Abstract :
The crystal structure of (C19H24N2O2C)2(CdCl4)2K(Cd0.5Cu0.5Cl4)2K has been determined by X-ray diffraction at 295 K. The compound
crystallises in orthorhombic P212121 space group with unit cell parameters: aZ12.7381(10), bZ13.5394(10), cZ25.6520(10) A ° , and ZZ4.
Packing of molecules in the unit cell is determined by single and bifurcated hydrogen bonds between cinchoninium cation, (cinchonineH2)2C,
CuCl2K
4 and CdCl2K
4 tetrahedral anions. Specific feature of the packing is a partial stacking of the quinoline rings of adjacent cinchoninium
cations. There are eight sites forMZCd 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 123.38) and have relatively long Cu–Cl
bonds (2.338 A ° ) at 295 K. Reflectance optical absorption band at 8700 cmK1 with a shoulder at 6200 cmK1 are assigned to (x2Ky2)/(z2) and
(x2Ky2)/(xy) transitions, and they follow the well-known relationship between band positions and geometry of tetrachlorocuprates. EPR
shows that the exchange coupling between Cu2Cions is very weak and an individual CuCl4 gives separate lines in EPR spectrum. The spectrum
below 200 Kis characterised with parameters gxZ2.047, gyZ2.127, and gzZ2.404 and above this temperature becomes dynamically averaged
to gsZ2.299 and gtZ2.065. 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 Cu2Cin weakly deformed tetrahedra. It is described as reorientations
of the CuCl4 between two Jahn–Teller distorted configurations, across the energy barrier of about 80 cmK1, resulting in dynamically averaged
geometry of the CuCl4-tetrahedra observed at room temperature.
q 2005 Elsevier B.V. All rights reserved.
Keywords :
X-ray diffraction , EPR , Cinchonine , Cd/CuCl4
