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Supramolecular Chemistry of Anions

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Research in the Supramolecular Chemistry of Anions is a newer project in the Dunbar Group. The project has developed into a highly interdisciplinary endeavor, encompassing coordination chemistry, computational chemistry, and, biochemistry. Anion-π interactions, i.e., the noncovalent forces between electron-deficient aromatic systems and anions, have been relatively unexplored as compared to cation-pi interactions, primarily due to the counter-intuitive nature of aromatic rings being attracted to a negative charge. The vital role of anions in many key chemical and biological processes and the involvement of pi rings in molecular anion recognition and transport processes, however, indicate that anion-pi contacts may be prominent players in fields as diverse as medicine and environmental chemistry. Our tutorial review in Chemical Society Reviews presents a good overview of the aims and scope of the field.1-4

The main question that we are posing is: "How does the interplay between anions and electron-deficient aromatic ligands mediate the structures and properties of supramolecular architectures"? An initial foray into the nature of supramolecular interactions involving anions - specifically the anion-pi interaction was initiated in response to our discovery of two related metallocyclophanes, the nuclearity of which is dictated by the identity of the encapsulated anion.1 Reactions of fully-solvated NiII ions with the ligand 3,6-bis(2'-pyridyl)-1,2,4,5-tetrazine (bptz) yield the square [Ni4(bptz)4(NCCH3)8⊂BF4]7+ (Figure 1, left) and pentagonal [Ni5(bptz)5(NCCH3)10⊂SbF6]9+ (Figure 1, right) motifs in the presence of [BF4]- and [SbF6]- anions, respectively. Intriguingly, the pentagonal motif can be converted to the square motif simply by adding the [BF4]- ions to the pentagon in solution. The reverse conversion may also be effected. The previous findings along with the observation of an alignment between the central tetrazine core of bptz and [BF4]- in the square motif's crystal structure, suggest that anion-pi interactions are an integral part of the template effect.

NiSqSide.tif NiPentFront.tif
NiSqTop.tif NiPentTop.tif

Figure 1. : Side (above) and top (below) views of the cationic [Ni4(bptz)4(NCCH3)8⊂BF4]7+ (square, left) and [Ni5(bptz)5(NCCH3)10⊂SbF6]9+ (pentagon, right) species. Non-encapsulated anions, solvent molecules and hydrogen atoms have been omitted for clarity. Atom colors: C = gray, N = blue, F = green, B = purple, Sb = yellow and Ni = tan.1

The aforementioned observations led us to explore the nature of anion-pi interactions in both experimental and computational systems, with a focus on the role of the electronic nature of the ligand and the anion identity. Our work revealed that when bptz reacts with salts of AgI, the ensuing structure depends on the identity of the anion which interacts strongly with the ligand through anion-pi interactions (the most striking example being the propeller-type structure with AgSbF6).2 Conversely if a ligand with a more electron-rich central ring is used, such as 3,6-bis(2'-pyridyl)pyridazine (bppn), a grid structure arises regardless of the anion used, and the structure maximizes pi-pi stacking interactions at the expense of anion-pi interactions (Figure 2). By using similar tricyclic ligands with varying binding motifs and electronic distributions we are aim to further probe the anion-templation effect in complexes of first-row transition metals.

agprop.tif aggrid.tif
bptz_esp.tif bppn_esp.tif

Figure 2. : Structures resulting from the reaction of AgSbF6 with bptz (propeller, top left) and bppn (grid, top right), with the electrostatic potential maps of bptz (lower left) and bppn (lower right). Electrostatic potential maps created at a 0.02 isodensity value and a color scale 126 (blue) to -63 (red) kcal/mol using Cerius2. Atom colors: C = gray, H = white, N = blue, F = green, B = purple, Sb = yellow and Ag = brown.2

As part of our broader interest in anion-pi interactions, we have recently been studying the interesting electron-deficient ligand HAT(CN)6 (1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile) (Figure 3, left) which co-crystallizes with [n-Bu4N][I] to afford {([n-Bu4N][I])3[HAT(CN)6]2}·3C6H6 (1). This material exhibits charge-transfer from the [I]- ions to the HAT(CN)6 rings as well as anion-pi interactions at the ring centroid.3,4 The crystal structure of 1 indicates four crystallographic positions partially occupied by three iodide anions (Figure 3, center); three of the four iodide positions are centered over the periphery of the HAT(CN)6 ring while the fourth position is located directly over the centroid on the opposite face of the ring (Figure 3, center). The resulting structure exhibits an ABCD type stacking with alternating layers of HAT(CN)6 molecules and iodide anions (Figure 3, right). The two distinct anion sites and the established anion-π interactions are the focus of ongoing studies in our group.

hatcn6_transparent.tif hatcn6_top.tif hatcn6_stack.tif

Figure 3. : (Left) Structure of HAT(CN)6; (center) {([n-Bu4N][I])3[HAT-(CN)6]2}.3C6H6 looking down the c axis (the three iodide ions are distributed among four crystallographic positions) and (right) space-filling diagram of the repeat layers in ([n-Bu4N][I])3HAT-(CN)6]2; the cations have been omitted for clarity. Atom colors: C = gray, H = white, N = blue, I = yellow.3

Our computational studies (with both Density Functional Theory and ab initio methods) are used to corroborate our experimental findings, and the results suggest that highly pi-acidic aromatic systems are more amenable to anion-pi interactions. Continuing research involves expanding the computational models to include multiple anions and multiple arenes to explore the anion-pi interaction in extended systems as well as single-point energy and geometry optimization computations on structures derived from crystallographic studies.

Perhaps even more fascinating than the role of anions in metallosupramolecular chemistry is the possible role of anion-pi interactions in biological systems due to the pivotal role of anions in many key chemical and biological processes.4 The presence of electron-poor aromatic moieties in biomolecules such as proteins and nucleic acids led us to ask whether anion-pi interactions may play a role in protein functions, e.g., anion-transport, or other enzyme activities. Current research in the Dunbar group aims at addressing the role of anion-π interactions in proteins and other molecules of biological interest via computational and statistical analyses of known protein structures.

References
1. Campos-Fernandez, C. S.; Schottel, B. L.; Chifotides, H. T.; Bera, J. K.; Bacsa, J.; Koomen, J. M.; Russell, D. H.; Dunbar, K. R. J. Am. Chem. Soc., 2005, 127, 12909-12923.
2. Schottel, B. L.; Chifotides, H. T.; Shatruk, M.; Chouai, A.; Bacsa, J.; Perez, L. M.; Dunbar, K. R. J. Am. Chem. Soc. 2006, 128, 5895-5912.
3. Szalay, P. S.; Galan-Mascaros, J. R.; Schottel, B. L.; Basca, J.; Perez, L. M.; Ichimura, I. S.; Chouai, A.; Dunbar, K. R. J. Cluster Sci. 2004, 15, 503-530.
4. Schottel, B. L.; Chifotides, H. T.; Dunbar, K. R. Chem. Soc. Rev. 2008, 37, 68-83.

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Department of Chemistry | Texas A&M University | State of Texas