Single-Crystal-to-Single-Crystal Transformation in a Copper(II) Complex: Structural and Spectroscopic Insights into Methanol Elimination and Hydrogen Bond Network Formation.

This study investigates the single-crystal-to-single-crystal (SCSC) transformations of a copper(II) complex, [Cu(L1)(acac)]·2CHOH (1), with an octahedral coordination geometry. The complex is synthesized using L1 (6-phenyl-1,3,5-triazine-4,2-diamine; CHN) and copper acetylacetonate. In this structure, copper is coordinated to four oxygen atoms from two monoanionic acetylacetonate (acac) ligands and two nitrogen atoms from two neutral L1 ligands. The transformation of complex 1 into complex 2 is achieved by heating at 80 °C for 48 h, leading to the removal of methanol. This elimination facilitates the formation of direct hydrogen bonds between the NH groups and nitrogen atoms of adjacent triazine rings, establishing a network of intermolecular interactions. Structural analysis revealed a 0.2 Å elongation of the copper-nitrogen bond in the L1 ligand as a result of methanol removal. Complementary characterization techniques, including FTIR, UV-vis spectroscopy, and Hirshfeld surface analysis, are employed to further elucidate the transformations. The impact of methanol elimination on the crystal structure is assessed, highlighting the changes in intermolecular interactions.