Preparation, characterization and solid state thermal studies of nickel (II) iodide complexes of ethane-1,2-diamine and its derivatives

Abstract

[NiL₃]I₂•2H₂O [L = N–methylethane–1,2–diamine (meen) and 2-methylpropane-1,2-diamine (ibn)], [NiL₃]I₂ [L = ethane–1,2–diamine (en), N-ethylethane-1,2-diamine (eten), N-propylethane-1,2-diamine (pren) and propane-1,2-diamine (pn)], [NiL₂(H₂O) ₂]I₂ (L = meen and eten) and [Ni₂L₂]I₂ [L = N-isopropylethane-1,2-diamine (ipren) and ibn] have been synthesized from solution and characterized. [NiL₂I₂] (L = en, meen, eten and pren] and [Ni(ibn) ₂]I₂ have been isolated in the solid state from their corresponding parent tris(diamine) complex by temperature arrest technique. [NiL₂(H₂O) ₂]I₂ (L = meen and eten) upon heating undergo deaquation-anation reaction. All the diamine species excepting [Ni(ipren)₂]I₂ and [Ni(ibn) ₂]I₂ which possess square-planar geometry are octahedral and both the diaqua and diiodobis(diamine) species possess trans-octahedral geometry showing variation only in [Ni(en) ₂I₂], which is having cis-octahedral geometry. [Ni(meen) ₂I₂] and [Ni(eten) ₂I₂] show irreversible endothermic phase transitions (160—190 °C; ΔH = 5.65 kJ mol⁻¹ and 204—225 °C; ΔH = 4.23 kJ mol⁻¹, respectively) without any visual color change with retention of trans-octahedral configuration. In case of [Ni(eten) ₂I₂] the post phase species reverts to the original diaqua species, [Ni(eten) ₂(H₂O) ₂]I₂ on keeping in open atmosphere whereas the rehydrated species of its meen analogue, [Ni(meen) ₂(H₂O) ₂]I₂ after deaquation does not show the phase transition. On the other hand, corresponding pren analogue shows reversible endothermic phase transition (68-111 °C; ΔH = 2.42 kJ mol⁻¹ for heating and 98-77 °C; ΔH = −1.4 kJ mol⁻¹ for cooling). The occurrence of this type of phase transition is due to the conformational changes in the diamine chelate rings. The square-planar [Ni(ibn) ₂]I₂ undergoes time dependent reversible phase transition (232-248 °C; ΔH = 5.99 kJ mol⁻¹) without any visual color change retaining the original geometry. Here also conformational changes in chelate rings are assumed to be responsible for this transition.