Thermal studies of 2-methyl-1, 2-propanediamine complexes of nickel(II) in the solid state

Summary The complexes, [NiL₂]Br₂ (L=2-methyl-1, 2-propanediamine), [NiL₂(H₂O)₂]SeO₄·2H₂O, [NiL₂]X₂·2H₂O (X=0.5SO₄ or 0.5SeO₄) and [NiL₂(NCS)₂] have been prepared and investigated thermally. Upon heating, [NiL₂]Br₂ exhibits reversible thermochromism from yellow to blue: [NiL₂]X₂·2H₂O (X=0.5SO₄ or 0.5SeO₄) undergoes dehydration accompanied with irreversible thermochromism from yellow to blue yielding [NiL₂X₂], whereas [NiL₂(H₂O)₂]SeO₄·2H₂O (blue) transforms irreversibly into [NiL₂SeO₄] (blue). [NiL₂]X₂ (X=0.5SO₄ or 0.5SeO₄), prepared from their corresponding diaquo complexes by the temperature arrest technique, shows irreversible thermochromism from yellow to blue without showing any peak in the d.t.a./d.s.c. curves. [NiL₂(NCS)₂] (blue) undergoes a thermally induced phase transition without any visual change. All the yellow species are square planar; the blue species are octahedral. These colour changes are due to configurational changes; the phase change in [NiL₂(NCS)₂] is probably due to conformational changes in the diamine chelate rings.