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.