Synthetic, Mechanistic, and Theoretical Studies on the Generation of Iridium Hydride Alkylidene and Iridium Hydride Alkene Isomers

Experimental and theoretical studies on equilibria between iridium hydride alkylidene structures, [(TpMe2)Ir(H){C(CH2R)ArO}] (TpMe2=hydrotris(3,5-dimethylpyrazolyl)borate; R=H, Me; Ar=substituted C6H4 group), and their corresponding hydride olefin isomers, [(TpMe2)Ir(H){R(H)CC(H)OAr}], have been carried out. Compounds of these types are obtained either by reaction of the unsaturated fragment [(TpMe2)Ir(C6H5)2] with o-C6H4(OH)CH2R, or with the substituted anisoles 2,6-Me2C6H3OMe, 2,4,6-Me3C6H2OMe, and 4-Br-2,6-Me2C6H2OMe. The reactions with the substituted anisoles require not only multiple CH bond activation but also cleavage of the MeOAr bond and the reversible formation of a CC bond (as revealed by 13C labeling studies). Equilibria between the two tautomeric structures of these complexes were achieved by prolonged heating at temperatures between 100 and 140?C, with interconversion of isomeric complexes requiring inversion of the metal configuration, as well as the expected migratory insertion and hydrogen-elimination reactions. This proposal is supported by a detailed computational exploration of the mechanism at the quantum mechanics (QM) level in the real system. For all compounds investigated, the equilibria favor the alkylidene structure over the olefinic isomer by a factor of between approximately 1 and 25. Calculations demonstrate that the main reason for this preference is the strong Ircarbene interactions in the carbene isomers, rather than steric destabilization of the olefinic tautomers.