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

Nitrogenase (N₂ase) is a critical enzyme which catalyzes the reaction of N₂ → NH₃ in nature. Studies on the spectroscopy and photochemistry of <i>trans</i>-[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 <i>trans</i> 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.