Observed enhancement of the reactivity of a biomimetic diiron complex by the addition of water - mechanistic insights from theoretical modeling

Abstract

The biomimetic diiron complex [Fe^IIIFe^IV(μ-O)₂(5-Me₃-TPA)₂](ClO₄)₃ (TPA = tris(2-pyridylmethyl)amine) has been found to be capable of oxidizing 9,10-dihydroanthracene in a solution of acetonitrile. Addition of water up to 1 M makes the reaction 200 times faster, suggesting that the water molecule in some way activates the catalyst for more efficient substrate oxidation. It is proposed that the enhanced reactivity results from the coordination of a water molecule to the iron(III) half of the complex, converting the bis-μ-oxo structure of the diiron complex to a ring-opened form where one of the bridging oxo groups is transformed into a terminal oxo group on iron(IV). The suggested mechanism is supported by DFT (B3LYP) calculations and transition state theory. Two different computational models of the diiron complex are used to model the hydroxylation of cyclohexane to cyclohexanol. Model 1 has a bis-μ-oxo diiron core (diamond core) while model 2 represents the “open core” analogue with one bridging μ-oxo group, a terminal oxo ligand on iron(IV), and a water molecule coordinated to iron(III). The computational results clearly suggest that the terminal oxo group is more reactive than the bridging oxo group. The free energy of activation is 7.0 kcal mol⁻¹ lower for the rate limiting step when the oxidant has a terminal oxo group than when both oxo groups are bridging the irons.