Bifunctional aliphatic PNP pincer catalysts for hydrogenation: Mechanisms and scope

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Bifunctional aliphatic PNP pincer catalysts for hydrogenation: Mechanisms and scope Zhihong Wei, Haijun Jiao* Leibniz-Institut f€ ur Katalyse e.V. an der Universit€at Rostock, Rostock, Germany *Corresponding author: e-mail address: [email protected]

Contents 1. Introduction 2. Coordination chemistry of pincer complex 2.1 Metal substitution 2.2 Phosphane moiety substitution 2.3 Catalyst stability 2.4 Catalyst interconversion 3. Hydrogenation reactions and mechanism 3.1 Hydrogenation of ester and aldehyde 3.2 Ketone hydrogenation 3.3 Enantioselective hydrogenation of ketones 3.4 Transfer hydrogenation and isomerization 3.5 Aqueous methanol dehydrogenation 3.6 Designed catalysts for aldehyde hydrogenation 4. Conclusion Acknowledgments References

2 4 5 7 10 11 20 22 35 38 41 45 53 55 58 59

Abstract Hydrogenation is one of the important transformation methodologies in academia and applied research; and homogeneous hydrogenation by using defined transition metal complexes provides the opportunity to fine-tuning the catalytic activity via either metal substitution or ligand modification. Recent studies in the synthesis, characterization and evaluation of transition metal based aliphatic HN(CH2CH2R2) chelating ligand complexes show excellent catalytic performances in the reactions of hydrogenation, transfer hydrogenation and isomerization of carbonyl compounds, as well as aqueous methanol dehydrogenation which includes formic acid dehydrogenation in the last step. It is found that B3PW91 computed gas phase kinetic and thermodynamic data have the closest and best agreement with the experiments, while those including solvation effects and/or dispersion corrections differ strongly from the experiments. Current

Advances in Inorganic Chemistry ISSN 0898-8838 https://doi.org/10.1016/bs.adioch.2018.10.002

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solvation models and dispersion corrections applied in these hydrogenation reactions are not sufficient enough for quantitative comparison. In this review, the results from earth-abundant (Fe, Mn) to precious (Ru, Os, Ir) metals based PNP complexes used in the hydrogenation of esters and aldehydes are summarized. It shows clearly that Feand Mn-based PNP catalysts can be as effective as Ru-, Os- and Ir-based PNP catalysts. The B3PW91 results perfectly explained the experimentally observed selectivity of transfer hydrogenation of α,β-unsaturated aldehydes and ketones, as well as the isomerization of allyl alcohol on the basis of self-transfer hydrogenation. Such interplay between computation and experiment not only streamlines the experimentally observed results, but also offers the understanding of the reaction mechanisms and reaction conditions.

1. Introduction Catalytic hydrogenation of unsaturated functional groups is of utmost importance in organic synthesis and plays a key role in the production of bulk chemicals, fine-chemical intermediates and products.1,2 Since Noyori’s milestone discovery of metal-ligand bifunctional catalysts for the asymmetric transfer hydrogenation of ketones by using Ru amine complex,3 two types of metal-ligand bifunctional pincer catalysts including reversible amino/ amido or aromatization/de-aromatization interconversion systems have been investigated and widely applied in the hydrogenation of C]O, C^N/C]N and C^C/C]C functional groups, transfer hydrogenation, isomerization and dehydrogenation of alcohols. The first aliphatic ligand, XN(CH2CH2PR2)2 [R ¼ Ph and X ¼ H, CH3, C6H11, abbreviated as R2PNxPR2 afterward], which has one hard N donor atom and two soft P donor atoms, was synthesized by Sacconi and Morassi.4 The magnetic, spectrophotometric, conductometric properties and molecular weight of NiII- and CoII-based complexes were reported initially. In 1984, Khan et al. synthesized the ill-defined complexes of [RhI(Cl) (Ph2PNHPPh2)] and [IrI(Cl)(Ph2PNHPPh2)] and applied them in the hydrogenation of cyclohexene at 0.4–1 H2 atmosphere over the temperature range of 20–50 °C in ethanol.5 Later, they synthesized a series of Ru-based amino Ru-Ph2PNHPPh2 complexes and applied them in cyclohexene hydrogenation at 30 °C and 1 H2 atmosphere in benzeneethanol mixture.6 Meanwhile, a series of Rh-, Ir-, Pd- and Pt-based amino M-Ph2PNHPPh2 complexes were synthesized.7 In 1989, group 6 metal Cr-, Mo- and W-based amino (mer-[M(CO)3(Ph2PNHPPh2)] and fac-[M(CO)3(Ph2PNHPPh2)], M ¼ Cr, Mo, W) complexes were reported by Ellermann et al.8 Meanwhile, Edward et al. synthesized the amido

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complex of [V(Cl)2(Me2PNPMe2)]9 and high yield routes for the synthesis of Me and Et substituted PNP ligand using trimethylsilyl protecting group on nitrogen were developed.10 Later, well-defined Ru-based chiral (bis (2-((2R,5R)-2,5-dimethyl-phospholanoethyl))amine)-PNP11 and Ir-based chiral C6H5CH(CH3)N(CH2CH2PPh2)2(R)-(+)- and (S)-()-PNP12 complexes were synthesized and applied in the reactions of asymmetric hydrogenation and transfer hydrogenation. In 2005, the chlorodihydride [IrIII(H)2(Cl) (iPr2PNHPiPr2)] complex was synthesized and isolated by Abdur-Rashid et al.,13,14 and the amido complex of dihydride [IrIII(H)2(iPr2PNPiPr2)] was obtained by the reaction of [IrIII(H)2(Cl)(iPr2PNHPiPr2)] with tBuOK. In addition, it was found that the amido complex of [IrIII(H)2(iPr2PNPiPr2)] readily forms the trihydride complex [IrIII(H)3(iPr2PNHPiPr2)] in the presence of iPrOH. Both amido [IrIII(H)2(iPr2PNPiPr2)] and amino [IrIII(H)3 (iPr2PNHPiPr2)] complexes are exceptionally active catalysts for transfer hydrogenation in the absence of base.14 Using [RuIICl2(C6H6)]2 and [HN (CH2CH2PPh2)2HCl], [RuII(Cl)2(Ph2PNHPPh2)] was synthesized and applied in the hydrogenations of various ketones and imines in the presence of tBuOK as base.13 Later, the chloromonohydride [RuII(H)(Cl) (CO)(Ph2PNHPPh2)] complex was synthesized and tested in the hydrogenation of esters in the presence of CH3ONa as base by Takasago International Corporation.15 Independently, a series of aliphatic PNP chloromonohydride [MII(H)(Cl)(CO)(iPr2PNHPiPr2)] and dihydride [MII(H)2(CO)(iPr2PNHPiPr2)] (M ¼ Os and Ru) complexes were synthesized by Gusev et al.16 and tested in alcohol dehydrogenation, transfer hydrogenation, dehydrogenative coupling and amine alkylation reactions. Early PNP complexes used in catalysis were mainly relying on the expensive and precious Ru-, Ir- and Os-based PNP pincer complexes,17–20 and several review papers focusing on noble metal pincer complexes are available.21–24 Inspired by the properties of Fe-based enzymes, several artificial Fe-based homogeneous catalysts were explored.25–27 It is suggested that noble metals can be replaced in an appropriate coordination sphere and spin state by non-noble metals.27 Beller et al. developed a set of FeII-PNP28–31 and MnI-PNP32–35 complexes in hydrogenation and dehydrogenation reactions under mild conditions. Hanson et al. developed the unusual cationic CoII-PNP complex.36 Since then incredible widespread developments of non-noble metal-based PNP complexes highlight their abilities in hydrogenation and dehydrogenation reactions. Since such pincer-type complexes allow the fine-tuning of electronic and steric properties by metal substitution and ligand modification without

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significant change in the coordination sphere,37 a set of Co,38–40 Fe,28,29 Mn,32–35 Mo41–43 and W41–43 pincer catalysts have been developed and applied in hydrogenation and dehydrogenation reactions. Due to the rapid development of non-noble metal-based catalysts, some comparative reviews focusing on their synthesis and application as catalysts with the respect of substrate scopes and reactivity have been reported recently.44–46 On the basis of amino/amido type aliphatic metal PNP pincer complexes developed in our laboratory, experimental and theoretical studies focusing on the reaction mechanisms and electronic structures have been reported. These studies not only rationally explained the existing experimental observations but also made predictions for new designed catalysts and related applications. In this review, we summarize recent progress in the fundamental aspect of coordination chemistry of aliphatic metal PNP pincer complexes as well as their applications and reaction mechanisms for the hydrogenation of aldehyde, ketone, ester as well as the reactions of transfer hydrogenation, isomerization, CO2 hydrogenation and aqueous methanol dehydrogenation.18,47,48 In this review, we also include some of our individual unpublished results for full comparison. Since the computed energetic data depend on conformational changes of the catalyst (especially the isopropyl substituents), some results have been recalculated for comparison based on the same conformations. If not otherwise mentioned, all results were obtained using the B3PW91 functional and the TZVP basis set (relativistic based LANL2DZ basis set for heavy elements).

2. Coordination chemistry of pincer complex The three donor atoms of the PNP ligand can form two chelating rings, and each chelating ring usually leads to an additional factor of about 105 in the equilibrium constant (formation constant) for the reaction,49 and this makes such pincer complexes much more stable. The electronic structure, geometry and steric property are the key factors that highly affect the stability, reactivity and selectivity.50 The nature of the PNP pincer metal complex provides two major strategies to tuning the electronic structure, i.e., geometry and steric property (Scheme 1). For example, the central metal (M) can alter the electronic structure and therefore affect the coordination around the metal center and the size of the substituents (R) at the phosphane center can change the steric properties. In this section, we summarize the structures of the reported PNP pincer complexes in detail.

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Scheme 1 Metal and phosphane moiety substitutions over aliphatic M PNP amino (2M) and amido (1M) complexes.

Scheme 2 Group 6–9 metals halide PNP 2MR–X complexes as pre-catalysts.

2.1 Metal substitution Since pincer complexes can be readily synthesized from metal halide precursors and HN(CH2CH2PR2)2 ligand, metal halide pincer complexes can be obtained directly. On the basis of group 6–9 metals, well-defined halide [M(L1)(L2)(X)(R2PNHPR2)] pincer complexes (X ¼ Cl or Br) were synthesized and applied as pre-catalysts in hydrogenation and dehydrogenation reactions in the presence of base. As shown in Scheme 2, several group 6–9 metal PNP complexes which fulfill the 18-electron rule were synthesized and characterized.43 On the basis of the 18-electron rule and charge neutrality, one CO and one linear-NO (considered as a 3e donor that lowers the metal oxidation state49) ligand were introduced in group 6 metal PNP complexes; and two CO ligands were introduced in group 7 metal PNP complexes, as well as one CO and one H ligand were introduced in group 8 metal PNP complexes. For group 9 metal PNP complexes, two H ligands were introduced. Table 1 lists the experimentally available and calculated geometry parameters of the synthesized iPr substituted [M(L1)(L2)(X)(iPr2PNHPiPr2)] pincer complexes (X ¼ Cl, X ¼ Br for Mn). As shown in Table 1, one can find that the bond lengths and the P–M–P bite angles are fine-tuned by metal substitution with iPr substituted

Table 1 Selected bond distances (Å) and P–M–P bite angle (degree) of [M(L1)(L2)(X)(iPr2PNHPiPr2)] 2M–X pincer complexes (B3PW91 computed values in square bracket). Group 6 metal Group 7 metal Group 8 metal Group 9 metal [51]

W43

[51]

Mn33

[52]

Mo43

M–N

2.278 [2.313]

2.268 [2.303]

2.128 [2.138]

M–P1

2.494 [2.498]

2.477 [2.484]

M–P2

2.492 [2.498]

[54]

Ru31 [54]

Os16 [54]

Co39

[54]

Ir14

[54]

2.070 [2.097]

2.195 [2.234]

2.186 [2.220]

2.040 [2.064]

2.188 [2.226]

2.237 [2.321]

2.229 [2.249]

2.326 [2.351]

2.328 [2.341]

2.145 [2.175]

2.276 [2.297]

2.475 [2.484]

2.236 [2.321]

2.210 [2.237]

2.310 [2.351]

2.328 [2.341]

2.144 [2.175]

2.276 [2.297]

M–Ltrans 2.038 [1.987]

1.961 [1.977]

1.756 [1.771]

1.713 [1.732]

1.833 [1.846]

1.839 [1.850]

1.368 [1.466]

1.630 [1.582]

M–Lanti 1.811 [1.796]

1.845 [1.796]

1.779 [1.741]

1.426 [1.511]

1.550 [1.597]

1.667 [1.624]

1.432 [1.469]

1.363 [1.574]

M–Cl

2.548 [2.571]

2.536 [2.551]

2.594 [2.594]

2.413 [2.405]

2.543 [2.589]

2.546 [2.579]

2.324 [2.365]

2.506 [2.566]

N–H

0.910 [1.023]

0.910 [1.023]

0.863 [1.024]

0.892 [1.024]

0.923 [1.016]

0.931 [1.026]

0.910 [1.023]

0.841 [1.024]

P–M–P 157.71 [157.83] 157.68 [158.04] 160.29 [166.52] 165.01 [167.99] 163.92 [164.94] 163.67 [164.93] 169.36 [171.77] 167.19 [168.00]

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(X 5 Br) Fe53

Metals

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phosphane moiety and periodic trends were observed. Generally, good agreements in bond lengths and PMP bite angle between X-ray molecular structure analysis and B3PW91 DFT computation have been found, apart from the N–H bond. It is found that the first row transition metal (3d) complexes [Mn(CO)2(Br)(iPr2PNHPiPr2)], [Fe(CO)(H)(Cl)(iPr2PNHPiPr2)] and [Co(H)2(Cl)(iPr2PNHPiPr2)] have shorter M–N, M–P and M–Cl(Br) bond lengths than the second row transition metal (4d) complexes [Mo (CO)(NO)(Br)(iPr2PNHPiPr2)] and [Ru(CO)(H)(Cl)(iPr2PNHPiPr2)] as well as the third row transition metal (5d) complexes [W(CO)(NO)(Br) (iPr2PNHPiPr2)], [Os(CO)(H)(Cl)(iPr2PNHPiPr2)] and [Ir(H)2(Cl) (iPr2PNHPiPr2)]. From 4d to 5d metal PNP complexes, orbitals get larger due to an increase in the principal quantum number, but this effect is counterbalanced by lanthanide contraction before 5d metals.49 As a result, the M–N, M–P and M–Cl bonds of 4d metal complexes are similar or longer than those of 5d metal complexes. For 3d and 4d [M(L1)(L2)(X) (iPr2PNHPiPr2)] complexes, the bond length decreases with the increase of the principal quantum number. For 5d [M(L1)(L2)(X)(iPr2PNHPiPr2)] ˚) complexes, the small difference between the M–N (2.186 vs. 2.188 A H ˚ and M–Cl bonds (2.546 vs. 2.506 A) of [Os(CO)(H)(Cl)(iPr2PN PiPr2)] and [Ir(H)2(Cl)(iPr2PNHPiPr2)] is consistent with their similar atom radius. In addition, it is found that 3d metal complexes have larger P–M–P bite angle than 4d and 5d metal complexes, and it increases with the increase of quantum number in the same row.

2.2 Phosphane moiety substitution Modifying the phosphane moiety with more or less bulky groups is another strategy to tuning the electronic structure, geometry and steric property. After the synthesis of the first Ph substituted Ph2PNHPPh2 ligand,4 other substitutions such as Me,9 Et,9,55,56 iPr,32,55,56 Cy,32,55,56 Ad (adamantyl)56 as well as chiral11,34 and non-symmetric57–60 ligand were widely explored. Owing to the rapid development of Mn pincer complexes in the last 3 years, versatile [Mn(CO)2(Br)(R2PNHPR2)] (2MnR–Br) complexes have been isolated. In Table 2, we summarized the geometry parameters of 2MnR–Br (R ¼ Et, Cy, iPr, and non-symm-iPr) pincer complexes which are characterized by X-ray structure analysis. It is found that the 2MnEt–Br32 and 2MnCy–Br33 complexes show similar geometry parameters and the bond lengths of 2MnCy–Br are slightly longer than that of 2MnEt–Br by 0.05–1.50%. The M–N bond of 2MniPr–Br33 is shorter than

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Table 2 Selected bond distances (Å) and P–M–P bite angle (degree) for a variety of substituents for the 2MnR–Br complex (B3PW91 computed values in square bracket). R Et32 Cy33 iPr33 Non-symm-iPr58

M–N

2.125 [2.152]

2.126 [2.149]

2.119 [2.147]

2.137

M–P1

2.278 [2.285]

2.305 [2.322]

2.302 [2.323]

2.330

M–P2

2.271 [2.286]

2.305 [2.322]

2.302 [2.323]

2.301

M–L1

1.779 [1.785]

1.787 [1.784]

1.777 [1.783]

1.782

M–L2

1.752 [1.757]

1.754 [1.752]

1.756 [1.754]

1.754

M–Br

2.561 [2.596]

2.563 [2.599]

2.575 [2.597]

2.568

N–H

0.821 [1.024]

0.824 [1.024]

0.846 [1.024]

0.999

P–Mn–P 165.20 [166.07] 164.73 [166.22] 166.03 [166.36] 174.34

that of 2MnEt–Br and 2MnCy–Br, while the M–P, M–Br, and N–H bonds of 2MniPr–Br are longer than those of 2MnEt–Br and 2MnCy– Br. The non-symmetric [NH(CH2CH2PiPr2)(o-CH2C6H4PiPr2)] (2-(diisopropylphosphino)-N-(2-(diisopropylphosphino)benzyl)ethanamine) ligand (non-symm-iPr) complexes of 2Mnnon-symm-iPr–Br58,60,61 have larger change in bond lengths than the 2MnEt–Br, 2MnCy–Br and 2MniPr–Br complexes. This is because one of the chelating rings for 2Mnnon-symm-iPr–Br complex is six-membered. Computationally, however, there are no significant differences in these bond lengths and angles apart from the Mn–P bonds in 2MnEt–Br; and they are approximately the same in most cases. Due to the same conformations of the isopropyl and cyclohexyl groups, the Mn–P distances in 2MnCy–Br and 2MniPr–Br are the same, but longer than that in 2MnEt–Br, and this is because of the smaller size of the ethyl group. The substituent effect has been reflected in their different catalytic activity. Although 2MnEt–Br and 2MnCy–Br show similar geometry parameters, the Cy group is more bulky and hindered than the Et group; and their steric difference has much influence on the activity of hydrogenation reactions.32 Beller et al. addressed the influence of the alkyl substituents at the phosphorus binding site on the catalytic performance of the complexes in hydrogenation reactions. It was found that 2MniPr–Br and 2MnCy–Br showed high conversion and yield (99%/98% for 2MniPr–Br and 99%/87% for 2MnCy–Br) in the hydrogenation of benzonitrile,33 while very low yields (6% for 2MniPr–Br and 2% 2MnCy–Br) for ester hydrogenation in the presence of base. However, the less hindered 2MnEt–Br improved the activity significantly for ester hydrogenation and 97% yield was reached.32

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Jones et al. also evaluated the stereo-electronic effects of ligand substitution on the catalytic activity in the Guerbet upgrade of ethanol to n-butanol catalyzed by the 2MnR–Br complex. It was found that the Ph substituted 2MnPh–Br showed the highest activity (34% of n-butanol yield) and bulkier substituted 2MniPr–Br and 2MnCy–Br showed slightly lower activity (30% and 27% of n-butanol yield, respectively), while most bulky tBu and Ad substituted catalysts showed very poor activity (only about 10% yield of the desired n-butanol).56 It is worth noting that the uncommon facial coordinated cationic tricarbonyl complex of fac-[2MnEt–3CO]+ can be isolated (Scheme 3). When fac-[2MnEt–3CO]+ is heated to reflux in toluene for an additional 16 h, 2MnEt can be obtained in higher yield. Calculation at the B3PW91/ LAN2DZ(Mn)/TZVP level suggests that fac-[2MnEt–3CO]+ is a kinetically controlled product and less stable than mer-[2MnEt–3CO]+ by 10.1 kcal/mol. Furthermore, the formation of 2MnEt from fac-[2MnEt– 3CO]+ is exergonic by 7.7 kcal/mol.32 Although mer-[2MnEt–3CO]+ is more stable than 2MnEt by 2.4 kcal/mol, CO release is entropy favored and the equilibrium will shift mer-[2MnEt–3CO]+ to 2MnEt under reflux conditions. Recently, a similar fac-[2ReiPr–3CO]+ complex was isolated by the groups of Beller and Sortais.62,63 DFT (PBE0-D3) calculations indicated that the formation of the amido complex of [Re(CO)3(iPr2PNHPiPr2)] (2ReiPr) from fac-[2ReiPr–3CO]+ is exergonic by 50.5 kcal/mol.62 Based on the conversion of fac-[2MnEt–3CO]+ to 2MnEt, 2ReiPr from fac-[2ReiPr–3CO]+ should also be isolated under reflux conditions. This needs experimental proof. Similar results were also found for the Fe-catalyzed methyl benzoate hydrogenation.64 By applying 1 mol% of catalyst [FeIIH(CO)(BH4) (iPr2PNHPiPr2)] (2FeiPr–BH4), [FeIIH(CO)(BH4)(Cy2PNHPCy2)] Cy II (2Fe –BH4) and [Fe H(CO)(BH4) (Et2PNHPEt2)](2FeEt–BH4) at 30 bar H2 and 60 °C, complex 2FeEt–BH4 produced 99% yield of benzyl alcohol, whereas under the same reaction conditions complexes

Scheme 3 Conversion of fac-[2MnEt–3CO]+.

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2FeiPr–BH4 and 2FeCy–BH4 only gave 50% and 30% yields of the product, respectively. In our recent work,40 a series of CoI and CoII pincer complexes bearing different iPr, Cy, and Ph substituents were prepared and applied in ester hydrogenation. It was found that [CoII(Cl)2(Ph2PNHPPh2)] showed high conversion (99%) and yield (96%), while [CoII(Cl)2(iPr2PNHPiPr2)] and [CoII(Cl)2(Cy2PNHPCy2)] only gave moderate conversion (65% and 67%) and yield (46% and 45%). Quintaine et al.61 tested acetophenone hydrogenation by using 1FeiPr and 2FeR–Br [R ¼ Cy, NH(CH2CH2PiPr2)(o-CH2C6H4PiPr2), NH(CH2CH2PPh2)(o-CH2C6H4PiPr2)] pincer complexes. It was found that the 1FeiPr and 2FeR–Br [R ¼ NH(CH2CH2PiPr2)(o-CH2C6H4PiPr2), NH(CH2CH2PPh2)(o-CH2C6H4PiPr2)] gave a conversion and yield of 99%, while 2FeCy–Br gave a lower conversion and yield (83%).

2.3 Catalyst stability The thermal stability of the catalyst is an important factor for its application in practice. As shown in Table 3, computations found that the dissociation of the equatorial CO from 2MiPr for 2MiPr–CO formation is highly endergonic by 23.8, 51.7 and 69.7 kcal/mol, resulting in the triplet ground state of 2FeiPr–CO, and the singlet ground state of 2RuiPr–CO and 2OsiPr–CO. The highly endergonic property indicates that the CO dissociation is hard to occur.65 For 2MniPr and 2ReiPr, we compared the dissociation of the equatorial CO ligand (2MiPr–CO) and the lability of one phosphine ligand (2MiPr– disP). It was found that CO dissociation from 2MniPr resulting in the triplet ground state of 2MniPr–CO is endergonic by 31.5 kcal/mol, while CO dissociation from 2ReiPr resulting in the singlet ground state of 2ReiPr–CO is endergonic by 62.4 kcal/mol. The de-coordination of one phosphine arm Table 3 B3PW91 computed thermal stability (kcal/mol) of 2M toward CO or one phosphine arm dissociation (triplet state energy in square bracket). 2MiPr 2MiPr–CO 2MiPr–disP

2FeiPr

41.2 [23.8]65

30.5 [23.5]54

2RuiPr

51.7 [59.0]65

34.8 [54.6]54

2OsiPr

69.7 [75.3]65

43.3 [65.6]54

2MniPr

39.0 [31.5]52

29.6 [29.2]52

2ReiPr

62.4 [77.7]52

40.9 [62.4]52

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from 2FeiPr, 2RuiPr and 2OsiPr is highly endergonic by 23.5, 34.8 and 43.3 kcal/mol and results in the triplet ground state of 2FeiPr–disP, and the singlet ground state of 2RuiPr–disP and 2OsiPr–disP.54 For 2MniPr, the de-coordination of one phosphine arm resulting in the singlet/triplet ground state 2MniPr–disP is endergonic by 29.6/29.2 kcal/mol.52 For 2ReiPr, the de-coordination of one phosphine arm resulting in the singlet ground state 2ReiPr–disP is endergonic by 40.9 kcal/mol.52 As shown in Table 3, our results indicate that 2MniPr is thermally more stable than 2FeiPr toward CO and one phosphine arm dissociation; this is in agreement with the experimentally observed long-term (longer than a month) stability for 2MniPr–Br as pre-catalyst in low temperature methanol reforming.66 In addition, phosphine ligand de-coordination is easier than CO ligand dissociation. The thermal stability of the catalyst has the decreasing order of 2OsiPr > 2ReiPr > 2RuiPr > 2MniPr > 2FeiPr toward ligand dissociation. The expected higher stability for the Os and Re PNP complexes requires experimental proof.

2.4 Catalyst interconversion For catalytic hydrogenation starting from the halide complexes, strong bases such as tBuOK are usually required to activate halide pincer complexes into active catalysts. It is established that the base is used to generate the active catalyst of the amido complex of [M(L1)(L2)(R2PNPR2)] (1M).31 As shown in Scheme 4, base abstracts the acidic proton at nitrogen to generate the conjugated [M(L1)(L2)(X)(R2PNPR2)] (2M) anion and to prompt halide removal with the formation of the formal N]M double bond. The lifetime of the anionic intermediate is too short to be detected, indicating the rapid X elimination.67 Therefore, the hydrogenation reaction can be operated by outer-sphere mechanism via a reversible H2 addition/elimination reaction between the amido complex of [M(L1)(L2)PNP] (1M) and amino complex of [M(L1)(L2)(H)PNHP] (2M). Since amino 2M complex can be formed from amido 1M complex and molecular H2 or H2 source (such as iPrOH for transfer hydrogenation), the reversible interconversion and concerted hydrogen shuttle between amino

Scheme 4 Metal PNP pincer complex activation and interconversion.

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complex 2M and amido 1M [2M ¼ 1M + H2] are the key features of the metal–ligand bifunctional catalyzed hydrogenation reaction.65 The barrier and energy of the interconversion are highly related with the catalytic activity of the catalysts as well as the reaction condition. For example, the equilibrium and reversibility of the interconversion must be maintained for the effectiveness of the catalysts and high pressure is needed for the reaction which has a higher barrier than that of the reversible interconversion. Schneider et al. found that 1FeiPr/1FeCy and H2 (1 atm.) can react rapidly to give 2FeiPr/2FeCy as the main product in C6D6 at room temperature. This reaction is fully reversible, which prevents the isolation of 2FeiPr/ 2FeCy due to facile H2 loss.68 Guan et al. found that when 2FeiPr–BH4 in toluene-d8 was heated at 100 °C, 2FeiPr was generated. However, it quickly converted to 1FeiPr as a result of losing H2. Alternatively, 2FeiPr could be produced from 2FeiPr–BH4 at room temperature via the trapping of BH3 by P(nBu)3. Under 1 atm. H2, the in situ generated 1FeiPr (from 2FeiPr–Br and t BuOK) was converted to 2FeiPr in >90% yield without much decomposition.69 A similar observation was also reported by Jones et al., using NMR.70 As shown in Table 4, gas phase calculations at B3PW91/TZVP65 and RI-B3PW91-D3BJ/def2-QZVPP//B3LYP/def2-SVP71 levels indicate that the reaction is thermo-neutral [ΔGr(B3PW91/TZVP) ¼ 0.9 kcal/mol; ΔGr(RI-B3PW91-D3BJ/def2-QZVPP//B3LYP/def2-SVP71) ¼ 2.9 kcal/ mol], in agreement with the experimental findings. The gas phase computed reaction energy for H2 elimination at B3PW91/TZVP and Table 4 Comparison of reaction Gibbs free energy (Δ Gr, kcal/mol) for 2FeiPr ¼ 1FeiPr + H2 using different models. 2FeiPr 5 1FeiPr + H2

Exp./C6H6 B3PW91/TZVP

ΔG‡

Δ Gr

Methods

54

Equilibrium

–

0.9 [0.3]

19.1 [17.5]65

65

RI-B3PW91-GD3BJ/def2-QZVPP/ /B3LYP/def2-SVP71

2.9a

17.7a

M06-SCRF(ethanol)/ECP10MDF(Fe)/ 6-311 ++G(d,p)72

9.1

24.0

M06-SCRF(THF)/6-311++G(d,p)/THF/ /B3LYP/6-31G(d,p)73

14.9

25.0

a

Thermal correction were obtained at 393.15 K.

ARTICLE IN PRESS Bifunctional aliphatic PNP pincer catalysts

13

B3LYP/def2-SVP levels is more reasonable and realistic than that obtained by using M06-SCRF/ECP10MDF(Fe)/6-311++G(d,p) [Δ Gr ¼ 9.1 kcal/ mol] in ethanol.72 Even a worse result [ΔGr ¼ 14.9 kcal/mol] was obtained from M06-SCRF/6-311 ++G(d,p) single-point energy on the B3LYP/ 6-31G(d,p) gas phase structures under consideration of solvation effects of THF.73 Such high endergonic reaction energies (9.1 and 14.9 kcal/mol) are too large to establish equilibrium, thus in disagreement with the experimental observation. It was noted that including solvation and dispersion corrections not only makes the reaction more endergonic, but also raises the barriers which also does not agree with the experimental finding at room temperature. For the Ru PNP pincer complex, the amino complex of 2RuiPr was found to be relatively stable and about 20% of 2RuiPr was dehydrogenated to 1RuiPr and H2 evolution when heating 2RuiPr in dioxane-d8 at 100 °C for 50 min in a sealed NMR tube. H2 was then shown to add back onto 1RuiPr to form 2RuiPr within few hours.31 Gauvin et al. dissolved 2RuiPr in toluene-d8, and no H2 release was observed in the closed system at room temperature even after 2 weeks. Heating the solution of 2RuiPr at 80 °C in a closed system leads to the formation of the amido 1RuiPr in 10% yield after 2 h. Under an argon stream and in an open system at 80 °C, however, H2 release from 2RuiPr took place smoothly with 1RuiPr formation and the 2RuiPr/1RuiPr ratio is 87/13 after 3 days.74 The constant H2 evolution is the main driving force to shift the equilibrium toward the formation of the amido 1RuiPr.74 The observed low yield of amido complex of 1RuiPr is consistent with DFT results calculated at B3PW91/LAN2DZ(Ru)/ TZVP (2RuiPr ¼ 1RuiPr + H2, ΔGr ¼ 2.3 kcal/mol) in the gas phase (Table 5), which reveals that the reaction is slightly endergonic and can form an equilibrium under H2 atmosphere in favor of 2RuiPr. However, including van der Waals dispersion correction by using the ωB97XD functional, the reaction becomes endergonic by 7.4 kcal/mol.31 This exchange reaction was calculated to be much more endergonic by using M06-SCRF/ECP28MWB(Ru)/6-311++G(d,p) (ΔGr ¼ 11.8 kcal/mol) including solvation effects in ethanol72 as well as by using mPW1K-SCRF (Δ Gr ¼ 18.8 kcal/mol) and at PBE0-SCRF (ΔGr ¼ 15.4 kcal/mol) in THF under 302 atm.16 or by using ωB97XD-SCRF (ΔGr ¼ 7.6 kcal/ mol).75 All these data including van der Waals dispersion correction and/or solvation model do not support the observed equilibrium; instead they not only make the reaction more endergonic but also raise the barriers. This also does not agree with the experimental finding at room temperature.

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Zhihong Wei and Haijun Jiao

Table 5 Comparison of reaction Gibbs free energy (Δ Gr, kcal/mol) for 2RuiPr ¼ 1RuiPr + H2 using different models. 2RuiPr 5 1RuiPr + H2 Methods

Exp./toluene B3PW91/LAN2DZ(Ru)/TZVP

65

ωB97XD/LAN2DZ(Ru)/TZVP31 M06-SCRF(ethanol)/ECP28MWB(Ru)/6–311 ++G(d,p) mPW1K-SCRF(THF)/Gen

a,16

PBE0-SCRF(THF)/Gena,16 ωB97XD-SCRF(methanol)/BSI

75

72

ΔGr

ΔG‡

Endergonic

–

2.3

21.1

7.4

24.4

11.9

29.4

18.8

29.2

15.4

–

7.6

27.1

a 6-31+ G(3pd) for the H atoms on metal and N atoms, Def2-QZVPP (with the corresponding ECP) for Ru, 6-311+ G(d) for P and Cl, and 6-31 +G(d,p) for all other atoms.

For the Os PNP pincer complex, the reaction energy is less endergonic by using the B3PW91 (ΔGr ¼ 5.4 kcal/mol), B3LYP (ΔGr ¼ 1.7 kcal/mol), BP86 (ΔGr ¼ 1.5 kcal/mol) and TZVP (LAN2DZ for Os) basis set,65 while stronger endergonic by using mPW1K-SCRF (ΔGr ¼ 19.7 kcal/mol) and PBE0-SCRF (Δ Gr ¼ 16.1 kcal/mol) with mixed basis sets.16 Considering the fact that 1OsiPr and 2OsiPr have been used as effective catalysts for hydrogenation and dehydrogenation reactions,16,19 B3LYP and BP86 seem more reasonable than B3PW91, while the results of mPW1K-SCRF (19.7 kcal/ mol)16 and PBE0-SCRF (16.1 kcal/mol)16 as well as B3PW91-SCRF (ΔGr ¼ 11.3 kcal/mol) are much worse, revealing the problematics of the current solvation model applied.65 For Mn PNP pincer complex, 2MniPr can be obtained from 2MniPr–Br in the presence of tBuONa in toluene-d8 under 5 bar H2 at room temperature. 2MniPr is found stable and can be stored under argon at low temperature for several weeks, while very slowly loses H2 to form 1MniPr at room temperature in solution.76 In a closed system (NMR tube) and in the absence of substrate, H2 release is rapid at 120 °C, leading to 80% conversion of 2MniPr into a mixture of 1MniPr, 2MniPr and a positional isomer 2MniPr’ within 15 min.76 The computed exergonic reaction Gibbs free energy (ΔGr ¼ 1.0 kcal/mol) in the gas phase is in agreement with the observed equilibrium.76 By applying a first-order kinetic model, the determined enthalpy barrier (ΔH‡) for the dehydrogenation of 2MniPr to 1MniPr is 19.1 kcal/mol, which is comparable to the computed gas phase results for B3PW91/TZVP (19.6 kcal/mol)33 and B3PW91/SDD(Mn)/6-31G** (21.2 kcal/mol).76

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Bifunctional aliphatic PNP pincer catalysts

Recently, we carried out benchmark calculations under the considerations of solvation effect and van der Waals dispersion correction by using different methods combined with the LAN2DZ(Mn)/TZVP basis set (Table 6). For the interconversion of the catalysts with hydrogen [2MniPr ¼ 1 MniPr + H2], experimental results as discussed above are available for comparison; i.e., this reaction is reversible and forms an equilibrium in C6D676; the Gibbs free energy of this reaction must be very small, slightly exergonic or endergonic. Indeed, our gas phase exergonic Gibbs free energy value (0.3 kcal/mol) predicts equilibrium and reversibility for this reaction, in reasonable agreement with the experimental results obtained in C6D6.76 In addition, the computed gas phase enthalpy barrier shows good agreement with the experimental estimated value (19.633/21.1 vs. 19.1 kcal/mol76). Next, we computed solvation corrections in non-polar benzene.77 The SMD solvation energy from an ideal gas to an ideal solution was parameterized by using experimental solvation free energies corrected to Table 6 Comparison of reaction Gibbs free energy (ΔGr, kcal/mol) and enthalpy barrier (ΔH‡, kcal/mol) for 2MniPr ¼ 1 MniPr + H2 using different models. 2MniPr 5 1 MniPr + H2 ΔH‡

ΔGr

Equilibrium 19.176

Exp./C6H6 B3PW91/LAN2DZ(Mn)/TZVP B3PW91/SDD(Mn)/6-31G** B3PW91/TZVP

76

33

B3PW91-SCRF(C6H6)/LAN2DZ(Mn)/TZVP77

0.3

21.1

1.0

21.2

0.2

19.6

1.1a

22.8

a

22.8

B3PW91-SCRF(C6H6) + GD3BJ/LAN2DZ(Mn)/ TZVP77

0.8

DLPNO-CCSD(T)/def2-TZVPP//B3PW91/LAN2DZ (Mn)/TZVP77

1.7

24.5

DLPNO-CCSD(T)-SCRF(C6H6)/def2-TZVPP/ /B3PW91/LAN2DZ(Mn)/TZVP77

6.0a

26.1

DLPNO-CCSD(T)-SCRF(C6H6) + GD3BJ/def2TZVPP//B3PW91/LAN2DZ(Mn)/TZVP77

5.2a

26.3

B3PW91-SCRF(CH3OH)/LAN2DZ(Mn)/TZVP52

5.2a [3.352]

B3PW91-SCRF + GD3BJ(CH3OH)/LAN2DZ(Mn)/ TZVP52 a

1M Θ Θ Θ GΘ /p ), p1M ¼ 24.46 atm. l ¼ Escrf + Gcorr + RT ln(p

a

52

4.3 [2.4 ]

27.0 26.7

ARTICLE IN PRESS 16

Zhihong Wei and Haijun Jiao

1 mol/L (1 M, equivalent to 1 mol of an ideal gas at a pressure of 24.46 atm.) for both gas and solution phases. For the number of moles changed reactions (Δ n), the concentration corrections [ΔG1atm.!1M ¼ RTln(p1M/pΘ) ¼ RTln(24.46) ¼ 1.89 kcal/mol] from standard state of gas phase (1 atm., 298.15 K) to solution (1 mol/L, 298.15 K) cannot be cancelled and omitted. Therefore, when computing reaction Gibbs free energies in solution, ΔG1atm.!1mol/L corrections were added. At B3PW91 level with and without dispersion in benzene solution, the reaction is endergonic by 1.1 and 0.8 kcal/mol and this is in agreement with the observed equilibrium; however, the computed enthalpy barrier (22.8 kcal/mol with and without dispersion) is 3.7 kcal/mol higher than the experimental value. At the DLPNO-CCSD(T) level in the gas phase as well as in benzene solution with and without dispersion, the reaction is endergonic by 1.7, 6.0 and 5.2 kcal/mol, respectively, and the enthalpy barrier becomes 5.4, 7.0 and 7.2 kcal/mol higher than the experimental value. This indicates that the enthalpy barrier is overestimated by about 5–7 kcal/ mol at the DLPNO-CCSD(T) level in the gas phase as well as in benzene solution with and without dispersion. For reaction of 2Mn ¼ 1 Mn + H2, the B3PW91 functional gives better match both in the gas phase and in liquid phase than DLPNO-CCSD(T). In addition, we computed solvation correction in polar CH3OH, calculations with pure solvation and solvation with dispersion give positive Gibbs free energy (B3PW91-SCRF(CH3OH)/LAN2DZ(Mn)/TZVP, 5.2 kcal/ mol; B3PW91-SCRF(CH3OH) + GD3BJ/LAN2DZ(Mn)/TZVP, 4.3 kcal/ mol), and these positive Gibbs free energies will shift the equilibrium much toward 2Mn, in disagreement with the experimentally observed equilibrium. In addition, the computed corresponding enthalpy barriers (B3PW91SCRF(CH3OH)/LAN2DZ(Mn)/TZVP, 27.0 kcal/mol; B3PW91-SCRF (CH3OH) +GD3BJ/LAN2DZ(Mn)/TZVP, 26.7 kcal/mol) are higher than the experimental one (19.1 kcal/mol) by 7.9 and 7.6 kcal/mol, respectively. For the interconversion of the catalysts with water [2MniPr–OH ¼ 1 MniPr + H2O], experimental results are available78 (Table 7), and this reaction has an endergonic reaction Gibbs free energy of 0.8 kcal/mol, indicating possible equilibrium and reversibility; and the reaction is endothermic by 7.9 kcal/mol. In the gas phase (B3PW91/LAN2DZ(Mn)/TZVP), the computed reaction Gibbs free energy and enthalpy are 1.1 and 12.1 kcal/ mol, in close agreement with the experimental values. In THF solution (B3PW91-SCRF(THF)/LAN2DZ(Mn)/TZVP), the reaction becomes exergonic (1.8 kcal/mol), in disagreement with the experiment, while

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Bifunctional aliphatic PNP pincer catalysts

Table 7 Calculated reaction Gibbs free energy (ΔGr, kcal/mol) and enthalpy (Δ Hr, kcal/ mol) for 2MniPr–OH ¼ 1 MniPr + H2O. 2MniPr–OH 5 1 MniPr + H2O

Exp./THF

78

B3PW91/LAN2DZ(Mn)/TZVP

52

B3PW91-SCRF(THF)/LAN2DZ(Mn)/TZVP77 B3PW91-SCRF(THF) + GD3BJ/LAN2DZ(Mn)/ TZVP77

Δ Gr

Δ Hr

0.8

7.9

1.1

12.1

1.8a 1.2

8.7

a

10.7

B3PW91-SCRF(CH3OH)/LAN2DZ(Mn)/TZVP52 2.7 [0.8a]

9.1

B3PW91-SCRF(CH3OH) + GD3BJ/LAN2DZ (Mn)/TZVP52

0.1 [1.9a]

B3PW91-SCRF(H2O)/LAN2DZ(Mn)/TZVP52

3.2 [1.3a]

8.4

B3PW91-SCRF(H2O) + GD3BJ/LAN2DZ(Mn)/ TZVP52

0.5 [1.4 ]

10.7

DLPNO-CCSD(T)/def2-TZVPP//B3PW91/ LAN2DZ(Mn)/TZVP77

3.0a

14.0

DLPNO-CCSD(T)-SCRF(THF)/def2TZVPP//B3PW91/LAN2DZ(Mn)/TZVP77

1.6a

10.7

DLPNO-CCSD(T)-SCRF(THF) + GD3BJ/def2TZVPP//B3PW91/LAN2DZ(Mn)/TZVP77

3.4a

12.5

a

11.2

a

Θ Θ Θ GΘ l ¼ Escrf + Gcorr + RT ln(p1M/p ), p1M ¼ 24.46 atm.

the reaction enthalpy (8.7 kcal/mol) is closer to the experiment. Furthermore, including dispersion (B3PW91-SCRF(THF) + GD3BJ/LAN2DZ (Mn)/TZVP) gives an endergonic reaction Gibbs free energy (1.2 kcal/ mol) and the reaction enthalpy is 10.7 kcal/mol. In addition, we computed solvation effects in polar methanol and water. In methanol and water, the reaction becomes more exergonic (B3PW91-SCRF(CH3OH), 2.7 kcal/mol, B3PW91-SCRF(H2O), 3.2 kcal/mol), while in solvation with dispersion the reaction is nearly thermal neutral (B3PW91-SCRF(CH3OH) + GD3BJ, 0.1 kcal/mol, B3PW91-SCRF(H2O) + GD3BJ, 0.5 kcal/mol). The computed enthalpy barriers in methanol and water (B3PW91-SCRF(CH3OH) + GD3BJ, 9.1 kcal/mol, B3PW91-SCRF(H2O), 8.4 kcal/mol) are more close to the experimental value than those including dispersion (B3PW91-SCRF

ARTICLE IN PRESS 18

Zhihong Wei and Haijun Jiao

(CH3OH) + GD3BJ, 11.2 kcal/mol, B3PW91-SCRF(H2O) + GD3BJ, 10.7 kcal/mol). These comparisons show that the results are model dependent from which it is not easy to reach a general conclusion.52 At the DLPNO-CCSD(T) level in the gas phase and in THF solution with and without dispersion, the computed reaction enthalpy is 14.0, 10.7 and 12.5 kcal/mol, respectively, larger than the experimental value by 6.1, 2.8 and 4.6 kcal/mol, respectively; and the computed reaction free energy is 3.0, 1.6 and 3.4 kcal/mol, respectively, larger than the experimental value by 2.2, 0.8 and 2.7 kcal/mol, respectively.77 This shows that both B3PW91 and DLPNO-CCSD(T) in gas phase and in liquid phase give reasonable results, and B3PW91 gives better results than DLPNO-CCSD(T) for 2Mn–OH ¼ 1Mn + H2O. Besides there are many discussions about the correction and scaling of the computed entropy quantity and several schemes are available,79 despite the fact that the physical basis for assuming any particular entropy error is unclear. Notably, some reactions exhibit large decreases in entropy barriers in solution vs. in the gas phase,80 while some others do not show any decreases at all.81 Nevertheless, we checked the correction of entropy contribution on the basis of the available experimental results for the reactions (2MniPr ¼ 1 MniPr + H2 and 2MniPr–OH ¼ 1 MniPr + H2O) using Martin’s scheme82 as well as the proposal by taking only 50–70% of the computed entropy contribution.79,83–87 As given in Table 8, the computed reaction free energy after entropy correction for 2MniPr ¼ 1 MniPr + H2 is endergonic by 4.0 kcal/mol, and this does not support the experimentally Table 8 Calculated reaction enthalpy, entropy and Gibbs free energy (kcal/mol, 298.15 K) as well as experimental data. Martin’s 50% entropy Energy Exp. B3PW91 approximation approximation

2MniPr ¼ 1 MniPr + H2 Δ Hr

–

8.3

8.3

8.3

T Δ Sr

–

8.7

4.3

4.4

Δ Gr

Equilibrium

0.3

4.0

3.9

2Mn

iPr

–OH ¼ 1 Mn

iPr

+ H2O

Δ Hr

7.9

12.1

12.1

12.1

T Δ Sr

7.1

11.0

6.7

5.5

Δ Gr

0.8

1.1

5.4

6.6

ARTICLE IN PRESS Bifunctional aliphatic PNP pincer catalysts

19

observed equilibrium,76 while the best agreement between theory and experiment is found for the gas phase result without such entropy correction (0.3 kcal/mol). For 2MniPr–OH ¼ 1 MniPr + H2O, the computed reaction free energy after entropy correction is endergonic by 5.4 kcal/mol, and this also does not agree with the experimentally determined value (0.8 kcal/ mol),78 while the uncorrected gas phase result (1.1 kcal/mol) has the best agreement with the experiment. All these show that the gas phase results are in closest agreement with the experimental data and those including solvation or/and dispersion corrections as well as those by using entropy correction show large deviations from experiments. Apart from water, there are also experimental data available for the reaction of 1MniPr and benzyl alcohol [1MniPr + PhCH2OH ¼ 2MniPr– OCH2Ph] and this provides another opportunity for benchmark calculation (Table 9). Experimentally, this reaction in benzene solution shows thermodynamic equilibrium and the estimated reaction enthalpy is 10.5 kcal/ mol.76 At B3PW91 in the gas phase and in benzene solution, the computed reaction enthalpy is 10.6 and 9.1 kcal/mol, respectively, indicating that the reaction enthalpy in the gas phase and in benzene solution is close to the experimental value (10.5 kcal/mol). The computed reaction free energy in the gas phase and in benzene solution is 2.8 and 3.8 kcal/mol, indicating that the reaction free energy in the gas phase and in benzene solution is reasonable. In addition, it was found that TPSS functional also provides reasonable reaction enthalpy (11.5 kcal/mol) and free energy (0.2 kcal/ mol) in benzene solution, while the reaction becomes more exothermic including dispersion (19.6 kcal/mol for B3PW91-SCRF(C6H6) + GD3BJ/LAN2DZ(Mn)/TZVP and 20.1 kcal/mol for TPSSTPSSSCRF(C6H6) + GD3BJ/LAN2DZ(Mn)/TZVP) and more exergonic (4.8 kcal/mol for B3PW91-SCRF(C6H6) + GD3BJ/LAN2DZ(Mn)/ TZVP and 5.0 kcal/mol for TPSSTPSS-SCRF(C6H6) + GD3BJ/ LAN2DZ(Mn)/TZVP), in disagreement with the experiment. At the highly correlated DLPNO-CCSD(T) level in the gas phase and in benzene solution, however, the computed reaction enthalpy is 21.2 and  21.3 kcal/mol, respectively, more than double of the experimental value (10.5 kcal/mol), and the reaction is very much exergonic by 7.9 and 9.8 kcal/mol, and this does not agree with the experimentally observed equilibrium. Adding dispersion correction at DLPNO-CCSD(T) level as well as by using different Minnesota (M06, M062X and M06L) and ωB97XD functional highly overestimated the reaction enthalpy and reaction free energy and does not agree with the experimental results at all.77

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Zhihong Wei and Haijun Jiao

Table 9 Calculated reaction Gibbs free energy (ΔGr, kcal/mol) and enthalpy (Δ Hr, kcal/ mol) for 1MniPr + PhCH2OH ¼ 2MniPr–OCH2Ph.77 1MniPr + PhCH2OH 5 2MniPr–OCH2Ph Δ Gr

ΔHr

Equilibrium 10.5

Exp./C6H6

2.8

10.6

3.8

9.1

4.8

19.6

0.2

11.5

TPSSTPSS-SCRF(C6H6) + GD3BJ/LAN2DZ (Mn)/TZVPa

5.0

20.1

DLPNO-CCSD(T)/def2-TZVPP//B3PW91/LAN2DZ (Mn)/TZVP

7.9

21.2

DLPNO-CCSD(T)-SCRF(C6H6)/def2TZVPP//B3PW91/LAN2DZ(Mn)/TZVPa

9.8

21.3

DLPNO-CCSD(T)-SCRF(C6H6) + GD3BJ/ def2-TZVPP//B3PW91/LAN2DZ(Mn)/TZVPa

19.9

31.4

M06-SCRF(C6H6)/LAN2DZ(Mn)/TZVPa

7.6

20.9

9.7

23.7

14.6

27.9

13.1

25.3

B3PW91/LAN2DZ(Mn)/TZVP B3PW91-SCRF(C6H6)/LAN2DZ(Mn)/TZVP

a

B3PW91-SCRF(C6H6) + GD3BJ/LAN2DZ(Mn)/TZVP TPSSTPSS-SCRF(C6H6)/LAN2DZ(Mn)/TZVP

M06 L-SCRF(C6H6)/LAN2DZ(Mn)/TZVPa M062X-SCRF(C6H6)/LAN2DZ(Mn)/TZVP

a

ωB97XD-SCRF(C6H6)/LAN2DZ(Mn)/TZVP a

Θ Θ Θ GΘ l ¼ Escrf + Gcorr + RT ln(p1M/p ),

a

a

a

p1M ¼ 24.46 atm.

3. Hydrogenation reactions and mechanism According to the orbital symmetry rule, a direct addition of H2 across an unsaturated bond is thermal forbidden. In contrast, transition metal catalysts have the appropriate orbitals to interact readily with H2, forming metal hydride species, allowing the transfer of the hydride to the unsaturated substrates. The mechanism of typical transition metal homogeneous hydrogenation can be considered as a two-step process (Scheme 5).2,47 Taking alkene hydrogenation as example, the first step is catalyst activation, and the second step is unsaturated bond activation. For monohydride/dihydride species formation, the homolytic cleavage of H2 takes place via oxidative

ARTICLE IN PRESS Bifunctional aliphatic PNP pincer catalysts

21

Scheme 5 The typical homogeneous hydrogenation catalyzed by inner-sphere mechanisms.

addition with the formal oxidation state of metal changes from I to I + 2. Then, the substrate coordinates at a vacant site by dissociation of a ligand, often solvent. This coordination in the inner sphere allows the insertion of the double bond of the alkene into the M–H bond. Subsequent reductive elimination results in the saturated product formation. In 1995, Noyori developed and coined the “metal–ligand bifunctional catalysis” which involves the N–H effect (Scheme 6). Also an outer-sphere mechanism was proposed for the hydrogenation and dehydrogenation, as well as transfer hydrogenation of polar multiple bonds based on M–H/ N–H bifunctional catalysts.3 Subsequently, the unsaturated substrate was hydrogenated via simultaneous hydride and proton transfer in an outer-sphere manner producing the hydrogenated substrate and 1M. However, Gordon and Dub recently proposed a revised Noyori mechanism for the related ketone hydrogenation that preferentially proceeds in a stepwise fashion via ion-pair intermediates in which the N–H function stabilizes the transition state through N–HO hydrogen bonding rather than reversible proton transfer (Scheme 6, blue lines).88 In this mechanism, the ligand was considered as innocent since the proton of N–H did not change during the whole cycle and the DFT calculations indicated that introducing the bifunctional ligand to the complex does not guarantee the active involvement of the amido complex in the catalytic cycle.89 Besides, it is worth noting that some metal complexes with methylated PNCH3P ligands were found as effective as the amino ones. For aqueous methanol dehydrogenation under strong base condition, the reaction rate by using [RuII(H)(CO)(Cl)(iPr2PNHPiPr2)] was only 2.4 times higher than

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Zhihong Wei and Haijun Jiao

Scheme 6 Metal–ligand bifunctional mechanism for (de)hydrogenation (X ¼ O, C or N).

that by using the methylated [RuII(H)(CO)(Cl)(iPr2PNCH3PiPr2)]. For CO2 hydrogenation, the methylated complexes [FeII(H)(CO)(BH4) (iPr2PNCH3PiPr2)] and [FeII(H)(CO)(BH4)(Cy2PNCH3PCy2)] are more active than the amino complex [FeII(H)(CO)(BH4)(iPr2PNHPiPr2)] as well as the amido complex [FeII(H)(CO)(iPr2PNPiPr2)], and [FeII(H)(CO) (Cy2PNPCy2)] complexes. For the hydrogenation of esters and alkenes, the unusual methylated complexes [CoII(CH2SiMe3)(Cy2PNCH3PCy2)] BArF4 and [CoII(CH2SiMe3)(Cy2PNHPCy2)]BArF4 have similar activity; in both cases the inner-sphere mechanism cannot be excluded since they have a vacant site available for the reaction.90–92

3.1 Hydrogenation of ester and aldehyde 3.1.1 Fe-, Ru- and Os-based PNP catalysts The catalyzed hydrogenation of esters using metal-based PNP ligands has been summarized in Scheme 7, and the details are given below. Since the second cycle of ester hydrogenation is the hydrogenation of the formed

ARTICLE IN PRESS Bifunctional aliphatic PNP pincer catalysts

23

Scheme 7 Experimentally applied metal PNP catalysts for hydrogenation of esters.

aldehyde intermediates, we did not pay attention to aldehyde hydrogenation despite the fact that many experimental studies are reported. One reason is that the barrier for ester hydrogenation is generally higher than that for aldehyde hydrogenation; and another reason is that both reactions have very similar mechanism; and there are no quantitative differences between these two reactions. By using [RuIIH(CO)(Cl)(Ph2PNHPPh2)] (Ru-MACHO) as precatalyst, various kinds of esters were reduced with good conversion and selectivity in the presence of NaOMe at 100 °C and 50 bar of H2.15 The catalyst [RuIIH(CO)(BH4)(Ph2PNHPPh2)] has been tested successfully in

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the hydrogenation of chiral esters of methyl L-Boc-alanine ester, methyl(S)-2(benzyloxy)propionate, methyl(R)-3-(Boc-amino)butanoate, methyl(S)-3(phenylamino)butanoate and methyl (R)-3-(tert-butyldimethylsilyloxy) butanoate; and good conversion and retention of the ee values were obtained without base at 80 °C and 55 bar of H2.93 Furthermore, [RuIIH2(CO)(Ph2PNHPPh2)], [RuIIH(CO)(Cl)(Ph2PNH PPh2)] and [RuIICl2(CO)(Ph2PNHPPh2)] were also found active for the selective hydrogenation of fluorinated esters. Under mild conditions the reaction proceeds rapidly to give the corresponding fluorinated alcohols or hemiacetals in good to excellent yields. The DFT analysis of methyl trifluoroacetate hydrogenation at ωB97XD/6-311++G**/SMD(CH3OH) suggests that the hydride transfer from [RuII(H)2(CO)(Ph2PNHPPh2)] to the ester occurs in an outer-sphere manner. The subsequent H–H coordination to the metal center and heterolytic cleavage of H2 resulted in hemiacetal formation. In their mechanism, the function of N–H is to stabilize the transition state via the N–H⋯O hydrogen bonding rather than the reversible proton transfer,94 and therefore an innocent mechanism is proposed.88,89 Schlaf et al. evaluated the amino complexes [OsII(H)2(CO) (iPr2PNHPiPr2)] and [OsIIH(CO)(Cl)(iPr2PNHPiPr2)] in the catalytic hydrogenation of hexyl octanoate and cis-3-hexenyl hexanoate to alcohols as model substrates for triglycerides. Both complexes achieved full conversion of the saturated ester at 220 °C and 55 bar of H2.19 The groups of Beller and Guan independently reported the hydrogenation of esters under relatively mild conditions by using [FeIIH(CO)(BH4) (iPr2PNHPiPr2)] as catalyst without base.29,69 In addition, Beller et al. examined the catalytic activity of differently substituted [FeIIH(CO)(BH4) (R2PNHPR2), R ¼ iPr, Cy and Et] for ester hydrogenation.64 To elucidate the reaction mechanism and to understand the different performance of both metal and ligand substituted catalysts [MII(CO) (H)2(R2PNHPR2), M ¼ Fe, Ru, Os; R ¼ iPr, Cy and Et], B3PW91 DFT calculations were carried out on the basis of the proposed outer-sphere mechanism (Scheme 8), despite the fact that the proposed innocent mechanism by Gordon and Dub might also be operative. The commonality of both mechanisms is that the first step is the M–H hydride transfer to the carbon center of the C]O group; and this step has been considered as the rate-determining step in ester hydrogenation. This enables our comparison without considering the next steps of the possibly more favored mechanisms. It is worth to note that by using the methylated [FeII(CO)(H)2(iPr2PNCH3PiPr2)]

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Scheme 8 Proposed outer-sphere mechanism for the M PNP catalyzed ester and aldehyde hydrogenation.

and [FeII(CO)(H)(BH4)(iPr2PNCH3PiPr2)] complexes, no conversion of methyl benzoate was achieved under 100 °C and 30 bar pressure of H2, indicating that a cooperative interaction involving the N–H unit of the pincer ligand and the Fe–H group is required for catalytic hydrogenation.29 In the proposed mechanism (Scheme 8), the first cycle is the hydrogenation of ester into hemiacetal; then the formed hemiacetal decomposes into aldehyde and alcohol; this step is accompanied by the regeneration of the amino catalyst (2M) by adding molecular H2 to the amido catalyst (1M). The second cycle is the hydrogenation of the formed aldehyde into alcohol, which is again accompanied by the regeneration of the amino catalyst (2M) from the amido catalyst (1M) and H2. Since the interconversion between 2M ¼ 1M + H2 has been discussed above in detail, no further discussion is needed; instead, only comparison has been done. Since the gas phase computed kinetic and thermodynamic data have the closest agreement with the available experimental values, we also used gas phase results for our discussion and comparison. In addition, it is also noted that for as close as possible comparison, we have recalculated the key step of this reaction by using the same conformations of the ground and transition states, and the computed

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energetic numbers might differ slightly from the previous one. Nevertheless, the general conclusion has not been altered. All these data are summarized in Table 10 for comparison. Using 2FeiPr [FeII(CO)(H)2(iPr2PNHPiPr2)] as active catalyst, we computed the hydrogenation of methyl benzoate and methyl acetate (Table 10). For the formation of hemiacetal, a stepwise mechanism has been found. The first step is hydride transfer, followed by an intermediate, and the second step is the proton transfer resulting in the formation of hemiacetal; the first step is rate-determining. For the formation of the hemiacetal, the computed Gibbs free energy barrier of the rate-determining hydride transfer step of methyl benzoate and methyl acetate on the basis of a stepwise mechanism is 29.6 and 28.2 kcal/mol, respectively, and the Gibbs free energy barrier of subsequent proton transfer is 23.2 and 24.3 kcal/mol, respectively. This reaction step is endergonic by 11.9 and 10.6 kcal/mol, respectively. For the decomposition of hemiacetal to the corresponding benzaldehyde and acetaldehyde, the reaction is exergonic by 8.2 and 4.1 kcal/mol from methoxy(phenyl)methanol and 1-methoxyethanol, respectively. In addition, we computed the hydrogenation activity of cyclohexyl and ethyl substituted catalysts 2FeCy–BH4 and 2FeEt–BH4 by using the same methodology (Table 10). Again, methyl benzoate hydrogenation to hemiacetal undergoes a stepwise mechanism corresponding to hydride and proton transfer for which the hydride transfer is the rate-determining step.54 The Gibbs free energy barrier of the hydride transfer for methyl benzoate hydrogenation to hemiacetal by using pre-catalyst 2FeiPr–BH4, 2FeCy–BH4, and 2FeEt–BH4 is 29.6, 30.2 and 26.3 kcal/mol, respectively, higher than that of the second proton transfer of 23.2, 24.1 and 20.2 kcal/ mol, respectively. It shows that the ethyl substituted 2FeEt has the lowest barrier, while that of 2FeCy is the highest. This demonstrates the substitution effect, i.e., the less bulky ethyl group has the least steric hindrance and hence the lowest barrier and highest activity. Indeed, this difference has been found in the experimental study,64 i.e., the reaction by using 2FeEt can be performed not only at lower catalyst loading but also at lower temperature, compared with that using 2FeiPr and 2FeCy. Indeed, Langer et al. also found that ethyl-substituted 2FeEt–BH4 shows higher activity than iPr-substituted 2FeiPr–BH4 in the hydrogenation of methyl benzoate to alcohols under 50 bar of H2.55 For the Ru- and Os-based catalysts on the basis of the same stepwise mechanism for methyl benzoate hydrogenation (Table 10), the barrier of the rate-determining hydride transfer step is 26.8 and 29.3 kcal/mol, while

Table 10 B3PW91 gas phase computed Gibbs free energy barriers and reaction energies for the hydrogenation of Ph-COOCH3 and Ph-CHO, as well as the interconversion between 2M and 1M (in kcal/mol). TS(Ph-CO-OCH3) TS(Ph-CHO) (2M 5 1M + H2)

FeII(CO)(H)2(iPr2PNHPiPr2)95 II

H

95

Fe (CO)(H)2(Cy2PN PCy2) II

H

95

Δ G‡ (N–H–O)

Δ G‡ (M–H–C)

ΔG‡ (N–H–O)

Δ G‡ (2M/1M)a

Δ G (1M + H2)

29.6

23.2

14.1

8.7

19.1 [20.0]

0.9

30.2

24.1

15.0

9.6

20.3 [16.4]

3.9

26.3

20.1

11.6

6.2

18.5 [18.1]

0.4

26.8

19.2

11.7

5.3

21.5 [21.6]

0.1

II

H

95,b

29.3

26.5

13.5

11.3

23.1 [20.0]

3.1

I

H

32

32.7

30.1

15.6

11.9

20.2 [20.4]

0.2

I

H

32

34.3

27.9

18.5

13.5

20.5 [19.2]

1.3

Mn (CO)2(H)(Et2PN PEt2)

31.8

25.8

15.9

12.8

20.3 [18.8]

1.5

IrIII(H)3(iPr2PNHPiPr2)95,b

29.6

27.8

14.3

12.5

26.8 [23.3]

3.5

RuII(CO)(H)2(iPr2PNHPiPr2)95,b Os (CO)(H)2(iPr2PN PiPr2)

Mn (CO)2(H)(iPr2PN PiPr2)

Mn (CO)2(H)(Cy2PN PCy2) I

a

H

32

Reverse reaction barriers in square brackets. TZVP all electron basis set are used, expected for Ru, Os and Ir (LANL2DZ).

b

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Fe (CO)(H)2(Et2PN PEt2)

Δ G‡ (M–H–C)

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that of the proton transfer step is 19.2 and 26.5, respectively. This indicates that the Ru-based catalysts are more active than the Fe- and Os-based catalysts; and both Fe- and Os-based catalysts have close activity. However, it is interestingly noted that ethyl substituted 2FeEt is as active as the isopropyl substituted 2RuiPr. On the basis of the same mechanism we also computed the hydrogenation of the formed benzaldehyde (Table 10), which also undergoes a stepwise mechanism corresponding to hydride transfer and proton transfer, and hydride transfer is the rate-determining step. The Gibbs free energy barrier of the hydride transfer is 14.1, 15.0 and 11.6 kcal/mol, respectively, for catalyst 2FeiPr–BH4, 2FeCy–BH4, and 2FeEt–BH4. The lowest barrier for 2FeEt differentiates once again the steric effect of the substituents. In addition, the hydrogenation of acetaldehyde to ethanol by using 2FeiPr has a barrier of 12.7 kcal/mol and is exergonic by 7.3 kcal/mol. Comparison among the Fe-, Ru- and Os-based catalysts shows that the ethyl substituted 2FeEt and the isopropyl substituted 2RuiPr have close barriers (11.6 and 11.7 kcal/ mol), and they are lower than that of 2FeiPr, 2FeCy and 2OsiPr. This demonstrated clearly that changing metal and varying the steric effect of the substituents can enhance the catalytic activity; and the base-metal 2FeEt can have similar catalytic activity as the precious metal 2RuiPr. The computed H2 elimination Gibbs free energy barrier for 2FeiPr– BH4, 2FeCy–BH4, and 2FeEt–BH4 is 19.1, 20.3 and 18.5 kcal/mol (Table 10), respectively, and they are lower than that of the rate-determining step of methyl benzoate hydrogenation, but higher than that of the ratedetermining step of benzaldehyde hydrogenation. This indicates that high pressure and high temperature are needed for the effective conversion for methyl benzoate hydrogenation. The lower barrier of benzaldehyde hydrogenation explains the fact that only benzyl alcohol has been observed as product experimentally and why benzaldehyde has not been observed as intermediate. 3.1.2 Mn-based PNP catalysts Based on the experimental finding that the cationic ethyl complex [MnI(CO)3(Et2PNHPEt2)]Br and the neutral ethyl complex [MnI(CO)2 (Br)(Et2PNHPEt2)] have the same catalytic performance, the active [MnI(CO)2(H)(Et2PNHPEt2)] catalyst formation and an outer-sphere mechanism were proposed (Scheme 8). DFT calculation suggested that the formation of the hemiacetal is the rate-determining step, and the

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computed free energy barrier is 31.8 kcal/mol for [MnI(CO)2(H) (Et2PNHPEt2)].32 To elucidate the reaction mechanism and to understand the different performance of catalysts [MnI(CO)2(H)(R2PNHPR2), R ¼ iPr, Cy and Et] for ester hydrogenation, DFT calculations were carried out at the B3PW91/TZVP level. For all complexes, the Gibbs free barrier for H2 elimination [2M ¼ 1M + H2] is around 20 kcal/mol and the reactions are slightly exergonic by 0.2–1.5 kcal/mol. This shows that a well-balanced equilibrium is established for H2 elimination from the amino complexes of [MnI(CO)2(H)(R2PNHPR2)] to the amido complexes of [MnI(CO)2 (R2PNPR2)]. For methyl benzoate hydrogenation to hemiacetal, a stepwise mechanism corresponding to hydride and proton transfer was located (Scheme 8). It was found that hydride transfer is the rate-determining step and the computed free Gibbs energy barrier is 32.7, 34.3 and 31.8 kcal/mol for [MnI(CO)2(H)(iPr2PNHPiPr2)], [MnI(CO)2(H)(Cy2PNHPCy2)] and [MnI(CO)2(H)(Et2PNHPEt2)], respectively, and higher than that of the proton transfer step (30.1; 27.9 and 25.8 kcal/mol, respectively). Subsequently, the formed hemiacetal dissociates to give benzaldehyde and methanol, while the amino complex is regenerated by H2 addition. In the second cycle, benzaldehyde is hydrogenated to benzyl alcohol in a similar stepwise process. Again, the hydride transfer has a higher free energy barrier than the second step of proton transfer, 15.6 vs. 11.9, 18.5 vs. 13.5, and 15.9 vs. 12.8 kcal/mol, respectively, for the [MnI(CO)2(H) (iPr2PNHPiPr2)], [MnI(CO)2(H)(Cy2PNHPCy2)], and [MnI(CO)2(H) (Et2PNHPEt2)] complexes. On the basis of the Gibbs free energy barriers of both cycles, we found that the initial reduction of ester to hemiacetal is the rate-determining step, and [MnI(CO)2(H)(Et2PNHPEt2)] has the lowest Gibbs free energy barrier followed by [MnI(CO)2(H)(iPr2PNHPiPr2)] and [MnI(CO)2(H)(Cy2PNHPCy2)], which is in agreement with the observed catalytic activity.32 Since the barrier of the rate-determining step of hemiacetal formation is higher than that of H2 elimination, high H2 pressure is needed to maintain the stability of the amino complexes for the hydrogenation reaction. For the hydrogenation of benzaldehyde, the barrier of the hydride transfer is lower than that of H2 elimination, and it is not necessary to have high H2 pressure. The high barrier of the rate-determining step rationalized the need of high reaction temperature. Under the reaction condition (110 °C, 30 bar H2, 24 h),32 the computed data rationally explain the formation of benzyl alcohol as the final product, instead of benzaldehyde as intermediate.

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As in case of Fe PNP complexes, the barrier of the rate-determining step of hemiacetal formation is higher than that of H2 elimination; high H2 pressure is needed to maintain the stability of the amino complexes for the hydrogenation reaction. It is noted that the barriers of the rate-determining step of Fe-based catalysts are lower than those of the Mn-based catalysts (3.1, 4.1 and 5.5 kcal/mol for the iPr, Cy and Et substituted catalysts, respectively); and this reveals that Fe-based catalysts are more active than the corresponding Mn-based catalysts, which is in agreement with the experimental observations, i.e., Fe-based catalysts perform at milder condition (60 vs. 110 °C) and shorter time reaction (6 vs. 24 h) than the Mn-based catalyst. 3.1.3 Ir-based PNP catalysts Our group also applied [IrIII(Cl)(H)2(iPr2PNHPiPr2)] and [IrIII(H)3 (iPr2PNHPiPr2)] complexes in ester hydrogenation. In the presence of 10 mol% of MeONa at 130 °C under 50 bar of H2, nearly complete conversion 94% and high yield (89% for [IrIII(Cl)(H)2(iPr2PNHPiPr2)] and 81% for [IrIII(H)3(iPr2PNHPiPr2)]) were obtained in toluene. For [IrIII(Cl)(H)2(iPr2PNHPiPr2)] as pre-catalyst, no reaction took place in the absence of base, and [IrIII(H)3(iPr2PNHPiPr2)] was active in the absence of base, although the yield is low (24%).20 DFT calculations reasonably explained the necessity of a strong base as the formation of the amido complex of [IrIII(H)2(iPr2PNPiPr2)] from [IrIII(Cl)(H)2(iPr2PNHPiPr2)] is endergonic by 30.7 kcal/mol. Furthermore, the stability of the heterolytic and homolytic H2 elimination products from the amido complex of [IrIII(H)2(iPr2PNPiPr2)] to the amino complexes of [IrIII(H)3 (iPr2PNHPiPr2)] and [IrI(H)(iPr2PNHPiPr2)] was calculated. The amido complex of [IrIII(H)2(iPr2PNPiPr2)] is 12.0 kcal/mol more stable than [IrI(H)(iPr2PNHPiPr2)]. This provides evidence for an outer-sphere mechanism, as the amido complex of [IrIII(H)2(iPr2PNPiPr2)] is thermodynamically more favorable. The elimination of H2 from the amino complex of [IrIII(H)3(iPr2PNHPiPr2)] to [IrIII(H)2(iPr2PNPiPr2)] has a Gibbs free energy barrier of 26.8 kcal/mol and is 3.5 kcal/mol endergonic. Thus, an outersphere mechanism was considered for methyl benzoate hydrogenation. In the first cycle of the hydrogenation of methyl benzoate, hemiacetal is formed with a computed energy barrier of 29.6 kcal/mol for the hydride transfer and 27.8 kcal/mol for the proton transfer, and the reaction is endergonic by 15.2 kcal/mol. The dissociation of hemiacetal into benzaldehyde and methanol is exergonic by 8.2 kcal/mol. In the second cycle,

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benzaldehyde is hydrogenated to benzyl alcohol with a free-energy barrier of 14.3 kcal/mol for hydride transfer and 12.5 kcal/mol for the proton transfer, and this reaction is endergonic by 0.2 kcal/mol. Our calculated results are in good agreement with the experimental results that benzaldehyde can be easier hydrogenated than ester methyl benzoate to give benzyl alcohol as the final product. Since the regeneration of [IrIII(H)3(iPr2PNHPiPr2)] from [Ir(H)2(iPr2PNPiPr2)] has a Gibbs free energy barrier of 23.2 kcal/mol, formation of hemiacetal is also found to be the rate-determining step on the whole potential surface. 3.1.4 Co-based PNP catalysts Using the Hanson pre-catalyst [CoII(CH2SiMe3)(Cy2PNHPCy2)]BArF4, Jones et al. reported the additive free catalytic hydrogenation of carboxylic acid esters to alcohols under 55 bar of H2 at 120 °C in THF. Methyl benzoate turned out to be challenging and gave only 15% yield of benzyl alcohol. After the hydrogenation of methyl benzoate, a new CoIII-based complex of [CoIII(κ1-PhCOO)(κ2-PhCOO)(Cy2PNHPCy2)]BArF4 was isolated. The formation of this complex might explain why methyl esters have low reactivity compared to those of the corresponding ethyl and benzyl esters. Their mechanistic study indicated that the methylated catalyst [CoII(CH2SiMe3)(Cy2PNCH3PCy2)]BArF4 works almost as good as [CoII(CH2SiMe3)(Cy2PNHPCy2)]BArF4. Therefore, an inner-sphere mechanism was proposed based on the CoII complex of [CoII(H) (Cy2PNHPCy2)].96 This is quite different from the above catalysts. Our group also tested both CoI and CoII pincer complexes in methyl benzoate hydrogenation and found that the pre-catalyst of [CoII(Cl)2 (Ph2PNHPPh2)] and [CoI(Cl)(iPr2PNHPiPr2)] was found active in the presence of 20 mol% of MeONa. Whenever the N–H position was blocked by a methyl substituent, no reaction occurred. Methyl benzoate was chosen as model substrate to test the catalytic activity of the differently substituted cobalt PNP pincer complexes.40 In the presence of 20 mol% of NaOMe under 50 bar of H2 at 140 °C after 48 h in dioxane, only moderate conversion (65% and 67%) and yields (46% and 45%) were detected, when 5 mol% of [CoII(Cl)2(iPr2PNHPiPr2)] and [CoII(Cl)2(Cy2PNHPCy2)] complexes were applied. In the case of the phenyl-substituted cobalt pincer complex of [CoII(Cl)2(Ph2PNHPPh2)], quantitative conversion of methyl benzoate (99%) and 96% yield of benzyl alcohol were obtained. Notably, applying 5 mol% of cobalt pincer complex [CoII(Cl)2(Ph2PNHPPh2)] at 100 °C still produced nearly quantitative yield of benzyl alcohol in only 6 h.40

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Therefore, it seems that an outer sphere mechanism might take place. DFT calculations at B3PW91/TZVP level showed that the singlet state of amido [CoIII(H)2(Ph2PNPPh2)] is much more stable than the corresponding triplet state by 12.6 kcal/mol. while the triplet state of amino [CoI(H)(Ph2PNHPPh2)] is more stable than the corresponding singlet state by 4.9 kcal/mol. It is very interesting to note that the singlet state of [CoIII(H)2(Ph2PNPPh2)] is only 0.9 kcal/mol higher in energy than the triplet state of [CoI(H)(Ph2PNHPPh2)], thus indicating that both heterolytic and homolytic H2 eliminations are thermodynamically possible and competitive. Similar results are also found for R ¼ iPr substituted catalysts, i.e., the singlet state of [CoIII(H)2(iPr2PNPiPr2)] is slightly more stable than the triplet state of [CoI(H)(iPr2PNHPiPr2)] by 2.9 kcal/mol. Such energetic properties of [CoIII(H)2(R2PNPR2)] and [CoI(H)(R2PNHPR2)] (R ¼ Ph and iPr) provide the assumption that depending on the applied reaction conditions, both outer- and inner-sphere mechanisms are possible. Although experimental results provided hints which support an outer-sphere mechanism, based on DFT computation an inner sphere cannot be excluded.40 Therefore, both CoIII and CoI species can be potential intermediates in the catalytic cycle and a multiple (complexity) mechanism should be expected.40 3.1.5 Acceptorless dehydrogenative coupling (ADC) of alcohols The reverse reaction of ester hydrogenation, ADC of alcohols to esters with hydrogen as only side product, attracts much attention in modern synthetic chemistry.97 Several Ru-,98,99 Ir-,98 Os-,16 Fe-,71,100 and Mn-76 based PNP complexes have been developed for ADC of a wide range of alcohols to esters (Scheme 9). Gusev et al. developed the [OsII(H)2(CO) (iPr2PNHPiPr2)] and [RuII(H)2(CO)(iPr2PNHPiPr2)] catalysts for ADC of alcohols and found that the complex [OsII(H)2(CO)(iPr2PNHPiPr2)] is an active catalyst for isoamyl alcohol, benzyl alcohol and hexanol, as well as the complex [RuII(H)2(CO)(iPr2PNHPiPr2)] shows similar activity for isoamyl alcohol at above 118 °C using only 0.1 mol% catalyst.16 Beller et al. applied the [RuII(H)(Cl)(CO)(Ph2PNHPPh2)], [RuII(H)2 (CO)(iPr2PNHPiPr2)] and [IrII(H)(Cl)(CO)(iPr2PNHPiPr2)] complexes in ADC of ethanol to ethyl acetate under mild conditions in the presence of base (EtONa) with low catalyst loading (25 ppm).98 The [RuII(H)(Cl) (CO)(Ph2PNHPPh2)] and [RuII(H)2(CO)(iPr2PNHPiPr2)] complexes showed high activity with TOF of 1134 and 1107 h1, respectively, while

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Scheme 9 Experimentally applied metal PNP catalysts for ADC of alcohols to esters.

the [IrII(H)(Cl)(CO)(iPr2PNHPiPr2)] complexes led to very low TOF (below 100 h1).98 By using [FeII(H)2(CO)(iPr2PNHPiPr2)], benzyl alcohol was dehydrogenated to benzyl benzoate with full conversion after 8 h in refluxing toluene and benzyl benzoate was isolated as the sole product.71 In addition, ADC of several primary alcohols to the respective esters is catalyzed by the amido complex [FeII(H)(CO)(iPr2PNPiPr2)] with catalyst loadings as low as 0.1 mol% and conversions between 62% and 90% within 20 h.71 Recently, Gauvin et al. reported the base free ADC from alcohols to esters by using the well-defined [MnI(CO)2(iPr2PNPiPr2)] pincer complex as active catalysts at 110–150 °C using 0.6 mol% catalyst. Elementary reactions of benzyl alcohol to metal-alkoxy complex ([MnI(CO)2(OCH2Ph) (iPr2PNHPiPr2)], MniPr–OCH2Ph) were investigated by multinuclear (1H, 13C, 31P, 15N and 55Mn) NMR methods. In addition, the molecular structure of MniPr–OCH2Ph was verified by X-ray structure analysis.76 The ADC reaction of alcohols to esters can undergo direct or nucleophilic C–O coupling mechanisms (Scheme 10). For the direct C–O coupling mechanism, alcohol is first dehydrogenated to aldehyde,89 which

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Scheme 10 ADC of benzyl alcohol to benzyl benzoate via direct and nucleophilic C–O coupling mechanisms (relative free energies for Mn- and Re-PNP (in bracket) are given in kcal/mol).

couples with a second alcohol to hemiacetal, and the last step is hemiacetal dehydrogenation to ester and H2.97,101,102 The alternative nucleophilic pathway involves the C–O coupling between aldehyde and metal-alkoxy. First, alcohol coordinates dissociatively to the metal center, and then the C–O coupling occurs between the carbon atom of aldehyde and the oxygen atom of metal-alkoxy. Finally, ester is formed via hemiacetal intermediate or via H transfer after O/H coordination exchange.77 With the help of in situ ESI-MS, the formation of ethanolate and 1-ethoxyethanolate intermediates was detected as intermediates in ADC of ethanol catalyzed by a series of pincer-type Ru and Os complexes on the basis of their m/z values.99,103 The collision induced dissociation mass spectrum of 1-ethoxyethanolate intermediates as well as ethyl acetate, acetaldehyde and H2 indicates that

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1-ethoxyethanolate species is the key species for ethyl acetate formation.99,103,76 In addition, they proposed that inner- and outer-sphere mechanisms are most likely competitive since the detected acetaldehyde can be presumably formed via β-hydrogen elimination from ethanolate, where the hemilabile pincer ligand plays a major role for vacant site formation.103 However, computational study by Wang et al. indicates that the bifunctional double hydrogen transfer pathway via an outer-sphere manner is energetically more favorable than the β-hydride elimination pathway via an innersphere mechanism.104,105 Very recently, Gauvin et al.74 studied the ADC mechanism of ethanol to ethyl acetate catalyzed by Ru pincer complex. It was found that the amido complex [RuII(CO)(H)(iPr2PNPiPr2)] can easily react with ethanol even at very low temperature (200 K), while the amino complex [RuII(CO)(H)2(iPr2PNHPiPr2)] does not react with ethanol under the same condition. Since aldehyde is detected as intermediate and the amido complex is not detected under the catalytic relevant condition, the nucleophilic C–O coupling pathway is supposed to be possible for Ru pincer catalyzed ADC of EtOH to ethyl acetate. In order to make a direct comparison with the experiment, we computed the ADC of benzyl alcohol to benzyl benzoate [2PhCH2OH ¼ PhCOOCH2Ph + 2H2] catalyzed by Mnand Re PNP complex under base free condition.77 For benzyl alcohol dehydrogenation to benzaldehyde, a two-step ionic pathway via outer-sphere mechanism is more favored kinetically, and the proposed inner-sphere mechanism via one phosphine ligand dissociation and β-hydride elimination with higher barrier can be discarded. For hemiacetal formation, the direct coupling between coordinated aldehyde and hydrogen-bonding stabilized benzyl alcohol is more favored kinetically than the nucleophilic path via dissociatively coordinated benzyl alcohol and benzaldehyde. The dehydrogenation of hemiacetal to ester proceeds via a two-step ionic pathway which is the rate-determining step. Both Mnand Re-based catalysts have close free energy barriers and similar catalytic activity.77

3.2 Ketone hydrogenation The catalyzed hydrogenation of ketones using metal-based PNP complexes has been summarized in Scheme 11; and the details are given below. As discussed in the sections of enantioselective hydrogenation of ketones, as well as transfer hydrogenation and isomerization of α,β-unsaturated aldehydes and ketones, ketone hydrogenation has a similar mechanism as the

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Scheme 11 Experimentally applied metal PNP catalysts for hydrogenation of ketones.

hydrogenation of ester and aldehyde. Therefore, we did not pay direct attention to the hydrogenation of ketones; and the summary is used for general information. In 1991, the first chiral PNP ligand was synthesized and 16-electron [RhIPNP(COD)]SbF6 complexes (COD ¼ 1,5-cyclooctadiene) were introduced for the enantioselective hydrogenation of various unsaturated C]C and C]O bonds; however, no reduction products were found.11 By using [RuII(Cl)2(Ph2PNHPPh2)] as pre-catalyst for the hydrogenation of acetophenone, benzophenone, cyclohexanone in the presence of tBuOK (10 mg), 100% yield was obtained under mild conditions (25 °C, under 3 atm. of H2) by Abdur-Rashid et al.13 They also applied [IrIII(Cl) (H)2(iPr2PNHPiPr2)]/tBuOK (1:10) in ketones hydrogenation, where acetophenone was fully converted and 97% phenylethanol was obtained. Benzophenone was converted to benzhydrol (98% isolated yield) under similar mild reaction conditions. Un-activated dialkyl ketones were readily converted to their respective alcohols, including sterically congested and electronically deactivated pinacolone. The chemo-selective hydrogenation of conjugated ketones, such as benzalacetone, β-ionone and 2-cyclohexen1-one, was also investigated. It was found that only the carbonyl group of benzalacetone and β-ionone was reduced to allyl alcohols, whereas

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hydrogenation of 2-cyclohexen-1-one resulted in a 1:1 mixture of the allyl and saturated alcohols.106 Kuriyama et al. developed the [RuII(H)(CO)(Cl)(Ph2PNHPPh2)] catalyst and applied it in ketone hydrogenation at 40 °C for 17.5 h and 30 bar of H2. It was found that phenylethanol from acetophenone was produced with full conversion.93 Compared with [RuII(Cl)2(Ph2PNHPPh2)], [RuII(H)(CO)(Cl)(Ph2PNHPPh2)] needs higher temperature (40 vs. 25 ° C) and H2 pressure (30 vs. 3 bar) as well as longer reaction time (17.5 vs. 3 h); however, a lower catalyst loading (S/C ¼ 2010 vs. 100) was used. Ketones were also hydrogenated under mild conditions by using [CoII(CH2SiMe3)(Cy2PNHPCy2)]BArF4 as reported by Hanson.36 Acetophenone hydrogenation gave nearly full yield (98% determined by GC) under 1 atm. of H2 at 25 °C after 24 h. Aliphatic 2-hexanone and aromatic 1-(2-bromophenyl)ethan-1-one, 1-(3-methoxyphenyl)ethan-1-one, 2,2,2-trifluoro-1-phenylethan-1-one and 2-indanone were reduced in high yields at 60 °C after 24–48 h. By using the amido complex of [MoI(CO)(NO)(iPr2PNPiPr2)], acetophenone was hydrogenated into phenylethanol in 32% yield after 3.5 h with 1 mol% loading of catalyst at 140 °C under 60 bar of H2 in toluene, but the maximum initial TOF was only 14 h1. Under the same conditions, however, benzaldehyde was not hydrogenated by [MoI(CO)(NO) (iPr2PNPiPr2)].43 Acetophenone was fully converted to phenylethanol by using the amido complex [FeII(H)(CO)(iPr2PNPiPr2)] at room temperature within 2 h with low catalyst loading (0.2 mol%) and under low H2 pressure (1 bar). The catalyst loading could be further reduced to 0.1 mol% at slightly higher pressure (5 bar) and longer reaction times (4 h) or at higher temperature (50 °C, 5 bar H2 and 2 h); and even 0.05 mol% of catalyst is practical for full conversion (50 °C, 5 bar H2 and 4 h). Apart from the high activity, the chemoselectivity of [FeII(H)(CO)(iPr2PNPiPr2)] is also remarkable, i.e., 100% for the chemo-selective hydrogenation of 5-hexen-2-one to the respective unsaturated alcohol.71 In contrast, Hanson’s cobalt catalyst of [CoII(CH2SiMe3)(Cy2PNHPCy2)]BArF4 is not chemo-selective, and only saturated alcohol was produced.36 The [MnI(Br)(CO)2(iPr2PNHPiPr2)] pre-catalyst is also found active for ketone hydrogenation in the presence of 1 mol% of catalyst and 3 mol% t BuONa. Compared with aldehyde hydrogenation, however, higher temperature (100 °C vs. 60 °C) and H2 pressure (30 bar vs. 10 bar) are needed.33

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Sortais et al. successfully developed the Re-catalyzed hydrogenation of ketones by using the well-defined complex mer-[ReI(CO)3(iPr2PNHPiPr2)] Br as pre-catalyst (0.5 mol%) at 70 °C and 30 bar of H2 in the presence of t BuOK.62 Their DFT(PBE0-D3) calculations demonstrated that CO dissociation from the neutral amido complex of [ReI(CO)3(iPr2PNPiPr2)] is slightly uphill (ΔG ¼ 6.3 kcal/mol) and forms the dicarbonyl amido complex of [ReI(CO)2(iPr2PNPiPr2)], which is the active form of the catalyst. The rate-determining step is H2 addition to the amido [ReI(CO)2(iPr2PNPiPr2)] to form the amino complex of [ReI(CO)2(H)(iPr2PNHPiPr2)] which has an energy barrier of 25.6 kcal/mol and is endergonic by 1.9 kcal/mol.

3.3 Enantioselective hydrogenation of ketones The catalyzed hydrogenation of ketones using metal-based PNP ligands has been summarized in Scheme 12, and the details are given below. Enantioselective hydrogenation of ketones was found by using [IrIII(Cl) (H)2(iPr2PNHPiPr2)]/tBuOK (1:10).106 The reduction of 4-tert-butylcyclohexanone resulted in a 1:2 non-thermodynamic mixture of the cis and trans alcohols, respectively. The hydrogenation of benzil formed mainly meso alcohol (meso:rac ¼ 3:1), whereas 2,5-hexanedione resulted in only rac-2,5hexanediol. A 6:1 mixture of endo:exo norborneol resulted from the hydrogenation of norcamphor. The ability of this catalyst system to reduce diketones and related substrates leading to diastereomeric products demonstrates the possible relevance of chiral derivatives for asymmetric hydrogenation reactions. By using the optically active [RuII(H)(CO)(Cl)(HN(CH2CH(CH3) PPh2)2)] for the asymmetric hydrogenation of acetophenone, 1-phenylethanol was obtained with acetophenone full conversion after 5 h at 40 °C and 30 bar of H2 bar and the enantiomeric excess (ee) is

Scheme 12 Experimentally applied metal PNP catalysts for hydrogenation of enantioselective ketones.

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54.0%.93 It is worth noting that the asymmetrical [RuII(H)(CO)(Cl)(HN (CH2CH(CH3)PPh2)2] is more active than [RuII(H)(CO)(Cl) (Ph2PNHPPh2)] for acetophenone hydrogenation since shorter time is needed (5 vs. 17.5 h).93 Four new ligands of R2PCH2CH2NHCHR0 CHR00 PPh2 (R ¼ iPr, Cy, 0 R ¼ Ph, CH(CH3)2, R00 ¼ Ph, H) were used to make the FeII complexes which were applied in reductive amination of α-dialkylphosphine acetaldehydes with enantiopure β-aminophosphines. It was found that the hydride complex with the rigid ligand (R0 ¼ R00 ¼ Ph) is efficient and highly enantioselective. Prochiral aryl ketones are reduced under mild conditions (THF, 0.1 mol% catalyst, 1 mol% tBuOK, 5–10 bar, 50 °C) to the (S)-alcohols, and the ee values are >90%. DFT calculations demonstrated that the transition-state structure of (S)-alcohol is 1.6 kcal/mol lower in energy than that of the (R)-alcohol; and this energy difference is the enantioselective.57 By using Ru-MACHO, methyl lactate was reduced at 30 °C and gave good enantioselectivity. It is also found that reaction temperature is important for the optical purity. The optical purity decreased from 99.2% to 35.9% ee at 80 °C, but the loss of optical purity was <1% ee at 40 °C. Even at a substrate/catalyst molar ratio (S/C) of 4000 at 30 °C, Ru-MACHO afforded a good yield of 1,2-propanediol with a loss of <1% ee.15 Recently, our group applied the chiral (bis(2-((2R,5R)-2,5dimethyl-phospholanoethyl))amine)-PNP* ligand in Mn-based asymmetric hydrogenation of several ketones under mild conditions (30–40 °C, 4 h, 30 bar H2). Gas-phase B3PW91/TZVP calculations were carried out to elucidate the enantioselective hydrogenation mechanism.34 Our computations show that the hydride complex [MnIH(CO)2PNHP] is very stable toward CO dissociation (>42 kcal/mol). The concerted H2 elimination from [MnIH(CO)2PNHP] to [MnI(CO)2PNP] has a Gibbs free energy barrier of 20.3 kcal/mol and is slightly endergonic (0.8 kcal/mol), thus indicating the facile reversibility and well-balanced equilibrium between [MnIH (CO)2PNHP] and [MnI(CO)2PNP] under H2 atmosphere. Then, three selected substrates, cyclohexyl methyl ketone, cyclopentyl methyl ketone and α-tetralone, were used to calculate the Gibbs free energy barrier for hydrogenation on the basis of the proposed outer-sphere mechanism. The enantioselective origin of pro-chiral ketones comes from the mutual approach of the C]O group perpendicular to the N–H and Mn–H groups in the position of the carbon center toward the Mn–H group and the oxygen atom toward the N–H group on the basis of the polarity and charge difference of the Mn–H hydride and N–H proton either in the R or

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in the S configuration. The barrier difference between the transition states of the R and S configurations determines the enantioselective degree. It is noted that compared with the stepwise mechanism of benzaldehyde hydrogenation by Mn PNP complexes with non-chiral substituents on the phosphorous center (Table 10), we located a concerted and strongly asynchronous transition state for all substrates, and this is confirmed by the additional intrinsic reaction coordinate (IRC) calculations. As shown in Scheme 13, an enantiomeric ratio of 97:3 for cyclohexyl methyl ketone (Gibbs free energy barrier of 31.4/33.5 kcal/mol for the R/S transition states), cyclopentyl methyl ketone (Gibbs free energy barrier of 31.4/33.5 kcal/mol for the R/S transition states) and α-tetralone (Gibbs free energy barrier of 33.5/35.6 kcal/mol for the S/R transition states) was computed. Although the computed enantiomeric ratios are higher than the experimentally determined data (92:8, 87:13, and 10:90, respectively), the experimentally observed selectivity is qualitatively reproduced. Experimentally, the hydrogenation of 1-cyclohexylethanone and 1-cyclopentylethanone has full conversion (>99%), while α-tetralone has rather low conversion (30%). Detailed analysis shows that the reaction of the aliphatic ketones is roughly thermal neutral (1.0 and 0.3 kcal/mol, respectively), while that of the aromatic ketone is endergonic (2.9 kcal/mol). This indicates that under equilibrium condition ketone rather than alcohol is more favored; and the low conversion of α-tetralone hydrogenation is thermodynamically controlled. In order to get insight into the steric effect of the enantioselectivity, we dissected the electronic activation energy of the R/S transition states into the geometrical strain energy and interaction energy by using the proposed

Scheme 13 Mn-catalyzed asymmetric hydrogenation of ketones (computed enantioselectivity in square bracket).

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activation strain model.107 For hydrogenation of cyclohexyl methyl ketone and cyclopentyl methyl ketone, the electronic energy of the transition state of the R isomer is lower in energy than that of the S isomer by 2.5 and 2.4 kcal/mol, respectively. For α-tetralone, however, the electronic energy of the transition state of the R isomer is higher in energy than that of the S isomer by 2.0 kcal/mol. Further analyses into the difference of geometrical strain energy and interaction energy reveal that the strain energy difference dominates the energy difference between the R and S transition states, and the strain energy difference comes mainly from the deformation of the catalyst instead of the substrate.

3.4 Transfer hydrogenation and isomerization Pincer complexes were also widely used in transfer hydrogenation reactions as it is an alternative way for hydrogenation reaction without direct H2 supply. Abdur-Rashid and co-workers found that Ir-based PNP complexes are very active for transfer hydrogenation of a variety of ketones in iPrOH.14,106 In the presence of catalytic amounts of base such as tBuOK at room temperature, [IrIII(Cl)(H)2(iPr2PNHPiPr2)] facilitates the efficient transfer hydrogenation of acetophenone to phenylethanol. In the absence of a base, no reaction was observed. This clearly demonstrated that [IrIII(Cl) (H)2(iPr2PNHPiPr2)] is not the active catalyst and it has to be activated by base at first. In addition, the trihydride [IrIII(H)3(iPr2PNHPiPr2)] and amido dihydride [IrIII(H)2(iPr2PNPiPr2)] complexes in the absence of a base are exceptionally active for the transfer hydrogenation of ketones in iPrOH.14 Detailed theoretical investigations on the mechanisms of transfer hydrogenation for the reaction of 3-pentanone with iPrOH catalyzed by the simplified model catalyst [IrIII(H)3(Me2PNHPMe2)] are reported at the B3LYP/LAN2DZ(Ir,P)/6-31G level. The catalytic reaction mainly consists of two steps. One is the [IrIII(H)3(Me2PNHPMe2)] catalyzed hydrogenation of 3-pentanone to pentan-3-ol with the formation of the amido complex [IrIII(H)2(Me2PNPMe2)]; and the another one is the [IrIII(H)2(Me2PNPMe2)] catalyzed dehydrogenation of iPrOH to acetone with the regeneration of the amino complex [IrIII(H)3(Me2PNHPMe2)].108 3.4.1 Ru-based PNP catalysts In our recent work we performed the transfer hydrogenation of α,βunsaturated aldehydes to allyl alcohols using [Ru(CO)(H)(BH4) (Ph2PNHPPh2)] as catalyst as well as iPrOH and EtOH as hydrogen source without additives under base free condition and found high conversion and

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selectivity under the reflux of alcohol in short reaction time.109 On the basis of the proposed outer-sphere mechanism, B3PW91/LAN2DZ(Ru)/TZVP density functional theory computations were carried out, and (E)but-2-enal (crotonaldehyde) and (E)-1-phenylbut-2-en-1-one were taken as model substrates (Scheme 14). For crotonaldehyde hydrogenation (R ¼ H), there are three competitive routes, one is the [3,4] route leading to ally alcohol; another one is the [1,2] route resulting aldehyde; and the third one is the [1,4] route producing vinyl alcohol which can tautomerize to aldehyde. It was found that for the crotonaldehyde (R ¼ H) as substrate and iPrOH as hydrogenation source (Scheme 15) the formation of the allylic alcohol from the [3,4] route is kinetically favored above that of butyraldehyde from the [1,4] route (12.3 vs. 13.7 kcal/mol) by 1.4 kcal/mol. The energy barrier of subsequent hydrogenation of allyl alcohol and butyraldehyde to butanol is 31.4 and 9.5 kcal/mol, and exergonic by 19.9 and 5.8 kcal/mol, respectively. This indicates that butyraldehyde, once formed, can be easily hydrogenated to saturated butanol. The small barrier difference between (E)-but-2-en-1-ol and butyraldehyde formation determines the selectivity for allylic alcohol (88%), which agrees with the experiment (89%) under optimized condition (iPrOH, 355 K); and this explains the short reaction time (5 min). After elongated reaction time to 25 min, the amount of saturated product increases; and this is because the back reaction, allyl alcohol dehydrogenation, has a low barrier (13.8 kcal/mol) and the reaction can have the saturated alcohol as final product. However, for (E)-1-phenylbut-2-en-1-one (R¼ Ph) as substrate and iPrOH as hydrogenation source (Scheme 16), the C]C bond reduction via

Scheme 14 Different hydrogenation phenylbut-2-en-1-one hydrogenation.

routes

for

crotonaldehyde

and

(E)-1-

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Scheme 15 Crotonaldehyde hydrogenation using iPrOH as hydrogen source [B3PW91; kcal/mol].

Scheme 16 (E)-1-Phenylbut-2-en-1-one hydrogenation using iPrOH as hydrogen source [B3PW91; kcal/mol].

1,4-addition is more favorable than the C]O hydrogenation via 3,4-addition both kinetically (12.9 vs. 21.2 kcal/mol) and thermodynamically (18.0 vs. 1.1 kcal/mol) by 8.3 and 19.1 kcal/mol. Thus, 1-phenylbutan-1-one was found as major product. For comparison, we computed the hydrogenation of 1-phenylbutan-1-one and (E)-1-phenylbut-2-en-1-ol into fully saturated alcohol of 1-phenylbutan-1-ol. The barriers are as high as 21.3 and 28.0 kcal/mol, and exergonic by 0.4 and 19.5 kcal/mol for 1-phenylbutan1-one and (E)-1-phenylbut-2-en-1-ol, respectively. This indicates that 1-phenylbutan-1-one and (E)-1-phenylbut2-en-1-ol cannot be easily hydrogenated to saturated alcohol of 1-phenylbutan-1-ol. This is in perfect agreement with the experimental data that hydrogenation of (E)-1-phenylbut-2-en-1-one into butyrophenone via the formation of (E)-1-phenylbut-1-en-1-ol as a result of 1,4-reduction is favored, while hydrogenation of crotonaldehyde to allylic alcohol is favored both kinetically and thermodynamically. It is also worth noting that the barriers of H2 elimination from the amino complex of [RuII(CO)(H)2(PPh2PNHPPh2)] to the amido complex of [RuII(CO)(H)(Ph2PNPPh2)] (21.3 kcal/mol and endergonic by 0.5 kcal/mol)30 are higher than those of alcohol dehydrogenation (18.1 and 15.6 kcal/mol for iPrOH and EtOH); and the catalysts are stable under the reaction conditions, and therefore no high pressure is needed.109 Since the barrier of alcohol dehydrogenation (18.1 and 15.6 kcal/mol for iPrOH and EtOH) is lower than the barrier of allyl alcohol formation

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(12.3 kcal/mol) as well as the barrier of the ketone formation (12.9 kcal/ mol), alcohol dehydrogenation should be the rate-determining step for such transfer hydrogenation. 3.4.2 Fe-based PNP catalysts Since M PNP pincer complexes have been successfully applied in reactions of hydrogenation using molecular H2 and transfer hydrogenation using alcohols as hydrogen sources. It is assumed that it might also be suitable for isomerization reaction from self-transfer hydrogenation reaction. Indeed, de Vries et al. found that both [FeII(CO)(H)(Cl)(iPr2PNHPiPr2)], activated in situ with KOtBu, and [FeII(CO)(H)(iPr2PNPiPr2)] are highly active for the isomerization of allylic alcohols to ketones without external hydrogen supply. High reaction rates were obtained at 80 °C, and it was also found that the catalyst is sufficiently active at room temperature for most substrates.110 In view of the need for a strong base for the amino complex [FeII(CO) (H)(Cl)(iPr2PNHPiPr2)] activation as well as the catalytic activity of the amido complex of [FeII(CO)(H)(iPr2PNPiPr2)] without strong base, the self-hydrogen-borrowing mechanism (Scheme 17) without an external hydrogen source was proposed and verified by DFT calculation. First, allylic alcohol is dehydrogenated to α,β-unsaturated ketone. Then, the C]C bond can be further hydrogenated to afford ketone proceed either by 1,4-addition to form the enol which can tautomerize into ketone or by direct 1,2-addition to ketone; and this is the same as proposed in Scheme 14 for the transfer hydrogenation of α,β-unsaturated ketone. DFT results indicated that the first step of the dehydrogenation of allyl alcohol to α,β-unsaturated ketone by the amido complex of [FeII(CO)(H) (iPr2PNPiPr2)] is determined by the hydride transfer from the alcohol to Fe. The transition state corresponding to hydride transfer has a Gibbs free energy barrier of 22.0 kcal/mol and the reaction is endergonic by 1.2 kcal/mol. The hydrogenation of α,β-unsaturated ketone to afford product saturated ketone is more favored both kinetically (13.7 vs. 22.0 kcal/mol) and

Scheme 17 Isomerization of allylic alcohol to ketone (B3PW91; R1 ¼ C5H11, kcal/mol).

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thermodynamically (22.9 vs. 1.2 kcal/mol) than the first step. Therefore, once α,β-unsaturated ketone has formed, the hydrogenation of the C]C double bond to saturated ketone is more favorable than the back reaction, both kinetically and thermodynamically by 7.1 and 21.7 kcal/mol. Furthermore, 1,4-addition is slightly more favored kinetically than 1,2-addition by 0.8 kcal/mol, and this small energy difference showed that both the 1,2- and 1,4-routes are competitive and possible. The isolated C]C double bond hydrogenation of allyl alcohol to saturated alcohol is kinetically (the barrier is 27.2 kcal/mol, it is exergonic by 24.5 kcal/mol) less competitive than that of the conjugated C]C double bond of α,β-unsaturated ketone by 13.5 kcal/mol. Furthermore, it was found that the consecutive hydrogenation of saturated ketone to saturated alcohol mediated by the complex [FeII(CO)(H)2(iPr2PNHPiPr2)] has an energy barrier of 25.4 kcal/mol and the reaction is exergonic by 2.8 kcal/mol. Using isopropanol as external hydrogen source can shift the reaction toward the saturated product octan-3-ol in the thermodynamically equilibrated state. The computed ratio between octan-3-one and octan-3-ol using isopropanol as a hydrogen source [iPrOH + octan-3-one ¼ acetone + octan-3-ol] is 10: 90 (determined by the equilibrium constant and the concentrations of iPrOH and substrate), in excellent agreement with the experimentally observed 9:90 after 24 h.110 Based on the amido complex of [FeII(CO)(H)(iPr2PNPiPr2)] we also calculated the inner-sphere mechanism through insertion of the C]C double bond of the alkene into the Fe–H bond. Since the overall isomerization from allyl alcohol to saturated ketone has an effective Gibbs free energy barrier of 52.1 kcal/mol, much higher than that for dehydrogenation of allyl alcohol (22.0 kcal/mol) or that for dehydrogenation of iPrOH (24.5 kcal/mol),110 this mechanism is kinetically hindered and can be discarded.

3.5 Aqueous methanol dehydrogenation Hydrogenation of CO2 to formic acid or methanol is a promising strategy for reversible H2 storage. By using amido complexes of [MoI(NO)(CO) (iPr2PNPiPr2)] and [WI(NO)(CO)(iPr2PNPiPr2)] as well as amino complexes of [WI(H)(NO)(CO)(iPr2PNHPiPr2)], CO2 was hydrogenated to HCOONa at 140 °C under 70 bar of H2 in the presence of Na[N(SiMe3)2] as stoichiometric agent (Scheme 18). However, the yield of HCOONa was only 4%, 2% and 3% for [MoI(NO)(CO)(iPr2PNPiPr2)], [WI(NO)(CO) (iPr2PNHPiPr2)] and [WI(H)(NO)(CO)(iPr2PNHPiPr2)], respectively.41

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Scheme 18 Mo-, W-based catalysts for CO2 hydrogenation.

Bernskoetter et al. first identified amido complexes of [FeII(H)(CO) (iPr2PNPiPr2)] and [FeII(H)(CO)(Cy2PNPCy2)] as well as [FeII(H)(CO) (OCHO)(iPr2PNHPiPr2)] and [FeII(H)(CO)(OCHO)(Cy2PNHPCy2)] catalysts for formic acid dehydrogenation (the reverse of CO2 hydrogenation). The impressive activity was obtained in the presence of Lewis acid, such as LiBF4.68 Then they applied these catalysts for CO2 hydrogenation.111 Surprisingly (Scheme 19), it was found that the methylated [FeII(H)(CO)(HBH3) (iPr2PNCH3PiPr2)], [FeII(H)(CO)(HBH3)(Cy2PNCH3PCy2)], [FeII(H) (CO)(H)(iPr2PNCH3PiPr2)] and [FeII(H)(CO)(OCHO)(iPr2PNCH3PiPr2)] are more active than the amino complex of [FeII(H)(CO)(HBH3) (iPr2PNHPiPr2)] as well as the amido complexes of [FeII(H)(CO) (iPr2PNPiPr2)] and [FeII(H)(CO)(Cy2PNPCy2)]. Furthermore, Lewis acid was found to raise the turnover number by more than an order of magnitude. Mechanistic investigations proved that the Lewis acid facilitates decarboxylation of formate species by disrupting an intramolecular hydrogen bond between the formate and amine ligand. For [FeII(H)(CO)(BH4) (iPr2PNCH3PiPr2)] catalyzed CO2 hydrogenation, the absence of bifunctional N–H moiety requires a distinct mechanism. It is proposed that Li+ facilitates the displacement of formate by H2 to generate the cationic complex of [FeII(H)(CO)(H2)(iPr2PNCH3PiPr2)]+, which is then deprotonated by 1,8-diazabicycloundec-7-ene (DBU) to regenerate the dihydride species of [FeII(H)(CO)(H)(iPr2PNCH3PiPr2)]. However, no evidence for formation of the H2 cationic complex has been observed during catalysis.111 Recently, Bernskoetter et al. also found that the presence of a methyl group on the central nitrogen donor of the PNP ligand can enhance the TON for Co-based PNP complexes catalyzed CO2 hydrogenation (Scheme 20) in the

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Scheme 19 Fe-based PNP catalysts applied for CO2 hydrogenation.

Scheme 20 Co-based PNP catalysts applied for CO2 hydrogenation.

presence of DBU and Lewis acid (LiOTf ). Steric effect of substituents has only limited influence on TON, i.e., 24,000 for [CoI(CO)2(Cy2PNCH3PCy2)]Cl and 29,000 for [CoI(CO)2(iPr2PNCH3PiPr2)]Cl (TON: 29,000). However, the presence of a bifunctional ligand in [CoI(CO)2(iPr2PNHPiPr2)]Cl has a significant deleterious impact on the catalytic CO2 hydrogenation which affords a TON of only 450.112 This clearly demonstrates that despite the recent and impressive successes in using bifunctional ligands to develop M PNP catalysis, such frameworks are not favored for FeII- and CoI-based CO2 hydrogenation.112 In 2013, our group applied well-defined ruthenium [RuII(CO)(H)(Cl) (Ph2PNHPPh2)] and [RuII(CO)(H)(Cl)(iPr2PNHPiPr2)]18 as well as iron

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[FeII(CO)(H)(Br)(iPr2PNHPiPr2)] and [FeII(CO)(H)(BH4)(iPr2PNHPiPr2)]113 catalysts in aqueous methanol reforming to carbon dioxide and hydrogen [CH3OH + H2O ¼ CO2 + 3H2] at low temperatures (65–95 °C) and ambient pressure in the presence of strong base (Scheme 21). Bernskoetter, Hazari and Holthausen also found that [FeII(CO)(H)(iPr2PNPiPr2)], [FeII(CO)(H)(Cy2PNPCy2)], [FeII(CO)(H)(OCHO)(iPr2PNHPiPr2)] and [FeII(CO)(H)(OCHO)(Cy2PNHPCy2)] can rapidly convert methanol to methyl formate and H2 in the absence of water.114 The catalytic system gives the highest TON when a Lewis acid was added. They also computed the accelerating effect of [Na(H2O)5]+ as Lewis acid for methanol dehydrogenation using Fe PNP complex under base-free condition, and they found that the effective free energy barrier of hydride transfer from formate to the metal center can be lowered by 2.4 kcal/mol.114 Later, mechanistic studies revealed that the anionic [RuII(CO) (H)2(iPr2PNPiPr2)]2 and [RuII(CO)(H)(OCH3)(iPr2PNPiPr2)]2 that are deprotonated at nitrogen in the pincer ligand backbone were resting states under strong basic conditions.31 Increasing KOH increases not only the reaction rate but also the ratio of [RuII(CO)(H)2(iPr2PNPiPr2)]2 and [RuII(CO)(H)(OCH3)(iPr2PNPiPr2)]2. DFT calculations further

Scheme 21 Rudehydrogenation.

and

Fe-based

PNP

(pre)catalysts

for

aqueous

methanol

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demonstrated that the ligand does not play a cooperative role in the catalytic cycle and the C–H coordination to Ru facilitates the C–H cleavage and hydride transfer from the deprotonated substrate (CH3O, HOCH2O– and HCOO) to the metal center (Scheme 22). The amino complex of [RuII(CO)(H)2(iPr2PNHPiPr2)] was proposed to be regenerated via protonation of [RuII(CO)(H)2(iPr2PNPiPr2)]2 by protonic species (CH3OH, HOCH2OH and HCOOH). Finally, the amino complex was dehydrogenated to regenerate the amido complex and liberate H2 gas.31 Inspired by recent achievements of Mn-based catalyst, our group reported the long-term stability of the [MnI(CO)2(Br)PNHPiPr2] catalyzed lowtemperature methanol reforming in the presence of base (Scheme 23).66 It was demonstrated that replacing KOH by other bases, such as tBuOK or LiOH, led either to a significant decrease in activity or to complete deactivation of the catalyst.66 Based on [IrIII(H)3(iPr2PNHPiPr2)] and [IrIII(H)2(Cl)(iPr2PNHPiPr2)], our group also developed Ir-based catalytic system which showed higher activity at low base concentration than that at high base concentration.115 However, higher stability at high base concentrations was observed. The mechanistic study indicated that the trihydride species of [IrIII(H)3(iPr2PNHPiPr2)] is the main species in the

Scheme 22 Proposed mechanism of aqueous methanol reforming under strong basic conditions (E ¼ PiPr2).

Scheme 23 Mndehydrogenation.

and

Ir-based

PNP

(pre)catalysts

for

aqueous

methanol

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strongly basic medium, while carbonyl species of [IrI(CO)(iPr2PNPiPr2)] and [IrIII(H)2(CO)(iPr2PNPiPr2)] were proposed as deactivated species were detected in less basic medium.115 In order to verify the mechanism, we carried out DFT calculations under base free and strong basic conditions at the B3PW91/LAN2DZ(Mn)/ TZVP level of density functional theory.52 On the basis of literature reports, we followed the proposed four-step mechanism for aqueous methanol dehydrogenation (Scheme 24). The first step is CH3OH dehydrogenation to CH2O and H2 [CH3OH ¼ CH2O + H2]; the second step is the condensation of CH2O and H2O to methanediol [CH2O + H2O ¼ HOCH2OH]; the third step is methanediol dehydrogenation to formic acid and H2 [HOCH2OH ¼ HCOOH + H2]; and the final step is formic acid dehydrogenation to CO2 and H2 [HCOOH ¼ CO2 + H2] as well as the simultaneous catalyst regeneration. In our calculation, we compared both the innocent and non-innocent mechanisms as well as inspected the role of strong base. 3.5.1 Mn-based PNP catalysts Under base free condition via non-innocent mechanism (Scheme 6), the rate-determining step is the catalyst regeneration and H2 release from [MnI(CO)2(H)(iPr2PNHPiPr2)] to [MnI(CO)2(iPr2PNPiPr2)] after formaldehyde formation (step one) and the effective free energy barrier is as high as 33.9 kcal/mol for [MnI(CO)2(H)(iPr2PNHPiPr2)]; and release and removal of H2 will therefore promote the reaction. For the innocent mechanism (Scheme 6), the rate-determining step is the formation of dissociatively coordinated aqueous complex [MnI(CO)2(OH) (iPr2PNHPiPr2)] after formaldehyde formation (step one), and the effective free energy barrier is 30.4 kcal/mol for [MnI(CO)2(H)(iPr2PNHPiPr2)], 3.6 kcal/mol lower than that of the non-innocent mechanism (33.9 kcal/ mol for [MnI(CO)2(H)(iPr2PNHPiPr2)]). This indicates that the innocent mechanism is kinetically more favored than the non-innocent one.

Scheme 24 Proposed mechanism for aqueous methanol dehydrogenation.

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Under strong basic conditions, the highest effective free energy barrier was found for H2 generation from 2MniPr and H2O (32.3 kcal/mol for 2MniPr). Compared with the base free reaction via the innocent mechanism (30.4 kcal/mol for 2MniPr), the highest barrier under strong basic conditions is 1.9 kcal/mol higher for 2MniPr. Taking K+ ion into consideration, the free energy barrier of hydride transfer from OCH3, HOCH2O– and HCOO– to the metal center is higher than that without K+ incorporated. However, the free energy barrier of K+-assisted H2 formation (14.6 kcal/mol for 2MniPr⋯K+) is significantly lower than that without K+ incorporation by 17.7 kcal/mol for 2MniPr. The rate-determining step under strong basic condition is the hydride transfer from formate to Mn center without K+ (20.1 kcal/mol) for 2MniPr. Consequently, the effective free energy barrier for aqueous methanol dehydrogenation under strong basic condition (20.1 kcal/mol for 2MniPr) is lower than that under base free condition via the more favored innocent mechanism (30.4 kcal/mol for 2MniPr) by 10.3 kcal/mol for Mn PNP complex, indicating the promotion role of strong base, in agreement with the experimental results. On the basis of our calculations, KOH plays a dual role, i.e., OH– facilitates the deprotonation of substrate and K+ stabilizes the transition state and lowers the free energy barrier of H2 formation. 3.5.2 Re-based PNP catalysts On the basis of [MnI(CO)2(Br)(iPr2PNHPiPr2)] catalyzed low-temperature aqueous methanol reforming discovered in our group,66 we also explored the potential activity and stability of the corresponding [ReI(CO)2(H) (iPr2PNHPiPr2)] catalyst in aqueous methanol dehydrogenation as well as formic acid dehydrogenation. Since the Re PNP complex is more stable than the Mn PNP complex (Table 3), a higher stability is expected for the Re complex in the catalytic system. Our computations show that both Mn and Re PNP complexes have the same catalytic mechanisms and it is therefore easy for direct and quantitative comparison (Table 11). Under base free condition via the non-innocent mechanism, the ratedetermining step is the catalyst regeneration from 2MiPr to 1MiPr (assisted by a methanol molecule) after formaldehyde formation. The effective free energy barrier for 2ReiPr (35.6 kcal/mol) is 1.7 kcal/mol higher than that of 2MniPr (33.9 kcal/mol). Under base free conditions via the innocent mechanism, the rate-determining step is MiPr–OH formation and the effective free energy barrier for 2ReiPr is comparable with that of 2MniPr (30.4 kcal/mol for 2MniPr and 30.6 kcal/mol for 2ReiPr, respectively).

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Table 11 Effective Gibbs free energy barriers (B3PW91; ΔG‡eff, kcal/mol) for aqueous methanol dehydrogenation. Mn complex Re complex Condition

Mechanism

ΔG‡eff

ΔG‡eff

Base free

Non-innocent

33.9

35.6

Innocent

30.4

30.6

without K+

32.3

29.4

20.1

18.4

KOH

+

K -assisted

Under basic condition, the rate-determining step is H2 generation from 2MiPr2 and H2O and the effective free energy barrier for 2ReiPr2 (29.4 kcal/mol) is lower than 2MniPr2 (32.3 kcal/mol) by 2.9 kcal/mol. Taking K+ ions into consideration, the rate-determining step is the hydride transfer from formate to the Mn center (20.1 kcal/mol) for 2MniPr2, and the H2 formation is the rate-determining step (18.4 kcal/mol) for 2ReiPr2. All these comparisons show that the base metal Mn-based PNP catalysts are as effective as the precious metal Re-based PNP catalysts. Therefore, aqueous methanol dehydrogenation via innocent mechanism is more favorable than via non-innocent mechanism under base free condition. Catalyst 2ReiPr shows similar activity as catalyst 2MniPr, while Re-based catalysts shows higher activity than Mn-based catalysts under basic conditions with and without K+ stabilization. K+ stabilizes the transition state through N⋯K+⋯O interaction and lowers the free energy barrier for H2 formation. Since the catalyzed reaction with Mn-based PNP catalysts is known experimentally, the computed activity of Re-based PNP catalysts needs experimental proofs. 3.5.3 Formic acid dehydrogenation with Mn- and Re-based PNP catalysts Within C1 chemistry, formic acid, like methanol, is another important molecule for practical use. By considering formic acid as H2 storage and CO2 capture, it is interesting to emphasize formic acid dehydrogenation. Since the last step of aqueous methanol dehydrogenation is formic acid dissociation into CO2 and H2, we can directly compare both innocent and non-innocent mechanisms as well as inspect the role of strong base without additional calculations (Table 12).

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Table 12 Effective Gibbs free energy barriers (B3PW91; ΔG6¼ eff, kcal/mol) for formic acid dehydrogenation. Mn complex Re complex Condition

Mechanism

ΔG‡eff

ΔG‡eff

Base free

Non-innocent

27.5

29.9

Innocent

18.9

18.6

Without K+

32.3

29.4

20.1

18.4

KOH

+

K -assisted

For formic acid dehydrogenation, the effective free energy barrier of the non-innocent mechanism determined by the resting state of 2MiPr-OOCH and the transition state of catalyst regeneration (27.5 kcal/mol for 1MniPr and 29.9 kcal/mol for 1ReiPr) is higher than that of the innocent mechanism determined by the resting state of 2MiPr-OOCH and the transition state of hydride transfer from the H-coordinated formate to the M center (18.9 kcal/mol for 2MniPr and 18.6 kcal/mol for 2ReiPr). The innocent mechanism is much more favorable than the non-innocent mechanism by 8.6 and 11.3 kcal/mol for Mn and Re PNP complex, respectively. Under strong basic conditions without K+ incorporation, the effective energy barrier of the rate-determining step is 32.3 and 29.4 kcal/mol for Mn- and Re-based catalyst, respectively. By including K+ incorporation, the effective energy barrier of the rate-determining step is significantly decreased by K+ stabilization. The highest free energy barrier is comparable with that under base free condition for the Mn-based catalyst (18.9 kcal/mol for 2MniPr vs. 20.1 kcal/mol for 2MniPr) and Re-based catalyst (18.6 kcal/mol for 2ReiPr vs. 18.4 kcal/mol for 2ReiPr) on the basis of the innocent mechanism. Thus, it is expected that formic acid dehydrogenation can be catalyzed by Mn- and Re-based PNP systems under both base free and strong basic condition. This needs experimental proofs.

3.6 Designed catalysts for aldehyde hydrogenation Encouraged by the promising results that base metal-hydride complexes [M(H)(L1)(L2)(R2PNHPR2)] under an appropriate coordination sphere are effective catalysts in many hydrogenation reactions, we were interested in isoelectronic analogues of early transition metals for catalytic reactions. Experimentally, the first group 5 metal amido [V(Cl)2(Me2PNPMe2)]

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complex9 and group 6 metal amino mer-[M(CO)3(Ph2PNHPPh2)] and fac[M(CO)3(Ph2PNHPPh2)] (M ¼ Cr, Mo, W)8 complexes have been synthesized by Ellermann and Edward, respectively. Berke et al. synthesized group 6 metal amino [MoI(NO)(CO)(H)(iPr2PNHPiPr2)] and [WI(NO)(CO)(H) (iPr2PNHPiPr2)] complexes and applied them in several hydrogenation reactions.42,43 On the basis of the 18-electron rule and under the consideration of the metal-hydride neutrality of the molecular systems, we designed group 5 metal PNP complex of [VI(NO)2(H)(iPr2PNHPiPr2)], [NbI(NO)2(H) (iPr2PNHPiPr2)] and [TaI(NO)2(H)(iPr2PNHPiPr2)] as well as group 6 metal PNP complex of [CrI(NO)(CO)(H)(iPr2PNHPiPr2)]. On the basis of group 5 and group 6 metal PNP complexes, we computed their activity in the reactions of phenyl-substituted C^N, C]N, C^C, C]C, and C]O functional groups.51,116 In Table 13, we summarized effective Gibbs energy barriers for catalysts interconversion as well as benzaldehyde hydrogenations for comparison. As shown in Table 10, Fe-, Mn- as well as Ru-, Os-, Ir-based PNP complexes have been proven to be effective catalysts in hydrogenation reactions of methyl benzoate and benzaldehyde. Practically, Fe-based PNP catalysts are as effective as Ru-based catalysts. Comparison on the basis of the computed barriers rationalizes the need of high temperature and high H2 pressure in ester hydrogenation, and ambient pressure and temperature in aldehyde hydrogenation. For group 6 metal-based PNP complexes, it was found that the barrier of benzaldehyde hydrogenation is higher than that of 2MiPr dehydrogenation by 4.1, 4.0 and 3.2 for the 2ViPr, 2NbiPr and 2TaiPr complexes, Table 13 B3PW91 computed effective Gibbs free energy barriers (ΔG‡, kcal/mol). 2MiPr 5 1MiPr + H2 Ph–CH]O + H2]Ph–CH2–OH Metals

ΔG‡effa

ΔG‡eff

V

17.7 [24.5]

21.8

Nb

15.4 [22.7]

19.4

Ta

16.9 [23.8]

20.1

Cr

18.5/20.9 [23.4]b

31.4

Mo

19.3 [22.8]

28.4

W

20.8 [24.1]

29.2

a

Reverse reaction barrier in square bracket. The CO ligand in the trans position with respect to the N–H group.

b

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respectively; and this indicates that high H2 pressure is needed for the hydrogenation of aldehydes for the 2ViPr, 2NbiPr and 2TaiPr complexes in order to maintain the stability of the catalysts. In addition, dehydrogenation of 2ViPr, 2NbiPr and 2TaiPr complexes is exergonic, and this indicates that the dehydrogenation step is more favored kinetically and thermodynamically, and H2 elimination is less favored. Thus, 2MiPr can be only stable under high H2 pressure and removal H2 from the system should shift 2MiPr back to 1MiPr. Since the barrier of benzaldehyde hydrogenation for the 2ViPr, 2NbiPr and 2TaiPr complexes is higher than that of the 2FeiPr, 2MniPr, 2RuiPr, 2OsiPr and 2IriPr complexes, high temperature is needed for effective hydrogenation. Therefore, the 2ViPr, 2NbiPr and 2TaiPr complexes are effective catalysts for hydrogenation reaction, albeit less active than the 2FeiPr, 2MniPr, 2RuiPr, 2OsiPr and 2IriPr. For group 5 metal-based PNP complexes, the same trends as for group 6 metal-based PNP complexes have been found. Since the barrier of benzaldehyde hydrogenation of group 6 metal complexes is also higher that of group 5 metal complexes by about 9 kcal/mol, even higher temperature and H2 pressure are needed. Compared with the 2FeiPr, 2RuiPr and 2OsiPr complexes, the barrier of benzaldehyde hydrogenation is about 17.4, 17.7 and 15.7 kcal/mol higher for 2CriPr, 2MoiPr and 2WiPr complexes, respectively. This is in good agreement with experimental results that 30 bar H2 is needed for the Fe PNP complex, while 60 bar H2 is needed for the Mo and W PNP complex for nitrile hydrogenation.

4. Conclusion In this review, we summarized the recent progress and development of homogeneously catalyzed reactions of hydrogenation, transfer hydrogenation and isomerization of carbonyl compounds as well as aqueous methanol dehydrogenation which includes formic acid dehydrogenation in the last step by using well defined aliphatic HN(CH2CH2PR2)2 transition metal complexes. Detailed and excellent comparison between theory and experiment has been made and such interplay between experiment and theory not only streamlines the experimentally observed results but also offers the understanding of reaction mechanisms. First of all, it was noted that the molecular structures of these complexes can be easily and simply reproduced computationally; and the agreement between computationally optimized and X-ray diffraction determined

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structures is excellent. However, it is not easy to compute the physical parameters, i.e., activation barriers and reaction energies. Since all reactions were performed under some appropriate conditions, like temperature, pressure and solvation, including solvation effect for appropriate solvents as well as van der Waals dispersion correction, are important aspects of modern computational chemistry. However, we found that current available solvation models and dispersion correction are not sufficient enough for quantitative energetic computations. Comparison with the available experimental result shows that solvation and dispersion corrections overestimate the barriers and make the reaction less exergonic or more endergonic, or in turn, underestimate the barriers of the reverse and make the reaction less endergonic or more exergonic. Curiously, it was found that B3PW91 gas phase computed results are in best and closest agreement with the experimental results for 2MniPr ¼ 1MniPr + H2, 2MniPr–OH ¼ 1MiPr + H2O and 1MniPr + PhCH2OH ¼ 2MniPr–OCH2Ph. Even the highly correlated DLPNO-CCSD(T)/def2-TZVPP method in the gas phase, in solution, as well as in solution including dispersion does not reproduce the experimentally determined energetic data. Therefore, all these B3PW91 computed gas phase results are used for discussion and interpretation of the experimental results. Structurally, defined aliphatic PNP metal complexes provide the opportunities for fine-turning the electronic and geometric properties for the catalytic activities, i.e., by changing the central metal (Mo, W, Mn, Re, Fe, Ru, Os, Co, Ir, etc.) and by modifying the ligands (CO, H and NO) as well as substituents at ligands (isopropyl, ethyl, phenyl, cyclohexyl and chiral phosphines). Taking the hydrogenation of methyl benzoate as example, it shows clearly that base Fe- and Mn-based PNP catalysts can be as effective as previous Ru-, Os- and Ir-based PNP catalysts, particularly; Fe-based catalysts with ethyl substituent and Ru-based catalysts with isopropyl substituent have nearly the same reaction barriers. Computationally, 2MniPr is thermally more stable than 2FeiPr toward CO and one phosphine arm dissociation; and this is in agreement with the experimentally observed long-term stability by using 2MniPr–Br as pre-catalyst for low-temperature aqueous methanol dehydrogenation. In addition, phosphine ligand de-coordination is easier than CO ligand dissociation. The thermal stability of catalyst has the decreasing order of 2OsiPr > 2ReiPr > 2RuiPr > 2MniPr > 2FeiPr toward ligand dissociation. The expected higher stability for the Os and Re PNP complexes needs

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experimental proof. In the hydrogenation of benzaldehyde, it shows that group 5 (V, Nb, Ta) and 6 (Cr, Mo, W) metal-based catalysts are effective under high H2 pressure, but less active than the group 7, 8 and 9 metal-based catalysts. Mechanistically, an outer-sphere non-innocent mechanism for the hydrogenation of methyl benzoate and benzaldehyde was proposed on the basis of the amino complex (2M), and the mechanism is stepwise rather than concerted and synchronous. In the stepwise hydrogenation mechanism, the first step is the M–H hydride transfer and the second step is the N–H transfer; and there is an intermediate in between. Energetically, the M–H transfer has a higher barrier than the N–H transfer; and the M–H transfer is the rate-determining step for the whole hydrogenation reaction. In contrast to the non-innocent mechanism in which the N–H function participates actively in the reaction, there is also an innocent mechanism in which the N–H function plays only the role of stabilizing the transition state and at the same time one additional H2 is involved in the reaction. Since both non-innocent and innocent mechanisms have M–H hydride transfer as the first step and this step is also the rate-determining step; the non-innocent barriers can be used for general comparison of differently substituted catalysts. However, it is noted that either stepwise or concerted non-innocent depends on the catalyst as well as substrates. For example, enantioselective hydrogenation of pro-chiral ketones has a concerted and strongly asynchronous transition state. For the base free ADC of benzyl alcohol to benzyl benzoate catalyzed by Mn- and Re-based pincer catalysts, the non-innocent outer-sphere mechanism incorporating the amido complex and the N–H bond is more kinetically favored than the innocent outer-sphere mechanism without the amido complex as well as the inner-sphere mechanism via the de-coordination of one phosphine ligand to create a vacant site for β-hydride elimination. The dehydrogenation of hemiacetal to ester represents the rate-determining step. Both Mn- and Re-based catalysts have close free energy barriers and similar catalytic activity. On the basis of the outer-sphere non-innocent mechanism and by using the phenyl substituted Ru-MACHO complexes, we also computed the transfer hydrogenation of α,β-unsaturated aldehyde and ketone by using ethanol or isopropanol as hydrogen source. Since there is no external molecular H2 supply, the innocent mechanism is not considered. It is noted that the computed kinetic and thermodynamic parameters on the basis of the

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outer-sphere non-innocent mechanism explain perfectly the observed selectivity for the hydrogenation of crotonaldehyde to allyl alcohol favored kinetically over short reaction time and in saturated alcohol favored thermodynamically over long reaction time. In addition, (E)-1-phenylbut2-en-1-one hydrogenation to 1-phenylbutan-1-one is the only kinetically and thermodynamically favored product; and this is in perfect agreement with the experiment. The hydrogenation of crotonaldehyde to butanol as well as (E)-1-phenylbut-2-en-1-one to 1-phenylbutan-1-one is determined by alcohol dehydrogenation. Under ambient pressure, the barrier of isopropanol or ethanol dehydrogenation is higher than the highest barrier for hydrogenation of crotonaldehyde to butanol as well as the highest barrier for (E)-1-phenylbut-2-en-1-one to 1-phenylbutan-1-one. In contrast, the barrier 1-phenylbutan-1-one to saturated alcohol is higher than the barrier of alcohol dehydrogenation. The same mechanism can also explain perfectly the observed self-transfer isomerization of allyl alcohol to ketone under ambient pressure by using isopropyl substituted amino 2FeiPr and amido 1FeiPr catalysts. For the aqueous methanol dehydrogenation reaction by using Mn- and Re-based catalysts, the reaction mechanism depends on the reaction conditions. Under base free condition, the innocent mechanism is kinetically more favorable than the non-innocent mechanism. Under strong base condition, KOH play a dual role, deprotonating substrate by OH– and stabilizing the rate-determining transition state by K+ in lowing the free energy barrier for H2 formation by N⋯K+⋯O interaction. Considering the special role of formic acid in H2 storage and CO2 hydrogenation, formic acid dehydrogenation should be accessible under base free and strong base conditions. Despite all these experimental and computational results, the mechanism for the reactions by using the catalysts in which the N–H functionality has been methylated is still ambiguous. In such cases, the N ligand is blocked by methyl substitution, and this suppresses both non-innocent and innocent mechanisms; therefore, an inner-sphere mechanism should be taken into account. In addition, the roles of base and Lewis acid should be considered.

Acknowledgments This work was supported by the state of Mecklenburg-Vorpommern and the Bundesministerium f€ ur Bildung und Forschung (BMBF), Germany. Z.W. is grateful for the financial support of the Leibniz Foundation (Leibniz Competition, SAW-2016LIKAT-1).

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