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

Abstract The heterometallic hydride [MoWCp 2 ( μ -H)( μ -PCy 2 )(CO) 4 ] was prepared in 30% yield through reaction of an equimolar mixture of [Mo 2 Cp 2 (CO) 6 ] and [W 2 Cp 2 (CO) 6 ] with a two-fold excess of PHCy 2 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 HBF 4 ·OEt 2 in dichloromethane to give the cationic derivative [MoWCp 2 ( μ -PCy 2 )(CO) 4 ](BF 4 ), then reaction of the latter with NaI in refluxing 1,2-dichloroethane to yield the iodide-bridged dicarbonyl complex [MoWCp 2 ( μ -I)( μ -PCy 2 )(CO) 2 ] and, finally, reaction of the latter with Na(Hg) in tetrahydrofuran solution. Reaction of this anion with (NH 4 )PF 6 gave the hydride [MoWCp 2 (H)( μ -PCy 2 )(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 [MoWCp 2 ( μ - κ 1 : η 2 -CH 2 Ph)( μ -PCy 2 )(CO) 2 ], which displays specific κ 1 -coordination to the W atom (Mo W = 2.580(1) A), while the analogous reaction with MeI gave a mixture of the related methyl-bridged complex [MoWCp 2 ( μ - κ 1 : η 2 -CH 3 )( μ -PCy 2 )(CO) 2 ] and its methoxycarbyne-bridged isomer [MoWCp 2 ( μ -COMe)( μ -PCy 2 )( 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 [MoWCp 2 ( μ -CR)( μ -PCy 2 )( 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 C H bond cleavage steps required for dehydrogenation of the alkyl ligands in these substrates.