Preparation, characterization and solid state thermal studies of nickel(II) squarate complexes of 1,2-ethanediamine and its derivatives

Abstract

[NiL₃]C₄O₄ [L = 1,2-ethanediamine (en) and 1,2-N,N′-dimethylethanediamine (N,N′-dmen)], [NiL₃]C₄O₄•nH₂O [for L = 1,2-N-methylethanediamine (meen) and 2-methyl-1,2-propanediamine (ibn); n = 1, and for L = 1,2-propanediamine (pn); n = 2] and [NiL₂C₄O₄]•2H₂O [L = 1,2-N-ethylethanediamine (eten), 1,2-N-propylethanediamine (pren), 1,2-N,N-dimethylethanediamine (N,N-dmen) and 1,2-N,N′-diethylethanediamine (N,N′-deten)] have been synthesized by the addition of the respective diamines to finely powdered nickel (II) squarate dihydrate, NiC₄O₄•2H₂O. All of the tris-species, except for [Ni(N,N′-dmen)₃]C₄O₄ were also prepared by adding diamines to NiC₄O₄•2H₂O dissolved in aqueous ammonia (25%). When the latter method was followed, the bis-species [NiL₂C₄O₄] [L = eten, pren, 1,2-N-isopropylethanediamine (ipren), N,N-dmen, N,N′-dmen, and N,N′-deten] were also obtained. [NiL₂C₄O₄] (L = en, meen, N,N′-dmen, pn, and ibn) were also prepared from their corresponding tris (diamine) species upon heating in the solid state. [Ni(ibn)₃]C₄O₄•nH₂O after dehydration underwent an irreversible endothermic (145-160 °C; ΔH = 6.5 kJ mol⁻¹) phase transition without showing any visual color change. [Ni(ipren)₂C₄O₄] exhibited a reversible phase transition (78-107 °C; ΔH = 1.4 kJ mol⁻¹ for heating and 105-72°C; ΔH = −1.3 kJ mol⁻¹ for cooling). The phase transitions are considered to have been due to conformational changes of the diamine chelate rings. All of the tris as well as bis (diamine) complexes possess an octahedral geometry, except for [Ni(ibn)₂]C₄O₄, which is square-planar. The bis-species [NiL₂C₄O₄] (L = eten, pren, N,N-dmen and N,N′-deten) exists in two isomeric forms.