Syntheses, structures and luminescence properties of novel metal–organic frameworks based on zinc(ii), cadmium(ii) or lead(ii) and a 2,2′-dimethoxy-functionalised biphenyl linker

Abstract

Three metal–organic frameworks (MOFs) based on zinc(II) (1), cadmium(II) (2) or lead(II) (3) and the functionalised biphenyl linker biphenyl-2,2′-dimethoxy-4,4′-dicarboxylate, [4,4′-(CO₂)₂-2,2′-(OMe)₂-1,1′-(C₆H₃)₂] (L), were synthesised. Compounds 1 and 3 exhibit different three-dimensional networks with the Zn₄O(O₂C)₆(H₂O) unit as the secondary building unit (SBU) in the Zn-based MOF [{(Zn₄O)(L)₃(H₂O)}·4.5DMF]_n (1) and Pb₆O₂(O₂C)₄ clusters as SBU in the Pb-based MOF [{(Pb₆O₂)L₄}·2EtOH]_n (3), as shown by X-ray analysis. The Cd-based MOF [{(Cd₃L₄)₂(CdL₂)}·8DMF·8Me₂NH₂·2Cl]_n (2) exhibits two interpenetrated three-dimensional networks formed from two different types of secondary building units, Cd₃(μ-O₂C)₆(O₂C)₂ and Cd(O₂C)₄. Compounds 1–3 are thermally stable up to 420, 360 and 380 °C, respectively (according to DTA–TG). All compounds show bluish emissions visible with the naked eye, which could be assigned to S_1,2 → S₀ transitions of the aromatic units by comparison with the luminescence properties of the free ligand.