The possible thione–thiol tautomerism of 2- and 4-mercaptopyridines, 2-mercaptopyrimidine, and 4,6-dimethyl-2-mercaptopyrimidine in solution is studied by means of absorption (UV–VIS) spectroscopy. In accordance with earlier observations, polar solvents and self-association shift the apparent tautomeric equilibrium significantly towards the thione form. In dilute solutions of nonpolar solvents the thiol form predominates. On standing, significant changes are observed in the absorption spectra of these tautomeric compounds in ethanol, dioxane, and water. The time course of the tautomerization, followed spectrophotometrically, reveals quantitative transformation of the thiol form to the corresponding symmetrical disulfides. The influence of concentration, temperature, and irradiation with indirect sunlight are discussed. This thione–disulfide process is reversible in water, starting either from the tautomeric thione or from its symmetrical disulfide, implying a possible importance in biological systems. Keywords: thione–thiol tautomerism, absorption spectra, thiol–disulfide oxidation, mercaptopyridines and pyrimidines.

Reaction of copper(I) chloride or bromide with equimolar amounts of the diphos ligand 1,3-propanebis(diphenylphosphine) and a heterocyclic thione (L) in acetonitrile/methanol solvent afforded mononuclear complexes of the type [CuX(dppp)(L)] with the diphosphine ligand acting as a chelating ligand. In contrast, copper(I) iodide under the same conditions gave the dimeric complex [Cu(mu-I)(dppp)](2), which contains doubly bridging iodo ligands. The structures of three complexes, namely, [CuCl(pymtH)(dppp)], [CuCl(pymtH)(dppp)], and [Cu(mu-I)(dppp)](2), have been established by single-crystal X-ray diffraction. Density functional calculations at the B3LYP level of theory provided a satisfactory description of the structural, bonding, electronic, and related properties of the [CuX(PH(3))(2)] and [CuX(1,3-pdp)] (1,3-pdp = 1,3-propane-di-phosphine) complexes and their dimers along with their associations with the pyrimidine-2-thione (pymtH) ligand. The interaction of the pymtH ligand with the Cu(I) metal center in these complexes corresponds to loose associations, the computed interaction energies predicted to be about 20 kcal/mol for all complexes in the series. The bonding mechanism of the thione ligand with the Cu(I) metal centers involves both a sigma-dative and pi-back-bonding components. The coordination of the pymtH ligand is further stabilized by X...H-N bond formation being more pronounced in the chloro than in the iodo derivatives. The Cu-X bond was also found to be a composite bond involving sigma- and pi-dative bonding components. Most important is the presence of pi-type MOs delocalized over the entire four-membered Cu(mu-X)(2)Cu ring, which supports a ring current and could probably account for the nearly equivalent Cu-X bonds in the rhombus. Moreover, all [Cu(mu-X)(PH(3))(2)](2) dimers exhibit a sigma-type MO corresponding to weak Cu.Cu interactions supporting through-ring intermetallic interactions, which seems to be responsible for the stabilization of the otherwise unstable antiaromatic Cu(mu-X)(2)Cu ring.

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Reactions of copper(I) halides with 1,1,1‐tris(diphenylphosphanylmethyl)ethane (triphos) in 1:1 molar ratio afforded mononuclear complexes of the type [CuX(triphos)] which on further treatment with one equivalent of the potassium thiolate salt of a heterocyclic thione gave rise to the formation of mixed‐ligand complexes of the formula [Cu(κ3‐triphos)(κ1‐thiolate)]. The molecular structures of [Cu(κ3‐triphos)(κ1‐py2S)]·C2H5OH (1) and [Cu(κ3‐triphos)(κ1‐pymt)] (2) have been established by single‐crystal X‐ray diffraction. The complexes adopt a rigid distorted tetrahedral geometry with the phosphane ligand acting in a tridentate chelating mode. The complexes are strongly luminescent in solution and in the solid state. The spectroscopic and electronic properties of the new copper(I) complexes have been computed by electronic structure computational techniques (TD‐DFT calculations), that contribute to the assignments of electronic spectra. The high‐energy absorptions in the absorption spectra of complexes 1 and 2, ranging from 210 to 270 nm, are assigned to MLCT/LLCT transitions and the same also holds true for the low‐energy absorptions found in the range 280 to 300 nm. In the lowest triplet excited state, T1, involved in the emission in the 456–502 nm range, the spin density is delocalized over the entire nuclear framework.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)