Asymmetric transformation of a double-standard, dicopper(I) helicate containing achiral bis(bidentate) Schiff bases

Reactions of the bis(bidentate) Schiff-bases N,N^'-bis(6-alkyl-2-pyridylmethylene)ethane-1,2-diamine (where alkyl = H, Me, ⁱPr) (L) with tetrakis(acetonitrile)copper(I) hexafluorophosphate and silver(I) hexafluorophosphate afforded, respectively, the double-stranded, dinuclear metal helicates [T-4-(R^*,R^*)]-(±)-[M₂L₂](PF₆)₂ (M = Cu, Ag). The helicates were characterized by ¹H and ¹³C NMR spectroscopy, conductivity, microanalysis, and single-crystal X-ray structure determinations on selected compounds. Intermolecular ligand exchange and intramolecular inversion rates for the complexes were investigated by ¹H NMR spectroscopy. Reversible intermolecular ligand exchange between two differently substituted helicates followed first-order kinetics. The rate constants (k) and corresponding half-lives (t_1/2) for ligand exchange for the dicopper(I) helicates were k = (1.6-1.8) × 10^-6 s^-1 (t_1/2 = 110-120 h) in acetoned₆, k = 4.9 × 10^-6 s^-1 (t_1/2 = 40 h) in dichloromethane-d₂, and k > 2 × 10^-3 s^-1 (t_1/2 < 5 min) in acetonitrile-d₃. Ligand exchange for the disilver(I) helicates occurred with k > 2 × 10^-3 s^-1 (t_1/2 < 5 min). Racemization of the dicopper(I) helicate by an intramolecular mechanism was investigated by determination of the coalescence temperature for the diastereotopic isopropyl-Me groups in the appropriate complex, and ΔG >> 76 kJ mol^-1 was calculated for the process in acetone-d₆, nitromethane-d₃, and dichloromethane-d₂ with ΔG = 75 kJ mol^-1 in acetonitrile-d₃. Complete anion exchange of the hexafluorophosphate salt of a dicopper(I) helicate with the enantiomerically pure Δ-(-)-tris(catecholato)arsenate(V) ([As(cat)₃]^-) in the presence of Dabco gave the two diastereomers (R,R)-[Cu₂L₂]- {Δ-(-)-[As(cat)₃]}₂ and (S,S)-[Cu₂L₂]{Δ-(-)-[As(cat)₃]}₂ in up to 54% diastereomeric excess, as determined by ¹H NMR spectroscopy. The diastereomerically and enantiomerically pure salt (R,R)-[Cu₂L₂]{Δ-(-)-[As(cat)₃]}₂ crystallized from the solution in a typical second-order asymmetric transformation. The asymmetric transformation of the dicopper- (I) helicate is the first synthesis of a diastereomerically and enantiomerically pure dicopper(I) helicate containing achiral ligands.