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

Abstract

The bis-bidentate ligand, obtained from Schiff base condensation of <i>RR</i>-1,2-cyclohexanediamine and 8-naphthylmethoxyquinoline-2-carbaldehyde (<b>L–L</b>), forms with [Cu^I(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 [Cu^I₂(<b>L–L</b>)₂]²⁺ complex, which does not decompose even on excess addition of either <b>L–L</b> or Cu^I, 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 [Cu^ICu^II(<b>L–L</b>)₂]³⁺ with respect to disproportionation to [Cu^I₂(<b>L–L</b>)₂]²⁺ and [Cu^II₂(<b>L–L</b>)₂]⁴⁺ is essentially due to a favorable electrostatic term. Cu^II forms with <b>L–L</b> 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 Cu^II and <b>L–L</b> 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, [Cu^II₂(<b>L–L</b>)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