Flexibility of CuCl4-tetrahedra in bis[cinchoninium tetrachlorocuprate(II)]trihydrate single crystals. X-ray diffraction and EPR studies

Crystal structure of bis[cinchoninium tetrachlorocuprate(II)] trihydrate, [ (C19H24N2O)CuCl4](2). 3H(2)O, has been determined by X-ray diffraction at 1 00 K and reexamined at 293 K. The compound crystallizes in orthorhombic sys tem with a P2(1)2(1)2(1) space group and unit cell parameters a = 15.3031(1 4), b = 36.415(3), and c = 7.8341(5) Angstrom at 100 K, and Z = 4. The asym metric unit consists of two (CuCl4)(2-) tetrahedral anions linked by hydrog en bonds to two doubly protonated cinchonine molecules and three water mole cules. The tetrahedra are strongly flattened, to approximately D-2d symmetr y, with different deformation for two inequivalent (CuCl4)(2-)-ions in the asymmetric unit. Them deformation of (CuCl4)(2-) and cinchoninium cations v aries with temperature due to a rearrangement of the bifurcated hydrogen bo nd network. This is a continuous process observed as a monotonic variation of the EPR spectral parameters and the unit cell dimensions. EPR spectra sh ow that very weak exchange coupling J(12) = 0.0030 cm(-1) operates between Cu2+ ions within asymmetric units, corresponding to the general formula of the compound, as well as between equivalent Cu2+ sites of different molecul es, whereas the coupling is negligible between inequivalent sites. The inte rmolecular J(12) coupling is temperature-independent indicating that the wh ole asymmetric unit behaves as a magnetic unit (pseudodimer) in the whole t emperature range.