Rational design of two bpy-bridged 3D and 2D CoII open frameworks with similar amino-acid-based Schiff bases

Abstract

Two novel bpy-bridged Co^II Schiff base complexes have been synthesized by the hydro(solvo)thermal reactions of corresponding amino-acid-based Schiff bases, bpy and Co(NO₃)₂·6H₂O. The following formulae identify the two complexes: {[Co(napala)(bpy)_0.5]·H₂O}_n (1) and [Co(napgly)(bpy)_0.5]_n (2) [H₂napala = N-(2-hydroxy-1-naphthylmethylidene)-D/L-alanine, H₂napgly = N-(2-hydroxy-1-naphthylmethylidene)-glycine and bpy = 4,4′-bipyridine]. These two compounds have been characterized using single-crystal X-ray diffraction, infrared, powder X-ray diffraction, thermogravimetric analysis, optical spectra analysis, and magnetic measurement. Complex 1 features an unprecedented threefold interpenetrated diamond network based on the fan-shaped Co^II₄(μ₂-napala)₄ molecular square node and bpy linker, which represents the first example of 3D framework among the amino-acid-based Schiff base complexes with salicylaldehyde or its derivatives. In 2, adjacent Co^II ions are bridged by μ₂-napgly²⁻ to form left- and right-handed [Co^II(μ₂-napgly)]_n helical chains. These two types of helical chains are sustained alternately by a symmetrical bpy co-ligand into a 2D grid-based layer. The solid-state fluorescence of complexes 1 and 2 are quenched almost completely compared with free mixed-ligands at room temperature. Moreover, magnetic studies show the dominant antiferromagnetic coupling between the Co^II centers mediated by the syn–anti-COO⁻-bridges in both complexes.