The synthesis of xanthene-based transition metal complexes and their application in the oxidation reaction of n-octane.

Abstract

The oxidation of alkanes into valuable products such as alcohols, ketones and aldehydes is very important to industry for detergents and perfumes. One of the challenges with alkanes is their inertness which results from the strong and localized C-C and C-H bonds. There are few methods that are known to transform alkanes into products of value. Therefore in this study xanthene-based ligands were used in an attempt to transform alkanes into products of value. Xanthene-based ligands are known to produce catalysts that are highly active and selective in reactions such as hydroformylation and hydrocyanation. These ligands are bidentate and their structure consists of a xanthene backbone with two phosphorus donor atoms and a rigid backbone. Five xanthene-based ligands were synthesized, characterized and complexed to cobalt and nickel. In this study modification at position X was done by using a sulphur atom, a methyl group as well as an isopropyl group in order to observe the effect this has on the activation of noctane. Crystal structures of ligand (4,5-bis(di-p-tolylphosphino)-9,9-dimethyl xanthene) and complex (Co(4,5-bis(di-p-tolylphosphino)-9,9-dimethylxanthene)Cl2) and (Ni(4,5-bis(di-ptolylphosphino)- 9,9-dimethylxanthene)Cl2) were obtained. The five cobalt and five nickel complexes were catalytically tested in the oxidation of n-octane, using three oxidants tert-butyl hydroperoxide, hydrogen peroxide and meta-chloroperbenzoic acid. This was carried out in tetrahydrofuran solvent at varying temperatures. Hydrogen peroxide and meta-chloroperbenzoic acid gave no substantial activation, while tertbutyl hydroperoxide showed activity. Modifications to the backbone at position X brought changes to the bite angle and minor changes to activity. Selectivity at 50 and 60 °C favoured the C-2 position with 2-octanone as the dominant product. Terminal position showed no products of alkane oxygenation. Alcohols (3-octanol and 4-octanol) were observed at higher temperatures. Steric factors had no significance effect on activity while temperature had a greater effect. The temperature that was best to work with was 50 °C since all catalysts were active. Sulphur had a deactivating effect on efficacy of both cobalt and nickel catalysts.