1,2,4,5-tetrazine: an unprecedented micro4-coordination that enhances ability for anion pi 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 mu(4)-coordination of the ligands (1,2,4,5-tetrazine, ttz; 3,6-dimethyl-1,2,4,5-tetrazine, Me(2)ttz) in [Ag(ttz)(X)] (X = NO(3), ; ClO(4), ), [Ag(2)(Me(2)ttz)(NO(3))(2)] (), [Ag(2)(Me(2)ttz)(H(2)O)(2)(ClO(4))(2)] (), [Ag(3)(Me(2)ttz)(H(2)O)(2)(CF(3)SO(3))(3)] () and [Cu(4)Cl(4)(Me(2)ttz)] (). In and , micro(4)-tetrazines and silver ions (AgN(4), Ag-N 2.42-2.53 A) compose a 3D framework of {4(2);8(4)} topology. Structures and were based on disilver-tetrazine ribbons, while in mu(4)-tetrazines interconnect silver-triflate chains. In , micro(4)-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 anionpi binding. Compounds and exhibit very short interactions of this type with corresponding OC(N) separations down to a record value of 2.78 A and Opi 2.61 A. For , pi-acidity of tetrazine was reflected by contacts Clpi of 3.30 A. 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 pi-acidity by increasing the number of Ag(i) ions N-coordinated to tetrazine.