MONOVALENT METAL-ION COMPLEXATION BY A BIBRACCHIAL LARIAT ETHER IN SEVERAL SOLVENTS

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 (NEt4ClO4). The decomplexation of Li+ in [LiL1]+, determined by Li-7 NMR spectroscopy, is characterized by k(d) (298.2 K) = 79.7, 1970 and 32 600 s-1, DELTAH(d)double dagger = 35.8, 20.4 and 36.4 kJ mol-1, and DELTAS(d)double dagger = -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, DELTAH(d)double dagger = 43.2, 53.4 and 55.8 kJ mol-1, and DELTAS(d)double dagger = -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.