Crystal structure, thermal, electric and magnetic study of [(CH2)7(NH3)2]CoCl2Br2

Abstract

1,7-Heptanediammonium-dichlorodibromo-cobaltate (II), with molecular formula [(CH2)7(NH3)2]CoCl2Br2, has been synthesized. It is triclinic $$ {\text{P}}\bar{1} $$ , a = 7.4042 (4) A, b = 10.3484 (5) A, c = 11.3554 (6) A, α = 66.289 (3)° β = 78.425 (2)°, γ = 86.546 (3)°. Its crystal structure contains distorted tetrahedral [CoCl2Br2]2− anions and zigzag [(CH2)7(NH3)2]2+ cations which are connected via a network of hydrogen bonds. The largest difference in Co–Cl and Co–Br bond lengths are 0.081 A and largest difference in Br–Co–Cl angles is 8.1°. The compound has been studied by thermal analysis, impedance spectroscopy and magnetic susceptibility. Thermal studies show three phase transitions at T 1 = 378 K, T 2 = 355 K, associated with solid–solid phase change and T 3 = 333.8 K ascribed to chain conformation and reorientation. Permittivity and ac conductivity as a function of temperature (300–395 K) and frequency (0.06 kHz < f < 100 kHz) is presented. AC conductivity is thermally activated in different phases. Conduction is mainly due to Cl− and Br− ion hopping in the high temperature range. Magnetic susceptibility yields an effective magnetic moment μ eff = 4.43 BM and Curie–Weiss temperature, Θ = −4.7 K which confirm tetragonal symmetry of [CoCl4]2− and weak antiferromagnetic interaction at low temperatures.