Building-up Remarkably Stable Magnesium Porphyrin Polymers Self-Assembled via Bidentate Axial Ligands: Synthesis, Structure, Surface Morphology, and Effect of Bridging Ligands

A series of supramolecular architectures of magnesium
tetranitrooctaethylporphyrins
mediated by several bidentate axial ligands have been synthesized
in excellent yields and structurally characterized. Six conjugated
axial ligand with increasing chain lengths have been utilized in the
present investigations in which the Mg···Mg nonbonding
distance between successive ions also increases from 0.73 to 2.70
nm in the series. To the best of our knowledge, this is the first
report where stable metallo-porphyrin polymers with such long spacers
have been synthesized in one pot so easily. Linear one-dimensional
(1D) polymeric chains were observed in the X-ray structure of the
six-coordinated complexes in which porphyrin units are aligned parallel
to each other to have so-called “shish kebab” like architectures
to maintain offset-stacked overlap. However, after an optimum Mg···Mg
nonbonding distance, these 1D chain do not continue, rather they form
five-coordinated porphyrin dimers with “wheel-and-axle”
like architectures which are then self-aggregated by π–π
interactions in a perpendicular manner to fill space created by large
bridging ligands more effectively which consequently results in spherical
structures. The structures of the molecules in solution and their
surface patterns on highly ordered pyrolytic graphite (HOPG) have
also been investigated.

本研究通过多种双齿轴向配体介导，合成了一系列四硝基八乙基卟啉镁（magnesium tetranitrooctaethylporphyrins）的超分子架构，产物产率优异且完成了结构表征。本研究采用了六种具有递增链长的共轭轴向配体，对应体系中相邻镁离子间的Mg···Mg非键距离也从0.73 nm增至2.70 nm。据我们所知，这是首次报道可通过如此简便的一锅法合成此类带有长间隔基的稳定金属卟啉聚合物。对六配位配合物的X射线晶体结构分析显示，其呈现线性一维（1D）聚合物链结构，卟啉单元相互平行排列，形成类似“烤串”的超分子架构以维持错位堆叠重叠。然而当Mg···Mg非键距离达到最优值后，此类一维链无法继续延伸，转而形成具有“轮轴”状结构的五配位卟啉二聚体；随后这些二聚体通过π-π相互作用以垂直方向发生自聚集，更高效地填充大体积桥联配体所形成的空间，最终得到球形结构。本研究同时考察了该类分子在溶液中的结构，以及其在高定向热解石墨（HOPG）表面的形貌特征。