Coordination Polymers Under Pressure

Coordination polymers are relevant to many fields in chemistry and material science with numerous potential applications. Structural diversity plays an important role in the design and optimization of coordination polymers. This is not limited to using fixed geometries and dimension-based linkers in the fabrication of coordination polymers of varying pore sizes and dimensions. Alternatively, various approaches are used in enhancing the properties of coordination polymers. For instance, the use of flexible ligands is specifically suitable for the synthesis of coordination polymers for specific applications.

Among available ligands, 4,4’-trimethylenedipyridine has been widely used as a flexible N-donor ligand due to its ability to adopt various conformations. Combined with the structural diversity provided by the secondary building units, flexible linkers provide an additional handle to tune topologies. Regardless of the kind of coordination polymers – with flexible or rigid linkers – the mechanical properties have not been studied in detail. Such studies enable understanding of the effects of external factors, such as pressure or temperature, on these polymers, which would further extend their application in a demanding environment. Unfortunately, little has been done concerning the effects of pressure at the molecular level.

To this end, Florida International University researchers Dr. Kaige Shi, Dr. Logesh Mathivathanan, Dr. Vadym Drozd and Professor Raphael Raptis have investigated the effects of pressure-induced deformation of trinuclear Cu-pyrazolato based coordination polymers. In particular, they synthesized three topologically isomeric coordination polymers by incorporating flexible 4,4’ trimethyenedipyridine linkers and tricopper pyrazolate building units. Their research work has been published in the American Chemical Society journal, Crystal Growth and Design.

In brief, the research team initiated their studies by a detailed examination of the one-dimensional and two-dimensional coordination polymers consisting of the aforementioned flexible linkers. Next, they characterized the three synthesized coordination polymers using both the single-crystal and powder-based X-ray diffraction techniques. In addition, the same methods were used to determine their elasticity and amorphization properties. Eventually, the response by the new coordination polymers to a range of applied pressures was monitored by both single-crystal and powder X-ray diffraction in a diamond anvil cell.

The authors observed reversible lattice contraction of the coordination polymers due to applied pressure of up to 4 GPa. However, at even higher pressure, there was a significant loss of crystallinity. These occurrences were attributed to the lattice compression that flattened the C-C-C angles under pressure.

In summary, Professor Raphael Raptis and his colleagues successfully demonstrated the effects of pressure on the structures of the coordination polymer. From the experiments, the linkers were observed to respond to external pressure by expanding along a crystallographic axis. Furthermore, amorphization pressure equivalent to that required to deform three-dimensional structures were obtained. Altogether, the work sheds light on the effects of pressure on coordination polymers that can be further improved to extend their applications in various fields.