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46121-AC3
Systematic and Theoretical Studies of Anion-pi Interactions for the Development of Supermolecules and New Materials

Kim R. Dunbar, Texas A&M University

9/1/2007 – 8/31/2008

During the past year of this project of the newly recognized anion-p interaction, we have made excellent progress in several of the proposed systems. A comprehensive crystallographic and theoretical study was undertaken to probe the preferred structural motifs of the Ag(I) complexes obtained from the reaction of the Ag(I)X salts (X = [PF₆]^-, [AsF₆]^-, [SbF₆]^-, [BF₄]^-) with 3,6-bis(2´-pyridyl)-1,2,4,5-tetrazine (bptz) or 3,6-bis(2´-pyridyl)-1,2-pyridazine (bppn), which exhibit different p-acidity of the central rings.[1] The bptz reactions lead to polymeric, dinuclear and propeller-type species (Figure 1a) depending on the anion, whereas the bppn reactions produce the grid-type structures [Ag₄(bppn)₄]⁴⁺ (Figure 1b), regardless of the anion present. In the bppn structures,π-π stacking interactions are maximized, whereas multiple, shorter, and therefore stronger, anion-p interactions between the anions and the tetrazine rings are present in the bptz complexes (shorter by ~0.2 Å than those encountered in the bppn complexes). Furthermore, all the Ag(I) bptz complexes have more than one tetrazine ring p-contact per anion (e.g., in [Ag₂(bptz)₃][SbF₆]₂ each anion interacts with 3 tetrazine rings; Figure 1a), whereas the bppn grids have only one pyridazine ring π-contact per anion (Figure 1b). The multiple anion-p interactions per anion established in the case of the bptz complexes (as compared to one per anion in the bppn complexes) and the difference in the preferred structural motifs between the bptz and bppn structures are in accord with the higher p-acidic character of the bptz central tetrazine ring as compared to the more electron-rich bppn pyridazine ring.

As a continuation of the metallocycles that we have prepared earlier with bptz, we recently isolated the Co(II) analog of [Ni₄(NCCH₃)₈(bptz)₄ÌBF₄][BF₄]₇, namely [Co₄(NCCH₃)₈(bptz)₄ÌBF₄][BF₄]₇.[2] Single crystal X-ray diffraction studies revealed that [Co₄(NCCH₃)₈(bptz)₄ÌBF₄][BF₄]₇ (Figure 2) indeed adopts the square motif with an encapsulated [BF₄]^- anion in the cage. The anion is positioned in a such a manner that the fluorine atoms of [BF₄]^- are pointing to the electron deficient carbon atoms of two bptz ligands. The distances between the anion and the tetrazine ring are 2.73-2.89 Å, which are close to the predicted distances from the gas-phase computations for the electron-deficient substituted tetrazine rings C₂N₄(CN)₂ and C₂N₄F₂ (Table 1; Figure 3). Mass spectrometric data revealed the parent ion peak at [Co₄(NCCH₃)₈(bptz)₄]⁸⁺ at m/z 188.53, an indication that the molecule remains intact in solution.

Table 1. B3LYP 6-31+G(d’) geometry optimization results for [BF4]-, anion-arene closest contact distances,  B to arene centroid distance, and total binding energy (Et).

Previous work performed in our group demonstrated that hexaazatriphenylene-hexacarbonitrile (HAT-(CN)₆) is capable of cocrystallizing in the presence of ([n-Bu₄][I]) or cobaltocenium hexafluorophosphate ([CoCp₂][PF₆]) to form the intensely colored compounds {([n-Bu₄N][I])₃[HAT-(CN)₆]₂}∙3C₆H₆ and {[CoCp₂][PF₆]}₃[HAT-(CN)₆]∙CH₃CN, respectively. Recently, we also crystallized HAT(CN)₆ with bromide ions, i.e., {([n-Bu₄N][Br])₃[HAT-(CN)₆]₂}∙3C₆H₆ (Figure 4). The intense colors of the solids for these compounds prompted us to undertake UV/visible spectroscopic titration experiments of HAT-(CN)₆ in the presence of the halide ions.[3]  Upon addition of [n-Bu₄N][I] to a solution of HAT-(CN)₆, an intensely green colored solution is observed, and gradual addition of increasing amounts of the salt, increases the intensity of the new absorption bands in the visible region.  Intensely colored solutions are also observed upon the addition of [n-Bu₄N][Br] or [n-Bu₄N][Cl] salts to HAT-(CN)₆.  Current work is in porgress to perform computational studies of HAT-(CN)₆ in the presence of these anions to correlate with the experimental data.

            In the same vein, UV/visible spectroscopic studies of the electron deficient molecule TCNE in the presence of [BF₄]^- ions indicate the appearance of new absorption bands in the visible region (Figure 5).[4] Similar studies are under investigation for 7,7,8,8-tetracyanoquinodimethane (TCNQ), 7,7,8,8-tetracyano-1,2,4,5-tetrafluoro-quinodimethane (TCNQF₄) and octacyanoquinodimethane (TCNQ(CN)₄). Companion computational studies have also been performed (Figure 6).

[1].         (a) B. L. Schottel, J. Bacsa, K. R. Dunbar, Chem. Comm., 2005, 46. (b) B. L. Schottel, H. T. Chifotides, M. Shatruk, A. Chouai, J. Bacsa, L. M. Pérez, K. R. Dunbar, J. Am. Chem. Soc., 2006, 128, 5895.

[2].         Giles, I., Dunbar, K. R. et al., manuscript in preparation.

[3]          Chifotides, H.T., Dunbar, K. R. et al., manuscript in preparation.

[4]          Funck, E. Dunbar, K. R. et al. manuscript in preparation.

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