Thermochromism in copper(II) complexes: thermal, spectroscopic, solid-state broadline ¹H NMR and room-temperature single-crystal x-ray analysis of bis(N,N′-dimethyl-1,2-ethanediamine)copper(II) dinitrate

Abstract

The complex bis(N,N′-dimethyl-1,2-ethanediamine)copper (II) dinitrate, Cu (NN′-dmen)₂(NO₃)₂, undergoes a first-order thermochromic phase transition at ca 148°C, changing from blue-violet to blue. The low-temperature phase has been characterized by X-ray crystallography. The copper ion possesses distorted (4+2) octahedral geometry with two five-membered chelate rings, in the δλ conformation, forming the CuN₄ plane [CuN = 2.000(6) and 2.033(7) Å] and oxygens from the two nitrate groups weakly coordinated in the two axial coordination sites [CuO = 2.506(8) Å]. The broad-line ¹H NMR indicates the onset of a dynamic disorder of diamine chelate rings at the phase transition temperature region. The electronic spectrum broadens on phase transition.