Unconventional Pyridyl Ligand Inclusion within a Flexible Metal-Organic Framework Bearing an N,N'-Diethylformamide (DEF)-Solvated Cd 5 Cluster Secondary Building Unit.

A cationic three-dimensional (3D) metal-organic framework (MOF) sustained by an N,N '-diethylformamide (DEF)-solvated zigzag-shaped Cd-5 cluster secondary building unit (SBU), [Et2NH2](2)[Cd-5(BTB)(4)(DEF)(2)] . 4.75DEF (1 a, H3BTB=benzene-1,3,5-tribenzoic acid) shows flexible framework behavior and undergoes solvate exchange with CHCl3 to yield [Et2NH2](2)[Cd-5(BTB)(4)(DEF)(2)] . xCHCl(3) (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 Cd-5 SBU, as exemplified by [Et2NH2](2)[Cd-5(BTB)(4)(DEF)(2)(PyCHO)(2)] . xSol (2 a, PyCHO=4-pyridinealdehyde) and [Et2NH2](2)[Cd-5(BTB)(4)(DEF)(2)(PyAc)(2)] . xSol (2 b, PyAc=4-acetylpyridine). Density functional theory (DFT) calculations were used to understand these counterintuitive observations.