Rotational flexibility of bridging ligands in paddle–wheel layer–pillar metal–organic frameworks studied by quantum calculations

Using quantum chemical (QC) calculations at the B3LYP/6-31G(d) level of theory, we investigate for three selected layer–pillar paddle–wheel metal–organic framework materials of composition M 2 II L 2 1 L 2 ( M II = Zn,Cu; L 1 = 2 , 3 , 5 , 6 -tetrafluorobenzene-1,4-dicarboxylate, 2,3,5,6-tetramethylbenzene-1,4-dicarboxylate; L2 = 1,4-diazabicylco[2.2.2]octane) the preferential orientations and the rotational energy barriers of both layer (L1) and pillar (L2) linkers. The calculations suggest unhindered rotational motion for pillar linker in all compounds studied (ΔEbarrier ⩽ 0.14 kcal mol−1). For layer linkers, the energy barriers for the rotation of benzene rings are found to vary significantly, indicating an hindered rotation in the case of the fluorine substituent (ΔEbarrier ≈ 14 kcal mol−1), and a static situation in the case of the methyl substituent (ΔEbarrier ≈ 75 kcal mol−1). The rotational dynamics are found not to depend on the type of metal ion. AIM analysis indicates that the rotational energy barriers are influenced by C–H⋯F hydrogen-bonding interactions between the dabco and fluorine-substituted linkers.