Systematic synthesis of tetranuclear osmium clusters by the reaction of trinuclear clusters with [OsH₂(CO)₄]; crystal structure of [Os₄H₃Br(CO)₁₃]

The complex [OsH₂(CO)₄] reacts with the clusters [Os₃(CO)_12-n(MeCN)_n] (n = 1 or 2) to afford the tetranuclear species [Os₄H₂(CO)₁₅] (1) and [Os₄H₂(CO)₁₄(MeCN)] (2), respectively. The pyrolysis of [Os₄H₂(CO)₁₄(MeCN)] (2) leads to the formation of [Os₄H₂(CO)₁₃] via the intermediate [Os₄H₂(CO)₁₄]. The acetonitrile ligand of complex (2) is readily displaced by P(OMe)₃ to give [Os₄H₂(CO)₁₄{P(OMe)₃}] (3) along with small quantities of [Os₃(CO)₁₄{P(OMe)₃}₂] and [Os₃(CO)₉{P(OMe)₃}₃]. With PPh₃ the reaction is so slow that fragmentation occurs and the only product isolated is [Os₃(CO)₁₀(PPh₃)₂]. The hydride-coupling reaction also occurs between [OsH₂(CO)₄] and [Os₃H(X)(CO)₉(NMe₃)] (X = Cl, Br, or I) and the complexes obtained [(4)-(6) respectively] have been fully characterised. These complexes decompose in solution via loss of HX to afford [Os₄H₂(CO)₁₃]. The reaction between [Os₃H(OR)(CO)₉(MeCN)] and [OsH₂-(CO)₄] proceeds smoothly for R = Me (7) or Ph (8) but for R = H the major product isolated is [Os₃H(OH)(CO)₁₀]. All the products were characterised by i.r., ¹H and ¹³C n.m.r. spectroscopy and structural assignments made. The structure of the cluster [Os₄H₃Br(CO)₁₃] (5) was confirmed by an X-ray analysis and shown to consist of an OS₃ triangle with a pendant OsH(CO)₄ unit co-ordinated to it. One edge of the triangle is bridged by both a hydride and a bromide ligand while another edge is bridged by a hydride.