Addition of H across the cobalt-phosphorus bond of (PPP)CoPMe (3) is demonstrated, where PPP is a monoanionic diphosphine pincer ligand with a central N-heterocyclic phosphido (NHP ) donor. The chlorophosphine Co complex (PP P)CoCl (2) can be generated through coordination of the chlorophosphine ligand (PP P, 1) to CoCl . Subsequent reduction of 2 with KC in the presence of PMe generates (PPP)CoPMe (3), in which both the phosphorus and cobalt centers have been reduced. The addition of 1 atm of H to complex 3 cleanly affords (PP P)Co(H)PMe (4), in which H has ultimately been added across the metal-phosphorus bond. Complex 4 was characterized spectroscopically and using computational methods to predict its geometry.

A tridentate [PPP] ligand has been used to construct a series of dimeric cobalt complexes and explore cooperative multielectron redox processes that are both metal- and ligand-centered. Reduction of (PPClP)­CoCl2 (1) with excess magnesium affords the CoICoI N-heterocyclic phosphido (NHP–)-bridged symmetric dimer [(μ-PPP)­Co]2 (2). Two-electron oxidation of 2 with FcPF6 generates an asymmetrically bridged dication [(μ-PPP)­Co]2[PF6]2 (3) in which the oxidation has occurred in a delocalized fashion throughout the Co2P2 core. In contrast, [(μ-PPP)­Co]2 + (5), which can be generated either by one-electron oxidation of 2 with FcPF6 or comportionation of 2 and 3, features an asymmetric geometry and localized mixed valence. Treatment of 1 with the milder reductants CoCp2 and KBEt3H does not lead to formation of 2, 3, or 5 but instead generates dimeric species [(PPP)­CoCl]2 (6) and [(PPP)­CoH]2 (7). Unlike 2–5, where the phosphine side arms of the tridentate [PPP] ligand span the two Co centers, complex 6 and 7 are connected solely by NHP– ligands that bridge the two (PPP)Co fragments.

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 andP, 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.

Addition of H 2 across the cobalt-phosphorus bond of (PPP)CoPMe 3 (3)isdemonstrated, where PPP is amonoanionic diphosphine pincer ligand with ac entral N-heterocyclic phosphido (NHP À )d onor.T he chlorophosphine Co II complex (PP Cl P)CoCl 2 (2)c an be generated through coordination of the chlorophosphine ligand (PP Cl P, 1)t oC oCl 2 . Subsequent reduction of 2 with KC 8 in the presence of PMe 3 generates (PPP)CoPMe 3 (3), in whichb oth the phosphorus and cobalt centers have been reduced. The addition of 1atm of H 2 to complex 3 cleanly affords (PP H P)Co(H)PMe 3 (4), in which H 2 has ultimately been added across the metalphosphorus bond. Complex 4 was characterized spectroscopically and using computational methods to predict its geometry.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

A cobalt N-heterocyclic phosphido complex is shown to cleave element–hydrogen bonds via a metal–phosphorus ligand cooperative pathway.