Reductive Termination of Cyanoisopropyl Radicals by Copper(I) Complexes and Proton Donors: Organometallic Intermediates or Coupled Proton-Electron Transfer?

Abstract

Cyanoisopropyl radicals, generated thermally by the decomposition of azobis(isobutyronitrile) (AIBN), participate in reductive radical termination (RRT) under the combined effect of copper(I) complexes and proton donors (water, methanol, triethylammonium salts) in acetonitrile or benzene. The investigated copper complexes were formed in situ from [Cu^I(MeCN)₄]⁺BF₄^- in CD₃CN or Cu^IBr in C₆D₆ using tris[2-(dimethylamino)ethyl]amine (Me₆TREN), tris(2-pyridylmethyl)amine (TPMA), and 2,2'-bipyridine (BIPY) ligands. Upon keeping all other conditions constants, the impact of RRT is much greater for the Me₆TREN and TPMA systems than for the BIPY system. RRT scales with the proton donor acidity (Et₃NH⁺ ≫ H₂O > CH₃OH), it is reduced by deuteration (H₂O > D₂O and CH₃OH > CD₃OD), and it is more efficient in C₆D₆ than in CD₃CN. The collective evidence gathered in this study excludes the intervention of an outer-sphere proton-coupled electron transfer (OS-PCET), while an inner-sphere PCET (IS-PCET) cannot be excluded for coordinating proton donors (water and methanol). On the other hand, the strong impact of RRT for the noncoordinating Et₃NH⁺ in CD₃CN results from the formation of an intermediate Cu^I-radical adduct, suggested by DFT calculations to involve binding via the N atom to yield keteniminato [L/Cu-N═C═CMe₂]⁺ derivatives with only partial spin delocalization onto the Cu atom.