A Series of Cation-Templated, Polycarboxylate-Based Cd(II) or Cd(II)/Li(I) Frameworks with Second-Order Nonlinear Optical and Ferroelectric Properties

This work presents the systematic investigation on the reactions of Cd(II) ions with various aromatic polycarboxylic acids in organic solvents under solvothermal conditions. By using salts such as LiNO3 or NH4NO3 as templates, eight new metal-polycarboxylate frameworks, namely, (Me2NH2)[CdLi(p-BDC)2]·CH3OH (1), (Me2NH2)2[Cd(p-BDC)2]·(Me2NH) (2), (Me2NH2)[CdLi(m-BDC)2] (3), (Me2NH2)[Cd(OH-m-HBDC)(OH-m-BDC)]·DMF·CH3OH·H2O (4), [Cd1.5Li(5-But-m-BDC)2(DMF)2] (5), (Me2NH2)[CdLi(OBA)2] (6), (Me2NH2)[CdLi(SDBA)2]·DMF·CH3OH (7), and (Me2NH2)[Cd3(H2O)3(OH)(SDBA)3] (8) (p-H2BDC = 1,4-benzenedicarboxylate, m-H2BDC = 1,3-benzenedicarboxylate, OH-m-H2BDC = 5-hydroxy-1,3-benzenedicarboxylate, 5-But-m-H2BDC = 5-tert-Butyl-1,3-benzenedicarboxylate, H2OBA = 4,4′-oxybis(benzoic acid) and H2SDBA = 4,4′-sulfonyldibenzoic acid) have been isolated in good yields from the reactions of corresponding polycarboxylate ligands with Cd(II) ions in DMF (or DMAC) at 160 °C. Their structures were determined by single-crystal X-ray diffraction analyses and further characterized by elemental analyses, IR spectra, and thermogravimetric analyses. Compounds 1, 2, 3, 5, and 6 are three-dimensional frameworks, of which 1 features an unprecedented 6-connected topological network, while 2 and 3 have structures with qtz and pcu topologies, respectively. Both 4 and 7 have a two-dimensional layer framework, whereas 8 exhibits a one-dimensional chain structure. Compounds 1, 3, 6, and 7 are the first example of Cd(II)/Li(I) heterometallic coordination polymers, in which the Li(I) ions bond to carboxylate ligands and also are involved in the extending of polymer frameworks. All of these compounds except 1 and 5 crystallize in acentric (or chiral) space groups and the bulk materials for 3, 4, and 6 display strong powder second harmonic generation efficiencies, approximately 4, 2, and 5 times that of a potassium dihydrogen phosphate (KDP) powder. In addition, 4, 7, and 8 all have ferroelectric properties. The fluorescence properties and thermal stabilities for these compounds are also investigated.