Magnetic and Electric Properties of Pyrazole-Based Metal–Organic Frameworks Grafted With a Sulfonic Moiety

Abstract

We report herein two multifunctional metal–organic frameworks (MOFs) that exhibit excellent mutually inclusive electrical and magnetic properties. Accordingly, two cobalt and nickel based MOFs (Co-MOF, Ni-MOF) were generated using a flexible bispyrazole based ligand and 2-sulphono terephthalic acid. The idea is to generate paramagnetic metal ion based magnetic MOFs, which can also be used to fabricate electrical devices by utilizing the immobilized free sulfonic groups and encapsulated H-bonded water clusters for active charge species generation and transportation. Further support comes from the intriguing structural features of the MOFs that include extensive H-bonded water clusters, free sulfonic acid moiety, or syn-anti bridged carboxylates, which make them highly suitable candidates for generating electrical and magnetic materials. Further complementary support for their candidature comes from the high thermal, chemical, and physical stability of the MOFs. The impedance spectroscopy data and I–V results unequivocally support the suitability of the MOFs for electronic device fabrication showing a befitting conductivity value of 1.80 × 10–4 S/m with an ideality factor of 1.06 for Ni-MOF. Interestingly, the Co-MOF shows a light-dependent behavior with conductivity values of 9.09 × 10–5 S/m (dark) and 6.31 × 10–4 S/m (light) and ideality factors of 0.78 (dark) and 0.92 (light). The MOFs, fitted with a free sulfonic acid moiety and extensive H-bonded water clusters, show high potential for proton exchange membrane fuel cells (PEMFCs) development with corroborating proton conductivity values of 1.95× 10–3 S/cm and 5.80 × 10–4 S/cm for Ni-MOF and Co-MOF, respectively, at 95% relative humidity and 85 °C. Moreover, the interesting structural aspects like syn-anti bridged carboxylates prompt us to explore the magnetic behavior of the MOFs. The Ni-MOF shows some interesting antiferromagnetic behavior. The Co-MOF reveals intriguing single molecule magnet behavior with a Ueff value of 34 K and moderate relaxation time of 3.5× 10–8 s.