Oxygen atom exchange between H₂O and non-heme oxoiron(IV) complexes: Ligand dependence and mechanism

Detailed studies of oxygen atom exchange (OAE) between H₂¹⁸O and synthetic non-heme oxoiron(IV) complexes supported by tetradentate and pentadentate ligands provide evidence that they proceed by a common mechanism but within two different kinetic regimes, with OAE rates that span 2 orders of magnitude. The first kinetic regime involves initial reversible water association to the Fe^IV complex, which is evidenced by OAE rates that are linearly dependent on [H₂¹⁸O] and H₂O/D₂O KIEs of 1.6, while the second kinetic regime involves a subsequent rate determining proton-transfer step between the bound aqua and oxo ligands that is associated with saturation behavior with [H₂¹⁸O] and much larger H₂O/D₂O KIEs of 5-6. [Fe^IV(O)(TMC)(MeCN)]²⁺ (1) and [Fe^IV(O)(MePy₂TACN)]²⁺ (9) are examples of complexes that exhibit kinetic behavior in the first regime, while [Fe^IV(O)(N4Py)]²⁺ (3), [Fe^IV(O)(BnTPEN)]²⁺ (4), [Fe^IV(O)(1Py-BnTPEN)]²⁺ (5), [Fe^IV(O)(3Py-BnTPEN)]²⁺ (6), and [Fe^IV(O)(Me₂Py₂TACN)]²⁺ (8) represent complexes that fall in the second kinetic regime. Interestingly, [Fe^IV(O)(PyTACN)(MeCN)]²⁺ (7) exhibits a linear [H₂¹⁸O] dependence below 0.6 M and saturation above 0.6 M. Analysis of the temperature dependence of the OAE rates shows that most of these complexes exhibit large and negative activation entropies, consistent with the proposed mechanism. One exception is complex 9, which has a near-zero activation entropy and is proposed to undergo ligand-arm dissociation during the RDS to accommodate H₂¹⁸O binding. These results show that the observed OAE kinetic behavior is highly dependent on the nature of the supporting ligand and are of relevance to studies of non-heme oxoiron(IV) complexes in water or acetonitrile/water mixtures for applications in photocatalysis and water oxidation chemistry.