A systematic investigation of structural transformation in a copper pyrazolato system: a case study

Dissolution-recrystallization structural transformation (DRST) is a powerful tool to unravel unequivocally the mechanism of dynamic conversion processes, based on the structures of the initial reactants, final products and sometimes intermediates isolated from the reaction mixture. Herein, we illustrate the details of the conversion processes of (Cu I pz) 3 into [Cu II (OH)pz] 6 (pzH = 4-chloro-3,5-diphenylpyrazole) through DRSTs. Based on crystal structure determination and spectroscopic methods, the most encountered species, (Cu I pz) 3 , is in equilibrium with (Cu I pz) 4 in solution with the tetramer becoming dominant at low temperature, indicating an entropy-controlled conversion between these two structural isomers. The Cu I trimer/tetramer in solution further experiences an oxidation if exposed to the open air, resulting in the formation of a pentanuclear mixed-valence intermediate Cu I 3 Cu II 2 (OMe) 2 pz 5 which can be isolated as single crystals at −20 °C and has been structurally characterized for the first time. The final product isolated from the solution is the fully oxidized hexanuclear [Cu II (OMe)pz] 6 , which is easily transformed into [Cu II (OH)pz] 6 in the presence of humidity. The mixed-valence Cu I 3 Cu II 2 (OMe) 2 pz 5 has been isolated as a possible key intermediate in the structural conversion from (Cu I pz) 3 / (Cu I pz) 4 to cyclic hexanuclear Cu II species.