Conformations and Binding Modes of 2,3,5,6-Tetra(2'-pyridyl)pyrazine

Oct 27, 2004 - Synopsis. The compound tppz (2,3,5,6-tetra(2'-pyridyl)pyrazine) is a versatile electron donor that displays a variety of binding modes...
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CRYSTAL GROWTH & DESIGN

Conformations and Binding Modes of 2,3,5,6-Tetra(2′-pyridyl)pyrazine Clifford W.

Padgett,†

William T.

Pennington,†

and Timothy W.

Hanks*,§

Department of Chemistry, Clemson University, Clemson, South Carolina 29634-1905, and Department of Chemistry, Furman University, Greenville, South Carolina 29613 Received August 2, 2004;

2005 VOL. 5, NO. 2 737-744

Revised Manuscript Received September 7, 2004

ABSTRACT: The compound 2,3,5,6-tetra(2′-pyridyl)pyrazine (tppz) is a remarkably versatile electron donor that displays a wide variety of binding modes. Quantum mechanical calculations on the 14 low energy conformations of tppz indicate a preference for four conformations in methanol and two others in either the gas phase or benzene. The prevalence of a species in solution and its predicted solubility, based on simple dipole moment considerations, provides an explanation for the isolation of different tppz polymorphs from the two solvents. Using a simple system for describing the tppz geometry, a review of previously reported crystal structures of complexes containing tppz as a ligand was conducted. The literature reveals a variety of tppz binding modes, including mono-, bi-, and tridentate coordination to a variety of Lewis acids. In addition to small molecular complexes, tppz-containing macrocycles and one-, two-, and three-dimensional extended chain systems have been characterized. Upon the basis of the analysis of tppz coordination behavior, several new binding modes are proposed that could be used for the construction of novel tppz complexes and supramolecular systems. Introduction The multi-ring heterocycle 2,3,5,6-tetra(2′-pyridyl)pyrazine (tppz) was first reported by Goodwin and Lions in 1959.1 The compound was designed as an analogue of terpyridine but with the idea that it might be able to form extended chain structures of the type shown in Figure 1. Reaction of the ligand with a variety of metal salts, however, resulted only in molecular complexes of the general form M(tppz)2 and M(tppz).1 They assumed that in each case the tppz acted as a tridentate ligand on one end, leaving the three nitrogens on the other side uncoordinated. The failure of tppz to bridge two metals was explained using simple models, which highlighted the steric problems that arise from having all of the pyridine rings coplanar (Figure 1). Complexes of tppz continued to attract modest interest as a reagent for chelating metals2 and for colorimetric studies,3 but it was not until X-ray crystallographic studies and photophysical measurements began appearing in the early 1990s that the potential of the ligand in supramolecular synthesis began to be appreciated. It was quickly shown that despite the steric difficulties noted in earlier reports, tppz could act as a bis(tridentate) bridging ligand between identical or different metal centers,4 resulting in interesting electronic and magnetic materials. In addition, a variety of binding modes were observed, offering new directions for the supramolecular construction of novel systems. Thus, while extended chain structures of the type shown in Figure 1 have remained elusive,5 other types of one-, two-, and three-dimensional coordination polymers have been characterized. Interest in tppz continues to increase. Recent patents * Corresponding author: Timothy W. Hanks, Department of Chemistry, Furman University, 3300 Poinsett Hwy, Greenville, SC 29613. Phone: 864-294-3373; fax: 864-294-3559; e-mail: Tim.Hanks@ Furman.edu. † Clemson University. § Furman University.

Figure 1. Hypothetical one-dimensional chain composed of bis(tridentate) tppz ligands.

make use of the ligand and its complexes: in the construction of methanol-based fuel cells,6 for sequestering iron for disease prevention,7 in molecular sensors,8 and in photodynamic therapy.9 Structural studies also continue, with a particular emphasis on the development of magnetic materials that exploit the extraordinary ability of tppz to mediate intermetallic coupling.10 The purpose of this paper is to examine the conformational behavior of tppz through computational analysis of the free ligand. Using a nomenclature that describes the tppz geometry, we then review previously published structures of tppz-containing complexes and salts. The results of this analysis are then used to suggest synthetic strategies for the construction of novel tppz complexes and supramolecular systems. In the following paper in this issue, we present examples of new systems constructed using these concepts.11 Conformations of tppz While orbital overlap would be optimized if each pyridyl ring in tppz were coplanar with the pyrazine, steric interactions make it impossible for all of them to do so at the same time. In fact, coplanarity of one pyridyl ring forces the adjacent pyridyl ring into a nearly perpendicular orientation and raises the energy of the entire system.12 Low energy conformations of tppz are predicted to have each pyridyl ring twisted from copla-

10.1021/cg049731r CCC: $30.25 © 2005 American Chemical Society Published on Web 10/27/2004

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Figure 2. Fourteen conformations of tppz. Each conformation is labeled by point group (above the structure) and by ring orientation as defined in the text (below the structure).

narity with the pyrazine by roughly 50 degrees. Therefore, there are fourteen conformations of tppz that lie in local energy minima. All of the conformers have the same basic shape but differ in the relative positions of the four pyridyl nitrogen atoms. The conformers can also be grouped into five families as shown in Figure 2. Interconversion of conformational isomers within families can be achieved through the concerted rotation of two adjacent pyridyl rings, a process with an energy barrier of less than 2 kJ/mol. Conversely, interconversion between conformers in different families requires that one ring slips past its adjacent partner. Molecular dynamics and trajectory studies indicate that this is most easily done by rotating one pyridyl ring perpendicular to the pyrazine ring plane, while the adjacent ring rotates parallel to the pyrazine with the nitrogen pointed at the perpendicular partner.12 The barrier to this motion is estimated to be on the order of 6 kJ/mol when the nitrogen atom of the parallel ring is pointed at its perpendicular partner and more than 20 kJ/mol if the rotation points a hydrogen atom at the perpendicular ring.13 To describe the various conformers, we will adopt the following conventions for naming: (1) The rings will be labeled A, B, C, and D, corresponding to the rings on the 2, 3, 5, and 6 positions of the pyrazine, respectively. The A ring is always defined as nitrogen “up” from the pyrazine ring plane or toward the reader. (2) Family 1 has all ring nitrogens up. Family 2 has the D ring nitrogen down. Family 3 has the C and D ring nitrogens down. Family 4 has the B and D ring nitrogens down, and Family 5 has the B and C ring

nitrogens down. All other combinations of nitrogen up and down are simple rotations of these conformers in space. (3) The letters following the number indicate the direction that the pyridyl nitrogen atom points relative to the center of gravity of the molecule. “X” indicates an “exo” conformation, where the nitrogen is pointed toward the exterior of the molecule (toward the pyrazine nitrogens, i.e., |Npz-C-C-Npy| < 90°) while “N” indicates an “endo” conformation, where it is pointed toward the interior of the molecule (|Npz-C-C-Npy| > 90°). (4) Many of the conformations are chiral. For example, Figure 2 shows the conversion of 1XNNX into the two enantiomers of 1XNXN. As of yet, there have been no reports in which complexes featuring only a single enantiomer have been isolated. Therefore, for most of this discussion, chirality is ignored. While the barrier to interconversion of these isomers is small, there can be significant differences in the relative internal energies and these differences are sensitive to the environment. Table 1 shows the calculated internal energies of the conformers in the gas phase, in methanol, and in benzene. These ab initio calculations were performed at the RHF/6-31++G** level without enforced symmetry.13,14 In certain cases, the runs failed to optimize or optimized to one of the other geometries. In methanol, four conformers are predicted to have significantly lower energy (> 10 kJ/mol) than the other eight conformers that could be optimized. The low energy geometries include the two members of group 1 and the two members of group 3. Of these, 3XNXN is predicted to be the lowest in energy by a small amount,

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Table 1. Energies (kJ/mol) of the 14 Geometry Optimized Conformers of tppz in the Gas Phase and Two Solvent Phases

1

Gas-phase data unavailable. 2 Benzene data unavailable. 3 Methanol data unavailable. 4 No data available.

although this is probably insignificant. The dipole moments of the four conformers are 4.6, 5.5, 3.8, and 0 D for 1XNXN, 1XNNX, 3XNNX, and 3XNXN, respectively. The first three conformers are the most polar of all of the tppz conformers, while the last is one of only four apolar species. Interestingly, it has been found that when crystals of tppz are grown from polar solvents, the polymorph that is obtained (tetragonal form, compound 6)15 has the apolar 3XNXN conformation. Thus, it is likely that all four conformations are present in polar solvents, but that 6 is isolated in the solid state because it is much less soluble than the other three conformations and precipitates more rapidly. Not surprisingly, the calculated energies for the gas phase structures and those in benzene are similar. In addition, the variations in energies between the various conformers in these two phases were small as compared to the variation observed when methanol was the solvent. Of the conformers that were fully optimized in benzene, 5NNNN is predicted to have the lowest energy by about 5 kJ/mol. In the gas phase, the closely related 4NNNN is predicted to be nearly 6 kJ/mol lower in energy than 5NNNN, which is almost 7 kJ/mol more stable than its nearest competitor. We were not able to fully optimize the benzene structure of 4NNNN, but it is likely that it is as stable, or more so, than is 5NNNN, since the results in benzene generally follow the gas-phase data. Crystallization of tppz from benzene or related apolar solvents indeed gives a polymorph (monoclinic form, compound 7)16 with the conformation of 5NNNN. It is reasonable to assume that 5NNNN, and probably 4NNNN, are the major species in apolar solvents. These results strongly suggest that the nature of the solvent might be used to influence the availability of particular coordination modes when tppz is placed in the presence of an electron acceptor. Review of Crystal Structures Containing tppz Nearly 50 X-ray crystal structures of tppz and its complexes have been reported in the literature prior to this report. They represent five distinct binding modes (defined as unique combinations of nitrogen coordina-

tion to a Lewis acid) and can be described in terms of eight different ligand conformations, five which are included in Figure 2 and three new conformations that are not energetically feasible in the pure ligand. Monodentate Coordination. There have been three structures reported where tppz displays monodentate coordination. While reporting on the structure of 6, Bock et al. also characterized the doubly protonated salt tppzH22+ 2Cl-.16 This complex shows strong hydrogen bonding to the chloride ions and thus is better described as the bis(monodentate) complex tppz‚2HCl, (8). The HCl coordinates to the nitrogens on the A and C rings, which are pointed down and up, respectively. Upon coordination, there is a rotation of the A and C rings out of the pyrazine plane as well as a rotation of the B and D rings into the plane. The net result is a decrease in the N‚‚‚N distance of the adjacent pyridyl rings. These authors also report the structure of the salt formed with the non-hydrogen-bonding counterion, tetraphenyl borate. The resulting complex displays dramatic changes in both the conformation of the ligand and in the binding mode. As the four ring nitrogens are involved in exceptionally short intramolecular hydrogen bonds, this complex is discussed below in the bidentate section.

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We have reported the structure of the bis(monodentate) complex of tppz and I2 (9).17 Here, the nitrogen donates electrons into the I-I σ* antibonding orbital, resulting in a halogen bond. Complex 9 is structurally very similar to 8, with a conformation of 5NNNN. Again, coordination of the electron acceptor results in a rotation of rings A and C out of the pyrazine ring plane, while B and D are rotated into the plane. This motion brings the geometry of the structure closer to that of the proposed transition state for the interconversion of the conformational families in Figure 2. Indeed, iodine has been shown to catalyze the solid-state interconversion of tppz polymorphs.12,17

While it was not characterized crystallographically, a second I2 adduct was also reported in this study. The complex was metastable, losing I2 readily. tppz‚nI2 was suggested (based on computational and thermal decomposition studies) to have the 3XNXN conformation, similar to the higher energy tetragonal form of tppz. Very recently, the X-ray structure of this complex was determined and it will be reported in the following paper in this issue.11 Bidentate Coordination. There have been nine crystal structures reported in which tppz acts as a bidentate ligand, either through two pyridine nitrogens or through one pyridine and one pyrazine. Three distinct binding modes have been identified (labeled R, β, and γ). The ligand can be mono-, bis-, or tris(bidentate), taking advantage of several conformations. Complexes of H+, Re, Pt, and Ru have been described. The bis(bidentate) tetaphenylborate salt [tppzH2]2+ 2(BPh4)- (10), formally adopts a 3NNNN configuration,

Padgett et al.

with rings A and B pointing toward each other and tightly bonding to the proton (the R bidentate mode). They are bowed slightly down out of the plane of the pyrazine. The C and D rings do the same thing, but are rotated so that the nitrogens are slightly above the plane of the pyrazine, giving the complex Ci symmetry. The 3NNNN conformation is not reasonable for the free ligand and is not listed in Table 1. The β bidentate coordination mode also involves two pyridine rings bowed out of the pyrazine ring plane. Stoeckli-Evans has reported a tetra-protonated tppz (11).18 We will characterize this salt as a tppz‚2H2Cl complex because of the strong hydrogen bonding to two of the four chloride counterions (the remaining chlorides form a hydrogen-bonded chain with water). The ligand adopts a 5XXXX conformation with each of the four pyridine ring nitrogens protonated. Rings A and D are rotated above the plane and their respective protons are hydrogen bonded to a single chlorine. The B and C rings are similarly situated below the ring, resulting in Ci symmetry. This unique binding mode has not been observed in any other complex, although it is structurally similar to the bowed tridentate coordination mode discussed below.

Metal complexes undergo bidentate coordination with tppz in one of two ways, either through the R coordination mode or between one pyridine and the central pyrazine ring (γ coordination). The β coordination mode is not possible with a single metal center as the bond distances would be too great. In a recent paper, Zubieta reported the structures of four closely related rhenium complexes that display either R coordination, γ coordination, or both.19 The R mode is observed when Re(CO)5Cl and tppz are heated to reflux in methanol. The complex takes on the conformational mode 2NNNN, with coordinated rings A and B pointed up as well as ring C. Conversely, when the same reagents are heated in toluene, the tppz coordinates through the pyrazine nitrogen and one pyridine nitrogen. Both a mononuclear complex and a symmetrical dinuclear complex have been isolated, depending on the metal-to-ligand ratio and the heating time. They show the 2NXNN and 4XXXX conformations, respectively. Finally, when the same reaction is run in benzene with prolonged heating and a 4-fold excess of Re(CO)5Cl, a trinuclear compound is formed that features both R and γ coordination (12). Formally displaying the unique 2NNXX tppz conformation (not one of the structures available to the free ligand),

Conformations of 2,3,5,6-Tetra(2′-pyridyl)pyrazine

12 offers interesting possibilities as a node for threedimensional tppz-containing structures. Indeed, the authors note the presence of polymeric side products from all reactions, particularly upon extended heating. These have not been characterized but likely display a combination of binding modes.

The authors did not venture an explanation for the origin of the different complexation modes, but this can be readily understood from the preferred conformations of tppz in polar and apolar solvents. In polar solvents, members of conformational families 1 and 3 are favored. In both of these families, adjacent pyridines have their nitrogen atoms on the same side of the pyrazine ring plane. Assuming a stepwise substitution of N-donor for CO ligand, coordination of one pyridine to the metal leaves the second perfectly situated to perform the second substitution, leading to the R coordination mode. Conversely, in apolar solvents, tppz prefers to adopt conformations from families 4 and 5 in which the nitrogens on adjacent pyridine rings are on opposite sides of the pyrazine ring plane. In this case, initial coordination of one pyridine ring leaves the nitrogen on the adjacent ring unable to reach the metal without a family-to-family ring rotation. The coordinated pyridine ring can most easily find a chelation partner in the pyrazine nitrogen, despite its weaker donor ability, giving γ coordination. The minor point that the nitrogen of the adjacent, uncoordinated pyridine ends up rotated toward the metal complex instead of away as might be initially expected can be explained by the ring rotations induced by pyridine coordination, as previously described. In a recent report, a platinum complex prepared in a CH2Cl2/EtOH solution was also found to adopt the R bidentate coordination mode, consistent with this interpretation.20 However, the compound Ru(phen)2tppz (phen ) phenazine) was prepared in methanol and found to have the γ bidentate coordination mode.21 In this case, the pyridine/pyridine coordination mode is probably prohibited due to the steric demands made by the phenazine ligands. Tridentate-Coordination Mode. By far the most widely characterized binding mode is the one that sparked the

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original synthesis of the ligand in 1959: tridentate coordination. There are three general classes of structures that arise from this binding mode, including the M(tppz) structures, M(tppz)2 structures, and M2(µ-tppz) structures. The simplest of these are the M(tppz) structures (not shown) that involve coordination of one mono-tridentate tppz to a single metal, leaving the other side free. Complexes containing Ir,4b Cu,22,23 Ru,24 and Zn23 have been characterized. Often, these have been isolated in conjunction with similar structures in which the tppz serves as a bridging ligand (M2(µ-tppz)). The major significance of these complexes is that they can serve as precursors for mixed metal complexes of the type M(µ-tppz)M′. While there have not yet been any crystal structures of such a mixed metal system, they are of particular interest for photophysical studies and several have been reported.4,5,25

In one interesting report, Stoeckli-Evans isolated the salt [(H2O)2Cu(µ-tppz)Cu(H2O)2]2+ 2(ClO4)-, where the tppz is bis(tridentate), from the reaction of tppz and Cu(ClO4)2.23 If the reaction was run in the presence of HCl, however, the chlorine-bridged dimer 13 resulted. The geometry about the copper is nearly square planar between the three tppz nitrogens and one of the chlorines. The second copper/chlorine bond is longer and is best described as a donation of a chlorine lone pair into an empty dz orbital. Unlike the bidentate complexes described previously, the two pyridine rings do not “bow” out of the pyrazine ring plane. Instead, the pyrazine ring twists to push one pyridine up and the other down. The pyridine rings then rotate in the opposite direction to bring their lone pairs close to the metal. This “twisting” of the pyrazine ring is a common feature of most (but not all) complexes in which the tppz is tridentate. Compound 13 has two unique features that are relevant to future supramolecular engineering designs.

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First, it displays two types of tppz binding modes, tridentate to the metal and monodentate to the protons. One pyridine nitrogen and one pyrazine nitrogen remain uncomplexed. Second, treatment of the complex with base could potentially open up the second tridentate site to further coordination, either to copper or to another metal. This means that a variety of 1D-mixed metal polymers might be assembled from this building block. In Goodwin and Lions’ initial publication on tppz,1 they isolated an iron complex that almost certainly possessed the M(tppz)2 structure. While much of the early work on tppz focused on using this binding mode for analytical applications, only three crystal structures displaying this motif have been reported. These include complexes of Cu,26 Ni,26 and Fe.27 The structures of the three complexes are quite similar. Each adopts a 3XNNX conformation (for example, compound 14), with all nitrogens pointed in the same direction (noncoordinating rings are in an endo orientation). Chelating rings A and D are rotated nearly into the pyrazine ring, forcing rings C and D to rotate out of the plane in response. Again, the pyrazine ring twists to help the three nitrogens into a near planar geometry (most easily seen tppz ligand on the right in 14).

Most investigations into the tppz ligand have focused on compounds of the general form M2(µ-tppz). Some 26 crystal structures have been reported featuring Pd,28 Ru,29 Pt,30 Ni,31 Co,32 Zn,33 Rh,34 and Cu,35 with the latter metal found in approximately half of the structures. For the tppz to act as a bis(tridentate) ligand, all four pyridine rings attempt to rotate into planarity with the central pyrazine. The resulting steric clash can be alleviated in two different ways. Most commonly, the pyridine rings are in the 4XXXX configuration, causing the pyrazine to twist in the same way as was observed for the M(tppz)2 complexes. An example of this is the complex CuCl2(µ-tppz)CuCl2 (15).35b In other cases, however, the rings are in a 5XXXX configuration, with the pyridines bowing out of the pyrazine ring plane, much like the geometry observed for the β-bidentate compound 11. In these examples, all Cu2+ salts, the pyrazine ring shows only minimal distortion from planarity (16).35c-e While coordination polymers of the type shown in Figure 1 have not yet been observed and may not be possible, the M2(µ-tppz) unit has been used to craft larger structures by using additional bridging ligands

or clusters. For example, a very interesting molecular rectangle consisting of parallel Rh2(µ-tppz) moieties linked by acetate ligands has been reported recently.34 A zinc macrocycle containing two tppz ligands and 10 ZnClx ions has also been characterized.33 Sletten, Julve, and co-workers have analyzed the structural, spectroscopic, and unusual magnetic properties of two polymers containing Cu2(µ-tppz) groups linked with copper azide units.35h The Zubieta group has prepared a series of one-, two-, and three-dimensional materials featuring molybdophosphate and -arsenate clusters held together with linear Cu2(µ-tppz) linkers.35c,e,g In these systems, the dimensionality of the system is defined by the number of nodes on the metal oxide cluster, while the Cu2(µ-tppz) unit “serves to provide charge compensation, space-filling, passivating, and structure-directing roles.”35g Prospects for New tppz Supramolecules The versatility of tppz as an electron donor has already proven exceptional, but many areas remain open for future exploration. For example, the handful of tppz salts that have been reported only hint at the possibilities of hydrogen-bonded networks incorporating the compound. A better understanding, and perhaps control over, the organization of such networks will depend on the characterization of other complexes between tppz and proton donors.11 Of particular importance will be investigations into the role of parameters such as solvent, temperature, and additives on the final ligand geometry. Other binding modes are also possible, such as those illustrated schematically in Figure 3. Structure 17 shows a tetra-protonated (or hydrogen bonded) species that is very different from salt 11. Here, there is

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Figure 3. Potential binding modes of tppz.

potential for intermolecular hydrogen bonding to two (effectively giving the β-bidentate coordination mode) or four other molecules. Structure 17 suggests that the pyridine rings would be rotated 90° from the pyrazine ring plane, but it is more likely that they will be at some intermediate angle. This could easily result in helical hydrogen-bonded extended chain structures with the direction and angle of the spiral controlled by the counterion/proton acceptor. The first example of a complex possessing this binding mode will be reported in the following paper in this issue.11 Another previously unreported binding mode is illustrated by structure 18. This structure, featuring bidentate coordination through the A/C and B/D pyridyl ring pairs, might be achieved and represents a fourth (∆) type of bidentate coordination. The distance between the two nitrogens is too large for binding a single metal center, so this coordination mode will only be observed in complexes between tppz and a multi-atom electron acceptor. An example of ∆-bidentate coordination has recently been characterized and will be described in the following paper in this issue.11 Other conceptual and synthetic challenges abound in tppz coordination chemistry. Compounds 19 and 20 represent as yet unrealized tri- and tetradentate structures. As with 18, these will necessarily involve multiatom electron acceptors. New synthetic strategies that encourage multiple coordination modes within a single tppz need to be developed. For example, 18-19, as well as many of the known coordination modes, leave Ndonors available for further reaction. Supramolecules and polymeric materials could take advantage of tppz as a node for cross-linking or the attachment of pendant functionalities. The important areas of magnetic and optical materials also require efficient synthesis and characterization of hetero bi- and perhaps trimetallic systems, as well as their organized assembly into oneand higher-dimensional materials. A subtle but important area of investigation will be a better understanding of the effect of the bow versus the twist geometry in the bis(tridentate) M2(µ-tppz) series as the degree of distortion of the central pyrazine ring will undoubtedly play a role in the intermetallic coupling of the complexes. Finally, as was mentioned in the introduction, many conformations of tppz are chiral. As yet, there are no reports that have taken advantage of this to isolate single enantiomers or diastereomers of tppz complexes. Chiral materials may have advantages in areas such as photodynamic therapies, where the compounds interact with biological systems. Summary There are 14 low energy conformations of the tppz ligand that can be described by the relative orientation of the four pyridine rings. While the barriers to inter-

conversion between the conformers are small, there are substantial, solvent-dependent differences in the internal energies of the conformers that influence both the polymorphic form that is crystallized from a particular solvent and the products of tppz coordination to some metals. A survey of the literature shows a host of tppz binding modes, not all of which can be described in terms of low energy geometries predicted for the uncomplexed ligand. To date, the majority of structures that have been described involve tridentate coordination of the ligand to one or two metal centers resulting in isolated small molecules. Notable exceptions include mono- and bidentate coordination to metals and other Lewis acids as well as macrocyclic and extended chain architectures. An examination of other low energy conformations of the free ligand suggests new binding modes that might be incorporated in future crystal engineering projects. Of particular interest would be complexes in which multiple binding modes are utilized, permitting the use of tppz as a node in three-dimensional supramolecular structures. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. CHE-0203402. Supporting Information Available: Rotatable threedimensional images in Quicktime format are available for structures 8-16. This material is available free of charge via the Internet at http://pubs.acs.org.

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