Charge Effects on the Reactivity of Oxoiron(IV) Porphyrin Species: A DFT Analysis of Methane Hydroxylation by Polycationic Compound I and Compound II Mimics

We analyze with DFT calculations the charge effects on the reactivity of the polycationic Compound I (Cpd I) and Compound II (Cpd II) mimics experimentally investigated by Bell and Groves (Bell, S. R.; Groves, J. T. J. Am. Chem. Soc. 2009, 131, 9640). Specifically, we consider the Cpd I model [(4-THPyP)•+(H2O)FeIV═O]5+ (THPyP = 5,10,15,20-tetrakis(N-hydro-4-pyridinium) porphyrinate), 1H2O, and its Cpd II counterparts [(4-THPyP)FeIV═O]4+, 2, and [(4-THPyP)(H2O)FeIV═O]4+, 2H2O. In gas phase simulations it is found that all positive charges enhance the reactivity. According to a detailed electronic structure analysis in the gas phase of the reactant complexes en route to the transition state, the positive charges stabilize the electron acceptor orbital (EAO) of the catalyst more than the electron donor orbital (EDO) of the substrate, thereby reducing the energy gap between these orbitals and, hence, the H-abstraction barrier. The effect of the peripheral charges residing on the pyridinium groups is dampened in water solution by screening effects of the solvent. However, the effect of the positive charge residing on the porphyrin ring of the Cpd I mimic is not much diminished by the solvent. The well-known stronger oxidizing capability of Cpd I mimics relative to their Cpd II counterparts is ascribed to the effect on the frontier orbitals of the positive charge on the ring, rather than to the presence of a hole per se.