Diiron oxo reactivity in a weak-field environment

Abstract

Concomitant deprotonation and metalation of a dinucleating cofacial Pacman dipyrrin ligand platform tBudmxH2 with Fe2(Mes)4 results in formation of a diiron complex (tBudmx)Fe2(Mes)2. Treatment of (tBudmx)Fe2(Mes)2 with one equivalent of water yields the diiron μ-oxo complex (tBudmx)Fe2(μ-O) and free mesitylene. A two-electron oxidation of (tBudmx)Fe2(μ-O) gives rise to the diferric complex (tBudmx)Fe2(μ-O)Cl2, and one-electron reduction from this FeIIIFeIII state allows for isolation of a mixed-valent species [Cp2Co][(tBudmx)Fe2(μ-O)Cl2]. Both (tBudmx)Fe2(μ-O) and [Cp2Co][(tBudmx)Fe2(μ-O)Cl2] exhibit basic character at the bridging oxygen atom and can be protonated using weak acids to form bridging diferrous hydroxide species. The basicity of the diferrous oxo (tBudmx)Fe2(μ-O) is quantified through studies of the pKa of its conjugate acid, [(tBudmx)Fe2(μ-OH)]+, which is determined to be 15.3(6); interestingly, upon coordination of neutral solvent ligands to yield (tBudmx)Fe2(μ-O)(thf)2, the basicity is increased as observed through an increase in the pKa of the conjugate acid [(tBudmx)Fe2(μ-OH)(thf)2]+ to 26.8(6). In contrast, attempts to synthesize a diferric bridging hydroxide by two-electron oxidation of [(tBudmx)Fe2(μ-OH)(thf)2]+ resulted in isolation of (tBudmx)Fe2(μ-O)Cl2 with concomitant loss of a proton, consistent with the pKa of the conjugate acid [(tBudmx)Fe2(μ-OH)Cl2]+ determined computationally to be -1.8(6). The foregoing results highlight the intricate interplay between oxidation state and reactivity in diiron μ-oxo units.