Anion−π Interactions as Controlling Elements in Self-Assembly Reactions of Ag(I) Complexes with π-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 π-acidity of the ligand central ring and the outer-sphere anion. The bptz reactions lead
to the polymeric {[Ag(bptz)][PF6]}∞ (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, π−π stacking interactions are maximized, whereas multiple and
shorter (therefore stronger) anion−π 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 π-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−π interactions in the bppn as compared
to the bptz complexes. From the systems studied herein, it is concluded that anion−π interactions play an
important role in the outcome of self-assembly reactions.