Thermochromism in copper(II) complexes: Spectroscopic, thermal, and electrical properties and room-temperature crystal structure of bis(2,2-dimethylpropane-1,3-diamine)copper(II) dinitrate

The compound bis(2,2-dimethylpropane-1,3-diamine)copper(II) dinitrate, Cu(dmpd)2(NO3)2, undergoes a first-order thermochromic phase transition at 142 °C, changing colour from blue-violet to blue upon heating. The crystal structure determination of the low-temperature phase demonstrates that the copper ion has a distorted (4 + 2) octahedral geometry, with the two six-membered chelate rings forming the basal plane (Cu-N = 2.020, 2.029 Å), and oxygens from the two nitrate groups weakly co-ordinated in the fifth and sixth co-ordination sites (Cu-O = 2.566 Å). The i.r. data have been interpreted in terms of the presence of semi-co-ordinated nitrate ions in the low-temperature phase and of unco-ordinated nitrate ions in the high-temperature phase. The electronic spectra reveal a weakening of the ligand field in the high-temperature phase. The breadline n.m.r. indicates the onset of a dynamic disorder of the chelate rings upon heating through the phase transition. The weakening of the crystal field may be due to this dynamic disorder and/or the presence of a small tetrahedral distortion. Electrical conductivity measurements show interesting relaxation processes associated with the phase transition.