Copper(II)-Sulfur Interactions in Pyridine- and Imidazole-containing Disulfide Complexes. Syntheses, Spectra, and Solution Equilibria

Abstract Synthetic, spectroscopic, and potentiometric studies on the copper(II) complexes of pyridine-, quinoline-, and imidazole-ring-containing disulfides and related compounds have been made with emphasis on copper(II)–sulfur interactions. In the presence of copper(II), disulfide ligands, (RCH2NHCH2CH2S–)2, where R=4-imidazolyl (IMAD), 2-pyridyl (PMAD), 6-methyl-2-pyridyl (MPAD), and 2-quinolyl (QMAD), are more stable in acid solution than in neutral–alkaline solution. A stable 1 : 1 copper(II) complex, Cu(IMAD)(ClO4)2·H2O, was isolated as crystals from aqueous solution, whereas a binuclear complex, Cu2(PMAD)Cl4, was obtained as crystals in ethanol. The Cu(II)–IMAD systems exhibit a single d-d peak at 585 nm indicative of a CuN4 chromophore but no charge transfer (CT) band in the region 300–900 nm. The other Cu(II)–disulfide systems exhibit d-d bands at 650 and ≈850 nm. A shoulder at 310–330 nm observed for the Cu(II)–PMAD and –MPAD systems at pH ≈4 and >6, respectively, is assigned to the S→Cu(II) CT transition. Thioether ligands, N-(2-pyridylmethyl)-l-methionine and N-(4-imidazolylmethyl)-Dl-methionine, show the S→Cu(II) CT peak at ≈310 nm at pH 9 in the 1 : 1 Cu(II)–ligand systems. Equilibrium constants of the proton-ligand and Cu(II)–ligand systems were determined by pH titrations at 25 °C and I=0.1(KNO3). The Cu(II)–disulfide systems were analyzed by considering monomeric species CuL, CuLH, and CuLH2 (L=ligand; H=proton) and a binuclear species Cu2L; the logβ110 values for CuL are 13.38, 13.32, and 9.46 for L=IMAD, PMAD, and MPAD, respectively. Conductivity measurements showed that the Cu(II)–IMAD complex isolated as crystals is monomeric in aqueous solution. Comparison of the electronic absorption and electron spin resonance spectral data and stability constants for various systems suggests that Cu(II)–PMAD has a square-pyramidal CuN4S chromophore with a sulfur atom in an equatorial position, whereas Cu(II)–IMAD has a planar CuN4 chromophore. The stability of disulfide bonds in copper(II) complexes in aqueous solution is briefly discussed on the basis of the modes of coordination.