Thermal studies of N-propylethane-1,2-diamine and N-isopropylethane-1,2-diamine complexes of nickel (II) in the solid state

Abstract

[NiL₃]X₂•2H₂O (X is Cl⁻, Br⁻ and ClO⁻₄; L is N-propylethane-1,2-diamine), [NiL₂(H₂O)₂](ClO₄)₂, [NiL₂X₂] (X is Cl⁻, Br⁻, 0.5SO²⁻₄ and 0.5SeO²⁻₄), [NiL′₂ (H₂O) ₂](ClO₄)₂•2H₂O (L′ is N-isopropylethane-1,2-diamine), [NiL′₂X₂] (X is Cl⁻, Br⁻, NCS⁻ and 0.5SeO²⁻₄) and [NiL′₂SO₄]•3H₂O have been synthesised from solution and their thermal studies have been carried out in the solid phase. [NiL₂X₂] (X is Cl⁻ and Br⁻), NiLCl₂ and NiL'SO₄ have been prepared pyrolytically from their respective parent species in the solid state. [NiL₂](ClO₄)₂ and [NiL′₂](ClO₄)₂ were prepared pyrolytically from their parents species, and also by storing [NiL₂(H₂O) ₂](ClO₄)₂ and [NiL′₂ (H₂O) ₂](ClO₄)₂•2H₂O in a CaCl₂ desiccator, respectively; these transformations are accompanied by a pink to yellow colour change due to octahedral to square-planar configurational changes. All other bis(diamine) species have octahedral geometry. Among them, [NiL₂X₂] (X is Cl⁻ and Br⁻) and [NiL′₂X₂] (X is Cl⁻, Br⁻ and NCS⁻) possess trans-octahedral configuration, whereas [NiL₂X] (X is SO²⁻₄ and SeO²⁻₄), [NiL′₂SO₄]•3H₂O and [NiL′₂SeO₄] have cis-configuration. On heating, trans-[NiL′₂Br₂] and trans-[NiL′₂(NCS) ₂] undergo reversible endothermic phase transitions (86–99°C, ΔH = 0.52 kJ mol⁻¹ for heating and 92–81°C, ΔH = −0.53 kJ mol⁻¹ for cooling for the former and 162–224°C, ΔH = 0.27 kJ mol⁻¹ for heating and 208–150°C, ΔH = −0.25 kJ mol⁻¹ for cooling for the latter), most likely due to conformational changes of the diamine chelate rings.