Catalytic Formation of Hydrogen and C-C Bonds on Illuminated Zinc Sulfide Generated from Zinc Dithiolenes

Article abstract of DOI:10.1021/ja00292a001

UV irradiation of bis(cis-1,2-dicyano-1,2-ethylenedithiolate)zinc bis(tetrabutylammonium) in aqueous tetrahydrofuran (THF) or 2,5-dihydrofuran (2,5-DHF) leads to the semiconductor n-ZnS which photocatalyses an efficient H2 evolution (Φ_300nm =/>0.1) coupled to a chemoselective dehydrodimerization of the cyclic ether.The reacion does not occur in pure water nor in dry ethers. n-ZnS, which immediately photocorrodes in pure water, becames very stable in the presence of 2,5-DHF. 2,3-DHF and six-membered ethers do not dehydrodimerize or prevent photocorrosion.The H2-evolution rate increases linearity with light intensity and does not change as a function of temperature (20-50 deg C) or pH (4-10).In the case of 2,5-DHF, the previously unknown 2,2'-, 2,3', and 3,3'-dehydrodimers are formed in the ratio of 1:2:1, respectively, on analytical and preparative scales.With THF, the reaction proceeds regioselectively with the exclusive formation of the 2,2'-dihydrodimer.A complete material balance shows that no water is consumed although the initial gas phase contains more than 90percent D2 when D2O is used.The latter value decreases with increasing turnover and is accompained by the formation of HD and H2.These results are compatible with the reduction of water to hydrogen by conduction band electrons.The dehydrodimers are produced by dimerization of the corresponding radicals formed most likely by hole oxidation of the cyclic ether with subsequent deprotonation.Comparison with reactions of OH<*> generated in homogeneous solution reveals a higher selectivity for the catalytic ZnS system.