Thermal studies of bis(2-aminoethyl)methylamine complexes of nickel(II) in the solid state

Abstract

[NiL 2]X 2· nH 2O (where L is bis(2-aminoethyl)methylamine, and n = 0 when X is Cl −, Br −, SCN − and CF 3COO −, n = 3 when X is 0.5SO 4 2− and 0.5SeO 4 2−) and NiLX 2· nH 2O ( n = 0 when X is Cl −, Br −, NO 3 −, n = 1 when X is CF 3COO − and n = 3 when X is 0.5SO 4 2− and 0.5SeO 4 2−) have been synthesized and investigated thermally in the solid state. All the bis, as well as monotriamine complexes possess octahedral geometry. [Nil 2](SCN) 2 upon heating undergoes irreversible phase transition (155–180°C; Δ H = 5.6 kJ mol −1) yielding an isomeric species which on further heating shows reversible phase transition (182–205°C for heating, Δ H = 2.8 kJ mol −1; 170–200°C for cooling, Δ H = −2.8 kJ mol −1) and [NiL 2]SO 3·3H 2O undergoes an endothermic phase transition after deaquation 172–200°C; Δ H = 6.8 kJ mol −1). Nil(NO 3) 2 also shows a phase transition (195–220°C; Δ H = 8.9 kJ mol −1). The phase transitions are proposed to be due to the conformational change in the chelate rings of the triamine. NiLAO 4·3H 2O (A = S or Se) exhibit thermochromism, blue-to-greenish-blue for elimination of two molecules of water and greenish-blue-to-light-green for elimination of the residual water molecule. The effect of methyl substitution in N 2 – position of bis(2-aminoethyl)amine is not noticeable with respect to thermal stability of the complexes, but is pronounced in the case of thermally induced solid-state isomerization.