Anion Binding Constants Research Articles

Relevance of the Cation in Anion Binding of a Triazole Host: An Analysis by Electrophoretic Nuclear Magnetic Resonance.

Triazole hosts allow cooperative binding of anions via hydrogen bonds, which makes them versatile systems for application in anion binding catalysis to be performed in organic solvents. The anion binding behavior of a tetratriazole host is systematically studied by employing a variety of salts, including chloride, acetate, and benzoate, as well as different cations. Classical nuclear magnetic resonance (1H NMR) titrations demonstrate a large influence of cation structures on the anion binding constant, which is attributed to poor dissociation of most salts in organic solvents and corrupts the results of classical titration techniques. We propose an approach employing electrophoretic NMR (eNMR), yielding drift velocities of each species in an electric field and thus allowing a distinction between charged and uncharged species. After the determination of the dissociation constants KD for the salts, electrophoretic mobilities are measured for all species in the host-salt system and are analyzed in a model which treats anion binding as a consecutive reaction to salt dissociation, yielding a corrected anion binding constant KA. Interestingly, dependence of KA on salt concentration occurs, which is attributed to cation aggregation with the anion-host complex. Finally, by the extrapolation to zero salt concentration, the true anion-host binding constant is obtained. Thus, the approach by eNMR allows a fully quantitative analysis of two factors that might impair classical anion binding studies, namely, an incomplete salt dissociation as well as the occurrence of larger aggregate species.

Anion Chelation via Double Chalcogen Bonding: The Case of a Bis-telluronium Dication and Its Application in Electrophilic Catalysis via Metal-Chloride Bond Activation.

Telluronium cations have long been known to engage their counteranions via secondary interactions. Yet, this property has rarely been exploited for anion binding. Motivated by such an application, we have now synthesized a bis-telluronium dication ([3]2+) that was obtained as a tetrafluoroborate salt by reaction of 2,7-di-tert-butyl-9,9-dimethylxanthene-4,5-diboronic acid with phenoxatellurine difluoride and BF3·OEt2. As confirmed by the formation of Te-(μ-BF4)-Te bridges in the structure of [3][BF4]2, [3]2+ functions as a bidentate Lewis acid toward anions. [3][BF4]2 has also been converted into the more exposed [3][BArF24]2 ([BArF24]- = [B(3,5-(CF3)2C6H3)4]-). The latter, which readily ionizes Ph3CCl, displays a chloride anion binding constant that exceeds that of a monofunctional model compound by almost 4 orders of magnitude. The unique properties of this new bis-telluronium dication are further highlighted by its ability to activate Ph3PAuCl and cis-(Ph3P)2PtCl2, leading to catalytic systems highly active in the cycloisomerization of propargylamide or enyne substrates.

The Coordination Chemistry of the N-Donor-Substituted Phosphazanes.

Phosph(III)azanes, featuring the heterocyclobutane P2 N2 ring, have now been established as building blocks in main-group coordination and supramolecular compounds. Previous studies have largely involved their use as neutral P-donor ligands or as anionic N-donor ligands, derived from deprotonation of amido-phosphazanes [RNHP(μ-NR)]2 . The use of neutral amido-phosphazanes themselves as chelating, H-bond donors in anion receptors has also been an area of recent interest because of the ease by which the proton acidity and anion binding constants can be modulated, by the incorporation of electron-withdrawing exo- and endo-cyclic groups (R) and by the coordination of transition metals to the ring P atoms. We observed recently that the effect of P,N-chelation of metal atoms to the P atoms of cis-[(2-py)NHP(μ-Nt Bu)]2 (2-py=2-pyridyl) not only pre-organises the N-H functionality for optimum H-bonding to anions but also results in a large increase in anion binding constants, well above those for traditional organic receptors like squaramides and ureas. Here, we report a broader investigation of ligand chemistry of [(2-py)NHP(μ-t NBu)]2 (and of the new quinolyl derivative [(8-Qu)NHP(μ-Nt Bu)]2 (8-Qu=8-quinolyl). The additional N-donor functionality of the heterocyclic substituents and its position has a marked effect on the anion and metal coordination chemistry of both species, leading to novel structural behaviour and reactivity compared to unfunctionalized counterparts.

Insights into the Complexity of Weak Intermolecular Interactions Interfering in Host-Guest Systems.

The recognition properties of heteroditopic hemicryptophane hosts towards anions, cations, and neutral pairs, combining both cation-π and anion-π interaction sites, were investigated to probe the complexity of interfering weak intermolecular interactions. It is suggested from NMR experiments, and supported by CASSCF/CASPT2 calculations, that the binding constants of anions can be modulated by a factor of up to 100 by varying the fluorination sites on the electron-poor aromatic rings. Interestingly, this subtle chemical modification can also reverse the sign of cooperativity in ion-pair recognition. Wavefunction calculations highlight how short- and long-range interactions interfere in this recognition process, suggesting that a disruption of anion-π interactions can occur in the presence of a co-bound cation. Such molecules can be viewed as prototypes for examining complex processes controlled by the competition of weak interactions.

Enhancement of Anion Binding in Lanthanide Optical Sensors

In the design of molecular sensors, researchers exploit binding interactions that are usually defined in terms of topology and charge complementarity. The formation of complementary arrays of highly cooperative, noncovalent bonding networks facilitates protein-ligand binding, leading to motifs such as the "lock-and-key". Synthetic molecular sensors often employ metal complexes as key design elements as a way to construct a binding site with the desired shape and charge to achieve target selectivity. In transition metal complexes, coordination number, structure and ligand dynamics are governed primarily by a combination of inner-sphere covalent and outer-sphere noncovalent interactions. These interactions provide a rich variable space that researchers can use to tune structure, stability, and dynamics. In contrast, lanthanide(III)-ligand complex formation and ligand-exchange dynamics are dominated by reversible electrostatic and steric interactions, because the unfilled f shell is shielded by the larger, filled d shell. Luminescent lanthanides such as terbium, europium, dysprosium, and samarium display many photophysical properties that make them excellent candidates for molecular sensor applications. Complexes of lanthanide ions act as receptors that exhibit a detectable change in metal-based luminescence upon binding of an anion. In our work on sensors for detection of dipicolinate, the unique biomarker of bacterial spores, we discovered that the incorporation of an ancillary ligand (AL) can enhance binding constants of target anions to lanthanide ions by as much as two orders of magnitude. In this Account, we show that selected ALs in lanthanide/anion systems greatly improve sensor performance for medical, planetary science, and biodefense applications. We suggest that the observed anion binding enhancement could result from an AL-induced increase in positive charge at the lanthanide ion binding site. This effect depends on lanthanide polarizability, which can be established from the ionization energy of Ln(3+) → Ln(4+). These results account for the order Tb(3+) > Dy(3+) > Eu(3+) ≈ Sm(3+). As with many lanthanide properties, ranging from hydration enthalpy to vaporization energy, this AL-induced enhancement shows a large discrepancy between Tb(3+) and Eu(3+) despite their similarity in size, a phenomenon known as the "gadolinium break". This discrepancy, based on the unusual stabilities of the Eu(2+) and Tb(4+) oxidation states, results from the half-shell effect, as both of these ions have half-filled 4f-shells. The high polarizability of Tb(3+) explains the extraordinarily large increase in the binding affinity of anions for terbium compared to other lanthanides. We recommend that researchers consider this AL-induced enhancement when designing lanthanide-macrocycle optical sensors. Ancillary ligands also can reduce the impact of interfering species such as phosphate commonly found in environmental and physiological samples.

Preorganization in bistriazolyl anion receptors

A series of 4,6-bis-(1,2,3-triazolyl)-pyrimidine and 4,6-bis-(1,2,3-triazolyl)-pyridine anion receptors were synthesized and the effect of the pyrimidine and pyridine moieties on their binding properties was examined. We found that intramolecular interactions preorganize the 4,6-bis-(1,2,3-triazolyl)-pyridine receptors resulting in higher anion binding constants in comparison with the nonpreorganized 4,6-bis-(1,2,3-triazolyl)-pyrimidine receptor.

A stimuli responsive system of self-assembled anion-binding Fe4L68+cages

A new cationic Fe4L6 cage molecule was synthesised from 4,4′-diaminobiphenyl, 2-formylpyridine and iron(II). The cage exists as a system of interconverting diastereomers in solution. The system adapts to the addition of anionic guest molecules, expressing a new combination of diastereomers that synergistically bind the guest molecules. Not only do the cage diastereomers interconvert, the volume of the individual cages adapts physically through the rotation of bonds, providing a tailored binding pocket for the guest lined with hydrogen-bond donors. A model for the resulting complex network of species was developed that allowed the system to be fully described. The anion binding constants and the kinetics of both diastereomer interconversion and guest exchange were measured.

Optical detection of anions using N-(4-(4-nitrophenylazo)phenyl)-N′-propyl thiourea bound silica film

The anion-sensing properties of N-(4-nitrophenylazo)-N′-propyl thiourea chromoionophores have been investigated as free agents in bulk solution and following immobilization on a sol–gel silica film. In addition, the effect of a variety of organic spacers on the immobilized chromoionophores was examined. The binding constants of free chromoionophores with anions were greater than those of the corresponding immobilized chromoionophores in acetonitrile solution and their binding constants revealed a trend were CH3CO2− > H2PO4− > Cl−. However, the use of an organic spacer with an immobilized chromoionophore can change the binding constants of anions. An octadecyl group spacer, for example, led to an increase in the difference between the binding constants of the acetate ion and other anions and also enhanced selectivity for the acetate ion.

From Helix to Macrocycle: Anion‐Driven Conformation Control of π‐Conjugated Acyclic Oligopyrroles

Anion-responsive pyrrole-based linear receptor oligomers were newly synthesized and their anion-driven dynamic conformation changes were investigated. Phenylene-bridged dimers and a tetramer of dipyrrolyldiketone boron complexes as π-conjugated acyclic anion receptors formed anion-driven helical structures in the solid and solution states. In fact, single-crystal X-ray analyses of the receptor-anion complexes exhibited various helical structures, such as [1+1]- and [1+2]-type single helices and a [2+2]-type double helix according to the lengths of oligomers and the existence of terminal aryl substituents. Anion-binding modes and behaviors of the oligomers in solution state were also examined by (1)H NMR and UV/Vis spectra along with ESI-TOF MS. Differences in the binding modes were observed in the solid and solution states. The oligomers showed augmented anion-binding constants and anion-tunable electronic and optical properties in comparison with the monomer receptor. A negative cooperative effect in the tetramer was observed in the second anion binding of the [1+2]-type single helix due to electrostatic repulsion between two anions captured in the helix. Further, an anion-template coupling reaction from the linear dimer provided a receptor macrocycle, which was obtained as a Cl(-) complex with distinct electronic and optical properties. The macrocycle exhibited extremely high anion-binding constants (>10(10) m(-1) in CH(2)Cl(2)) through multiple hydrogen bonding.

Design, Synthesis, Photophysics, and Anion-Binding Studies of Bis(dicyclohexylphosphino)methane-Containing Dinuclear Gold(I) Thiolate Complexes with Urea Receptors

A series of bis(dicyclohexylphosphino)methane (dcpm)-containing gold(I) thiolate complexes with urea receptors, 1-3, has been successfully designed and synthesized, and their photophysical and anion-binding properties have been studied. The linker between the thiolate and the urea group, and the electronic environment of the urea moiety, have been found to exert a great influence on the photophysical and anion-binding properties of the complexes. Complex 3 displays an intense long-lived orange-red luminescence at around 620 nm in the solid state and in the glass state at 77 K, which is considerably red shifted from the band seen in a solution of dichloromethane at room temperature, suggesting the presence of AuAu interactions. Upon introducing an electron-withdrawing NO(2) group, complex 1 was found to show high selectivity and sensitivity for F(-) through a drastic color change from yellow to red. The anion-binding constants of the complexes have been determined from electronic absorption and (1)H NMR spectroscopy titration studies and the data were found to fit well to a 1:1 binding model for the interactions between the complexes and the anions. Complexes 1 and 2 show the same anion selectivity trend of F(-) > AcO(-) > H(2)PO(4)(-) > Cl(-) approximately Br(-) approximately I(-), which is consistent with the trend in anion basicity. In addition, complex 1 has been shown to exhibit higher binding affinity for anions compared with those of complexes 2 and 3, probably due to the higher acidity of the urea moiety as a result of the introduction of the NO(2) group.

Recognition of Anions and Neutral Guests by Dicationic Pyridine-2,6-dicarboxamide Receptors

Dicationic N-methylated at pyridyl or quinolyl moieties derivatives of three isomers of N,N'-bis(pyridyl)pyridine-2,6-dicarboxamide (o-, m-, and p-1) and of N,N'-bis(3-quinolyl)pyridine-2,6-dicarboxamide (4) strongly bind anions in MeCN (log K in the range 3.5-6.5) with pronounced selectivity for Cl(-) and also bind neutral urea and amide guests with log K in the range 1.1-2.8. Crystal structures of the triflate salts of m-1, p-1, and 4 show that amide NH and pyridinium o-CH groups are directed inside the receptor cleft with their four protons forming a circle of radius ca. 2.35 A optimal for inclusion of Cl(-). The binding of anions to these protons is evident from the crystal structure of a mixed triflate/chloride salt of p-1, calculated (DFT/B3LYP 6-31G**) structures of 1:1 complexes of all receptors with Cl(-), and results of (1)H NMR titrations. In the crystal structure of o-1 pyridinium N-Me(+) groups are directed inside the receptor cleft impeding the anion complexation, but calculations demonstrate that simple rotation of pyridinium rings in opposite directions by ca. 30 degrees creates a cavity to which the Cl(-) ion can fit forming 4 hydrogen bonds to amide NH and aliphatic CH groups of N-Me(+). The results of (1)H NMR titrations confirm this type of binding in solution. Anions quench the intense fluorescence of 4, which allows their fluorescent sensing in the muM range. A new methodology for determination of anion binding constants to strongly acidic receptors by inhibitory effects of anions on the receptor deprotonation by an external base has been developed. High affinity and selectivity of anion complexation by dicationic pyridine-2,6-dicarboxamides is attributed to the rigid preorganized structure of receptors, the high acidity of NH and CH groups, and the electrostatic charge effect.

Anion Binding of N‐(o‐Methoxybenzamido)thioureas: Contribution of the Intramolecular Hydrogen Bond in the N‐Benzamide Moiety

N-(o-methoxybenzamido)thioureas (2 X/2 Y) are found to show an enhanced anion binding affinity with binding constants over 10(7) mol(-1) L orders of magnitude for AcO(-) and a redshifted absorption of the anion binding complexes in acetonitrile (MeCN) relative to those of N-benzamidothioureas (1) that bear no o-OMe in the N-benzamide moiety, despite the electron-donating character of o-OMe. Absorption of the anion-2 X/2 Y complex was shown to be of the same charge-transfer nature as that of the anion-1 complex, but its dependence on substituent X is interestingly influenced by the o-MeOHNC=O six-membered-ring intramolecular hydrogen bond identified in 2 X/2 Y. Such an intramolecular hydrogen bond is suggested to be responsible for the enhanced anion binding affinity. In the presence of this intramolecular hydrogen bond, the anion binding constant of 2 X was found to be independent of substituent X at the N-phenyl ring, as in the case of 1, whereas that of 2 Y showed an amplified dependence on substituent Y at the N'-phenyl ring, but to a lower extent than that of 1. A similar ring intramolecular hydrogen bond was purported to exist in 2 Za, 2 Zd, and 2 Ze, which bear NHMe, F, and Cl as the ortho substituent in the N-benzamide moiety. In terms of the current roles of thiourea in not only anion recognition and sensing but also organocatalysis and crystal engineering, the present finding would be of significance for a wider structural diversity of smart thiourea derivatives with predesigned functions.

N-(Acetamido)thiourea based simple neutral hydrogen-bonding receptors for anions

N-(Acetamido)-N'-phenylthioureas (4-6) were found to be efficient anion receptors with higher anion affinity than their N-benzamido-N'-phenylthiourea counterparts (1 and 2). The N'-phenylthiourea moiety in 4-6 was shown to be the chromophore with an absorption maximum at ca. 270 nm. It was found that, in the presence of anions, the absorption at ca. 270 nm of 4-6 (except 5f) in acetonitrile (MeCN) was blue shifted and enhanced while a red-shifted shoulder appeared at ca. 295 nm, together with an isosbestic point at ca. 240 nm. The 1:1 anion binding constants of 4-6, for example at 10(6)-10(7) M(-1) order of magnitude for AcO(-) in MeCN, were found to be higher than those of 1 and 2, although the acidity of the thioureido -NH protons in 4-6 is lower than that in 1 and 2. (1)H NMR data indicates that the N-N single bond in 4-6 is twisted but less than that in 1 and 2. A conformation change at the N-N single bond of 4-6 was suggested to occur upon anion binding which leads to a planar hydrogen-bonding network in the anion binding complex in which a charge transfer takes place with the N-acyl moiety being the electron acceptor. Variations in the CD signals of a proline derivative 6 bearing a chiral center in the N-amido moiety provide direct evidence for this conformation change upon its binding with anions in MeCN. The amplified effect of substituent X at the N'-phenyl ring of 5 on the anion binding constant supports the conclusion of anion-binding switched charge transfer in the anion binding complex. (1)H NMR and absorption titrations for 5 indicated that the anion-receptor interaction was of a hydrogen-bonding nature until the N'-phenyl substituent X is as electron-withdrawing as m-CF(3) (5e). With X being the more electron-withdrawing p-NO(2) (5f), deprotonation of the thioureido -NH occurs in the presence of anion. Results reported here confirm that N-amidothioureas derived from both N-aliphatic and N-aromatic amides can in general be a family of efficient hydrogen-bonding receptors, with the aliphatic N-amido derivatives being more efficient. This provides a wider structural diversity for designing thiourea-based functional molecules such as anion receptors and organocatalysts. Preliminary experiments confirm that 6 could catalyse efficiently the reduction of nitrostyrene in CH(2)Cl(2) and MeCN.

Modular Functional Integration of a Two-Input INH Logic Gate with a Fluorophore−Spacer−Receptor1−Spacer−Receptor2 Conjugate

The synthesis and photophysical characterization of a fluorophore-spacer-receptor 1-spacer-receptor 2 system, which combines the 1,8-naphthalimide fluorophore with amine and urea receptor units, is reported. Photoinduced electron transfer (PET) from the amino group was blocked by protonation, leading to a drastic fluorescence enhancement (ca. 20 times). Interaction of the urea receptor with anions (F (-), AcO (-), H 2PO 4 (-)) via hydrogen bonding or urea NH deprotonation resulted in significant fluorescence quenching of the 1,8-naphthalimide chromophore in an appropriately chosen model compound (ca. 30-45%). In the presence of both chemical input species, protons and anions, the fluorescence was also quenched. The binding of the anions by NH (+) ammonium receptor has been assumed. The apparent anion binding constants of the protonated conjugate follow the basicity trend of the anions: AcO (-) approximately F (-) > H 2PO 4 (-). The investigated system constitutes an example for the flexible and modular realization of functionally integrated INH logic at the molecular level, using protons and anions as chemical input species and the fluorescence of a PET-active signaling unit as output.

Enhanced Anion Binding of N-(Anilino)thioureas. Contribution of the N-Anilino −NH Proton Acidity

We report here that N-anilino-N'-phenythioureas in general function as a new family of thiourea-based efficient anion receptors superior to classical N-alkyl(aryl)thioureas, when the N-anilino-NH proton is acidic enough; that is, the N-phenyl substituent is not less electron-withdrawing than m-Cl. Changes due to anion binding in the absorption spectra of these N-anilinothioureas are much more substantial than those of N-alkyl(aryl)thioureas, and anion binding constants in MeCN, at 10(6)-10(7) mol(-1) L order of magnitude for AcO(-) for example, are much higher despite a similar acidity of the thioureido-NH protons. Crystal structure and (1)H NMR data show that the N-aniline chromophore is electronically decoupled from the thiourea anion binding site by the N-N bond, and an intramolecular hydrogen bond exists in MeCN but not in DMSO between the N-anilino-NH nitrogen atom and the other thioureido-NH proton. Conformation changes in the N-anilinothioureas upon anion binding were assumed to occur and lead to a much higher increment in the electron-donating ability in the N-aniline chromophore that the charge transfer (CT) is enhanced or switched on, compared to not switching on a CT in the case of N-phenylthioureas. The anion binding constant shows a stronger dependence on the N-phenyl substituent than on the N'-phenyl substituent, opposite to that observed with N-benzamidothioureas, and the CT band position of the anion binding complex depends much more on the N-phenyl substituent than that of the anion binding complexes of N-benzamidothioureas. The implications of these findings for new anion-receptors design and thiourea-based organocatalysts development are discussed.

Gelation is crucially dependent on functional group orientation and may be tuned by anion binding

The gelation ability of a series of chiral bis(urea) gels alternates between even and odd chain length and for the even numbered spacers the rheological characteristics can be tuned by the addition of anions according to the anion binding constant.

Highly effective electrochemical anion sensing based on oxoporphyrinogen

The effect of anion binding on the oxidation potential of an anion receptor, N 21, N 23-dibenzyl-5,10,15,20-(3,5-di- t-butyl-4-oxo-cyclohexa-2,5-dienylidene)porphyrinogen, 1 in o-dichlorobenzene is reported. The anion binding site of 1, at its inner pyrrolic amine hydrogens, is an integral part of the highly conjugated macrocycle, thus predicting larger potential shifts upon anion binding. Accordingly, cathodic shifts up to 600 mV are observed upon anion binding and such potential shifts correlate well with the anion binding constants.

Anion binding in (arene)ruthenium(ii)-based hosts

Asymmetric ruthenium(II) complexes of a flexible aminomethylpyridine derivative exhibit diastereotopic ligand methylene protons, as measured by NMR spectroscopy; binding of external anions renders these protons equivalent possibly by increasing dynamically averaged symmetry; the amount of anion needed to raise average symmetry correlates to the anion binding constant.

A unique NH-spacer for N-benzamidothiourea based anion sensors. Substituent effect on anion sensing of the ICT dual fluorescent N-(p-dimethylaminobenzamido)-N′-arylthioureas

A series of N-(p-dimethylaminobenzamido)-N'-(substituted-phenyl)thioureas (substituent = p-CH3, H, p-Cl, p-Br, m-Br, m-NO2, and p-NO2) were designed as anion sensors in order to better understand the -NH-spacer via a substituent effect investigation. In these molecules the dual fluorescent intramolecular charge transfer (ICT) fluorophore p-dimethylaminobenzamide as the signal reporter was linked to the anion-binding site, the thiourea moiety, via an N-N single bond. Correlation of the NMR signals of the aromatic and -NH protons with substituents in these molecules indicated that the N-N single bond stopped the ground-state electronic communication between the signal reporter and the anion-binding site. Dual fluorescence was observed in highly polar solvents such as acetonitrile with the former five derivatives. The fact that the CT emission wavelength and the CT to LE emission intensity ratio of the sensors were independent of the substituent existing in the anion-binding moiety suggested that the substituent electronic effect could not be communicated to the CT fluorophore in the excited-state either. Yet in acetonitrile both the CT dual fluorescence and the absorption of the sensors were found to be highly sensitive toward anions. A conformation change around the N-N bond in the sensor molecules was suggested to occur upon anion binding that established the electronic communication between the signal reporter and the anion-binding site. The anion binding constants of the N-(p-dimethylaminobenzamido)thiourea sensors were found higher than those of the corresponding traditional N-phenylthiourea counterparts and the substituent effect on the anion binding constant was much higher than that in the latter. "-NH-" was shown to be a unique spacer that affords N-benzamidothiourea allosteric anion sensors.

Uridine Binding by Zn(II) Macrocyclic Complexes: Diversion of RNA Cleavage Catalysts

Zn(II) complexes of 1-oxa-4,7,10-triazacyclododecane (12[ane]N3O), 1,5,9-triazacyclododecane (12[ane]N3), and 1-hydroxyethyl-1,4,7-triazacyclononane (9[ane]N3OH) promote cleavage of the RNA analogue, 2-hydroxypropyl-4-nitrophenyl phosphate (HpPNP) at pH 8.0, I=0.10 M (NaCl), 25 degrees C with second-order rate constants of 8.9x10(-3), 9.0x10(-3), and 3.3x10(-3) M-1 s-1, respectively. Cleavage of HpPNP by these catalysts is inhibited by uridine with inhibition constants (Ki) of 1.2, 0.46, and 45 mM, respectively, under these conditions. Binding constants derived from these inhibition constants are 2-200-fold larger than those for binding of related Zn(II) complexes to phosphate diesters under similar conditions, suggesting that uridine sequences in RNA will inhibit Zn(II)-catalyzed cleavage by competing with phosphate diester binding sites. Further studies are carried out that utilize pH-potentiometric titrations to monitor uridine binding to five Zn(II) macrocyclic complexes in aqueous solution at 25 degrees C, I=0.10 M (NaCl). The data are consistent with binding of the Zn(II) complexes to the N3-deprotonated form of uridine to give log KU.-values of 5.29, 4.57, 4.56, 3.47, and 2.65 for the Zn(II) complexes of 12[ane]N3, 12[ane]N4, 12[ane]N3O, 15[ane]N3O2, and 9[ane]N3OH, respectively (12[ane]N4=1,4,7,10-tetraazacyclododecane, 15[ane]N3O2=1,4-dioxa-7,10,13-triazacyclopentadecane). For the five Zn(II) complexes studied, there is a linear relationship between uridine anion binding constants and hydroxide binding constants.