Stabilization of the Monovalent Nickel Complex with 1, 4, 8, 11-Tetraazacyclotetradecane in Aqueous Solutions by N- and C-Methylation. An Electrochemical and Pulse Radiolysis Study

The divalent nickel complexes with 1, 4, 8, 11-tetraazacyclotetradecane (L₁), 1, 4, 8, 11-tetramethyl-l, 4, 8, 11-tetraazacyclotetradecane (L₂), meso-5, 7, 7, 12, 14, 14-hexamethyl-l, 4, 8, 11-tetraazacyclotetradecane (L₃), and l, 4, 5, 7, 7, 8, 11, 12, 14, 14-decamethyl-l, 4, 8, 11-tetraazacyclotetradecane (L₄) were reduced by reactions with e_aq^-and CO₂^-and by electrochemical reactions in aqueous solutions. The redox potentials of the NiLi²⁺/NiLi⁺couples are -1.58, -1.15, -1.42, and -0.98 V vs. SCE for i = 1, 2, 3, and 4, respectively. The UV absorption bands of NiL, ⁺are attributed to CTTS transitions. The kinetics of reduction of Co(NH₃)₆³⁺, Ru(NH₃)₆³⁺, O₂, and N₂O by NiLi⁺are reported and discussed. The self-exchange rates of reaction between NiLi⁺and NiLi²⁺were calculated by using the Marcus cross relation. The EPR spectra of NiL₂⁺and NiL₄⁺are reported. The complexation of NiLi²⁺by OH^-was studied. The results are discussed in detail. NiL₂⁺and NiL₄⁺are suggested as new, powerful, easily attainable single-electron-reducing agents that can be used over a wide pH range in aqueous solutions.