Synthesis, structural and spectroscopic characterization of the thermochromic compounds A2CuCl4: [(Naphthyl ethylammonium)2CuCl4]

Two A2CuCl4 type (A = organic ammonium cation) compounds, namely bis-(R)-(+)-naphthyl-1-ethylammonium tetrachlorocuprate(II) (1) and bis-(R)-(+)-naphthyl-2-ethylammonium tetrachlorocuprate(II) (2) were synthesized. Compounds 1 and 2 crystallize in the monoclinic crystal system with the non-centrosymmetric space groups P21 and C2 respectively, with Z = 2. In both compounds the rigid regio-isomeric ligands surround the CuCl42− anion, forming an isolated distorted tetrahedral structure. Temperature dependent single crystal X-ray and FT-IR measurements show diverse ‘motifs’ in the NH⋯Cl interactions and concurrence with the structural phase transition in the DSC study. The observed structural transition is not due to a change in geometry around CuCl42−, normally seen in the literature, but is driven by ‘motifs’ conceived in the ligand itself. A polarizing microscopic study revealed that these reversible transitions are of a thermochromic nature.

Bis-(R)-(+)-naphthyl-1-ethylammonium tetrachlorocuprate(II) (1) and bis-(R)-(+)-naphthyl-2-ethylammonium tetrachlorocuprate(II) (2) were synthesized and characterized by various spectral techniques. These compounds show thermochromism originating from the rigid regio-isomeric ligands around the Cu(II) center. The molecular structures of both compounds at different temperatures were determined by single crystal X-ray crystallography. One of the driving forces for such an occurrence lies in the observation of a new motif in the form of NH⋯Cl interactions amongst the ligands.Figure optionsDownload as PowerPoint slide