1,2,4,5-Tetrazine: An unprecedented μ₄-coordination that enhances ability for anion⋯π interactions

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)(H ₂O)₂(ClO₄)₂] (4), [Ag ₃(Me₂ttz)(H₂O)₂(CF ₃SO₃)₃] (5) and [Cu₄Cl ₄(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.