Cobalt N-Heterocyclic Phosphenium Complexes Stabilized by a Chelating Framework: Synthesis and Redox Properties.

Two cobalt complexes containing coordinated N-heterocyclic phosphenium (NHP) ligands are synthesized using a bidentate NHP/phosphine chelating ligand, [PP]. Treatment of Na[Co(CO)] with the chlorophosphine precursor [PP]Cl (1) affords [PP]Co(CO) (2), which features a planar geometry at the NHP phosphorus center and a short Co-P distance [1.9922(4) Å] indicative of a Co═P double bond. The more electron-rich complex [PP]Co(PMe) (3), which is synthesized in a one-pot reduction procedure with 1, CoCl, PMe, and KC, has an even shorter Co-P bond [1.9455(6) Å] owing to stronger metal-to-phosphorus back-donation. The redox properties of 2 and 3 were explored using cyclic voltammetry, and oxidation of 3 was achieved to afford [[PP]Co(PMe)] (4). The electron paramagnetic resonance spectrum of complex 4 features hyperfine coupling to both Co and P, suggesting strong delocalization of the unpaired electron density in this complex. Density functional theory calculations are used to further explore the bonding and redox behavior of complexes 2-4, shedding light on the potential for redox noninnocent behavior of NHP ligands.