Tungsten alkylidyne compound

Our interest in dimetallacyclobutadienes was triggered when Schrock reported the synthesis of alkylidyne tungsten compounds in a metathesis-like reaction involving W=W bonds [Eq. (2)] (18). 

A.J. Carty, Guelph-Waterloo Centre What are the frequencies of the metal-carbon (v(M=C)) stretching frequencies in the tungsten alkylidyne compounds One might expect these to be quite high in view of the x-ray data showing short M=C bonds and evidence of multiple bond reactivity. 

We referred earlier to the significance of reactions at the alkylidyne carbon atoms of the dimetal species. Our studies in this area are in a preliminary stage, but Schemes 1 and 2 summarise some chemistry at the bridged carbon centres for the compounds (1 ) and (3,)(12). It will be noted that protonation of the neutral bridged al ylidyne compounds yields cationic alkylidene species in which one C—C bond of the tolyl group is n2 co-ordinated to tungsten, a feature revealed by both n.m.r. and X-ray diffraction studies. 

Thus (2 ) is prepared by adding [Rh(C2Hlt)2(acac)] to (6), the latter being preformed by treating the mononuclear tungsten alkylidyne compound with [Rh(C0)2(acacl] (6). Similarly, step-wise addition of [Rh(C0)2(n-C9H7)] to r(n-C5H5)(0C)2W = CR] yields ( )(9). 

A number of reactions between M2(OR)6 compounds and alkynes have been found to give alkylidyne compounds by a metathesis of the M=M and C=C bonds (see Sections 15.3.6.4) and evidence has been presented275 for an equilibrium between a ditungstatetrahedrane and a methyl-idyne-tungsten complex supported by BulO ligands (equation 72). 

Further developments are likely as the chemistry of the compounds described above is explored. Moreover, entirely new dimensions may be added. For example, the synthesis of tungsten-alkylidyne complexes with carba-borane ligands with cage structures smaller than the icosahedral C2B9 fragment should result in the isolation of new electronically unsaturated metal cluster and electron-deficient molecules of types as yet unknown. 

It is important to note that this reaction works only with W2(OCMe3)6 and not with any other M2(OR)6 molecule and that while it appears to be general for aliphatic R it does not proceed for R = QH5. It goes under very mild conditions and is virtually quantitative. Since the (Me3CO)3W=CR compounds are alkyne metathesis catalysts, still other tungsten alkylidyne compounds, not obtainable in the above reaction, can be obtained by the following reaction ... 

Molybdenum, tungsten, and rhenium alkylidyne compounds similar to the complexes involved in alkene metathesis can metathesize alkynes via metallacyclobuta-diene intermediates ... 

NMR resonances of the alkylidyne carbon atom and selected IR absorptions for new carbyne complexes are given in Table 11. In other studies, the Mo and NMR spectra of molybdenum and tungsten alkylidyne complexes were determined by Enemark and co-workers (23). The metal atoms in the alkylidyne complexes were found to be less shielded than those in nitrido complexes but more shielded than those in compounds containing metal-metal multiple bonds. 

Bocarsly et al. reported the electronic absorption and emission spectra of a series of tungsten alkylidyne complexes of the type W(CR)X(CO)2L2 (R = Ph, CMe3, and 2-naphthyl X = halide Lj = tmeda, 2py, dppe) 24). For example, the absorptions at 450 and 327 nm for the complex W(CPh)Br(CO)2(tmeda) were assigned to d n Mc and transitions. Emission from the excited state occurs at 630 nm. The lifetime of the electronically excited state was measured to be 0.180 //seconds. Emission was observed only for the aryl-substituted carbyne complexes. (The photoreactivity of these compounds is discussed in Section IV, B.)... 

The tungsten alkylidyne complex (t-BuO)3W(C-t-Bu) [130-132] is capable of catalyzing alkyne metathesis polymerization of cyclic alkynes. In addition, it can be used in acyclic diene metathesis (ADIMET) [133, 134]. The synthesis of the corresponding Mo-compound Mo(CCH2SiMe3)(OAd)3 (Ad=adamantyl) has been reported by C.C. Cummins et al. [135]. Though the active species is not known yet, a mixture of Mo(CO)6 and 4-chlorophenol is nowadays used for purposes of convenience [136]. 

This shows that the t)rpe of product isolated is dependent on the nature of the cyclopentadienyl(alkylidyne)metal reagent used. Thus protonation of an equimolar mixture of the tungsten compoimds 2a and 3a affords 21a as the only product, whereas when the cyclopentadienyl-molybdenum complex 3d is employed with 2a, a mixture of B9 and Bio products results. On the other hand mixtures of 2a with 3b, which bears the sCMe group, yield only complex 22a upon protonation. ot) fhe overall form of the dimetal species 21 and 22 is related to the species 20, however, unlike the latter compound no CH2R boron cage substituent is observed in the products from 2a. The compoimds... 

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