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.