Anion−π Interactions in Supramolecular Architectures (2013)

The study of the noncovalent force between π-acidic aromatic systems and anions, referred to as the anion-π interaction, has recently emerged as a new branch of supramolecular chemistry. The anion-π contact is complementary to the cation-π interaction, a prominent noncovalent force involved in protein structure and enzyme function. Until recently, the scientific community had overlooked the anion-π interaction due to its ostensibly counterintuitive nature. Pioneering theoretical studies in 2002, however, established that anion-π interactions are energetically favorable (~20-70 kJ/mol) and prompted a flurry of reports in support of their existence. The interest in anion-π contacts was further fueled by the importance of anions in key chemical and biological processes and the involvement of π-rings in anion recognition and transport. Anion-π interactions hold great promise for the design of selective anion receptors, hosts or scaffolds, colorimetric sensors, and catalysts and may also affect biological functions. Currently, the area of anion-π research is highly topical in the scientific community and on a meteoric rise in the chemical literature. This Account highlights our leading findings in this burgeoning area. Our work has focused on comprehensive investigations of several unprecedented supramolecular systems, in which the anions and their close anion-π contacts are the driving elements of the final architectures. We surveyed several heterocyclic π-acidic aromatic systems amenable to anion-π contacts and explored the subtle interplay between ligand π-acidity, anion identity, and metal ions in mediating the ensuing self-assembled architectures. The reactions we performed between solvated first-row transition metal ions and the π-acidic ligands bptz (3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) or bmtz (3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine) resulted in unprecedented metallacycles. Our investigations revealed that the identity of the encapsulated ion dictates the metallacycle nuclearity and close anion-π contacts are critical for the metallacycle stability. Our X-ray crystallographic, NMR spectroscopic, and mass spectrometric (MS) studies demonstrated that the tetrahedral ([BF4](-), [ClO4](-)) and octahedral ([SbF6](-), [AsF6](-), [PF6](-)) anions template discrete molecular squares and pentagons, respectively. The metal ions occupy the vertices, and bptz or bmtz moieties span the edges of the metallacycles. The encapsulated anions occupy the π-acidic cavities of the metallacycles and establish multiple close directional F/O···C(tetrazine) contacts with the edges. The observation of notable (19)F solid-state NMR chemical shifts reflects the short contacts of the encapsulated anions, findings that we corroborated by DFT calculations. The solution NMR data support the conclusion that bona fide metallacycle templation and interconversion between the metallacycles in solution occurs only in the presence of the appropriate anions. The NMR, MS, and CV data underscore the remarkable metallapentacycle stability despite the angle strain inherent in pentagons formed by octahedral metal ions. The low anion activation energies of encapsulation (ΔG(‡) ~ 50 kJ/mol) suggest that anion-π contacts assist the anion templation. We also studied reactions of Ag(I)X (X(-) = [PF6](-), [AsF6](-), [SbF6](-), [BF4](-)) with bptz or bppn (3,6-bis(2-pyridyl)-1,2-pyridazine) to assess the effect of the ligand π-acidity on the preferred structures. The X-ray data revealed that the higher π-acidity of the tetrazine ring in bptz leads to propeller-type products [Ag2(bptz)3](2+) exhibiting prominent short anion-π contacts. By contrast, the less π-acidic bppn preferentially favors grids [Ag4(bppn)4](4+) which exhibit maximized π-π interactions. Finally, we explored the reactions of the extended π-acidic heterocycle HAT(CN)6 (1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile) with the Cl(-), Br(-), I(-) ions which lead to highly colored solutions/crystals. X-ray crystallographic studies of the HAT(CN)6/halide complexes revealed unprecedented multisite short peripheral charge-transfer and centroid anion-π contacts. In solution, the charge-transfer contacts were evidenced by electronic absorption, (13)C and halogen NMR, as well as MS data. The distinctly colored complex entities exhibit extraordinarily high association constants, which render them promising for anion-sensing receptor applications.