A Diazo Linker Ligand Promotes Flexibility and Induced Fit Binding in a Microporous Copper Coordination Network

Flexible organic linkers represent an intuitive and effective strategy to design flexible metal-organic materials. We report herein a systematic study concerning the effect of varying the central bond of mixed pyridyl-benzoate linkers, L, upon the flexibility of three isostructural kdd topology microporous coordination networks (CNs) of formula ML₂: X-kdd-1-Cu, 1 = L = (E)-4-(pyridin-4-yldiazenyl)benzoate; X-kdd-2-Cu, 2 = L = (E)-4-(2-(pyridin-4-yl)vinyl)benzoate; the previously reported X-kdd-3-Cu, 3 = L = 4-(pyridin-4-ylethynyl)benzoate. As revealed by single crystal x-ray diffraction (SCXRD) and gas sorption studies, X-kdd-1-Cu, exhibited gate-opening during CO₂ and hydrocarbon (C2 and C8) sorption experiments whereas the other two CNs did not. Insight into these phase transformations was gained from in situ variable-pressure and variable temperature powder X-ray diffraction (PXRD), SCXRD, and modeling. Rotation of ligand 1 around the diazo bond, torsion angle changes between phenyl and carboxylate moieties, and deformation of the Cu-based rod building blocks enabled activated X-kdd-1-Cu to form new phases with C8 isomers and CH₂Cl₂, CH₂Cl₂ inducing contraction of the activated phase. Computational studies suggest that 1 enables flexibility thanks to its lower barrier of deformation versus 2 or 3. This study teaches that diazo moieties could offer a general strategy to enhance the flexibility of CNs.