Conversion of a Hydrido–Butenylcarbyne Complex to η²‑Allene-Coordinated Complexes and Metallabenzenes

Treatment of OsCl₂(PPh₃)₃ with HCCCH­(OH)­Et produces the cyclic complex Os­(PPh₃)₂Cl₂(CHC­(PPh₃)­CH­(OH)­CH₂CH₃) (<b>1</b>), which can undergo dehydration to give the hydrido–alkenylvinylidene complex Os­(PPh₃)₂HCl₂(CC­(PPh₃)­CHCHCH₃) (<b>2</b>). Reaction of <b>2</b> with HBF₄ generates the hydrido–butenylcarbyne complex [OsHCl₂(CC­(PPh₃)CH­(Et))­(PPh₃)₂]­BF₄ (<b>3</b>). The complex <b>3</b> evolves into the unstable metallabenzene [(PPh₃)₂(RCN)­ClOs­(CHC­(PPh₃)­CHCHCH)]­BF₄ (<b>4</b>; RCN = benzonitrile, 2-cyanobenzaldehyde, 3-methoxyacrylonitrile, 2-cyanoacetamide) via triple hydrogen eliminations in the presence of excess nitriles in refluxing CHCl₃ in an air atmosphere. The ligand substitution reaction of <b>4</b> with excess CO affords the stable metallabenzene product [(PPh₃)₂(CO)­ClOs­(CHC­(PPh₃)­CHCHCH)]­BF₄ (<b>5</b>). The key intermediates, η²-allene-coordinated osmium complexes [(PPh₃)₂(RCN)­ClOs­(CHC­(PPh₃)­CHCCH₂)]­BF₄ (<b>6</b>; RCN = benzonitrile, 2-cyanobenzaldehyde, 3-methoxyacrylonitrile, 2-cyanoacetamide) can be captured by performing the conversion at room temperature. Remarkably, in the absence of nitriles, reaction of <b>3</b> with excess CO only generates the vinylethenyl complex [(PPh₃)₂(CO)₂ClOs­(CHC­(PPh₃)­CHCHCH₃)]­BF₄ (<b>7</b>). The complexes <b>1</b>–<b>3</b>, <b>5</b>, <b>6a</b>, and <b>7</b> have been structurally characterized by single-crystal X-ray diffraction. Detailed mechanisms of the conversions have been investigated with the aid of density functional theory (DFT) calculations. DFT calculations suggest that the high stablility of the carbonyl coordinated complexes in the conversion inhibits the further transformation to metallabenzene product. However, the transformation is both kinetically and thermodynamically favorable in the presence of the relatively weaker nitrile ligand, which is consistent with the experimental conversion of <b>3</b> to <b>5</b> via unstable metallabenzenes <b>4</b> observed for in situ NMR experiments