Reduction chemistry of aryl- and alkyl-substituted bis(imino)pyridine iron dihalide compounds: Molecular and electronic structures of [(PDI)2Fe] derivatives

Sodium amalgam reduction of the aryl-substituted bis(imino)pyridine iron dibromide complex, (EtPDI)FeBr2 (EtPDI = 2,6-(2,6-Et2-C6H3N=CMe)2C5H3N), under a dinitrogen atmosphere in pentane furnished the bis(chelate)iron compound, (EtPDI)2Fe. Characterization by X-ray crystallography established a distorted four-coordinate iron center with two K2-bis(imino)pyridine ligands. Reducing the steric demands of the imine substituent to either a less sterically encumbered aryl ring (e.g., C6H4-4-OMe) or an alkyl group (e.g., Cy, Pr, c/s-myrtanyl) also yielded bis(chelate) compounds from sodium amalgam reduction of the corresponding dihalide. Characterization of the compounds with smaller imine substituents by X-ray diffraction established six-coordinate, pseudo-octahedral compounds. In one case, a neutral bis(chelate)iron compound was prepared by reduction of the corresponding iron dication, [(PDI)2Fe]2+, providing chemical confirmation of electrochemically generated species that were previously reported as too reducing to isolate. Magnetic measurements, metrical parameters from X-ray structures, Mossbauer spectroscopy, and open-shell, broken symmetry DFT calculations were used to establish the electronic structure of both types (four- and six-coordinate) of neutral bis(chelate) compounds. The experimentally observed S = 1 compounds are best described as having high-spin ferrous (SFe = 2) centers antiferromagnetically coupled to two bis(imino)pyridine radical anions. Thus, the two-electron reduction of the diamagnetic, low-spin complex [(PDI)2Fe]2+ to [(PDI)2Fe] is ligand-based with a concomitant spin change at iron.