Enlarging an Isoreticular Family: 3,3′,5,5′-Tetramethyl-4,4′-bipyrazolato-Based Porous Coordination Polymers

The solvothermal reaction between
the rigid spacer 3,3′,5,5′-tetramethyl-4,4′-bipyrazole
(H2Me4BPZ) and a number of transition metal
salts promoted the formation of the coordination polymers [M­(Me4BPZ)] (M = Zn, 1; Co, 2; Cd, 3; Cu, 4). Ab initio X-ray powder
diffraction analyses revealed the main structural aspects of the four
materials. 1 and 2 are representative examples
of the so-called isoreticular strategy: isostructural to [M­(BPZ)]
and [M­(BDP)] (H2BPZ = 4,4′-bipyrazole; H2BDP = 1,4-bis­(1H-pyrazol-4-yl)­benzene), they feature
three-dimensional (3-D) porous networks containing square-shaped channels.
In 3, tetrahedral Cd­(II) ions are arranged within homochiral
helices reciprocally linked by radial Me4BPZ spacers, overall
creating a 3-D nonporous network. Finally, the 3-D porous framework
of 4 comprises square Cu4 nodes linked to
eight neighboring ones by the bridging spacers. Thermogravimetric
analyses, coupled to variable-temperature X-ray powder diffraction,
demonstrated the remarkable thermal robustness of all the materials,
decomposing above 300 °C, and their stability for consecutive
heating–cooling cycles. N2 and CO2 adsorption
measurements at 77 and 273 K, respectively, were employed to probe
the permanent porosity of the materials and to give a coherent picture
of their textural properties including specific surface areas, micro-
and mesopore volumes, as well as size distributions.