A series of framework coordination polymers reveals the use of 1,2,4,5-tetrazines as efficient bridging ligands towards silver(I) and copper(I) ions. All four nitrogen atoms were functional as lone pair donors leading to an unprecedented μ₄-coordination of the ligands (1,2,4,5-tetrazine, ttz; 3,6-dimethyl-1,2,4,5-tetrazine, Me₂ttz) in [Ag(ttz)(X)] (X = NO₃, 1; ClO₄, 2), [Ag₂(Me₂ttz)(NO₃)₂] (3), [Ag₂(Me₂ttz)(H₂O)₂(ClO₄)₂] (4), [Ag₃(Me₂ttz)(H₂O)₂(CF₃SO₃)₃] (5) and [Cu₄Cl₄(Me₂ttz)] (6). In 1 and 2, μ₄-tetrazines and silver ions (AgN₄, Ag–N 2.42–2.53 Å) compose a 3D framework of {4²;8⁴} topology. Structures 3 and 4 were based on disilver–tetrazine ribbons, while in 5μ₄-tetrazines interconnect silver–triflate chains. In 6, μ₄-ligands connect inorganic layers sustained by copper–chloride squares, hexa- and octagons (1.974(2) and 1.981(2) Å). Multiple N-coordination to metal ions enhances the ability of the electron deficient tetrazine system for anion⋯π binding. Compounds 1 and 2 exhibit very short interactions of this type with corresponding O⋯C(N) separations down to a record value of 2.78 Å and O⋯π 2.61 Å. For 6, π-acidity of tetrazine was reflected by contacts Cl⋯π of 3.30 Å. Results of high level ab initio calculations (RI-MP2/aug-cc-pVTZ) were in good agreement with experimental results, and were suggestive of the progressive enhancement of the π-acidity by increasing the number of Ag(I) ions N-coordinated to tetrazine.

Pure gas adsorption isotherms of CH₄ and N₂ and their binary mixtures were measured at 273 K, 298 K and 323 K and up to 2 MPa on two different microporous metal–organic frameworks (MOFs), i.e. the commercially available Basolite^® A100 and the recently reported copper-based triazolyl benzoate MOF ³ _∞[Cu(Me-4py-trz-ia)] (1). The Tòth isotherm model and the vacancy solution model were used to describe the experimentally determined isotherms and proved to be well suited for this purpose. While 1 shows a more homogeneous surface with a nearly constant isosteric heat of adsorption of 18–18.5 kJ mol⁻¹ for CH₄ and 12–15 kJ mol⁻¹ for N₂, the isosteric heat of adsorption at zero coverage for Basolite^® A100 is 19 kJ mol⁻¹ for CH₄ and 16.2 kJ mol⁻¹ for N₂, decreasing significantly with increasing loading. Binary adsorption isotherms were measured gravimetrically to determine the total adsorbed mass of CH₄ and N₂. The van Ness method was successfully applied to calculate partial loadings from gravimetrically measured binary adsorption isotherms. Further studies by volumetric–chromatographic experiments support the good correlation between experimental data and predictions by the vacancy solution model (VSM-Wilson) and the ideal adsorbed solution theory (IAST) from pure gas isotherms. The experimental selectivities were determined to be α CH₄/N₂ = 4.0–5.0 for 1, slightly higher than for Basolite^® A100 with α CH₄/N₂ = 3.4–4.5. These values are in good agreement with predictions for ideal selectivities based on Henry's law constants. From the experimental selectivities the potential of both MOFs in gas separation of CH₄ from N₂ can be derived.