Structural Chemistry, Vol. 12, Nos. 3/4, 2001

1040-0400/ 01/ 0800-0283$19.50/ 0 2001 Plenum Publishing Corporation

283

Molecular Recognition by Cesium Tetrabenzo-24-Crown-8

Jeffrey C. Bryan1,3 and Benjamin P. Hay2

Received October 10, 2000; revised November 6, 2000; accepted November 26, 2000

Two crystal structures of cesium tetrabenzo-24-crown-8 complexes are reported. Solvent molecules4-methylmorpholine (1) and ethylene glycol (2) are observed to coordinate cesium within twoclefts created by the cation–crown ether complex. Careful examination of the structures suggeststhat while both complexes exhibit sterically crowded clefts, the binding of cesium to the crownether is perturbed only in 2. C — H· · ·p hydrogen bonding is observed between the clefts andthe included guests. The ethylene glycol complex 2 forms a complex O — H· · ·O hydrogen bondnetwork between free and bound glycol and nitrate.

KEY WORDS: Cleft; crown ether; cesium complex; crystal structure.

INTRODUCTION

Solvent extraction research has long been con-cerned with the ability to recognize certain cations overothers and has met with considerable success over thepast few decades [1]. More recently, anion recognitionhas generated considerable interest and some success[2]. Less researched are the solvent recognition abil-ities of cation-extractant complexes. Since the extrac-tant rarely completes the cation’s inner coordinationsphere, other ligands, such as anions, solvent moleculesor even parts of other extractant molecules, are oftenobserved to also fill this void [3]. These “secondaryinteractions” are often overlooked, as most work isfocused on the interaction of the ion with the extrac-tant. Once bound, the stereoelectronic environment ofthe cation–extractant complex presents a new topogra-phy for potential guest species. Additional ligands thatcan approach the crown–extractant complex and interactfavorably with both the cation and nearby portions ofthe extractant may exhibit especially favorable bindingcharacteristics.

1 Chemical and Analytical Sciences Division, Oak Ridge National Lab-oratory, Oak Ridge, Tennessee 37831-6119.

2 Environmental Molecular Sciences Laboratory, Pacific NorthwestNational Laboratory, PO Box 999, Richland, Washington 99352.

3 To whom all correspondence should be addressed.

Our past work on the selective extraction of cesiumion with large crown ether molecules [4] led us toexamine the structure of cesium ion when complexedby tetrabenzo-24-crown-8. As illustrated in Fig. 1, thiscrown does not complete the cation’s coordinationsphere, but leaves two U-shaped clefts available for addi-tional ligation [5]–[8]. In this paper, we present twostructures that further explore the binding characteristicsof these clefts, by placing 4-methylmorpholine and ethy-lene gycol in them.

METHODS

General

Solvents and reagents were obtained from commer-cial sources and used as received. Tetrabenzo-24-crown-8 was prepared as previously reported [9].

X-Ray Structure Determination

A summary of crystallographic data is given inTable I. The crystals are sensitive to solvent loss, crum-bling to powder within minutes of being removed fromthe mother liquor. To avoid this undesirable fate, thecrystals were mounted directly from the mother liquorand immediately transferred to the diffractometer’s cold

Bryan and Hay284

Fig. 1. Ball and stick representation of a generic cesium(tetrabenzo-24-crown-8) structure. Hydrogen atoms are omitted for clarity.

stream. All data were obtained on a Nonius CAD4diffractometer equipped with a graphite monochroma-tor and MoKa radiation (l c 0.71073 A). Structureswere solved by direct methods, and expanded usingFourier techniques (SHELXTL [10]). Geometric analysiswas performed by PLATON [11]. Except as noted below,all non-hydrogen atoms were refined anisotropically. AllH atoms were given an isotropic displacement param-eter equal to 1.2 (CH, CH2) or 1.5 (CH3, OH) timesthe equivalent isotropic displacement parameter of theatom to which it is bound. All H atoms were refinedusing a riding model, although the methyl and hydro-xyl H atoms were allowed to rotate about the adjacentC—C or C—O bond. Full-matrix least-squares refine-ment against |F |2 of the quantity Sw(F2

o − F2c)2 was

used to adjust the refined parameters. Complete crystal-lographic details are available as supplementary mate-rial, and have been deposited at the Cambridge Crystal-lographic Data Centre (CCDC) [12].

Cesium(tetrabenzo-24-crown-8)bis(4-methylmorpholine)Nitrate (1)

Crystals were prepared by adding 21 mg (0.11mmol) CsNO3 in 0.18 mL H2O to 50 mg (0.09 mmol)

Table I. Summary of Crystallographic Data

Complex 1 2

Formula C42H57CsN3O14.5 C40H56CsNO19

Formula weight 968.8 987.8Crystal dimensions

(mm) 0.41 × 0.33 × 0.12 0.26 × 0.24 × 0.12Temp. (8C) −110(2) −100(2)a (A) 13.515(2) 13.219(8)b (A) 13.594(2) 14.156(8)c (A) 13.6608(2) 14.359(5)a (deg) 90.176(7) 110.02(4)b (deg) 93.724(10) 91.33(4)g (deg) 114.914(7) 116.44(5)V (A3) 2270.1(4) 2211(2)Z 2 2Space group P1 P1rcalc, g cm−3 1.42 1.48m (mm−1) 0.88 0.92Max./ min.

transmission 0.922/ 0.749 0.898/ 0.797Ra 0.030 0.057wR2b 0.085 0.162S 1.03 1.06

a R c (S || Fo | − | Fc || )/ (S | Fo | ), based on F 2o > 2jF 2

o.b wR2 c

f(S[w(F 2

o − F 2c )2])/ (Sw(F 2

o)2), for all data.

tetrabenzo-24-crown-8 in 10 mL 4-methylmorpholine,and allowing the subsequent mixture to slowly evap-orate. A total of 9162 reflections were collected (v ≤138, ±h, ±k, ± l, v ≤ 25.58, ±h, +k, ± l). Data reductionwas performed by XCAD4 [13], and an empiricalabsorption correction, based on a set of w scans, wasapplied [10]. The data were averaged over 1 symme-try (Rint c 1.6%) giving 7788 independent reflections.Two oxygen atoms on the nitrate anion of 1 are disor-dered (78:22). Residual lattice electron density is mod-eled as disordered water. Assignments of site occupancyfactors for these water molecules are somewhat arbi-trary, but are based on roughly equivalent Ueq’s, and ≤100% occupancy in any one volume. Hydrogen atoms onthe water molecules were not located. All non-hydrogenatoms, except minor disorder components, were refinedanisotropically.

Cesium(tetrabenzo-24-crown-8)tris(ethylene glycol)Nitrate (2)

Crystals were prepared by adding 21 mg (0.11mmol) CsNO3 in 0.18 mL H2O to 50 mg (0.09 mmol)tetrabenzo-24-crown-8 in 10 mL EtOH/ THF/ ethyleneglycol (10:10:1), and allowing the subsequent mixtureto slowly evaporate. A total of 6831 reflections were

Structures of Cesium Tetrabenzo-24-Crown-8 285

collected (+h, ±k, ± l). Data reduction was performed byXCAD4 [13], and an empirical absorption correction,based on a set of w scans, was applied [10]. The datawere averaged over 1 symmetry (Rint c 3.2%) giving6504 independent reflections. The carbon atoms on theh1-ethylene glycol ligand in 2 exhibit unusual elonga-tion, possibly a result of unresolved disorder. Attemptsto model this disorder were not successful. Location ofhydroxyl hydrogen atoms in 2 was based on examinationof the difference electron density map and considerationof plausible hydrogen bonding.

RESULTS AND DISCUSSION

The structures of cesium(tetrabenzo-24-crown-8)-bis (4- methylmorpholine) nitrate (1) and cesium-(tetrabenzo-24-crown-8)tris(ethylene glycol) nitrate (2)are illustrated in Figs. 2 and 3, with selected bond dis-tances and angles given in Tables II and III. The metaland crown portions of the two structures reported hereare quite similar to previously reported structures ofcesium and rubidium complexed by the same crownether (tetrabenzo-24-crown-8) [5]–[8]. In all structures,

Fig. 2. ORTEP representation (50% probability ellipsoids) of cesium-(tetrabenzo-24-crown-8)bis(4-methylmorpholine) nitrate (1). Unla-beled atoms are carbon. Lattice water, the minor nitrate orientation,and most hydrogen atoms are omitted for clarity. Symmetry code: (i)1 − x, 1 − y, −z.

Fig. 3. ORTEP representation (35% probability ellipsoids) of cesium-(tetrabenzo-24-crown-8)tetrakis(ethylene glycol) nitrate (2). Unlabeledellipsoids are carbon. Hydroxyl hydrogen atoms are represented bycircles, while almost all other hydrogen atoms are omitted for clar-ity. Hydrogen bonds are indicated by dashed lines. Symmetry code:(i) x − 1, y − 1, z.

the crown ether folds around the cation, with approx-imate S4 symmetry, reminiscent of K+ structures withnonactin [14] or dibenzo-30-crown-10 [15]. The crownthus covers a large percentage of the surface of thecation, but leaves two arc-shaped volumes on oppositesides of the cation exposed. These volumes are roughlyorthogonal to each other and are shaped into cleftsby arene rings from opposite sides of the tetrabenzo-24-crown-8 macrocycle (Fig. 1). In previously reportedstructures, these clefts were filled by: 1,4-dioxane, water[5], 1,2-dichloroethane [6], acetonitrile, dichloromethane[7], and picrate [8].

Crystal Structure of [Cesium(tetrabenzo-24-crown-8)-bis(4-methylmorpholine)][Nitrate]

Knowing that 1,4-dioxane binds to the cation-crown complex of cesium(tetrabenzo-24-crown-8) [5],we thought it worthwile to see if the closely related4-methylmorpholine would also fit into its clefts.

Bryan and Hay286

Table II. Selected Bond Distances (A)

1 2

Cs — O(1) 3.259(2) 3.328(5)Cs — O(2) 3.310(2) 3.375(6)Cs — O(3) 3.284(2) 3.368(6)Cs — O(4) 3.380(2) 3.273(5)Cs — O(5) 3.258(2) 3.222(5)Cs — O(6) 3.287(2) 3.318(6)Cs — O(7) 3.253(2) 3.482(6)Cs — O(8) 3.345(2) 3.361(6)Cs — O(9) 2.967(2) 3.248(7)Cs — O(10) 2.970(3) 3.039(7)Cs — O(11) 3.440(9)Cs — O(14) 3.149(9)Average Cs — Ocea 3.30(4) 3.34(7)Cs — centroid(Oce)b 0.03 0.07

a Oce represents the crown ether oxygen atoms. Uncertainties quotedare j values based on the statistical distribution of M — Oce.

b The calculated centroid of the eight crown ether oxygen atoms.

The structure of cesium(tetrabenzo-24-crown-8)bis(4-methylmorpholine) nitrate (1) features one 4-methyl-morpholine ligand occupying each cleft (Fig. 2). Both4-methylmorpholine ligands bind cesium through theiroxygen donor atom, rather than the more sterically inhib-ited amine nitrogen atom. This structure is similar to the1,4-dioxane containing complex, in that 4-methylmor-

pholine replaces 1,4-dioxane in one cleft, but replacesboth the water and 1,4-dioxane ligands in the othercleft. The 4-methylmorpholine ligands also exhibit sig-nificantly shorter Cs—O bond lengths at 2.967(2) and2.970(3) A for 1 vs. 3.103(3) and 3.117(4) A for 1,4-dioxane. It is difficult to anticipate what the differencein bond length should be, since, to the best of our knowl-edge, this is the first example allowing direct compari-son of these two ligands. Another difference between 1and the 1,4-dioxane complex is that the clefts are widerin the former structure. The distance between the cen-troids of the arene rings that make up the “walls” of theclefts are roughly 7.9 A apart in 1, but only 7.6 A in thedioxane complex [5]. This extra width is likely relatedto the shorter Cs—O bond lengths to 4-methylmorpho-line, drawing more steric bulk into the narrow part of thecleft and pushing the arene rings apart.

Remarkably, the widening of the cleft does not seemto affect the interactions between cation and crown. TheCs—O lengths to the crown ether oxygen atoms rangefrom 3.253(2) to 3.380(2) A with an average value of3.30 A (Table II), indicating that the crown cavity issymmetrically formed. The distance between the cesiumion and the calculated centroid of the eight crown etheroxygen atoms is only 0.03 A, suggesting that the metalion is well centered in the crown cavity. These val-

Table III. Hydrogen Bond Distances (A) and Angles (deg)a

D — H· · ·Ab H⊥c H· · ·A D· · ·A /–D — H· · ·A

1 C7 — H7B· · ·O12i 2.45 3.311(5) 1461 C23 — H23B· · ·O13ii 2.41 3.301(6) 1491 C33 — H33B· · ·Cg2 2.72 3.53 4.12 1201 C34 — H34B· · ·Cg4 2.68 2.71 3.69 1721 C35 — H35A· · ·Cg4 2.97 3.59 4.37 1371 C36 — H36A· · ·Cg2 2.80 2.81 3.54 1311 C38 — H38B· · ·Cg3 2.78 2.79 3.57 1361 C39 — H39B· · ·Cg1 2.98 3.53 4.33 1401 C40 — H40A· · ·Cg1 2.80 2.90 3.83 1581 C41 — H41A· · ·Cg3 2.72 3.52 4.16 1252 O9 — H9· · ·O19i 2.01 2.832(9) 1672 O10 — H10· · ·O18iii 1.91 2.745(11) 1692 O16 — H16· · ·O15iv 1.78 2.615(15) 1722 O18 — H18· · ·O16v 1.80 2.639(12) 1742 O19 — H19· · ·O17 2.01 2.771(11) 1502 C40 — H40B· · ·Cg2 2.77 2.78 3.67 1502 C41 — H41B· · ·Cg4 2.65 2.73 3.70 167

a No esd’s are given since the positional parameters of H atoms were not refined. CgX refers to the calculated centroid of the crownarene rings (X c 1, Cl–6; 2, C9–14; 3, C17–22; 4, C25–30). Hydrogen bonding parameters to nitrate in 2 are given in Table IV.

b Symmetry codes: (i) 1 − x, 1 − y, −z; (ii) 1 − x, 1 − y, 1 − z; (iii) x − 1, y − 1, z; (iv) x + 1, y, z; (v) 2 − x, 2 − y, 1 − z.c H⊥ is the perpendicular distance between the H atom and the arene plane.

Structures of Cesium Tetrabenzo-24-Crown-8 287

ues are quite similar to those in previously reportedcesium(tetrabenzo-24-crown-8) structures [5]–[8].

Like the 1,4-dioxane structure, the methylenehydrogen atoms on 4-methylmorpholine form C—H· · ·p bonds with the arene walls of the cleft. These inter-actions likely facilitate binding of 4-methylmorpholineto the crown-cation complex. The metrical parametersof these interactions are presented in Table III. Thesedistances are systematically shorter in 1 than in the 1,4-dioxane structure (the shortest H· · ·centroid distances are3.1 A versus 2.7 A in 1), consistent with the fact that 4-methylmorpholine sits lower in the cleft and thereforeexperiences greater steric congestion.

Similar to all reported structures of cesium nitratewith tetrabenzo-24-crown-8, the nitrate anion is excludedfrom the inner coordination sphere of cesium [5]–[7].The anion does experience some level of hydrogen bond-ing with the slightly electropositive methylene H atomson the crown ether backbone (Table III). These weakinteractions may help stabilize the nitrate in the secondcoordination sphere.

Crystal Structure of Cesium(tetrabenzo-24-crown-8)-tris(ethylene glycol) Nitrate

The structure of 2 differs from all other cesium(tetra-benzo-24-crown-8).nitrate structures [5]–[7] in that nitrateis part of cesium’s inner coordination sphere. Nitrateshares one cleft with an ethylene glycol ligand and bothare monodentate toward cesium. The other cleft is occu-pied by an h2-ethylene glycol ligand (Fig. 3).

At first glance it may be surprising to see mixeddenticity between the two ethylene glycol ligands inwhat appears to be roughly equivalent binding environ-ments. There may be several reasons for this observa-tion. The cleft containing the h2-ethylene glycol ligand(the “upper” cleft as drawn in Fig. 3) is more stericallycongested than the (“lower”) cleft containing two lig-ands. This difference apparently induces an asymmetrybetween the clefts in 2, which can be partially quantifiedby measuring the separation of the arene centroids for thetwo clefts. The separation is 7.73 A in the upper cleft,while it is only 7.31 A in the lower cleft. Another indi-cation of significant steric congestion in the upper cleftare the close contacts made between the glycol ligandand the arene walls. Like 1, the h2-glycol ligand formsC—H· · ·p bonds with the walls of the clefts. The dis-tances between these H atoms and the adjacent areneplane (H ⊥) are comparable (2.7 A, Table III) to theshortest such bonds observed in 1.

Extensive O—H· · ·O hydrogen bonding is observedin this structure, and is likely its most significantattribute. Most of these interactions are illustrated inFigs. 3 and 4, and metrical parameters presented inTable III. Each glycol oxygen atom acts as a hydrogenbond donor and those not bound to cesium also act as anacceptor. Not all of these interactions will be discussedin detail, but, taken together, they form a complex three-dimensional framework.

The hydrogen bonds involving nitrate will be dis-cussed, as they may shed light on the future design ofanion or ion-pair recognition agents. A hydrogen bondexists between the h1-glycol ligand (O14) and nitrate(O12). The other end of that glycol ligand (O15) is ahydrogen bond donor toward a nitrate ion in an adja-cent complex forming a ring (see Fig. 4). Nitrate (O13)is further stabilized by a hydrogen bond from one of thetwo lattice glycol molecules (O17). Metrical parametersfor these interactions are given in Table IV, within the“primary bond” section.

It is interesting to note that the hydrogen bonds tonitrate do not align themselves along the nitrate N—Ovectors, but rather appear to be oriented toward the lonepairs on the nitrate oxygen atoms (/–N—O· · ·H ∼ 1108)[16], and thereby asymmetrically bifurcate two nitrateoxygen atoms. The “secondary” hydrogen bonds formed(H14· · ·O11 and H15· · ·O13, Fig. 3) are longer thanthose already mentioned, and have poorer directionality(lower /–O—H· · ·O; see Table IV). H17 seems to be anextreme example. While its primary hydrogen bond isapparently oriented toward a lone pair on O13, it is farfrom O11 (3.05 A). It is likely that the least amount ofhydrogen bonding is observed toward O11 because it isalready bound to cesium.

Examination of interactions between alcohols andnitrate in the Cambridge Structural Database (CSD) [17]reveals a markedly invariant asymmetric bifurcated geom-etry. On average, the hydrogen bonding is very similar towhat is observed in 2, with one hydrogen bond signifi-cantly shorter than the other (Table IV). The CSD dataalso suggest a high degree of planarity between the nitrateand the hydrogen donor atom, which can be measured bythe O—N—O· · ·H torsion angle. As can be seen in TableIV, these values are very close to 1808. The small degree ofscatter in the metrical data from the CSD suggests a signif-icant orientation preference exists for O—H· · ·O hydro-gen bonds to nitrate, which may prove useful in the futuredesign of nitrate-specific receptors. Optimization of a gas-phase methanol-nitrate dimer at the HF/ 6-31G∗ level oftheory [18] yields the same geometry (Fig. 5; Table IV).

Bryan and Hay288

Fig. 4. Stereo packing diagram of 2, illustrating the extensive hydrogen bonding in the crystal lattice. Hydroxylhydrogen atoms are represented by circles, while all other hydrogen atoms are omitted for clarity. Hydrogenbonds are indicated by dotted lines.

This is the first structure of CsNO3 with tetrabenzo-24-crown-8 to contain a bond between cesium and nitrate[5]–[7], and it appears to be a relatively tentative one.At 3.440(9) A, it is the longest Cs—Onitrate distancereported [19]. Only a few examples of ion-paired cesiumnitrate complexes have been reported, and while both h1

complexes are typically 3.1–3.2 A [19]. The unusuallylong distance observed in 2 is possibly a result of theextensive hydrogen bonding to nitrate.

The binding of glycol and nitrate ligands to cesium-(tetrabenzo-24-crown-8) apparently disrupts the bondsbetween cesium and the ether oxygen atoms. These

Table IV. Calculated and Observed Hydrogen Bonding to Nitrate (A, deg)a

H14 H15 H17 CSDb HF/ 6-31G*

Primary bondH· · ·O 2.01 2.08 1.95 1.90(15) 1.92O — H· · ·O 169 173 157 163(10) 175N — O· · ·H 108 101 123 111(8) 111O — N — O· · ·H −156 171 167 166(10) 180

Secondary bondH· · ·O 2.70 2.48 3.05 2.65(20) 2.64O — H· · ·O 130 128 155 133(12) 132N — O· · ·H 76 95 69 75(6) 76O — N — O· · ·H 162 −172 −172 170(7) 180

a No esd’s are given since the positional parameters of H atoms were not refined.b Mean values and standard deviations (parentheses) for the metric parameters of 98 ROH-nitrate interactions observed in the CSD(Ref. 17).

Structures of Cesium Tetrabenzo-24-Crown-8 289

Fig. 5. Calculated (HF/ 6-31G∗) structure of the methanol–nitratedimer.

distances range from 3.222(5) to 3.482(6) A (TableII), a much wider range than observed for 1 or anyother cesium(tetrabenzo-24-crown-8) structure [5]–[8].The average Cs—Oce bond distance and the displace-ment of cesium from the crown donor centroid in 2are also higher (Table II), indicating a more poorlyformed cavity and weaker binding between cesium andthe crown ether.

CONCLUSIONS

The two structures of cesium(tetrabenzo-24-crown-8) presented here both experience steric congestion fromthe guest moieties filling their clefts. The 4-methylmor-pholine complex (1) does so by forming a strong bondto cesium, and synergistic C—H· · ·p bonds to the wallsof the clefts. When the clefts are filled with ethyleneglycol and nitrate in complex 2, the bonding betweencesium and the crown oxygen atoms is partially dis-rupted. Complex 2 also exhibits an unusually weak inter-action between cesium and nitrate, which may be partlydue to the extensive hydrogen bonding to nitrate.

SUPPLEMENTARY MATERIAL AVAILABLE

Tables of atomic positional and thermal parametersas well as complete listings of bond lengths and angles(12 pages) are available from the author on request.

ACKNOWLEDGMENTS

This research was sponsored in part by the Divisionof Chemical Sciences, Geosciences, and Biosciences,Office of Basic Energy Sciences, U.S. Department

of Energy, under contract DE-AC05-00OR22725 withOak Ridge National Laboratory, managed and operatedby UT-Battelle, LLC, and in part by the LaboratoryDirected Research and Development Program at PacificNorthwest National Laboratory. This research was per-formed in part in the William R. Wiley EnvironmentalLaboratory at PNNL funded by the Office of Biologicaland Environmental Research in the U.S. Department ofEnergy.

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