Synthesis, characterization and catalytic activity of the water-soluble tungsten complex [W(CO)(3)(MeCN)(TPPMS)(2)], TPPMS = (C6H5)(2)P(m-C6H4SO3Na)center dot 2H(2)O: the unprecedented transformation of the complex into a hybrid (homogeneous/heterogeneous) catalyst precursor during two-phase catalytic hydrogenation upon changes in reaction conditions

The water-soluble tungsten complex: [W(CO)(3)(MeCN)(TPPMS)(2)] where TPPMS = (C6H5)(2)P(m-C6H4SO3Na . 2H(2)O), was prepared and characterized using sp ectroscopic techniques. The complex is soluble and stable in water for long periods as demonstrated by UV-VIS studies. When this water-soluble tungste n complex was used as a catalyst precursor during the two-phase catalytic h ydrogenation of small olefins and aromatic molecules (I.-hexene, styrene, 2 ,3-dimethyl-1-butene, 2,3-dimehtyl-2-butene and benzene) the expected selec tivity for a homogeneous catalyst precursor was not obtained. Under the cho sen reaction conditions, the hydrogenation of benzene proceeded at a much f aster rate (benzene: 2.1; 1-hexene: 1.2; styrene: 1.3; 2,3-dimethyl-1-buten e: 1.5 and 2,3-dimethyl-2-butene: 1.6 mol(mol W)(-1) h(-1)). The Hg test, c arried out in another set of experiments, showed inability of the system fo r catalytic hydrogenation. Upon changes in the pH of the catalytic solution , the hydrogenation proceeded either homogeneously or heterogeneously. At p H < 6, the catalysis proceeds heterogeneously, while at pH > 6 the process is homogeneous (Scheme 1). Hydrogen pressure also facilitates this transiti on. Temperature, substrate to catalyst ratio (S/C) and stirring do not affe ct the heterogeneous nature of the system. The heterogeneous catalytic solu tion is blue while the homogeneous one remained yellow. (C) 2001 Elsevier S cience B.V. All rights reserved.