cis- and trans-bis(pyridine)chromium(III) complexes containing mixed-valence chrysenesemiquinonate-chrysenecatecholate ligands:: Synthesis, characterization and interpretation of the electronic absorption spectra using ZINDO/S method

Two new geometrical isomers of [Cr(PY)(2)(chrySQ)(chryCat)] (chrySQ = chrysenesemiquinonate; chryCat = chrysenecatecholate; py = pyridine) were synthesized by two different synthetic procedures. In the first, an acetonitrile solution containing a stoichiometric mixture of Cr(CO)(6), chrysenequitione and pyridine was photolyzed with a Hg-lamp. The second procedure was based on substituting one of the chrysenesemiquinonate ligand in the tris-[Cr(chrySQ)(3)] complex with two pyridine ligands. In both procedures two isomeric forms of [Cr(py)(chrySQ)(chryCat)] were isolated with the trans-isomer obtained in higher yield. The structures of the two isomers have been modeled using parameterized PM3 semiempirical method. Theoretical harmonic vibrational frequencies of the cis- and trans-isomers have been computed and compared with the experimental vibrational frequencies. Variable-temperature magnetic susceptibility has been studied for the two isomers in the 10-300 K temperature range. Theoretical modeling of the magnetic data indicated strong antiferromagnetic exchange interaction between Cr-III (S = 3/2) and chrySQ (S = 1/2) with J = -365 +/- 6 and -395 +/- 4 cm(-1) for the cis- and trans-isomers, respectively. The electrochemical behavior of cis- and trans- [Cr(py)(2)(chrySQ)(chryCat)] complexes were studied by cyclic voltammetry in acetonitrile solvent. Both complexes showed two one-electron redox processes attributable to reversible reduction and oxidation of the chrySQ and chryCat ligands. Reduction of the Cr(III) to Cr(II) was observed for both complexes near- 1300 mV. The electronic spectra of the two isomers were dominated by charge-transfer (LMCT, MLCT and ILCT) transitions. In addition, a low-energy intervalence charge-transfer (IVCT) transition was observed for the cis-isomer at 1085 nm. Theoretical studies of the electronic spectra by ZINDO/S-CI method were useful in interpreting the observed electronic transitions. (c) 2007 Elsevier Ltd. All rights reserved.