Self-assembly of d–f coordination frameworks based on 1H-benzimidazole-5-carboxylic acid: synthesis, structure and luminescence

Abstract

By the addition of different secondary ligands [acetate (OAc), Br⁻ and oxalate (ox)], or by the absence of such ligands, four different types of heterometallic coordination compounds were obtained by reaction of 1H-benzimidazole-5-carboxylic acid (H₂bic) with transition metal (d) and lanthanide (f) ions under hydrothermal conditions, namely, [TbAg₃(Hbic)₂(bic)₂]·4H₂O (1), [LnCu₂(Hbic)₄Br] [Ln = Er (2), Ln = Yb (3)], [EuAg₂(Hbic)₂(bic)(OAc)(H₂O)] (4), [TbAg(Hbic)₃·(ox)_0.5] (5). Compounds 1, 2 and 3 stand out by having octahedrally coordinated lanthanide centers rather than the more usual 8 or 9 coordinated metal centers. Complex 1 exhibits an interesting 2D wave-like layered network constructed from infinite six-coordinated terbium carboxylate chains and Ag₃(Hbic)₂(bic)₂ units. Isostructural compounds 2 and 3 display two-dimensional (2D) mildly ruffled sheets constructed by interconnecting μ₂-Br ligands to adjacent ribbon-shaped Cu₂(Hbic)₄ chains. Addition of auxiliary acetate and oxalate ions led to lanthanide ions with normal coordination numbers, and Eu₂(OAc)₂ and Tb(ox) subunits are formed. Complex 4 is a 2D structure with a (4,4) net that is constructed by interlinking dinuclear Eu₂(OAc)₂ subunits with Ag₂(bic)(Hbic)₂(H₂O) subunits. Complex 5 represents a 2D heterometallic coordination framework with an (8,3) net that is constructed by interconnection of Ag(Hbic)₃ subunits and Tb(ox) subunits. The luminescence properties of 1, 4 and 5 are discussed.