Charge-Modulated Associates of Anionic Donors with Cationic π-Acceptors: Crystal Structures of Ternary Synthons Leading to Molecular Wires Jianjiang Lu* and Jay K. Kochi
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 291–296
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204 ReceiVed May 5, 2008; ReVised Manuscript ReceiVed October 2, 2008
ABSTRACT: Tetramethylpyrazine is effectively transformed via controlled N-protonation or N-methylation to afford a series of mono- and dicationic π-acceptors of particular use in the study of charge modulation during the self-assembly of various anionic donors and π-acid acceptors into ternary synthons and infinite linear chains (molecular wires). Thus, when the charges of the anionic donor (D) and the cationic acceptor (A) are unequal (either as 1:2 or as 2:1), the corresponding crystal structures of the donor/ acceptor associates all contain structure motifs of discrete synthons consisting of (A · · · D · · · A) or (D · · · A · · · D) triads. On the other hand, when the anionic donor and cationic acceptor are of equal charge (either as 1:1 or as 2:2), the corresponding crystal structures all contain infinite chain arrangements: ( · · · D · · · A · · · D · · · A · · · D · · · ), which are the same as those we previously found in the cocrystallization of various anionic donors with neutral π-acids. Fully appreciated anion/π interactions are found in all of these solids owing to the absence of aromatic protons in the π-acids, as well as the presence of strong electrostatic anion/cation interactions. It is noteworthy in such associates that halide anions (Cl-, Br-, and I-) show different patterns of donor/acceptor interactions. For example, in the case of the diprotonated tetramethylpyrazinium dication, the chloride donor is π-bonded over the edge of the aromatic ring, whereas bromide and iodide donors are precisely located over the aromatic ring centrosymmetrically. We hope these structural studies will provide additional stimulus for the design of new anion receptors based on the use of charged aromatic acceptors. Introduction Controlling the nucleation process especially in the early stages of crystal growth of two or more molecular components is one of the most important aspects of crystal engineering because it involves the competition among all possible molecular recognition and interaction patterns.1,2 As such, the formation of synthons [tectons or secondary building units (SBU) etc.] generates ordered thermodynamically stable structures from the chaotic precursor state and guides the propagation direction of the molecular components into infinite three-dimensional structures (crystals). As such, understanding the structural factors leading to the supramolecular synthons lies at the core of such self-assembly and crystal growth processes. Recently, considerable attention has focused on the supramolecular anion interactions with π-acids3,4 of relevance to anion binding in biological systems as well as the design new anion receptors.5,6 However, the general identification and utilization of anion/π-synthons for crystal engineering of novel functional materials are still lacking, despite the existence of numerous crystal structures with such interactions extant in the CCDC database.7,8 We recently demonstrated how charge-transfer (CT) interactions between anions and aromatic π-acids play an important role in the stabilization of ternary anion/π-complexes that are responsible for the direction of crystal growth of anions and neutral π-acids into infinite chain structures (wires).9 We suggested that such a unique structural development derived from the ternary synthons of the donor (anion) and acceptor (aromatic π-acid) shown in Scheme 1 that propagate into infinite one-dimensional molecular wires. In particular, two ternary synthons exist as either (a) 2:1 associates in which a pair of anionic donors interacts with each π-acceptor or (b) 1:2 associates in which each anion ties up two π-acceptors; the * Corresponding author.
combination of both synthons is responsible for the growth of the tecton into infinite molecular chain structures (Scheme 1). To structurally identify these ternary synthons in either a 1:2 or a 2:1 ratio, we altered the π-acid acceptor by simply changing the positive charge on the aromatic ring because such ionic complexes with an anionic donor are expected to engender an electrostatic force such that anion/π charge-transfer interactions can be easily identified.10,11 Indeed, charge-transfer complexes between N-alkylated pyridinium cations with different anions have been well-known for some 50 years;12,13 tetramethylpyrazine (TMP) was chosen because (1) one or more ring nitrogens can be either protonated or methylated (Scheme 2) to generate diprotonated tertramethyl-pyrazinium (H2TMP2+) and pentamethylpyrazinium (PMP+); (2) the analogous geometrical arrangements of tetramethylpyrazinium compared to either tetracyanopyrazine (TCP) or trinitrobenzene (TNB) as used in our previous study9 will allow both faces of the cationic aromatic ring to interact with anions; and (3) a fully methylated substituted aromatic ring eliminates the interferences from aromatic C-H bonds that commonly beset anion/π-interactions. In addition, there is also a topical issue about the structural patternsofhalide/π-acidinteractionsbecauseHayandco-workers8,14 have identified three different positions of a halide anion donor π-bonded to an aromatic acceptor, as shown in Scheme 3 based on theoretical calculations. Thus, chloride was predicted to occupy the over-the-edge mode, whereas bromide and iodide were favored to exhibit the over-the-center mode. Since no systematic experiment of this structural pattern has been carried out, it is also our interest in this study to learn more about the interaction mode of halide anions as affected by the charge on the aromatic acceptor. Results and Discussion 1. Synthesis and Spectral Observations. Diprotonated tertramethylpyrazinium (H2TMP2+) was generated in situ by mixing the neutral donor TMP with the corresponding Bro¨nsted
10.1021/cg800465e CCC: $40.75 2009 American Chemical Society Published on Web 11/26/2008
292 Crystal Growth & Design, Vol. 9, No. 1, 2009
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Scheme 1. Repeating Tecton (right) Which Is Comprised of a Pair of Synthons (left) in the One-Dimensional Co-Crystallizations of Anions and π-Acids9
Scheme 2. Molecular Representations of Cationic π-Acids (Acceptors): H2TMP2+ (middle) and PMP+ (right) Relative to the Parent TMP (left)
Scheme 3. Molecular Illustrations of the Patterns of Halide/ π-Acid Interaction as σ-Adduct (left), Over-the-Edge (middle), and Over-the-Center (right) According to Hay et al.14a
acid. Pentamethylpyrazinium (PMP+) was prepared from tertramethylpyrazine and methyl iodide, and the synthetic details are summarized in Table 3 of the Experimental Section. For the donor/acceptor complexes of the cationic aromatic acceptor H2TMP2+ with different anions, the crystals formed spontaneously after the solvent was slowly removed under vaccum from the mixture of TMP with either corresponding Bro¨nsted acid or a mixture of acid with the appropriate halometallate salt. Two categories of crystals were thus obtained: (a) H2TMP2+ with Cl-, Br-, I-, HSO4-, and ClO4- in a 1:2 ratio and (b) H2TMP2+ with CdX42- (X ) Cl, Br) in 1:1 ratio. For the cationic aromatic acceptor PMP+ with different anions, two different categories of crystals were obtained by the slow evaporation of an aqueous solution of PMP+I- in the presence of either I2 or CdI2 to yield the following adducts: (c) PMP+ with I- and I3- in 1:1 ratio and (d) PMP+ with CdI42in 2:1 ratio. In roughly half the cases examined in this study, the charge-transfer nature of the anion/cationic π-interaction was visually indicated by the crystal color as well as the color change observed in solution.9 Clear charge-transfer bands were identified in PMP+I- (λCT ) 362 nm) and (PMP+)2CdI42- (λCT ) 368 nm). However, UV-vis measurements of the H2TMP2+Br-2 or H2TMP2+I-2 complex in solution revealed no clearly resolved absorption band despite the observation of color changes in solution indicative of charge-tranfer interactions.9 For example, a colorless solution of H2TMP2+ was prepared by the addition of an acetonitrile solution of TMP to either 1 equiv of H2SiF6 or 2 equiv of hydrochloric acid. The bromide/CT complex was generated in situ by addition of aqueous hydrogen bromide (1-10 equiv), and it was observed as a bright yellow solution. However, the charge-transfer band was partially obscured by the strong local absorption band of H2TMP2+ cation at λmax ≈ 300 nm.
2. Supramolecular Self-Assemblies of Anions and Cationic π-Acids into Ternary Synthons and Infinite (One-Dimensional) Wires. When predetermined charge ratios of an aromatic cationic acceptor (A) are mixed with an anionic donor (D) at 1:2 and 2:1, they yield either the (DAD) or the (ADA) ternary synthon depicted in Figure 1. Such an observation prevails in all the crystal structures of anions examined in this study. The absence of one-dimensional chain structures in these crystals indicates that these ternary synthons are quite stable in the solution and that the anion/π-interactions are fully appreciated in these particular systems owing to the elimination of aromatic hydrogen bonds, as well as the presence of strong electrostatic anion/cation interactions. However, when the charge ratio is either 1:1 or 2:2, the infinite 1D chain structures shown in Figure 2 are obtained. This additional observation suggests that the positively charged aromatic π-acid has the same ability as the parent (neutral) π-acids such as tetracyanopyrazine or trinitrobenzene to induce the infinite chain structure despite the absence of the acceptor counterion.9 As such, we conclude that any strong electrostatic interaction between the donor and the acceptor does not, by itself, markedly affect the charge-transfer interaction pattern for anion/π-acid recognition. 3. X-ray Analyses of π-Bindings of Various Anions to the Dicationic π-Receptor: H2TMP2+. Crystals of H2TMP2+ with different anionic donors were obtained and characterized by X-ray single crystal analysis.15 In general, the diprotonated pyrazine ring experiences an average increase of the aromatic C · · · N bonds by ∼0.20 Å, and the aromatic C · · · C bonds increase by ∼0.10 Å relative to its neutral parent. As such, strong anion/π interactions as well as anion/N-H hydrogen bonds exist in these solids, as follows: A. Halide Donors. Structures of the halide/cationic π-acid interactions in H2TMP2+Cl-2 (1) and H2TMP2+Br-2 (2) are shown in Figure 3. The corresponding iodide analogue H2TMP2+I-2 (3) is isostuctural with 2 as described in Figure S1, Supporting Information. In all three structures, the H2TMP2+ aromatic ring is located between two centrosymmetrical halide ions consistent with the (DAD) ternary structure in Figure 1 (upper row). Chloride is located in an over-theedge pattern, whereas bromide and iodide both exist in the overthe-center pattern. Chloride, bromide, and iodide anions have close contacts with the carbon center with the shortest X · · · C distances of 3.23, 3.57, and 3.65 Å, respectively. Chloride also interacts with two nearby water molecules via H-bonding and with Cl · · · O distances of 3.01 Å. In compounds 2 and 3, bromide and iodide anions are H-bonded to the protonated N atoms with the distances of X · · · N at 3.33 and 3.43 Å, respectively. B. Bisulphate and Perchlorate Donors. In H2TMP2+ (HSO4-)2 (4) and in H2TMP2+(ClO4-)2 (5), the π-binding of HSO4- occurs via a pair of O centers with the atom-to-atom contacts at 2.91 and 3.02 Å, and the π-binding of ClO4- also similarly occurs via a pair of O centers with the atom-to-atom
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Crystal Growth & Design, Vol. 9, No. 1, 2009 293
Figure 1. Space-filling examples of anion/cationic π-ternary subunits (synthons): Upper, π-acid (A) ) H2TMP2+ with anionic donors (D-) ) Cl-, Br-, I-, HSO4-, and ClO4-; lower, π-acid (A) ) PMP+ and anionic donor (D-) ) CdI42-.
Figure 2. One-dimensional chain (ORTEP) structures obtained from the co-crystallization of anionic donor (salt) and cationic π-acid acceptor (salt) in equimolar amounts, as indicated. (See Han et al.9 for the identification of R and φ.)
Figure 3. π-Bonding of chloride (left) and hydrogen- and π-bonding of bromide (right) to H2TMP2+.
contacts at 2.96 and 3.06 Å, shown in Figure 4 (left and right), respectively. Consideration of the closest nonbonded O-C centers in Figure 4 (right) indicates that the perchlorate anion π-binding is somewhat weaker than that of bisulfate. Moreover, in (4), strong hydrogen bonds are found to form a bisulfate chain with the O · · · O distances at 2.58 Å and between anion/N-H
at 2.67 Å. The perchlorate/N-H hydrogen bond in (5) is found with the O · · · N distance at 2.71 Å. C. Tetrachlorodicadmiate and Tetrabromodicadmiate Donors. H2TMP2+CdCl42- (6) and H2TMP2+CdBr42- (7) are crystalline isostructures. A pair of tetrahedral dianions are also located above and below the aromatic plane of the H2TMP2+ π-acceptor, but they merely relate to each other via a twofold rotation symmetry (C2) rather than a center symmetry (i). For example, the anions are π-bonded to the periphery of H2TMP2+ by three Cl centers with atom-to-atom contacts ranging from 3.15 to 3.24 Å (Figure 5). Hydrogen bonds are also found between Cl and N(H) atoms with Cl · · · N contact at 3.10 and 3.13 Å. Similarly, CdBr42- is also π-bonded to H2TMP2+ with atom-to-atom contacts ranging from 3.27 to 3.36 Å (Figure S2, Supporting Information) and H-bonded to H2TMP2+ with Br · · · N contacts at 3.27 and 3.28 Å. It is noteworthy here that the reported analogous16 crystal structure of H2TMP2+CdI42- with centrosymmetrical space group (P21/c),16a is different from the chiral crystals (6) and (7) (space group P212121). A careful examination of these three
294 Crystal Growth & Design, Vol. 9, No. 1, 2009
Figure 4. Bisulphate (left) and perchlorate (right) π-bonding to H2TMP2+ to H2TMP2+.
Lu and Kochi
Figure 6. π-Bonding of I- (upper left), I3- (upper right), and CdI42-(lower) to PMP+. Table 1. Contraction of the Van der Waals Separation upon the Formation of Various Anion/Cationic π-Acid Associates anion/π-acid -
Figure 5. Hydrogen and π-bonding of CdCl42- to H2TMP2+.
crystal structures revealed that all structures contain similar arrangements of noncentro-symmetrical anion/π bonded chains (Figures 2a,b and S2, Supporting Information) and layers that are constructed of parallel anion/π chains stacked together through H-bonds (Figure S3b, Supporting Information). However, the layer packing styles are quite different. As shown in Figure S3c (Supporting Information), in H2TMP2+CdI42-, adjacent layers (parallel to the ab plane) are related by inversion symmetry through the point (1/2, 1/2, 1/2), whereas in 6 and 7, adjacent layers (parallel to the ac plane) are related by two additional 21 screw operations which prevent the inversion symmetry. It is quite interesting to see that such a dramatic change of crystal packing results simply by replacing the halide anion from chloride or bromide to iodide. Moreover, our study shows that isostructural crystals of 6 or 7 can be obtained by replacing the metal ion from Cd2+ to either Co2+, Zn2+, or Hg2+. These crystals may have interesting second harmonic generation (SHG) possibilities.17 4. X-ray Structural Analyses of π-Bindings of Various Anions to the Cationic π Receptor: PMP+. Three crystal structures were obtained from the interaction of PMP+ monocation with three iodide-derived anions. From comparisons of the crystal data, the dimension of the charged acceptor ring relative to the parent TMP donor shows progressively increasing CAr · · · NAr bond distances from the methylated nitrogen atom by ∼0.3 Å and the CAr · · · CAr bond by ∼0.15 Å, but virtually no change of the CAr · · · NAr bond distance to the uncharged nitrogen atom. No hydrogen bonding is evident in these structures, consistent with the absence of a strong hydrogenbonding donor. Iodide, Triidoide, and Tetraiododicadmiate Donors. In PMP+I- (8), PMP+I3- (9), and (PMP+)2CdI42- (10), each of the anionic donors I-, I3-, and CdI42- is π-bonded to the
2+
Cl /H2TMP Br-/H2TMP2+ I-/H2TMP2+ Cl-/TCP Br-/TCP I-/TCP HSO4-/H2TMP2+ ClO4-/H2TMP2+ CdCl42-/H2TMP2+ CdBr42-/H2TMP2+ CdI42-/H2TMP2+ Cd2Cl62-/TCP CdBr42-/TCP CdI42-/TCP I-/PMP+ I3-/PMP+ CdI42-/PMP+
atom sitea Cl Br I Cl Br I O O Cl Br I Cl Br I I I I
Rexp (Å)a
Rvdw (Å)
CR (%)b
3.23 3.51 3.65 3.07c 3.15c 3.49c 2.91 2.96 3.15 3.27 3.47e 3.41 3.41d 3.57d 3.58 3.68 3.57
3.50 3.55 3.80 3.50 3.55 3.80 3.22 3.22 3.50 3.55 3.80 3.50 3.55 3.80 3.80 3.80 3.80
8 1 4 12 11 8 10 8 10 8 9 3 4 6 6 3 6
a From the anion donor to the closest carbon site on the π-acid. b CR ) (1 - Rexp/Rvdw) × 100%. c Reference 4f d Reference 9. e Reference 16a.
periphery of PMP+by iodine centers with atom-to-atom contacts at 3.58, 3.68, and 3.57 Å, respectively (Figure 6). In compounds 8 and 9, infinite anion/cation chains joined by anion/πinteractions are found in the crystal structures shown in Figure 2c,d. In compound 10, the isolated (ADA) ternary structure is identified as shown in Figure 6 (lower). The two acceptors are related to each other by twofold rotation symmetry (C2 axis through Cd and midpoint of two π-bonded iodine atoms of the anion). 5. Van der Waals Contractions Diagnostic of Anion/ π-Interactions.9,18,19 Crystallographic data of the various anion/ cationic π-interactions are summarized in Table 1 from the semiquantitative evaluation of the normalized contraction previously designated as CR by Han et al.9 These analyses reveal a considerable span of van der Waals contractions, with the value of CR ) 10% representing one limit of strong π-binding relative to CR < 1% for the weakest interaction of the various cocrystals examined in this study. Included are the comparisons of H2TMP2+ and PMP+ relative to TCP in binding strengths that are relevant to the evaluation of the charge-transfer effect on anion/cationic π-acid interactions. We found that, in general, H2TMP2+ and PMP+ are weaker acceptors than TCP for binding the monoanionic halogen donors. However, H2TMP2+ and PMP+ show increasing binding strength for binding the dianionic tetrahalometallate donors relative to monoanionic halide. By contrast, TCP shows the reverse trend. Indeed,
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Crystal Growth & Design, Vol. 9, No. 1, 2009 295
the-edge to over-the-center mode, as predicted theoretically by Hay and co-workers.14a Conclusions
Figure 7. Degree of displacement of an interaction atom of anion according to Hay et al.,14a from the center of an arene is given by doffset. This parameter is calculated from distances (dcentroid and dplane). Table 2. Comparison of Cl-, Br-, and I- Interaction Modes to H2TMP2+ halides X
X · · · C (N) distance (Å)
dcentroid (Å)
dplane (Å)
doffset (Å)
R (deg)
3.23-3.83 3.51-3.65 3.65-3.75
3.26 3.33 3.45
3.17 3.32 3.45
0.79 0.22 0.12
76.0 86.9 88.1
-
Cl BrI-
Table 3. Crystals from Anion/Cationic π-Acid Ionic Associations no.
π-acid cations
anions
1
H2TMP2+ Cl-
2 3 4
H2TMP2+ BrH2TMP2+ IH2TMP2+ HSO4-
5 6 7 8 9 10
H2TMP2+ H2TMP2+ H2TMP2+ PMP+ PMP+ PMP+
ClO4CdCl42CdBr42II3CdI42-
color
co-crystal
H2TMP2+ (Cl-)2(H2O)2 yellow H2TMP2+ (Br-)2 yellow brown H2TMP2+ (I-)2 pale yellow H2TMP2+ (HSO4-)2 colorless H2TMP2+ (ClO4-)2 colorless H2TMP2+ CdCl42yellow H2TMP2+ CdBr42yellow PMP+ Ired PMP+ I3yellow (PMP+)2CdI42colorless
color colorless yellow red pale yellow colorless colorless yellow yellow red yellow
H2TMP2+ is a stronger acceptor than TCP for binding the dianionic tetrahalometallates donors. As such, we identify the stronger electrostatic interactions between dianion donors and cationic acceptors as the principle basis for such a distinctive trend. 6. Halide/π Interaction Modes. The π-bonding of halides over the aromatic acceptor can be semiquantitatively evaluated by four parameters: (a) the distance dcenteriod between halide and the center of the ring, (b) the distance dplane from the halide to the ring plane, (c) the doffset distance (dcentroid2 - dplane2)1/2, and (d) the angle R shown in Figure 7. Typically, some of such interaction details of chloride, bromide, and iodide with the H2TMP2+ π-acid acceptor are summarized in Table 2. Thus, the magnitude of dcenteriod for chloride, bromide, and iodide to the center of the aromatic ring are 3.26, 3.33, and 3.45 Å and the corresponding R angles are 76.0, 86.9, and 88.1°, respectively; these data clearly establish the unambiguous strength of halide/cationic π-interactions. The results also show that with H2TMP2+, the Cl-/π-interaction pattern lies in the over-the-edge mode, whereas Br-/π interaction and I-/π interaction patterns are in the over-the-center mode. In particular, for the case of I-/H2TMP2+, the atom-to-atom distances between iodide and four aromatic carbon atoms are all within the sum of the van der Waals distance. The magnitude of doffset for the iodide position is 0.12 Å, which represents one of the closest anion/π-interaction in over-the-center mode according to the listings in CCDC database and literature survey.8,14a As such, this study clearly demonstrates for the first time the progressive shift of the halide/π-interaction mode from chloride to bromide and iodide to coincide with the transition from over-
Tetramethylpyrazine is transformed into mono- and dicationic π-acceptors via controlled N-protonation or N-methylation; the resultant cationic acceptors are employed in the solid-state synthesis of ternary synthons and infinite chain structures (molecular wires) by predetermined charge-ratio modulations with various anionic donors. When the donor/acceptor charge ratio is either 2:1 or 1:2, ternary synthons consisting of (DAD) or (ADA) triads are produced, whereas when the donor/acceptor charge ratio is either 1:1 or 2:2, infinite chains consisting of ( · · · D · · · A · · · D · · · A · · · D · · · ) are formed. Halide anions show different interaction patterns with the diprotonated tetramethylpyrazinium (H2TMP2+) in which the anionic chloride donor exists in over-the-edge mode whereas bromide and iodide anions exhibit the over-the-center mode. We hope that this strategy can be used for the design of novel crystalline materials by using anion/π synthons, and it may help in the design of new anion receptors based on charged aromatic molecules. Experimental Section Chemicals and General Methods. Tetramethylpyrazine, methyl iodide, inorganic acids, iodine, and cadmium dihalide were obtained from commercial sources and used as received. Solvents were purified according to standard operation procedure. Anion/π- complexes between H2TMP2+ and different anions were synthesized by dissolving 1 equiv of TMP with either 2 equiv of the corresponding Bro¨nsted acid or combining 2 equiv of the Bro¨nsted acid with 1 equiv of cadmium dihalide in aqueous (for I-, CdCl42-, and CdBr42-) or ethanol solution (for Cl-, Br-, HSO4-, and ClO4-). Crystals were isolated after the solvent was allowed to slowly evaporate under vacuum. Pentamethylpyrazinium iodide (PMP+I-) was synthesized by refluxing TMP in neat methyl iodide as the solvent. Yellow crystals were isolated by filtration and recrystallized from acetonitrile. Evaporation of aqueous solution of 2 equiv PMP+I- with either 2 equiv I2 or 1 equiv CdI2 led to crystals suitable for X-ray diffraction. X-ray Crystallography. The intensity data were collected on a Siemens/Bruker SMART APEX equipped with 1K CCD detector using Mo KR radiation source (λ ) 0.71073 Å) in a cold nitrogen stream at 173(2) K. The frames were integrated with the Bruker SAINT software package,20 and a semiempirical absorption correction using multiplemeasured reflections was applied using the program SADABS.21 The structures were solved by direct method22 and refined by full matrix least-squares procedure with IBM Pentium and SGI O2 computers. The crystal parameters and information pertaining to the data collection and refinement of the crystals for 1-10 are summarized in Table 3. Selected bond distances and angles are provided in the corresponding figures of each structure.
Acknowledgment. We thank the R.A. Welch Foundation and National Science Foundation for financial support. Supporting Information Available: Hydrogen and π-bonding of I- to H2TMP2+ (Figure S1); hydrogen and π-bonding of CdBr42and CdI42- to H2TMP2+ (Figure S2); comparison of crystal structures between H2TMP2+CdCl42- and H2TMP2+CdI42- (Figure S3); andcrystallographic parameters and details of the structure refinements (Table S1). Crystallographic information in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
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Lu and Kochi (11) (a) See also Berryman, O. B.; Hof, F.; Hynes, M. J.; Johnson, D. W. Chem. Commun. 2006, 506. (b) Mascal, M.; Yakovlev, I.; Nikitin, E. B.; Fettinger, J. C. Angew. Chem., Int. Ed. 2007, 46, 8782. (12) For ionic CT complexes of N-alkylated pyridium with iodide see: (a) Kosower, E. M. J. Am. Chem. Soc. 1958, 80, 3253. (b) Kosower, W. M.; Skorcz, J. A. J. Am. Chem. Soc. 1960, 82, 2195. (13) For other ionic CT complexes see also:(a) Bockman, T. M.; Kochi, J. K J. Am. Chem. Soc. 1989, 111, 4669. (b) Magueres, P. L.; Hubig, S. M.; Lindeman, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2000, 122, 10073. (14) (a) Berryman, O. B.; Bryantsev, V. S.; Stay, D. P.; Johnson, D. W.; Hay, B. P. J. Am. Chem. Soc. 2007, 129, 48. (b) Albrecht, M.; Wessel, C.; de Groot, M.; Rissanen, K.; Luchow, A. J. Am. Chem. Soc. 2008, 130, 4600. (15) For comparision, no di-protonated pyrazinium salts are found in the CCDC database to reflect their instability in the solid state. For monoprotonatedpyraziniumsaltsandtheirproperties,seeforexample:Katrusiak, A.; Szafranski, M. J. Am. Chem. Soc. 2006, 128, 15775. (16) For all prior structures of H2TMP2+ forming anion complexes with motifs analogous to those reported here, see: (a) Baily, R. D.; Pennington, W. T. Acta Crystallogr., Sect. C 1995, 51, 226. (b) Modec, B.; Brencic, J. V.; Zubieta, J. J. Chem. Soc., Dalton Trans. 2002, 1500. (c) Hu, S. Z. Jiegou Huaxue (Chin. J. Struct. Chem.) 2000, 19, 234. (17) Lindeman, S. V.; Hecht, J.; Kochi, J. K J. Am. Chem. Soc. 2003, 125, 11597. (18) The X · · · C distance is taken as the closest atom-to-atom contact. Note that the van der Waals radii are (in Å) 1.70 (C), 1.52 (O), 1.80 (Cl), 1.85 (Br), and 2.1 (I). (19) Lommerse, J. P. M.; Stone, A. J.; Raylor, R; Allen, F. H. J. Am. Chem. Soc. 1996, 118, 3108. (20) SAINT, Program for Area Detector Absorption Correction; Siemens Analytical X-Ray Instruments Inc.: Madison, WI, 1994-1996. (21) Sheldrick, G. M. SADABS, Program for Siemens Area Detector Absorption Correction; University of Gottingen: Gottingen, Germany, 1996. (22) Sheldrick, G. M. SHELXS-97, A Program for Crystal Structure Solution; University of Gottingen: Gottingen, Germany, 1997.
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