Synthesis and diphosphine ligand fluxionality in Os3(CO)10(bmi): Kinetic evidence for nondissociative diphosphine isomerization and X-ray crystal structure of 1,1-Os3(CO)10(bmi)

The triosmium cluster 1,2-Os3(CO)10(MeCN)2 reacts rapidly with the diphosphine ligand 2,3-bis(diphenylphosphino)-N-p-tolylmaleimide (bmi) at room temperature to give bmi-bridged cluster 1,2-Os3(CO)10(bmi) (2b) as the major product, along with the chelating isomer 1,1-Os3(CO)10(bmi) (2c) and the hydride-bridged cluster HOs3(CO)9[μ-(PPh2)Cdouble bond; length as m-dashC{PPh(C6H4)}C(O)N(tolyl-p)C(O)] (3) as minor by-products. All three cluster compounds have been isolated and fully characterized in solution by IR and NMR spectroscopies (1H and 31P), and X-ray crystallography in the case of 2c. Cluster 2b is unstable and readily isomerizes to 2c in quantitative yield on mild heating. The kinetics for the conversion of 2b → 2c have been measured over the temperature range of 318–348 K in toluene solution, and based on the observed activation parameters a nondissociative isomerization process that proceeds via a transient μ2-bridged phosphine moiety is presented. Near-UV photolysis of cluster 2c at room temperature affords HOs3(CO)9[μ-(PPh2)Cdouble bond; length as m-dashC{PPh(C6H4)}C(O)N(tolyl-p)C(O)] (3) with a quantum yield of 0.017. The reactivity of clusters 2b, 2c, and 3 is discussed with respect to related diphosphine-substituted Os3(CO)10(P–P) clusters prepared by our groups.