A structural investigation of anion-triazole interactions: Observation of "π-pockets" and "jr-Sandwiches"

Eight mononuclear nickel(II) complexes of the ligands 4-amino-3,5-di(2- pyridyl)-1,2,4-triazole (adpt) and 4-pyrrolyl-3,5-di(2-pyridyl)-l,2,4-triazole ipldpti with the anions ClO₄, BF₄, PF₆ and SbF₆ have been prepared. In all cases the metal/ligand ratio is 1:3, and the complexes are of the form [NiL₃](A)₂-soiven(s where L = adpt or pldpt and A = one of the aforementioned anions. Five of these complexes have been structurally characterized by X-ray crystallography: four of these contain pldpt and strong anion-π interactions are observed, with two motifs present in all four structures. One of the anions occupies a "π-pocket" formed by two coordinated triazole rings and one coordinated pyridine ring. The other anion only interacts with one triazole ring, which is involved in the pocket around the first anion, such that the triazole ring is "sandwiched" by two anions. Surprisingly, in all four of these complexes, the two triazole centroid⋯anion distances in the anion-triazole-anion interactions [2.917(7)-3.005(10) Å] are significantly shorter than in any of the other types of triazole-anion interactions [3.164(5)-3.456(9) Å].