Triosmium clusters with 2-pyridylphosphines as ligands

Abstract

Room-temperature reactions of [Os₃(CO)₁₀(MeCN)₂] with the tertiary phosphines (L), diphenyl(2-pyridyl)phosphine, phenyldi(2-pyridyl)phosphine, and tri(2-pyridyl)phosphine, led to clusters of the type[Os₃(CO)₁₁L]1, [Os₃(CO)₁₀L₂]2, and [Os₃(CO)₁₀(µ-L)]3 which is the major product in each case. Clusters 1 and 2 have ligands L co-ordinated through the phosphorus atom alone, whereas 3 contain diaxially co-ordinated bridging ligands through one P and one pyridyl substituent. Two isomers of 3[L = PPh(2-C₅H₄N)₂] are in rapid exchange in solution above room temperature and the clusters 3[L = PPh₂(2-C₅H₄N) or P(2-C₅H₄N)₃] gain a time-averaged plane of symmetry by a process which is the counterpart of this isomerisation; dynamic transfer of pyridine co-ordination between two metal atoms is proposed on ¹H, ¹³C-{¹H} and ³¹P-{¹H}NMR evidence. Protonation of 3[L = PPh₂(2-C₅H₄N)] arrests this process. Thermal treatment of 3[L = PPh₂(2-C₅H₄N)] led to a major product [Os₃(σ-Ph){µ₃-PPh(2-C₅H₄N)}(CO)₉](60%)(crystal structure reported) which contains the five-electron donating phenyl(2-pyridyl) phosphido bridge and a terminal phenyl ligand. Minor products are the cluster [Os₃(µ-C₅H₄N)-(µ-PPh₂)(CO)₁₀](6%)(crystal structure reported), [Os₂(µ-C₅H₄N)(µ-PPh₂)(CO)₆](trace), which is derived from the former, and [Os₃(µ-PhCO){µ₃-PPh(2-C₅H₄N)}(CO)₉](trace). These clusters are generated by cleavage of either phosphorus–phenyl or phosphorus–2-pyridyl bonds; cleavage of a bond within the five-membered ring forming the bridge is less favoured than that of an exocyclic substituent.