Crystal structural, electrochemical and computational studies of two Cu(II) complexes formed by benzotriazole derivatives

Two Cu(II)-containing complexes, Cu(pbbt)Cl₂]₂ · CH₃OH (1) and Cu(bbbt)_1.5Cl₂]_n (2) (pbbt = 1,1′-(1,3-propylene)bis-1H-benzotriazole, bbbt = 1,1′-(1,4-butanediyl)bis-1H-benzotriazole), have been synthesized and characterized. Single crystal X-ray diffraction shows that 1 exhibits discrete binuclear structure, in which two Cu(II) ions are bridged together through two Cl⁻ anions and two pbbt ligands. Whereas 2 displays infinitely extended two-dimensional reticulate grid structure with hexagon units in which six Cu(II) ions act as corners and six bbbt ligands serve as sides. The electrochemical studies show that their redox processes in the potential range of 0.1-0.9 V are quasi-reversible and controlled by diffusion. The diffusion coefficients decrease with increase in the molecular weight of the complexes and the size of the molecules. Further investigation exhibits that their electrochemical properties obtained from experiments are consistent with their crystal data and the computed parameters. For example, the observed one-pair well-defined redox waves in the cyclic voltammetry (CV) diagrams correspond to the X-ray diffraction results that all of the Cu(II) ions in the title complexes are equivalent, respectively; the cathodic shifts (as compared with CuCl₂) of the reduction potentials of 1 and 2 are in agreement with the computed results: the LUMO energies of the complexes (−3.789 eV for 1 and −4.330 eV for 2) are higher than that of CuCl₂ (−6.942 eV).