Osmabenzenes from Osmacycles Containing an η²‐Coordinated Olefin

Osmabenzenes can be easily synthesized from two η 2 ‐coordinated olefin osmacycles in the presence of benzonitrile by means of facile hydrogen‐transfer conversions (see graphic). Mechanisms for the formation of osmabenzenes are proposed based on DFT calculations. magnified image Treatment of HCCC(CH 3 )(OH)CHCH 2 with [OsCl 2 (PPh 3 ) 3 ] in dichloromethane yielded the η 2 ‐olefin‐coordinated osmacycle [Os{CHC(PPh 3 )C(CH 2 )‐η 2 ‐CHCH 2 }Cl 2 (PPh 3 ) 2 ] ( 9 ). Transformations of osmacycle 9 by treatment with benzonitrile under various conditions have been investigated. Reaction of 9 with excess benzonitrile at room temperature afforded the dicationic osmacycle [Os{CHC(PPh 3 )C(CH 2 )‐η 2 ‐CHCH 2 }(PhCN) 2 (PPh 3 ) 2 ]Cl 2 ( 11 ) by ligand substitution, which reacted further to the intramolecularly coordinated η 2 ‐allene complex [Os{CHC(PPh 3 )C(CH 3 )(η 2 ‐CCH 2 )}(PhCN) 2 (PPh 3 ) 2 ]Cl 2 ( 12 ). In contrast, heating a chloroform solution of 9 to the reflux temperature in the presence of excess benzonitrile generated osmabenzene [Os{CHC(PPh 3 )C(CH 3 )CHCH}(PhCN) 2 (PPh 3 ) 2 ]Cl 2 ( 14 ). Complexes 11 , 12 and 14 are in fact isomers. In the absence of excess benzonitrile, the isolated dicationic 12 and 14 readily dissociate the benzonitrile ligands in solution to produce the neutral complex [Os{CHC(PPh 3 )C(CH 3 )(η 2 ‐CCH 2 )}Cl 2 (PPh 3 ) 2 ] ( 13 ) and the monocationic osmabenzene [Os{CHC(PPh 3 )C(CH 3 )CHCH}Cl(PhCN)(PPh 3 ) 2 ]BPh 4 ( 15 ), respectively. Mechanisms for the formation of osmabenzene 14 from 11 and 12 are proposed based on DFT calculations.