Diphosphine isomerization and C-H and P-C bond cleavage reactivity in the triosmium cluster Os3(CO)10(bpcd):: Kinetic and isotope data for reversible ortho metalation and X-ray structures of the bridging and chelating isomers of Os3(CO)10(bpcd) and the benzyne-substituted cluster HOs3(CO)8(μ3-C6H4)[μ2,η1-PPhC=C(PPh2)C(O)CH2C(O)]

The coordination and reactivity of the diphosphine ligand 4,5-bis(diphenylphosphino)-4-cyclopentene1,3-dione (bpcd) with Os-3(CO)(10)(MeCN)(2) (1) has been explored. The initial substitution product 1,2-Os-3(CO)(10)(bpcd) (2b) undergoes a nondissociative, intramolecular isomerization to furnish the bpcd-chelated cluster 1,1-Os-3(CO)(10)(bpcd) (2c) over the temperature range of 323-343 K. The isomerization 7 reaction is unaffected by trapping ligands, yielding the activation parameters Delta H-double dagger = 25.0(0.7) kcal/mol and Delta S-double dagger = -2(2) eu. Thermolysis of 2c in refluxing toluene gives the hydrido cluster HOs3(CO)(9)[mu-(PPh2)C=C{PPh(C6H4)C(O)CH2C(O)] (3) and the benzyne cluster HOs3(CO)(8)(mu(3)-C6H4)[mu(2), eta(1)-PPhC= C(PPh2)C(O)CH2C(O)] (4). Time-concentration profiles obtained from sealed-tube NMR experiments starting with either 2c or 3 suggest that both clusters are in equilibrium with the unsaturated cluster 1,1-Os-3(CO)(9)(bpcd) and that the latter cluster serves as the precursor to the benzyne-substituted cluster 4. The product composition in these reactions is extremely sensitive to CO, with the putative cluster 1,1-Os-3(CO)(9)(bpcd) being effectively scavenged by CO to regenerate 2c. Photolysis of cluster 2c using near-UV light affords 3 as the sole product. These new clusters have been fully characterized in solution by IR and NMR spectroscopy, and the molecular structures of clusters 2b,c, and 4 have been determined by X-ray crystallography. Reversible C-H bond formation in cluster 3 is demonstrated by ligand trapping studies to give 1,1-Os-3(CO)(9)L(bpcd) (where L = CO, phosphine) via the unsaturated intermediate 1,1-Os-3(CO)(9)(bpcd). The kinetics for reductive coupling in HOs3(CO)(9)Lu-(PPh2))C=C{PPh(C6H4)}C(O)CH2C-(O)] and DOs3(CO)(9)[mu-(PPh2-d(10))C=C{P(Ph-d(5))(C6D4)}C(O)CH2)C(O)] in the presence of PPh3 give rise to a k(H)/k(D) value of 0.88, a value that supports the existence of a preequilibrium involving the hydride (deuteride) cluster and a transient arene-bound Os-3 species that precedes the rate-limitino, formation of 1,1-Os-3(CO)(9)(bpcd). Strong proof for the proposed hydride (deuteride)/arene preequilibrium has been obtained from photochemical studies employing the isotopically labeled cluster whose C1,1-Os-3(CO)(10)(bpcd-d(4,ortho)), bpcd phenyl groups each contain one ortho hydrogen and deuterium atom. Generation of 1,1-Os-3(CO)(9)(bpcd-d(4,ortho)) at 0 degrees C gives rise to a 55:45 mixture of the corresponding hydride and deuteride clusters, respectively, from which a normal KIE of 1.22 is computed for oxidative coupling of the C-H(D) bond in the ortho metalation step. Photolysis Of 1,1-Os-3(CO)(10)(bpcd-d(4,ortho)) at elevated temperature and then-nolysis of the low-temperature photolysis hydride/deuteride mixture afford an equilibrium mixture of hydride (67%) and deuteride (33%), yieldincr a K-eq value of 0. 49, which in conjunction with the k(H)/k(D) ratio rom the C-H(D) ortho-metalation step allows us to establish a k(H)/k(D) value of 0.60 for the reductive coupling from the participant hydride/deuteride clusters. These data, which represent the first isotope study on ortho metalation in a polynticlear system, are discussed relative to published work on benzene activation at mononuclear rhodium systems. UV-vis kinetic data on the transformation 3 - 4 provide activation parameters consistent with the rate-limiting formation of the unsaturated cluster 1,1-Os-3(CO)(9)(bpcd), preceding the irreversible P-C cleavaue manifold. The ortho metalation of the bpcd ligand in 3 and formation of the benzyne moiety 4 are discussed relative to ligand degradation reactions in this genre of cluster.