Syntheses, structural diversities and characterization of a series of coordination polymers with two isomeric oxadiazol-pyridine ligands

Abstract

In this work two positional-isomeric oxadiazol-pyridine ligands 3-(5-methyl-1,3,4-oxadiazol-2-yl)pyridine (L₁) and 4-(5-methyl-1,3,4-oxadiazol-2-yl)pyridine (L₂) have been designed and synthesized. A series of novel coordination polymers, namely [Cd₂(μ₂-L₁)₂(μ₂-NCS)₄]_n (1), {[Cd(L₁)(μ₂-dca)₂(H₂O)]·H₂O}_n (2), {[Cu(μ₂-L₁)₂(NCS)₂]·0.5H₂O}_n (3), {[Ag₂(μ₂-L₁)(μ₃-L₁)₂]·2PF₆}_n (4), {[Ag₃(μ₂-L₁)₄(μ₂-CF₃SO₃)(CF₃SO₃)]·CF₃SO₃} (5), {[Cd(L₂)₂(μ₂-NCS)₂]}_n (6), [Ag(μ₂-L₂)(μ₂-CF₃SO₃)]_n (7) and {[Ag(μ₂-L₂)]·BF₄}_n (8) have been isolated. Both 1 and 2 are 2D Cd^II coordination polymers containing infinite {Cd–NCS–Cd} chains (for 1) or infinite {Cd–dca–Cd} layers (for 2), respectively. 3 is a 2D Cu^II coordination polymer, in which central metal ions are bridged via a bidentate bridging L₁ ligand. While when different Ag^I salts were introduced into the reaction system, 1D Ag^I coordination polymers 4 and 5 with diverse coordination modes can be isolated. Furthermore, when the isomeric oxadiazol-pyridine L₂ is used to replace L₁ in the reaction system, 6–8 can be isolated. 6 is a 2D Cd^II coordination polymer containing {Cd–NCS–Cd} layers. 7 is a 2D neutral Ag^I coordination polymer while 8 is a 2D cationic Ag^I coordination polymer. Variable temperature magnetic susceptibility measurements (2–300 K) reveal anti-ferromagnetic interactions between central copper(II) ions for 3. Solid-state luminescent properties of 1, 2 and 4–8 have been investigated indicating strong fluorescent emissions. Additionally, luminescent measurements illustrate that complex 8 exhibits highly sensitive luminescence sensing for Cr₂O₇²⁻ ions in aqueous solutions with high quenching efficiency K_sv = 2.08 × 10⁴ L mol⁻¹ and low detection limit (0.19 μM (S/N = 3)). 8 also represents the first report of a coordination polymer based on oxadiazol-pyridine derivatives with a luminescence response to Cr₂O₇²⁻ anion pollutants in aqueous solutions.