Single Crystal X-Ray and Optical Studies on Bis(1,5-diazacyclooctane)copper(II) Nitrate, a Thermochromic Copper(II) Complex Involving Hydrogen Bonds between the Ligand and the Counter Ion

Abstract A thermochromic copper(II) complex of 1,5-diazacyclooctane (daco), [Cu(daco)2](NO3)2, has been subjected to single-crystal X-ray and optical studies. This complex is orange as isolated, but it turns violet discontinuously when heated above 90 °C and returns to orange on cooling. Crystal structure of the low-temperature form has been determined at room temperature; the complex crystallizes in the monoclinic space group P21⁄n with a=7.9040(7), b=15.694(1), c=7.0886(7) Å, β=95.36(1)°; V=875.5(1) Å3, and Z=2. The CuN4 chelate plane is planar and Cu–N coordination bonds are rather short. The structure has revealed the presence of weak hydrogen bonds bridging imino groups of the ligands with the nitrate ions and forming a macrocycle-like ring system, which results in enhancement of the ligand field strength. Single-crystal visible and infrared spectra for both low- and high-temperature forms have heen obtained and compared. A high-energy d–d transition band(s) causing the orange color at room temperature is in contrast to an extremely flattened broad band(s) for the violet form. The IR spectra of the two forms clearly differ in the intensity of νNH stretching peak, implying that the thermochromic phase transition involves a change in the nature of the hydrogen bonds of the imine ligands. On the basis of these observations and comparison to reported studies on a similarly thermochromic copper(II) complex, a mechanism is suggested involving dynamic fluctuation of the ligand field and geometry due to thermal motion of the anion/ligand hydrogen bond system.