Anion鈭捪€ Interactions in Supramolecular Architectures
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- 作者：Helen T. Chifotides ; Kim R. Dunbar
- 刊名：Accounts of Chemical Research
- 出版年：2013
- 出版时间：April 16, 2013
- 年：2013
- 卷：46
- 期：4
- 页码：894-906
- 全文大小：817K
- 年卷期：v.46,no.4(April 16, 2013)
- ISSN：1520-4898
文摘
The study of the noncovalent force between 蟺-acidic aromatic systems and anions, referred to as the anion鈭捪€ interaction, has recently emerged as a new branch of supramolecular chemistry. The anion鈭捪€ contact is complementary to the cation鈭捪€ interaction, a prominent noncovalent force involved in protein structure and enzyme function. Until recently, the scientific community had overlooked the anion鈭捪€ interaction due to its ostensibly counterintuitive nature. Pioneering theoretical studies in 2002, however, established that anion鈭捪€ interactions are energetically favorable (20鈥?0 kJ/mol) and prompted a flurry of reports in support of their existence. The interest in anion鈭捪€ contacts was further fueled by the importance of anions in key chemical and biological processes and the involvement of 蟺-rings in anion recognition and transport. Anion鈭捪€ 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鈭捪€ 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鈭捪€ contacts are the driving elements of the final architectures. We surveyed several heterocyclic 蟺-acidic aromatic systems amenable to anion鈭捪€ contacts and explored the subtle interplay between ligand 蟺-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 蟺-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鈭捪€ contacts are critical for the metallacycle stability. Our X-ray crystallographic, NMR spectroscopic, and mass spectrometric (MS) studies demonstrated that the tetrahedral ([BF4]鈭?/sup>, [ClO4]鈭?/sup>) and octahedral ([SbF6]鈭?/sup>, [AsF6]鈭?/sup>, [PF6]鈭?/sup>) 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 蟺-acidic cavities of the metallacycles and establish multiple close directional F/O路路路Ctetrazine contacts with the edges. The observation of notable 19F 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 (螖G鈥?/sup> 50 kJ/mol) suggest that anion鈭捪€ contacts assist the anion templation.
We also studied reactions of Ag(I)X (X鈥?/sup> = [PF6]鈭?/sup>, [AsF6]鈭?/sup>, [SbF6]鈭?/sup>, [BF4]鈭?/sup>) with bptz or bppn (3,6-bis(2-pyridyl)-1,2-pyridazine) to assess the effect of the ligand 蟺-acidity on the preferred structures. The X-ray data revealed that the higher 蟺-acidity of the tetrazine ring in bptz leads to propeller-type products [Ag2(bptz)3]2+ exhibiting prominent short anion鈭捪€ contacts. By contrast, the less 蟺-acidic bppn preferentially favors grids [Ag4(bppn)4]4+ which exhibit maximized 蟺鈥撓€ interactions.
Finally, we explored the reactions of the extended 蟺-acidic heterocycle HAT(CN)6 (1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile) with the Cl鈥?/sup>, Br鈥?/sup>, I鈥?/sup> 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鈭捪€ contacts. In solution, the charge-transfer contacts were evidenced by electronic absorption, 13C 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.
Our work has focused on comprehensive investigations of several unprecedented supramolecular systems, in which the anions and their close anion鈭捪€ contacts are the driving elements of the final architectures. We surveyed several heterocyclic 蟺-acidic aromatic systems amenable to anion鈭捪€ contacts and explored the subtle interplay between ligand 蟺-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 蟺-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鈭捪€ contacts are critical for the metallacycle stability. Our X-ray crystallographic, NMR spectroscopic, and mass spectrometric (MS) studies demonstrated that the tetrahedral ([BF4]鈭?/sup>, [ClO4]鈭?/sup>) and octahedral ([SbF6]鈭?/sup>, [AsF6]鈭?/sup>, [PF6]鈭?/sup>) 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 蟺-acidic cavities of the metallacycles and establish multiple close directional F/O路路路Ctetrazine contacts with the edges. The observation of notable 19F 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 (螖G鈥?/sup> 50 kJ/mol) suggest that anion鈭捪€ contacts assist the anion templation.
We also studied reactions of Ag(I)X (X鈥?/sup> = [PF6]鈭?/sup>, [AsF6]鈭?/sup>, [SbF6]鈭?/sup>, [BF4]鈭?/sup>) with bptz or bppn (3,6-bis(2-pyridyl)-1,2-pyridazine) to assess the effect of the ligand 蟺-acidity on the preferred structures. The X-ray data revealed that the higher 蟺-acidity of the tetrazine ring in bptz leads to propeller-type products [Ag2(bptz)3]2+ exhibiting prominent short anion鈭捪€ contacts. By contrast, the less 蟺-acidic bppn preferentially favors grids [Ag4(bppn)4]4+ which exhibit maximized 蟺鈥撓€ interactions.
Finally, we explored the reactions of the extended 蟺-acidic heterocycle HAT(CN)6 (1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile) with the Cl鈥?/sup>, Br鈥?/sup>, I鈥?/sup> 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鈭捪€ contacts. In solution, the charge-transfer contacts were evidenced by electronic absorption, 13C 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.