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

It was recently discovered that (Ph2PPrPDI )Mn (PDI= pyridine diimine) exists as a superposition of low-spin Mn(II) thatis supported by a PDI dianion and intermediate-spin Mn(II) that isantiferromagnetically coupled to a triplet PDI dianion, afinding thatencouraged the synthesis and electronic structure evaluation of latefirst row metal variants that feature the same chelate. The addition ofPh2PPrPDI to FeBr2resulted in bromide dissociation and theformation of [(Ph2PPrPDI)FeBr][Br]. Reduction of this precursorusing excess sodium amalgam afforded (Ph2PPrPDI)Fe, whichpossesses an Fe(II) center that is supported by a dianionic PDIligand. Similarly, reduction of a premixed solution ofPh2PPrPDI andCoCl2yielded the cobalt analog, (Ph2PPrPDI)Co. EPR spectroscopyand density functional theory calculations revealed that thiscompound features a high-spin Co(I) center that is antiferromagnetically coupled to a PDI radical anion. The addition ofPh2PPrPDI to Ni(COD)2resulted in ligand displacement and the formation of (Ph2PPrPDI)Ni, which was found to possess a pendentphosphine group. Single-crystal X-ray diffraction, CASSCF calculations, and EPR spectroscopy indicate that (Ph2PPrPDI)Ni is bestdescribed as having a Ni(II)-PDI2-configuration. The electronic differences between these compounds are highlighted, and acomputational analysis ofPh2PPrPDI denticity has revealed the thermodynamic penalties associated with phosphine dissociation from5-coordinate (Ph2PPrPDI)Mn, (Ph2PPrPDI)Fe, and (Ph2PPrPDI)Co