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The compound bis(2-amino-4-oxo-6-methylpyrimidinium) tetrachlorocuprate(II) 1 contains CuCU 4  2-  square-planar anions, and bis(2-amino-4-chloro-6-methylpyrimidinium) hexachlorodicuprate( II) 2 quasi-planar CU 2 CI 6  2-  anions. Both compounds show thermochromic behaviour. This phenomenon has been studied by X-ray crystallography at variable temperature, with the result that no major change is observed in the geometry of the copper atom. Thus it is possible to assume some influence of the hydrogen bonds and o f the different geometries of intermediate states on the color of the compounds due to the modifications provoked in the L→M charge transfer and in the energy o f the metal d-d transitions. Magnetic measurements of the compounds give information on magneto-structural correlations. Compound 1 is ferromagnetic (T c  ≅ 20 K) due to the perpendicular arrangement of the square anions that allows exchange pathways only via Cu-Cl -Cu , H-bonding or cationic n electron interactions. Compound 2 shows a very complicated behavior at low temperature with local antiferromagnetic fluctuations. Crystal data: 1 C 10 H 10 N 6 O 2 Cl 4 Cu, triclinic, P1; 393(2) K: α = 11.053(2), b = 11.334(2), c =14.038(3) (Å), α = 95.76(3), β = 101.35(3), γ=90.15(3)(°) ; 293(2) K: a = 11.022(2), b = 11.289(2), c = 14.001(3) (Å), α = 95.86(2), β = 101.34(2), γ=90.09(3)(°) ; 155(2) K: a = 11.008(2), b = 11.231 (2), c = 13.967(3) (A), a = 95.86(2), β = 101.37(2), γ=89.99(2)(°) ; Z = 4. 2 (C 5 H 7 N 3 Cl 3 Cu) 2  , monoclinic, P2 1 /c; 293(2) K: a = 5.998(1), b =18.669(4), c = 9.466(2) (Å), β =100.93(3)(°) ; 150(2) K: a = 5.971(1), b = 18.655(4), c = 9.383(2) (Å), ß =101.64(1)(°);Z=4

zeitschrift für naturforschung b

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)