Rates of formation and dissociation of complexes of manganese(II) with the ligands 1,10-phenanthroline, 2,2′-bipyridine, and 2,2′2″-terpyridine in anhydrous methanol

Abstract

Rate constants for the formation (k_f) and dissociation (k_d) of complexes of Mn^II with the ligands 1,10-phenanthroline (phen), 2,2′-bipyridine (bipy), and 2,2′,2″-terpyridine (terpy) have been measured, at low temperatures in anhydrous methanol, by the stopped-flow method. At 298·1 K and I= 0·20 mol l^–1(NaClO₄) : log (k_f/l mol^–1 s^–1)(extrapolated)= 4·95, 4·24, and 3·72; ΔH_f^‡= 10·6 ± 0·4, 11·7 ± 0·1, and 12·8 ± 1·0 kcal mol^–1; ΔS_f^‡=–0·4 ± 1·6, 0·1 ± 0·1 and 1·3 ± 3·9 cal K^–1 mol^–1; log (k_d/s^–1)= 1·15, 1·57, and –1·33; ΔH_d^‡= 13·5 ± 0·3. 13·4 ± 0·5. and 13·7 ± 0·6 kcal mol^–1; and ΔS_d^‡=–8·2 ± 0·9, –6·4 ± 1·9, and –18 ± 2 cal K^–1 mol^–1 for phen, bipy, and terpy respectively. The kinetically determined stability constants in methanol for the complexes (logK₁= 3·8, 2·7, and 5·0 for phen, bipy, and terpy respectively) are very similar to values determined in aqueous solution. Values of ΔH_f^‡ are significantly larger than the activation enthalpy associated with Mn^II–methanol solvent exchange and possible reasons for this unexpected behaviour are discussed.