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.