Construction of hybrid d(10) metal-organic frameworks by flexible aromatic dicarboxylate and N-donor ligands : syntheses, structures and physical properties

Six novel metal coordination polymers with flexible aromatic dicarboxylate ligands and N-donor ligands, Zn-2(DPA)(2)(mbix)(H2O) (1), Cd-2(DPA)(2)(mbix) (2), ZnCd(DPA)(2)(bix) (3), Cd(SDBA)(mbix)(H2O)(2) (4), Cd(SDBA)(bimb) (5) and [Cd(SDBA)(4,4'-bipy)(0.5)(H2O)]center dot H2O (6) (H(2)DPA = diphenic acid, H(2)SDBA 4,4'-dicarboxybiphenyl sulfone, mbix 1,3-bis(imidazol-1-ylmethyl) benzene, bix 1,4-bis(imidazol-1-ylmethyl) benzene, bimb 4,4'-bis(imidazol-1-ylmethyl) biphenyl and 4,4'-bipy 4,4'-bipyridine), have been synthesized and structurally characterized by elemental analysis, IR, TGA and X-ray single crystal diffraction. Complex 1 has a two-dimensional (2D) (3,4)-connected layer network with a novel (6(2).8)(4.6(2).8(3))(4.6.8) topology, whereas complex 2 crystallizes in the chiral space group P6(1) and has a guest-free three-dimensional (3D) chiral framework structure with one-dimensional (1D) homochiral chains. Complex 3 shows a complicated 2D framework consisting of a heteroquadrinuclear cluster which comprises a centrosymmetric linear array of two Zn atoms and two Cd atoms which are bridged by carboxylate groups of DPA ligands. Complex 4 presents a puckered chiral layer structure with rare heterostranded double helices. Complex 5 exhibits a 2D chiral structure with an open channel by incorporating the auxiliary bimb ligand, while complex 6 features an uncommon self-penetrating 2D network containing double-stranded helices. The results indicate that the dicarboxylate DPA(2-) and SDBA(2-) ligand can adopt varied coordination modes in the formation of the complexes and the configuration and flexibility of the ligands play a key role in directing the related properties of the complexes. The photoluminescence properties of 1-6 were studied in the solid state at room temperature. The nonlinear optical measurements showed that 2 and 5 displayed a second-harmonic-generation (SHG) response of 0.7 and 0.8 times that for urea, respectively. Moreover, the ferroelectric properties of 5 have also been investigated.