Interconversion of Metallabenzenes and Cyclic η²‐Allene‐Coordinated Complexes

Treatment of the osmabenzene [Os{CHC(PPh(3))CHC(PPh(3))CH}Cl(2)(PPh(3))(2)]Cl (1) with excess 8-hydroxyquinoline produces monosubstituted osmabenzene [Os{CH C(PPh(3))CHC(PPh(3))CH}(C(9)H(6)NO)Cl(PPh(3))]Cl (2) or disubstituted osmabenzene [Os{CHC(PPh(3))CHC(PPh(3))CH}(C(9)H(6)NO)(2)]Cl (3) under different reaction conditions. Osmabenzene 2 evolves into cyclic η(2)-allene-coordinated complex [Os{CH=C(PPh(3))CH=(η(2)-C=CH(2))}(C(9)H(6)NO)(PPh(3))(2)]Cl (4) in the presence of excess PPh(3) and NaOH, presumably involving a P-C bond cleavage of the metallacycle. Reaction of 4 with excess 8-hydroxyquinoline under air affords the S(N) Ar product [(C(9)H(6)NO)Os{CHC(PPh(3))CHCHC}(C(9)H(6)NO)(PPh(3))]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(PPh(3))CH=CH(2)}(C(9)H(6)NO)(CO)(PPh(3))(2)]Cl (6). Complex 4 also reacts with PPh(3) in the presence of acid and results in a transformation to [Os{CHC(PPh(3))CHCHC}(C(9)H(6)NO)Cl(PPh(3))(2)]Cl (7) and [Os{CH=C(PPh(3))CH=(η(2)-C=CH(PPh(3)))}(C(9)H(6)NO)Cl(PPh(3))]Cl (8). Further investigation shows that the ratio of 7 and 8 is highly dependent on the amount of the acid in the reaction.