Spin-transition compounds are coordination complexes that can present two stable or metastable high-spin and low-spin states at a given temperature (thermal hysteresis). The width of the thermal hysteresis (difference between the maximum and minimum temperature between which the compound exhibits bi-stability) depends on the interactions between the coordination complexes within the compound, and which may be modulated by the absence or presence of solvent within the structure. The new compound [Fe(3-bpp) ][Au(CN) ] (1, 3-bpp=2,6-di-(1H-pyrazol-3-yl)pyridine) was synthesized and its properties were compared with those of the solvated compound [Fe(3-bpp) ][Au(CN) ] ⋅2 H O (1.H O) already described. 1 has a two-steps thermal hysteresis of 45 K, in contrast to the compound 1.H O which exhibits a gradual conversion without hysteresis. This hysteretic transition is accompanied by a reversible reconstructive structural transition and twinning. This stepped behaviour is also observed in the photomagnetic properties despite the low efficiency of photoswitching. Single-crystal photocrystallographic investigations confirm this low conversion, which we attributed to the high energy cost to form the high-spin structure, whose symmetry differs from that of the low-spin phase.

COVID-19 is rapidly spreading and there are currently no specific clinical treatments available. The absence of an immediate available vaccine against SARS-CoV-2 made it hard for health professionals to tackle the problem. Thus, the need of ready to use prescription drugs or herbal remedies is urgent. SARS-CoV-2 main protease (Mpro) and Angiotensin Converting Enzyme2 (ACE2) protein structure are made available to facilitate finding solutions to the present problem. In this brief research, we compare the efficacy of some natural compounds against COVID-19 Mpro and ACE2 to that of Hydroxy-Chloroquine in silico.<br>Molecular docking investigations were carried out using AutoDock. Virtual screening was performed using AutoDock Vina and the best ligand / protein mode was identified based on the binding energy. Amino Acids residues of ligands interactions were identified using PyMOL. According to present research results, Quercetin, Hispidulin, Cirsimaritin, Sulfasalazine, Artemisin and Curcumin exhibited better potential inhibition than Hydroxy-Chloroquine against COVID-19 main protease active site and ACE2. Our provided docking data of these compounds may help pave a way for further advanced research to the synthesis of novel drug candidate for COVID-19.<br>