Kinetics and mechanism of oxidation of nitrogen(III) by [ethylenebis(biguanide)]silver(III) cation and its conjugate bases in aqueous perchlorate media

Abstract

The complex cation [ethylenebis(biguanide)]silver(III), [Ag(H2L)]3+. oxidised NIII quantitatively to NV, being reduced to AgI. Free ethylenebis(biguanide) was recovered in near-quantitative yield. In the investigated pH range (1–7.5) the reaction is first order in [complex] and [NIII]T. The reaction proceeds through four parallel paths: [Ag(H2L)]3+–HNO2(k1), [Ag(H2L)]3+–NO2–(K2), [Ag(HL)]2+–NO2–(k3) and [AgL]+–NO2–(k4), where [Ag(HL)]2+ and [AgL]+ are two conjugate bases of [Ag(H2L)]3+. The respective rate constants (102k dm3 mol–1 s–1) are k1= 2.4, k2= 42.5, k3= 11.4 and K4= 4.4 at 30 °C and I= 1.0 mol dm–3(NaClO4). The associated ΔH‡ values are all moderately low and ΔS≠ are significantly negative. Nitrogen(III) reduces the silver(III) complex much faster than do alcohols and hydrogen peroxide. No silver(I)-catalysed path was detected. The present reactions appear to be outer sphere in nature.