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

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 ] + BF 4- in CD 3 CN or Cu I Br in C 6 D 6 using tris[2-(dimethylamino)ethyl]amine (Me 6 TREN), 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 Me 6 TREN and TPMA systems than for the BIPY system. RRT scales with the proton donor acidity (Et 3 NH + ≫ H 2 O > CH 3 OH), it is reduced by deuteration (H 2 O > D 2 O and CH 3 OH > CD 3 OD), and it is more efficient in C 6 D 6 than in CD 3 CN. 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 Et 3 NH + in CD 3 CN 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=CMe 2 ] + derivatives with only partial spin delocalization onto the Cu atom.