Dinuclear europium(3+), terbium(3+) and gadolinium(3+) complexes of a branched hexaazacyclooctadecane ligand containing six 2,2'-bipyridine pendant units

Abstract

The dinuclear Eu[sup 3+], Tb[sup 3+], and Gd[sup 3+] complexes of a branched hexaazacyclooctadecane ligand containing six 2,2[prime]-bipyridine (bpy) pendant units (1) have been prepared and characterized as chloride salts. Elemental analysis and FAB[sup +] mass spectrometry show unambiguously that in the solid state two lanthanide ions are complexes by one molecule of 1. In CH[sub 3]CN and alcohol (ethanol/methanol, 5:1 v/v) solution, 1 exhibits a bpy-centered absorption band at 285 nm and a bpy-centered phosphorescence (77 K, [lambda] = 435 nm, [tau] = 1 s). The [Gd[sub 2](1)]Cl[sub 6] complex shows an absorption band at 305 nm and a phosphorescence band at 455 nm with [tau] = 1.8 ms (at 77 K) in CH[sub 3]CN solution, whereas in alcohol solution it displays an absorption band at 286 nm and two phosphorescence bands ([lambda] = 435 nm, [tau] = 1.2 s; [lambda] = 455 nm, [tau] = 3.9 ms). The [Eu[sub 2](1)]Cl[sub 6] and [Tb[sub 2](1)]Cl[sub 6] complexes show an absorption band at 306 nm in CH[sub 3]CN and at 285 in alcohol and H[sub 2]O. In all the solvents used the metal-centered luminescence, characteristic of the Eu[sup 3+] and Tb[sup 3+] metal ions, can be observed. The excitation spectramore » (recorded at the wavelength of the metal centered emission) of the Eu[sup 3+] and Tb[sup 3+] complexes show a maximum at 306 nm in all solvents, which coincides with the absorption maximum in CH[sub 3]CN but not in alcohol and H[sub 2]O solution. The results obtained indicate that in CH[sub 3]CN solution an average of almost five bpy units per molecule of 1 are coordinated to the two metal ions in the [M[sub 2](1)]Cl[sub 6] complexes (M = Eu[sup 3+], Tb[sup 3+], Gd[sup 3+]), whereas in alcohol and water most of the pendant bpy branches are not coordinated. 29 refs., 5 figs., 2 tabs.« less