MIMICKING THE HDS ACTIVITY OF PROMOTED TUNGSTEN CATALYSTS - A HOMOGENEOUS MODELING STUDY USING A 2-COMPONENT TUNGSTEN RHODIUM SYSTEM/

Reaction of W(CO)(5)THF with (triphos)Rh[eta(3)-S(C6H4)CH=CH2] (1), ob tained by insertion of the 16e(-) fragment [(triphos)RhH] into the C-2 -S bond of benzo[b]thiophene (BT), gives the dimer (triphos)Rh[eta(3)- (CO)(5)WS(C6H4)CH=CH2] (2; triphos = MeC(CH2PPh2)(3)). Unlike 1, the h eterometal dimer 2 reacts with H-2 (30 atm) above 70 degrees C in THF, undergoing the desulfurization of the C-S-inserted BT. As a result, a mixture of the hydride carbonyl species (triphos)RhH(CO), ethylbenzen e, and WSx (x(av) = 1.5) is obtained. High-pressure NMR spectroscopy i n the temperature range from 20 to 70 degrees C shows that the desulfu rization step is preceded by the formation by the dimer (triphos)RhH(m u-H)[mu-o-S(C6H4)C2H5]W(CO)(4) (5), in which the Rh and W centers are held together by bridging 2-ethylthiophenolate and hydride ligands. Co mplex 5 has been characterized in both the solid state (single-crystal X-ray analysis) and solution (multinuclear NMR spectroscopy). The des ulfurization of 5 occurs also by thermolysis in THF at 120 degrees C u nder a nitrogen atmosphere. Reaction of 5 with CO (30 atm, 40 degrees C) gives the complex [(triphos)Rh(CO)(2)][(CO)(5)W(o-S(C6H4)C2H5)] (8) , in which the thiolate ligand is eta(1)-S bound to the tungsten atom in the complex anion [(CO)(5)W(o-S(C6H4)C2H5)](-). The hydrogenation o f 8 (30 atm of H-2, >70 degrees C) gives exclusively free 2-ethylthiop henol. The carbonylation of 2 (30 atm of CO, room temperature) results in the formation of [(triphos)Rh(CO)(2)][(CO)(5)W(o-S(C6H4)CH=CH2)] ( 3), in which the 2-vinylthiophenolate ligand is eta(1)-S bound to the tungsten atom. The possible similarity in the C-S bond cleavage mechan ism in the desulfurization of 5 to those occurring in the HDS over pro moted heterogeneous catalysts is discussed.