine with hydrogen over heterogeneous HDN catalysts. These complexes serve as models of the simultaneous activation of H2 and organonitrogen compounds on a single metal atom.

+

[CpRu(MeCN)3 ]

+ Ru

5 min

+ N

N

20 h RT

Ru+

ð20Þ N

The complex Mo(PMe3)6 provides a remarkable set of reactions with six-membered N-heterocycles in relation to HDN on heterogeneous Mo-based catalysts. Reaction with pyridine proceeds by activation of the C–H bond  to the N atom to yield the side-bonded pyridyl derivative [MoH(PMe3)4(2-C5H4N)]; acridine affords the commonly encountered arene-bonded [Mo(PMe3)3(6-C–acridine)] and more interesting, quinoline gives the only known complex 6-bonded to the heterocyclic ring, viz, [Mo(PMe3)3(6N-Q)], which isomerizes thermally to the more stable [Mo(PMe3)3(6-C–Q)] (Equation (21)). The 6-N-derivative reacts further with H2 to yield 1,2,3,4-tetrahydroquinoline (THQ), while the 6-C isomer does not react with hydrogen under similar conditions, demonstrating the importance of binding the heterocycle to the metal in order to reduce it.149 N Mo(PMe3) 6

N

80 °C

+

Mo N

Me3 P

PMe3 PMe3

80 °C

Mo Me3 P

ð21Þ

PMe3 PMe3

A rather rare coordination mode of pyridines is the 2(CTC) mode, which is exclusively stabilized by the electronrich [Os(NH3)5]2þ fragment, for example, 2,6-lutidine. The olefin-like complex easily rearranges to the 1(N) mode by a one-electron oxidation. Interestingly, the 2(CTC) mode of lutidine rearranges with time to give an Os(II) lutidinium ylide. In contrast, the 2(C,C) mode is maintained when [Os(NH3)5(2-lutidine)]2þ is protonated to give a lutidinium derivative.66,66a,66b The bonding modes of pyridines described so far represent reasonable models for the adsorption and activation of pyridines, quinolines, and acridines on HDN catalysts, but none of them promotes C–N bond activation, which ultimately leads to nitrogen removal. In contrast, a curious 2(N,C) coordination mode has been discovered, which

791

792

Hydrodesulfurization and Hydrodenitrogenation

effectively activates the py ring toward C–N bond scission.4,4a,4b,7,7a,159,159a,159b The complex (silox)3Ta[2(N,C)py] 64 (silox ¼ But3SiO) was prepared by reaction of Ta(silox)3 with pyridine, as illustrated in Equation (22). The structure of 64 is best viewed as a Ta(V) metallaaziridine, where the aromaticity of py has been substantially perturbed; analogous reactions with 2-picoline and lutidine give similar 2(N,C) products. Similar derivatives of other pyridines have been synthesized. Reduction of (silox)3NbCl2 with Na/Hg in py affords a kinetic 2(N,C)–py product which thermally undergoes C–N insertion (vide infra). R N

R

N (silox)3Ta

(silox)3Ta +

ð22Þ

R = H, Me

A related Ta compound containing (dipp)2ClTa(2(N,C)–2,4,6-NC5H2But3) (dipp ¼ 2,6-OC6H3Pr2i) was obtained through an indirect route involving insertion of a nitrile into a tantallacyclopentadiene complex. The 2(N,C) quinoline analog (dipp)3Ta(2(N,C)–Q) (dipp ¼ 2,6-OC6H3Pri2) is obtained in an analogous manner to the py derivative, via an 1H-bonded intermediate.146 Besides being precursors to C–N bond scission, intermediates containing 2(N,C) py ligands intervene in C–H-activation reactions leading to complexes containing 2(N,C)– NC5H4 ligands like Cp* Lu(2(N,C)–NC5H4) and Cp* Sc(2(N,C)-NC5H4). Similar Ti derivatives with 2-substituted pyridines have been described. The reaction of py with a triosmium cluster results in a trinuclear compound [Os3(H)(-NC5H4)(CO)10], in which py uses the N atom and the C atom for coordination to two metal centers.158 Transition metal complexes of other relevant organonitrogen compounds such as isoquinoline, 5,6-benzoquinoline, 7,8-benzoquinoline, acridine, and phenanthridine are known, and they contain the ligand coordinated in the 1(N) or the 6-arene fashion. The triosmium cluster Os3(CO)10(CH3CN)2 reacts with polyaromatic N-heterocycles such as 5,6-benzoquinoline and phenanthridine in an analogous manner to py and Q, yielding 3,2(N,C,C) complexes.4,4a,4b,7,7a

1.27.3.2 Reactions of N-heterocycles in Transition Metal Complexes Related to HDN When N-heterocycles bind to a transition metal complex, their reactivity may be enhanced with respect to the free molecules. The reactions depend on the electronic and geometric characteristics of the metal-containing fragment, as well as on the nature of the organonitrogen substrate; however, no general trends relevant for HDN can be extracted from the accumulated literature. Nevertheless, two types of reactions merit further discussion, namely N-heteroaromatic ring hydrogenation and the rare activation of C–N bonds by metal complexes.

1.27.3.2.1

Hydrogenation of N-heteroaromatic compounds

The hydrogenation of N-heteroaromatic compounds is of interest in relation to HDN, since selective hydrogenation of the nitrogen-containing rings always takes place prior to C–N bond breaking (see Scheme 2).4,4a,4b,7,7a,160 Few examples are available of metal complex-catalyzed hydrogenation of pyridines, which is not easy due to their high aromatic character. Rh6(CO)16 under water-gas shift conditions (CO þ H2O) reduces pyridine but with low efficiency, and the reaction mechanism is not known. Early reports on the activity of RhCl3(py)3/NaBH4 for pyridine reduction were not followed up. On the other hand, detailed work was carried out on the hydrogenation of 2-methylpyridine to 2-methylpiperidine by use of [Cp* Rh(NCMe)3]2þ as the catalyst precursor. Mechanistic studies, based on NMR spectroscopy, deuteration experiments, and isolation of reaction intermediates, suggest that hydrogen addition to both CTN and CTC bonds is reversible, and that initial CTN bond reduction is the key step of the cycle by breaking the aromaticity of the ring. Partially hydrogenated pyridines are readily dehydrogenated back under the reaction conditions, confirming that re-aromatization is thermodynamically favored over C–N bond breaking during HDN.161 Much more success has been achieved in the hydrogenation of polynuclear N-heterocycles; the regioselective reduction of the N-containing ring in quinoline, isoquinoline, indole, benzoquinolines, acridine, and other related molecules can be achieved with relative ease under moderate conditions. Simple metal carbonyls of Mn, Cr, Mo, W, Fe, Ru, Rh display some hydrogenation activity for a variety of polynuclear N-heterocycles under H2, H2/CO, or CO/H2O but conditions are harsh and turnovers low; metallic particles, rather than molecular species, may be involved. This makes them unattractive for practical use or for mechanistic studies. Cp and phosphine complexes

Hydrodesulfurization and Hydrodenitrogenation

MLn +H2 , –THQ

H

+Q, –THQ

H

NH MLn

N MLn H

N MLn H H

N H

NH

H H

MLn

MLn

+H2

Ln M

H

+H2

H N MLn

N H

Scheme 16 General mechanisms of the homogeneous hydrogenation of quinoline.

of Ru and Rh are more efficient for N-heterocycle hydrogenation under moderate conditions, and some of them have been extensively investigated, particularly, [CpRh(NCMe)3]þ, RhCl(PPh3)3, [Rh(COD)(PPh3)2]þ, RuCl2(PPh3)3, RuHCl(PPh3)3, MHCl(CO)(PPh3)3 (M ¼ Ru, Os), [MH(CO)(NCMe)2(PPh3)2]þ (M ¼ Ru, Os). Scheme 16 summarizes the most important catalytic cycles for quinoline reduction deduced for monohydride and dihydride systems on the basis of kinetic measurements, in situ spectroscopic studies, isolation of intermediates, and theoretical calculations; further details may be found in previous reviews.4,4a,4b,7,7a,160,161 The most interesting features of these mechanisms are: (i) the importance of 1-N binding of Q to initiate the cycle, in line with heterogeneous proposals involving vertical adsorption of the heterocycle onto catalytic surfaces; (ii) the possibility of 2(CTN)bonded intermediates being involved in the hydrogenation step; (iii) the reversible hydrogenation of the CTN bond, followed by migration of the metal fragment to the C(3)TC(4) bond, which is also reversibly hydrogenated; and (iv) binding of the product THQ through either the N atom or the carbocyclic ring. The mechanisms of Scheme 16 are easily adapted to explain the hydrogenation of pyridine and of other polynuclear N-heterocycles, the rates of which follow a trend that decreases with increasing basicity and steric hindrance at the nitrogen atom: phenanthridine > acridine > quinoline > 5,6-benzoquinoline > 7,8-benzoquinoline > indole > 2-Me–quinoline > 2-Me– pyridine. Isoquinoline is only reduced with difficulty, and this has been associated with its high basicity; indole is also difficult to hydrogenate, and very few catalysts have been reported for this reaction, maybe due to the fact that indole tends to bind to metals through the C(2)TC(3) bond, rather than 1-N, and thus the activation of the CTN bond is not very marked. Water-soluble catalysts composed of RuCl3?3H2O, m-monosulfonated, or trisulfonated triphenylphosphine (TPPMS, TPPTS), and a basic co-catalyst such as aniline or quinoline, which are active for the biphasic reduction of sulfur-containing heterocycles, can also be employed for the hydrogenation of N-heterocycles. The active species is thought to be [RuHCl(PR3)2(L)2] (PR3 ¼ TPPMS, TPPTS; L ¼ Q, THQ, aniline), and the mechanism is the typical one for monohydride catalysts.84,84a,84b Other water-soluble complexes containing sulfonated ligands, viz, (Na2–PPPDS ¼ [{NaO3S(C6H4)CH2}2C(CH2PPh2)2]), [(sulphos)Rh(COD)], and [(DPPDS)Rh(H2O)2]Na [(sulphos)Ru(NCMe)3]–SO3CF3 (sulphos ¼ [NaO3S(C6H4)CH2C(CH2PPh2)3]) are also very efficient in catalyzing the regioselective hydrogenation of Q under reasonable reaction conditions in liquid biphasic media, or when supported on silica or on polymers, and in both cases high catalytic activities for the hydrogenation of quinoline have been found.7 No reports are available on the homogeneous hydrogenation of carbazoles, which would be interesting substrates analogous to dibenzothiophenes. Regioselectivity for the reduction of the heterocycle has been the norm in all cases with the catalysts described above. However, the complex RuH2(H2)2(PCy3)2 hydrogenates quinoline, isoquinoline, and acridine selectively at the carbocyclic ring; it was suggested that this remarkable shift in selectivity may be due to 4-arene coordination being involved in the hydrogenation mechanism. Indeed the complex RuH2(4C–acridine)(PCy3)2 was isolated and shown to function as a catalyst precursor, thereby strongly indicating that it is an intermediate in the cycle.150

793

794

Hydrodesulfurization and Hydrodenitrogenation

1.27.3.2.2

Metal-mediated C–N bond-activation reactions relevant to HDN

A crucial step in HDN is the scission of C–N bonds of amines and N-heteroaromatics, but few examples are available of metal complexes that can model such reactions. One interesting case is the activation of py when coordinated to metals in the unusual 2(N,C) metallaaziridine mode, which has indeed resulted in C–N bond cleavage. The complex (ArO)2ClTa(2(N,C)-2,4,6-NC5H2But3) (ArO ¼ 2,6-Pri2PhO) reacts with 1 equiv. of LiHBEt3 in THF at low temperature to cleanly afford the corresponding C–N insertion product (ArO)2Ta[TNCtBuTCHCtBuTCHCHtBu], characterized by X-ray diffraction. Additional experiments with other carbon nucleophiles (RMgCl, MeLi) in place of LiHBEt3 have confirmed that the reaction occurs via initial addition of the hydride to the Ta atom, followed by an intramolecular endo-attack of the metal hydride onto the Py C atom, as depicted in Equation (23).146

N

N

Ta O

Ta

H–

Cl

O

O

H

N

endo -attack H

ð23Þ

Ta

O

O

O

The most interesting conclusion that may be derived from this chemistry is that 2(N,C) bonding produces a perturbation of the formally sp2-C atom of py toward sp3-hybridization, rendering it susceptible to nucleophilic attack by the hydride. However, this is not a general reaction, as the presence of the But groups on py is crucial for C–N bond cleavage. Neither the related Ta complex (silox)3Ta(2(N,C)–N–py) (silox ¼ OSiBut3) nor the quinoline derivative (ArO)2TaCl(2(N,C)-Q) undergoes C–N insertion upon nucleophilic addition. On the other hand, the 2(N,C)-bonded Q ligand can be stoichiometrically hydrogenated to THQ under mild conditions, but free Q is not reduced. Another interesting related case of metal-assisted ring opening of py makes use of Nb complexes, as shown in Equation (24). The low-valent (silox)3Nb fragment coordinates py in the 2(N,C) mode to yield (silox)3Nb(2(N,C)–NC5H5), which readily undergoes C–N insertion by thermolysis in benzene at 70  C [56]. The stoichiometry of the reaction yields 0.5 equiv. of py and 0.5 equiv. of (silox)3NbTCHCHTCHCHTCHNTNb(silox)3 as a thermodynamic mixture of cis,cis-, trans,cis-, trans,trans-, and cis,trans-isomers. (silox)3NbCl 2

Na/Hg, py 25 °C

Nb(xolis)3

N H

70 °C C6H6 –Py

H Nb(xolis)3 H

N Nb(silox)3

ð24Þ

Within its own limitations, these reactions represent the only examples of ring-opening reactions of pyridines by metal complexes that can be related to HDN catalysis. The fact that in these cases C–N scission occurs without prior hydrogenation of the heterocycle is in contrast with the heterogeneous catalysis literature, which points to a dominant mechanism via prehydrogenation, but it has also been argued that C–N bond activation of the unsaturated molecule could be envisaged as a less H2-consuming route for nitrogen extrusion.146 On the other hand, C–N bond hydrogenolysis of piperidine to various bis(piperidinyl)alkanes is promoted by Rh6(CO)16 in solution under water-gas shift conditions, but the real nature of this process is not clearly defined, and the harsh experimental conditions (150  C, 60 atm CO) and the proven capability of heterogeneous catalysts to promote similar reactions of pyridines and piperidines have raised doubts on the homogeneous nature of the process. In addition, ring-substituted anilines H2N–Ph–X react with (silox)3Ta by oxidative addition of the N–H and/or the C–N bond, depending on the substituents present on the ring (Equation (25)): NH 2 Ta (silox) 3 +

NH

X

ð25Þ

+ (silox)3 Ta

(silox)3Ta X

X

H

NH2

The propensity for C–N versus N–H activation correlates well with substituent Hammet parameters; groups that increase the basicity of aniline increase the relative rate of N–H activation, suggesting that nucleophilic attack by the amine at an empty dxz/dyz orbital of Ta(silox)3 precedes oxidative addition. On the other hand, electron-withdrawing substituents decrease the rate of N–H activation and increase the rate of C–N activation, similarly to the effects observed on electrophilic aromatic substitution. Arylamine N–H versus C–N activation is therefore a consequence of

Hydrodesulfurization and Hydrodenitrogenation

energetically similar pathways; electrophilic attack on the nitrogen lone pair is dominant in N–H scission, whereas nucleophilic attack on the arene ring is most important to C–N bond cleavage. This is a very interesting organometallic model of the last step in the heterogeneous HDN of quinoline, which involves the C–N bond cleavage of 2-Prn–aniline. Considering the composition of classical HDN catalysts (e.g., Ni–Mo–S or Ni–W–S), it is intriguing that this reaction takes place on an early transition metal complex. Related examples include the intramolecular oxidative addition of a C–N bond in the unstable intermediate ‘‘[PhC(NSiMe3)2]2Zr’’ to yield {[PhC(NSiMe3)2]Zr (2-PhCNSiMe3)(-NSiMe3)}2162 and the C–N bond cleavage of neutral and anionic amides by dinuclear Nb complexes.163 Late metals also cleave C–N bonds. For instance, RuHCl(CO)(PPh3)3 reacts with primary and secondary allylamines to yield the corresponding insertion products Ru(CH2CH2CH2NHR)Cl(CO)(PPh3)3 (R ¼ H, alkyl), but for tertiary amines (e.g., N,N-dimethylallylamine) the C–N bond is cleaved instead of inserting the olefinic moiety into the M–H bond, and the only metal-containing product is the stable -allyl complex Ru(3-C3H5)Cl(CO)(PPh3)2; the nitrogen atom is eliminated with the hydride in the form of dimethylamine.164 N,N,N9,N9-tetraethylethylenediamine undergoes single or double intramolecular dealkylation reactions in the presence of [RuCl2(diene)]x under moderate conditions, according to Equation (26).165

[RuCl2(diene)]n + Et2N

NEt2

Cl H Et N Ru N Cl R Et

ð26Þ

R = Et for NBD R = H for COD

Alkyl–ammonium or –imminium tetraphenylborate salts readily undergo oxidative addition to (Cy3P)2Ni(2-CO2) or (Cy3P)2Ni(-N2)Ni(PCy3)2 at or below room temperature to yield the corresponding Ni(II) derivatives containing coordinated NH3 or imine, besides the hydrocarbon moiety that remains bound to the metal in the form of a 3-allyl ligand. This chemistry illustrates reaction pathways related to those taking part in HDN processes using Ni-based heterogeneous catalysts, where the organonitrogen substrates may be protonated by surface –OH or –SH groups prior to denitrogenation. The reaction on Ni phosphine compounds, however, does not seem to be general, since changing the substituent on the imminium cation from –C3H5 to –CH2Ph causes a switch from C–N to N–H activation. An extensive theoretical study of the mechanism of the Ni-induced C–N bond activation indicates that the active catalyst is the (bis)phosphine Ni complex; for allylammonium salts, the reaction proceeds by an associative rather than a dissociate mechanism involving coordination of the allylammonium cation to the metal center followed by oxidative addition of the C–N bond to Ni(0). The resulting NH3 remains coordinated to Ni(II) in a pentacoordinated intermediate, and finally a phosphine is lost to generate the final product. The reaction of the imminium salts, on the other hand, follows a dissociative mechanism.166–168

1.27.3.3 Conclusion Metal complexes are reasonable models for mimicking some of the steps involved in heterogeneous HDN. The 1-N and 6-coordination modes of aromatic N-heterocycles are involved in their hydrogenation, whereas 2(N,C) binding can be related to C–N bond breaking and hydrogenolysis. While the hydrogenation of the heterocyclic rings is effectively accomplished by late transition metals, C–N bond cleavage of heteroaromatic rings is best accomplished with early transition metals. The fact that Nb complexes are capable of oxidatively adding the C–N bond of anilines takes the organometallic modeling a step closer to the more conventional HDN mechanisms. For non-aromatic amines, electron-rich late metals (Ru, Ni) are better suited for cleaving C–N bonds, more in line with the components of real HDN catalysts, particularly in the case of nickel. The mechanisms reported so far in these cases imply either an intramolecular nucleophilic attack by a metal hydride, or protonation of the N atom, followed by elimination of ammonia or an amine, similarly to the accepted denitrogenation mechanisms. Analogous reactions on primary aliphatic amines would be most welcome. Considering the complexity of the HDN mechanisms, the homogeneous modeling studies must taken with the necessary caution. Nonetheless, many reactions described involving transition metal complexes with N-heterocycles or other N-ligands show some striking analogies with related reactions occurring on the surface of heterogeneous catalysts. Organometallic HDN modeling is far behind HDS modeling, but the research described in this section opens the way for further work on this field of molecular analogs of surface

795

796

Hydrodesulfurization and Hydrodenitrogenation

species and interactions. The development of this area is to be encouraged as an additional contribution to the understanding of the complex issues involved, and to the design of novel catalysts of better performance in practical applications.

1.27.4 Concluding Remarks A number of exciting concepts have emerged over the last two decades connecting organometallic with surface chemistry and homogeneous with heterogeneous catalysis, in relation to the important HDS and HDN reactions. Organometallic modeling has become a powerful method to study many aspects of the complex HDS and HDN mechanisms, which is best used in conjunction with the modern analytical arsenal of solid-state and surface chemistry. As with any modeling, intrinsic limitations must always be kept in mind and extrapolations must be made with great caution. Solution chemistry of well-defined complexes necessarily ignores the influence of supports and other surface cooperative effects that are very important in heterogeneous catalysis. At the same time, solvent effects that have no relevance in reactions on solid catalysts are introduced when dealing with homogeneous solutions. Some molecular geometries and rearrangements that appear very reasonable in metal complexes, and particularly in clusters, may not be available in more rigid extended solids, and vice versa. Thus, the best possible chemical sense must be employed when trying to use organometallic models in order to explain surface phenomena. Still, this is a fascinating example of how two traditionally separated fields can come together in an effort to solve a complex fundamental chemical problem related to important environmental and industrial issues.

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