Comparison of Mechanochemistry vs Solution Methods for Synthesis of 4,4′-Bipyridine-Based Coordination Polymers

Coordination polymers (CPs) are inherently modular because they are typically comprised of metal-based nodes and organic linker ligands. The resulting ability to control the pore size and pore chemistry results in CPs being of interest in storage, separation, and catalysis. CPs are generally synthesized by solution methods such as solvent diffusion and solvothermal methods; however, these methods can be time-consuming and lead to solvent waste. Mechanochemistry offers a greener and more sustainable approach, but it remains understudied in the context of CPs. This is especially the case for CPs, which contain 4,4′-bipyridine (bipy), the most widely used linker ligand. Herein, we compare the mechanochemistry vs slurry vs solution methods for the synthesis of five CPs formed by a 1:1 ratio of bipy and M^II(NO₃)₂·xH₂O, (M^II = Co, Ni, Zn, x = 6; M^II = Cu, x = 3; M^II = Cd, x = 4). We observed that ball milling and twin-screw extrusion both offer high-yield, low-waste routes to the same CP products prepared from solution, but the water slurry approach was found to be ineffective. The isolated CPs were found to exhibit high thermal stability (≥250 °C) and good compression resistance (≥350 bar). In addition, they have potential utility as sorbents for vapor separation or as precursors to form higher-dimensional CP structures.