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 N^III quantitatively to N^V, being reduced to Ag^I. 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 [N^III]_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.