Magnetic and luminescent properties of Cd(II)- and Fe(II)-anion radical frameworks: various networks or structures influenced by metal ion sizes or in situ forming mechanisms of anion radical ligand

The preparation, X-ray crystallography, EPR, and magnetic and luminescent properties of three new metal-anion radical frameworks are described herein. The anion radical ligand (bipo(-center dot)) and co-ligand, 1,4-benzenedicarboxylate (BDC2), coordinate to Cd(II) centers, leading to an interpenetrated three-dimensional (3D) metal-organic frameworks (MOF), [Cd-3(bipo(-center dot))(4)(BDC)](n) (1). Although [Fe-3(bipo(-center dot))(4)(BDC)](n) (2) and 1 possess the same space group (tetragonal, P4(2)/nbc), and similar rigid spirocycle-like chain and powder X-ray diffraction (PXRD) patterns, 2 exhibits an interesting spirocycle-like one-dimensional (1D) chain network, in which BDC2- behaves not as bridging co-ligand but as a counteranion, according to smaller Fe(II) ion size and long Fe1 center dot center dot center dot Fe1' distance between rigid chains. Thus, metal ion sizes induce different networks of isomorphous 1 and 2. Moreover, the different forming mechanisms of bipo(-center dot) ligand also result in different structural MOFs: the in situ deprotonation of initial Hbipo(-center dot) leads to an 1D Fe(II) MOF (2); the in situ aldol condensation and then deprotonation of initial hydrochloride salt of imidazo[1,2-a] pyridin-2(3H)-one results in a two-dimensional (2D) Fe(II) MOF, [Fe-3(bipo(-center dot))(2)(mu(2)-OH)(2)(mu(2)-H2O)(BDC)](n) (3). More noteworthy, in 2, BDC2(-center dot) suffers large distortion, mainly attributed to its strong electrostatic interaction with the Fe(II) cation on the rigid spirocycle chain. Compounds 2 and 3 exhibit broad EPR signals, ascribed to the strong iron-anion radical antiferromagnetic coupling. 1 shows an unusual magnetic phase transition at similar to 70 K, and intense fluorescence emission which can even be excited by visible light (460 nm). Both 2 and 3 show strong antiferromagnetic interaction between Fe(II) cation and radical. Compared to 1, two Fe(II) MOFs exhibit obvious blue-shift fluorescence emissions.