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 ?-oxo complex ( ^tBudmx)Fe₂(?-O) and free mesitylene. A two-electron oxidation of ( ^tBudmx)Fe₂(?-O) gives rise to the diferric complex ( ^tBudmx)Fe₂(?-O)Cl₂, and one-electron reduction from this Fe^IIIFe^III state allows for isolation of a mixed-valent species [Cp₂Co][( ^tBudmx)Fe₂(?-O)Cl₂]. Both ( ^tBudmx)Fe₂(?-O) and [Cp₂Co][( ^tBudmx)Fe₂(?-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(?-O) is quantified through studies of the pK _a of its conjugate acid, [( ^tBudmx)Fe₂(?-OH)]⁺, which is determined to be 15.3(6); interestingly, upon coordination of neutral solvent ligands to yield ( ^tBudmx)Fe₂(?-O)(thf)₂, the basicity is increased as observed through an increase in the pK _a of the conjugate acid [( ^tBudmx)Fe₂(?-OH)(thf)₂]⁺ to 26.8(6). In contrast, attempts to synthesize a diferric bridging hydroxide by two-electron oxidation of [( ^tBudmx)Fe₂(?-OH)(thf)₂]⁺ resulted in isolation of ( ^tBudmx)Fe₂(?-O)Cl₂ with concomitant loss of a proton, consistent with the pK _a of the conjugate acid [( ^tBudmx)Fe₂(?-OH)Cl₂]⁺ determined computationally to be -1.8(6). The foregoing results highlight the intricate interplay between oxidation state and reactivity in diiron ?-oxo units.

SUBMITTER: Johnson EJ 

PROVIDER: S-EPMC6598509 | biostudies-literature | 2019 Jul

REPOSITORIES: biostudies-literature