Osmium(VIII) catalyzed and uncatalyzed oxidation of a hemorheologic drug Pentoxifylline by alkaline copper(III) periodate complex: A comparative kinetic and mechanistic approach

Abstract

The oxidation of a hemorheologic drug, Pentoxifylline (PTX) by diperiodatocuprate(III) (DPC) has been investigated both in the absence and presence of Osmium(VIII) catalyst in aqueous alkaline medium at a constant ionic strength of 0.30 mol dm −3 spectrophotometrically. The stoichiometry was same in both the cases, i.e., [PTX]/[DPC] = 1:2. In both catalyzed and uncatalyzed reactions, the order of the reaction with respect to [DPC] was unity while the order with respect to [PTX] was <1 over the concentration range studied. The rate increased with an increase in [OH −] and decreased with an increase in [ IO 4 - ] in both the cases. The order with respect to [Os(VIII)] was unity. The reaction rates revealed that Os(VIII) catalyzed reaction was about sixfold faster than the uncatalyzed reaction. The reaction products were identified by TLC and spectral studies including LC–MS. Suitable mechanisms were proposed. The reaction constants involved in different steps of the reaction mechanisms were calculated for both cases. The catalytic constant ( K C) was also calculated for catalyzed reaction at different temperatures. The activation parameters with respect to slow step of the mechanism and also the thermodynamic quantities were determined. Kinetic experiments suggest that [Cu(H 2IO 6)(H 2O) 2] is the reactive copper(III) species and [OsO 4(OH) 2] 2− is the reactive Os(VIII) species.