Novel tetrazole-based metal–organic frameworks constructed from in situ synthesize bifunctional ligands: syntheses, structure and luminescent properties

Abstract

By control of the in situtetrazole ligand reaction and by the addition of different secondary ligands such as 2,2′-bipy, 4,4′-bipy or INA, five interesting frameworks, namely, Cd(tza)(2,2′-bipy)·H₂O (1), [Cd₂(btm)₂(2,2′-bipy)₂]·2.24H₂O (2a), [Zn₂(btm)₂(2,2′-bipy)₂]·3H₂O (2b), Cd(btm)(4,4′-bipy)_1/2 (3), [Zn(5MT)(INA)]₂·2H₂O (4) and Mn₂(btm)₂(2,2′-bipy)₂ (5) (H₂tza = 1H-tetrazolate-5-carboxylate; bipy = bipyridine; H₂btm = bis(tetrazole) methane; 5MT = 5-methyl-tetrazole; HINA = isonicotinic acid), were synthesized and structurally characterized by single-crystal X-ray diffraction. Compound 1 exhibits a two-dimensional (2D) framework constructed of the dinuclear [Cd₂O₂] clusters and μ₃-κN4:κN6,O1:κO1 bridging tza. Compounds 2a and 2b display 2D frameworks built up by the linkage of infinite one-dimensional (1D) helical zigzag -M-btm-M- chains (M = Cd, Zn, respectively) with dinuclear units [MN₂]_2.. Compounds 1, 2a and 2b all exhibit a 2D four-connected (4,4) topological network. Compound 3 represents a three-dimensional (3D) framework constructed of 2D layers formed by the linkage of [Cd₂N₄] cluster units and μ₄-κN2:κN4:κN8:κN1, N5 tetrazolate groups as "double-bridges", which are pillared through 4,4-bipy, finally giving a pcu (4¹² 6³) topology. Compound 4 exhibits a 3D two-fold interpenetrating diamond-like network constructed by infinite -5MT-Zn-5MT- chains and INA as bridging coligands. Compound 5 is a 2D framework which is built up by Mn1-tetrazole helical chains, [Mn2N₂]₂ clusters and μ₃-κN11:κN14, N15:κN17 and μ₃-κN3, N7:κN4:κN10 tetrazole groups. It is noted that both H₂tza and H₂btm ligands are in situ generated from the same precursor, malononitrile. These results show the influence of the reactant ratio, the nature of the secondary ligands and the metal ions in the construction of metal frameworks based on tetrazole. The photoluminescent properties of 1, 2a, 2b, 3, 4 and thermal stabilities of 1, 3, 4, 5 are investigated in the solid state at room temperature.