Methods for determining electronic structures and mechanisms of bis(imino)pyridine iron pre-catalysts

The electronic structures of bis(imino)pyridine iron compounds ( iPrPDI)FeL; (iPrPDI =2,6-(2,6-iPr 2C6H3N=CMe)2C5H3N; L = ligand) were explored and are of relevance to a variety of fundamental transformations including oxidative addition, olefin coordination, processes relevant to nitrogen fixation and catalytic reactions including hydrogenation and polymerization of olefins. The known redox-activity of the bis(imino)pyridine ligand complicates the determination of the electronic structures of pre-catalysts and intermediates and raises questions regarding the electron flow in catalysis by these systems. This system allows for a series of unusual high spin (pseudo) square planar iron compounds to be isolated, which display single molecule magnet behavior. To understand both iron and ligand oxidation states, fundamental studies of contributions to iron and nitrogen K-edge X-ray absorption (XAS) and iron Kβ X-ray emission (XES) spectroscopies have been undertaken. The results establish that iron and nitrogen K-edge XAS are sensitive to ligand oxidation state, but iron Kβ XES is not. The overall results of synthetic, spectroscopic and computational studies suggest that the redox-active bis(imino)pyridine ligand allows for iron to maintain the preferred iron(II)-(III) redox couple and avoid high iron oxidation states.