Thermodynamic Trends in Carbon-Hydrogen BondActivation inNitriles and Chloroalkanes at Rhodium

Several transition-metal systems have been used to establish correlations between metal-carbon and carbon-hydrogen bonds. Here, the [Tp0RhL] fragment, where Tp0=tris(3,5-dimethylpyrazolyl)borate and L = neopentyl isocyanide, is used to investigate C-H bond activation in a series of linear alkylnitriles and chloroalkanes. Using a combination of kinetic techniques, relative free energies can be found for the compoundsTpRhL(CH3)H, Tp 0RhL[(CH2)nCN]H (n=1-5), andTp0RhL[(CH2)mCl]H (m=1,3, 4, 5). It is found that theCNandCl substituents dramatically strengthen theM-Cbondmore than anticipated if in the R-position, with the effect on bond strength diminishing substantially as the Xgroupmoves further from themetal (i.e,β,γ,δ). Examination ofM-CvsC-Hbond strengths shows that the Tp0RhL(CH2X)H compounds (X= phenyl, vinyl, CN, Cl) all show a good correlation, as do the alkyl, aryl, and vinyl derivatives. The compounds in the former group, however, have strongerM-C bonds than expected based on the C-H bond strengths and consequently, their correlation is separate from the other unsubstituted compounds.