Theoretical examination of soluble organometallic complexes utilized as models for hydrodesulfurization catalysts

Hydrodesulfurization (HDS) entails the conversions of organosulfur compounds found in petroleum feedstocks to hydrocarbons and H2S. Although current HDS procedures effectively desulfurize most organosulfur compounds, they are inefficient with aromatics such as thiophene. In studying ways to improve the process, the experimental research community has relied heavily on organometallic chemistry in order to model complex HDS reactions. One of the more promising models is based on complexes characterized by the eta 2-S,C-coordination mode in which a transition metal fragment (ML n) inserts into one of the S---C bonds of a thiophenic molecule. Herein, a thorough investigation of transition-metal- inserted thiophene complexes is discussed.;In chapter 2, molecular orbital calculations are reported for several transition-metal-inserted thiophene complexes. The results of the calculations provide explanations for observed differences between the complexes, including metallathiacycle geometry, delocalization of the metallathiacycle bonding, and reactivity toward electrophiles and nucleophiles. Results of molecular mechanics calculations on metal-inserted complexes are reported in chapter 3. Steric factors which contribute to distortions in ring geometry of (C 5Me5)Rh(PMe3)(eta2-S,C-2,5-Me 2C4H2S), benzothiophene and dibenzothiophene analogs, and other metallathiacycles are discussed. The results are supported by X-ray structural characterization of the products formed via insertion of [(C 5Me5)Rh(PMe3)] into a S---C bond of unsubstituted thiophene, benzothiophene, and dibenzothiophene. Chapter 4 describes the results of ab initio calculations on benzothiophene (BT), 2- and 3-methyl-substitute derivatives, and several organometallic complexes containing these thiophenic moieties in order to understand the usual preference for insertion of metal fragments into the sulfur-carbon(vinyl) bond of BT rather than the sulfur-carbon(aryl) bond. Several possible explanations for the insertion selectivity are discussed, including the orbital structure of BT, steric interactions between the metal and the heterocycle, and metal-carbon bond strengths of the inserted products. The last chapter describes the results of calculations on bimetallic thiophene-bridged complexes, formed by coordination of a second metal fragment to a canonical metal-inserted complex. The discussion describes the interactions between mononuclear metallathiacycles and a second metal-center, as well as the effects coordination has on the M---C and M---S bonds of the heterocycle. In addition, the stability of bimetallic thiophene-bridged complexes and their possible role in desulfurization reactions is discussed.