Temperature- and solvent-controlled dimensionality in a zinc 6-(1H-benzoimidazol-2-yl)pyridinecarboxylate system

Abstract

Several 0D, 1D, and 3D metal–organic hybrid frameworks of Zn^II have been synthesized by using the asymmetric ligand 6-(1H-benzoimidazol-2-yl)pyridinecarboxylic acid (HL) with control of the temperature and solvent in a solvothermal synthesis technique. In DMF, the structure at room temperature confirms that the resulting compound Zn(L)₂·2(H₂O) (1) is the 0D mononuclear structure, above ∼80 °C, C–N bond cleavage of partial DMF molecules induces the formation of the 1D chain-like compound {[Zn(L)(HCOO)]·2(H₂O)}_n (2), but on warming up to 120 and 160 °C, due to the decomposition of a large number of DMF molecules, the same reaction, respectively, gives two 3D metal–organic coordination polymers with 3D channels reported previously {[Zn(HCOO)₃]⁻·[H₂N(CH₃)₂]⁺}_n (3) and [Zn(HCOO)₂]_n (4), in which the 3D channels of 3 contain guest [H₂N(CH₃)₂]⁺ motifs, while at high temperature the 3D channels of 4 are an open stable system without any guest molecules. Replacing DMF in the above reaction by ethanol or acetonitrile, only 0D compound 1 is obtained in the range from room temperature to 160 °C. However, the same reaction in DMSO provides a 1D wave-like compound {[Zn(L)(DMSO)]·(ClO₄)·H₂O}_n (5), in which the solvent DMSO molecule coordinates to the central metal Zn^II ion. Furthermore, in three new complexes 1, 2 and 5, through π⋯π stacking and hydrogen bonding interactions, 3D supramolecular networks are assembled. It is worth noting that the existence of the twelve-membered cyclic water–perchlorate dimers [(ClO₄)⋯(H₂O)]₂²⁻ with chair-conformation in the solid state of 5 not only increases the stability of the supramolecular structure, but also results in a stronger fluorescence signal compared with ligand HL, 1 and 2.