Octahedral Copper(II) and Tetrahedral Copper(I) Double-Strand Helicates: Chiral Self-Recognition and Redox Behavior

The racemic form of the linear multidentate ligand L (RRL þ SSL) gives dinuclear complexes of 2:2 stoichiometry both with CuII, acting as a bisterdentate ligand, and with CuI, acting as a bis-bidentate ligand. Single crystal X-ray diffraction studies have shown that the CuII complex exists as double-strand homochiral helicate molecules: P,P-[Cu2 II(RRL)2]4þ and M,M-[Cu2 II- (SSL)2]4þ; in which the two trans-1,2-cyclohexanediamine subunits have the same chirality for of the two strands. Each CuII metal center is six-coordinated according to a cis-octahedral geometry and interacts with a NNO donor subunit of each strand. The CuI complex, when crystallized from THF in the presence of racL, gives a double-strand homochiral helicate complex and in the solid state forms a racemic mixture of the homochiral metal complexes M, M-[Cu2 I(RRL)2]2þ and P,P-[Cu2 I(SSL)2]2þ. When crystallizing from a MeCN solution, CuI and racL give rise to the heterochiral nonhelicate dimeric complex [Cu2 I(RRL)(SSL)]2þ, in which the two strands of the dimer have inverse configuration of the trans-1,2-cyclohexanediamine subunits and are assembled side-by-side. In both structural architectures, the CuI centers are four-coordinated by two nitrogen atoms from each strand, according to a distorted tetrahedral geometry. In MeCN solution, the dinuclear CuII complex disassembles to give the mononuclear species, which, on reduction at a platinum electrode in a cyclic voltammetry experiment, gives two CuI mononuclear complexes that quickly assemble to give the dinuclear CuI complex. This complex undergoes two consecutive one-electron oxidation processes, but the dinuclear CuII species that forms decomposes in less than 1 s. On the contrary, the [Cu2 I(racL)2]2þ complex is stable in MeCN solution and undergoes two one-electron oxidation processes to give a form of dinuclear CuII complex that lasts in solution for more than 20 s.