Ligand-Tuned Coordination Frameworks: Structural Insights from Gel-Crystal Formation, Hydrogen-Bond-Donating Friedel–Crafts Catalysis and Optical Sensing

The judicious selection of the amide-functionalized pyridine-based linker, bis(4-pyridinyl)-1,4-benzenedicarboxamide, and zinc facilitates the development of a series of coordination frameworks exhibiting unique self-assembly characteristics and functional features. Incorporation of an additional amine functionality (5-aminoisophthalic acid) facilitates hierarchical self-assembly to form a 2D coordination polymer (CP), Zn-AIP-BPBD, and a soft coordination polymer gel (Zn-AIP-BPBD-gel), whereas the non-amine counterpart (isophthalic acid) solely yields a crystalline CP (Zn-IP-BPBD). In addition to the structural role in framework construction, the presence of hydrogen-bond-donor functionalities in the organic linkers provides effective pathways of substrate activation. Herein, we designed coordination frameworks that include hydrogen-bond-donating (HBD) sites originating from organic linkers to facilitate substrate activation and improve catalytic transformations. Both Zn-AIP-BPBD and Zn-AIP-BPBD-gel demonstrate significant efficacy in hydrogen-bond-donating Friedel–Crafts alkylation reactions. Furthermore, the effects of this HBD site were shown by the generation of superior optical sensors. Both Zn-AIP-BPBD and Zn-AIP-BPBD-gel were employed as effective sensors for the jaundice biomarker, bilirubin. The high KSV value (5.32 × 104 M–1) and a nanomolar detection limit (1.26 nM) for bilirubin demonstrate its potential for bilirubin sensing.