Stabilization of an elusive tautomer by metal coordination

The solid-state isolation of the different tautomers of a chemical com­pound can be a challenging problem. In many cases, tautomers with an energy very close to the most stable one cannot be isolated (elusive tautomers). In this article, with reference to the 4-methyl-7-(pyrazin-2-yl)-2H-[1,2,4]triazolo[3,2-c][1,2,4]triazole ligand, for which the elusive 3H-tautomer has an energy only 1.4 kcal mol(−1) greater than the most stable 2H form, we show that metal com­plexation is a suc­cessful and reliable way for stabilizing the elusive tauto­mer. We have pre­pared two com­plexes of the neutral ligand with CuBr(2) and ZnBr(2), namely, aqua­bromido­bis­[4-methyl-7-(pyrazin-2-yl)-3H-[1,2,4]triazolo[3,2-c][1,2,4]triazole]copper(II) bromide trihydrate, [CuBr(C(8)H(7)N(7))(2)(H(2)O)]Br·3H(2)O, and di­bro­mido­[4-methyl-7-(pyrazin-2-yl)-2H-[1,2,4]triazolo[3,2-c][1,2,4]triazole][4-methyl-7-(pyrazin-2-yl)-3H-[1,2,4]triazolo[3,2-c][1,2,4]tri­azole]zinc(II) monohydrate, [ZnBr(2)(C(8)H(7)N(7))(2)]·H(2)O. The X-ray analysis shows that, in both cases, the elusive 3H-tautomer is present. The results of the crystallographic analysis of the two com­plexes reflect the different coordination preferences of Cu(II) and Zn(II). The copper(II) com­plex is homotautomeric as it only con­tains the elusive 3H-tautomer of the ligand. The com­plex can be described as octa­hedral with tetra­gonal distortion. Two 3H-triazolotriazole ligands are bis-chelated in the equatorial plane, while a water mol­ecule and a bromide ion in elongated axial positions com­plete the coordination environment. The zinc(II) com­plex, on the other hand, is heterotautomeric and con­tains two bromide ions and two monodentate ligand mol­ecules, one in the 2H-tautomeric form and the other in the 3H-tautomeric form, both coordinated to the metal in tetra­hedral geometry. The observation of mixed-tautomer com­plexes is unprecedented for neutral ligands. The analysis of the X-ray mol­ecular structures of the two com­plexes allows the deduction of possible rules for a rational design of mixed-tautomer com­plexes.