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 [CuI2(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.