Preparation of Core–Shell Coordination Molecular Assemblies via the Enrichment of Structure-Directing “Codes” of Bridging Ligands and Metathesis of Metal Units

A series
of molybdenum- and copper-based MOPs were synthesized
through coordination-driven process of a bridging ligand (3,3′-PDBAD, L1) and dimetal paddlewheel clusters.
Three conformers of the ligand exist with an ideal bridging angle
between the two carboxylate groups of 0° (H2α-L1), 120° (H2β-L1), and of 90° (H2γ-L1), respectively. At
ambient or lower temperature, H2L1 and Mo2(OAc)4 or Cu2(OAc)4 were crystallized into a molecular square with
γ-L1 and Mo2/Cu2 units. With proper temperature elevation, not only
the molecular square with γ-L1 but also a lantern-shaped cage with α-L1 formed simultaneously. Similar to how Watson–Crick
pairs stabilize the helical structure of duplex DNA, the core–shell
molecular assembly possesses favorable H-bonding interaction sites.
This is dictated by the ligand conformation in the shell, coding for
the formation and providing stabilization of the central lantern shaped
core, which was not observed without this complementary interaction.
On the basis of the crystallographic implications, a heterobimetallic
cage was obtained through a postsynthetic metal ion metathesis, showing
different reactivity of coordination bonds in the core and shell.
As an innovative synthetic strategy, the site-selective metathesis
broadens the structural diversity and properties of coordination assemblies.

一系列基于钼和铜的金属有机聚合物（MOPs）通过桥连配体（3,3′-PDBAD，L1）与双金属桨叶簇的配位驱动过程合成。该配体存在三种构象，分别对应于两个羧酸基团之间的理想桥连角为0°（H2α-L1）、120°（H2β-L1）以及90°（H2γ-L1）。在常温或更低温度下，H2L1与Mo2(OAc)4或Cu2(OAc)4结晶形成带有γ-L1和Mo2/Cu2单元的分子正方形。通过适当升高温度，不仅形成了带有γ-L1的分子正方形，同时亦形成了具有α-L1的灯笼状笼结构。类似于沃森-克里克碱基对稳定双链DNA的螺旋结构，核-壳分子组装体具有有利的氢键相互作用位点。这种结构由壳中的配体构象所决定，编码并提供了中央灯笼状核心的形成与稳定，而这种核心的形成在缺乏这种互补相互作用时并未观察到。基于晶体学意义，通过后合成金属离子置换反应获得了一种杂金属笼结构，展示了核心和壳层中配位键的不同反应活性。作为一种创新的合成策略，位选择性置换反应扩展了配位组装体的结构多样性和性质。