1,2,4,5-Tetrazine: an unprecedented mu4-coordination that enhances ability for anion···pi interactions

Abstract

[eng] 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 m4-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, m4-tetrazines and silver ions (AgN4, Ag-N 2.42-2.53A˚ ) compose a 3D framework of {42;84} topology. Structures 3 and 4 were based on disilver-tetrazine ribbons, while in 5 m4-tetrazines interconnect silver-triflate chains. In 6, m4-ligands connect inorganic layers sustained by copper-chloride squares, hexa- and octagons (1.974(2) and 1.981(2)A˚ ). Multiple N-coordination to metal ions enhances the ability of the electron deficient tetrazine system for anion ◊ ◊ ◊ p binding. Compounds 1 and 2 exhibit very short interactions of this type with corresponding O◊ ◊ ◊ C(N) separations down to a record value of 2.78A˚ and O◊ ◊ ◊ p 2.61A˚ . For 6, p-acidity of tetrazine was reflected by contacts Cl ◊ ◊ ◊ p of 3.30A˚ . 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 p-acidity by increasing the number of Ag(I) ions N-coordinated to tetrazine.