Comparison of Rh−OCH3 and Rh−CH2OH Bond Dissociation Energetics from Methanol C−H and O−H Bond Reactions with Rhodium(II) Porphyrins

Reaction of methanol in toluene with tetramesityl rhodium(II) porphyrin ((TMP)RhII•) produces a 1H NMR-observable equilibrium with rhodium methoxide ((TMP)Rh−OCH3(CH3OH)) and rhodium hydride ((TMP)Rh−H) complexes. Equilibrium concentrations for each of these species, obtained from integration of 1H NMR spectra, were used in determining the equilibrium constant, K(298 K) = [Rh−OCH3(CH3OH)][Rh−H]/[RhII•]2[CH3OH]2 = 3.0(0.3), and free energy change, ΔG 0(298 K) = −0.65(0.5) kcal mol−1, for the reaction. Equilibrium thermodynamic measurements in CD2Cl2 give ΔG 0(298 K) = −5.5(0.2) kcal mol−1 for association of methanol with (TMP)Rh−OCH3 to form the six-coordinate 18-electron complex (TMP)Rh−OCH3(CH3OH). Equilibrium measurements in conjunction with (TMP)Rh−H and CH3O−H bond energetics are used to evaluate the (TMP)Rh−OCH3 bond dissociation free energy (Rh−OCH3 BDFE(298 K) = 38 (1.3) kcal mol−1), which is 15 kcal mol−1smaller than the Rh−H BDFE and approximately equal to the Rh−CH2OH BDFE.