3436
Ind. Eng. Chem. Res. 2000, 39, 3436-3441
Experimental Evidence for Alkali-Metal Ion Cation-π Interactions Using Bibracchial Lariat Ether Complexes Leonard J. Barbour,† Stephen L. De Wall,‡ Eric S. Meadows,‡ and George W. Gokel*,‡ Bioorganic Chemistry Program and Department of Molecular Biology & Pharmacology, Washington University School of Medicine, 660 S. Euclid Avenue, Campus Box 8103, St. Louis, Missouri 63110, and Department of Chemistry, University of Missouri, 601 S. College Avenue, Columbia, Missouri 65211
Attempts to demonstrate cation-π interactions between alkenes and/or alkynes and such alkalimetal cations as Na+ and K+ have proved largely unsuccessful. Recently, however, lariat ethers having side arms terminated by either indole or phenol have afforded solid complexes that exhibit clear evidence for cation-π complexation when studied by X-ray crystallography. Introduction The alkali-metal cation family comprises group I in the periodic table and includes lithium, sodium, potassium, rubidium, cesium, and francium. Sodium is one of the most common elements on earth, and sodium and potassium are two of the most biologically important cations. From the chemical perspective, they are differently sized but otherwise featureless spheres defined by a single positive charge. The discovery of macrocyclic “crown” polyethers by Pedersen1 and the cryptands by Lehn2 revolutionized our ability to experimentally address alkali-metal cation interactions with a variety of donor groups. The importance of crown ether and cryptand compounds is apparent from the award of the 1987 Nobel Prize in Chemistry to Pedersen, Lehn, and Cram, the pioneers in this area. Despite the attention the subject of alkali-metal cations has received during the past 3 decades, their interactions with the π electrons of double bonds, triple bonds, and aromatic rings have remained elusive. Much effort has been expended on calculations that predict significant strength for the complexation interaction between an arene and an alkali-metal cation. Finding experimental, rather than computational, evidence for such interactions has proved to be an enormous challenge.3 We made our first effort in this area more than a decade ago but only recently obtained the type of compelling experimental evidence for which we had long hoped. Lariat Ethers. Crown ethers, as designed and prepared by Pedersen, were essentially two-dimensional macrocycles. To be sure, the elaboration of structures has been enormous,4 but in most cases the single macrocyclic unit defines the system. In contrast, cryptands were specifically designed to be threedimensional structures that could completely envelop a cation, particularly an alkali-metal cation. The structures of dibenzo-18-crown-6 and [2.2.2]cryptand are as follows: * To whom correspondence should be addressed. Tel.: 314/ 362-9297. Fax: 314/362-9298 or -7058. E-mail: ggokel@molecool. wustl.edu. † University of Missouri. ‡ Washington University School of Medicine.
The complexation of cations by “host” or “receptor” compounds such as crowns or cryptands can be expressed by the following equilibrium relationship. The complexation rate, kcomplex, could equally well be called k1 or kf (for forward rate). Likewise, krelease could be called kdecomplex, k-1, or kbackward. The important point is that the equilibrium binding constant, usually referred to as KS, is the ratio of k1/k-1, i.e., KS ) k1/k-1. kcomplex
receptor + M+ y\ z complex k release
Cation complexation by crown ethers is generally fast because access to the macrocycle is relatively unhindered.5 Cation decomplexation is likewise fast because the cation complex is “two-dimensional” in the sense that the macroring forms a belt about the ion. Solvent can approach the bound cation and displace it from the enveloping ring. Complexation by crowns is thus a dynamic process, but their cation selectivities are less dramatic than those for cryptands.6 The three-dimensional cryptands provide an enveloping capsule for a cation that excludes solvent. This leads to high selectivity ratios for differently sized cations, but the cost of the enhanced selectivity is poorer complexation dynamics. It is important to recognize that the equilibrium position in complexation is determined both by how readily the cation is trapped and by how easily it is released. The question of dynamics is especially important for cation transport because the transmembrane conveyance of a cation is a multistep process.7 On the external surface of the membrane, strong and rapid cation binding is desirable. The complexation and decomplexation rates are less important while the complex is within the membrane where their ratio, i.e., KS, is the critical issue. At the internal membrane surface, release of the cation becomes the important issue and kdecomplex becomes crucial. Data (determined in an aqueous solution) that support these assertions are shown
10.1021/ie990780o CCC: $19.00 © 2000 American Chemical Society Published on Web 08/23/2000
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3437 Table 1. Kinetic and Equilibrium Binding Data for 18-Crown-6 and [2.2.2]Cryptand compound 18-crown-6a,b [2.2.2]cryptandb,c a
kcomplex (M-1 s-1)
krelease (s-1)
KS
4.3 × 7.5 × 106
3.7 × 106 38
115 2 × 105
108
Data for H2O. b See ref 5. c Data for a methanol solution.
Figure 1. Schematic representation of the lariat ether concept. The cation is shown as a shaded sphere that is coordinated in the first step by the crown ether and in the second step by the side arm.
in Table 1. In many studies, the need for solubility of both the organic receptor molecule (crown and cryptand) and the salt has led to a choice of methanol as the solvent. In general, rates are faster and binding is stronger in methanol than in water. Lariat ethers8 were developed in order to capitalize on the dynamics of crown ethers in combination with the strength of three-dimensional enveloping interactions.9 We felt that nature provided excellent guidance on this issue. In general, a covalent structural framework defines natural organization. Natural systems retain flexibility, however, because adjustments are required for binding and release, for catalysis, and for replication. Proteins are excellent examples.10 They assemble into chains of amino acids that occur in a specified order. In all cases, the backbone or main chain is a polymer of ∼(NH-CHR-CO)∼ repeating units. The backbone provides a regular array of carbonyl donor groups and potential hydrogen-bond sites. Proteins are distinguished by the order in which the 20 common amino acids occur in the chain. Because the side chains of the amino acids differ, the overall structure is different, and this defines the so-called “primary structure”. The three-dimensional structure of a protein is defined by the sum of all possible steric and electronic effects. Such effects include side-arm size, rigidity, polarity, and orientation. Furthermore, interactions within the backbone (largely H-bond formation) and among the side chains (salt bridge and H-bond formation and hydrophobic contacts) are critical to the shape and function. We were guided by these concepts as follows. We felt that a simple crown macroring would impart to the system a rudimentary “hole size” selectivity while retaining binding dynamics. The side arm and its accompanying donor(s) were expected to provide a third dimension of solvation. It was anticipated that, kinetically, this would be a two-step process but that each step would be facile and fast. The anticipated overall complexation process is illustrated schematically. As Figure 1 shows, complexation occurs in a fashion that might be described as “rope and tie”. Moreover, the molecular models of crown/side-arm structures resemble a looped rope or lariat, hence our choice of the name. Classes of Lariat Ethers. Once the basic macrocycle/side-arm notion was adumbrated, it was clear that ring size, side-arm identity, side-arm length, and numerous other variables could be manipulated. A fundamental question is how the side arm(s) would be
Figure 2. Examples of two types of lariat ethers.
attached and what would be the effect of each option on the dynamics of the system. Because a crown ether constitutes a basic building block of the lariat ethers, the ways in which side arms can be attached to the macroring are limited. Simple crown ethers possess alternating ethyleneoxy, -(CH2CH2O)-n units. It is possible to form trivalent oxygen (as in protonated water, H3O+), but this would be an unstable arrangement. Linkage of the side arm to carbon is synthetically feasible but suffers from two problems. The first issue is of “sidedness”. Carbon is tetrahedral, potentially chiral, and noninvertable. Attachment of a side arm at carbon would distinguish one face of the macrocycle from the other. The second issue is that significant conformational restrictions are imparted to the system by such a structural arrangement. A number of molecules were prepared in which the point of attachment was carbon.11 Clear evidence was obtained for binding cooperation between the macroring and the side arm. Still, binding enhancements were modest, and this class of structures seemed less promising. Others have carried forward the “carbon-pivot” structures with greater success (Figure 2).12 Most of our efforts have focused on developing compounds in which the side arms are attached at a nitrogen atom. These so-called nitrogen-pivot molecules are relatively flexible and achiral, and they are synthetically accessible. When these molecules were elaborated into two-armed compounds, we coined the additional term “bibracchial lariat ethers” or BiBLEs. At the time we began the studies that are reported here, it was well-known that silver cations could interact with π bonds.13 Silver-impregnated gas chromatographic columns were commonplace.14 Clear experimental demonstrations of cation-π interactions between the alkalimetal cations and olefins were completely unknown. During recent years, numerous calculations have appeared concerning the possibility of cation-π interactions between cations and a variety of arenes. In general, these calculations support the possibility of cation-π interactions between benzene, phenol, or indole and either Na+ or K+. Such interactions are the most widely studied because among the 20 common amino acids the three having potential π-donor side chains are phenylalanine (Phe, F), tyrosine (Tyr, Y), and tryptophan (Trp, W). The side chain of the fourth aromatic amino acid, histidine (His, H), is imidazole, a σ donor. None of the naturally occurring amino acids possess alkenyl or alkynyl side chains, so these have not been studied by biochemists. Cation-π interactions were assessed by Burley and Petsko,15 who analyzed the Protein Data Bank database. They found numerous
3438
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000
close contacts between such residues as protonated lysine (-NH3+) and the benzene ring. Search for Alkali Metal-Olefin Cation-π Interactions. A combination of theoretical and experimental studies has produced information about ammonium ion-arene interactions.16,17 A number of theoretical studies have been conducted that support cation-π interactions between alkali-metal cations and aromatic residues.18 Experimental evidence has been extremely limited. The earliest experimental data resulted from mass spectrometric studies reported in 1981 by Sunner, Nishizawa, and Kebarle.19 Lisy and co-workers have reported additional mass spectrometric evidence of alkali metal-arene cation-π interactions.20 Important and suggestive crystal structure work has also been reported.21 None of these reports gives the unambiguous information that can be gleaned from a small-molecule X-ray crystal structure. Our goal was, therefore, to obtain high-resolution structures so that these interactions could be clearly understood. We thought that bibracchial (two-armed) lariat ethers comprised the ideal vehicle to probe for alkali metal to olefin interactions. We reasoned that the crown macroring would provide a two-dimensional ring of solvation about an alkali-metal cation. In this situation, the apical positions would be unencumbered. Side arms having appropriately placed donor groups should, therefore, be appropriate to fill these empty, apical sites. If the only available donors were π bonds, the π electrons should complete the solvation sphere. If the interactions were reasonably strong, it should be detectable by a variety of methods. Ligand Syntheses. The choice was made to use 4,13-diaza-18-crown-6 derivatives in the studies described below. Their structures are shown in Chart 1 as compounds 1-8. The side arms are as follows: n-propyl, 1;22 allyl, 2;23 propargyl, 3;23 cyanomethyl, 4;23 benzyl, 5;23,24 phenethyl, 6; indolylethyl, 7;25 and hydroxyphenethyl, 8.26 The BiBLEs were prepared by either of two basic approaches. 4,13-Diaza-18-crown-6 was alkylated using either the incipient side arm as an electrophile or the previously reported, single-step synthesis. In the latter, a primary aliphatic amine, R-NH2 in which R is the incipient side arm, is treated with triethylene glycol diiodide in the presence of sodium carbonate and acetonitrile (Scheme 1).22 Cation Binding Studies. The value of solid-state data is great, but unambiguous solution data are vital to the study of cation complexation. We had extensive experience in assessing cation complexation by use of ion-selective electrode methods. The first data were obtained on compounds 1-3, which have n-propyl, allyl, and propargyl side arms, respectively. By keeping the side-arm length constant and systematically increasing the extent of unsaturation, we felt we would obtain evidence for cation-π interactions between double and triple bonds and either sodium or potassium if such interactions were possible. The sodium binding constants (log KS) were measured at 25 °C in an anhydrous methanol solution and were found to be as follows: 1, 2.86; 2, 3.04; 3, 3.61. These decadic logarithms translate to equilibrium binding constants of the following: 1, 725; 2, 1100; 3, 4100. We were encouraged by the fact that cation binding strength increased with unsaturation. This was clearly the hoped-for result. Our interpretation was that so-
Chart 1
Scheme 1
dium was bound in the macroring and the side arms constituted additional, apical donors. The cylindrical symmetry of the propargyl group’s triple bond was thought to make the π system more accessible to the ring-bound cation. Measurement of the K+ binding constants afforded a similar trend. Thus, the experimentally determined K+ complexation constants (log KS) were as follows: 1, 3.77; 2, 4.04; 3, 4.99. Higher binding strength (KS range ) 5900-10 000) for K+ over Na+ complexation was expected. The important observation was that, like the Na+ binding constants, the K+ binding strength increased with the degree of unsaturation. Crystal Structures of BiBLEs Having Unsaturated Side Arms. A collaborative effort27 with Fronczek et al. permitted us to obtain crystal structures of several free receptor molecules and their complexes. In particular, we obtained two structures of N,N′-bis(propargyl)4,13-diaza-18-crown-6 (3). One was of 3‚KSCN, and the other was of hydrated 3, i.e. 3‚4H2O.23 We also obtained the solid-state structures of bis(benzyl)crown 5 in the
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3439 Table 2. Equilibrium Constants and Thermodynamics of Cation Binding Affinitiesa log KS no.
side arm
Na+
1 2 3 4
18-crown-6 CH2CH2CH3 CH2CHdCH2 CH2-CtCH CH2-CtN: CH2CH2OCH3
4.34 2.86 3.04 3.61 2.69 4.77
a
sodium cation
potassium cation
K+
∆H
T∆S
∆H
T∆S
6.09 3.77 4.04 4.99 3.91 5.52
-7.40 ( 0.11 -2.82 ( 0.05 -3.56 ( 0.23 -4.97 ( 0.04 -4.87 ( 0.08 -7.20 ( 0.05
-1.50 ( 0.09 1.08 ( 0.04 0.59 ( 0.20 -0.05 ( 0.12 -1.20 ( 0.10 -0.73 ( 0.08
-11.3 ( 0.02 -6.28 ( 0.27 -7.34 ( 0.02 -4.97 ( 0.04 -9.54 ( 0.11 -8.81 ( 0.03
-3.03 ( 0.04 -1.14 ( 0.30 -1.84 ( 0.01 -0.05 ( 0.12 -4.21 ( 0.09 -1.28 ( 0.02
In anhydrous CH3OH at 15-41 °C as described in ref 29. log KS values are in M-1, enthalpy values in kcal/mol, and entropies in eu.
presence and absence of K+. Regrettably, in no case examined at that time was there any evidence for donor group participation by the side arm π electrons. Determination of Binding Thermodynamics. It is sometimes the case that solid-state structural data reflect crystal packing forces that overwhelm other weak interactions. The solution binding data described above for 1-3 were suggestive of side-arm participation in the π sense. We, therefore, decided to determine thermodynamic parameters for the complexation process. It was thought that these would reveal an increase in the enthalpic term as the extent of side-arm unsaturation increased. Izatt, Christensen, and their co-workers had proved the importance of obtaining thermodynamic data for complexation and had amassed an enormous amount of information that was available for comparison.6,28 We lacked the apparatus to undertake calorimetric measurements on these systems. We had previously obtained acceptable thermodynamic data using the van’t Hoff relationship.29 The so-called van’t Hoff isochore or van’t Hoff equation is d ln K/dT ) ∆H/RT2. A plot of ln KS vs 1/T (in degrees Kelvin) gives a plot having a slope of -∆H/R and an intercept of ∆S/R. Typically, the equilibrium constant is plotted vs inverse temperature (abscissa). Normally, the equilibrium complexation constant is determined at several temperatures over a range of 15-30 °C. Using this method, the results shown in Table 1 were obtained for BiBLEs 1-4 (Table 2). Compound 4 has two cyanomethyl (-CH2CtN:) side arms. Cyanomethyl is isosteric with propargyl, but the side arms differ in dipole and the nucleophilicity of the distal terminus. The cyanomethyl side arm is expected to be a better σ donor because of the electron pair present on the nitrogen atom. The data were both revealing and disappointing. We expected the triple bonds in derivatives 3 and 4 to interact with the ring-bound cation in a similar fashion. The binding constants differed by about a power of 10 for either cation. Remarkably, the values determined for ∆H for 3 and 4 were nearly the same for Na+ binding but dramatically different for K+. The sodium binding constants differed because the entropic component of the free energy differed substantially. We attributed this difference to hydrogen bond interactions between the nitrile lone pair and methanol solvent. The enthalpic trend for sodium seemed to support the side-arm hypothesis, but the trend was quite different for potassium binding. We reluctantly concluded that if cation-π interactions were important in this system, they applied only to Na+ binding. This is possible because Na+ is more charge dense than K+, but such a conclusion is obviously both equivocal and unsatisfying. Structures of BiBLEs Having Indolyl and Phenolic Side Arms and Their Complexes. Compounds 7 and 8 are analogues of 6, but they differ from the other
structures in the side-arm extension. Thus, the phenyl group and macroring nitrogen are separated by a single methylene in 5 but by two methylene groups in 6. The same spacing of potential π donor and macroring was incorporated into 7 and 8. An increase in the side-chain distance was suggested by two facts. First, CPK molecular models showed that the arene could interact with a ring-bound cation in a more nearly parallel fashion when the spacer chain was ethylene. Second, several previous structural studies showed no evidence for sidearm participation when the spacer was a single methylene. Obviously, the latter result could simply mean that such interactions are unimportant in these systems. Despite this outcome, we were encouraged by the fact that side-arm participation was observed when such donors as oxygen were appropriately placed on side arms so that they could participate in the Na+ or K+ solvation sphere. Solid-state structures were obtained for 4,13-diaza18-crown-6 derivatives 7 and 8. Compound 7 has CH2CH2 indole side arms and 8 has CH2CH2 phenol side arms (see structures shown above). The arenes are attached as they are in the amino acids Trp and Tyr. Thus, indole is ethylenated at the 3-position (7), and phenol is 4-substituted (8). The uncomplexed host molecules exhibited structures approximately as expected. The macrorings of 7 and 8 were in a conformation similar to that found for N,N′-dibenzyldiaza-18crown-6 (5) in the solid state. In all three cases, two methylene groups on opposite sides of the ring were rotated inward to fill the molecular void. This is apparent in Figure 3 where the structures of uncomplexed 5 and 8 are shown as ORTEP plots. No side-arm interaction between the benzyl groups of 5 was apparent upon complexation with K+. Crystal structures were obtained of complexes 7‚K+ 25 and 8‚ K+.26 In both cases, the arene side arms showed clear evidence for what might be called π-sandwich formation. Such interactions are, of course, well-known in the organometallic literature. Indeed, such compounds as ferrocene30 and dibenzenechromium31 are archetypes for entire families of such structures. The observation of unequivocal side-arm cation-π interactions is an important milestone in alkali-metal ion chemistry. Some details of the interactions are recorded in Table 3. Note that the heteroatom-donor and cation-π distances are either identical or virtually the same in these two KI complexes. Conclusions The long-standing problem of characterizing alkalimetal cation-π interactions is finally yielding to experimental effort. Theoretical calculations have certainly been encouraging. The results of mass spectral work have always been clear, but there was a lingering
3440
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000
Literature Cited
Figure 3. Crystal structures of lariat ethers: (top) unbound structure of N,N′-dibenzyldiaza-18-crown-6 (5); (middle) unbound structure of N,N′-diethylphenoldiaza-18-crown-6 (8); (bottom left) structure of the 7‚KI complex; (bottom right) structure of the 8‚ KI complex. The K+ (gray sphere) is shown coordinated by the indole (left) or phenol (right), while the iodide anion (black sphere) is hydrogen-bonded to the nitrogen of indole or the oxygen of phenol. Table 3. Structural Information for K+-arene π Complexes complex
interaction
distance (Å)
7‚KIa
O-K average N-K average K f centroid O-K average N-K average K f centroid
2.70 ( 0.06 3.06 3.45 2.70 ( 0.06 3.04 3.44
8‚KIb
a
Data from ref 30. b Data from ref 31.
concern about whether these results accurately represented the situation outside the gas phase. The present results show conclusively that such interactions occur and that the bonding parameters are largely as expected. A challenge that remains as this paper is written is to understand a discrepancy between experiment and calculation. Theoretical treatments predict that indole should bind to an alkali metal with the benzocentroid, but all of the crystal structures obtained in our laboratory show that the pyrrolo centroid is the donor group. We and others are undertaking additional experiments and calculations in an attempt to better understand the documented phenomenon of alkali-metal cation-π interactions. Acknowledgment We warmly thank the National Institutes of Health (GM 36262) and the National Science Foundation (CHE9805840) for grants that supported this work.
(1) (a) Pedersen, C. J. Cyclic polyethers and their complexes with metal salts. J. Am. Chem. Soc. 1967, 89, 2495. (b) Pedersen, C. J. Cyclic polyethers and their complexes with metal salts. J. Am. Chem. Soc. 1967, 89, 7077. (2) (a) Lehn, J. M.; Sauvage, J. P. Cation and cavity selectivities of alkali and alkaline-earth “cryptates”. J. Chem. Soc., Chem. Commun. 1971, 440. (b) Dietrich, B.; Lehn, J. M.; Sauvage, J. P. Diaza-polyoxa-macrocyclic and macrobicyclic compounds. Tetrahedron Lett. 1969, 2885. (3) Ma, J. C.; Dougherty, D. A. The cation-π interaction. Chem. Rev. 1997, 97, 1303-1324. (4) Gokel, G. W.; Korzeniowski, S. H. Macrocyclic Polyether Syntheses; Springer-Verlag: Berlin, 1982; p 415. (5) Liesegang, G. W.; Vasquez, A.; Purdie, N.; Eyring, E. M. Ultrasonic absorption kinetic studies of the complexation of aqueous Li+, Na+, Rb+, Tl+, Ag+, NH4+, and Ca2+ by 18-crown-6. J. Am. Chem. Soc. 1977, 99, 3240. (6) Izatt, R. M.; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen, J. J.; Sen, D. Thermodynamic and kinetic data for cation-macrocycle interaction. Chem. Rev. 1985, 85, 271-339. (7) Moyer, B. A. Complexation and Transport. In Molecular Recognition: receptors for cationic guests; Gokel, G. W., Ed.; Elsevier Science Ltd.: Oxford, 1996; Vol. 1, pp 377-416. (8) Gokel, G. W.; Dishong, D. M.; Diamond, C. J. Lariat ethers. 2. The remarkable solvent dependence of binding constants in macrocyclic polyethers bearing secondary donor groups on flexible arms. J. Chem. Soc., Chem. Commun. 1980, 1053. (9) Gokel, G. W.; Trafton, J. E. Cation Binding by Lariat Ethers. In Cation Binding by Macrocycles; Inoue, Y., Gokel, G. W., Eds.; Marcel Dekker: New York, 1990; pp 253-310. (10) Creighton, T. E. Proteins: Structures and Molecular Properties, 2nd ed.; W. H. Freeman and Co.: New York, 1993. (11) Gokel, G. W. Lariat Ethers. Inclusion Phenomena; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Oxford University Press: Oxford, U.K., 1991; Vol. 4, pp 287-328. (12) (a) Huber, V. J.; Ivy, S. N.; Lu, J.; Bartsch, R. A. Lariat ethers with a novel proton-ionizable group. Synthesis and solvent extraction of alkali metal cations. Chem. Commun. 1997, 14991500. (b) Katsuta, S.; Kimura, T.; Kudo, Y.; Nakagawa, R.; Takeda, Y.; Ouchi, M. Extraction equilibria between benzene and water of uni- and bivalent metal picrates with lariat 16-crown-5: effect of the sidearms on the extraction ability and selectivity. J. Inclusion Phenom. Mol. Recognit. Chem. 1998, 31, 89-98. (c) Kita, K.; Kida, T.; Nakatsuji, Y.; Ikeda, I. Synthesis and complexation properties of C-pivot lariat ethers containing 16-crown-5, 19-crown-6 and 22crown-7 rings toward alkali metal cations. J. Chem. Soc., Perkin Trans. 1 1998, 22, 3857-3866. (d) Bartsch, R. A.; Hwang, H.-S.; Talanov, V. S.; Talanova, G. G.; Purkiss, D. W.; Rogers, R. D. New Proton-Ionizable Lariat Ethers with Picrylamino-Type Side Arms and Their Alkali Metal Salts. Synthesis and Structural Studies. J. Org. Chem. 1999, 64, 5341-5349. (13) (a) McKechnie, J. S.; Newton, M. G.; Paul, I. C. Crystal and molecular structure of a silver-bullvalene complex. J. Am. Chem. Soc. 1967, 89, 4819-25. (b) Ganis, P.; Dunitz, J. D. Structures of compounds with average size rings. XIV. Structure of silver nitrate-trans-cyclodecene adducts. Helv. Chim. Acta 1967, 50, 2379-86. (c) Robinson, D. J.; Kennard, C. H. Crystal structure of racemic geijerene-silver nitrate adduct. Chem. Commun. 1968, 914-15. (d) Michel, D.; Meiler, W.; Angele, E. Study of silver olefin complexes in solution by carbon-13 NMR spectroscopy. Z. Phys. Chem. (Leipzig) 1974, 255 (2), 389-94 (Chem. Abstr. 1974, 81, 119333). (14) See: Bessarabov, D. G.; Theron, J. P.; Sanderson, R. D. Novel application of membrane contactors: solubility measurements of 1-hexene in solvents containing silver ions for liquid olefin/paraffin separations. Desalination 1998, 115, 279-284 and references therein. (15) Burley, S. K.; Petsko, G. A. Amino-aromatic interactions in proteins FEBS 1986, 203, 139-143. (16) Kearney, P. C.; Mizoue, L. S.; Kumpf, R. A.; Forman, J. E.; McKurdy, A.; Dougherty, D. A. Molecular recognition in aqueous media. New binding studies provide further insights into the cation-π interaction and related phenomena J. Am. Chem. Soc. 1993, 115, 9907-9919. (17) McCurdy, A.; Jiminez, L.; Stauffer, D. A.; Dougherty, D. A. Biomimetic catalysis of SN2 reactions through cation-π
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3441 interactions. The role of polarizability in catalysis. J. Am. Chem. Soc. 1992, 114, 10314-10321. (18) (a) Kumpf, R. A.; Dougherty, D. A. Science 1993, 261, 1708-1710. (b) Mecozzi, S.; West, A. P., Jr.; Dougherty, D. A. Cation-π interactions in simple aromatics: electrostatics provide a predictive tool. J. Am. Chem. Soc. 1996, 118, 2307-2308. (c) Donini, O.; Weaver, D. F. Development of a modified force field for cation-amino acid interactions: Ab initio-derived empirical correction terms with comments on cation-π interactions J. Comput. Chem. 1998, 19, 1515-1525. (19) Sunner, J.; Nishizawa, K; Kebarle, P. Ion-solvent interactions in the gas phase. The potassium ion and benzene. J. Phys. Chem. 1981, 85, 1814-1820. (20) Cabarcos, O. M.; Weinheimer, C. J.; Lisy, J. M. Size selectivity by cation-π interactions: Solvation of K+ and Na+ by benzene and water. J. Chem. Phys. 1999, 110, 8429-8435. (b) Cabarcos, O. M.; Weinheimer, C. J.; Lisy, J. M. Competitive solvation of K+ by benzene and water: cation-π interactions and π-hydrogen bonds. J. Chem. Phys. 1998, 108, 5151-5154. (21) (a) Wouters, J. Cation-π (Na+-Trp) interactions in the crystal structure of tetragonal lysozyme. Protein Sci. 1998, 7, 2472-2475. (b) King, B. T.; Noll, B. C.; Michl, J. Cation-π interactions in the solid state: crystal structures of M + (benzene)2CB11Me12- (M ) Tl, Cs, Rb, K, Na) and Li+(toluene)CB11Me12-. Collect. Czech. Chem. Commun. 1999, 64, 1001-1012. (22) (a) Gatto, V. J.; Arnold, K. A.; Viscariello, A. M.; Miller, S. R.; Gokel, G. W. Novel synthetic access to 15- and 18-membered ring diaza-bibracchial lariat ethers (BiBLEs) and a study of sidearm-macroring cooperativity in cation binding. Tetrahedron Lett. 1986, 327-330. (b) Gatto, V. J.; Arnold, K. A.; Viscariello, A. M.; Miller, S. R.; Gokel, G. W. Syntheses and binding properties of bibrachial lariat ethers (BiBLEs): survey of synthetic methods and cation selectivities. J. Org. Chem. 1986, 51, 5373-5384. (23) Arnold, K. A.; Viscariello, A. M.; Kim, M.-S.; Gandour, R. A.; Fronczek, F. R.; Gokel, G. W. N,N′-Bis(substituted)-4,13-diaza18-crown-6 derivatives having π-donor-group-side arms. Correlation of thermodynamics and solid state structures. Tetrahedron Lett. 1988, 3025-3028. (24) Gatto, V. J.; Gokel, G. W. Syntheses of calcium-selective, substituted diazacrown ethers: a novel, one-step formation of bibracchial lariat ethers (BiBLES). J. Am. Chem. Soc. 1984, 106, 8240-8244. (25) De Wall, S. L.; Meadows, E. S.; Barbour, L. J.; Gokel, G. W. Solution- and solid-state evidence for alkali metal cation-π interactions with indole, the Side Chain of Tryptophan. J. Am. Chem. Soc. 1999, 121, 5613-5614. (26) De Wall, S. L.; Barbour, L. J.; Gokel, G. W. Cation-π complexation of potassium cation with the phenolic side chain of tyrosine. J. Am. Chem. Soc. 1999, 121, 8405-8406.
(27) (a) Fronczek, F. R.; Gatto, V. J.; Schultz, R. A.; Jungk, S. J.; Colucci, W. J.; Gandour, R. D.; Gokel, G. W. Unequivocal evidence for sidearm participation in crystalline lariat ether complexes. J. Am. Chem. Soc. 1983, 105, 6717. (b) Fronczek, F. R.; Gatto, V. J.; Minganti, C.; Schultz, R. A.; Gandour, R. D.; Gokel, G. W. Ester sidearm participation in a crystalline lariat ethersodium bromide complex. J. Am. Chem. Soc. 1984, 106, 7244. (c) Gandour, R. D.; Fronczek, F. R.; Gatto, V. J.; Minganti, C.; Schultz, R. A.; White, B. D.; Arnold, K. A.; Mazzochi, D.; Miller, S. R.; Gokel, G. W. Solid-state structural chemistry of lariat ether and BiBLE cation complexes: metal-ion identity and coordination number determine cavity size. J. Am. Chem. Soc. 1986, 108, 4078-4088. (d) Miller, S. R.; Cleary, T. P.; Trafton, J. E.; Smeraglia, C.; Fronczek, F. R.; Gokel, G. W. Tribracchial lariat ethers: syntheses, binding, and formation of an intramolecular macroring-sidearm complex in the absence of any cation. J. Chem. Soc., Chem. Commun. 1989, 806-808. (28) (a) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Thermodynamic and kinetic data for macrocycle interactions with cations and anions. Chem. Rev. 1991, 91, 1721-85. (b) Izatt, R. M.; Bradshaw, J. S.; Pawlak, K.; Bruening, R. L.; Tarbet, B. J. Thermodynamic and kinetic data for macrocycle interaction with neutral molecules. Chem. Rev. 1992, 92, 1261-354. (c) Chen, X.; Izatt, R. M.; Oscarson, J. L. Thermodynamic data for ligand interaction with protons and metal ions in aqueous solutions at high temperatures. Chem. Rev. 1994, 94, 467-517. (d) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Thermodynamic and kinetic data for macrocycle interaction with cations, anions, and neutral molecules. Chem. Rev. 1995, 95, 2529-86. (29) Arnold, K. A.; Echgoyen, L.; Gokel, G. W. Determination of thermodynamic parameters in lariat ether complexes using ionselective electrodes. J. Am. Chem. Soc. 1987, 109, 3713-3715. (30) (a) Bublitz, D. E.; Rinehart, K. L., Jr. Synthesis of substituted ferrocenes and other π-cyclopentadienyl-transition metal compounds. Org. React. 1969, 17, 1-154. (b) Bruce, M. I. Organometallic structures. Transition metals. Annual survey covering the year 1970. Organomet. Chem. Rev., Sect. B 1972, 10, 75-122. (31) Fritz, H. P.; Fischer, E. O. The spectra of bisbenzenemetal complexes in the far-infrared. J. Organomet. Chem. 1967, 7, 1217.
Received for review October 26, 1999 Accepted May 4, 2000
IE990780O
