New osmium cluster compounds containing the heterocyclic ligand 2,3-bis-(diphenylphosphino)quinoxaline (dppq): Ligand isomerization and crystal structures of dppq, the isomeric clusters Os3(CO)10(dppq), and HOs3(CO)9[μ-2,3-PhP(η1-C6H4)(Ph2P)quinoxaline]

Treatment of the labile cluster 1,2-Os3(CO)10(MeCN)2 (1) with the diphosphine ligand 2,3-bis(diphenylphosphino)quinoxaline (dppq) at room temperature affords 1,2-Os3(CO)10(dppq) (2b) as the kinetic product of ligand substitution in 84% yield. 2b isomerizes to the thermodynamically more stable dppq-chelated cluster 1,1-Os3(CO)10(dppq) (2c) as the sole observable product under CO at temperatures below 358 K. The kinetics for the conversion of 2b → 2c have been investigated by NMR spectroscopy in CDCl3 over the temperature range 323–353 K, and the reaction was found to exhibit a rate law that is first order in 2b. The calculated activation parameters [ΔH≠ = 25.4(4) kcal/mol; ΔS≠ = −3(1) eu] support an intramolecular isomerization scenario, one that involves the migration of phosphine and CO groups about the cluster polyhedron. The disposition of the dppq ligand in the isomeric Os3(CO)10(dppq) clusters has been established by X-ray crystallography and 31P NMR spectroscopy. Photolysis of 2c at 366 nm leads to CO loss and ortho metalation of one of the aryl groups on the Ph2P moiety to furnish the hydride cluster HOs3(CO)9[μ-PhP(η1-C6H4)(Ph2P)quinoxaline] (3). The isomerization behavior exhibited by 2b follows that of related diphosphine-substituted Os3 clusters prepared by us.