Osmabenzenes from Osmacycles Containing an η2-Coordinated Olefin

• Published in: Chemistry : a European journal Vol. 15; no. 25; pp. 6258 - 6266
• Main Authors: Gong, Lei, Chen, Zhening, Lin, Yumei, He, Xumin, Wen, Ting Bin, Xu, Xin, Xia, Haiping
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 15.06.2009; WILEY‐VCH Verlag
• Subjects: benzonitrile; isomerization; metallacycles; osmabenzene; osmium
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200900214

Osmabenzenes can be easily synthesized from two η2‐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. Treatment of HCCC(CH3)(OH)CHCH2 with [OsCl2(PPh3)3] in dichloromethane yielded the η2‐olefin‐coordinated osmacycle [Os{CHC(PPh3)C(CH2)‐η2‐CHCH2}Cl2(PPh3)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(PPh3)C(CH2)‐η2‐CHCH2}(PhCN)2(PPh3)2]Cl2 (11) by ligand substitution, which reacted further to the intramolecularly coordinated η2‐allene complex [Os{CHC(PPh3)C(CH3)(η2‐CCH2)}(PhCN)2(PPh3)2]Cl2 (12). In contrast, heating a chloroform solution of 9 to the reflux temperature in the presence of excess benzonitrile generated osmabenzene [Os{CHC(PPh3)C(CH3)CHCH}(PhCN)2(PPh3)2]Cl2 (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(PPh3)C(CH3)(η2‐CCH2)}Cl2(PPh3)2] (13) and the monocationic osmabenzene [Os{CHC(PPh3)C(CH3)CHCH}Cl(PhCN)(PPh3)2]BPh4 (15), respectively. Mechanisms for the formation of osmabenzene 14 from 11 and 12 are proposed based on DFT calculations. Osmabenzenes can be easily synthesized from two η2‐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.