The Aryl Chlorine−Halide Ion Synthon and Its Role in the Control of the Crystal Structures of Tetrahalocuprate(II) Ions

The role of the arylchlorine−halide (C−Cl···X-) synthons in the development of crystal structures of the type
(nCP)2CuX4·mH2O (nCP+ = n-chloropyridinium; n = 2, 3, and 4; X = Cl- and Br-; m = 0 except for (3CP)2CuBr4·H2O] are
investigated. On the basis of detailed analysis of the synthonic interactions in these systems, it is concluded that the C−Cl···Cl-
synthon plays a more significant role as a crystal engineering tool in these structures than the C−Cl···Br- synthon. In addition, it
is shown that the C−Cl···X- interactions are weaker than the C−Br···X- synthons in the analogous (nBP)2CuX4 salts (nBP+ =
n-bromopyridinium). For all of the (nCP)2CuCl4 salts, the nCP+ cations are involved in nearly linear C−Cl···Cl- interactions with
Cl···Cl- distances less or equal to the sum of the van der Waals radii. Supramolecular chain networks, based on the C−Cl···Cl-
and N−H···Cl- synthons, are formed with all three nCP+ cations. In addition, a polymorph of the 4CP+ salt contains two-dimensional
networks based on these synthons. In contrast, the role of the linear C−Cl···Br- synthons is less well-defined. A supramolecular
structure developed on the basis of the C−Cl···Br- and N−H···Br- synthons is observed only for (4CP)2CuBr4. In (2CP)2CuBr4
and (3CP)2CuBr4·H2O, formation of dimeric species occurs through a combination of hydrogen and halogen bonding. The role of
a previously undescribed synthon, the N(π)···X- synthon, where the N−X vector lies roughly normal to the plane of the pyridinium
ring, is explored in the further development of the three-dimensional structures.

本研究探讨了芳基氯-卤离子（arylchlorine−halide，C−Cl···X⁻）合成子(synthon)在(nCP)₂CuX₄·mH₂O型晶体结构构建中的作用，其中nCP⁺为n-氯吡啶鎓阳离子（n=2、3、4；X⁻为Cl⁻与Br⁻；除(3CP)₂CuBr₄·H₂O外m均为0）。基于对上述体系中合成子相互作用的详细分析，本研究得出结论：相较于C−Cl···Br⁻合成子，C−Cl···Cl⁻合成子在这类晶体结构中作为晶体工程工具的作用更为显著。此外，研究表明，在类似的(nBP)₂CuX₄盐（nBP⁺为n-溴吡啶鎓阳离子）中，C−Cl···X⁻相互作用弱于C−Br···X⁻合成子。对于所有(nCP)₂CuCl₄盐，其nCP⁺阳离子均与Cl⁻形成近乎线性的C−Cl···Cl⁻相互作用，且Cl···Cl⁻距离小于或等于范德华半径之和。基于C−Cl···Cl⁻与N−H···Cl⁻合成子，三种nCP⁺阳离子均形成了超分子链网络。此外，4CP⁺盐的一种多晶型体含有基于此类合成子的二维网络结构。与之相对，线性C−Cl···Br⁻合成子的作用则尚不明确。仅在(4CP)₂CuBr₄中观察到基于C−Cl···Br⁻与N−H···Br⁻合成子构建的超分子结构。在(2CP)₂CuBr₄与(3CP)₂CuBr₄·H₂O中，通过氢键与卤键的协同作用形成了二聚体物种。本研究还探讨了一种此前未被报道的合成子——N(π)···X⁻合成子（其中N−X矢量大致垂直于吡啶鎓环平面）在三维结构构建中的作用。