Six new metal–organic frameworks with multi-carboxylic acids and imidazole-based spacers: syntheses, structures and properties

Abstract

Six new metal–organic frameworks [Cu(obba)(bimb)·(obbaH₂)]_n (1), [Cu(obba)(bimb)]_n (2), [Zn₂(obba)₂(bimb)₂(DMF)₂(H₂O)_3.5]_n (3), [Ni₃(2,2′,4,4′-bptcH)₂(bimb)₂(H₂O)₂·(H₂O)₂]_n (4), [Ni₂(bimb)₃(H₂O)₆·(aobtc)·(DMF)₂·(H₂O)₂]_n (5) and [Cd(3,3′,4,4′-bptcH₂)(H₂O)·(bimb)]_n (6), were obtained by reactions of 4,4′-bis(1-imidazolyl)biphenyl (bimb) and multi-carboxylic acids of 4,4′-oxybis(benzoic acid) (obbaH₂), 2,2′,4,4′-biphenyltetracarboxylate acid (2,2′,4,4′-bptcH₄), azoxybenzene-3,3′,5,5′-tetracarboxylic acid (aobtcH₄), and 3,3′,4,4′-biphenyltetracarboxylate acid (3,3′,4,4′-bptcH₄) with corresponding metal salts under hydro/solvothermal conditions, respectively. Complexes 1–3 have entangled structures with different topologies: 1 is a 3-fold interpenetrating NbO three-dimensional (3D) network; 2 is a 3-fold interpenetrating dmp 3D net; 3 is a 6-fold interpenetrating dia 3D chiral net containing rare 1D helical chains with the same handedness. Complex 4 is an uninodal 6-connected network with a Schäfli symbol of (4⁸6⁴8³) based on the trinuclear Ni(II) subunits, while complexes 5 and 6 are 1D chains. Interestingly, compound 6 represents the rare example of MOFs that exhibit high photocatalytic activity for dye degradation under visible light and shows good stability towards photocatalysis. Complexes 3 and 6 exhibit intense blue emissions in the solid state at room temperature whereas 3 appears to be a good candidate of novel hybrid inorganic–organic NLO material.