Unconventional Pyridyl Ligand Inclusion within a Flexible Metal-Organic Framework Bearing an N,N'-Diethylformamide (DEF)-Solvated Cd5 Cluster Secondary Building Unit.

A cationic three-dimensional (3D) metal-organic framework (MOF) sustained by an N,N'-diethylformamide (DEF)-solvated zigzag-shaped Cd5 cluster secondary building unit (SBU), [Et2 NH2 ]2 [Cd5 (BTB)4 (DEF)2 ] ⋅ 4.75DEF (1 a, H3 BTB=benzene-1,3,5-tribenzoic acid) shows flexible framework behavior and undergoes solvate exchange with CHCl3 to yield [Et2 NH2 ]2 [Cd5 (BTB)4 (DEF)2 ] ⋅ xCHCl3 (1 b) accompanied by changes to pore sizes and shapes. Unexpectedly, the DEF solvates coordinated to the central Cd2+ could not be replaced by strongly donor pyridyl and dipyridyl ligands. Additionally, more electron-deficient pyridyls preferentially coordinated to the flanking Cd2+ of the Cd5 SBU, as exemplified by [Et2 NH2 ]2 [Cd5 (BTB)4 (DEF)2 (PyCHO)2 ] ⋅ xSol (2 a, PyCHO=4-pyridinealdehyde) and [Et2 NH2 ]2 [Cd5 (BTB)4 (DEF)2 (PyAc)2 ] ⋅ xSol (2 b, PyAc=4-acetylpyridine). Density functional theory (DFT) calculations were used to understand these counterintuitive observations.