Structural Isomers of Aryl-Substituted η 3-Propargyl Complexes:  η 2-1-Metalla(methylene)cyclopropene and η 3-Benzyl Complexes

Hydride abstraction from C5Me5(CO)2Re(η 2-PhC⋮CCH2Ph) (1) gave a 3:1 mixture of η 3-propargyl complex [C5Me5(CO)2Re(η 3-PhCHC⋮CPh)][BF4] (5) and η 2-1-metalla(methylene)cyclopropene complex [C5Me5(CO)2Re(η 2-PhCCCHPh)][BF4] (6). Observation of the η 2-isomer requires 1,3-diaryl substitution and is favored by electron-donating substituents on the C3-aryl ring. Interconversion of η 3-propargyl and η 2-1-metalla(methylene)cyclopropene complexes is very rapid and results in coalescence of Cp* 1H NMR resonances at about −50 °C. Protonation of the alkynyl carbene complex C5Me5(CO)2ReC(Ph)C⋮CPh (22) gave a third isomer, the η 3-benzyl complex {C5Me5(CO)2Re[η 3(α,1,2)-endo,syn-C6H5CH(C⋮CC6H5)]}[BF4] (23) along with small amounts of the isomeric complexes 5 and 6. While 5 and 6 are in rapid equilibrium, there is no equilibration of the η 3-benzyl isomer 23 with 5 and 6.