Kinetics of silver(I)-catalyzed oxidation of formic acid by the (ethylenebis(biguanidine))silver(III) cation in acid perchlorate media

Abstract

In the presence of added Ag⁺, the title reaction follows the rate law-d[complex]/dt = k^o = Kk₁[Ag⁺]²[HCO₂H]/([H⁺] + K[HCO₂H]), where k^o is the observed zero-order rate constant for a large excess of Ag⁺,H⁺, and HCO₂H over the complex. 10³k₁ was found to be 1.0 ± 0.1M^-1 min^-1 at 30°C and I=1.0M. Corresponding ΔH^* and ΔS^* values are 13 ± 1 kcal/mol and -37 ± 3eu, respectively. Under these conditions, K, the formation constant of Ag(HCO₂) from HCO₂H and Ag⁺, was found from kinetic results to be 0.1 ± 0.01. The acid dissoication constants (pK₁ and pK₂) of the complex cation under similar conditions in a 50% (v/v) aqueous-DMF medium are 3.0 ± 0.2 and 6.8 ± 0.3. There is evidence fro the Ag⁺ ion acting here as the actual oxidant for HCO₂H with use of the complex as an oxidant regenerating Ag⁺ from Ag⁰ formed as an intermediate. Under the experimental conditions where Ag(II) is detectable in concentrations less than about 10^-4M, the whole reaction course is EPR silent. This has been discussed in terms of possible reaction schemes.