Thermodynamic Trends in CarbonâHydrogen Bond Activation in Nitriles and Chloroalkanes at Rhodium.

Several transition-metal systems have been used to establish correlations between metal−carbon and carbon−hydrogen bonds. Here, the [Tp′RhL] fragment, where Tp′ = 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 compounds TpRhL(CH3)H, Tp′RhL[(CH2)nCN]H (n = 1−5), and Tp′RhL[(CH2)mCl]H (m = 1, 3, 4, 5). It is found that the CN and Cl substituents dramatically strengthen the M−C bond more than anticipated if in the α-position, with the effect on bond strength diminishing substantially as the X group moves further from the metal (i.e, β, γ, δ). Examination of M−C vs C−H bond strengths shows that the Tp′RhL(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 stronger M−C bonds than expected based o...