Wavelength dependent photochemistry of an iron dinitrogen hydride complex via multiple spectroscopies – competing ejection of axial ligands

Abstract

Nitrogenase (N₂ase) is a critical enzyme which catalyzes the reaction of N₂ → NH₃ in nature. Studies on the spectroscopy and photochemistry of trans-[Fe^II(DMeOPrPE)₂(N₂)H][BPh₄] (1) and its isotopologues (2–6) provide a possible first step to evaluate the geometries and properties of the real N₂ase–N₂ structure(s). In this article, we have used FT-IR, FT-Raman, synchrotron-based nuclear resonant vibrational spectroscopy (NRVS) and DFT calculations to examine and assign the normal modes of these complexes. In addition, we have monitored their wavelength dependent photochemistry using mid-IR, near-IR, NRVS, and Mössbauer spectroscopies. Two distinct photolysis pathways are observed with mid-IR at (nominal) 4 K – (1) the cleavage of Fe–N₂ bond in UV or visible light photolyses, which presents a unipolar disappearance of the N₂ peak at 2094 cm⁻¹ and is recombinable; (2) the ejection of trans hydrogen atom with UV irradiation, which has a pair of bipolar peaks with the disappearance of N₂ at 2094 cm⁻¹ and the appearance of a new species at 2056 cm⁻¹ and is non-recombinable. The latter peak is well aligned with the N₂ peak in an Fe^I reference complex (7). The combination of mid IR monitored photolysis/recombination and NRVS monitored photolysis form the central evidence for the conclusions in this article. In particular, the Fe^II–N₂ and Fe^I–H·dissociations are in competition with each other in UV or UV-inclusive photolyses of this dinitrogen hydride complex. In addition, near-IR and Mössbauer also provide consistent evidence about Fe^I. This wavelength dependent photochemical work is the first one on a reaction active N₂ase–N₂ model complex and it also demonstrates the competition ejection between two axial ligands (H· and N₂). It offers valuable information for future studies on real N₂ase–N₂ and its photolysis products.