First Report on Crystal Engineering of Hg(II) Halides with Fully Substituted 3,4-Pyridinedicarboxamides: Generation of Two-Dimensional Coordination Polymers and Linear Zig-Zag Chains of Mercury Metal Ions

Two new ligands N,N,N′,N′-tetraisopropyl/tetraisobutyl-3,4-pyridinedicarboxamide L1–L2 and six of their Hg­(II)­X2 complexes (where X = Cl–, Br–, and I–) have been synthesized and characterized.
Single crystal X-ray structures of three complexes of L1 (1–3) with HgCl2/Br2/I2 and two complexes of L2 (4–5) with HgCl2/Br2 show that these are two-dimensional (2D) coordination polymers,
with three different (one new) topologies and five coordinated Hg­(II)
ions, in square pyramidal coordination. Complex 6, of L2 with HgI2, is a dimer with a four coordinated
tetrahedral Hg­(II) ion. The ligands behave as two- or three-connecting
linkers for forming coordination polymers (CPs) but only one-connector
in the dimer. Various types of hydrogen bonding and other noncovalent
interactions have been calculated, analyzed, and discussed for all
the complexes and the ligand L1. Although the large size
and soft character of iodide coupled with the steric effects of large
isobutyl groups are mainly responsible for a change in the primary
structure of 6, a significant role of the semilocalized
LP···π and C–H···O noncovalent
interactions into this has also been found. The latter transform this
dimer into a stable 2D hydrogen-bonded network instead of a coordination
polymer. The unique halide bridged 2D structure of 1 forms
one-dimensional zig-zag chains of metal ions owing to the mercurophilic
interactions. Weaker interactions of the same kind further extend
along the basic skeleton of 2D CP and are facilitated by LP···π,
strong C–H···Cl and C–H···N­(py)
interactions.

本研究合成并表征了两种新型配体——N,N,N′,N′-四异丙基/四异丁基-3,4-吡啶二酰胺（L1~L2），以及它们与Hg(II)X₂形成的六种配合物（其中X代表Cl⁻、Br⁻、I⁻）。对L1分别与HgCl₂、HgBr₂、HgI₂形成的三种配合物（1~3），以及L2分别与HgCl₂、HgBr₂形成的两种配合物（4~5）的单晶X射线衍射结构分析显示，上述五种配合物均为二维（2D）配位聚合物（coordination polymer），具有三种不同的拓扑结构（其中一种为新型拓扑），中心Hg(II)离子均为五配位，呈四方锥配位构型。而L2与HgI₂形成的配合物6则为二聚体，其中的Hg(II)离子为四配位，呈四面体配位构型。该类配体可作为二连接或三连接的连接体用于构建配位聚合物（coordination polymers，以下简称CPs），但在二聚体中仅作为单连接体。针对所有配合物与配体L1，研究人员对各类氢键及其他非共价相互作用进行了计算、分析与讨论。尽管碘离子的大尺寸与软质特性，加之大体积异丁基的位阻效应，是导致配合物6的一级结构发生改变的主要原因，但研究同时发现，半定域孤对电子···π（LP···π）与C–H···O非共价相互作用在此过程中也发挥了重要作用。上述非共价相互作用将该二聚体转化为稳定的二维氢键网络，而非配位聚合物。配合物1独特的卤桥桥联二维结构因亲汞相互作用，形成了一维金属离子锯齿链。同类较弱的相互作用进一步沿二维配位聚合物的基本骨架延伸，并通过LP···π、强C–H···Cl与C–H···N(吡啶)相互作用得以强化。