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)CH₂SiMe₂NSiMe₂CH₂P^tBu₂)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.