Theoretical study of the water oxidation mechanism with non-heme Fe (Pytacn) iron complexes. Evidence that the FeIV(O)(Pytacn) species cannot react with the water molecule to form the O-O bond

Abstract

Recent studies have shown that non-heme iron complexes [Fe(L N4)X2], where LN4 stands for a tetradentate nitrogen based aminopyridine ligand (LN4 = Pytacn, mcp or mep, Pytacn = 1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane, mcp = N,N-dimethyl-N,N-bis(2-pyridylmethyl)cyclohexane-trans-1,2-diamine, mep = N,N-dimethyl-N,N-bis(2-pyridylmethyl)ethylendiamine), and X are monodentate ligands (X = Cl, CH3CN, CF3SO3 -, or H2O), catalyze the oxidation of water using cerium(IV) ammonium nitrate (CAN) as oxidant. Spectroscopic monitoring of catalytic water oxidation with [Fe(CF3SO3)2(Pytacn)] established [Fe IV(O)(OH2)(Pytacn)]2+ as an intermediate along the catalytic pathway, raising the question if these high valent species could be directly responsible for the O-O bond formation event. Herein, this question is addressed by a computational analysis of the thermodynamic and kinetic parameters associated with the reaction of non-heme iron complexes [Fe IV(O)(OH)(Pytacn)]+, [FeIV(O)(OH 2)(Pytacn)]2+, and [FeIV(OH)(OH)(Pytacn)] 2+ with water. Two different mechanisms have been studied for [FeIV(O)(OH)(Pytacn)]+; the nucleophilic water attack assisted by the hydroxyl group as internal base, which is the lowest energy path, and the external nucleophilic water attack. For [FeIV(OH)(OH) (Pytacn)]2+, only the attack assisted by the internal base has been studied, while in the case of [FeIV(O)(OH2)(Pytacn)] 2+, the only viable mechanism is the external nucleophilic water attack. Up to four water molecules were needed to be included in modeling of the O-O bond formation event for a proper description of the external nucleophilic water attack. The lowest Gibbs energy barrier and reaction free energy found for the direct water nucleophilic attack to the oxo ligand are of 32.2 and 28.3 kcal·mol-1 for [FeIV(O)(OH)(Pytacn)]+, 52.0 and 40.5 kcal·mol-1 for [FeIV(O)(OH 2)(Pytacn)]2+, and 28.3 and 28.3 kcal·mol -1 for [FeIV(OH)(OH)(Pytacn)]2+, respectively. These energy barriers and endergonic reaction energies are too high for the reaction to proceed and inconsistent with the relatively rapid reaction rates determined experimentally (δG‡ (exp.) = 17.6 kcal·mol-1). Therefore, this study provides strong evidence against the O-O bond formation by these species. The energetic accessibilities of FeV(O) and FeVI(O) intermediates have also been investigated, showing that FeV is the higher oxidation state accessible under catalytic conditions, consistent with our previous results