Acetaldehyde complexes of molybdenum

Polyhedron Vol. 5, No. l/2, pp. 451-459, Printed in Great Britain

1986

ACETALDEHYDE J. TREVOR

0

COMPLEXES

0277-5387/86 S3.00+ 1986 Pegamon Pnm

.OO Ltd

OF MOLYBDENUM

and MARK J. WINTER*

GAUNTLETT

Department of Chemistry, The University, Sheffield S3 7HF, U.K. (Received 17 June 1985) Abstract-Addition of LiEt3BH to a solution of MoMe(CO),(r&H,) allows the successive observation of the formyl alkyl [Mo(CHO)Me(CO),(+Z,H~)]-, the hydrido acyl [Mo(COMe)H(CO),(u-CsH&]-, and the ultimate product, a species formulated as a complex of coordinated acetaldehyde, [Mo(CO),(q*-MeCHO)(q-C,H5)] -. All three anions are spectroscopically characterized. Addition of Me1 to the acetaldehyde complex in the presence of PPh, or CO results in smooth formationof trans-MoMe(CO),(PPh,)(+,H,) or MoMe(CO)&-C,H,) together with the evolution of acetaldehyde ; the latter case is therefore a stoicheiometric synthesis of acetaldehyde. The reaction of PhCH,Br with [Mo(CO),(#MeCHO)(r&H,)]provides a high-yield synthesis of Mo(CO),($-CH,C,H&-C,HS) while the q3-ally1 complex Mo(CO),($-C,H,)(r,i-C,H,) is formed in the corresponding reaction of ally1 bromide. The position of alkylation is different using [Me,O] [BF,] and the arises from alkylation of the aldehyde primary product Mo(~2-CHMeOMe)(CO)2(+,H,) oxygen atom. This species reacts with PPhJ with 0 + MO bond cleavage and formation of Mo(CHMeOMe)(CO)2(PPh,)(q-C,H,). According to the conditions of workup the cationic ylide complex [Mo(CHMePPh,)(CO),(PPh&-C,H,)] [BF,] can also be formed. Reaction of [Mo(t72-MeCHO)(CO)2(r&Hs)]with several oxidizing agents, such as Fe(III), [Fe(qC5H5)2]+, [C,H,]+ or [Mo(CO),(@,H,)]+, leads to the q2-acetaldehyde complex Mo2(CO),(~-?2-MeCHO)(+C,H,), in which the carbon atom of the aldehyde is bound to a single metal but the oxygen atom bridges both metal atoms. Formally the aldehyde is a four-electron donor to the dimolybdenum system. The same product arises in the reaction ofboth [M(CO),(+,H,)] [BF,] (M = MO or W) with [Mo(CO),(q2-MeCHO)(@,H,)] -, no heterobimetallic aldehyde compounds are formed suggesting that Mo,(CO),@-q2MeCHO)(r&H,), is formed after an initial electron-transfer process.

A short time ago we reported syntheses of cyclic carbene complexes 1 and 2 through the iodideinduced cyclization of M([CH,],X)(CO),(QC,H,) (3) (Scheme 1).‘v2Other nucleophiles such as CN-, SPh- or [W(CO),(q-C,H,)]are suitable for this cyclization24 and a related reaction leading to a cationic species is also known.5*6 Perhaps naively we thought that a nucleophilic hydride source might induce a similar such cyclization so forming a hydride carbene of type 4. It occurred to us that such a species might react with nucleophiles and that it would be possible to observe hydride to carbene migrations. This is important since, although such processes are often invoked to explain aspects of carbene chemistry, well-defined examples are rare. * Author to whom correspondence should be addressed.

One such hydride source is LiEt,BH. In fact, the major isolated product from the reaction of LiEtsBH and 3 is compound 5 rather than 4.’ It is possible to view formation of 5 as proceeding by an analogous cyclization step forming the hydrido carbene 4 which subsequently undergoes a hydride to carbene migratory insertion reaction in which the ring oxygen atom functions as an intramolecular source of nucleophile (Scheme 2, path A). Compound 5 is also obtained from the reaction of carbene 1 with LiEt3BH, although probably by a different mechanism.s These processes are quite complex and one of the ways we attempted to understand these results was to initiate a study on the interactions of hydride-donor reagents with molybdenum complexes containing simpler alkyl functions. A suitable reaction is that of MoMe(CO)&-C,H,) (6) with

451

J. T. GAUNTLETT

452

and M. J. WINTER

e

oc-, rAfl

oi’aco

-0 I’

X

>

oc-M

4 r’l 1,

/

\

co

e.

+Pi /\*

<

oc”co 1 M=Mo

2M=W Scheme 1. Nucleophile-induced

-0 H-

40

// \

Oc'

cyclization of Mo[(CH,),X](CO),(r,4,H,).

==a cO

4

7 R=Me IIR=Et JZR=CH

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oc+q I/ 0

cf 5 5

x

453

Acetaldehyde complexes of molybdenum

Br

_H

oc14

d // $T-\/

CF

-

B

0

? Bf

I _H oc-

,!“\o 5

OC' 5

Scheme 2. Two possible pathways for the formation of Mo[~~-CH(CH,),O](CO),(~~-C,H,) reaction of Mo[(CH,),Br](CO),(q-C,H,) with LiEt,BH.

LiEt,BH, the initial aim being to identify the position of hydride attack at the metal complex. This reaction proceeds readily at room temperature and the ultimate product is a spectroscopically characterized anion formulated as [Mo(CO),(q’MeCHO)(+,H,)] - (7).‘*lo We have succeeded in isolating 7 with a 12-crown-4 lithium complex as counter ion as a solid but have yet to obtain suitable crystals for X-ray analysis. Until we have a crystal structure of this molecule we choose to depict this molecule with the representation 7, a metallaoxirane, in common with representations of other aldehyde or ketone complexes that have been crystallographically characterized.’ l--l4 However, the alternative representation as 8 also seems reasonable. It is noteworthy that the IR spectrum of the crown complex is identical to that of the parent species in THF solution, suggesting that 7 exists as solvent-separated ion pairs in solution rather than contact ion pairs. It is also possible to write dimeric

from the

structures for this species: such a possibility is currently being examined but the chemistry of 7 delineated thus far can be rationalized satisfactorily using 7 as a representation. The anion 7 is not the primary product: by initiating the reaction at -78°C and allowing the reaction to warm slowly to room temperature it is possible to identify two intermediates, the formyl alkyl 9 and the hydrido acyl 10 (Scheme 3) by monitoring the ‘H and l %ZNMR and the IR spectra at low temperatures. ‘JO Formyl complexes arising from hydride additions to carbonyls are now wellknown, although there are comparatively few molybdenum examples. ’ 5-1s They are characterized by high-frequency signals in the ‘H NMR spectrum,‘g in this case a signal is observed at 6 14.3. The relative intensities of the bands in the carbonyl IR spectrumZo and the presence of only a single carbonyl signal in the 13C NMR spectrum are in accord with the truns geometry depicted. The

J. T. GAUNTLETT

454

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and M. J. WINTER

LiEt3BH

7

>

IO

Scheme 3. Stoicheiometric synthetic cycle for acetaldehyde based on MoMe(CO)&C,H,).

rearrangement to the hydrido acyl is unusual although there are some examples of complexes containing both hydride and acyl ligands.21-24 Compound 10 is characterized by a low-frequency signal (6 - 5.15), indicative of a metal hydride. We presume that the driving force converting 9 to 10 is the higher bond energy associated with an MO-H bond relative to an Mo-alkyl bond. Compound 10 undergoes a formal migration of the hydride to the acyl carbon (one could also view this as a hydride to carbenoid atom migration). We do not yet know whether this step is inter- or intramolecular:The important feature of this last step is that a hydride ligand and an acyl ligand combine at a metal to produce a ligated aldehyde. The closely related complexes [Mo(CO)&‘RCHO)(&H,)](11, R = Et; and 12, R = CH,Ph) are formed in the reactions of MoR(CO)&C,H,) with LiEt,BH. Although not fully spectroscopically characterized the IR spectra in the carbonyl region, at least, are identical to that of 7 in THF. Formation of 7 in this reaction allows us to comment on the formation of 5 in the reaction of 1 with LiEt3BH. There are two items of information relevant here. The results of monitoring by lowtemperature NMR spectroscopy suggest that the initial product is a formyl complex (13) [G(THF) 14.21. The results of IR monitoring clearly shows

that the ultimate product, 5, is produced at the expense of an intermediate whose IR spectrum in the carbonyl region is identical to that of 7. This species is almost certainly 14. Although we have not unambiguously identified a hydride resonance in the NMR spectrum of this reaction upon warming it does seem likely that 15 is an intermediate by analogy to the formation of 7. Therefore, compound 5 is formed through ring closure from an anionic aldehyde complex 14 (Scheme 2, path B) rather than migration of hydride to a carbene ligand (Scheme 2, path A). Having established a working model for the mechanism of formation of 5 an investigation of the chemistry of 7 was clearly required, with views to contrasting its reactivity with that of [Mo(CO)& C,H,)]-, to see if the aldehyde could be liberated, and to determine whether a modified system could catalytically generate aldehydes. Addition of Me1 to a THF solution containing 7 results in a very slow reaction whose only identified ultimate product is a low yield of 6, some of which at least arises from reaction of the very small quantities of [Mo(CO),(q-C,H,)]generally present in solutions of 7 with MeI. Even a prolonged reaction time does not result in any observed products in the reaction of 7 with PPh,: in particular, there is no tendency for the PPh, to exchange with the MeCHO

Acetaldehyde complexes of molybdenum

ligand in the anion 7.” However, when 7 is treated with Me1 in the presence of PPh, the phosphine complex trans-MoMe(CO),(PPh,)(q-C,H,) (16) is formed smoothly. We regard this as proceeding by nucleophilic replacement of the I- of Me1 by anion 7, giving an undetected intermediate MoMe(CO),(MeCHO)(@,H,) (17) (Scheme 4) from which MeCHO is evolved through substitution of the labile MeCHO by the two-electron ligand PPh,. An alternative two-electron ligand is provided by CO itself. Addition of Me1 to a solution of 7 maintained under a CO atmosphere (1 atm) provides the alkyl6in very good yield, together with MeCHO, which may be analysed by GLC and GC-mass spectrometry. lo These two reactions probably share the same intermediate (17). The last result is particularly interesting in that it closes a stoicheiometric cycle for the synthesis of acetaldehyde (Scheme 3). Both these two reactions employ an external or intermolecular source of a two-electron ligand. It is also possible to arrange matters so that the new twoelectron ligand is provided by an intramolecular source. Addition of ally1 bromide to a solution of 7 swiftly results in formation of the known q3-ally1 derivative, Mo(CO)~(~~-C~H,)(~-CSH,) (18), isolated in moderate yield after chromatography and crystallization. 25 The probable intermediate is 19, a labile complex from which MeCHO is displaced by

7

RX

455

intramolecular attack of the pendant ene group at the metal. An interesting related phenomenon is observed in the reaction of 7 with benzyl bromide. Here the major product is the q3-benzyl Mo(C0)2(~3-CH2Ph)(~-CSHS) (20), isolated in 64% yield after chromatography and crystallization. This is a known compound, 26 but the previous synthesis is of low yield and relatively arduous. The $-benzyl Mo(#-CH2Ph)(CO)3(q-C5H5) is isolated in very low yield as a byproduct, probably arising from reaction of the small quantities of [Mo(CO),(qC,H,)] - present in the solution of 7 with PhCH,Br. Compound 20 will be formed from the intermediate 21: in this case the aromatic ring itself is the intramolecular source of nucleophile. These reactions rely upon the nucleophilic replacement of halide from an organic halide: effectively this is the formal addition of an R+ group to the metal. Another source of R+ is provided by the reagent [Me,O][BF,], but it is quite common for it to display a different reactivity with organotransition-metal substrate when compared to RX. This is the case here. Addition of solid [Me,O][BF,] to a THF solution of 7 at -78°C followed by warming towards ambient temperature provides a solution whose IR spectrum in the carbonyl region is superimposable upon that of 5, suggesting that methylation occurs at the oxygen

R-

eaI

MO1' \

I7 R=Me

0

II A H Me

19 R =aJlyl 21 R =PhCH2

6

\

R=Me;PP% \

Phi$'

20

18

Scheme 4. Reactions of [Mo(q2-MeCHO)(CO),(t&HJ-

16 with C,H,Br, PhCH,Br and MeI-PPh,.

456

J. T. GAUNTLETT

-iUlCl* M.

atom and that the product is 22. Regrettably the product is extremely sensitive to air and decomposes upon attempted chromatography, so it remains unisolated as a pure material. Compound 5 reacts with PPh, to give 23, in which the MO t 0 bond is cleaved by the two-electron PPh, ligand. This is entirely reasonable, since one can regard the oxygen-molybdenum interactionin 5 as an ether coordinated to a metal and therefore expected to be very labile. Analogus behaviour is observed for 22, supporting the structural assignment. This reaction is conveniently carried out by allowing a solution of 7 in THF to warm up from -78°C to ambient temperature in the presence of solid PPh, and solid [Me,O] [BF,]. Neither reagent is particularly soluble under these conditions, but during the time taken to warm up the IR spectrum of the reaction mixture changes first to one indicating formation of 22 (that is, identical to the IR spectrum of the reaction involving [Me,O][BF,] alone) but whose bands in turn are replaced by two new bands typical of a trans-MoR(CO),(PPh,)(?-C,H,) molecule and which overlap with those of the cyclic alkyl 23. Provided the reaction mixture is filtered through alumina to remove excess [Me,O][BF,]

_ __-_. .-__

J. WINICK

23 before removal of solvent the expected transMo(CHMeOMe)(CO),(PPh&C,H,) (24) is isolable, although somewhat reactive. The ‘H NMR spectrum of 24 is informative and, in particular, a signal (G(CDC1,): 5.15 [d of q, 1 H, J(HH) 6.5, J(HP) 3 Hz]} is as expected for the CH proton coupled to the protons of the methyl group and the trans P atom. If the reaction is not filtered prior to solvent removal a further reaction proceeds (during workup) with excess [Me,01 [BF,] and a cationic ylide (25) is isolable along with lesser quantities of other minor neutral products. We believe that attack of excess [Me,O] [BF,] upon 24 removes the OMe- function (Scheme 5) and the resulting transient cationic

i >

ii

.. . /

25

III

24

Scheme 5. Reactions of [Mo($-MeCHO)(CO),(r-C,H,IIwith [Me,O][BF,]-PPh,. (i) [MeJO][BF,] alone, (ii) [Me,O][BF,] and PPh, (filtration before solvent removal), and (iii) [Me,01 [BF,] and PPh, without filtration before solvent removal.

457

Acetaldehyde complexes of molybdenum alkylidene is stabilized by a second molecule of PPh,. Clearly [Me,01 [BF,] alkylates 7 preferentially at the oxygen of coordinated acetaldehyde rather than at the metal as is the case for Me1 although there is evidence for alkylation at the metal to a very minor degree, small quantities of the known methyl complex MoMe(C0)2(PPh,)(r&H5) (16) are also isolable from the reaction.27*28 The product obtained by oxidation of 7 with any one of several oxidizing agents was unexpected to us. Addition of, for instance, [Fe(q-C,H,),] + to a THF solution of 7 results in the formation of 26 in 11% yield. The structure of 26 is the subject of an X-ray crystallographic study” and shows the MeCHO ligand to bridge the dimolybdenum centre in an asymmetric fashion. There is also a semi-bridging carbonyl of the type known for many related molecules of MO, and W2.30-34 The Mo-MO bond length is consistent with a single Mo-MO bond.34*35 One can view the bonding as coordination of the C=O bond to one molybdenum atom and donation of an oxygen lone pair to the second. Acetaldehyde therefore functions as a four-electron ligand in this molecule. The analogous complex Mo~(CO)~(~-~~PhCH2CHO)(@,H,)2 is also available by oxidation of the benzyl species (12). In a formal sense, this molecule is an addition product of MeCHO across the Mo-MO triple bond of [Mo(CO),(@,H,)]~. However, we have, as yet, been unable to synthesize the molecule by this route either by photochemical or thermal means. There is literature precedent for some sulphur-containing analogues. Addition of R’R’C=S takes place smoothly across dimolybdenum of [Mo(CO),(qC5H5)12 for a wide range of R’ and RZ to give species 27 with a Mo,CS core whose structure is completely analogous to the Mo,CO core of 26.36 These molecules are fluxional on the NMR time scale upon warming but analogous behaviour is not observed for 26 since decomposition occurs upon warming to 60°C in C6D6. One possibility for the reaction mechanism is for the oxidizing agent to accept an electron from the anion 7, giving a 17-electron species (Scheme 6) which dime&es with loss of MeCHO to give 26. In support of this acetaldehyde is detected in the reaction mixture by GC-mass spectrometry. Other one-electron acceptors such as Fe(III), provide the LW-M + or [Mo(Co),(r~-C,H,)]+ same product but in each case there are byproducts whose nature depends upon the actual oxidizing agent employed. Reaction of [Mo(CO),(~&H,)][BF,]~’ with 7 in THF similarly results in the formation of 26(21x), together with quantities of [Mo(CO)~(~&H,)],

(28) (13x), but in this case there is the question as to whether both MO atoms of 26 originate from the anionic species or one from each reagent. This question is probably answered by the result of employing ~(CO),(~-C,H,)][BFJ3’ in place of the MO cation. The only product containing an aldehyde ligand isolated is 26, containing two MO atoms. There is no sign of either of the two possible heterobimetallic species 31 or 32. The other products isolated from chromatography are all three complexes, (+Z,H,)(OC)3M’-M2(C0)3(r&HS) (2g-30). This implies that 26 is formed by an electrontransfer process as for the other oxidizing agents. The formation of all these three possible hexacarbonyl compounds can be rationalized similarly and by making the assumption that when the 17-electron

31

M:Mo,Mlt=W

32 M”=W , &MO species combine in all possible permutations to form metal-metal bonds (Scheme 7) one or two moles of acetaldehyde are then lost and disproportionation reactions give the isolated hexacarbonyls. In no case is it necessary to invoke a direct metal-metal bond formation between 7 and ~(CO),(~-C,H,)] [BF,] as the initial step, but this does not dismiss this possibility as accounting for at least some of the heterobimetallic product.

458

J. T. GAUNTLETT

and M. J. WINTER

26 Scheme 6. Reaction of [Mo(r,r’-MeCHO)(CO),(r&H~)]-

oc-

with [Mo(CO),(r]-C,H,)][BF,].

IA

-FBF3 / \ rf r

Scheme 7. Reaction of [Mo(q2-MeCHO)(CO)2(‘]-C,H,II-

Acknowledgements-We should like to acknowledge generous support from the S.E.R.C. and The Royal Society, and the S.E.R.C. for a research studentship (to J.T.G).

3. 4.

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5. 6. 7.

;28

M’-&MO

29

M’=M”=W

30

M’=Mo

M’LW

with [W(CO),(t&H,)][BF,].

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