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)Rh(II•)) produces a (1)H NMR-observable equilibrium with rhodium methoxide ((TMP)Rh-OCH(3)(CH(3)OH)) and rhodium hydride ((TMP)Rh-H) complexes. Equilibrium concentrations for each of these species, obtained from integration of (1)H NMR spectra, were used in determining the equilibrium constant, K(298 K) = [Rh-OCH(3)(CH(3)OH)][Rh-H]/[Rh(II•)](2)[CH(3)OH](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 CD(2)Cl(2) give ΔG(0)(298 K) = -5.5(0.2) kcal mol(-1) for association of methanol with (TMP)Rh-OCH(3) to form the six-coordinate 18-electron complex (TMP)Rh-OCH(3)(CH(3)OH). Equilibrium measurements in conjunction with (TMP)Rh-H and CH(3)O-H bond energetics are used to evaluate the (TMP)Rh-OCH(3) bond dissociation free energy (Rh-OCH(3) BDFE(298 K) = 38 (1.3) kcal mol(-1)), which is 15 kcal mol(-1) smaller than the Rh-H BDFE and approximately equal to the Rh-CH(2)OH BDFE.