Four-Coordinate Cobalt Pincer Complexes: Electronic Structure Studies and Ligand Modification by Homolytic and Heterolytic Pathways

A family of cobalt chloride, methyl, acetylide and hydride complexes bearing both intact and modified tert-butyl substituted bis­(phosphino)­pyridine pincer ligands has been synthesized and structurally characterized and their electronic structures evaluated. Treatment of the unmodified compounds with the stable nitroxyl radical, TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxidanyl) resulted in immediate H- atom abstraction from the benzylic position of the chelate yielding the corresponding modified pincer complexes, (tBumPNP)­CoX (X = H, CH3, Cl, CCPh). Thermolysis of the methyl and hydride derivatives, (tBuPNP)­CoCH3 and (tBuPNP)­CoH, at 110 °C also resulted in pincer modification by H atom loss while the chloride and acetylide derivatives proved inert. The relative ordering of benzylic C–H bond strengths was corroborated by H atom exchange experiments between appropriate intact and modified pincer complexes. The electronic structures of the modified compounds, (tBumPNP)­CoX were established by EPR spectroscopy and DFT computations and are best described as low spin Co­(II) complexes with no evidence for ligand centered radicals. The electronic structures of the intact complexes, (tBuPNP)­CoX were studied computationally and bond dissociation free energies of the benzylic C–H bonds were correlated to the identity of the X-type ligand on cobalt where pure σ donors such as hydride and methyl produce the weakest C–H bonds. Comparison to a rhodium congener highlights the impact of the energetically accessible one-electron redox couple of the first row metal ion in generating weak C–H bonds in remote positions of the supporting pincer ligand.