Diiron oxo reactivity in a weak-field environment

Concomitant deprotonation and metalation of a dinucleating cofacial Pacman dipyrrin ligand platform dmxH₂ with Fe₂(Mes)₄ results in formation of a diiron complex ( dmx)Fe₂(Mes)₂. Treatment of ( dmx)Fe₂(Mes)₂ with one equivalent of water yields the diiron μ-oxo complex ( dmx)Fe₂(μ-O) and free mesitylene. A two-electron oxidation of ( dmx)Fe₂(μ-O) gives rise to the diferric complex ( dmx)Fe₂(μ-O)Cl₂, and one-electron reduction from this Fe^IIIFe^III state allows for isolation of a mixed-valent species [Cp₂Co][( dmx)Fe₂(μ-O)Cl₂]. Both ( dmx)Fe₂(μ-O) and [Cp₂Co][( dmx)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 ( dmx)Fe₂(μ-O) is quantified through studies of the pK _a of its conjugate acid, [( dmx)Fe₂(μ-OH)]⁺, which is determined to be 15.3(6); interestingly, upon coordination of neutral solvent ligands to yield ( dmx)Fe₂(μ-O)(thf)₂, the basicity is increased as observed through an increase in the pK _a of the conjugate acid [( dmx)Fe₂(μ-OH)(thf)₂]⁺ to 26.8(6). In contrast, attempts to synthesize a diferric bridging hydroxide by two-electron oxidation of [( dmx)Fe₂(μ-OH)(thf)₂]⁺ resulted in isolation of ( dmx)Fe₂(μ-O)Cl₂ with concomitant loss of a proton, consistent with the pK _a of the conjugate acid [( dmx)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.