Energetics of homogeneous intermolecular vinyl and allyl carbon-hydrogen bond activation by the 16-electron coordinatively unsaturated organometallicfragment [Tp ' Rh(CNCH2CMe3)]

Reaction of the complex Tp'Rh(CNneo)(CH=CH2)Cl (neo = CH2CMe3, Tp' = hydrid otris-(3,5-dimethylpyrazolyl)borate) with Cp2ZrH2 leads to the formation of Tp'Rh(CNneo)(CH=CH2)H. This complex is also formed upon photolysis of a so lution of Tp'Rh(CNneo)(PhN=C=Nneo) containing ethylene or by thermal reacti on of Tp'Rh(CNneo)(c-hexyl)H with ethylene. The vinyl hydride complex rearr anges to the more stable eta(2)-ethylene complex with a half-life of 8 h at 22 degrees C. Photolysis of a solution of Tp'Rh(CNneo)(PhN=C=Nneo) in liqu id propylene produces the allylic activation product Tp'Rh(CNneo)(CH2CH=CH2 )H, which rearranges (t(1/2) = 3 days at 22 degrees C) to the eta(2)-propyl ene complex. Allylic activation is also seen with isobutylene, but loss of olefin is observed at 22 degrees C in benzene solution to generate Tp'Rh(CN neo)(Ph)H (t(1/2) = 16.6 h). Photolysis of a tert-butylethylene solution of Tp'Rh(CNneo)(PhN=C=Nneo) produces the trans vinyl hydride complex, which l oses tert-butylethylene to generate Tp'Rh(CNneo)(Ph)H (t(1/2) = 113 days at 22 degrees C). A combination of kinetic selectivity and reductive eliminat ion experiments have allowed for calculation of relative Rh-C bond strength s for both the rhodium allyl and vinyl hydride complexes and for the inclus ion of these new data in an analysis of bond strength correlations. The res ults show that the trend for relative Rh-C bond strengths parallels the tre nd of hydrocarbon C-H bond strengths, i.e., Rh-Ph > Rh-vinyl > Rh-methyl > Rh-alkyl (1 degrees) > Rh-cycloalkyl (2 degrees) > Rh-benzyl > Rh-allyl, bu t that differences in M-C bond strengths typically exceed the differences i n C-H bond strengths.