Diiron oxo reactivity in a weak-field environment

Concomitant deprotonation and metalation of a dinucleating cofacial Pacman dipyrrin ligand platform (tBu)dmxH(2) with Fe-2(Mes)(4) results in formation of a diiron complex ((tBu)dmx)Fe-2(Mes)(2). Treatment of ((tBu)dmx)Fe-2(Mes)(2) with one equivalent of water yields the diiron mu-oxo complex ((tBu)dmx)Fe-2(mu-O) and free mesitylene. A two-electron oxidation of ((tBu)dmx)Fe-2(mu-O) gives rise to the diferric complex ((tBu)dmx)Fe-2(mu-O)Cl-2, and one-electron reduction from this (FeFeIII)-Fe-III state allows for isolation of a mixed-valent species [Cp2Co][((tBu)dmx)Fe-2(mu-O)Cl-2]. Both ((tBu)dmx)Fe-2(mu-O) and [Cp2Co][((tBu)dmx)Fe-2(mu-O)Cl-2] 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 ((tBu)dmx)Fe-2(mu-O) is quantified through studies of the pK(a) of its conjugate acid, [((tBu)dmx)Fe-2(mu-OH)](+), which is determined to be 15.3(6); interestingly, upon coordination of neutral solvent ligands to yield ((tBu)dmx)Fe-2(mu-O)(thf)(2), the basicity is increased as observed through an increase in the pK(a) of the conjugate acid [((tBu)dmx)Fe-2(mu-OH)(thf)(2)](+) to 26.8(6). In contrast, attempts to synthesize a diferric bridging hydroxide by two-electron oxidation of [((tBu)dmx)Fe-2(mu-OH)(thf)(2)](+) resulted in isolation of ((tBu)dmx)Fe-2(mu-O)Cl-2 with concomitant loss of a proton, consistent with the pK(a) of the conjugate acid [((tBu)dmx)Fe-2(mu-OH)Cl-2](+) determined computationally to be -1.8(6). The foregoing results highlight the intricate interplay between oxidation state and reactivity in diiron mu-oxo units.