1,2,4,5-Tetrazine: an unprecedented mu(4)-coordination that enhances ability for anion center dot center dot center dot pi interactions

摘要

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 = NO3, 1; ClO4, 2), [Ag-2(Me(2)ttz)(NO3)(2)] ( 3), [Ag-2(Me(2)ttz)(H2O)(2)(ClO4)(2)] (4), [Ag-3(Me(2)ttz)(H2O)(2)(CF3SO3)(3)] (5)and[Cu4Cl4(Me(2)ttz)] (6). In 1 and 2, mu(4)-tetrazines and silver ions (AgN4, Ag-N 2.42-2.53 angstrom) compose a 3D framework of {42; 84} topology. Structures 3 and 4 were based on disilver-tetrazine ribbons, while in 5 mu(4)-tetrazines interconnect silver-triflate chains. In 6, mu(4)-ligands connect inorganic layers sustained by copper-chloride squares, hexa-and octagons (1.974(2)and 1.981(2)angstrom). Multiple N-coordination to metal ions enhances the ability of the electron deficient tetrazine system for anion center dot center dot center dot pi binding. Compounds 1 and 2 exhibit very short interactions of this type with corresponding O center dot center dot center dot C(N) separations down to a record value of 2.78 angstrom and O center dot center dot center dot pi 2.61 angstrom. For 6, pi-acidity of tetrazine was reflected by contacts Cl center dot center dot center dot pi of 3.30 angstrom. 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.