Silver(II) complexes of tetraazamacrocycles: Studies on e.p.r. and electron transfer kinetics with thiosulfate ion

The e.p.r. studies of Ag ^II complexes of three tetraazamacrocycles, 1,4,8,11-tetraazacyclotetradecane (cyclam) (I), 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclo-teradecane-4,11-diene (Me ₆CD) (II) and 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclo- teradecane (Me ₆Cy) (III) and the electron transfer between [Ag ^II(cyclam)] ²⁺ and thiosulfate ion are described. The e.p.r. studies reveal that the spectra are almost the same as those reported earlier, particularly, for polycrystalline material and are typical of a d ⁹ planar Ag ^II complexes. Previous e.p.r. studies on these square planar polycrystalline complexes showed no ligand hyperfine splitting. Reinvestigation of the e.p.r. spectra of these complexes in both the solid state and in solution at room (T ∼ 297 K) and at low (T = 120 K) temperature reveals resolved hyperfine structures in solution for [Ag(Me ₆CD)] ²⁺ and [Ag(Me ₆Cy)] ²⁺ complexes, Surprisingly, such a structure was not observed in solutions of [Ag(cyclam)] ²⁺. Computer simulations of the hyperfine structure observed in solutions are in good agreement with structural formulae proposed. The oxidation of thiosulfate ion by [Ag(cyclam)] ²⁺ follows the rate law: -d/dt[Ag ^II(cyclam) ²⁺] = (k ₁Q ₁[H+] + k ₂Q ₂K _a2)/([H+] + K _a2)[Ag ^II(cyclam) ²⁺][S ₂O ₃ ^2-] and an inner-sphere mechanism is proposed, based on the spectral and kinetic evidence.