A series of MOFs based on a tricarboxylic acid and various N-donor ligands: syntheses, structures, and properties

Abstract

Eight highly connected metal–organic frameworks (MOFs) based on flexible tricarboxylate ligands, namely, Na[Zn₅(L)₂(μ₃-OH)₅] (1), (HTEA)[Zn₃(L)₂(μ₃-OH)]·0.25H₂O (2), [Zn₃(L)₂(BIME)(H₂O)₂] (3), [Cd₃(L)₂(BIME)(H₂O)₂] (4), [Pb₇(L)₄(μ₃-OH)₂(H₂O)₂]·8H₂O (5), [Zn₄(L)₂(BET)(μ₄-O)] (6), [Zn₂(HL)₂(PBIB)]·2H₂O (7), and [Zn₃(L)₂(PBIB)₂]·4H₂O (8) (H₃L = (3,5-dicarboxylphenyl)-(4-(2′-carboxylphenyl)benzyl) ether, TEA = triethylamine, BIME = 1,2-bis(imidazol-1-yl)ethane, BET = 1,2-bis(1,2,4-triazole-1-yl)ethane, and PBIB = 1,4-bis(imidazol-1-ylmethyl)benzene) have been synthesized under hydrothermal conditions. Compound 1 exhibits a three-dimensional (3D) (3,12)-connected framework based on decanuclear [Zn₁₀(μ₃-OH)₁₀(COO)₁₂] units. Compound 2 displays a 3D (3,6)-connected framework based on trinuclear [Zn₃(μ₃-OH)(CO₂)₆] units. Compounds 3 and 4 display unusual 2-fold interpenetrated 3D (3,6)-connected frameworks with the same (4²·6)₂(4⁴·6²·8⁸·10) topology. Compound 5 features a 3D tetranodal (3,3,4,8)-connected framework based on pentanuclear [Pb₅(COO)₈(μ₃-OH)₂(H₂O)₂] units. Compound 6 exhibits an unprecedented 2D (3,8)-connected network with (3·4²)(4⁶·5⁷·6⁸·7³·8) topology based on tetranuclear [Zn₄(μ₄-O)(COO)₆] units. Compound 7 reveals a wavy 2D sheet with (4⁴·6²) topology. Further, each sheet is interdigitated by two others in a parallel fashion to yield an unusual 2D → 3D interdigitated framework. Compound 8 displays a 3D rare trinodal (3,4,6)-connected framework with (3·4·5)₂(3²·10³·11)(3²·4²·5²·10³·11⁴·12·13) topology. These compounds have been characterized by IR spectra, UV-vis spectra, TGA, and powder XRD patterns. The optical band gaps and photoluminescent behaviors of 1–8 have been investigated in detail. In addition, surface photovoltage spectroscopies of compounds 1–4 indicate that these compounds possess positive surface photovoltage (SPV) responses in the range of 300–600 nm.