Synthesis of Unsaturated Os₃W₂ and Metastable Os₄W Oxo‒Ethylidyne Clusters by Solid-State Pyrolysis. Direct C‒O Bond Cleavage of a Coordinated Ketenyl Ligand

Heterometallic ketenyl complex Os₃(CO)₁₀(μ-H)[C(O)CH₂(W(CO)₃Cp)] (1a) possessing a pendant CpW(CO)₃ substituent was prepared by condensation of Os₃(CO)₁₀(NCMe)₂ with the metal-aldehyde complex CpW(CO)₃CH₂CHO. Pyrolysis of la in the solid state at 185 °C afforded two heteropentametallic cluster compounds, Cp₂W₂Os₃(CO)₁₂(μ-O)(μ₃-CMe) (2) and CpWOs₄(CO)₁₂(μ-O)(μ₃-CMe) (3), demonstrating a unique example of C‒O bond scission of a ligated ketene fragment and a cluster aggregation process. Complex 2 crystallizes in orthorhombic space group Pnma with a = 18.913(4) Å, b = 16.229(3) Å, c = 9.129(2) Å, Z = 4, R = 0.054, and R_w = 0.055 for 2551 observed reflections. The W₂OS₃ cluster that contains 72 cluster valence electrons has a distorted square-pyramidal array of metal atoms with one Os‒H‒Os hydride ligand, one bridging oxo ligand associated with a W‒W double bond, and an ethylidyne ligand bridging a unique W₂Os triangle. Crystals of 3 are monoclinic of space group P2₁/n with a = 15.941(2) Å, b = 19.395(4) Å, c = 16.369(4) A, β = 94.72(2)°, Z = 8, R = 0.050, and R_w = 0.041 for 6568 observed reflections. This molecule possesses a WOs₄ trigonal-bipyramidal skeleton with an oxo ligand bridging a W‒Os edge and an ethylidyne ligand capping a WOS₂ face. Isomerization of 3 occurred upon dissolution at room temperature to afford edge-bridging tetrahedral cluster CpWOs₄(CO)₁₂(μ₃-O)(μ₃-CMe) (4), in which the oxo ligand formally migrated from an edge-bridging to a face-bridging position. Pyrolysis of 4 in the solid state at 190 °C regenerated 3 in high yield, suggesting the existence of a delicate equilibrium between these two cluster complexes. Crystal data for 4: space group P2₁/n, a = 9.492(3) Å, b = 16.990(2) Å, c = 15.887(3) Å, β = 98.79(2)°, Z = 4. The structure was solved by direct methods and refined to R and R_w values of 0.039 and 0.032 for 4455 observed reflections with I > 2σ(I).