Hydride, alkyl and carbyne derivatives of the unsaturated heterometallic anion [MoWCp2(μ-PCy2)(μ-CO)2

Abstract

The heterometallic hydride [MoWCp2(μ-H)(μ-PCy2)(CO)4] was prepared in 30% yield through reaction of an equimolar mixture of [Mo2Cp2(CO)6] and [W2Cp2(CO)6] with a two-fold excess of PHCy2 in xylene solution, in a sealed tube at 453 K. The sodium salt of the title anion was then prepared from the latter hydride in a three-step process first involving dehydrogenation with HBF4·OEt2 in dichloromethane to give the cationic derivative [MoWCp2(μ-PCy2)(CO)4](BF4), then reaction of the latter with NaI in refluxing 1,2-dichloroethane to yield the iodide-bridged dicarbonyl complex [MoWCp2(μ-I)(μ-PCy2)(CO)2] and, finally, reaction of the latter with Na(Hg) in tetrahydrofuran solution. Reaction of this anion with (NH4)PF6 gave the hydride [MoWCp2(H)(μ-PCy2)(CO)2], which in solution displays an equilibrium mixture of two isomers, one with terminal carbonyls and the hydride ligand bridging the metal atoms (B), another one with a semibridging carbonyl and the hydride ligand terminally bound to the W atom (T). The prevalence of isomer T was higher than the observed ones in the corresponding homometallic analogues, and there was a clear thermodynamic preference of the hydride ligand for the W site, estimated in some 20 kJ/mol according to density functional theory (DFT) calculations. The title anion reacted selectively with benzyl chloride at room temperature, to give the agostic benzyl-bridged derivative [MoWCp2(μ-κ1:η2-CH2Ph)(μ-PCy2)(CO)2], which displays specific κ1-coordination to the W atom (MoW = 2.580(1) Å), while the analogous reaction with MeI gave a mixture of the related methyl-bridged complex [MoWCp2(μ-κ1:η2-CH3)(μ-PCy2)(CO)2] and its methoxycarbyne-bridged isomer [MoWCp2(μ-COMe)(μ-PCy2)(μ-CO)] in a ratio of ca. 5:1, with the latter corresponding to a chemical behaviour intermediate between those of its homonuclear analogues. Photolysis of the above alkyl complexes with visible-UV light at room temperature resulted in fast decarbonylation followed by dehydrogenation, to give the corresponding carbyne-bridged derivatives [MoWCp2(μ-CR)(μ-PCy2)(μ-CO)] (R = H, Ph) in good yield. This suggests that the cooperative action of Mo and W atoms greatly reduces the thermal barrier of the CH bond cleavage steps required for dehydrogenation of the alkyl ligands in these substrates.