Jinbiao Cai

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

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.