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, TPANHPh, 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), TPAMe, 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.