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 api^3– [{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 E_a ≈ (1.2 ± 0.2) × 10³ cm^–1] and Eu³⁺(⁵D₀) [E_a ≈ (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 [{Tb_1 – xEu_x(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.