FICN-6 was synthesized
via a solvothermal reaction of copper sulfate and the H
2pba ligand. Through the morphology analysis by scanning electron microscope (SEM), it was found that the hexagonal shape of FICN-6 can be clearly seen at 10 µm size (
Fig. 1a). Single crystal X-ray diffraction studies revealed that FICN-6 crystallizes in the orthorhombic
Cmca space group. FICN-6 has a two-fold interpenetration structure (
Fig. 1b). For each network, there are two different types of secondary building units (SBUs), one is the four-connected Cu
2(O
2CR)
4 paddle-wheel dimer and the other is the Cu
3(OH)(pyz)
3(O
2CR) planar triangular trimer. The Cu
3 trimer is coordinated with three pyrazolate groups and a carboxylate group from four pba
2− ligands, so both SBUs are four-connected. Topological analysis found that each isolated network adopts in
nbo topology (
Fig. 1d). Remarkably, the two interpenetrated networks are not only packed by non-covalent interactions, but also bridged with sulfate anions on the Cu
3 trimer SBU. The Cu
3 trimer is capped by a sulfate anion from the axial direction, which also coordinates to another Cu
3 trimer from the other network. Therefore, the Cu
3 trimers from two distinct networks are bridged by two sulfate anions to form an eight-connected Cu
6(OH)
2(pyz)
6(SO
4)
2(O
2CR)
2 cluster (
Fig. 1c). The parallel distance between the two Cu
3 trimers is about 3.20 Å. The two-fold interpenetration structure reduce the size of the pore and cavity in the framework. Viewed from the
b axis, the MOF has quadrilateral channels with diagonals of approximately 1.1 nm × 0.8 nm (
Fig. 1b). The powder X-ray diffraction pattern (PXRD) of FICN-6 is consistent with that simulated from single-crystal structure, indicating the phase purity of bulk sample (
Fig. 1e). Although the single crystal structure of FICN-6 gives a solvent accessible volume of 65.0% as calculated by PLATON, nitrogen and CO
2 sorption experiments indicate a low BET surface area of about 18 m
2/g (Fig. S6 in Supporting information). The low surface area may be attributed to the strong breathing effect, which is common for MOFs with large quadrilateral channels [
34,
35]. Thermogravitic analysis indicated that FICN-6 remains stable below 300 ℃ (Fig. S3 in Supporting information).