Preparation of core-shell coordination molecular assemblies via the enrichment of structure-directing "codes" of bridging ligands and metathesis of metal units.
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A series of molybdenum- and copper-based MOPs were synthesized through coordination-driven process of a bridging ligand (3,3'-PDBAD, L(1)) and dimetal paddlewheel clusters. Three conformers of the ligand exist with an ideal bridging angle between the two carboxylate groups of 0 (H2-L(1)), 120 (H2-L(1)), and of 90 (H2-L(1)), respectively. At ambient or lower temperature, H2L(1) and Mo2(OAc)4 or Cu2(OAc)4 were crystallized into a molecular square with -L(1) and Mo2/Cu2 units. With proper temperature elevation, not only the molecular square with -L(1) but also a lantern-shaped cage with -L(1) 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.