Monomeric Cu(II) complexes with tetradentate bis(benzimidazole) ligand; synthesis, EPR, spectral and electrochemical studies

摘要

A series of 1:1 copper(II) complexes with the tetradentate ligand 1,2bis (2 benzimidazolyloxamethyl) benzene (L) have been prepared with stoichiometry of Cu(L)X-2. nH(2)O (X = Cl-, NO3-, OAc- and HCOO-). X-band EPR spectra of the above complexes examined as a frozen DMSO/DMF solution indicate g(parallel to) > g(perpendicular to) > 2.0 and show four or less than four g(parallel to) lines and a broadening of the g(perpendicular to) component suggesting a lowered site symmetry for the copper(II) ion. The g(perpendicular to) component of all the complexes show nitrogen superhyperfine structure. A clear five line pattern is found in the g(perpendicular to) region with B-N similar or equal to 15G +/- 1 for Cu(L) X(2 .)nH(2)O complexes indicating that two nitrogen from the benzimidazole ring are coordinated to the central Cu-II in the equatorial basal plane while the other two positions in the sphere are occupied by the anionic ligand (X). Interestingly with ClO4- as the anion, a 1 : 2 complex results with stoichiometry of [Cu(L)(2)] (ClO4)(2). The X-band EPR spectra of [Cu(L)(2)](ClO4)(2) complex shows a clear nine line SHF structure on the g(perpendicular to) component indicating the presence of four nitrogen atoms coordinated to the central Cu-II atom; this confirms the retention of two ligands around Cu-II in solution. Molecular orbital coefficients beta(1) and beta(1)' have been evaluated from the expression used by Manoharan and Rogers. The trends of beta(1) and beta(1)' indicate increasing electron delocalization on nitrogen atom as we change the exogenous ligand from formate/acetate-chloride to nitrate. The covalency parameter, alpha(2), has also been evaluated and reflects the decreasing trend of covalency in the order, nitrate > chloride greater than or equal to acetate > formate for complexes containing these ligands. This trend of order is in conformity with that obtained from MO coefficients. All the complexes exhibit a visible band with lambda(max) in the 625-690 nm region, in DMF; suggesting a distorted geometry for copper(II) ion in the present series of complexes. Cyclic voltammetric studies of copper(II) complexes reveal that the E-1/2 for the Cu-II/Cu-I couple shifts to most negative value with HCOO- and OAc- anionic ligands implying that these anion stabilize the Cu-II state whereas E-1/2 data for NO3- reveals that this anion destabilizes the Cu-II state. A plot of visible band energy vs E-1/2 shows that as the energy of the d(x2-y2) orbital decreases, an anodic shift in Cu-II --> C-I reduction potential is observed. (C) 1998 Elsevier Science Ltd. All rights reserved. [References: 34]