RESUMO
The activation of inert CâH bonds by transition metals is of considerable industrial and academic interest, but important gaps remain in our understanding of this reaction. We report the first experimental determination of the structure of the simplest hydrocarbon, methane, when bound as a ligand to a homogenous transition metal species. We find that methane binds to the metal center in this system through a single M···H-C bridge; changes in the 1JCH coupling constants indicate clearly that the structure of the methane ligand is significantly perturbed relative to the free molecule. These results are relevant to the development of better CâH functionalization catalysts.
Assuntos
Metano , Osmio , Ligantes , MetaisRESUMO
The intermediates and transition structures in the degenerate thermal rearrangements of bicyclo[4.4.0]deca-2,4,7,9-tetraene (1c), bicyclo[5.5.0]dodeca-2,4,8,10-tetraene (11b), and bicyclo[5.4.0]undeca-2,4,8,10-tetraene (14) have been located by (U)B3LYP/6-31G(d) calculations. The singlet-triplet energy differences (ΔE(ST)) in the diradical intermediates (tricyclo[4.4.0.0(2,7)]deca-3,8-dien-5,10-diyl (2c), tricyclo[5.5.0.0(5,11)]dodeca-2,8-dien-4,10-diyl (12b), and tricyclo[5.4.0.0(5,11)]undeca-2,8-dien-4,10-diyl (15)) have been computed, using both UB3LYP and (6/6)CASPT2 calculations. ΔE(ST) in 2c, in which a four-membered ring is anti-bridged by two allylic radicals, is computed to be larger by a factor of 5 than ΔE(ST) in 15, in which the anti-bridged ring is five-membered, and by a factor of 10 than that in 12b, in which the anti-bridged ring is six-membered. The reasons for the much larger interaction between two allylic radicals through the bonds of the four-membered ring in 2c than through the bonds of the five-membered ring in 15 or the six-membered ring in 12b are discussed, and the consequences of the large, through-bond stabilization of the singlet state of 2c are described.
