Polynuclear Iron Compounds with Hydrocarbon Ligands

31.5 Polynuclear Iron Compounds with Hydrocarbon Ligands J. L. DAVIDSON Heriot-Watt University

31.5.1

INTRODUCTION

615

31.5.2 TRINUCLEAR IRON COMPLEXES 31.5.2.1 Complexes Derived from Alkynes 31.5.2.2 Polyene Complexes 31.5.2.3 Group Vand VI Heteroatom Ligands

617 617 623 625

31.5.3 TETRANUCLEAR IRON COMPLEXES 31.5.3.1 Alkene and Alkyme Complexes 31.5.3.2 Sulphur Bridged Complexes 31.5.3.3 [Fe^S^iv-CsHsU] and Related Complexes 31.5.3.4 [Fe4(CO)4(7}-C5H5)4]

636 636 637 638 641

31.5.3.5 31.5.4

Antimony Halide Complexes

643

PENTA- AND HEXA-NUCLEAR CARBIDO CARBONYL CLUSTERS

644

REFERENCES

648

31.5.1 INTRODUCTION The ability of second and in particular third row transition metals to form stable polynuclear complexes has facilitated the development of an extensive chemistry of transition metal cluster compounds in recent years. Early studies of the reactions of iron carbonyls with alkynes led to the serendipitous discovery of one of the first high nuclearity carbonyl clusters [Fe 5 (CO)i 5 C] containing a carbide ligand encapsulated within a square pyramidal array of iron atoms.1 Despite this, within the iron triad of the Group VIII metals the cluster chemistry of iron remains the least well developed. The responsibility for this state of affairs can be attributed to the stability of metal clusters which generally increases with increasing atomic weight of the metal. This in turn can be attributed to the enhanced kinetic stability associated with third row elements in particular, combined with, in many cases, stronger metal-metal bonding in the heavier elements.2 However, as the bond enthalpy values in Table 1 illustrate, the increased stability of metal-metal bonds found on descending a triad is accompanied by a corresponding increase in the stability of metal-carbon bonds. Clearly it is not possible to discuss the stability of the cluster unit exclusively in terms of the metal-metal interaction. However, the large range of known osmium carbonyl clusters contrasts with the sole neutral iron representative [Fe 3 (CO)i 2 ]. Moreover the thermal decomposition of the osmium analogue [Os 3 (CO)i 2 ] gives higher carbonyls in good yield (equation I) 3 ' 4 while [Os3(CO)12]

-^

[Os5(CO),«]

+

[Os5(CO)15C]

+ +

615

[Os6(CO)18] [Os8(CO)23]

+ +

[Os7(CO)2i] [Os8(CO)2iC] (1)

Polynuclear Iron Compounds with Hydrocarbon Ligands

616

Table 1 Bond Enthalpy Values in Metal Carbonyls1'8

Metal Carbonyl Methyl

Metal Carbonyl

Metal Carbonyl Methyl

Cr

Mn

Fe

Co

Ni

— 107.6 —

67.0 99.2 116.8

80.37 117.63 —

9.21 136.0 —

— 146.9 —

Mo

Tc

Ru

Rh

— 152.0

— —

117.2 172.5

112.2 164.9

W

Re

Os

Ir

127.7 187.5 222.7

130.2 191.7 —

127.8 189.6 —

— 178.3 —

MnkJmol" 1 . 1. J. Lewis and B. F. G. Johnson, Pure Appl. Chem., 1975, 44, 43.

[Fe 3 (CO)i 2 ] on thermolysis readily undergoes fragmentation to yield [Fe(CO) 5 ] and metallic iron.4 The observed variation in stability is also reflected in the behaviour of the cluster unit towards a variety of ligands. This is illustrated in Scheme 1 where reactions of the [M3(CO)i2] clusters with inorganic ligands are summarised.2 Thus in reactions leading to substitution of two twoelectron carbonyl groups by two- four- and six-electron donors, it is clear that the integrity of the [Fe3(CO)w] unit is only maintained when the carbonyls are replaced by nitrogen,5"7 phosphorus and sulphur7 donor ligands. However, the more robust triruthenium and triosmium clusters are retained in reactions with a much wider variety of substrates. High nuclearity iron compounds

[Os3(CO)12XY] (a)X = Y = C1, Br,I (b)X = Cl, Br; Y = Ph3Au X = Cl; Y = SnCl3 XY

Oxidative addition reaction

[M 3 (CO) 12 ] —2CO

i

Substitution reactions

^

4-electron donor

1

2-electron donor

I

I

(a) [M 3 (CO) 1 2 - r t Ln] L = phosphine, arsine, stibine, n = 1, 2, 3 M = Fe, Ru, Os

[H 2 Os 3 (CO), 0 ]

(b)[M 3 (CO) 1 0 YX] X = H; Y = Cl, Br, SR, OR; M = Ru, Os X = H; Y = CNMe 2 ; M = Fe (c)[M 3 (CO)i 0 NSiMe 3 ] M = Fe, Ru Scheme 1

6-electron donor

I [M 3 (CO)i 0 XY] (a) X = Y = Cl; M = Os (b) X = Y = OR, SR R = alkyl, aryl M = Os,Ru (c) X = Y = NO; M = Ru

Polynuclear Iron Compounds with Hydrocarbon Ligands

617

are found more commonly with bridging ligands which augment or replace the relatively weak iron-iron bonds. Two of the most stable tetranuclear iron clusters so far isolated are [Fe4(S4)(77-05^15)4] and [Fe4(CO)4)(77-05115)4] which appear to derive their stability from the presence of bridging sulphur and carbonyl ligands repectively. Contrasting behaviour is also found in reactions with simple alkenes such as monoenes and dienes. [Fe3(CO)i2] undergoes facile cluster cleavage to give chiefly mononuclear species, e.g. [Fe(CO)3diene], while [Ru3(CO)i2] and [Os3(CO)i2] yield a variety of trinuclear products containing bridging hydrocarbon ligands.2'8'9 Even polyenes do not appear to stabilize iron clusters. Although polynuclear complexes of iron have been isolated from the reactions of iron carbonyls with such ligands, discrete Fe(CO) 3 and/or Fe 2 (CO) 6 units are invariably present attached to different parts of the hydrocarbon moiety.8'9 However, despite the absence of stable iron clusters containing alkene ligands, heteroatom analogues, e.g. R 2 C = N R , do form a limited number of complexes. Unlike alkenes, alkyne ligands are able to stabilize clusters by utilizing both sets of 7r-orbitals in bridging three or more atoms simultaneously. Alternatively trinuclear iron clusters can be stabilized by bridging acyclic and cyclic hydrocarbon moieties resulting from alkyne oligomerization during the reaction.10'11 To summarise the position, it appears that two main types of polynuclear iron complex exist: (a) cluster complexes which are held together by a combination of weak metal-metal bonds and bridging ligands and (b) complexes in which mono- or di-iron units are independently linked via a- and/or vr-bonds to a central organic or inorganic species.

31.5.2 TRINUCLEAR IRON COMPLEXES 31.5.2.1 Complexes Derived from Alkynes Reactions between alkynes and iron carbonyls are frequently very complex, particularly those involving terminal alkynes. A variety of unusual species has been isolated from these reactions in which cyclic hydrocarbon ligands resulting from alkyne oligomerization are present and a number of careful studies over the years have resulted in some mechanistic rationalisation of the complex reaction pathways involved. Trinuclear complexes have been isolated in some cases but these are frequently unstable with respect to thermal decomposition to mono- and di-nuclear products. Consequently mild conditions and/or short reaction times are necessary for the isolation of such species. Thus isolation of Fe3 derivatives almost invariably requires the use of [Fe3(CO)i2] or [Fe2(CO)9] rather than the less reactive pentacarbonyl [Fe(CO)s]. A number of studies have established that disubstituted alkynes react with [Fe2(CO)9] or [Fe 3 (CO)i 2 ] to yield successively complexes (1), (2) and (3) in Scheme 2, the structures of which have been confirmed by X-ray diffraction studies for R = Ph.10'11 The initially formed monoalkyne complex (1) results from replacement of one terminal and two bridging carbonyl groups in the parent [Fe 3 (CO) 12 ] cluster by an alkyne ligand. Further reaction of [Fe 3 (CO) 9 (PhC=CPh)] with diphenylacetylene gives the violet bis(alkyne) complex (2) which can be thermally converted into the black isomeric metallocyclic species (3). The ring closure process, which involves Fe—C bond cleavage and concomitant C—C bond making, illustrates how the linkage of two alkynes can be achieved within the framework of an organoiron cluster. Further reaction, e.g. with CO, however does not preserve the cluster since only mono- and di-nuclear products are obtained. A comparison with [M 3 (CO)i 2 ]/diphenylacetylene reactions (M = Ru, 12 Os 13 ) reveals little difference in the early stages of the reaction except that the reactivity varies in the order Fe > Ru > Os. However, a wider range of trinuclear products is ultimately obtained with the ruthenium and osmium systems in addition to which the clusters are more resistant to fragmentation, monoand di-nuclear products being obtained with difficulty. The structure of (1) established by X-ray diffraction studies revealed for the first time the ability of an alkyne to bond to a triangular array of metal atoms.14 The mode of bonding, termed ^3 J_ r;2 is such that the alkynic C = C bond bisects the equilateral triangle formed by the three iron atoms (4). An alternative bonding mode, ^h2 (5), is found in the triruthenium complex (6) in which the alkyne lies with its axis parallel to an Ru—Ru bond.15 The metal-alkyne interaction in (1) has been discussed in terms of a localised o--bond between iron atom Fe(a) and the triply-bridging carbon C(a) complemented by a two-electron three-centre Ai-bond involving the remaining x-electrons of the C = C bond with the other two metals. 14 The vacant sp2 orbital of the doubly-bridging carbon C(b) consequently participates in a three-centre bent M—C—M bond. The alkyne formally functions as a four-electron donor which is reflected in the drastically modified C = C distance of 1.41 A. Consequently the cluster can be considered

618

Polynuclear Iron Compounds with Hydrocarbon Ligands R RC=CR

(CO).

R

(CO) 3 Fe— I—Fe(CO) 3 Fe (CO) 3

Fe a (CO a ) 2

(CO) 2 Fe b

(CO)3Fe

Fe(CO) 2

Violet isomer (2)

Scheme 2

M Ru(CO) 3

(CO) 3 Ru M M (4)

(5)

(6)

electron deficient since the three carbonyl groups in [Fe 3 (CO)i 2 ] displaced by the alkyne furnish six electrons to the cluster. The differing alkyne orientations in (1) and (6), i.e. (4) and (5), have been explained in terms of the number of valence electrons available to the metal cluster.15 The frontier orbitals of [M 3 (CO)9] consist of three low-lying Walsh type orbitals lai + le above which lie an empty set of acceptor orbitals 2a i + 2e. Extended Hiickel calculations (for M = Fe) indicate that the interaction of the 7T- and 7r*-orbitals of an alkyne is greatest when the perpendicular orientation (4) is adopted (Figure 1). In the parallel orientation, the dominant feature appears to be a destabilizing interaction involving the filled 7r-orbitals TTSI and TTS2 with les- However, addition of two more electrons to the system to give [Fe 3 (CO) 9 (HC=CH)] 2 ~ which is isoelectronic with (6) changes the situation. The electrons enter the highest symmetric MO which is the high energy destabilized symmetric TT*S in the perpendicular structure (4). In (6) however, the lai orbital becomes occupied and this is only slightly destabilized. The net result is that the parallel orientation found in [RuH 2 (CO)io(HC=CH)] becomes more favourable. The nsWrj2 mode of bonding has also been found in the bis(alkyne) derivative (2) (Scheme 2). Each alkyne, which could in this situation be described as a dimetallated alkene, donates two electrons to the unique iron atom. 16 Consequently each metal atom achieves a stable eighteen-electron configuration unlike the iron atoms in the electron deficient precursor. This may account for the shorter C = C distance of 1.37 A in (2) relative to 1.41 A in (1) since four electrons are available for four-centre bonding in the former compared with only two for a three-centre interaction in the latter.

Polynuclear Iron Compounds with Hydrocarbon Ligands /

619

\.

Figure 1 Interaction diagram for Fe3(CO)9 and acetylene for the parallel and perpendicular conformations discussed in the text (reproduced with permission from /. Am. Chem. Soc.)

The X-ray structure of (3) confirms that alkyne cyclization has occurred to produce a ferracyclopentadiene ring and that in the process, two terminal carbonyls have been converted into bridging groups. 16 In common with an increasing number of di- and poly-nuclear CO bridged species,17 the Fe—CO bridge linkages are asymmetric. According to variable temperature 13 C NMR studies the bridging CO groups undergo exchange with the terminal carbonyls CO a between - 6 4 and +96 °C as a result of the longer bond of the asymmetric bridges opening to allow rotation of the two Fe a (CO) 3 moieties.18 The carbonyls attached to the unique iron Feb do not participate in any fluxional process. Similar studies of the violet isomer (2) were precluded by irreversible isomerization to the ferracyclopentadiene form (3). 19 A triiron complex (7) related to (3) containing a ferracyclopentadiene ring has also been isolated from the decomposition of [Fe2(CO)6(??-C3H5)2] in hydrocarbon solvents at ambient temperature. 20 The fully characterized products of the reaction with yields normalized to a hypothetical 10 mol quantity of [Fe2(CO)6(??-C3H5)2] are given in Table 2. The trinuclear species and other dinuclear ferracyclopentadiene products are effectively derived from one or two C3H4 derivatives which indicates hydrogen abstraction at some stage during the reaction. A trinuclear iron complex (8) has also been isolated from the reaction in equation (2). 21 The structure, which has been confirmed by X-ray diffraction studies, contains an acetylide ligand Ph CpFe(CO)2C=CPh

[Fe2(C

°H

(CO)3Fe~

Fe(CO)3

(8)

(2)

620

Polynuclear Iron Compounds with Hydrocarbon Ligands Table 2 Products of the Thermal Decomposition of [Fe 2 (CO)6(7 ? -C 3 H5)2] 1

Yield* (mol)

Product

Fe (metal)

1.6

Carbon monoxide

0.2

Product

Yield* (mol)

Et J

|^Fe(CO)3

0.5

(CO)3 OH Propylene

7.3

[Fe(CO)5]

2.2

^Fe(CO,3

0.6

Fe (CO)3 (CO), 0.07

[Fe3(CO)12]

e(CO)2 (7) 07

7

(CO)2 a

Normalized to a hypothetical 10 mol quantity of [Fe2(CO)6(r?C 3 H 5 ) 2 ]. 1. C. F. Putnik, J. J. Welter, G. D. Stucky, M. J. D'Aniello, Jr., B. A. Sosinsky, J. F. Kirner and E. L. Muetterties, J. Am. Chem. Soc, 1978, 100,4107.

bonded in a M3-772 mode reminiscent of the bridging aikyne in (1). However, the phenylethynyl group, which was originally bonded to the cyclopentadienyliron moiety, has become a-bonded to an iron tricarbonyl group and 7r-bonded to the remaining two iron atoms. Not surprisingly the linkage of the bridging CO group to the same two iron atoms is asymmetric. It is interesting to note that the C = C distance (1.299 A) is remarkably short for a bridging triple bond and contrasts with that found in [Co 2 (CO) 6 (M2-PhC^CPh)] (1.46 A). This difference could be explained if the a-carbon lone pair MO of — C = C R is antibonding with respect to the C = C bond. Thus cr-donation of this lone pair should lead to a slight shortening of the triple bond, an effect which may also occur in coordinated nitriles^ Dinuclear ferracyclopentadienes [Fe2(CO)6CR=CRCR=CR] are more commonly obtained from reactions of iron carbonyls with alkynes than their trinuclear analogues (3). When the aikyne is 2,4-hexadiyne one of the products of this type of reaction is a trinuclear species (9). 23 However, the structure is based on a dinuclear unit with the third iron atom independently bonded to a cyclopentadienone ring. Differentiation between sixteen alternative isomeric structures awaits X-ray diffraction studies of this molecule.

(CO)3F Fe(CO)3 (9) Unlike disubstituted alkynes, terminal alkynes do not produce simple trinuclear complexes

Polynuclear Iron Compounds with Hydrocarbon Ligands

621

such as (1) and (2) when reacted with iron carbonyls. Instead mono-, di-, tri- and tetra-nuclear complexes are isolated containing, in many cases, unusual hydrocarbon ligands constructed from two or more alkynes {vide infra). It is therefore surprising to find that a complex, assigned structure (10) or (11), has been obtained from the thermal decomposition of the bromovinyl complex [Fe 2 (Br)(CH=CHBr)(CO) 6 ] in alcohol solution (MeOH, EtOH) at 40 °C. 2 4 The reaction of [Fe 3 (CO)i 2 ] with methylacetylene in refluxing heptane is significantly more complex, reproducibly yielding at least fourteen products including three trinuclear derivatives, [Fe3(CO)8(MeC2H)2] (cf. 3), [Fe 3 (CO) 8 (MeC 2 H) 3 ] (12) and [Fe 3 (CO) 8 {(CEt)C 5 H 2 Me 2 (C 2 H 3 )}] (13). 25 Complex (12) contains a planar heterocyclic ring constructed from an iron atom and a 1,3,6-trimethylhexa-l,3,5-triene-l,5-diyl ligand formed by trimerization of the alkyne. Two of the alkynes have linked head-to-tail, the third tail-to-tail, a process which is apparently accompanied by a 1,2hydrogen shift. The molecule is stereochemically non-rigid as a result of localized carbonyl exchange between the axial and equatorial sites on either Fe(l) or Fe(3). The other trinuclear product (13) exhibits two unusual structural features; a methinyltriiron cluster Fe3CEt structurally related to the ubiquitous cobalt complexes [Co 3 (CO) 9 CX], and a cyclopentadienyl ligand bearing one vinyl and two methyl substituents. 26 Variable temperature 13 C NMR studies have established that a two stage fluxional process occurs in solution involving scrambling of the six terminal carbonyl ligands (—74 to +18 °C). Above this temperature, the bridging carbonyls also participate in the exchange process which represents the first example of terminal carbonyls undergoing scrambling prior to exchange of bridging carbonyls. (CO) 3

(CO)4Fe;

OC \

H

(10)

(11)

(CO) 2 (CO)

H (CO) 3 Fe

O (12)

(13)

Extension of these synthetic studies to the reaction of ethyl- and «-propyl-alkynes with [Fe 3 (CO)i 2 ] gave, in addition to analogues of (12) and (13), a new trinuclear complex [Fe3(CO) 7 (RC 2 H) 4 ] (14). 27 X-ray studies of (14) (R = Et) have confirmed the presence of two hydrocarbon units bonded to the trinuclear cluster, an ethylallyl unit bonded to the three iron atoms, and a 775-l,2,3-triethylcyclopentadienyl ligand bonded to one iron. This obviously requires the cleavage of a C = C triple bond, an uncommon phenomenon in alkyne-transition metal chemistry, but not one without precedent.28 The formation of (14) could conceivably proceed via the thermal rearrangement of [Fe 3 (CO) 9 (RC 2 H) 4 ] (13) (R = Et, Prn) in which CO expulsion is accompanied by interaction of the cyclopentadienyl ligand with the bridging CEt or CPrn group. Chemical and mass spectral evidence apparently support this proposal.

Polynuclear Iron Compounds with Hydrocarbon Ligands

622

(CO) 3 Fe :

H H (14)

A variety of trinuclear complexes have also been isolated from the reactions of [Fe3(CO)i2] with the phosphino alkyne PPh 2 C=CCF 3 and again X-ray diffraction studies have generally been required to establish unequivocally the structures of these derivatives.29 At room temperature in benzene the reaction gives several products; one mononuclear species, [Fe(CO)3(PPli2O=CCF3)2] and a variety of trinuclear derivatives (Scheme 3). The isolation of phosphine substituted complexes [Fe 3 (CO)ii(PPh 2 C=CCF 3 )] and [Fe3(CO)10(PPh2C=CCF3)2] indicates that phosphine coordination occurs prior to P—C bond cleavage and alkyne oligomerization. On warming, the uncoordinated alkynes in the latter dimerize giving products containing ferracyclopentadiene (15) and ferracyclobutene rings (16). On refluxing in 80-100 petrol, (16) undergoes thermal rearrangement with CO expulsion to give (17) which, although structurally similar to (16), contains two metal bonds. The low temperature reaction of [Fe3(CO)i2] with Pli2PC=CCF3 also gives the trinuclear complex (18) containing a methoxycarbonyl ester group which apparently results from the combination of a carbonyl ligand with methoxide ion. The source of the latter is presumably the methanol used to stabilize the [Fe3(CO)i2] cluster.

[Fe3(CO)i2]

+

Ph 2 PC=CCF 3

—»•

[Fe3(CO)3L2]

+

[Fe3(CO),,L]

+

[Fe3(CO),0L2] / CF 3

-Fe(CO) 3

Fe(CO) 3

Fe(CO) 3 Ph 2 (16) (15)

reflux 80-100 petrol

+ CO 2 Me

(17)

(18) Scheme 3

Polynuclear Iron Compounds with Hydrocarbon Ligands

623

31.5.2.2 Polyene Complexes Iron clusters are readily fragmented in reactions with simple alkenes to give a wide range of mono- and di-nuclear complexes.2 Consequently it is not surprising to find that polyene complexes containing three or more iron atoms are relatively rare. 5 The few existing trinuclear compounds all exhibit a common characteristic, namely they consist of a polyene to which are attached either three separate iron carbonyl fragments or one dinuclear fragment and an isolated iron carbonyl moiety. It appears that a genuine polyene Fe3 cluster complex has not yet been isolated from reactions of iron carbonyls with polyenes. An early report documents the reaction of iron pentacarbonyl with cycloheptatriene which produces three organometallic products in yields which depend on the reaction conditions.30 At 135-140 °C the two reactants give cycloheptatrieneirontricarbonyl (19), its hydrogenation product cyclohepta-l,3-dieneirontricarbonyl and a trinuclear complex which according to spectroscopic evidence has structure (20). Thus two cycloheptatrieneirontricarbonyl moieties are linked by an iron tricarbonyl group ^-bonded to each C 7 H 8 ring. This complex is not obtained at lower temperatures (ca. 110 °C) and presumably results from the further reaction of cycloheptatrieneirontricarbonyl with [Fe(CO) 5 ] at 135 °C. Reaction of the non-alternant hydrocarbon 3,5-dimethylaceheptylene (21) with excess [Fe 3 (CO)i 2 ] or [Fe(CO)5] produces red-brown crystals of [Fe 3 (CO) 8 (Ci 6 Hi 4 )] (22).31 An X-ray diffraction study revealed that unlike (20) the complex contains a dinuclear fragment Fe2(CO)s bonded to one face of the hydrocarbon while the third iron is bonded to the other face. The crystals contain an ordered racemic mixture of two enantiomeric forms in which the isolated Fe(CO)3 is bonded to four adjacent carbon atoms of the dimethylated seven-membered ring. The bonding of the Fe2(CO)5 fragment, which is precisely analogous to that in [Fe2(CO)5(azulene)], consists of an Fe(CO)2 unit linked to the five-membered ring via an 77-cyclopentadienyl-iron bond while the Fe(CO)3 group is bonded to three carbon atoms of the remaining seven-membered ring via an ?73-allyl iron linkage. (CO)3Fe / = \

(19)

/ = \ Fe(CO)3

(20) Fe(CO)3

(CO) 2 Fe-Fe(CO) 3 (21)

(22)

The photochemical reaction of [Fe(CO)s] with cyclooctatetraene is more complex and proceeds via mononuclear derivatives to give a trinuclear compound (23) containing a ligand derived from cyclooctatetraene dimer (Scheme 4). 32 The dimerization occurs by addition of cyclooctatetraene to [Fe(CO)3(774-C8H8)] (24) to give two products (25) and (26), both of which react with [Fe(CO)s] to give (23), a reaction which is thought to involve a diradical intermediate (27). This requires two bond ruptures, between C(l) and C(14) and between C(l 1) and C(16) in complex (25), and on complexation to an Fe2(CO)6 moiety the diradical is subsequently stabilized. This assumes that the metal is not actively involved in the rearrangement process, i.e. rearrangement occurs prior to coordination of the Fe2(CO)6 group. An alternative and perhaps more reasonable view is that coordination of an iron carbonyl fragment to the tricyclic ring of (25) possibly via the C(14)—C(15) alkenic bond promotes the bond cleavage at C(12)—C(16) and C(l)—C(14) necessary to produce (23). The formation of (23) from (26) furthermore suggests that the latter first rearranges to (25) or to a precursor related to (25) prior to forming (23). The trinuclear species

624

Polynuclear Iron Compounds with Hydrocarbon Ligands

(23) has also been observed to undergo thermolysis to give two isomeric complexes [Fe2(CO)6Ci6Hi6] and a mononuclear complex [Fe(CO)3Ci6Hi6] which were not structurally characterised. [Fe(CO)5]

+ Fe(CO)3 (24) hv (C 8 H 8 )

(CO) 3 Fe 4

1

14

(26)

(25)

(CO) 3 Fe-h

WOiv

(CO) Fe-F ( C 0 ) 33Fe

Fe(CO) 3 e(CO) 3

(27)

(23) Scheme 4

The reaction in Scheme 5 produces five organometallic products, only one of which (28) is trinuclear. 33 Compound (28) is apparently formed via insertion of [Fe(CO) 5 ] into the carbonbromine bond of the diene in (29) but the details of the coupling reaction (30) - • (28) are not clear. The intermediacy of vinylacetylene was proposed in order to account for the presence of such a fragment in (28). The dinuclear alkyne bridged fragment in (28) can be compared with [Fe2(CO)6(alkyne)] complexes isolated previously from reactions of iron carbonyls with alkynes bearing bulky substituents. In the case of [Fe2(CO)6(Bu t C=CBu t )] X-ray diffraction studies have established the presence of a metal-metal double bond which is required to maintain the eighteen-electron configuration of the iron tricarbonyl fragments.34 Br Br Br Fe(CO)4 Fe(CO)3

Fe (CO)3

(29)

(30) / Me

HTJL_=^ / 7 \

(CO)3Fe=Fe(CO)3

Fe (CO)3 (28) Scheme 5

Polynuclear Iron Compounds with Hydrocarbon Ligands

625

The photolytic reaction of o-bromostyrene with [Fe(CO)5] leads to di- and tri-nuclear organometallic products (31) and (32) which result from dehydrobromination of the organic species.35 As depicted in Scheme 6 the reaction again proceeds via initial coordination of an iron carbonyl fragment, but in this case elimination of HBr gives a ferracyclopentadiene complex (31) structurally related to compounds obtained by Hiibel from the reaction of diphenylacetylene with [Fe3(CO)i2].36 However, Hubel's original attempts to coordinate an additional iron tricarbonyl group to the remaining diene portion of the C$ ring were unsuccessful which led to the suggestion that the aromatic nature of the fused benzene ring in complexes of this type is not appreciably disturbed. The ready conversion of (31) into (32) argues against this conclusion and suggests that the six 7r-electrons originally delocalized over the benzene ring have become localized onto two diene fragments following coordination of two iron atoms to give (31). Extended Hiickel calculations on the basic metallocycle (33) tend to support this conclusion since the orbitals on the two metal atoms are nicely set up for maximum interaction with the x-orbitals of the diene so as to promote formation of a delocalised 7r-system extending over all five atoms of the FeC4 metallocyclic ring. 37 This will obviously have a detrimental effect on the aromaticity of the arene ring. The X-ray structure of (32) clearly shows that following coordination of the third Fe(CO)3 moiety to the C 6 ring, the aromaticity of the latter is lost completely since a distinctly non-planar ring system is present.38 Moreover the C—C bond distances shown in formula (34) exhibit an alternation commonly found in 1,3-dienes bound to a metal atom.

(31)

(CO) 3 Fe-

_ ^ Fe(CO) 3

(33)

(34)

31.5.2.3 Group V and VI Heteroatom Ligands

A variety of trinuclear iron complexes are known in which the metal atoms are linked via bridging ligands containing a heteroatom such as nitrogen, phosphorus, arsenic, antimony or sulphur. In some cases the heteroatom does not coordinate to any of the iron atoms but in most instances it plays a crucial role in stabilizing the metal cluster by participating in bridge formation. Simple heteroatom ligands such as NR, NR2, PR2 and SR have already been discussed in a wider context in Chapter 31.1, whereas in this section we shall deal principally with more complex ligands.

626

Polynuclear Iron Compounds with Hydrocarbon Ligands

In theory it should be possible for metals, particularly those in polynuclear clusters, to bind ligands containing heterounsaturated linkages such as C==O, C = N , C = S or even N = N . In the case of a heterounsaturated ligand one metal atom could be used to
[Fe2(CO)8]2-

+

[W(CO)5I]

[Fe3H(CO) n ]-

[(0

f__

[Fe(CO)5]

_(ii) moist RC=N

+ NOI

(iii)|

R = Me (a) Ph(b) n

Pr (c)

R

H

"I"

I"

/T N \

R

(CO)3Fe—[—Fe(CO)3

L

Fe c

^

J

( cO) 3 Fe^-^Fe(CO)3

L

(35)

4|

~/7\ Fe

^ °)3 Ba

H^

(co)3 (36)

H+

OH-JJH+

R H \ / /HTN\ (CO) 3 Fe^p^Fe(CO) 3 \i/

65 °c "

071

„ ^ R -CT-R / / \ (CO) 3 Fe—-/—7Fe(CO) 3

IM, Scheme 7

The structures of the isomeric species (37a) and (38a) have been established by X-ray diffraction methods. 40 The mode of bonding between the acetimidoyl group and the Fe3 triangle in (37a) is reminiscent of the linkage of the alkyne in (2) (Section 31.5.2.1) except that the nitrogen ligand functions as a five-electron donor. The ethylidenimido group in (38a) also donates five electrons to the metal cluster but the bonding in this case can be compared with that of the vinylidene ligand in [Os 3 H2(C=CH 2 )(CO)9]. The latter can also exist in an isomeric form [Os 3 H(HC=CH)(CO)9] analogous to (37a) in which the acetylene is bonded in a M3II7?2 manner. However, the two forms do not interconvert thermally unlike the iron complexes where the irreversible formation of (38a) from (37a) is observed at 65 °C in hexane. The anions (35a) and (36a) do not isomerize (80 °C/MeCN) although the partial isomerization which occurs when (37a) is deprotonated with OH~ to give mixtures of (35a) and (36a) must involve a different species such as [FeH(MeCN)(CO)9] which is probably a product of kinetic rather than thermodynamic control. The proposed isomerization mechanism is given in Scheme 8. Deuterium labelling studies have also revealed that a unique hydrogen exchange occurs in (38a) which equilibrates the Fe—H and

Polynuclear Iron Compounds with Hydrocarbon Ligands

627

methine C—H position (equation 3). However no H-D scrambling was observed in [Fe 3 D(MeC = N H ) ( C O ) ] up to 65 °C. It was therefore suggested that the exchange occurs via a coordinatively unsaturated nitrene intermediate (39). Other aspects of the chemistry of (37) and (38) are displayed in Scheme 9.41 Thus hydrogenation or carbonylation of (38) yields nitrene cluster complexes (40) and (41) respectively which are Me

H

r

\\

C

/ T\

Me»-|W N

(CO) 3 Fe^P^Fe(CO) 3 ^FeX

—

(co)3

(co)3Fe_U^Fc(CO)3

L

/ {chh

\ \

i H

r

\

(CO)3Fe

/—7Fe(CO) 3 Fe

*—

H

\

(CO)3Fe

H

fl—Fe(CO)3 Fe

(CO)3

[

(CO)3

Scheme 8

Me x I / H

[Fe3D(N=CHMe)(CO)9]

^=^ (CO)3Fe

— Fe(CO)3 ^=^ [Fe3H(N=CDMe)(CO)9] (3) (CO)3 (39)

[Fe3(NR')(CO)io (41)

R

(CO)3Fe Fe (CO)3 (40)

Scheme 9

(43)

628

Polynuclear Iron Compounds with Hydrocarbon Ligands

interconvertible when treated with CO or hydrogen. Complex (40) can also be converted into a hydridonitrene (42) which is accessible by a different route, that of addition of R ! N O 2 to [Fe 3 H(CO)n]~. The acetimidoyl complex (37) interestingly undergoes air oxidation to give a jii3-i72-organonitrile complex (43) in which the bonding of the nitrile resembles the mode of attachment of the acetylide ligand to the triiron cluster in [Fe3(r72-C=CPh)(CO)7(r7-C5H5)] (8) (Section 31.5.2.1). The crystal structure of (43) (R = Pr n ) has confirmed the presence of the triply-bridging nitrile ligand, the first known example.22 As with /u2-?72-bonded alkynes the triple bond distance [1.260(3) A] is lengthened significantly relative to the free ligand (1.16 A) and is in fact close to that of a carbon-nitrogen double bond (1.28 A). This effect also results in a large shift in the C = N stretching mode of 610 cm" 1 to low frequency relative to the free ligand. An interesting feature is the non-linearity of the N—C(CN)—C(l) linkage since the substituent on nitrogen is bent back some 20° in the manner found in alkyne and V-acetylide complexes. A comparison with complexes (1) and (8) discussed in Section 31.5.2.1 illustrates this feature as the profiles of the coordinated ligands in Figure 2 show.

I.

1.98% Fe-

2.66

(8)

2.63

.2.08

-Fe 249

(43)

(1) 2

Figure 2 Comparison of selected structural parameters for 1x3 ±r) bonded ligands (bond lengths in A)

Several other trinuclear iron complexes are known in which an unsaturated nitrogen-containing hydrocarbon bridges two or more metal atoms. One example [Fe3(CH2NCO)(CO)4(77-C5H5)3] was unexpectedly isolated in 3.8% yield from the reaction of chloromethylisocyanate with Na[Fe(CO)2(r7-C5H5)] in THF. 42 Spectroscopic data [IR vco 2023 and 1973, vCo (bridge) 1764 and vCo (CH 2 NCO) 2120 cm" 1 ; *H NMR 5 4.85 (s, 5), 4.48 (s, 10) and 7.07 (s, 2) p.p.m.] are in accord with structure (44) in which the CH 2 NCO ligand is a-bonded through carbon to one iron atom and 772-bonded via the C = N linkage to the other two iron atoms. A complex originally assigned the stoichiometry [Fe 3 H(CO)nNMe 2 ] but later corrected to [Fe 3 H(CO)i 0 CNMe 2 ] 43 has been obtained from the reaction of benzoyl chloride and [Fe 3 (CO)i 2 ] using dimethylformamide as solvent (80 °C, 24 h). The *H NMR [5 4.0 (s, 6), -17.8 (s, 1) p.p.m.] and IR spectra (*>co 2080-1978 and j>FeH 770 cm" 1 ) suggest the presence of two equivalent methyl groups and a bridging hydride ligand. The Mossbauer spectrum is very similar to that of [Fe 3 (CO)i 2 ] and indicates that two of the iron atoms are equivalent. On this basis structure (45) was proposed in which the two bridging carbonyls of [Fe 3 (CO)i 2 ] have been replaced by a bridging hydride and a bridging dimethylimmoniocarbene ligand CNMe 2 .

(CO)4Fe

(44)

(45)

Carbene complexes or species derived from carbenes are frequently obtained from reactions involving diazo compounds R 2 CN 2 and metal complexes. However, attempts to generate diarylcarbenes from diphenyl- and di-p-tolyl-diazomethane in reactions with [Fe(CO)s] or [Fe 3 (CO)i 2 ] gave organonitrogen complexes rather than the sought for carbenes. 44 Two types

Polynuclear Iron Compounds with Hydrocarbon Ligands

629

of compound were isolated; a dinuclear species [Fe2(CO)6(M2-NHN=CR2)2] and black trinuclear complexes [Fe3(CO)9()Lt3-NN=CR2)2] (46). The X-ray structure of the latter (R = Ph) shows that two diazo ligands are present, each of which is attached via the terminal nitrogen to the three iron atoms simultaneously, one above and one below the Fe3 plane. 45 In accord with the diamagnetism of the compounds only two iron-iron bonds are present. Nitrene bridged complexes [Fe3(CO)9()Lt3-NR)2] (R = alkyl) obtained from the reactions of iron carbonyls with organoazides exhibit identical structural features. 46

(46) Azotoluene also reacts with iron carbonyls [Fe(CO)s] or [Fe2(CO)9] either thermally or on irradiation with UV light (equation 4) in benzene to give a trinuclear species tentatively formulated as an o-semidine derivative (47) in low yields (0.13-0.16%).47 The osemidine ligand is also present in dinuclear complexes [Fe2(M2-HNC6H4NAr)(CO)6] obtained from these and related reactions, and apparently results from the 1,3-rnigration o f a MeC6H4N group from nitrogen to an ortho carbon atom with concomitant migration of the ortho hydrogen in the opposite direction. However the mechanistic details of the process are not understood. Me

Me-^QN-N=N-/QN—Me

[Fe2(C )9]>

°

0

HN

NC6H4Me-/?

(4)

(CO) 3 FeA v -/-—Fe(CO) 3 (CO)A

(47)

Diazenes also react with iron carbonyls to give organonitrogen complexes, and in the case of diazonorbornene a trinuclear [(c/5-diazene)Fe3(CO)9] cluster has been isolated and assigned a structure (48), 48 closely related to that of (47). Similar species have also been prepared from the reactions of [Ru3(CO)i2] with cyclic diazenes. Extension of these studies to reactions of iron carbonyls with diazirines led to a variety of mono- and di-nuclear compounds. Minor amounts (0.8%) of the brown trinuclear cluster (49) were also isolated when [Fe2(CO)9] was treated with the diazirine in THF solution.

Fe (CO)3 (48) C.O.M.C. VOL.

4—u

Fe (CO)3 (49)

630

Polynuclear Iron Compounds with Hydrocarbon Ligands

Complexes of this type have no counterparts in phosphorus chemistry, no doubt because of the inaccessibility of cyclic phosphorous compounds containing P—P double bonds. However, heterocycles containing P—P single bonds have been reacted with iron carbonyls and'in the case of l,2,3,4-tetrakis(trifluoromethyl)-l,2-diphosphacyclobut-3-ene both mono- and tri-nuclear products have been isolated (Scheme 10). 49 The structure of the trinuclear product could not be assigned unequivocally on the basis of spectroscopic evidence and two possibilities (50a) and (50b) fit the available data. The electronic spectrum exhibits pronounced shifts compared with that of the free ligand but this does not unequivocally establish that the P—P bond has necessarily undergone fission as in (50a).

CF3

CF 3 p _p

CF/

CF3 20 ^(benzene/

X

CF 3

CF3 j>__^Fe(CO)4 X

CF/

CF 3

CF3 CF3

CF3

CF.CF,

CF

\ )={

; (H ^

/ 3

/X\

(CO)J^

+

^Fe(CO)3

Fe (CO) 4 ^50a)

CF

or

3

(CO) 3 Fe-

-Fe(CO)3 Fe (CO) 4 (50b)

Scheme 10

The magnitude of 31 P NMR chemical shifts and coupling constants has enabled King to characterize structurally the various products resulting from the reaction of 1,2,3-triphenyl1,2,3-triphosphaindane (51) with [Fe 3 (CO)i 2 ]. 50 Depending on the solvent (hexane, benzene or toluene), the reaction temperature (69,80 or 110 °C) and the reaction time, a variety of products was isolated including three trinuclear species, each with the same basic formula [Fe3(CO)c){C6H4(PPh)3}] (Scheme 11). The predominant isomer significantly exhibits a doublet of doublets (>5 -32.8) in the 3 1 P[ 1 H]NMR spectrum similar to the resonances at 5 - 32.2 in the free ligand (51) which is attributed to the presence of the two essentially equivalent phosphorus atoms in (52). This indicates that one PPh group in (52) is bonded directly to the o-phenylene ring and to another phosphorus atom. The other PPh groups are presumably attached to iron atoms and act as three-electron bridging atoms as in (52a) and (52b). The 31 P[*H] resonances of the second isomer are all shifted downfield relative to the free ligand which suggests coordination of all three PPh groups as in the proposed structure (53). However the pattern of the large ! / ( P , P) couplings (ca. 183 Hz) in the spectrum of the third isomer indicates that the ligand (51) is still intact. The resonance due to the central atom is shifted downfield by ca. 95 p.p.m. while, surprisingly, the other peak is shifted upfield by about 50 p.p.m. This contrasts with the commonly observed shift on coordination to lower field. This is attributed to the requirement in the proposed structure (54) for the three phosphorus lone pairs to point inward to accommodate the Fe3 triangle. The resulting bond angle changes relative to the free ligand can therefore explain the anomalous upfield shift. Organometallic complexes of transition metal carbonyls and ligands of the type/^fars (55, E = AsMe 2 ), f4fos (55, E = PPh 2 ), f4asp (56), f6fars (57, E = AsMe 2 ) and f6fos (57, E = PPh 2 ) have been extensively studied by Cullen and coworkers.51 A number of mono-, di- and tri-nuclear iron derivatives have been isolated from reactions of (55), (56) and (57) with [Fe 3 (CO)i 2 ], some

Polynuclear Iron Compounds with Hydrocarbon Ligands (CO)3 Ph ,

(CO)3

k

or

(CO)3Fe Fe«

(52a)

(C0)3^

+

631

F

(52b)

Fe(CO)3

(CO)3Fe

(CO)3Fe^l—

(54)

of which have been subjected to X-ray diffraction studies. The product type isolated is apparently determined by the nature of the donor ligand rather than the size of the fluorocarbon ring. Thus f4fars has the ability to displace two terminal carbonyl groups from [Fe3(CO)i2] (Scheme 12) to give [Fe3(CO)iof4fars] (58) in which the bridging carbonyls of the dodecacarbonyl are retained. The presence of a plane of symmetry in the product was originally suggested by Mossbauer studies which gave a three line spectrum. The two outer lines constitute a quadrupole doublet from the two equivalent iron atoms similar to [Fe3(CO)i2] and the proposed structure (58) was subsequently confirmed by X-ray diffraction studies.52 The thermal rearrangement of (58) to the nonacarbonyl (59) occurs rapidly in refluxing cyclohexane but a more efficient route to the latter is via the direct reaction of [Fe3(CO)i2] with f4fars, also in refluxing cyclohexane.51 The large number of VQO bands in the IR spectrum, the four methyl group singlets in the *H NMR spectrum and the extreme complexity of the 19 F NMR spectrum suggest a low symmetry for (59). The Mossbauer spectrum, which could be resolved into five independent lines, suggested the presence of three inequivalent iron atoms. The structure revealed by X-ray studies 53 confirms that this results from cleavage of a C—As bond in the fluorocarbon ligand to give bridging AsMe2 groups and a direct ironfluorocarbon linkage. Further rearrangement of (59) is observed on refluxing in cyclohexane for six hours, the main product being a fluorine free complex [Fe 3 (CO)9AsC 3 H 8 ]. 51 The lH NMR spectrum of this derivative shows two singlets each due to three protons and an AB quartet due to two inequivalent hydrogen atoms which led to postulated structures (60a) and (60b). The four line Mossbauer spectrum, however, is more in accord with the latter than the former. The reactions of [Fe 3 (CO)i 2 ] with f4fos or related di-/-phosphines with larger bridging fluoroalicyclic groups, e.g. f6fos, do not lead to trinuclear complexes. Instead mono- ([Fe(CO)3(ligand)], [Fe(CO)4(ligand)]) and di-nuclear complexes ([Fe2(CO)6)(ligand)]) are formed preferentially. These differences led Cullen to study the reactions of f4asp and f6asp with [Fe3(CO)i2]. It was subsequently found that f4asp behaves more like f4fars than f4fos in yielding [Fe3(CO)io(f4asp)] and [Fe3(CO)9(f4asp)], although the isolation of the mononuclear chelate complex [Fe(CO)3(f4asp)] also indicates a chemical similarity to f4fos.54 As with [Fe3(CO)9(f4fars)] (59), the structure of [Fe3(CO)9(f4asp)] results from As—C bond fission (rather than P—C bond cleavage) and formation of an iron-carbon bond.55 However, unlike complex (59) the f4asp derivative does not undergo further reaction in refluxing hexane with loss of the fluorocarbon ring. It was concluded that the principal factor determining whether trinuclear species are formed in preference to mono- and di-nuclear derivatives in the reactions of iron carbonyls with these ligands is the size of the chelate bite. Thus the smaller bite of f4fos and f6fos leads to mono- and di-nuclear complexes [Fe(CO)3(ligand)] and [Fe2(CO)6(ligand)] where five-membered chelate rings are found. The larger chelate bite in f4fars and f4asp, in contrast, results in the preferential formation of (58) in which the ligand can act as a bridge between two iron atoms and thus form a larger six-membered ring.

Polynuclear Iron Compounds with Hydrocarbon Ligands

632

F

F. F

F )=\ F E E

Ph2P

+

(57)

(56)

(55)

[Fe3(CO)i2]

F

F X F

(CO)3Fe

f4fars 110 h

(58) cyclohexane reflux 5.5 h 90% yield

1

cyclohexane reflux 6 h

Me As

V

(CO)3Fe;

'(CO)3Fe. AsMe

cyclohexane reflux 1 h

or

(CO)3Fe

Fe(CO)3 .H H (60b)

(60a) Scheme 12

Polynuclear Iron Compounds with Hydrocarbon Ligands

633

Arsenic atoms have also been observed to function as bridging ligands towards transition metals. However, an unsuccessful attempt to prepare the cubane-like molecule [Fe4(jLt3-As)4(r7-CsH5)4] which has nickel and cobalt analogues gave instead an unusual trinuclear cation (61) (equation 5). 56 The structure of (61) consists of a central As4OsFe core with a non-bonded tetrahedral array of arsenic atoms, two of which are bonded to both the core iron atom and a terminal [Fe(CO)2(??-C5H5)] unit. Thus complex (61) may be considered as a substituted derivative of the known AS4O6 molecule, one oxygen of the latter being replaced by a bridging iron. The reactions of antimonyl halides with the dimer [Fe2(CO)4(r?-C5H5)2] under mild conditions yield a variety of mono- and di-nuclear compounds, the nature of which appears to depend on the solvent and the reaction conditions. Under more forcing conditions trinuclear ionic products of stoichiometry [(Fe(CO)2(T?-C5H5)}3(SbCl)] + (62) can be isolated.57 In contrast the addition of Na[Fe(CO)2(T7-C5H5)] to SbCl3 in THF proceeds in a relatively straightforward manner to give the same cation (62) which was crystallized from dichloromethane as the FeCll" salt containing a solvent molecule of crystallization.58 X-ray diffraction studies confirm, not surprisingly, the absence of direct metal-metal bonds since the three [Fe(CO) 2 07-C 5 H 5 )] groups are independently linked to the central antimony atom which achieves four coordination and a distorted tetrahedral geometry by coordination of a single chlorine atom. The average Fe—Sb bond length of 2.54 A was the first reported for an iron-antimony compound. Compounds with this structure form part of a series of iron-antimony complexes of the type [Fe(CO)2(r7-C5H5)]wSbX4_w (X = Cl, Br, I, CR3, Ph, Bun; n = 1-3, not all combinations) which have been studied by iron-57 and antimony-121 Mossbauer spectroscopy.59 57 Fe and 121 Sb Mossbauer parameters for the trinuclear compounds (n — 3) together with corresponding values for selected mono- and di-iron compounds (n = 1,2) are given in Tables 3 and 4. For a series such as this, the greater donor strength of the [Fe(CO)2(?7-C5H5)] moiety is expected to dominate the trend in s-electron density so that the isomer shift 6sb should become more negative as n increases. This trend is observed for X = Ph but with [(Fe(CO)2(?7-C5H5)J2SbX2]+ (X = Cl, Br), both complexes have more negative values of 5sb than the triiron derivatives [{Fe(CO)2(?7-C5H5)}3SbX]+. This implies that the s-character in the X—Sb bonds is so low that replacement of the halide X by an iron group does not lead to any increase in the total ^-character of the Fe—Sb bonds. Thus the isomer shift is dominated by an increase in p-shielding. Moreover the 121Sb isomer shifts fall between the two ranges of values normally associated with the two oxidation states Sb(III) and Sb(V). Consequently, assignment of a formal oxidation state to the antimony atom in the complexes studied would be meaningless. A comparison of <5pe values for isoelectronic antimony and tin complexes reveals a slight increase in the former which may be construed as an argument in favour of some ^-interaction in the Fe—Sb bond. However, the lack of significant reduction in the Fe—Sb bond length in (62) compared to the Fe—Sn distance in related tin complexes supports the premise that 7r-bonding, if present, has little effect on the physical and chemical properties of the complexes.58 Cp ^

Cp(CO)2Fe [Fe 2 (CO) 4 (,-C 5 H 5 ) 2 ]

+

AsF3

,ino

As

- ^

/ \

F e

CO ^ Fe(CO)2Cp As

/ \

^

^As^As^ (61) Cl Sb Cp(CO)2Fe I Fe(CO)2Cp CpFe(CO)2 (62) Bridging oxygen-donor ligands have not been found in polynuclear iron complexes but /u-COR groups bonded through carbon are known. The well known basicity of the oxygen atom in a bridging carbonyl group renders the ligand open to attack by electrophiles, and alkylation of

Polynuclear Iron Compounds with Hydrocarbon Ligands

634

Table 3

121

Sb Mossbauer Parameters of some Iron-Antimony Complexes1 6 (mms" 1 )

Compound [Cl2Sb|Fe(CO)2(77-C5H5)|2] [PF6] Br2Sb{Fe(CO)2(7?-C5H5)!2] [PF6] •ClSb{Fe(CO)2(i?-C5H5)}3][FeCl4] •BrSb{Fe(CO)2(i7-C5H5)}3] [PF6] ISb{Fe(CO)2(r;-C5H5)|3][l3] ISbiFe(CO)2(r7-C5H5)i3] [PF6] PhSb{Fe(CO)2(77-C5H5)}2][PF6] Ph2Sb|Fe(CO)2(77-C5H5)}2][PF6] [Ph3SbFe(CO)2(T7-C5H5)][PF6]

-9.3 -9.6 -8.8 -8.6 -8.8 -8.8 -7.9 -7.0 -6.7

± ± ± ± ± ± ± ± ±

0.2 0.3 0.1 0.3 0.3 0.2 0.1 0.2 0.2

e2qQ (mms" 1 )

77

T (mms" 1 )

x2

29.0 ± 1.0 26.6 ± 0.4 -23.9 ± 1.7 -23.8 ± 0.7 -20.5 ± 1.0 -22.4 ± 0.2 Unresolved -7.0 ± 0.4 9.6 ± 0.5

0.46 ± 0.05 0.44 ± 0.06 0.0 0.0 0.0 0.0 0.0 0.43 ±0.16 0.0

2.8 ± 0.1 3.0 ± 0.1 3.1 ± 0.1 3.1 ± 0.1 3.1 ± 0.1 2.9 ± 0.1 3.2 ±0.1 2.8 ± 0.1 2.8 ±0.1

206 203 198 225 191 205 205 116 149

1. W. R. Cullen, D. J. Patmore, J. R. Sams and J. C. Scott, Inorg. Chem., 1974,13, 649.

Table 4

57

Fe Mossbauer Parameters of some Iron-Antimony Complexes1

Compound [Ph3SbFe(CO)2(7?-C5H5)][PF6] Ph2Sb{Fe(CO)2(i?-C5H5)}2][PF6] Cl2Sb{Fe(CO)2(7?-C5H5))2][PF6] Br2Sb|Fe(CO)2(77-C5H5)}2][PF63 ClSb|Fe(CO)2(77-C5H5)i3][FeCl4] BrSb{Fe(CO)2(77-C5H5)J3[Pf6] [ISb|Fe(CO)2(7?-C5H5)b3[PF6] [ISb{Fe(CO)2(r?-C5H5)}3][l3] [PhSb{Fe(CO)2(7?-C5H5)b][PF6]

8 (mms" 1 )

AEQ (mms" 1 )

r (mms" 1 )

0.41 0.39 0.40 0.40 0.39 0.38 0.39 0.40 0.41

1.86 1.74 1.86 1.83 1.73 1.72 1.71 1.74 +1.73

0.23 0.26 0.25 0.26 0.25 0.26 0.26 0.27 0.26

1. W. R. Cullen, D. J. Patmore, J. R. Sams and J. C. Scott, Inorg. Chem., 1974,13, 649.

[Fe 3 (CO)iip- and [Fe 3 H(CO)n]- proceeds readily (Scheme 13) to give products (63) and (64) containing a M - C O R ligand.60 X-ray diffraction studies carried out on (64) (R = Me) have confirmed the structure illustrated.61 The IR spectrum of the intermediate anion [Fe 3 (COMe)(CO)io]~~ exhibits a bridging CO stretch around 1746 cm" 1 and a band at 1360 cm" 1 due to ^coMe suggesting a similar structure to the neutral hydride. The structure of (64) is related to that of the hydride anion [Fe3H(CO)i i]~, but on methylation the unique CO moves from a double metal bridge to a triple metal semi-bridging position. This shift possibly occurs because 0-alkylation increases the acceptor character of the CO ligand and in consequence promotes close approach to the third iron atom. This is accompanied by a concomitant shift of another CO towards the opposite face of the Fe 3 triangle. Variable temperature 13C NMR data have been obtained for the anion [Fe 3 (COMe)(CO)i 0 ]~ and can be reconciled with the proposed solid state structure at the slow exchange rate limit. At higher temperatures complete exchange between face bridging carbonyl and the terminal CO ligands is observed. The spectrum of [Fe 3 H(COMe)(CO)i 0 ] at - 1 2 0 °C shows seven peaks in a ratio of 1 : 1 : 1 : 2 : 2 : 2 : 2 due to the COMe ligand, two axial and two equatorial carbonyls of the Fe(CO)4 moiety and three pairs of equivalent carbonyl carbon atoms of the Fe(CO) 3 groups. A similar pattern is observed for [Fe 3 H(CO)n]~ at low temperatures. However, [Fe 3 H(COMe)(CO)io] retains this pattern up to —20 °C whereas the spectrum of the anionic hydride collapses to a single peak at ca. — 80 °C as a result of CO exchange. Obviously methylation of the bridging CO slows down the exchange process considerably. If [Fe 3 H(CO)i i]~ is treated with fluorosulphuric acid at —90 °C (cf. Scheme 13), a red-violet solution is obtained which exhibits NMR spectra similar to those described above.62 This is attributed to the presence of (64) (R = H) which decomposes above ca. —30 °C. An interesting contrast exists between this species and [Os 3 H(CO)n]~ where only metal-bonded hydrogen atoms are found. It may be that each represents the thermodynamically favoured isomeric form which is different for different metals. In view of this the authors point out that COH linkages may exist in other metal carbonyl hydrides. Further independent studies of [Fe 3 H(COMe)(CO)io] and its ruthenium and osmium analogues have subsequently been reported.63 All three clusters react readily with hydrogen (1 atm) under mild conditions (M = Fe, Ru, 60 °C; M = Os, 120 °C) to form the corresponding trihydride

Polynuclear Iron Compounds with Hydrocarbon Ligands

( C O

635

O £ F^

!

(CO)3Fe

RX/CH 2 C1 2 RX = MeSO3F

(CO)3Fe

o [Fe 3 (CO) n ] 2 -

RX

(CO)3Fe

[Fe 3 H(CO) n ]Scheme 13

[M3H3(COMe)(CO)9] although the reaction does not go to completion with M = Fe and in the absence of hydrogen, the product reverts back to [FeH(COMe)(CO)9]. An interesting structural difference between the mono- and the tri-hydride is the presence in the latter of a true triplybridging ^3-COMe ligand since 13C NMR data confirm that six equivalent equatorial and three equivalent carbonyls are present. The ability of sulphur to function as a bridging atom in organometallic complexes is well known. Complexes containing bridging mercapto ligands are widespread while bridging sulphur atoms are found less frequently. The reactions of [Fe2(CO)4(r7-CsH5)2] with dialkyl and diaryl disulphides RSSR generally yield mono- [FeSR(CO)2(^-CsH5)] or di-nuclear products [Fe2(SR)2(CO)2(?7-C5H5)2], but under certain conditions (refluxing benzene or xylene) some disulphides (R = Me, Et, SBu1, SBz) also give small quantities of a third product [Fe 3 S(SR)( C O ^ ^ - C s H s ^ ] . 6 4 On the basis of spectroscopic data, structure (65) was originally proposed in which both bridging sulphur and mercapto ligands are present. The 57 Fe Mossbauer spectra of the /-butyl- and benzyl-mercapto derivatives contain two resonances with the peak at lower velocity being the slightly broader and less intense of the two. This indicates inequivalence of the iron atom positions but the spectra were insufficiently resolved to allow confirmation of the 2:1 intensity ratio required by structure (65). An X-ray diffraction study subsequently established a more plausible structure (66) in which the carbonyl bridges are accompanied by metal-metal

CpFe

(66)

636

Polynuclear Iron Compounds with Hydrocarbon Ligands

bonds while the mercapto ligand only bridges two adjacent iron atoms.65 The reaction of thiophene with iron carbonyls results in desulphurization by the iron to yield [Fe2(CO)6(C4H4)] containing a ferracyclopentadiene ring [(33), Section 31.5.2.2]. In contrast [Fe 2 (CO) 9 ] reacts with tetrahydrothiophene to yield [Fe 3 (CO) 8 (C 4 H 8 S)2] (67) in which the organosulphur ligands remain intact. 66 This has been confirmed by X-ray diffraction studies which show that the triangular Fe 3 cluster has bridging C 4 H 8 S ligands on two edges and two very unsymmetrical carbonyl bridges on the other. Thus the sulphur utilizes both its lone pairs in this bridging mode and all six valence electrons are therefore involved in bond formation. Bridging ligands of this type are rare since diorganosulphides R 2 S generally function as terminal ligands towards transition metals.

(CO)2Fe

(67)

31.5.3 TETRANUCLEAR IRON COMPLEXES Tetranuclear iron complexes containing hydrocarbon ligands are noticeably less numerous than triiron derivatives and very few Fe 4 compounds have been obtained from the reactions of alkynes or alkenes with iron carbonyls. Instead, a recurring feature of the complexes described in this section is the presence of sulphur ligands which are able to stabilize tetranuclear structures by simultaneously binding to three or four iron atoms.

31.5.3.1 Alkene and Alkyne Complexes The reactions of substituted alkynes with [Fe3(CO)i2] which lead to a variety of trinuclear complexes (Section 31.5.2.1) also give in most cases very low yields (ca. 1%) of tetranuclear complexes [Fe 4 (CO)n(R 1 C=CR 2 ) 2 ] (R 1 = H, R 2 = Me, Et, PrJ; R1 = R 2 = Me) (68). 67 An X-ray diffraction study of the but-1-yne complex shows that the four metal atoms are situated at the vertices of a tetrahedrally distorted square. The two alkyne molecules, which interestingly have not undergone condensation with each other, are bonded in a M4-7?2 fashion to the four iron atoms. This mode of bonding, in which the alkyne is attached via c-bonds to two metal atoms and ?72-bonded to the other two was first discovered in the cobalt-alkyne cluster [Co4(CO)io(EtC=CEt)]. 6 8 Many examples of the latter type are known and, in contrast with (68), can be prepared in relatively high yield by addition of alkynes to [Co 4 (CO)i2]. The alkyne C = C distances in (68) (1.374 and 1.397 A) are somewhat shorter than that in the cobalt complex (1.44 A). This is somewhat surprising since the iron compound is electron deficient. It might be expected (CO)2 -Fe

(68)

637

Polynuclear Iron Compounds with Hydrocarbon Ligands

that electron donation from the alkynes to the cluster would be more efficient because of this leading to a longer C = C bond. This appears to be the case in [Fe 3 (CO) 9 PhC=CPh] (1), as discussed in Section 31.5.2.1. The reactions of azulene with iron carbonyls [Fe(CO)5], [Fe2(CO)9] or [Fe3(CO)i2] have been shown to yield azulenediiron pentacarbonyl and a polynuclear complex originally formulated as [Fe 5 (CioH 8 )2(CO)i3], an iron cluster compound. Later X-ray studies led to this being reformulated as [Fe 4 (CioH 8 )2(CO)io] [(69) in Scheme 14] and as illustrated the complex cannot be classified as a cluster compound since only two iron atoms are linked together in an Fe 2 (CO) 4 unit.69 The molecule consists of a 4-em/o,4/-e«do-diazulene ligand in which the two five-membered rings function as substituted r;5-cyclopentadienyl systems linked via the Fe 2 (CO) 4 unit. The cw-l,3-diene systems remaining in the seven-membered rings are each coordinated to discrete Fe(CO) 3 groups. The Fe 2 (CO) 4 group is reminiscent of the analogous dinuclear group found in [Fe2(CO)4(77-C5H5)2] and interestingly the Fe—Fe bond lengths are identical (2.5 A). However, the basic Fe 2 (CO) 4 unit differs in that the Fe(CO) 2 Fe bridge is planar in the latter whereas the dihedral angle between the two Fe—CO—Fe planes in (69) is 154° 40'.

Fe(CO)3

Fe(CO)3 (69)

The relationship of (69) to the other products of the reaction of azulene with iron carbonyls can be seen in Scheme 14 which attempts to rationalize the observations reported to date. Thus on coordination of azulene to iron, a diradical (70) is formed initially which can either react with iron carbonyl (path i) to give (71) or dimerize (path ii) to give (72). Subsequent coordination of two Fe(CO)3 groups to the latter gives complex (69).

31.5.3.2 Sulphur Bridged Complexes The reactions of dimethylthiocarbamoyl chloride [ClC(S)NMe2] with a number of neutral and ionic iron carbonyls in THF give a variety of unusual products resulting from dehalogenation of the carbamoyl compound.70 The sole tetranuclear complex was obtained from [Fe 2 (CO)8p~ and, on the basis of spectroscopic evidence, assigned structure (73). This was later confirmed by X-ray diffraction studies71 which showed that both Fe 2 (CO) 6 units were linked at right angles by a tetrahedral sulphur atom which donates a total of six electrons to the four iron atoms. Complex (73) can be considered as being derived from two Me 2 NC=S ligands, one of which has undergone C = S bond cleavage to give bridging sulphur and dimethylimmoniocarbene ligands. Another immoniocarbene bridged iron complex [Fe 2 (CO) 6 (CNEt 2 ) 2 ] has been found to result from the fission of the C = C triple bond in E t 2 N C = C N E t in the presence of iron carbonyls.72 C.O.M.C. VOL. 4—u*

638

Polynuclear Iron Compounds with Hydrocarbon Ligands (C0) 3 }CNMe2

(73)

The structure of (73) bears a very close relationship to that of (74) obtained from the reaction of 2-mercaptopyridine with [Fe 3 (CO)i 2 ]. 73 The complex in this instance results from cleavage of a C—S single bond to yield both bridging sulphur and 2-metallated pyridine ligands. A third species which belongs to this class of complex, i.e. containing a tetrahedral Fe2SFe2 core, has been isolated from the reactions of thiols with iron carbonyls74 and shown to have structure (75) by X-ray methods when R = Me. 75 It has been pointed out that such complexes are chiral in solution since the *H NMR spectrum of the isopropyl derivative exhibits two methyl group signals up to 135 °C. 74 Thus the chiral framework is rigid unlike the two isomeric forms (syn and anti) of related dinuclear iron complexes [Fe2(M"SR)2(CO)6] (R = alkyl, aryl) which can interconvert in solution. 76 A different but not entirely unrelated tetranuclear complex (76) has been isolated from the reaction of [Fe2(CO)9] with tetrathionaphthalene (equation 6). 77 Spectroscopic features of the complex (IR and electronic spectra) are in accord with the structure illustrated in which the tetrathiolene ligand bridges two Fe2(CO)6 units.

(CO)3

(CO)3

(CO),

,co^

^?\

J-Fe

1

©CixC//*'"" (74)

3

R

^x I X / / (75)

(76)

31.5.3.3 [Fe4(S4Xi?-C5H5)4] and Related Complexes

The desire to understand the behaviour of non-heme iron-sulphur proteins in biochemical systems has stimulated wide ranging studies of iron-sulphur clusters including the cubane-like Fe 4 S 4 system.78 This cluster unit is found in naturally occurring iron-sulphur proteins such as ferrodexins and a number of synthetic analogues have now been prepared. Not surprisingly, the synthesis of a cyclopentadienyl complex [Fe4(S4)(77-C5H5)4] (77) containing the same structural unit generated some interest and led to the proposal that this complex could serve as a model for the redox centre of the reduced form of the high potential iron-sulphur protein isolated from the photosynthetic bacterium Chromatium.19 Complex (77) was first isolated by chromatographic means from the mixture of products obtained by refluxing [Fe2(CO)4(r7-CsH5)2] and cyclohexene sulphide in benzene. 80 ' 81 X-ray diffraction studies interestingly reveal that the complex can

v

Polynuclear Iron Compounds with Hydrocarbon Ligands

639

crystallize in both orthorhombic and monoclinic space groups. 80 ' 81 The molecular structure of (77) consists of two interpenetrating tetrahedral arrays of both sulphur and iron atoms, the latter bonded to one 77-C5H5 ligand, three sulphur atoms and one other iron atom in order to attain a closed shell configuration. The presence of only two metal-metal bonds results in a distortion of the Fe4S4 core from the idealised cubic configuration of Tj symmetry to the observed tetragonal structure with Did symmetry. Thus four FeS bonds are close to 2.2 A while two are ca. 2.25 A long. Although the molecular packing in the monoclinic and orthorhombic phases are obviously different, the corresponding bond lengths are quite similar.

(77) The publication of the structure of (77) stimulated further studies of the complex, dealing in particular with its redox properties. Electrochemical work showed that at a platinum bead electrode, four electrochemically reversible waves are exhibited (equation 7) indicating that the cluster remains intact in five distinct oxidation states.82'83 Attempts to isolate the (—1) and (+3) oxidation states by electrochemical methods were unsuccessful88 but in hot acetonitrile with NH4PF6 as supporting electrolyte, [Fe 4 (S 4 )(r/-C 5 H 5 )4] + and [Fe4(S4)(r7-C5H5)4]2+ were obtained as PF;~ salts in 60-70% yield. Chemical oxidising agents AgBF4,12 and Br2 similarly gave the mono- and di-cations.84 X-ray studies of both [Fe4(S4)(T?-C5H5)4]Br (77)+ and [Fe4(S4)(7?-C5H5)4](PF6)2 (77) 2+ have been carried out with a view to establishing the stereochemical consequences of oxidising the Fe4S4 core. 83 ' 84 The basic structure of the monocation is related to that of the neutral parent (77) but a distortion of the Fe4S4 core from tetragonal (£>2^-42m) to orthorhombic (Z>2-222) geometry is observed. The two equivalent Fe—Fe bonding and four equivalent Fe—Fe non-bonding distances found in the former (2.64 and 3.36 A respectively) are replaced by three pairs of Fe—Fe distances of 2.65, 3.19 and 3.32 A as a result of preferential shortening of two of the non-bonded distances. The dication was unexpectedly found to exhibit tetragonal (Z>2
*±1A1V

(7?)2+

^Hmv

(?7)

^±O33V

(7?)+

zO33^

(7?)-

(7)

A molecular orbital description of the [Fe4(S4)(^-CsHsJJ system has been developed to account for the structural alterations resulting from oxidation of the parent cluster.83 The metal species (77) has 20 metal electrons which are distributed under assumed T (77) + ] from 3.13 to 3.08 A is consistent with the removal of an electron in an antibonding orbital with respect to the tetrametal cluster. The qualitative energy level diagram in Figure 3 was constructed on this basis. This diagram further predicts that the non-isolable monoanion (77)~ will have orthorhombic D2 geometry while the dianion (77) 2 ~ will exhibit either D2d or orthorhombic Z>2 geometry. Attempts to define more clearly the nature of the molecular orbital occupied by the unpaired electron in (77) + using Mossbauer spectroscopy were only moderately successful, and the technique was unable to distinguish between the structures adopted by the mono- and the di-cation.85

640

Polynuclear Iron Compounds with Hydrocarbon Ligands

Table 5

Selected Mean Distances and Bond Angles for Complexes [Fe4(S4)(T/-C5H5)4]"+ (n = 0, 1, 2) [Fe4(S4)(r?-C5H5)4] (monoclinic phase)

Reference Idealized geometry of Fe4S4 core Fe—Fe(A)

[Fe4(S4)(T7-C5H5)4] (orthorhombic phase) [Fe4(S4)(77-C5H5)4] + [Fe 4 (S 4 )(77-C 5 H 5 ) 4 ] 2+

1 D2d~42m

2

| 2 | 2.650(6) |4| 3.363(10)

2 2.631(2) 4 3.366(2)

S(A)

41 2.880(13) 2| 3.334(9)

14 2.884(3) |2 3.344(6)

Fe—S(A)

8 2.204(8) 4 2.250(10)

|8 2.206(2) |4 2.256(3)

Fe—S—Fe(°)

4 73.9(2) 8 98.0(3)

4| 73.3(1) 8| 98.0(1)

S—Fe—S(°)

8 80.5(3) 4 98.2(2)

8| 80.5(1) 4| 98.7(1)

S

1. 2. 3. 4.

3 D2-222

D2d-42m 2 2 2 2 2 2 4 4 4 4 4 4 4 4 4

4 D2d-42m

2.652(4) 3.188(3) 3.319(3) 2.879(6) 3.062(6) 3.389(8) 2.185(5) 2.212(5) 2.246(5) 74.2(2) 92.1(2) 96.3(2) 80.5(2) 87.5(2) 100.9(2)

4 2.834(3) 2 3.254(3) |2 2.820(6) |4 3.304(5) |4 2.156(3) |8 2.208(4) |8 81.0(1) |4 94.9(1) |4 79.3(1) |8 98.4(1)

C. H. Wei, G. R. Wilkes, P. M. Treichel and L. F. Dahl, Inorg. Chem., 1966, 5, 900. R. A. Schunn, C. J. Fritchie, Jr. and C. T. Prewitt, Inorg. Chem., 1966, 5, 892. T. Toan, W. P. Fehlhammer and L. F. Dahl, J. Am. Chem. Soc, 1977, 99, 402. T. Toan, B. K. Teo, J. A. Ferguson, T. J. Meyer and L. F. Dahl, J. Am. Chem. Soc, 1977, 99, 408. [Fe 4 S 4 ]-

[Fe4S4]

[Fe4S4]+

[Fe4S4p+

[Fe4S4p+

21

20

19

18

17

D2

Did

Di

Did

Did

Number of Metal Cluster Electrons Point Group Symmetry _

^*'*Wm

b2

b|**,_

Antibonding

e

--;#*f' /

s

,--+~-^,' b3

1 ^ '

/ bz^7

a2

y>*rx

\

^ * - .

12

*i

\

/

\

Bonding

"-^^

12

b3

12

e

/

12

12

3a+bi+b 2 +b 3 2ai+bi+b2+e 3a+b!+b2+b3 2a1+b1+b2+e 2ai+bi+b2+e Total Bond Order 1.5 2 2.5 3 3.5 Figure 3 Qualitative energy level diagram for the tetrametal cluster orbitals in the [Fe4(S4)(77-C5H5)4] system (reproduced with permission from /. Am. Chem. Soc.)

Polynuclear Iron Compounds with Hydrocarbon Ligands

641

The reaction of [^2(03)4(77^5^)2] with elemental sulphur under reflux in non-polar solvents gives, in addition to the Fe 4 S 4 cluster complex (77), a related complex of stoichiometry [Fe4(S6)(r?-C5H5)4] (78). 86 The structure of the latter is similar in many respects to that of (77). Each iron atom has approximately octahedral coordination (three sulphur atoms and an 77-C5H5 group) while two Fe—Fe bonding interactions of 2.65 A are found in addition to four longer non-bonding distances. Two of the triply-bridging sulphur atoms in (77) have been replaced by two disulphide groups and consequently the symmetry of the iron core is reduced from D2d to

c2. As with [Fe4(S4)(?7-C5H5)4], interest has centred primarily on the redox properties of the cluster. Cyclic voltammetry exhibits two reversible one-electron redox potentials while oxidation of AgSbF 6 yields a paramagnetic monocation (79) with a strong ESR signal (g = 2.027). Controlled potential electrolysis has been employed to isolate a dication salt while an apparent trication was detected at more positive potentials. The structure of the monocation (79), established by X-ray diffraction studies, was surprisingly found to consist of two iron-sulphur clusters coordinated to a silver(I) ion and the stoichiometry of the oxidation can therefore be written as in equation (8). The only significant modification to the Fe4S6 core resulting from oxidation appears to be a reduction of one non-bonding Fe—Fe distance from 3.41 to 3.00 A indicating that as with [Fe4(S4)(?7-C5H5)4] + the electron has been removed from an antibonding MO. On oxidation the positive charge apparently becomes localized on the end of the molecule opposite the disulphide groups. The latter obviously retain their basic character on account of their ability to coordinate to Ag + . Another aspect of the chemical reactivity of the disulphide bridges is revealed in the reaction of (78) with triphenylphosphine which abstracts one sulphur from each disulphide bridge (equation 9).

Fe"

(78)

2[Fe4(S6)(77-C5H5)4]

Cp groups omitted for clarity

+

[Fe4(S6)(77-C5H5)4]

3AgSbF 6 +

2PPh 3

.—• -^f^

[JFe4(S6)(77-C5H5)4)2Ag][SbF6]3 [Fd4(S4)(r;-C5H5)4]

+

+

2SPPh 3

2Ag

(8) (9)

The methyl cyclopentadienyl analogue of (78) has also been prepared and interestingly, two methyl group signals are observed in the ! H NMR spectrum. This and the complexity of the ring l H NMR resonances confirm that the rigid chiral framework of the cluster is retained in solution [cf. (64)].^

31.5.3.4 [Fe4(CO)4(77-C5H5)4] The tetranuclear iron cluster [Fe^CCO^-CsHs^] (80) was first prepared by King by a tedious method which involved refluxing [Fe2(CO)4(r;-C5H5)2] in xylene for 12 days followed by exhaustive Soxhlet extraction with diethyl ether for a further seven days to give the product in ca. 14% yield.87 Higher yields (56%) were subsequently obtained by photolyzing the dimer in refluxing xylene for seven days. 88 However, the most efficient synthesis, in which a slight molar excess of PPh 3 is added to the refluxing xylene solution of the dimer, has been reported to give the tetramer in 56% yield after only seven hours. 89 The structure of the tetramer (80) according to X-ray dif-

642

Polynuclear Iron Compounds with Hydrocarbon Ligands

fraction studies is superficially similar to the iron sulphur cluster [Fe4(S4)(77-05115)4] but unlike the latter, the iron atoms are located at the corners of a regular tetrahedron which conforms closely to cubic 7^-43m symmetry. 90 Moreover each iron atom is bonded directly to the other three, the average Fe—Fe distance being 2.520 A. The carbonyl groups, each of which is bonded symmetrically to three iron atoms on each face of the cube, also form a tetrahedral array. In addition to X-ray work a number of spectroscopic studies have been carried out on (80) including mass and Raman spectroscopy. The mass spectrum, in addition to showing a parent ion and ions due to successive loss of carbonyl ligands, also exhibits a series of dimetallic ions [Fe2(CO)n(77-C5H5)3] (n = 0, 1, 2), one of which (n = 0) may possess a triple-decker structure.91 The Raman spectrum has been utilized to estimate an Fe—Fe force constant of 1.3 X 10~8 N A" 1 , 92 while photoelectron spectroscopy reveals a pattern of molecular orbitals very similar to those1 of ferrocene.93 However, the ionization potentials of these levels are significantly lower while a weak band at 8.89 eV (a t mode) corresponds to the HOMO of the cluster molecule.

Chemically, interest has again centred on the redox properties of the cluster which can be oxidised by excess bromine to give the paramagnetic monocation [^64(00)4(77-05^)4] + isolable as the tribromide salt.87 Reduction of the latter back to the neutral species is readily accomplished by addition of hydrazine. The chloride and pentaiodide salts of the monocation are also known94 while photolysis of (80) in the presence of halocarbons gives [^4(00)4(77-05^)4] + as the first oxidation product in quantitative yields.95 This is the only photoreaction observed for any irradiation of wavelength greater than 300 nm while in the absence of charge-acceptor solvents no photooxidation is found. Complex (80) is essentially inert to ligand substitution since photolytic replacement of CO in the presence of potential nucleophiles could not be achieved. However, in common with many other di- and poly-nuclear carbonyl bridged complexes, reactions with electrophiles are observed. The well known basicity of bridging carbonyl ligands is manifested in their ability to form donor-acceptor linkages with electrophiles such as AIX3 and BX3 (X = alkyl or halide). In reactions with (80) only 1:4 adducts could be isolated with A1X3 (X = Et, Pr 1 ) 96 while the stronger acid AlBr3 afforded isolable 1:1, 1:2, 1:3 and 1:4 adducts when appropriate ratios of the reactants were mixed in aromatic solvents.97 The poorer electrophiles BF3 and BCI3 gave only 1:2 adducts under these conditions.97 In each case the IR spectrum of the adduct-forming CO group exhibited characteristic bands at 1300-1480 cm" 1 , some 300 cm" 1 lower than the uncomplexed car bony Is. Cyclic voltammetric studies of the neutral cluster (80) have shown that electrochemically reversible oxidation to both mono- and di-cations can be accomplished while reduction to the monoanion also occurs.98 As with [Fe4(S4) (77-05^)4] the cluster remains intact throughout the oxidation-reduction processes. However, attempts to isolate the dication resulted in cluster fragmentation to Fe 2 + and [Fe(77-C5H5)(CO)2S]+ (S = solvent).99 An X-ray study of the PF^ salt of the monocation (80) + reveals that the central [Fe4(CO)4] core has acquired an idealised D2d-42m tetragonal geometry as a result of a distortion from the tetrahedral geometry of the neutral parent. 99 Thus two opposite Fe—Fe tetrahedral edges of 2.484(2) and 2.506(2) A are slightly longer than the four chemically equivalent Fe—Fe edges of 2.467(2) and 2.478(2) A. Interestingly the average CO bond length (1.188 A) is 0.015 A less than the corresponding value (1.203 A) in the neutral cluster. This is reflected in a shift of the CO stretching frequencies in the IR spectrum to higher energy (ca. 80 cm" 1 ) on oxidation. Moreover the mass spectrum exhibited a relatively high abundance of species formed by carbon-oxygen bond fission.91 The MO description of [Fe 4 (S 4 ) (77-05^)4] (77) and its derivatives discussed earlier has also

Polynuclear Iron Compounds with Hydrocarbon Ligands

643

+

been applied to (80) and (80) . The highest filled MOs are e + ti + t 2 which are largely nonbonding. It was concluded on the basis of the structural observations that the electron removed on oxidation comes from the highest orbital of this set which is somewhat antibonding between the iron atoms and which is also antibonding with respect to the carbonyl C—O bonds. Mossbauer spectra of (80) and (80) + recorded at 4.2 K are consistent with this conclusion.100 Oxidation to the monocation apparently results in a change in the quadrupole interaction and a small magnetic hyperfine interaction but no change in the isomer shift. The unpaired electron in the monocation interacts with all the iron atoms which appear to be equivalent on the basis of the spectra obtained. The most recent study of this cluster system is concerned with the catalytic properties of the neutral complex.101 The high stability of [Fe^CO^iy-CsHs^], i.e. its resistance to fragmentation to smaller units, makes it an ideal candidate for studies of this type. With many other clusters, particularly those containing first row metals, doubts can frequently be raised as to whether or not catalysis is promoted by the parent cluster or fragments derived from it. [Fe^CO^rj-CsHs)^ turns out to be an efficient catalyst for the selective hydrogenation of alkynes to alkenes at 100-130 °C and 100-1000 p.s.i. and the reduction of terminal alkynes to alkenes. These and other hydrogenations are listed in Table 6. The stability of the cluster was assessed by monitoring the concentration during a pent-1-yne hydrogenation (125 °C, 600 p.s.i.) and no detectable change occurred throughout 1410 turnovers. These and other experiments clearly indicate that the active catalyst is the cluster and not lesser fragments derived from it. Table 6

Homogeneous Hydrogenations Catalyzed by [Fe4(CO)4(77-CsH5)4]a in Benzene1 Temp. Pressure Time Conversidn (°C) (psi) (h) (%)

1-Pentyne 1-Pentyne 1-Pentyne 1-Pentyne 1-Pentyne 2-Pentyne 1-Pentene 1,3-Cyclooctadiene Nitrobenzene Nitrobenzene Methyl acrylate Acrylonitrile Ethyl crotonate Benzonitrile Methyl ethyl ketone Nitropropane

120 120 120 120 120 120 130 100 100 130 100 100 100 100 100 130

100 105 460 685 980 504 460 100 100 300 100 100 100 100 100 300

6 88 46 120 70 40 57 24 26 24 24 24 24 24 24 24

85 99 83 98 62 4.4 61 3.7 6 31 32 26 2.3 0 0 0

Product (yield) (%) 1-pentene (73), 2-pentene (8.8), pentane (3.8) 1-pentene (84), 2-pentene (10.1), pentane (3.0) 1-pentene (61), 2-pentene (4.2), pentane (17) 1-pentene (73), 2-pentene (2.6), pentane (21.4) 1-pentene (48), 2-pentene (0), pentane (13.7) 2-pentene (4.2), pentane (0.2) 2-pentene (55), pentane (6) cyclooctane (2.9), cyclooctene (0.8) aniline (6) aniline (31) methyl propanoate (32) propionitrile (26) ethyl butanoate (2.3) — — —

a Each reaction employed 0.05 mmol of catalyst, benzene (15 ml), substrate (20.3 mmol). 1. C. U. Pittman, Jr., R. C. Ryan, J. McGee and J. P. O'Connor, /. Organomet. Chem., 1979,178, C43.

31.5.3.5 Antimony Halide Complexes The reactions of antimony halides with [Fe2(CO)4(r?-C5H5)2] yield a variety of compounds, the nature of which appears to depend on the solvent and the reaction conditions.58 A 2:1 adduct obtained from a 1:1 mixture of SbC^ and the iron dimer in dichloromethane has been subjected to X-ray diffraction studies and found to have structure (81). 102 This contains two octahedrally coordinated antimony atoms linked by two bridging chlorines each of which is also coordinated to an iron atom. The octahedron in each case is completed by three fac chlorine atoms terminally bound and a chlorine which acts as a bridge between the antimony and a [Fe(CO)2(r?-C5H5)] moiety. We can compare this structure with that of the charge transfer complex [Fe4 {C1(CO)2(^-C5H5)}4] [(SbCl 3 ) 4 ] (82) isolated from the reaction in equation (10) which also gives the trinuclear complex [FestCO^-CsHsWSbCl)] discussed in Section 31.5.2.3.103 Compound (82) is apparently formed by the cosublimation of the intermediate [FeCl(CO)2(?7-C5H5)] and unreacted SbC^ during the purification process. The structure is related to that of (81) in that a central chlorine bridged array of antimony atoms is present to which the [Fe(CO)2(??-C5H5)] groups are each attached via bridging chlorine atoms. The antimony atoms are again octahedrally coordinated but this time by three terminal and, unexpectedly, by three triply-bridging chlorine

644

Polynuclear Iron Compounds with Hydrocarbon Ligands

atoms, the latter of which also coordinate to iron and in the process achieve tetrahedral coordination. Thus the basic [Sb4(77-Cl)4] unit consists of two interpenetrating Sb 4 and Cl 4 tetrahedra which together form a distorted cube. CpFe(CO)2 \ ^eCCOaCp

CPlCO)l

>a

c<|fl>''

aAU'x?

MCO lCP

'

b

I °4^%'

Cp(CO)2Fe c r ^ ^ C l Qx

CpCCO)2Fe^xSb
Fe(CO)2CP

(81) SbCl 3

+

>e(CO)2Cp

(82)

Na[Fe(CO) 2 (irC 5 H 5 )]

— • [{FeCl(CO)2(7,-C5H5Jj4][(SbCl3)4] (82) +

[{Fe(CO)2(77-C5H5)}3SbCl][FeCl4]

(10)

(62)

31.5.4 PENTA- AND HEXA-NUCLEAR CARBIDO CARBONYL CLUSTERS The carbido iron cluster [FesCCO^C] (83) was first isolated in milligram quantities in 1962 as a minor product (<0.5%) from the reactions of [Fe3(CO)i2] with alkynes P h C = C M e and P r C = C H in petroleum ether. The spectroscopic features of the complex did not provide an indication of the novel nature of the complex and the structure, unique at the time, was established by X-ray diffraction1 studies. The molecule consists of a pentameric array of iron atoms situated at the corners of a square pyramid with an average Fe—Fe distance of 2.64 A. Each iron atom, which is coordinated to three carbonyl groups, achieves an eighteen-electron configuration by additional bonding to a carbide atom located below the centre of the basal plane of the four equivalent iron atoms. The unique apical iron-carbon distance is 1.96 A while the four basal iron-carbon distances have an average value of 1.89 A. Following this report, a number of polynuclear ruthenium ([Ru 5 (CO)i 5 C], [Ru 6 (CO)i 7 C], [Ru 6 (CO)i 4 C(arene)]), osmium ([Os 5 (CO)i 5 C], [Os 8 (CO) 2 iC]) 4 and rhodium ([Rh 6 C(CO) 1 5 ]-) carbido clusters have been isolated. In'the case of [Ru 6 (CO)i 7 C], [Os 5 (CO)i 5 C] and [Os 8 (CO) 2 iC], evidence has been presented to suggest that the source of the carbon atom is a carbonyl group,4'104 a conclusion which may also apply to the iron complex (84). (CO)3 Fe

/

/ /

\ \

/iCOh/ F

/

( C °)2

\

1

\

^ r~rH—-~\

^

\

l^^Fe(CO)3

(CO)3F^^^ == 22\1 / (CO)3 (83)

(CO)2_ /^P\^>=0 (co)3

--- \\~/~ A/?i

O

^ \ y X X\: (CO)2Fe

^l^f(CO)2

(COh^O (84)

Polynuclear Iron Compounds with Hydrocarbon Ligands

645

More recently a second iron carbonyl carbide [Fe 6 (CO)i 6 C] 2 - (84) has been synthesized independently by three different routes (equations II, 1 0 5 12 106 and 13 107 ). The structure of this complex as a tetramethylammonium salt has also been established by X-ray diffraction studies. 105 ' 107 The dianion contains an octahedral array of iron atoms inside which is encapsulated the carbido carbon atom bonded to all six metal atoms. One iron is coordinated to three terminal carbonyl ligands while the others are attached to two CO groups. The three remaining carbonyls are attached in a semibridging manner. A slight distortion present in the Fe 6 skeleton appears to be related to the presence of the semibridging groups.

[Fe(CO)5]

+

Na[Mn(CO)5]

[Fe(CO)5]

+

^ *

[MnFe2(CO)12]-

[Na(diglyme)2]+[V(CO)6][Fe(CO)5]

. ^ W

* £ *

n ^ V

[Fe 6 (CO) l6 p-

[Fe*(CO)16p-

[FedCO) 14 p-

(II) (12) (13)

Complex (84) was the first anionic carbonyl carbide to be isolated and its high stability led to predictions that a series [Fe 6 (CO)i 7 C], [Fe 6 (CO) 1 6 Cp- and [Fe 6 (CO)i 4 C] 4 - might exist by analogy with the known series based on [Co 6 (CO)i 6 ]. Similarly [Fe 5 (CO)i 5 C] could have anionic derivatives [Fe 5 (CO)i 4 C] 2 - and [Fe 5 (CO) 1 3 C] 4 - and the djanion of the first type (85) was subsequently isolated from the reaction in equation (14) (base = NaOH, NaBH 4 or Na-amalgam). 108 This may be contrasted with the reaction of [Fe 3 (CO)i 2 ] with base which either gives [Fe 3 H(CO)n]- with NaBH 4 or undergoes cluster fragmentation on treatment with NaOH or sodium amalgam. In none of the above reactions did [Fe 5 (CO)i 5 C] undergo reduction to the dianion [Fe 5 (CO)i 5 C] 2 - although this possibility has been predicted. An earlier report of the synthesis of [Fe 5 (CO)i 4 Cp- from the reaction of [Fe(CO) 5 ] with Na[Mo(CO) 3 (7?-C 5 H 5 )] in refluxing diglyme 109 now appears to be erroneous since the product originally formulated as the dianion proves to have spectral features identical with those of [Fe 6 (CO)i6C] 2 ~. 108 This is not surprising in view of the similarity of the reagents employed in this reaction to those in equations (11) and (12). A more recent synthesis of (85) has been reported (equation 15) but full details of the preparation are not yet available. 110

[Fe5(CO)15C]

«

^

[Fe5(CO)14Cp-

(14)

(85) [Fe5(CO)15C]

+

2

[Fe(CO)4] -

—

[Fe5(CO),4C]2-

(15)

Attempts to obtain neutral hexanuclear iron carbonyl carbides by either protonation (96% H 2 SO 4 , 85% H 3 PO 4 , 6M HC1) or oxidation (Ph 3 C + PF^, AgBF4) of [Fe 6 (CO) 16 C] 2 - salts have so far failed. Instead, these reagents convert the dianion to the neutral pentairon cluster [Fe5(CO)i5C] and thus provide a much improved synthesis of this compound.108 This has enabled some chemistry of [Fe5(CO)i5C] to be developed which is summarised in Scheme 15. The inability of strong acids to protonate (83) contrasts with the ease with which many polynuclear hydrocarbon complexes undergo protonation. In particular, the carbide carbon appears unreactive and therefore cannot be considered as a centre of negative charge in the molecule.108 Moreover 13 C NMR resonances of carbon atoms in carbido carbonyl clusters have been found to exhibit significant downfield shifts which is consistent with their assignment as shielded carbonium ions rather than as carbides. 111 Some evidence for the electrophilicity of carbido carbon atoms is provided by the oxidation of [Fe 6 (CO)i 6 C] 2 ~ with tropylium bromide in methanol.(Scheme 16).111 The novel tetranuclear product (86) which was characterised by X-ray diffraction studies apparently results from the removal of two vertices from the octahedral dianion. The apparent electrophilicity of the exposed carbon results in attack by carbon monoxide (liberated during the reaction) and the metallaketene formed undergoes methanolysis to yield the final product. Complex (86) is readily hydrogenated

646

Poly nuclear Iron Compounds with Hydrocarbon Ligands H2S04t .CWXW.X---

[FerfCOuCF-

<85»

. . . . ^ ^ H .

[Fe5(CO)15C]

•

2 ^

+

"

^

3

Fe^

^

\j

(83)

'

[Fe5(CO),C(L)]

(CO)

(cS) 3

L = PR 3 ,P(OR)3

+

[WCO )n C(W,

(CO) 3 Fe^e(CO)3

[Fe3(CO)12C(L3)]

(C

Fe °)3

Scheme 15

Fe W ^ f ^ ^ | y>^ Fe Fe

^


Fe [Fe 6 (CO) 16 C] 2 -

Fe

II

Jco Fev/

A)

Fe^/

|\Ar—T1 N

,MeOH

N

M

OMe

IN A / > _ r _ o

FI^" ° X

Fe

Fe

[Fe4(CO)12(CCO2Me)] (86) Scheme 16

(120 °C, 22 p.s.i.) in THF to give methyl acetate ( « 50% yield). Labelling experiments ( 13 CO) demonstrated that the methylidyne carbon in II was derived from CO and, since this is the origin of the acetyl methyl carbon, effectively a carbonyl group has undergone a two stage conversion to an organic moiety. This may have implications for the mechanism of the Fischer-Tropsch synthesis which originally was thought to involve surface carbide hydrogenation. This proposal later fell into disfavour but the methyl acetate synthesis mediated by complex (86) tentatively suggests that coordinated carbido groups may indeed be involved in the process. Despite this encouraging result the carbido carbon in [Fes(CO)i5C]~ does not exhibit high reactivity in the catalytic hydrogenation of carbon monoxide. 110 This has led to the synthesis of related heteronuclear carbonyl carbido clusters based on (84) which would hopefully prove more reactive in this respect. The complexes isolated are detailed in Scheme 17. The oxidative reactions of some of the heteroclusters which are given in Scheme 18 illustrate that loss of an iron atom occurs readily to provide a route to a new class of cluster [MM4C]. The structures of [RhFe4-

Polynuclear Iron Compounds with Hydrocarbon Ligands [CKCOMNC^,

[CrFe5(co)i7q2

[Mo(CO)3(QH,0)3l>

[MoFe5(CO)i7C]2-

1W(C0¥CH]CN)

[WFe 5 (CO) 17 cp-

^

^

1Rh2CI (C0)<1

'

[Fe 5 (CO) 1 4 C]^-

— tRh^"JM

".

_

•

[Fe 6 (CO) I S Cr

•

[RhFe 5 (CO) 1 6 C]-

.

[RhFe 5 (CO) l 4 C(cod)]-

^

641

^

^

[IrFe 5 (CO) l 4 C(cod)]-

^

^

•

[NiFe 5 (CO) 1 3 C(cod)F-

|

«

•

[NiFe 5 (CO) 1 5 Cp-

•

[PdFe 5 (CO) ) 4 C ( j ,-C3H 5 )r

l [Cu(CH3CN)< " >F< •

[CuFe5(CO)14C(MeCN)r

">d2CI'("CjH');1

Scheme 17

[MFesCCO^C]-"

Fe3+

M = Cr

> [CrFe 4 (CO) 1 6 C]

M = M

[MoFe 4 (GO) 1 6 C]

°>

M = Rh

>

[RhFe 4 (CO) 1 4 C]-

Scheme 18

(CO) 1 4 C]- (87) and [MoFe 5 (CO)i 7 C] 2 - (88), established by X-ray diffraction studies, are related to those of [Fe 5 (CO)i 5 C] and [Fe 6 (CO)i 6 C] 2 - respectively. The rhodium atom in (87) occupies a basal site of the square pyramidal array of metals while the carbido ligand projects 0.19 A below the basal plane. In (88) the molybdenum occupies an apical site of the metal octahedron and the carbido carbon is displaced 0.10 A above the Fe 4 plane towards the Mo atom. Unlike (85), only two bridging carbonyl ligands are present. The catalytic reactivity of these heteronuclear species towards carbon monoxide hydrogenation has yet to be determined. Since the reactivity of the carbide atom in the cluster in Scheme 16 was attributed to its electrophilicity, studies of the anionic carbido clusters in Schemes 17 and 18 should be particularly informative in this respect.

-VFe(CO)3

Fc(CO)i

(CO)

(COhFe

(87)

e(CO)2

648

Polynuclear Iron Compounds with Hydrocarbon Ligands

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Comprehensive Organometallic Chemistry