Wavelength dependent photochemistry of an iron dinitrogen hydride complex via 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 trans-[FeII(DMeOPrPE)2(N2)H][BPh4](1) and its isotopomers 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, and synchrotron-based nuclear resonant vibrational spectroscopy (NRVS) 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) 4K - 1) the cleavage of Fe-N2 bond at UV or visible light photolyses, which presents a unipolar disappearance of the N2 peak at 2094 cm-1; 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 which is aligned with the N2 peak in a similar Fe(I) complex. To assist the detailed investigation, NRVS, near-IR and Mössbauer were performed – NRVS finds a low spin Fe(II) and no H detachment in a visible photolysis while the latter two studies reveal an additional low spin Fe(I) species in the UV photolysis product. In summary, the N2 and H dissociations are in competition with each other in UV or UV-inclusive photolyses of this dinitrogen hydride complex. This wavelength dependent photochemical work is the first one on a reaction active N2ase-N2 model complex, which for sure offers valuable information for future studies on real N2ase-N2 and its photolysis products.