Chemistry of Unsaturated Group 6 Metal Complexes with Bridging Hydroxy and Methoxycarbyne Ligands. 4. Carbonyl, Isocyanide, and Diphosphine Derivatives of the Complexes [M2(η5-C5H5)2(μ-COMe)(μ-PR2)2]BF4 (M = W, R = Ph; M = Mo, R = Et)

Abstract

The unsaturated complexes [M2Cp2(μ-COMe)(μ-PR2)2]BF4 [M = W, R = Ph (1a); M = Mo, R = Et (1b); Cp = η5-C5H5] react with CO under pressure [ca. 4 atm (1a) or ca. 60 atm (1b)] to give the corresponding electron-precise dicarbonyl derivatives [M2Cp2(μ-COMe)(μ-PR2)2(CO)2]BF4, which display a parallel arrangement of the carbonyl ligands (W−W = 2.9020(5) Å for the ditungsten complex). The reactions with CNtBu take place rapidly at room temperature to give analogously the corresponding derivatives with parallel isocyanide ligands, cis-[M2Cp2(μ-COMe)(μ-PR2)2(CNtBu)2]BF4. At low temperature, however, a less stable isomer can be detected, which in the ditungsten system is identified as the isomer having a transoid arrangement of isocyanide groups, trans-[M2Cp2(μ-COMe)(μ-PR2)2(CNtBu)2]BF4, while for the dimolydenum system that isomer is identified as the ketenylimine complex [Mo2Cp2{μ-η1:η1-C(OMe)C(NtBu)}(μ-PEt2)2(CNtBu)]BF4, a product resulting from the C−C coupling between carbyne and isocyanide ligands; these products rearrange slowly at room temperature to yield the corresponding and more stable cis isomers. The reactions of compounds 1a,b with the diphosphine Me2PCH2PMe2 (dmpm) yield analogously addition derivatives of formula [M2Cp2(μ-COMe)(μ-PR2)2(μ-dmpm)]BF4, with the diphosphine ligand being placed preferentially either trans to the carbyne or trans to a diphenylphosphide bridge depending on the experimental conditions. The structure of the first isomer was confirmed through an X-ray study on the ditungsten complex (W−W = 2.917(1) Å). Diazoalkanes N2CRR′ (R = H, R′ = SiMe3; R = R′ = Ph) react with the ditungsten carbyne complex 1a without loss of dinitrogen nor coupling to the methoxycarbyne ligand to yield, after spontaneous demethylation of the latter, the neutral diazoalkane complexes [W2Cp2(μ-PPh2)2(κ1-N2CRR′)(CO)]. Carbonylation of the isocyanide complex cis-[W2Cp2(μ-COMe)(μ-PPh2)2(CNtBu)2]BF4 does not induce any coupling process but just partial substitution to give the carbonyl isocyanide derivative cis-[W2Cp2(μ-COMe)(μ-PPh2)2(CO)(CNtBu)]BF4, whereas its reduction with sodium amalgam induces an N−C(tBu) bond cleavage, yielding the cyanide derivative cis-[W2Cp2(μ-COMe)(μ-PPh2)2(CN)(CNtBu)]. The latter complex can be methylated with [Me3O]BF4 to give the methylisocyanide derivative cis-[W2Cp2(μ-COMe)(μ-PPh2)2(CNMe)(CNtBu)]BF4, and this reduction/methylation sequence can be repeated once more to give the bis(isocyanide) complex cis-[W2Cp2(μ-COMe)(μ-PPh2)2(CNMe)2]BF4 via the corresponding cyanide intermediate.