Anion-pi interactions as controlling elements in self-assembly reactions of Ag(I) complexes with pi-acidic aromatic rings.

Reactions of 3,6-bis(2'-pyridyl)-1,2,4,5-tetrazine (bptz) and 3,6-bis(2'-pyridyl)-1,2-pyridazine (bppn) with the AgX salts (X = [PF6]-, [AsF6]-, [SbF6]-, and [BF4]-) afford complexes of different structural motifs depending on the pi-acidity of the ligand central ring and the outer-sphere anion. The bptz reactions lead to the polymeric [[Ag(bptz)][PF6]]infinity (1) and the dinuclear compounds [Ag2(bptz)2(CH3CN)2][PF6]2 (2) and [Ag2(bptz)2(CH3CN)2][AsF6]2 (3), as well as the propeller-type species [Ag2(bptz)3][AsF6]2 (4) and [Ag2(bptz)3][SbF6]2 (5a and 5b). Reactions of bppn with AgX produce the grid-type structures [Ag4(bppn)4][X]4 (6-9), regardless of the anion present. In 6-9, pi-pi stacking interactions are maximized, whereas multiple and shorter (therefore stronger) anion-pi interactions between the anions and the tetrazine rings are established in 1-5b. These differences reflect the more electron-rich character of the bppn pyridazine ring as compared to the bptz tetrazine ring. The evidence gleaned from the solid-state structures was corroborated by density functional theory calculations. In the electrostatic potential maps of the free ligands, a higher positive charge is present in the bptz as compared to the bppn central ring. Furthermore, the electrostatic potential maps of 3, 4, and 5b indicate an electron density transfer from the anions to the pi-acidic rings. Conversely, upon addition of the [AsF6]- ions to the cation of 7, there is negligible change in the electron density of the central pyridazine ring, which supports the presence of weaker anion-pi interactions in the bppn as compared to the bptz complexes. From the systems studied herein, it is concluded that anion-pi interactions play an important role in the outcome of self-assembly reactions.