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Asymmetric Transformation of a Double-Stranded, Dicopper(I) Helicate Containing Achiral Bis(bidentate) Schiff Bases

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journal contribution
posted on 20.02.2006, 00:00 authored by Nicolle C. Habermehl, Patricia M. Angus, Nathan L. Kilah, Lasse Norén, A. David Rae, Anthony C. Willis, S. Bruce Wild
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 (t1/2) for ligand exchange for the dicopper(I) helicates were k = (1.6−1.8) × 10-6 s-1 (t1/2 = 110−120 h) in acetone-d6, k = 4.9 × 10-6 s-1 (t1/2 = 40 h) in dichloromethane-d2, and k > 2 × 10-3 s-1 (t1/2 < 5 min) in acetonitrile-d3. Ligand exchange for the disilver(I) helicates occurred with k > 2 × 10-3 s-1 (t1/2 < 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.