Structure and luminescence of the neutral dinuclear lanthanide(III) complexes [{Ln(api)}2] {H3api = 2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine}

Abstract

Dinuclear compounds of lanthanide(III) (Ln3+) ions with api3– [{Ln(api)}2] {H3api = 2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazoline} were synthesized and characterized in microcrystalline form and frozen aqueous solutions in order to establish the ability of triply charged chelates to stabilize Ln3+–Ln3+ couples. The crystal structure of the [{Sm(api)}2] complex confirms the dinuclear sandwich binding in which the pair of Ln3+ ions is trapped between the two chelates to yield an eight-fold ca. square antiprismatic co-ordination geometry on each Ln3+ ion. Luminescence decay dynamic studies revealed thermal quenching of Tb3+(5D4) [thermal barrier Ea ≈ (1.2 ± 0.2) × 103 cm–1] and Eu3+(5D0) [Ea ≈ (4.7 ± 0.2) × 103 cm–1] emission from [{Tb(api)}2] and [{Eu(api)}2] respectively. Luminescence decay dynamics of homodinuclear [{Ln(api)}2] (Ln = La, Gd, Eu or Tb) and heterodinuclear [{Tb1 – xEux(api)}2] complexes in polycrystalline and frozen aqueous solutions as well as the small proton NMR relaxivity of [{Gd(api)}2] and low toxicity (to mice) of [{La(api)}2] show qualitatively that members of the [{Ln(api)}2] system are stable in water and merit further study in search of potentially useful tunable biomedical probes.