Thermochromism in nickel (II) complexes: thermal, IR spectroscopic, solid-state 1H NMR and single crystal X-ray analysis of diaqua-bis(2,2-dimethyl-1,3-propanediamine) nickel (II) nitrate

The newly synthesized complex [Ni(dmtn)₂(H₂O) ₂](NO₃)₂ (dmtn = 2,2-dimethyl-1,3-propanediamine) undergoes a reversible thermochromic phase transition in the range of 180–202°C (ΔH = 10.4kJ mol⁻¹) on heating after dehydration (dehydration temperature range 43–102°C). The complex has been characterized by X-ray crystallography at room temperature. The crystals are monoclinic, space group C2m, a = 20.385(2), b = 6.9004(4), c = 6.9552(3)Å, β = 102.60(5)°, V = 954.8(1)Å3, Z = 2 and R = 0.046. The geometry of the nickel atom in the complex is tetragonally distorted octahedral with two H₂O molecules coordinated at the trans position. The diamine chelate rings are in the trans chair-chair conformations. The broad-line ¹H NMR of partially deuterated [Ni(dmtn-d₄)₂(NO₃)₂] (deuterated amine protons) indicates the onset of dynamic disorder of the diamine chelate rings at the phase transition temperature. The high temperature IR spectral measurement shows the complex retains its trans configuration after dehydration and phase transition with NO⁻₃ ions coordinated as a monodentate ligand