Synthesis and reactions of thiophene, 2,5-dihydrothiophene, benzo[b]thiophene and selenophene transition metal complexes: models for catalytic hydrodesulfurization

S-coordinated thiophene (Th) complexes Cp[superscript]'(CO)[subscript]2Re(Th), Cp[superscript]' = Cp or Cp* and Th = T, 2-MeT, 3-MeT, 2,5-Me[subscript]2T, Me[subscript]4T and dibenzothiophene (DBT), have been synthesized. The compounds Cp[superscript]'(CO)[subscript]2Re(Th) (Th = T, 2-MeT and 3-MeT) react with Fe[subscript]2(CO)[subscript]9 to give thiophene-bridged dinuclear complexes Cp[superscript]'(CO)[subscript]2Re([mu]2-[eta][superscript]4(S)-Th)Fe(CO)[subscript]3. These are the first S-coordinated thiophene complexes that undergo further reactions. The first X-ray structures of a simple nonchelated S-bound thiophene complex Cp*(CO)[subscript]2Re(T) and a thiophene-bridged compound Cp*(CO)[subscript]2Re([mu]2-[eta][superscript]4(S)-T)Fe(CO)[subscript]3 are also reported. In kinetic studies of S-coordinated thiophene substitution by PPh[subscript]3, Cp(CO)[subscript]2Re(Th) + PPh[subscript]3 → Cp(CO)[subscript]2Re(PPh[subscript]3) + Th, rate constants (10[superscript]7k, s[superscript]-1) for thiophene dissociation decrease with increasing methyl substitution: T(3,000) > 3-MeT(1,200) > 2-MeT(91) > 2,5-Me[subscript]2T(13) > Me[subscript]4T(2.7) > DBT(1.6);Several S-coordinated 2,5-dihydrothiophene (2,5-DHT) transition metal complexes were synthesized to investigate the possibility of metal promoted butadiene formation as proposed in a HDS mechanism. Thermal decomposition of W(CO)[subscript]5(2,5-DHT) and Re[subscript]2(CO)[subscript]9(2,5-DHT) at 110°C liberate the butadiene and free 2,5-DHT (relative ratio 1:4);The benzo(b) thiophene complexes Cp[superscript]'(CO)[subscript]2Re(BT) exist as S-and 2,3-[eta][superscript]2-bound BT isomers in rapid equilibrium with each other. Replacing Cp by Cp* shifts the equilibrium in favor of the [eta][superscript]2-isomer. A structural determination of Cp*(CO)[subscript]2Re(2,3-[eta][superscript]2-BT) confirms this new bonding mode. However, methyl-substituted BT (2-MeBT or 3-MeBT) are shown to coordinate only through the sulfur atom;In the Se analogs of thiophene complexes, Cp*(CO)[subscript]2Re(Sel), the selenophene (Sel) coordinates to Re through Se or [eta][superscript]2 through the olefin depending of the number of methyl groups on the selenophene ring. While Sel is [eta][superscript]2-bound in Cp*(CO)[subscript]2Re(Sel), the analogous 2,5-dimenthyl selenophene (2,5-Me[subscript]2Sel) complex is Se-coordinated, and the compound containing 2-methyl selenophene (2-MeSel) exists as an equilibrium mixture of the Se- and [eta][superscript]2-isomers. The uncoordinated Se atom in Cp*(CO)[subscript]2Re(Sel) is capable of binding to a second metal to give dinuclear complexes in which the selenophene is [eta][superscript]2-bonded to the Re and Se-bonded to the second metal. The structure of Cp*(CO)[subscript]2Re([mu]2-[eta][superscript]2(Se)-Sel)W(CO)[subscript]4(PPh[subscript]3) is reported;These studies and the discovery of several novel bonding modes ([eta][superscript]2-BT, [mu]2-[eta][superscript]4(S)-Th, and [mu]2-[eta][superscript]2(Se)-Sel) suggest possible new modes of thiophene adsorption and activation in HDS catalysis.