Characterization of Cu(II) bipyridine complexes in halogen atom transfer reactions by electron spin resonance

Electron spin resonance was used to investigate structural features of CuII complexes with dNbpy (4,4‘-di(5-nonyl)-2,2‘-bipyridine) and dnNbpy (4,4‘-di-n-nonyl-2,2‘-bipyridine) ligands in methyl isobutyrate (MIB) and toluene. With 1 or 2 equiv of either dNbpy or dnNbpy ligands, CuIIBr2 forms predominantly the neutral complexes CuII(dNbpy)Br2 and CuII(dnNbpy)Br2, respectively. Bromine atom transfer between [CuI(dnNbpy)2]+[CuIBr2]- and ethyl 2-bromoisobutyrate (EBriB) in MIB does not yield CuII(dnNbpy)Br2. Instead, it leads to the complex [CuII(dnNbpy)2Br]+[CuIBr2]-. The latter species is also formed in an equilibration reaction of CuII(dnNbpy)Br2 with [CuI(dnNbpy)2]+[CuIBr2]- (K ≥ 100 M-1/2, 23 °C, MIB).