Cluster activation via cyclometallation. Formation of the cyclometallated triosmium carbonyl cluster [Os3H2(CO)9{P(C6H4)Ph}] and its reactions with phosphorus donor ligands. The X-ray crystal structures of [Os3H2(CO)9{P(C6H4)Ph}] and [Os3H(CO)9{P(OMe)3}(PPh2)]

Abstract

Cyclometallation of [Os₃H(CO)₁₀(PPh₂)](1) proceeds smoothly in n-octane, under reflux, to give [Os₃H₂(CO)₉(P(C₆H₄)Ph}](2) in good yield. An X-ray analysis of this product reveals that the phosphorus atom of the µ-P(C₆H₄)Ph group bridges an edge of the osmium triangle, while the α-carbon of the orthometallated ring occupies an axial co-ordination site on the third Os atom. Although not located directly, one hydride bridges the same Os–Os [2.936(2)Å] as the bridgehead P atom while the second bridges the longest Os–Os edge [3.028(3)Å]. Addition of phosphorus donor ligands to (2) results in the reversal of cyclometallation to give substituted clusters [Os₃H(CO)₉L(PPh₂)][L = P(C₆H₄Me-p)₃(3), P(OMe)₃(4), or PPh₂H (5)]. For each of these substituted clusters only a single structural isomer is obtained, but ¹H and ³¹P n.m.r. spectroscopy reveals that it is a different isomer for (3) compared to those for (4) and (5). The structure of [Os₃H (CO)₉{P(OMe)₃}(PPh₂)](4) has been established by an X-ray analysis, which shows that the phosphite ligand occupies an equatorial site on the Os atom of the triosmium triangle which is not involved in the co-ordination to the bridging phosphido- and hydrido-ligands. N.m.r. data are consistent with a similar structure for the phosphine derivative (5), while the magnitudes of the phosphorus-phosphorus coupling indicate that the bulky phosphine in (3) occupies a site on one of the OS atoms associated with the phosphido- and hydrido-bridges. On heating, under reflux, in high-boiling solvents, complexes (3) and (4) decarbonylate and undergo metallation to give [Os₃H₂(CO)₈L{P(C₆H₄)Ph}][L = P(C₆H₄Me-p)₃(6) or P(OMe)₃(7)]. Complex (6) is isolated as a single isomer whereas (7) exists as a mixture of three isomers, the structures of which have been established by n.m.r. spectroscopy. In contrast, when complex (5) is heated in dichloromethane, under reflux, decarbonylation occurs, and metallation of the phosphine group, rather than of the phosphido-ligand, takes place to afford [Os₃H₂(CO)₈(PPh₂)₂](8) in good yield.