Metal-Mediated Reductive-Coupling of 2-Ethynylpyridine to Cluster-Bound 1,3-Functionalized Allenes

High Resolution Image Download MS PowerPoint Slide Reductive-coupling of 2-ethynylpyridine (HC≡C-py, py = 2-NC 5 H 4 ) upon reaction with [Os 3 (CO) 8 (μ-H) 2 (μ-dppm)] ( 1 ) affords three isomeric clusters containing 1,3-functionalized allene ligands, namely [Os 3 (CO) 7 (μ-dppm){μ 3,κ 1 (N)κ 1 (N)-pyC(Me)CC(H)py}] ( 2 ) and [Os 3 (CO) 7 (μ-dppm){μ 3,κ 1 (N)κ 1 (N)-pyCH 2 CHCC(H)py}] ( 3-4 ) which have been separated and characterized by single crystal X-ray diffraction studies. Each contains an open 50-electron triosmium framework, the functionalized allene ligand acting as an 8-electron donor via coordination of three carbons and the two pyridyl nitrogens. Cluster 2 contains a trisubstituted allene, while 3 and 4 (which interconvert) contain disubstituted allenes. DFT studies have been used to assess their relative energies. They are proposed to result from initial migratory alkyne insertion into a hydride, leading to competitive formation of either α– and β–alkenyl complex, followed by insertion of a second hydride to afford mono- or disubstituted alkylidenes, which react with a second equivalent of 2-ethynylpyridine, likely following its conversion to a metal-bound vinylidene.