An unusual mixed-valence cobalt dimer as a catalyst for the anti-Markovnikov hydrophosphination of alkynes

Abstract

The reaction of [Co(PMe₃)₄] (1) with a redox-active NNN pincer ligand (L₁) led us to isolate a unique binuclear cobalt complex ([(PMe₃)₂Co^II(L₁³⁻)Co^I(PMe₃)₃] (2)) anchored by a three-electron reduced L₁ in its unusual coordination mode. Such an unprecedented binuclear pincer/allyl cobalt complex consisting of the mixed-valence oxidation state of cobalt ions (+1 and +2) is confirmed by various analytical techniques (XPS, EPR, and UV–vis), and the experimentally determined electronic structure is well corroborated by detailed theoretical studies based on DFT calculations. Complex 2 efficiently catalyzes the hydrophosphination of alkynes regioselectively, and affords the anti-Markovnikov product in good to excellent yields without any additional strong bases or organolithium reagents. Both internal and terminal alkynes including propargylic alcohols were amenabled to accessing vinyl trivalent phosphines with exclusive E-selectivity. The isotopic labeling experiments confirmed the 1,2-anti-Markovnikov addition of P–H from HPPh₂ across the alkyne stem. The preliminary ¹H and ³¹P NMR investigations revealed that the Co^I nucleus in 2 is likely responsible for the catalytic outcome of the hydrophosphination of alkynes. Overall, the study discloses the importance of the exemplary structural platform offered by the redox-active pincer ligands in isolating unusual multi-nuclear complexes and the need to explore the multi-nuclear complexes as a catalyst besides the traditional mononuclear catalyst.