| Escalation in the price of crude oil has brought about an alarming increase in the price of oil-based raw materials. As an alternative to the current dependence of petroleum, chemists have been focusing considerable attention on the use of coal as a source of energy and petrochemical production. The heterogeneously catalyzed Fischer-Tropsch syntheses are presently the most widely used industrial processes for conversion of coal-derived synthesis gas to organic chemicals. Only a few reports of analogous homogeneous systems have been published. One approach to the construction of a suitable homogeneously catalyzed system involves proposing a hypothetical mechanism that incorporates steps having precedence in organometallic chemistry and to optimize those steps that appear to be the most troublesome. In many of the proposed mechanisms for CO reduction an important step is the insertion of CO into a metal-hydrogen bond. However, only one example of such an insertion has been demonstrated, suggesting that another mode of initial C-H bond formation must be sought. One possibility is via nucleophilic attack by H('-) on coordinated CO. In this research, the reactivity of several transition metal carbonyl complexes with hydridic reducing agents was studied. For a series of dimeric cobalt complexes, Co(,2)(CO)(,6)L(,2) (L = CO, P(C(,6)H(,5))(,3), P(n-C(,4)H(,9))(,3)), reaction with LiAlH(,4) produced C(,1) to C(,3) hydrocarbons. The rate of product formation in the early stages of reaction increased in the following order: P(n-C(,4)H(,9))(,3) < P(C(,6)H(,5))(,3) < CO. For the iron carbonyl dimers, ((eta)('5)-C(,5)H(,5))Fe(CO)(,2) (,2) and (CH(,3))(,2) ( ('5)-C(,5)H(,4))Fe(CO)(,2) (,2), reaction with LiAlH(,4) also produced C(,1) to C(,3) hydrocarbons, while analogous reactions with complexes containing only bridging carbonyl groups result in no hydrocarbon formation. Although no detailed mechanistic information was obtained, the results indicate that an early step in the reduction process involves nucleophilic attack of the hydride on the carbon atom of a terminal carbonyl ligand to give a formyl intermediate. A plausible reaction sequence was proposed. The complex Mg (eta)('5)-C(,5)H(,5))Fe(CO)(,2) (,2)(C(,4)H(,8)O)(,4) reacts with H((eta)('5)-C(,5)H(,5))Fe(CO)(P(C(,6)H(,5))(,3)) at 40(DEGREES)C to produce ethylene and a much smaller amount of methane. Carbon-13 labeling studies suggest that, in this reaction, the pathway leading to methane formation is not the same as that for the production of ethylene. |
