Structural isomers of aryl-substituted eta(3)-propargyl complexes: eta(2)-1-Metalla(methylene)cyclopropene and eta(3)-benzyl complexes.

Hydride abstraction from C(5)Me(5)(CO)(2)Re(eta(2)-PhC triple bond CCH(2)Ph) (1) gave a 3:1 mixture of eta(3)-propargyl complex [C(5)Me(5)(CO)(2)Re(eta(3)-PhCH-C triple bond CPh)][BF(4)] (5) and eta(2)-1-metalla(methylene)cyclopropene complex [C(5)Me(5)(CO)(2)Re(eta(2)-PhC-C=CHPh)][BF(4)] (6). Observation of the eta(2)-isomer requires 1,3-diaryl substitution and is favored by electron-donating substituents on the C(3)-aryl ring. Interconversion of eta(3)-propargyl and eta(2)-1-metalla(methylene)cyclopropene complexes is very rapid and results in coalescence of Cp (1)H NMR resonances at about -50 degrees C. Protonation of the alkynyl carbene complex C(5)Me(5)(CO)(2)Re=C(Ph)C triple bond CPh (22) gave a third isomer, the eta(3)-benzyl complex [C(5)Me(5)(CO)(2)Re[eta(3)(alpha,1,2)-endo,syn-C(6)H(5)CH(C triple bond CC(6)H(5))]][BF(4)] (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 eta(3)-benzyl isomer 23 with 5 and 6.