Neutral-Ligand Complexes of Bis(imino)pyridine Iron:  Synthesis, Structure, and Spectroscopy

A family of bis(imino)pyridine iron neutral-ligand derivatives, (<sup>iPr</sup>PDI)FeL<i><sub>n</sub></i> (<sup>iPr</sup>PDI = 2,6-(2,6-<sup>i</sup>Pr<sub>2</sub>−C<sub>6</sub>H<sub>3</sub>NCMe)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>N), has been synthesized from the corresponding bis(dinitrogen) complex, (<sup>iPr</sup>PDI)Fe(N<sub>2</sub>)<sub>2</sub>. When L is a strong-field ligand such as  <sup>t</sup>BuNC or a chelating alkyl diphosphine such as DEPE (DEPE = 1,2-bis(diethylphosphino)ethane), a five-coordinate, diamagnetic compound results with no spectroscopic evidence for mixing of paramagnetic states. Reducing the field strength of the neutral donor to principally σ-type ligands such as <sup>t</sup>BuNH<sub>2</sub> or THT (THT = tetrahydrothiophene) also yielded diamagnetic compounds. However, the <sup>1</sup>H NMR chemical shifts of the in-plane bis(imino)pyridine hydrogens exhibit a large chemical shift dispersion indicative of temperature-independent paramagnetism (TIP) arising from mixing of an <i>S</i> = 1 excited state via spin−orbit coupling. Metrical data from X-ray diffraction establish bis(imino)pyridine chelate reduction for each structural type, while Mössbauer parameters and NMR spectroscopic data differentiate the spin states of the iron and identify contributions from paramagnetic excited states.