Topological identification of the first uninodal 8-connected lsz MOF built from 2,2′-difluorobiphenyl-4,4′-dicarboxylate pillars and cadmium(II)-triazolate layers

Using polynuclear metal clusters as nodes, many high-symmetry high-connectivity nets, like 8-connnected bcu and 12-connected fcu, have been attained in metal-organic frameworks (MOFs). However, construction of low-symmetry high-connected MOFs with a novel topology still remains a big challenge. For example, a uninodal 8-connected lsz network, observed in inorganic ZrSiO₄, has not been topologically identified in MOFs. Using 2,2′-difluorobiphenyl-4,4′-dicarboxylic acid (H₂L) as a new linker and 1,2,4-triazole (Htrz) as a coligand, a novel three-dimensional Cd^II-MOF, namely poly[tetrakis(μ₄-2,2′-difluorobiphenyl-4,4′-dicarboxylato-κ⁵O¹,O^1′:O^1′:O⁴:O^4′)tetrakis(N,N-dimethylformamide-κO)tetrakis(μ₃-1,2,4-triazolato-κ³N¹:N²:N⁴)hexacadmium(II)], [Cd₆(C₁₄H₆F₂O₄)₄(C₂H₂N₃)₄(C₃H₇NO)₄]_n, (I), has been prepared. Single-crystal structure analysis indicates that six different Cd^II ions co-exist in (I) and each Cd^II ion displays a distorted [CdO₄N₂] octahedral geometry with four equatorial O atoms and two axial N atoms. Three Cd^II ions are connected by four carboxylate groups and four trz^- ligands to form a linear trinuclear [Cd₃(COO)₄(trz)₄] cluster, as do the other three Cd^II ions. Two Cd₃ clusters are linked by trz^- ligands in a μ_1,2,4-bridging mode to produce a two-dimensional Cd^II-triazolate layer with (6,3) topology in the ab plane. These two-dimensional layers are further pillared by the L^2- ligands along the c axis to generate a complicated three-dimensional framework. Topologically, regarding the Cd₃ cluster as an 8-connected node, the whole architecture of (I) is a uninodal 8-connected lsz framework with the Schläfli symbol (4²²·6⁶). Complex (I) was further characterized by elemental analysis, IR spectroscopy, powder X-ray diffraction, thermogravimetric analysis and a photoluminescence study. MOF (I) has a high thermal and water stability.