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-cyclopentene-1,3-dione (bpcd) with Os₃(CO)₁₀(MeCN)₂ (1) has been explored. The initial substitution product 1,2-Os₃(CO)₁₀(bpcd) (2b) undergoes a nondissociative, intramolecular isomerization to furnish the bpcd-chelated cluster 1,1-Os₃(CO)₁₀(bpcd) (2c) over the temperature range of 323−343 K. The isomerization reaction is unaffected by trapping ligands, yielding the activation parameters ΔH^⧧ = 25.0(0.7) kcal/mol and ΔS^⧧ = −2(2) eu. Thermolysis of 2c in refluxing toluene gives the hydrido cluster HOs₃(CO)₉[μ-(PPh₂)CC{PPh(C₆H₄)}C(O)CH₂C(O)] (3) and the benzyne cluster HOs₃(CO)₈(μ₃-C₆H₄)[μ₂,η¹-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-Os₃(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-Os₃(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-Os₃(CO)₉L(bpcd) (where L = CO, phosphine) via the unsaturated intermediate 1,1-Os₃(CO)₉(bpcd). The kinetics for reductive coupling in HOs₃(CO)₉[μ-(PPh₂)CC{PPh(C₆H₄)}C(O)CH₂C(O)] and DOs₃(CO)₉[μ-(PPh₂-d₁₀)CC{P(Ph-d₅)(C₆D₄)}C(O)CH₂C(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 Os₃ species that precedes the rate-limiting formation of 1,1- Os₃(CO)₉(bpcd). Strong proof for the proposed hydride (deuteride)/arene preequilibrium has been obtained from photochemical studies employing the isotopically labeled cluster 1,1-Os₃(CO)₁₀(bpcd-d_4,ortho), whose bpcd phenyl groups each contain one ortho hydrogen and deuterium atom. Generation of 1,1-Os₃(CO)₉- (bpcd-d_4,ortho) at 0 °C gives rise to a 55:45 mixture of the corresponding hydride and deuteride clusters, re- spectively, 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-Os₃(CO)₁₀(bpcd-d_4,ortho) at elevated temperature and thermolysis of the low-temperature photolysis hydride/deuteride mixture afford an equilibrium mixture of hydride (67%) and deuteride (33%), yielding a K_eq value of 0.49, which in conjunction with the k_H/k_D ratio from 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 polynuclear system, are discussed relative to published work on benzene activation at mono- nuclear 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-Os₃(CO)₉(bpcd), preceding the irreversible P−C cleavage 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.