Constrained metal’s local geometry can be provided by a tridentate pincer ligand to offer complexes with frontier molecular orbitals in appropriate symmetry and energy. Due to favorable occupancy in d-orbitals, many d8 square planar pincer systems have been developed and explored for various organometallic reactions and catalysis applications. To expand their window in chemical reactions, prominent metal–ligand cooperation (MLC) has been corroborated by utilizing a redox noninnocent moiety such as pyridine. Our group recently reported a distinctive MLC behavior of a PPP pincer ligand (PPP– = P[2-PiPr2-C6H4]2–) with nickel. By adding a π-acidic ligand, such as CO, a transformation of a phosphide-nickel(II) moiety to phosphinite-nickel(0) occurs and accompanies a two-electron redox change in the nickel center. Since such cooperative reactivity is expected with cobalt, a (PPP)Co-L complex was prepared and studied. The metalation of PPPH with CoBr2 generates a trigonal bipyramidal cobalt(II) complex (PPHP)CoBr2, which could be transformed to a (PPP)Co-L as a square planar cobalt precursor complex via its deprotonation. Addition of trimethylamine, however, produces a dinuclear cobalt(I) complex, {(PPP)CoBr}2 containing a P–P bond. This reaction involves a single electron reduction at a cobalt center and oxidation at a phosphide group upon deprotonation. Detailed redox activity of a PPP ligand system cooperated with a cobalt center through P–P bond formation and cleavage will be discussed.

Redox–active behavior of a PPP ligand with a cobalt center

256th National Meeting (2018)

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