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

Abstract

The racemic form of 5 ((RR)5 + (SS)5) gives dinuclear complexes of 2:2 stoichiometry both with Cu(II), acting as a bis-terdentate ligand, and with Cu(I), acting as a bis-bidentate ligand. Single crystal X-ray diffraction studies have shown that the Cu(II) complex exists as double-strand homochiral helicate molecules: P,P-[Cu(2)(II)((RR)5)(2)](4+) and M,M-[Cu(2)(II)((SS)5)(2)](4+); in which the two trans-1,2-cyclohexanediamine subunits have the same chirality for of the two strands. Each Cu(II) metal center is six-coordinated according to a cis-octahedral geometry and interacts with a NNO donor subunit of each strand. The Cu(I) complex, when crystallized from THF in the presence of (rac)5, gives a double-strand homochiral helicate complex and in the solid state forms a racemic mixture of the homochiral metal complexes M,M-[Cu(2)(I)((RR)5)(2)](2+) and P,P-[Cu(2)(I)((SS)5)(2)](2+). When crystallizing from a MeCN solution, Cu(I) and (rac)5 give rise to the heterochiral nonhelicate dimeric complex [Cu(2)(I)((RR)5)((SS)5)](2+), 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 Cu(I) centers are four-coordinated by two nitrogen atoms from each strand, according to a distorted tetrahedral geometry. In MeCN solution, the dinuclear Cu(II) complex disassembles to give the mononuclear species, which, on reduction at a platinum electrode in a cyclic voltammetry experiment, gives two Cu(I) mononuclear complexes that quickly assemble to give the dinuclear Cu(I) complex. This complex undergoes two consecutive one-electron oxidation processes, but the dinuclear Cu(II) species that forms decomposes in less than 1 s. On the contrary, the [Cu(2)(I)((rac)5)(2)](2+) complex is stable in MeCN solution and undergoes two one-electron oxidation processes to give a form of dinuclear Cu(II) complex that lasts in solution for more than 20 s.