Crystal structure and EPR studies of (cinchonineH2)2(CdCl4)(Cd/CuCl4) crystals with thermochromic and Jahn-Teller effect

The crystal structure of (C19H24N2O2+)2(CdCl4)2−(Cd0.5Cu0.5Cl4)2− has been determined by X-ray diffraction at 295 K. The compound crystallises in orthorhombic P212121 space group with unit cell parameters: a=12.7381(10), b=13.5394(10), c=25.6520(10) Å, and Z=4. Packing of molecules in the unit cell is determined by single and bifurcated hydrogen bonds between cinchoninium cation, (cinchonineH2)2+, CuCl42− and CdCl42− tetrahedral anions. Specific feature of the packing is a partial stacking of the quinoline rings of adjacent cinchoninium cations. There are eight sites for M=Cd or Cu in the crystal unit cell. In the four of them the CdCl4 tetrahedra are located. In the others Cd or Cu can be located and an average geometry (Cd/CuCl4) is described by the X-ray diffraction data. EPR allows to observe only the CuCl4 complexes and we have proved that there is not a random distribution of individual Cd and Cu among the (Cd/CuCl4) sites but they are coupled in dimers, so that asymmetric unit can be identified as (cinchonineH2·MeCl4)2 moiety. Specific feature of the structure is that CuCl4 tetrahedra are relatively weakly deformed from ideal tetrahedral geometry Td towards the D2d symmetry (flattening angle 123.3°) and have relatively long Cu-Cl bonds (2.338 Å) at 295 K. Reflectance optical absorption band at 8700 cm−1 with a shoulder at 6200 cm−1 are assigned to (x2−y2)→(z2) and (x2−y2)→(xy) transitions, and they follow the well-known relationship between band positions and geometry of tetrachlorocuprates. EPR shows that the exchange coupling between Cu2+ ions is very weak and an individual CuCl4 gives separate lines in EPR spectrum. The spectrum below 200 K is characterised with parameters gx=2.047, gy=2.127, and gz=2.404 and above this temperature becomes dynamically averaged to g∥=2.299 and g⊥=2.065. This continuous type transition to the dynamic phase, without a thermal effect, is accompanied by the change in the crystal colour from green to yellow which is characteristic for thermochromic transitions observed in tetrachlorocuprates. The dynamics is related to the Jahn-Teller effect operating in the triplet ground state T2 of Cu2+in weakly deformed tetrahedra. It is described as reorientations of the CuCl4 between two Jahn-Teller distorted configurations, across the energy barrier of about 80 cm−1, resulting in dynamically averaged geometry of the CuCl4-tetrahedra observed at room temperature.