New metal-anion radical framework materials: CoII compounds showing ferromagnetic to antiferromagnetic phase transition at about 344 K, and ZnII compounds exhibiting terminal anion ligand induced direct white-light-emission

Abstract

The preparation, X-ray crystallography, EPR, magnetic and luminescent investigation of new metal-anion radical framework materials based on a new anion radical ligand generated by in situ deprotonation of a stable zwitterionic radical are described herein. N,O,N-tripodal anion radical ligand (bipo⁻˙) links metal cations, giving rise to four isostructural one-dimensional metal–organic frameworks, [M(bipo⁻˙)(L)]_n [M = Zn, L = HCOO⁻ (1), SCN⁻ (1a), N₃⁻ (1b); M = Co, L = Br⁻ (3)]. The tripodal bipo⁻˙ ligand and one co-ligand, 1,4-benzenedicarboxylate, coordinate to metals leading to two isostructural two-dimensional metal–organic frameworks, [M(bipo⁻˙)(BDC)_0.5]_n [M = Zn (2) and Co (4)]. The two Co^II compounds are the first examples that exhibit unusual ferromagnetic to antiferromagnetic phase transition with transition temperature over room temperature, which can be demonstrated by the cooling and warming measurements of susceptibility. Compound 4 also exhibits long-range magnetic ordering with the critical temperature at about 44 K proved by ac susceptibility measurements. The metal-radical frameworks exhibit distinctly different fluorescence emissions. Especially, the isomorphous one-dimensional Zn^II compounds show interesting terminal anion ligand-induced photoluminescent color changes, including direct and invariable white-light-emission with terminal SCN⁻ ligand.