DOI: 10.1021/cg100444n
Design Considerations for the Group 15 Elements: The Pnictogen 3 3 3 π Interaction As a Complementary Component in Supramolecular Assembly Design
2010, Vol. 10 3531–3536
Virginia M. Cangelosi, Melanie A. Pitt, W. Jake Vickaryous, Corinne A. Allen, Lev N. Zakharov, and Darren W. Johnson* Department of Chemistry, Materials Science Institute, and the Oregon Nanoscience and Microtechnologies Institute (ONAMI ), University of Oregon, Eugene, Oregon 97403-1253 Received April 2, 2010; Revised Manuscript Received June 9, 2010
ABSTRACT: An exhaustive structural survey was analyzed in combination with new structural reports on pnictogen 3 3 3 π interactions. The Cambridge Structural Database was searched for examples of pnictogen atoms (As, Sb, and Bi) that lie within the sum of their van der Waals radii with an arene ring. The results were analyzed, and it was concluded that the interaction distance is shorter for the heavier pnictogens due to the increasingly diffuse nature of the lone pair of electrons on the atom. A weak correlation between the distance and the angle of the interaction was found, implying that the interaction is partially covalent in nature but is primarily a dispersion interaction. These arene 3 3 3 π interactions can be used as driving forces for supramolecular assembly, and the crystal structures of two new arsenic-containing assemblies are presented. The combined analysis of these structures and the structural survey suggest that our supramolecular design strategy can be fine-tuned to take advantage of the pnictogen 3 3 3 π interaction. Introduction The great success of supramolecular chemistry in the preparation of rationally designed supermolecules from simple components1-5 has led to a concomitant surge of interest in the study of the main group elements.6 Main group metals occupy an interesting position in the periodic table: the nature of their bonding is generally coordinative, while their geometric preferences often more closely resemble the nonmetals.6 As the main group metals see increasing use in supramolecular chemistry, their coordination preferences have been incorporated into predictive design strategies that have led to the formation of rather spectacular self-assembled structures.6 The development of these design strategies has, however, been hampered by the unpredictable coordination preferences of these elements.7 Secondary bonding interactions also introduce new challenges to rational design strategies. Weak interactions with arene rings,8 secondary coordination to Lewis basic elements,9,10 and steric strain all play a role in determining the overall structure of supramolecular assemblies; the main group elements and pnictogens in particular appear to have a unique susceptibility to unusual coordination behavior. Interactions between arene rings and pnictogen (Group 15) metal centers are of particular interest in the development of supramolecular design strategies with main group elements. While this type of interaction has been known for quite some time,11-18 only recently has it been used as a specific design element in supramolecular assemblies.19-23 A better understanding of the nature of the pnictogen 3 3 3 π interaction is necessary for improved supramolecular design and an important consideration in Group 15 coordination chemistry. For instance, the pnictogen 3 3 3 π interaction is an important complementary force for specific Group 15 chelation. Here, we present the results of an exhaustive Cambridge Structural Database (CSD) search for close pnictogen 3 3 3 π interactions.
Additionally, we present two new crystal structures that feature As 3 3 3 π interactions, one of which is supramolecular in nature, and discuss ways to fine-tune Group 15 ligand design to take advantage of these interactions. Experimental Section
*To whom correspondence should be addressed. E-mail: dwj@uoregon. edu.
General Procedures. 1H NMR spectra were measured using a Varian INOVA-300 spectrometer, referenced to tetramethylsilane (0 ppm), and reported in ppm. Single crystal X-ray diffraction studies were performed on a Bruker SMART APEX diffractometer. Commercially available reagents were used as received. Caution: Arsenic compounds are highly toxic and should be handled with care! (This accounts for the small scale of the reactions reported herein.) H(L4)24 and H2(L6)25 were prepared according to literature procedures. As(L4)3. 2-Mercaptomethylnaphthalene (H(L4)) (0.170 g, 0.977 mmol) was dissolved in THF (30 mL). AsCl3 (27.8 μL, 0.326 mmol) was added and the solution was stirred at 25 °C for 1 h 45 min. Saturated aqueous Na2CO3 (10 mL) was added and stirred vigorously for 15 min. The biphasic solution was extracted with CH2Cl2 (3 15 mL), and the combined organics were dried with brine and concentrated in vacuo to yield a white solid (0.180 g, 0.303 mmol, 93%). X-ray quality crystals were grown by dissolving the product in CHCl3 (10 mL) and layering with hexanes.26 Colorless needles appeared in six days. 1H NMR (300 MHz, CDCl3, TMS): δ 7.79 (m, 1H, CH ), 7.69 (d, 1H, CH, J = 8.7 Hz), 7.65 (m, 1H, CH), 7.55 (s, 1H, CH), 7.46 (m, 2H, CH), 7.34 (dd, 1H, CH, J = 8.0, 1.8 Hz), 4.04 (s, 2H, CH2). As2(L6)2Cl2. 1,4-Bis(mercaptoethyl)benzene (H2(L6)) (0.116 g, 0.584 mmol) was dissolved in 10 mL of CHCl3 in a scintillation vial. AsCl3 (49.8 μL, 0.584 mmol) was added and the mixture was sonicated. Layering of hexanes over the reaction mixture yielded X-ray quality crystals of the desired macrocycle.27 Because of the low yielding crystallization, no crystalline yield is reported. However, the reaction went to completion (>99% yield) by NMR spectroscopy. 1H NMR (300 MHz, CDCl3, TMS): δ 7.19 (s, 4H, CH), 3.86 (t, 4H, CH2, J = 6.5 Hz), 2.86 (t, 4H, CH2, J = 6.5 Hz). X-ray Crystallography. X-ray diffraction intensities for As(L4)3 and As2(L6)2Cl2 were collected on a Bruker SMART Apex CCD diffractometer at 173 K using Mo KR radiation (λ = 0.71073 A˚).28 The crystallographic data and details of the data collections and refinements of the structures are given in the CIF files (Supporting Information). Absorption corrections in all cases were applied by SADABS.29 Structures were solved using direct methods, completed by subsequent
r 2010 American Chemical Society
Published on Web 06/25/2010
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Figure 2. Representations of the X-ray crystal structures of supramolecular assemblies featuring As 3 3 3 π interactions: As2(L1)3 (a), As2(L2)3 (b), and As2(L3)2Cl2 macrocycles dimerized around solvent guest molecules in the solid state (c).
Chart 1. Thiol and Dithiol Ligands Used to Prepare ArsenicContaining Supramolecular Assemblies Figure 1. Examples of structures from the CSD with particularly short pnictogen 3 3 3 π distances: hexaethylbenzene 3 3 3 AsCl3 adduct with perpendicular orientation (a, b),31 cyclophane 3 3 3 SbCl3 adduct with tilted geometry (c),32 and hexamethylbenzene 3 3 3 Bi cluster adduct with perpendicular geometry (d).11. difference Fourier syntheses, and refined by full matrix least-squares procedures on F2. In both structures non-H atoms were refined with anisotropic thermal parameters, and H atoms were found from the residual densities and refined with isotropic thermal parameters. All calculations were performed using the Bruker SHELXTL package.30 CSD Search. Searches were performed using ConQuest version 1.10 with CSD database version 5.29 updates (Jan 2008). The searches screened for C6-arene rings and the pnictogens with intermolecular contacts that were shorter than the sum of the van der Waals radii (1.7 A˚ for aryl C, 1.85 A˚ for As, 2.0 A˚ for Sb, and 2.0 A˚ for Bi). The pnictogen atoms were limited to having three bonds, and the structures were excluded if the pnictogens were bonded to another pnictogen or metal, charged, or part of a cluster. The cone correction was applied using Vista version 2.1d. Centroids were determined using Mercury version 2.2 and the hapticity of the interaction was defined as the number of contacts between the pnictogen and carbons in the arene ring that are less than the sum of the van der Waals radii.
Results and Discussion Pnictogen 3 3 3 π interactions were first discovered through the observation that combinations of benzene or naphthalene with antimony trichloride produced highly crystalline solids. The unusually high solubility of pnictogen trihalides in neutral organic solvents has also been attributed to noncovalent interactions between the metal and the solvent.13 This interaction appears to be quite strong, but its geometry is also variable. Examples of the closest pnictogen 3 3 3 arene interactions for As(III), Sb(III), and Bi(III) found in a survey of the Cambridge Structural Database are shown in Figure 1. Of note is the fact that for As(III) and Bi(III), the closest interactions with arene rings are found when the metal is oriented perpendicular to the face of the ring (Figure 1a,b,d), while for Sb(III), the closest interaction places the metal in a tilted orientation with respect to the arene (Figure 1c). Here, a metal’s orientation is defined as perpendicular if its lone pair of electrons (or the C3 axis for EX3) is perpendicular to the arene ring. A number of computational studies on these so-called “Menshutkin complexes” describe the interaction as a ligandto-metal charge transfer interaction and suggest that donor orbitals on the ligand interact with p acceptor orbitals on the metal center.18,33,34 While these calculations do provide insight into the fact that there is likely some degree of charge transfer character to the interaction, there is still disagreement regarding the bonding strength, and the calculations are unsatisfactory to completely explain the bonding observed between pnictogens and neutral arene rings. Specifically, antimony complexes bear
off-center interactions much more frequently than do arsenic or bismuth, but these calculations provide no explanation for this phenomenon. A new computational study by Mehring et al. describes the Bi 3 3 3 π interaction as mainly a dispersion interaction with some π-to-σ* donor-acceptor character,35 but does not address the difference in strength of the interaction for the other pnictogens. We have recently reported the development of a supramolecular design strategy for the preparation of helicates, mesocates, and macrocycles from difunctional benzylic dithiolates bridged by rigid and semirigid aromatic spacers coordinated to trigonal pyramidal pnictogen centers.19,20,22,23,36,37 The primary thiolate coordination sphere is supplemented by intramolecular secondary bonding interactions between the pnictogen and the areneligand scaffold. This stands in contrast to previous examples of pnictogen 3 3 3 π interactions, which tend to be intermolecular adducts. Several examples of previously reported supramolecular As2L3 and As2L2Cl2 structures prepared through this strategy are shown in Figure 2. The assembly shown in Figure 2a is the first example of pnictogen 3 3 3 arene interactions so intimately involved in the overall structure of a supramolecular coordination complex.22 This design strategy was shown to be general for ligands bearing benzylic thiolate donor groups, and one example of an extended As2L3 cryptand structure is shown in Figure 2b.20 Several examples of As2L2Cl2 macrocycles based on this design strategy have been reported as well,19,23,36,38 and one unusual example is capable of interacting further with aromatic solvent molecules in the solid state to form inclusion complexes (Figure 2c).19 The nature of the pnictogen 3 3 3 π interaction clearly has some bearing on its utility in supramolecular assembly design. The possibility of covalent character (if the interaction were caused, for example, by interactions between π-electrons and
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Table 1. Mean Interaction Distances and Angles for Pnictogen 3 3 3 π Interactions pnictogen mean deviation deviation mean θ deviation deviation type d (A˚) (A˚) (%) (deg) (deg) (%) As - CSD Sb - CSD Bi - CSD As2L3 As2L2Cl2
3.45 3.34 3.23 3.27 3.28
0.11 0.18 0.23 0.06 0.07
3.19 5.48 7.26 1.91 2.19
161.7 154.5 151.6 151.5 148.7
14.22 13.55 11.75 11.21 10.81
8.79 8.77 7.75 7.40 7.27
Figure 3. A charge transfer model for the pnictogen 3 3 3 π interaction in supramolecular assemblies. Locations of σ* orbitals in the primary coordination sphere about an As(III) center where the generic ligand L replaces the sulfur ligand (a). View down the As-As axis of As2(L1)3 showing the possibility of interactions (red arrows) between the π systems of the arene spacers with the σ* orbitals of As (b).
Figure 5. Mean pnictogen 3 3 3 π distances observed in the CSD. See Table 1 for data values.
Figure 4. Schematic for measurement of pnictogen 3 3 3 π interaction distances and angles. The interaction distance d is the distance between the carbon (or centroid for higher hapticity interactions) of observed close contacts and the pnictogen center, while the interaction angle θ refers to the angle between the carbon, the pnictogen center, and the ligand (L2) opposite the interaction. The As 3 3 3 π interaction is depicted here as an η1 contact.
the As-S σ* orbital) would imply that very specific structural arrangements are required for this interaction to be used as a design element. Conversely, a primarily dispersion interaction would allow considerably more latitude in how the interaction could be incorporated into assembly design. A closer examination of the As2(L1)3 assembly provides a visual depiction of how the pnictogen 3 3 3 π interaction might be caused by overlap between the As-S σ* orbital and the arene’s π-electrons to create an interaction with some covalent character.39 Figure 3a shows the primary ligand coordination sphere about the As(III) center in more detail. In particular, the σ* orbitals associated with the As-S bonds are drawn in their approximate locations, 180° from the As-S bonds, though not to scale. Figure 3b is a view down the As-As axis of the supramolecular assembly, showing how the π systems of the arene ring of the ligand spacer tilts in what appears to be an interasubstituent on Bi is with these σ* orbitals, stabilizing the overall assembly. CSD Structural Survey. To better understand these geometric preferences, a systematic survey of the CSD was undertaken. Specifically, we wished to establish if pnictogen size contributes to the distance and angle of interaction with arene rings. Structures included in this survey are those containing neutral As(III), Sb(III), or Bi(III) complexes in close proximity (i.e., less than the sum of their van der Waals radii) to C6aromatic rings. As shown in Figure 4, the distance for the interaction d and the interaction angle θ were measured, tabulated, and compared with the interactions observed in
supramolecular pnictogen 3 3 3 π assemblies. Some interactions occur between the pnictogen center and more than one carbon in the aromatic ring. In the case of these higher hapticity (η2-η6) interactions, the distance was measured to the centroid of the interaction. However, for each interaction, the angle was treated as an η1 interaction and measured separately to each carbon for ease of comparison between structures. Examination of the distances (as measured to the centroid) found in the CSD search broken down by pnictogen type shows two important facts about the pnictogen 3 3 3 π interaction (Table 1, Figure 5). The first is that the average distance decreases with increasing pnictogen weight. As 3 3 3 π interactions average 3.45 A˚, while interactions with Sb and Bi are 3.34 and 3.23 A˚, respectively. The second notable feature is that as the pnictogen weight increases the distribution of the interaction distance increases considerably. The standard deviation for As 3 3 3 π interactions is 0.11 A˚, or 3.2%, which increases to 7.3% for bismuth. This data, while not particularly conclusive, does suggest that increasing pnictogen size results in a wider range of possible interactions. The angular preferences associated with this interaction (measured to the nearest carbon rather than to the centroid)40 provide a slightly different picture of the pnictogen 3 3 3 π interaction (Table 1). The mean angle associated with the pnictogen 3 3 3 π interaction (Figure 6) decreases for the heavier pnictogens, from 162° for As to 152° for Bi. The standard deviation decreases slightly, but remains around 8-9% for each pnictogen. These results imply that the As 3 3 3 π interaction is more covalent than the Sb 3 3 3 π or Bi 3 3 3 π interaction. Conversely, a more bent angle is more frequently observed in the arsenic-containing supramolecular assemblies reported by our laboratory. This is most likely due to the geometric constraints applied by the relatively short mercaptomethylene tether.
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Figure 6. Mean angles for pnictogen 3 3 3 π interactions observed in the CSD. See Table 1 for data values.
Figure 7. Frequency distribution of bond angles for arene 3 3 3 pnictogen-ligand bond angles after cone correction.
For linear interactions, the distribution of angles is influenced by the increasing possible points of interaction as the angle decreases from 180°. A cone correction41,42 was applied to the data from the CSD search and the weighted data support this preference for this angular distribution (Figure 7). The corrected data shows a preference for more linear angles of 170-180° for As and Sb. For Bi, the correction does not imply a clear preference toward linear. Again, this suggests that the interaction may be more covalent in character for As and Sb than for Bi. While there does appear to be some preference for the angle to approach a linear orientation, comparison between distance/ angle pairs is necessary to determine whether there is a relationship between the length of the interaction and its approach to linearity. If the orbital overlap model shown in Figure 3 is correct, there should be a strong correlation between the pnictogen 3 3 3 π interaction distance d and the interaction angle: as d decreases, the angle should approach 180°. The challenge in determining a correlation is that angles are better measured to the carbon, and distances are better measured to the centroid. A plot of the distance(centroid)/angle(centroid) pairs (Figure S5e-h, Supporting Information) reveals an essentially random distribution of distances and angles, while a plot of the
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Figure 8. Stick representations of the X-ray crystal structures of As(L4)3 (a) and As2(L5)2Cl2 (b).
distance(carbon)/angle(carbon) pairs reveals only a slight correlation (Figure S5a-d, Supporting Information). The negligible correlation between d and θ suggests that the pnictogen 3 3 3 π interaction is primarily a dispersion interaction, corroborating the studies35 that indicated that a charge transfer model is insufficient to describe this type of interaction. This is also consistent with our observations of DFT underestimating the As 3 3 3 π interaction distance.20 The slight decrease in d as the pnictogen’s size is increased is likely due to decreased repulsions between the π system of the arene ring and the lone pair of electrons on the pnictogen. For bismuth, relativistic effects may also be involved.43 This is possibly the result of increased delocalization of these electrons.18 The fact that this interaction is primarily due to dispersion interactions, especially for bismuth, is actually quite useful for the design of supramolecular assemblies, as the interaction’s geometric preferences are less likely to dominate the final structure as much as the primary coordination sphere and the structure of the chosen ligands. Influence on Self-Assembly. A closer look at the similarities and differences between the “adduct” pnictogen 3 3 3 π structures and supramolecular pnictogen 3 3 3 π structures provides a great deal of insight into how ligand properties may be adjusted to achieve desired structural outcomes. In systems less constrained than rigid As2L3 assemblies, the overall structure of pnictogenbased assemblies depends heavily on the nature of competing arene 3 3 3 arene interactions, particularly edge-to-face CH 3 3 3 π interactions. To test this trend, we synthesized a new mononuclear AsL3 complex. Compare the two structures shown in Figure 8. The first (Figure 8a), a mononuclear As(L4)3 complex was prepared by reacting H(L4) (2-mercaptomethylnaphthalene), AsCl3, and Na2CO3. X-ray diffraction of crystals of the complex26 reveals that it contains only one As 3 3 3 π interaction of moderate length (3.53 A˚). The naphthalene portions of all three ligands are simultaneously engaged in an extended network of edge-to-face arene interactions that appear to be the dominant feature of this structure. In contrast, the previously published As2(L5)2Cl2 assembly36 shown in Figure 8b bears the expected four As 3 3 3 π contacts (where H2(L5)=2,6-bis(mercaptomethyl)naphthalene). In this macrocycle, the η2-As 3 3 3 π contacts are shorter than that found in the mononuclear naphthalene complex at 3.42 A˚. While the monofunctional and difunctional naphthalene ligands differ only in the number of mercaptomethyl groups, the different secondary bonding interactions in the two structures suggest
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the alkyl tether in the supramolecular As2(L6)2Cl2 macrocycle gives the As center the ability to seek out the most favorable orientation - this appears to be much closer to the “tilted” geometry frequently observed in crystalline adducts and in DFT calculations.18,23 This “tilted” orientation has the effect of drawing the two As(III) centers closer together. Figure 9. Representation of the X-ray crystal structure of As2(L6)2Cl2 showing the effect of increased “chelate” ring size on the supramolecular macrocycle. Use of the ligand 1,4-bis(mercaptoethyl)benzene (H2(L6)) increases the size of the “chelate” ring created when the As(III) center interacts with the arene ring (a). An expanded view of one side of this complex (b) shows that the added conformational freedom afforded by this spacer allows the metal center to interact with the ring in an η3 manner rather than η2 as in the analogous bis(mercaptomethyl) complex As2(L1)3. The preference of the ethylene spacer groups to minimize gauche interactions also contributes to the placement of the As(III) center over the arene ring.
strong macrocyclic effects. Within the macrocyclic complex, each arsenic center is bound to two thiolates and one chloride, while the arsenic center in As(L4)3 is bound to three thiolates. The differences in these coordination spheres could affect the length of the As 3 3 3 π contacts in these assemblies. However, DFT calculations on the strength of the Bi 3 3 3 π interaction reveal that the better an electron acceptor the r specific, the longer the interaction is.35 If the As 3 3 3 π interaction is similarly affected by the identity of the substituent, one would expect a longer distance in the Cl-substituted macrocycle. Here, a shorter interaction is observed, suggesting that macrocyclic effects, not substituent effects, are dominating the structure. The intramolecular nature of the As 3 3 3 π interaction in As2(L1)3, As2(L2)3, As2(L3)2Cl2, and As2(L5)2Cl2 necessarily induce endohedral directionality in each stereochemically active lone pair of electrons on arsenic. Analogous systems bearing nitrogen or C-H groups as a bridgehead are known to exist as “in-in,” “in-out,” and “out-out” isomers,44 and the stability of any one of these isomers is directly related to the macrocycle’s ring size and amount of steric strain on the system. An organic πprismand bearing nitrogen bridgeheads covalently linked by a p-diethylbenzene spacer45 is most stable in the “in-in” conformation, suggesting that relief of steric strain imposed by the macrobicyclic framework contributes to the exclusive formation of this isomer in our supramolecular systems. The restrictions imposed on the geometry of the interaction between the metal center and the neighboring arene ring by the effective size of the chelate ring may compete with the driving force toward forming a given pnictogen 3 3 3 π interaction. The chelate ring formed by the mercaptomethyl(arene) system is effectively five-membered if the point of interaction between the metal and the arene is included in counting the ring system. The interaction angles observed for these structures are much more bent than would be expected for an As 3 3 3 π interaction, given the typical angles found in the structural survey. These sharper angles are the result of the geometric constraints inherent in the short mercaptomethylene tether. The addition of one more methylene group to the spacer between the phenylene scaffold and the thiol group increases the effective size of the “chelate” ring, allowing the pnictogen center more freedom to adopt the most favorable interaction geometry (Figure 9). To expand the size of the chelate ring, we synthesized H2L6, which possesses an ethylene linker between the thiol and arene ring and would provide for a 6-member “chelate” ring with As. X-ray quality crystals of As2(L6)2Cl2 were prepared by reacting H2(L6) (H2(L6)=1,4-bis(mercaptoethyl)benzene) with AsCl3 in CHCl3 and layering the mixture with hexanes.27 The increased length of
Conclusions Our As2L3 and As2L2Cl2 assemblies showcase the remarkable power of weak, noncovalent interactions to influence the overall structure of supramolecular assemblies. As this design strategy grows in order to generate even larger and more sophisticated structures, the following results of our CSD structural survey may provide insight to guide this process. First, the choice of pnictogen can influence the overall structure because the secondary bonding interaction distance decreases slightly as the pnictogen size is increased. This is likely a result of increased delocalization of the lone pair of electrons on the pnictogen and the resultant decreased repulsions between this lone pair and the neighboring arene ring and the increasing Lewis acidity of the heavier pnictogens. The range of possible interaction distances also increases as the pnictogen increases in size, presenting additional opportunities to fine-tune supramolecular structures. Second, the average interaction angle decreases as the pnictogen size increases. This implies that the pnictogen 3 3 3 π interaction is more covalent for As and Sb than for Bi. From our previously and newly reported structures, we can conclude that the complexity of the assembly itself influences the potential nature of the interaction. While As2L2Cl2 macrocycles and As2L3 cryptands have similar As 3 3 3 π interaction distances and angles, they are significantly shorter and smaller than those found in the CSD search. Increasing the effective size of the chelate ring allows for shorter, and presumably more favorable, As 3 3 3 π interactions than does the mercaptomethylene spacer. The larger chelate rings are comparably more flexible, allowing greater freedom in the positioning of the pnictogen. Finally, competition with other weak interactions such as edge-to-face aromatic interactions has an influence on the final structure of the assembly in the solid state. Only one As 3 3 3 π interaction was observed in the mononuclear As(L4)3 complex, while four shorter As 3 3 3 π interactions were observed in the macrocyclic As2(L5)2Cl2 complex (two per As). In the mononuclear complex, the other naphthalene ligands were involved in an extended network of edge-to-face interactions that dominated the crystal structure and were preferred over the possible As 3 3 3 π interactions. The new insight on the pnictogen 3 3 3 π interaction gleaned from this CSD search and the two new structures reported herein will allow fine-tuning of ligand design strategies for Group 15 elements. While our laboratory has had success in chelating arsenic using benzylic thiolates, this geometry does not allow for the most commonly observed angles for pnictogen 3 3 3 π contacts and limits the complexity of our assemblies. We are currently exploring the balance of close pnictogen 3 3 3 π contacts and the rigidity of our ligands to discover the most favorable combination - one which will allow for specificity for the Group 15 elements and the generation of more sophisticated supramolecular assemblies. Acknowledgment. We gratefully acknowledge funding from the National Science Foundation (CAREER award CHE-0545206). D.W.J. is a Cottrell Scholar of Research Corporation. This material is based upon work supported
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by the U.S. Department of Education under Award No. P200A070436 (V.M.C.) and an NSF Integrative Graduate Education and Research Traineeship (M.A.P. and W.J.V.). Supporting Information Available: X-ray data in CIF form, ORTEP and packing diagrams, table of structures used in the CSD survey. This information is available free of charge via the Internet at http://pubs.acs.org.
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