Exploring the Reactivity of Four-Coordinate PNPCoX with Access to Three-Coordinate Spin Triplet PNPCo

The compounds (PNP)CoX, where PNP is (tBu2PCH2SiMe2)2N- and X is Cl, I, N3, OAr, OSO2CF3 and N(H)Ar, are
reported. Some of these show magnetic susceptibility, color, and 1H NMR evidence of being in equilibrium between
a blue, tetrahedral S = 3/2 state and a red, planar S = 1/2 state; the equilibrium populations are influenced by
subtle solvent effects (e.g., benzene and cyclohexane are different), as well as by temperature. Attempted oxidation
to Co(III) with O2 occurs instead at phosphorus, giving [P(O)NP(O)]CoX species. The single O-atom transfer reagent
PhIO likewise oxidizes P. Even I2 oxidizes P to give the pendant phosphonium species (tBu2P(I)CH2SiMe2NSiMe2CH2PtBu2)CoI2 with a tetrahedral S = 3/2 cobalt; the solid-state structure shows intermolecular PI···ICo
interactions. Attempted alkyl metathesis of PNPCoX inevitably results in reduction, forming PNPCo, which is a spin
triplet with planar T-shaped coordination geometry with no agostic interaction. Triplet PNPCo binds N2(weakly) and
CO (whose low CO stretching frequency indicates strong PNP → Co donor power), but not ethene or MeCCMe.