Kinetic and thermodynamic studies of aquation reactions in [RuL2(mac)]q+ complexes: [mac = 1,4,8,11-tetraazacyclotetradecane (cyclam) or 1,4,7,10-tetraazacyclododecane (cyclen); and L = Cl−, OH−, OH2

Abstract

We report here the synthesis, characterization and kinetic studies of [RuL2(mac)]n+ complexes [mac = 1,4,8,11-tetraazacyclotetradecane (cyclam) or 1,4,7,10-tetraazacyclododecane (cyclen); and L = Cl−, OH−, OH2]. The dichloride complexes release one Cl− producing [RuCl(L)(mac)]n+ or two Cl− producing [Ru(L)2(mac)]n+ (L = OH2, OH−) in aqueous solution within pH 1–7. The product of these reactions were characterized by ultraviolet–visible (UV–Vis) spectroscopy. The chloride affinity of Ru(III) depends on the macrocyclic ring size and geometric structure. The k1 and thermodynamic activation parameters (at pH 4.4, T = 25 °C) of cis-[RuCl2(cyclen)]+ complex are k1 = 3.6 × 10−3 s−1 (25 °C); ΔH# = 40.2 kJ mol−1 and ΔS# = − 138 J K mol−1; under similar conditions, for cis-[RuCl2(cyclam)]+ these values are 2.4 × 10−3 s−1, ΔH# = 50.8 kJ mol−1 and ΔS# = − 119 J K mol−1. The rate of chloride aquation reactions for cis-[RuCl2(cyclen)]+ is about 1.5 times faster than for the cyclam complex. The reactivity difference could be due to a combination of the σ-trans effect, the nephelauxetic effect and the effect of solvation.