On the Lewis Acidity of the Oxoiron(IV) Unit in a Tetramethylcyclam Complex

The correlation between oxidation state and Lewis acidity is well established for hexaquairon complexes in the +II and +III oxidation state, in which the higher oxidation state leads to a lower pK_a for the bound H₂O ligand. This article addresses the Lewis acidity of the oxoiron(IV) complex [Fe^IV(O)(TMC)(OH₂)]²⁺ (1-OH₂; TMC=1,4,8,11-tetramethylcyclam) by determining the pK_a of the H₂O ligand. We establish that 1-OH₂ has a pK_a of 6.9±0.5, a value that falls in between those found for [Fe^III(OH₂)₆]³⁺ and [Fe^II(OH₂)₆]²⁺. This intermediate value can be readily rationalized by the presence of the highly basic oxide ligand that mitigates the Lewis acidity of the iron(IV) center. Although the oxo ligand occupies only one position in 1-OH₂, anti to all four methyl groups that protrude from the same face of the nonplanar TMC ligand, its conjugate base 1-OH exists as a mixture of syn and anti tautomers, which are related by proton transfer between the oxo and the hydroxo ligands.