Rapid nitrite reduction enabled by secondary sphere hydrogen bonds within non-heme iron complexes

A non-heme iron(ii) complex bearing a ligand with secondary sphere hydrogen bond (H-bond) donors, (tris(6-phenylaminopyridylmethyl)amine, TPA^NHPh, rapidly reduces nitrite (NO₂ ^-) to nitric oxide (NO) in the absence of exogenous additives, affording a Fe(iii)₂(μ-O)₂ diamond core. An electronically analogous complex containing a ligand without H-bonds (tris(6-methylpyridylmethylamine), TPA^Me, also reduces NO₂ ^- to NO and forms an Fe(iii)₂(μ-O)₂ core, but is four orders of magnitude slower, highlighting the impact of H-bonds to promote NO₂ ^- reduction. We compare the structural and spectroscopic differences of the two Fe(iii)₂(μ-O)₂ complexes and show that H-bonding interactions weaken the Fe-O bonds, perturb the electronic structure of the Fe₂O₂ cores, and thereby engender distinct reductive stability profiles.