Interconversion of Metallabenzenes and Cyclic η2‐Allene‐Coordinated Complexes

AbstractTreatment of the osmabenzene [Os{CHC(PPh3)CHC(PPh3)CH} Cl2(PPh3)2]Cl (1) with excess 8‐hydroxyquinoline produces monosubstituted osmabenzene [Os{CH C(PPh3) CHC(PPh3)CH}(C9H6NO)Cl(PPh3)]Cl (2) or disubstituted osmabenzene [Os{CHC(PPh3)CHC(PPh3)CH} (C9H6NO)2]Cl (3) under different reaction conditions. Osmabenzene 2 evolves into cyclic η2‐allene‐coordinated complex [Os{CHC(PPh3)CH=(η2‐CCH2)}(C9H6NO)(PPh3)2]Cl (4) in the presence of excess PPh3 and NaOH, presumably involving a PC bond cleavage of the metallacycle. Reaction of 4 with excess 8‐hydroxyquinoline under air affords the SNAr product [(C9H6NO)Os{CHC(PPh3)CHCHC} (C9H6NO)(PPh3)]Cl (5). Complex 4 is fairly reactive to a nucleophile in the presence of acid, which could react with water to give carbonyl complex [Os{CHC(PPh3)CHCH2}(C9H6NO) (CO)(PPh3)2]Cl (6). Complex 4 also reacts with PPh3 in the presence of acid and results in a transformation to [Os {CHC(PPh3)CHCHC}(C9H6NO)Cl (PPh3)2]Cl (7) and [Os{CHC(PPh3) CH=(η2‐CCH(PPh3))}(C9H6NO) Cl(PPh3)]Cl (8). Further investigation shows that the ratio of 7 and 8 is highly dependent on the amount of the acid in the reaction.