Ortho-Metalation Dynamics and Ligand Fluxionality in the Conversion of Os3(CO)10(dppm) to HOs3(CO)8[-PhP(C6H4-2,1)CH2PPh2]: Experimental and DFT Evidence for the Participation of Agostic C-H and -Aryl Intermediates at an Intact Triosmium Cluster
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The mechanism for the stepwise conversion of Os3(CO) 10(dppm) to HOs3(CO)9[-PhP(C 6H4)CH2PPh2] and HOs 3(CO)8[-PhP(C6H4- 2,1)CH2PPh2] has been investigated. The octacarbonyl cluster HOs3(CO)8[- PhP(C6H4-2,1)CH 2PPh2] is in rapid equilibrium with the isomer containing a metalated phenyl ring that is bound to a single osmium, HOs 3(CO)8[-PhP(C6H4- 1)CH2PPh2], which in turn captures CO to give HOs3(CO)9[-PhP(C6H4)CH 2PPh2] in a reaction that is first-order in cluster and CO with a rate constant of 23.9(3) 10-3 M-1 s -1 at 288 K. The kinetics for the transformation of HOs 3(CO)9[-PhP(C6H4)CH 2PPh2] to Os3(CO)10(dppm) have been studied in toluene over the temperature range 317-340 K and found to be first-order in starting cluster and independent of CO. Important insight into the reductive coupling process was obtained from the carbonylation kinetics employing DOs3(CO)9[-(Ph-d2)P(C 6H3D)CH2PPh2-d4], which was prepared from Os3(CO)10(dppm-d8) and where all of the ortho sites on the aryl groups contained deuterium. Here, a significant inverse isotope effect (kH/kD = 0.50) was found, whose origin actually derives from an inverse equilibrium isotope effect, and this supports a preequilibrium phase of the reaction involving a hydride (deuteride) cluster and an intact Os3 cluster containing a coordinated aryl moiety, prior to the rate-limiting formation of the unsaturated cluster Os3(CO)9(dppm-Pa,Pe). Experimental proof for the intermediacy of an extremely labile cluster-aryl complex(es) in the proposed preequilibrium step has been demonstrated by photochemical experiments using the isotopically labeled cluster Os 3(CO)10(dppm-d4), where each aryl group contains one ortho hydrogen and deuterium atom. Photolysis of Os 3(CO)10(dppm-d4) in toluene-d8 gives a 70:30 mixture of the octa- and nonacarbonyl HOs3(CO) 8,9[-(Ph-d1)P(C6H3D)CH 2PPh2-d4] and DOs3(CO) 8,9[-(Ph-d1)P(C6H4)CH 2PPh2-d4], respectively, over the temperature range 243-298 K. These data indicate that the intermediates formed after CO loss are kinetically labile and that the ortho metalation leads to a thermodynamically equilibrated mixture of hydride and deuteride clusters. DFT calculations corroborate the experimental findings and provide crucial details on the nature and lability of those cluster species that do not lend themselves to direct spectroscopic observation. 2010 American Chemical Society.
