Pseudocryptand Hosts for Paraquats and Diquats.

H-bonding interaction of acidic moieties (CH2OH, COOH) at the 5- and 5'-positions of bis(1,3-phenylene)-32-crown-10 (1) with di- or tritopic anions leads to enhanced formation of inclusion complexes with N,N'-dialkyl-4,4'-bipyridinium salts ("paraquats", 2); the enforced folding of the crown ethers into pseudocryptands thus leads to pseudo-pseudorotaxanes. Strikingly, in the presence of the most effective anion (trifluoroacetate, TFA), the apparent bimolecular association constants for crown-paraquat complexation increase by more than an order of magnitude and approach those for covalent cryptands derived from the crown ether. Even though they may form pseudocryptands, the picolinate, nicotinate, and isonicotinate diesters 6 of cis-(4,4')-bis(hydroxymethyl)dibenzo-30-crown-10 do not exhibit enhanced binding of either diquat or paraquat relative to the starting diol in contrast to the picolinate ester of isomeric 5,5'-bis(hydroxymethyl)bis(m-phenylene)-32-crown-10, which displayed a higher binding constant than the starting diol. The results for the analogous reverse esters 7 derived from cis-(4,4')-dicarboxydibenzo-30-crown-10 and pyridylmethanols reveal weaker complexes with diquat than the normal esters 6; however, surprisingly, two reverse esters 7 complex paraquat more strongly than isomers 6.

Complexation equilibria involving salts in non-aqueous solvents: ion pairing and activity considerations.

Complexation of anions, cations and even ion pairs is now an active area of investigation in supramolecular chemistry; unfortunately it is an area fraught with complications when these processes are examined in low polarity organic media. Using a pseudorotaxane complex as an example, apparent K(a2) values (=[complex]/{[salt](o)-[complex]}{[host](o)-[complex]}) for pseudorotaxane formation from dibenzylammonium salts (2-X) and dibenzo-[24]crown-8 (1, DB24C8) in CDCl(3)/CD(3)CN 3:2 vary with concentration. This is attributable to the fact that the salt is ion paired, but the complex is not. We report an equilibrium model that explicitly includes ion pair dissociation and is based upon activities rather than molar concentrations for study of such processes in non-aqueous media. Proper analysis requires both a dissociation constant, K(ipd), for the salt and a binding constant for interaction of the free cation 2(+) with the host, K(a5); K(a5) for pseudorotaxane complexation is independent of the counterion (500 M(-1)), a result of the complex existing in solution as a free cation, but K(ipd) values for the salts vary by nearly two orders of magnitude from trifluoroacetate to tosylate to tetrafluoroborate to hexafluorophosphate anions. The activity coefficients depend on the nature of the predominant ions present, whether the pseudorotaxane or the ions from the salt, and also strongly on the molar concentrations; activity coefficients as low as 0.2 are observed, emphasizing the magnitude of their effect. Based on this type of analysis, a method for precise determination of relative binding constants, K(a5), for multiple hosts with a given guest is described. However, while the incorporation of activity coefficients is clearly necessary, it removes the ability to predict from the equilibrium constants the effects of concentration on the extent of binding, which can only be determined experimentally. This has serious implications for study of all such complexation processes in low polarity media.

Competitive interactions of two ion-paired salts with a neutral host to form two non-ion-paired complexes.

It is demonstrated that our reported equilibrium treatments that take into account ion-paired guest and non-ion-paired complexes can be applied to competitive complexations. Satisfactory results were obtained for a system with two cationic guests [N,N'-dimethyl-4,4'-biyridinium bis(hexafluorophosphate) (1) and dibenzylammonium hexafluorophosphate (2)] having a common counterion and a single neutral host dibenzo-24-crown-8 (3), even though for this system one exchange process is slow and the other fast on the 1H NMR time scale. The competitive complexation protocol presented here provides a convenient method for the determination of KapKipd (the product of the ion-pair dissociation constant of the guest salt and the association constant for the host with the resultant free cation) for new systems from ion-paired guests that form complexes that are not ion paired.

Regioselective routes to disubstituted dibenzo crown ethers and their complexations.

Two isomers of bis(carbomethoxybenzo)-24-crown-8 (cis-BCMB24C8, 1, and trans-BCMB24C8, 2) were synthesized regiospecifically with acceptable to excellent yields. Cyclization in the presence of a template reagent, KPF(6), led to an essentially quantitative yield of the potassium complex of the crown ether 1; the isolated cyclization yield of pure was a remarkable 89%! The methods not only avoid the very difficult separation of the isomers, but also greatly shorten the synthesis time by eliminating syringe pump usage during cyclization. The complexations of the isomeric BCMB24C8 with dibenzylammonium hexafluorophosphate (10) were studied by NMR; association constants (Ka) for 1 and 2 with the dibenzylammonium cation are 190 and 312 M(-1), respectively. The X-ray crystal structures of crown ether and the complexes 1.KPF(6), 2.KPF(6) and pseudorotaxane 2.10 were determined.

Remarkably improved complexation of a bisparaquat by formation of a pseudocryptand-based [3]pseudorotaxane.

Significant improvement of complexation of a bisparaquat guest was achieved by the formation of a pseudocryptand-based [3]pseudorotaxane.

Formation of dimers of inclusion cryptand/paraquat complexes driven by dipole-dipole and face-to-face pi-stacking interactions.

Dimers of inclusion complexes were formed from a new cryptand and viologens (paraquats) driven by dipole-dipole and face-to-face pi-stacking interactions as shown by mass spectrometric characterization and X-ray analysis.

Ion pairing in fast-exchange host-guest systems: concentration dependence of apparent association constants for complexes of neutral hosts and divalent guest salts with monovalent counterions.

An equilibrium treatment of complexation of neutral hosts with dicationic guests having univalent counterions includes two possible modes: (1) dissociation of the ion pair prior to interaction of the free dication with the host to produce a complex that is not ion paired and (2) direct complexation of the ion pair to produce an ion paired complex. This treatment is easily modified for complexation of neutral guests by dianionic hosts, or divalent hosts by neutral guests. The treatment was tested by a study of fast-exchange host-guest systems based on paraquats or viologens (G(2+)2X(-)) and crown ethers (H). The bis(hexafluorophosphate) salts of viologens are predominantly ion paired in acetone; the value of the dissociation constant of paraquat bis(hexafluorophosphate) was determined to be 4.64 (+/- 1.86) x 10(-4) M(2). The complex based on dibenzo-24-crown-8 and paraquat bis(hexafluorophosphate) is not ion paired in solution, resulting in concentration dependence of the apparent association constant K(a,exp), (= [complex]/[H][G(2+)2X(-)]) which is well fit by the treatment, according to mode (1), yielding K(ap) = 106 (+/-42) M(-1). However, the four complexes of two different bis(m-phenylene)-32-crown-10 derivatives and bis(p-phenylene)-34-crown-10 with paraquat derivatives are all ion paired in solution and therefore K(a,exp) is not concentration dependent for these systems, mode (2). X-ray crystal structures support these solution-based assessments in that there is clearly ion pairing of the cationic guest with its PF(6)(-) counterions in the solid states of the latter four examples in which access of the counterions to the guests is granted by the relatively large cavities of the hosts and dispositions of the guest species within them.

Water assisted formation of a pseudorotaxane and its dimer based on a supramolecular cryptand.

Water acts as a "molecular clip" to form a supramolecular cryptand structure that improves complexation of a diammonium salt by pseudorotaxane formation, and leads to a novel dimer in the solid state.

Ion pairing and host-guest complexation in low dielectric constant solvents.

We report an equilibrium treatment for complexation of ionic species in low dielectric constant media that explicitly includes ion pairing of one of the components. Experimental validation was achieved through study of pseudorotaxane formation between dibenzylammonium salts and dibenzo-24-crown-8. In particular, we show that concentration-dependent fluctuations in the apparent K(a,exp) values as usually reported are attributable to ion pairing, with dissociation constant K(ipd), and that the constant K(ap) for complexation of the free cationic guest species, G(+), by the host crown ether is independent of counterion. More generally, using a simple extension of our model, we show the ability to diagnose the relative extent of ion pairing of the complex, which may be readily applied to other host-guest systems involving ionic species.

Crowned dendrimers: pH-responsive pseudorotaxane formation.

With the end goal of incorporating the unique structural and physical properties of dendrimers into supramolecular assemblies, bis(m-phenylene)-32-crown-10-functionalized poly(propyleneimine) dendrimers of the first and third generations have been synthesized and their interaction with paraquat diol has been investigated. Using (1)H NMR, we determined that binding to the 4 or 16 crown ether sites occurred in an anti-cooperative fashion, most likely a result of steric influences. Upon protonation of the tertiary amines in the dendritic interior, binding became independent, i.e., statistical, and the average apparent association constant increased by nearly 5-fold; this effect is attributed to rigidification of the dendrimer, which makes its binding sites more accessible and less crowded.

Supramolecular pseudorotaxane polymers from complementary pairs of homoditopic molecules.

Self-assembly of supramolecular pseudorotaxane polymers from complementary homoditopic building blocks comprised of bis(dibenzo-24-crown-8) esters derived from the hydroxymethyl crown ether and aliphatic diacid chlorides (CxC, x = number of methylene units in the diacid segment) and 1,10-bis[p-(benzylammoniomethyl)phenoxy]alkane bis(hexafluorophosphate)s (AyA, y = number of methylene units in the linker) has been studied. (1)H NMR spectroscopic studies of bis[(2-dibenzo-24-crown-8)methyl] sebacate (C8C) with dibenzylammonium hexafluorophosphate (6) showed that the two binding sites of the ditopic host are equivalent and independent (no positive or negative cooperativity). Likewise the binding sites in 1,10-bis[p-(benzylammoniomethyl)phenoxy]decane bis(hexafluorophosphate) (A10A) were shown to behave independently with dibenzo-24-crown-8 (1a). Then using (1)H NMR spectroscopy on dilute equimolar solutions (<1 mM) of CxC and AyA association constants were estimated for the formation of the linear (lin-CxC*AyA) and cyclic (cyc-CxC*AyA) dimers, thus enabling effective molarities to be estimated for the various systems. Finally (1)H NMR spectroscopy was used to semiquantitatively or qualitatively demonstrate the formation of linear supramolecular polymers lin-[CxC*AyA](n) in more concentrated solutions (up to 2.0 M) of the complementary pairs of CxC and AyA. The sizes of the assemblies (n values) are not as great as the dilute solution studies predict; this is attributed to the deleterious effect of ionic strength and exo complexation at high concentrations. However, as expected from the dilute solution results, linear extension is indeed favored with the longer building blocks, meaning that "monomer" end-to-end distance is a key factor in reducing the amount of cyclic species that form. Viscosity experiments clearly demonstrate the formation of large noncovalent polymers lin-[CxC*AyA](n) in concentrated solutions. Cohesive film and fiber formation also indicate that supramolecular polymers of sufficient size to enable entanglement self-assemble in these solutions.

Cooperative host/guest interactions via counterion assisted chelation: pseudorotaxanes from supramolecular cryptands.

Addition of di- or tritopic hydrogen bond accepting anions to solutions of bis(5-hydroxymethyl-1,3-phenylene)-32-crown-10 and paraquat di(hexafluorophosphate) serves to enhance host/guest interaction. In particular, addition of Et4N+CF3COO- effectively boosts Ka 14-fold, as estimated by 1H NMR studies. Similar increases in apparent Ka values are observed upon addition of n-Bu4N+OTs-. Evidenced by crystal structures, the increased association results from chelation of the OH moieties of the crown by the di- or tritopic anions, forming supramolecular bicyclic macrocycles and stabilizing the complex in a cooperative manner.

Cooperative self-assembly of dendrimers via pseudorotaxane formation from a homotritopic guest molecule and complementary monotopic host dendrons.

Interaction of the homotritopic guest 1,3,5-tris[p-(benzylammoniomethyl)phenyl]benzene tris(hexafluorophosphate) (1a) with dibenzo-24-crown-8 (DB24C8) leads to the sequential self-assembly of [2]-, [3]-, and [4]-pseudorotaxanes 7a, 8a, and 9a, respectively. The self-assembly processes were studied using NMR spectroscopy. In CD(3)CN and CD(3)COCD(3) the individual association constants K(1), K(2), and K(3) for 1:1, 1:2, and 1:3 complexes were determined by several methods. Via Scatchard plots, the three NH(2)(+) sites of 1a were shown to behave independently in binding DB24C8. K values (4.4 x 10(2), 1.4 x 10(2), and 41 M(-)(1), respectively, in CD(3)CN) directly determined from signals for the individual complexes (7a, 8a, and 9a) were somewhat higher than those estimated from the Scatchard plot because of concentration dependence, but the ratios of association constants followed the expected statistical order (K(1):K(2):K(3) = 3:1:(1)/(3)). These are believed to be the first evaluations of association constants leading to a [4]-pseudorotaxane. In the less polar CDCl(3), association constants could not be determined because approximately 90% of the dissolved tritopic guest, which by itself is insoluble, was present as the fully loaded [4]pseudorotaxane 9a! Self-assembly of homotritopic guest 1a with benzyl ether dendrons of the first, second, and third generations functionalized at the "focal point" with DB24C8 moieties (3-5) produces pseudorotaxane dendrimers. The self-assembly processes were studied using (1)H NMR spectroscopy. In CD(3)COCD(3) for all three generations the individual association constants K(1), K(2), and K(3) for [2]-, [3]-, and [4]-pseudorotaxane complexes 7c-e, 8c-e, and 9c-e indicated that the self-assembly was cooperative; that is, the ratios of the individual association constants exceeded the expected statistical ratios. Scatchard plots confirmed this behavior. Self-assembly processes in the less polar CDCl(3) were kinetically slow, requiring ca. 1, 2, and 3 days, respectively, for the first, second, and third generation systems to reach equilibrium with 1a; the slow rate is attributed to the insolubility of the homotritopic guest 1a in this medium and the steric demands of the resulting dendrimers. However, only dendrimers of 1:3 stoichiometry, that is, the nanoscopic [4]pseudorotaxanes 9, were formed! Moreover, it is noteworthy that the extent of dissolution of 1a (reflective of the overall association constant which is too high to measure) increases with generation number, presumably because of the more effective screening of the ionic guest by the larger dendrons and perhaps favorable pi-pi and CH-pi interactions. Such cooperative effects suggest a number of applications that can take advantage of the pH-switchable nature of these self-assembly processes.