Monovalent metal-ion complexation by a bibracchial lariat ether in several solvents

Abstract

The stabilities of monovalent metal complex ions, [ML¹]⁺, of 7,13-bis(2-methoxyethyl)-1,4,10-trioxa-7,13-diazacyclopentadecane (L¹) have been determined by potentiometric titration. The stability varies with the identity of M⁺ in the sequence Li⁺(9.13, 7.0, 3.01, 2.23), Na⁺(8.17, 7.1, 4.89, 3.50), K⁺(5.24, 5.0, 4.69, 3.31), Rb⁺(4.39, 4.2, 3.97, 2.84) Cs⁺(3.77, 3.6, 3.46, 2.31) and Ag⁺(7.08, 12.2, 9.86, 8.37), where the figures in parentheses are log(K/dm³ mol^–1) and K is the stability constant for [ML¹]⁺ in acetonitrile, propylene carbonate, methanol and dimethylformamide, respectively, at 298.2 K and I= 0.05 mol dm^–3(NEt₄CIO₄). The decomplexation of Li⁺ in [LiL¹]⁺, determined by ⁷Li NMR spectroscopy, is characterized by K_d(298.2 K)= 79.7, 1970 and 32 600 s^–1, ΔH_d^‡= 35.8, 20.4 and 36.4 kJ mol^–1, and ΔS_d^‡=–88.6, –113 and –36.5 J K^–1 mol^–1, respectively, in propylene carbonate, methanol and dimethylformamide. Sodium-23 NMR spectroscopy yields k_d(298.2 K)= 124, 60.0 and 61.8 s^–1, ΔH_d^‡= 43.2, 53.4 and 55.8 kJ mol^–1, and ΔS_d^‡=–60.0, –32.0 and –23.5 J K^–1 mol^–1 for the decomplexation of [NaL¹]⁺ in acetonitrile, propylene carbonate and pyridine, respectively. These variations in complex-ion stability and lability are discussed in terms of ligand, metal-ion and solvent characteristics.