PUB - Publikationen an der Universität Bielefeld

Selenium(II) dimethanethiolate, Se(SMe)(2), was synthesized by reaction of SeO2 with HSMe. Basic spectroscopic data for Se(SMe)(2) and selenium(II) bis(2-methyl-2-propanethiolate), Se((SBu)-Bu-t)(2), were recorded and interpreted with the support of ab initio calculations. Both compounds are thermodynamically unstable relatively to selenium and the corresponding disulfide. The UV/vis spectra of both compounds are qualitatively similar, the two bands being attributed to n(Se)-sigma*(Se-S) transitions. The bands at 369 and 397 cm(-1) in the IR spectra of Se(SMe)(2) and Se((SBu)-Bu-t)(2), respectively, are assigned to nu(as)(SeS2). The Se-77 NMR shifts of Se(SMe)(2) (784 ppm) and Se((SBu)-Bu-t)(2) (556 ppm) differ substantially from each other and show positive temperature gradients. Calculations at the GIAO-HF/962+(d) level reproduced the difference of the Se-77 NMR chemical shifts between Se(SMe)(2) and Se((SBu)-Bu-t)(2). At the same level, the effect of conformational changes on Se-77 shifts were studied for Se(SMe)(2). In the solid state Se(SMe)(2) forms long intermolecular Se...S contacts while Se((SBu)-Bu-t)(2) does not. Both compounds exhibit anti-conformations of the methyl and tert-butyl groups with respect to the SeS2 plane. MP2/LANL2DZ(d) geometry optimizations, single point energy and frequency calculations performed for Se(SMe)(2) show, that syn- (C-s) and anti-conformers (C-2) represent minima on the potential energy surface, the latter being by 8 kJ mol(-1) lower in energy than the former. Both conformers are stabilized by intramolecular pi-type n(S-1)-sigma*(Se-S-2) orbital interactions. The energy of the transition state for the mutual conversion of the two conformers was calculated to be 31 kJ mol(-1) above that of the syn conformer, allowing a rapid interconversion of the two conformers at room temperature. Intermolecular interactions between Se(SMe)(2) molecules were also studied by means of calculations at the MP2/LANL2DZ(d) level. For Se((SBu)-Bu-t)(2) MP2/LANL2DZ(d) geometry optimizations and single point energy calculations revealed a C-2-symmetric anti- and a C-1 symmetric syn-conformer, the latter being 21 kJ mol(-1) higher in energy than the former. Se(SMe)(2) and Se((SBu)-Bu-t)(2) exchange thiolate groups with other selenium(II) dithiolates, tellurium(II) dithiolates and with thiols, if catalytic amounts of p-CH3C6H4SO3H are added.