Abstract
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)4] with the chlorophosphine
precursor [PP]Cl (1) affords [PP]Co(CO)2 (2), which features a planar geometry at the NHP+ phosphorus center and a short Co–P distance [1.9922(4) Å]
indicative of a CoP double bond. The more electron-rich complex
[PP]Co(PMe3)2 (3), which is synthesized
in a one-pot reduction procedure with 1, CoCl2, PMe3, and KC8, 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(PMe3)2]+ (4). The electron paramagnetic resonance spectrum of complex 4 features hyperfine coupling to both 59Co and 31P, 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.