Comparing the Electronic Structure of Iron, Cobalt, and Nickel Compounds That Feature a Phosphine-Substituted Bis(imino)pyridine Chelate.

It was recently discovered that ( Ph2PPr PDI)Mn (PDI = pyridine diimine) exists as a superposition of low-spin Mn(II) that is supported by a PDI dianion and intermediate-spin Mn(II) that is antiferromagnetically coupled to a triplet PDI dianion, a finding that encouraged the synthesis and electronic structure evaluation of late first row metal variants that feature the same chelate. The addition of Ph2PPr PDI to FeBr 2 resulted in bromide dissociation and the formation of [( Ph2PPr PDI)FeBr][Br]. Reduction of this precursor using excess sodium amalgam afforded ( Ph2PPr PDI)Fe, which possesses an Fe(II) center that is supported by a dianionic PDI ligand. Similarly, reduction of a premixed solution of Ph2PPr PDI and CoCl 2 yielded the cobalt analog, ( Ph2PPr PDI)Co. EPR spectroscopy and density functional theory calculations revealed that this compound features a high-spin Co(I) center that is antiferromagnetically coupled to a PDI radical anion. The addition of Ph2PPr PDI to Ni(COD) 2 resulted in ligand displacement and the formation of ( Ph2PPr PDI)Ni, which was found to possess a pendent phosphine group. Single-crystal X-ray diffraction, CASSCF calculations, and EPR spectroscopy indicate that ( Ph2PPr PDI)Ni is best described as having a Ni(II)-PDI 2- configuration. The electronic differences between these compounds are highlighted, and a computational analysis of Ph2PPr PDI denticity has revealed the thermodynamic penalties associated with phosphine dissociation from 5-coordinate ( Ph2PPr PDI)Mn, ( Ph2PPr PDI)Fe, and ( Ph2PPr PDI)Co.