Conversion of a Hydrido–Butenylcarbyne Complex
to η<sup>2</sup>‑Allene-Coordinated Complexes and Metallabenzenes
- Publication date
- Publisher
Abstract
Treatment
of OsCl<sub>2</sub>(PPh<sub>3</sub>)<sub>3</sub> with
HCCCH(OH)Et produces the cyclic complex Os(PPh<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub>(CHC(PPh<sub>3</sub>)CH(OH)CH<sub>2</sub>CH<sub>3</sub>) (<b>1</b>), which can undergo dehydration to
give the hydrido–alkenylvinylidene complex Os(PPh<sub>3</sub>)<sub>2</sub>HCl<sub>2</sub>(CC(PPh<sub>3</sub>)CHCHCH<sub>3</sub>) (<b>2</b>). Reaction of <b>2</b> with HBF<sub>4</sub> generates the hydrido–butenylcarbyne complex [OsHCl<sub>2</sub>(CC(PPh<sub>3</sub>)CH(Et))(PPh<sub>3</sub>)<sub>2</sub>]BF<sub>4</sub> (<b>3</b>). The complex <b>3</b> evolves into the unstable metallabenzene [(PPh<sub>3</sub>)<sub>2</sub>(RCN)ClOs(CHC(PPh<sub>3</sub>)CHCHCH)]BF<sub>4</sub> (<b>4</b>; RCN = benzonitrile, 2-cyanobenzaldehyde, 3-methoxyacrylonitrile,
2-cyanoacetamide) via triple hydrogen eliminations in the presence
of excess nitriles in refluxing CHCl<sub>3</sub> in an air atmosphere.
The ligand substitution reaction of <b>4</b> with excess CO
affords the stable metallabenzene product [(PPh<sub>3</sub>)<sub>2</sub>(CO)ClOs(CHC(PPh<sub>3</sub>)CHCHCH)]BF<sub>4</sub> (<b>5</b>). The key intermediates, η<sup>2</sup>-allene-coordinated
osmium complexes [(PPh<sub>3</sub>)<sub>2</sub>(RCN)ClOs(CHC(PPh<sub>3</sub>)CHCCH<sub>2</sub>)]BF<sub>4</sub> (<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
[(PPh<sub>3</sub>)<sub>2</sub>(CO)<sub>2</sub>ClOs(CHC(PPh<sub>3</sub>)CHCHCH<sub>3</sub>)]BF<sub>4</sub> (<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
