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Spontaneous Resolution of a Racemic Nickel(II) Complex and Helicity Induction via Hydrogen Bonding: The Effect of Chiral Building Blocks on the Helicity of One-Dimensional Chains

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posted on 07.04.2008, 00:00 by Guang-Chuan Ou, Long Jiang, Xiao-Long Feng, Tong-Bu Lu
The reactions of a racemic four-coordinated nickel(II) complex [Ni(α-rac-L)](ClO4)2 (containing equal amount of SS and RR enantiomers) with l- and d-phenylalanine in acetonitrile/water gave two less-soluble six-coordinated enantiomers of {[Ni(f-SS-L)(l-Phe)](ClO4)}n (Δ-1) and {[Ni(f-RR-L)(d-Phe)](ClO4)}n (Λ-1), respectively. Evaporation the remaining solutions gave two six-coordinated diastereomers of {[Ni3(f-RR-L)3(l-Phe)2(H2O)](ClO4)4}n (a-2) and {[Ni3(f-SS-L)3(d-Phe)2(H2O)](ClO4)4}n (b-2), respectively (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, Phe = phenylalanine anion). The reaction of [Ni(α-rac-L)](ClO4)2 with dl-Phe gave a conglomerate of c-1; in which, the SS and RR enantiomers preferentially coordinate to l- and d-Phe respectively to give a racemic mixture of Δ-1 and Λ-1, and the spontaneous resolution occurs during the reaction, in which each crystal crystallizes to become enantiopure. Removing Phe from Δ-1 and Λ-1 using perchloric acid gave two enantiomers of [Ni(α-SS-L)](ClO4)2 (S-3) and [Ni(α-RR-L)](ClO4)2 (R-3). Dissolving S-3 and R-3 in acetonitrile gave two six-coordinated enantiomers of [Ni(f-SS-L)(CH3CN)2](ClO4)2 (S-4) and [Ni(f-RR-L)(CH3CN)2](ClO4)2 (R-4), while dissolving [Ni(α-rac-L)](ClO4)2 in acetonitrile gave a racemic twining complex [Ni(f-rac-L)(CH3CN)2](ClO4)2 (rac-4). Δ-1 and Λ-1 belong to supramolecular stereoisomers, which are constructed via hydrogen bond linking of [Ni(f-SS-L)(l-Phe)]+ and [Ni(f-RR-L)(d-Phe)]+ monomers to form 1D homochiral right-handed and left-handed helical chains, respectively. The reaction of S-3 with d-Phe gave {[Ni(f-SS-L)(d-Phe)](ClO4)}n (5), which shows a motif of a 1D hydrogen bonded zigzag chain instead of a 1D helical chain. Compound a-2/b-2 contains dimers of [{Ni(f-RR-L)}2(l-Phe)(H2O)]3+/[{Ni(f-SS-L)}2(d-Phe)(H2O)]3+ and 1D zigzag chains of {[Ni(f-RR-L)(l-Phe)]+}n/{[Ni(f-SS-L)(d-Phe)]+}n. The homochiral nature of Δ-1/Λ-1, a-2/b-2, S-3/R-3, and S-4/R-4 are confirmed by the results of circular dichroism (CD) spectra measurements.