Abstract
The tetranuclear Cu4OBrnCl(6−n)(pm)4 complexes, where pm = 3-pyridylmethanol (3-pm), 4-pyridylmethanol (4-pm) and n = 0−6 with trigonal bipyramidal coordination of copper(II) were prepared and their infrared and electronic absorption spectra as well as cyclic voltammograms in nitromethane solutions were measured. The molecules exhibit strong single infrared Cu4O stretching absorptions between 580 and 530 cm−1 which are in linear correlations with the number of halides, n. Far infrared absorptions assigned to Cu-Br, Cu-Cl, and Cu-N stretching vibrations appear at higher wavenumbers for 3-pm complexes compared with those for 4-pm complex molecules. Two strongly overlapped d–d bands in the region 11300–13100 cm−1 assigned to $$e(d_{x^2 - y^2 } ,d_{xy} ) \to a_1 (d_{z^2 } )$$ and e(d xz, d yz) → a 1(d z 2) transitions in the trigonal bipyramidal coordination of the copper(II) atoms were resolved by Gaussian analysis. Both, 3-pm and 4-pm ligands produce in the complex molecules almost the same ligand field. The maxima of the Gaussian d–d components vary with the parameter n and are for both series of the complexes deviated from linearity, more for the low-energy d–d bands than for high-energy analogues. The electrochemical reduction in nitromethane is significantly less reversible for 4-pm complex molecules compared with that of 3-pm analogues. Formal reduction potentials E′ c for 3-pm complexes were observed in the range 478 (n = 0)−599 (n = 6) mV. The plot E′ c vs. n is significantly deviated from linearity. It is suggested that the reducing electrons enter the half-filled d z 2 orbital of the copper(II) atom. The different spectral and electrochemical results obtained for 3-and 4-pm complexes are explained by different structural distortions, bond lengths, and charge distributions produced by halide ligand variations. The results are also discussed with previously reported data on analogous pyridine, 3-methylpyridine and 4-methylpyridine complexes.
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