Metal-controlled structural variations of coordination architectures constructed from flexible 1H-benzimidazole-1-propionic acid

Abstract

Treatment of a flexible bifunctional ligand 1H-benzimidazole-1-propionic acid (Hbiap) with appropriate transition metal salts led to the formation of six new complexes, [Co(biap)₂]_n (1), [Cu₂(biap)₄(H₂O)]_n·7n(H₂O) (2), [Ag(H_0.5biap)₂]_n·2n(H₂O) (3), [Zn(biap)₂]_n (4), [Cd₁₂(biap)₂₄]·8H₂O (5) and [Hg(Hbiap)Cl₂] (6). X-ray crystallographic studies reveal that complexes 1 and 2 exhibit two different 1D chain structures due to the distinct coordination geometries of central Co(II) and Cu(II). Complex 3 consists of a 1D zigzag [Ag(H_0.5biap)₂]_n chain constructed via O⋯H⋯O interactions. In 4, two sets of 1D [Zn(biap)]_n chains along two perpendicular directions are interwoven at the Zn(II) nodes to afford a 2D puckered sheet with a (4,4) topology. The crystal structure of 5 manifests an unprecedented nano-sized dodecanuclear Cd(II) cage in a garland-like appearance, which can be thought of as a head-to-tail 1D looped chain enclosing a hollow cavity. Complex 6 features a 1D infinite wavy chain generated by the linkage of [Hg(Hbiap)₂] and HgCl₂ fragments through μ₂-Cl⁻ groups. Of these complexes, the flexible organic ligand is in favor of the gauche conformation in 1–5, and the anti conformer can only be observed in 6. A comparison of the structural features for these complexes suggests that the coordination preferences of the metals, connectivity of the metal nodes as well as the binding modes and protonation behavior of Hbiap are responsible for the interesting architectural variations. The four coordination complexes with d¹⁰ metal centers display solid-state luminescent emissions at room temperature, which originate from either intraligand π–π* transitions (4 and 5) or ligand to metal charge transfer (LMCT) transitions (3 and 6).