Reversible Isomerization of a Diphosphine Ligand about a Triosmium Cluster: Synthesis,Kinetics,and X-ray Structures for the Bridging and Chelating Isomers of Os_3(CO)_(10)[(Z)-Ph_2PCH=CHPPh_2]

摘要

Substitution of the MeCN ligands in the activated cluster Os_3(CO)_(10)(MeCN)_2 (1) by the unsaturated diphosphine ligand (Z)-Ph_2PCH=CHPPh_2 proceeds rapidly at room temperature to furnish the ligand-bridged cluster l,2-Os_3(CO)_(10)[(Z)-Ph_2PCH=CHPPh_2] (2b).Heating 2b leads to the formation of the thermodynamically more stable chelating isomer 1,1-Os_3(CO)_(10)-[(Z)-Ph_2PCH=CHPPh_2] (2c).The molecular structure of each isomer of 2 has been crystallographically determined,and the ~(31)P NMR data have been recorded.The kinetics for the ligand isomerization have been investigated by UV-vis and ~(31)P NMR spectroscopy in toluene solution over the temperature range of 358-383 K.The reversible nature of the diphosphine isomerization is confirmed by ~(31)P NMR measurements,and reported within are the forward (k_1) and reverse (k_(-1)) first-order rate constants for the bridge-to-chelate rearrangement.On the basis of the activation parameters and lack of CO inhibition on the reaction rate,a nondissociative,intramolecular mechanism involving the migration of one P group from an adjacent osmium center to the other P-substituted osmium center via a mu_2-bridged phosphine species is presented.