Dicopper Double-Strand Helicates Held Together by Additional π–π Interactions

The bis-bidentate ligand, obtained from Schiff base condensation of RR-1,2-cyclohexanediamine and 8-naphthylmethoxyquinoline-2-carbaldehyde (L−L), forms with [CuI (MeCN)4]ClO4 a double strand helicate complex, made especially stable by the presence of four definite interstrand π−π interactions involving a quinoline subunit and a naphthylmethoxy substituent of the two strands. The [CuI2(L−L)2]2+ complex, which does not decompose even on excess addition of either L−L or CuI, undergoes a two electron oxidation in MeCN, through two one-electron fully reversible steps, separated by 260 mV, as shown by cyclic voltammetry (CV) studies. The high stability of the mixed valence complex [CuICuII(L−L)2]3+ with respect to disproportionation to [CuI 2(L−L)2]2+ and [CuII2(L−L)2]4+ is essentially due to a favorable electrostatic term. CuII forms with L−L a stable species, with a 1:1 stoichiometric ratio,but, in the absence of crystallographic data, it was impossible to assess whether it is of mono- or dinuclear nature. However, CV studies on an MeCN solution containing equimolar amounts of CuII and L−L showed the presence in the reduction scan of two fully reversible waves, separated by about 250 mV, which indicated the presence in solution of a dicopper(II) double strand helicate complex, [CuII2(L−L)2]4+. This work demonstrates that additional interstrand π−π interactions can favor the formation of unusually stable dicopper(I) and dicopper(II) helicate complexes.