Synthesis and structural study of the thermochromic compounds bis(2-amino-4-oxo-6-methylpyrimidinium) tetrachlorocuprate(II) and bis (2-amino-4-chloro-6-methylpyrimidinium) hexachlorodicuprate(II)

The compound bis(2-amino-4-oxo-6-methylpyrimidinium) tetrachlorocuprate(II) 1 contains CuCl42- square-planar anions, and bis(2-amino-3-chloro-6-methyl pyrimidinium) hexnchlorodicuprate(II) 2 quasi-planar Cu2Cl62- anions. Both compounds show thermochromic behaviour. This phenomenon has been studied by X-ray crystallography at variable temperature, with the result that no maj or change is observed in the geometry of the copper atom. Thus it is possib le to assume some influence of the hydrogen bonds and of the different geom etries of intermediate states on the color of the compounds due to the modi fications provoked in the L-->M charge transfer and in the energy of the me tal d-d transitions. Magnetic measurements of the compounds give informatio n on magneto-structural correlations. Compound 1 is ferromagnetic (T-c cong ruent to 20 K) due to the perpendicular arrangement of the square anions th at allows exchange pathways only via Cu-Cl ... Cu , H-bonding or cationic p i electron interactions. Compound 2 shows a very complicated behavior at lo w temperature with local antiferromagnetic fluctuations. Crystal data: 1 C1 0H16N6O2Cl4Cu, triclinic, P (1) over bar;393(2) K: a=11.053(2), b=11.334(2) , c =14.038(3) (Angstrom), alpha = 95.76(a), beta = 101.35(3), gamma = 90.1 5(3)(degrees); 293(2) K: a = 11.022(2), b = 11.289(2), c = 14.001(3) (Angst rom), alpha = 95.86(2), beta = 101.34(2), gamma = 90.09(3)(degrees); 155(2) K: a = 11.008(2), b = 11.231(2), c = 13.967(3) (Angstrom), alpha = 95.86(2 ), beta = 101.37(2), gamma = 89.99(2)(degrees); Z=4. 2 (C5H7N3C13Cu)(2), mo noclinic, P2(1)/c; 293(2) K: a = 5.998(1), b =18.669(4), c = 9.966(2) (Angs trom), beta = 100.93(3)(degrees); 150(2) K: a = 5.971(1), b = 18.655(4), c = 9.383(2) (Angstrom), beta = 101.64(1)(degrees); Z=4.