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

Graphical Flexible friend : A metal–organic framework sustained by a Cd 5 cluster shows flexible behavior and coordination preference toward electron-deficient rather than electron-rich pyridyl ligands. Density functional theory calculations offer binding energies of the electron-deficient 4-pyridinealdehyde (1.09 eV) and 4-acetylpyridine (1.36 eV) to the flanking Cd centers, and are larger than those of the electron-rich ligands, such as 4-iodopyridine (0.32 eV) and 4,4′-bipyridine (0.61 eV). 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), [Et 2 NH 2 ] 2 [Cd 5 (BTB) 4 (DEF) 2 ] ⋅ 4.75DEF ( 1 a , H 3 BTB=benzene-1,3,5-tribenzoic acid) shows flexible framework behavior and undergoes solvate exchange with CHCl 3 to yield [Et 2 NH 2 ] 2 [Cd 5 (BTB) 4 (DEF) 2 ] ⋅ x CHCl 3 ( 1 b ) accompanied by changes to pore sizes and shapes. Unexpectedly, the DEF solvates coordinated to the central Cd 2+ could not be replaced by strongly donor pyridyl and dipyridyl ligands. Additionally, more electron-deficient pyridyls preferentially coordinated to the flanking Cd 2+ of the Cd 5 SBU, as exemplified by [Et 2 NH 2 ] 2 [Cd 5 (BTB) 4 (DEF) 2 (PyCHO) 2 ] ⋅ x Sol ( 2 a , PyCHO=4-pyridinealdehyde) and [Et 2 NH 2 ] 2 [Cd 5 (BTB) 4 (DEF) 2 (PyAc) 2 ] ⋅ x Sol ( 2 b , PyAc=4-acetylpyridine). Density functional theory (DFT) calculations were used to understand these counterintuitive observations.