The reaction of copper(II) and 2-hydrazino-2-imidazoline leading to the in situ formation of bisimidazoline (biz). Crystal structure and vibrational spectroscopy of mixed-valence [Cu(biz)2][Cu2Br4] complex

The reaction of Cu(OH)₂ and 2-hydrazino-2-imidazoline hydrobromide surprisingly resulted in complex compound where Cu(II) ions are chelated by a new ligand, namely bisimidazoline (biz). As has been found in the X-ray analysis, the Cu(biz)₂]²⁺ cations are accompanied by Cu₂Br₄]²⁻ anions, which makes the whole compound of metal-mixed-valency type. Both ions are centrosymmetric and quasi-planar. The Cu(II) coordination environment is a rectangle with almost equal Cu–N bond lengths (1.984(3), 1.987(3) Å). The electrostatic interaction of both complex ions is strengthened by two strong N–H···Br and four weaker (C–H···Br, C–H···N) hydrogen bonds. The relatively simple IR and Raman spectra were interpreted with help of quantum calculations carried out at the B3LYP/LanL2DZ level. The characterization of computed normal vibrations and correlating observed bands is given in terms of approximate D_2h symmetry. The most intense band resulting from the Cu–N stretching vibration (B_3u) was located at 342 cm⁻¹, by ⁶³Cu and ⁶⁵Cu isotope substitution. The chemical reactions leading to the formation of presented compound are also proposed.