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

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.

外消旋四配位镍(II)配合物[Ni(α-rac-L)](ClO₄)₂（含有等量的SS和RR对映异构体）在乙腈/水混合溶剂中与L-和D-苯丙氨酸反应，分别得到两种溶解性更差的六配位对映异构体{[Ni(f-SS-L)(L-Phe)](ClO₄)}ₙ（Δ-1）与{[Ni(f-RR-L)(D-Phe)](ClO₄)}ₙ（Λ-1）。
蒸发剩余溶液，分别得到两种六配位非对映异构体{[Ni₃(f-RR-L)₃(L-Phe)₂(H₂O)](ClO₄)₄}ₙ（a-2）与{[Ni₃(f-SS-L)₃(D-Phe)₂(H₂O)](ClO₄)₄}ₙ（b-2）（其中配体L为5,5,7,12,12,14-六甲基-1,4,8,11-四氮杂环十四烷，Phe⁻为苯丙氨酸根阴离子）。
[Ni(α-rac-L)](ClO₄)₂与dl-苯丙氨酸根的反应生成了c-1型外消旋聚集体：其中SS和RR对映异构体分别优先与L-和D-苯丙氨酸根配位，得到Δ-1与Λ-1的外消旋混合物，且反应过程中发生自发拆分，每一颗晶体均为对映纯产物。
使用高氯酸从Δ-1和Λ-1中脱去苯丙氨酸根，得到两种对映异构体[Ni(α-SS-L)](ClO₄)₂（S-3）与[Ni(α-RR-L)](ClO₄)₂（R-3）。
将S-3与R-3溶于乙腈，得到两种六配位对映异构体{[Ni(f-SS-L)(CH₃CN)₂](ClO₄)₂}（S-4）与{[Ni(f-RR-L)(CH₃CN)₂](ClO₄)₂}（R-4）；而将[Ni(α-rac-L)](ClO₄)₂溶于乙腈则得到外消旋缠结配合物{[Ni(f-rac-L)(CH₃CN)₂](ClO₄)₂}（rac-4）。
Δ-1与Λ-1属于超分子立体异构体，二者分别通过[Ni(f-SS-L)(L-Phe)]⁺与[Ni(f-RR-L)(D-Phe)]⁺单体的氢键连接，构建出一维同手性右手螺旋链与左手螺旋链。
S-3与D-苯丙氨酸根反应得到{[Ni(f-SS-L)(D-Phe)](ClO₄)}ₙ（5），该化合物呈现一维氢键锯齿链结构，而非一维螺旋链。
化合物a-2/b-2包含[{Ni(f-RR-L)}₂(L-Phe)(H₂O)]³⁺/[{Ni(f-SS-L)}₂(D-Phe)(H₂O)]³⁺二聚体，以及{[Ni(f-RR-L)(L-Phe)]⁺}ₙ/{[Ni(f-SS-L)(D-Phe)]⁺}ₙ的一维锯齿链结构。
Δ-1/Λ-1、a-2/b-2、S-3/R-3以及S-4/R-4的同手性属性，均通过圆二色性(Circular Dichroism, CD)光谱测试结果得到了验证。