Second Ligand-Directed Assembly of Photoluminescent Zn(II) Coordination Frameworks

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Abstract

Three new coordination polymers, {[Zn(H2Bpz)(Tph)]·1/2(H2Bpz)}n (1), [Zn2(H2Bpz)(Bpz)(Iph)]n (2), and {[Zn2(H2Bpz)(HBtc)2(H2O)2]·7/6(H2O)}n (3) (H2Bpz = 3,3‘,5,5‘-tetramethyl-4,4‘-bipyrazole; H2Tph = terephthalic acid; H2Iph = isophthalic acid; H3Btc = benzene-1,3,5-tricarboxylic acid), have been isolated from parallel hydrothermal reactions of ZnSO4 and H2Bpz with H2Tph, H2Iph, and H3Btc, respectively. These crystalline solids have been structurally defined, of which 1 was revealed to construct in a 3D architecture with porosity. Interestingly, because of its large porosity, 1 allows the concurrent 2-fold interpenetration and inclusion of free H2Bpz as guest molecule. In contrast to 1, polymer 2 structures in a 2D framework of note for containing both neutral and deprotonated H2Bpz as μ2-and μ4-bridging linkers, respectively. This case demonstrates a rare usage of H2Bpz as ligand in the construction of MOFs. The partial deprotonation of H2Bpz may be a consequence of less acidity of H2Iph than H2Tph, by which the influence of a second ligand on the reactivity of the main ligand is reified. Likewise, 3 is also assembled in a 2D framework, containing H2Bpz and monoacid HBtc2- as ligands, which indicates the stronger acidity of H3Btc. Moreover, to probe the effect of substitution of second ligands, we have investigated the luminescence of 1−3 in the solid state at room temperature, revealing that the polymers are photoluminescent and their emissions are varied with the alteration of second ligands.

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