We present a systematic investigation of the structural and electronic changes that occur in an Fe(0)–N2 unit (Fe(depe)2(N2); depe = 1,2-bis(diethylphosphino)-ethane) upon the addition of exogenous Lewis acids. Addition of neutral boranes, alkali metal cations, and an Fe2+ complex increases the N–N bond activation (Δ νNN up to 172 cm−1), decreases the Fe(0)–N2 redox potential, polarizes the N–N bond, and enables −N protonation at uncommonly anodic potentials. These effects were rationalized using combined experimental and theoretical studies.

Flavodiiron nitric oxide reductases (FNORs) are a subclass of flavodiiron proteins (FDPs) capable of preferential binding and subsequent reduction of NO to NO. FNORs are found in certain pathogenic bacteria, equipping them with resistance to nitrosative stress, generated as a part of the immune defense in humans, and allowing them to proliferate. Here, we report the spectroscopic characterization and detailed reactivity studies of the diiron dinitrosyl model complex [Fe(BPMP)(OPr)(NO)](OTf) for the FNOR active site that is capable of reducing NO to NO [Zheng et al., J. Am. Chem. Soc. 2013, 135, 4902-4905]. Using UV-vis spectroscopy, cyclic voltammetry, and spectro-electrochemistry, we show that one reductive equivalent is in fact sufficient for the quantitative generation of NO, following a semireduced reaction mechanism. This reaction is very efficient and produces NO with a first-order rate constant k > 10 s. Further isotope labeling studies confirm an intramolecular N-N coupling mechanism, consistent with the rapid time scale of the reduction and a very low barrier for N-N bond formation. Accordingly, the reaction proceeds at -80 °C, allowing for the direct observation of the mixed-valent product of the reaction. At higher temperatures, the initial reaction product is unstable and decays, ultimately generating the diferrous complex [Fe(BPMP)(OPr)](OTf) and an unidentified ferric product. These results combined offer deep insight into the mechanism of NO reduction by the relevant model complex [Fe(BPMP)(OPr)(NO)] and provide direct evidence that the semireduced mechanism would constitute a highly efficient pathway to accomplish NO reduction to NO in FNORs and in synthetic catalysts.

We report an experimental and computational analysis of the effects of hydrogen bonding to a metal dinitrogen complex. A series of H-bond donors over a wide pK a range (Δ 20) interact with the nitrogen unit of a Re I-(N 2) complex at room temperature. Analysis by 15 N NMR, IR spectroscopy, association equilibria, and DFT studies indicates that the H-bonding interaction polarizes and weakens the N−N bond. These results provide insight into the role of the secondary sphere residues in nitrogenase enzymes.

Pendent borane Lewis acids provide an avenue for changing a ligand's field strength through acid/base interactions; this strategy was highlighted within a series of biologically-relevant high spin iron hydrides.