Anion Coordination Chemistry E-Book
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- Herausgeber: Wiley-VCH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Building on the pioneering work in supramolecular chemistry from the last 20 years or so, this monograph addresses new and recent
approaches to anion coordination chemistry. Synthesis of receptors, biological receptors and metallareceptors, the energetics of anion binding, molecular structures of anion complexes, sensing devices are presented and computational studies addressed to aid with the understanding of the different driving forces responsible for anion complexation. The reader is promised an actual picture of the state of the art for this exciting and constantly evolving field of supramolecular anion coordination chemistry. The topics range from ion channels to selective
sensors, making it attractive to all researchers and PhD students with an interest in supramolecular chemistry.
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Veröffentlichungsjahr: 2012
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Table of Contents
Related Titles
Title Page
Copyright
Preface
List of Contributors
Chapter 1: Aspects of Anion Coordination from Historical Perspectives
1.1 Introduction
1.2 Halide and Pseudohalide Anions
1.3 Oxoanions
1.4 Phosphate and Polyphosphate Anions
1.5 Carboxylate Anions and Amino Acids
1.6 Anionic Complexes: Supercomplex Formation
1.7 Nucleotides
1.8 Final Notes
References
Chapter 2: Thermodynamic Aspects of Anion Coordination
2.1 Introduction
2.2 Parameters Determining the Stability of Anion Complexes
2.3 Molecular Recognition and Selectivity
2.4 Enthalpic and Entropic Contributions in Anion Coordination
References
Chapter 3: Structural Aspects of Anion Coordination Chemistry
3.1 Introduction
3.2 Basic Concepts of Anion Coordination Chemistry
3.3 Classes of Anion Hosts
3.4 Acycles
3.5 Monocycles
3.6 Cryptands
3.7 Transition-Metal-Assisted Ligands
3.8 Lewis Acid Ligands
3.9 Conclusion
Acknowledgments
References
Chapter 4: Synthetic Strategies
4.1 Introduction
4.2 Design and Synthesis of Polyamine-Based Receptors for Anions
4.3 Design and Synthesis of Amide Receptors
References
Chapter 5: Template Synthesis
5.1 Introductory Remarks
5.2 Macrocyclic Systems
5.3 Bowl-Shaped Systems
5.4 Capsule, Cage, and Tube-Shaped Systems
5.5 Circular Helicates and meso-Helicates
5.6 Mechanically Linked Systems
5.7 Concluding Remarks
References
Chapter 6: Anion–π Interactions in Molecular Recognition
6.1 Introduction
6.2 Physical Nature of the Interaction
6.3 Energetic and Geometric Features of the Interaction Depending on the Host (Aromatic Moieties) and the Guest (Anions)
6.4 Influence of Other Noncovalent Interactions on the Anion–π Interaction
6.5 Experimental Examples of Anion–π Interactions in the Solid State and in Solution
6.6 Concluding Remarks
References
Chapter 7: Receptors for Biologically Relevant Anions
7.1 Introduction
7.2 Phosphate Receptors
7.3 Carboxylate Receptors
7.4 Conclusion
References
Chapter 8: Synthetic Amphiphilic Peptides that Self-Assemble to Membrane-Active Anion Transporters
8.1 Introduction and Background
8.2 Biomedical Importance of Chloride Channels
8.3 The Development of Synthetic Chloride Channels
8.4 Approaches to Synthetic Chloride Channels
8.5 The Development of Amphiphilic Peptides as Anion Channels
8.6 Structural Variations in the SAT Modular Elements
8.7 Conclusions
Acknowledgments
References
Chapter 9: Anion Sensing by Fluorescence Quenching or Revival
9.1 Introduction
9.2 Anion Recognition by Dynamic and Static Quenching of Fluorescence
9.3 Fluorescent Sensors Based on Anthracene and on a Polyamine Framework
9.4 Turning on Fluorescence with the Indicator Displacement Approach
References
Index
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The Editors
Prof. Dr. Kristin Bowman-James
Department of Chemistry
University of Kansas
1251 Wescoe Hall Drive
Lawrence, KS 66045
USA
Prof. Dr. Antonio Bianchi
University of Florence
Department of Chemistry
Via della Lastruccia 3
50019 Sesto Fiorentino
Italy
Prof. Dr. Enrique García-España
Instituto de Química Molecular
Departamento de Química Inorgánica
C/ Catedrático José Beltrán 2
46980 Paterna (Valencia)
Spain
The photograph of Professor Bowman-James on the back cover of the book was kindly supplied by David F. McKinney/KU University Relations © 2011 The University of Kansas/Office of University Relations.
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Preface
While Park and Simmons provided the first seminal report of the supramolecular chemistry of anions in 1968, it was Jean-Marie Lehn who suggested in 1978 that it was truly a form of coordination chemistry. At that time supramolecular chemistry, which refers to the interactions of molecular and ionic species beyond the covalent bond, was in its formative years. The term supramolecular chemistry was built on the lock and key concept first proposed by Emil Fischer in 1894. The actual term, however, was coined by Jean-Marie Lehn at the early stages of the development of this field. In many respects this concept can be merged with another key chemical concept, that of coordination chemistry, also introduced in the late nineteenth century by Alfred Werner. All three men, Fischer, Lehn, and Werner, were recognized for their seminal contributions to science with Nobel Prizes.
As pointed out in Chapter 1, anions were of interest to chemists as early as the 1920s. Yet in the early years of supramolecular chemistry, the focus on anions began only as a small seedling that has now grown into a giant tree with many branches. Anion coordination chemistry now impinges on numerous fields of science, including medicine, environmental remediation, analytical sensing, as well as many aspects of the global field of nanotechnology. Scientists from all areas of chemistry and beyond have joined forces to explore this exciting new field.
By the early 1990s, there were a number of texts devoted to various aspects of supramolecular chemistry, but none that focused entirely on anions. At that time the three of us realized the need for such a text, and we gathered the expertise of anion researchers far and wide to contribute to the book that was published in 1997, Supramolecular Chemistry of Anions. Since that time a small number of excellent texts and many reviews have been published, focusing on anions and reporting advances in this rapidly evolving field. In this sequel to our earlier text, using the same strategy of enlisting the aid of noted scientists in the field, we have tried to incorporate some of the imagination and excitement that has gone into the science of anions in the last 15 years. The chapters are laid out in a manner similar to that in our first volume, covering basic topics in anion coordination. Chapter 1 approaches the historical development of anion chemistry from a slightly different viewpoint than usual, by covering both biological and supramolecular developments. It is followed by two chapters outlining what we consider to be the core foundations of anion coordination, thermodynamic and structural aspects. Synthetic aspects of some of the more commonplace receptors are reviewed in Chapter 4. The following two chapters explore some of the more recent and exciting aspects that illustrate the growth of the field: the use of anions as synthetic templates in Chapter 5 and anion-π interactions in Chapter 6. Chapters 7 and 8 focus on biological implications of anions and include an overall view of hosts for biologically relevant anions and receptors designed for membrane transport, respectively. The book concludes with a chapter exploring an important application of anion coordination, sensors for anions.
This book has been possible only because of the outstanding scientists who have contributed exceptionally well-written chapters. We extend our warm thanks for the time and effort that they have dedicated to this process. We would also like to thank the many funding agencies worldwide that have made this research possible. K.B.-J would like to express appreciation to the National Institutes of Health and the Department of Energy, and especially the National Science Foundation grant CHE CHE0809736 for the current funding. EGE thanks the Spanish Ministry of Science and Innovation and Science (MCINN), Projects CONSOLIDER CSD 2010-00065, CTQ 2009-14288-C04-01 and Generalidad Valenciana (GVA), project Prometeo 2011/008.
Last but not least, we would like to take this opportunity to acknowledge our families, research groups, and students. Our families have provided patience and encouragement throughout the making of this book. Our students and other researchers in our groups have made significant contributions to some of the science reported here. We would also like to thank the many researchers in the anion community who have conducted the outstanding science that has now become part of this book.
Lawrence, Kansas, USA
Kristin Bowman-James
Florence, Italy
Antonio Bianchi
Valencia, Spain
Enrique García-España
List of Contributors
1
Aspects of Anion Coordination from Historical Perspectives
Antonio Bianchi, Kristin Bowman-James, and Enrique García-España
1.1 Introduction
Supramolecular chemistry, the chemistry beyond the molecule, gained its entry with the pioneering work of Pedersen, Lehn, and Cram in the decade 1960–1970 [1–5]. The concepts and language of this chemical discipline, which were in part borrowed from biology and coordination chemistry, can be to a large extent attributed to the scientific creativity of Lehn [6–8]. Recognition, translocation, catalysis, and self-organization are considered as the four cornerstones of supramolecular chemistry. Recognition includes not only the well-known transition metals (classical coordination chemistry) but also spherical metal ions, organic cations, and neutral and anionic species. Anions have a great relevance from a biological point of view since over 70% of all cofactors and substrates involved in biology are of anionic nature. Anion coordination chemistry also arose as a scientific topic with the conceptual development of supramolecular chemistry [8]. An initial reference book on this topic published in 1997 [9] has been followed by two more recent volumes [10, 11] and a number of review articles, many of them appearing in special journal issues dedicated to anion coordination. Some of these review articles are included in Refs [12–52]. Very recently, an entire issue of the journal Chemical Society Reviews was devoted to the supramolecular chemistry of anionic species [53]. Since our earlier book [9] the field has catapulted way beyond the early hosts and donor groups. Because covering the historical aspects of this highly evolved field would be impossible in the limited space here, a slightly different approach will be taken in this chapter. Rather than detail the entry of the newer structural strategies toward enhancing anion binding and the many classes of hydrogen bond donor groups that have come into the field, only the earlier development will be described. This will be linked with aspects of naturally occurring hosts, to provide a slightly different perspective on this exciting field.
Interestingly enough, the birth of the first-recognized synthetic halide receptors occurred practically at the same time as the discovery by Charles Pedersen of the alkali and alkaline-earth complexing agents, crown ethers. While Pedersen submitted to JACS (Journal of the American Chemical Society) his first paper on crown ethers in April 1967 entitled “Cyclic Polyethers and their Complexes with Metal Salts” [1], Park and Simmons, who were working in the same company as Pedersen, submitted their paper on the complexes formed by bicyclic diammonium receptors with chloride entitled “Macrobicyclic Amines. III. Encapsulation of Halide ions by in, in-1, (k + 2)-diazabicyclo[k.l.m]alkane-ammonium ions” also to JACS in November of the same year [54].
These cage-type receptors (1-4) were called katapinands, after the Greek term describing the swallowing up of the anionic species toward the interior of the cavity (Figure 1.1). The engulfing of the chloride anion inside the katapinand cavity was confirmed years later by the X-ray analysis of the structure of Cl− included in the [9.9.9] bicyclic katapinad [55]. However, while investigations on crown ethers rapidly evolved and many of these compounds were prepared and their chemistry widely explored, studies on anion coordination chemistry remained at the initial stage. Further development waited until Lehn and his group revisited this point in the late 1970s and beginning of the 1980s [56–62].
Figure 1.1 In–in and out-out equilibria, and halide complexation in katapinand receptors.
Nevertheless, evidence that anions interact with charged species, modifying their properties, in particular their acid–base behavior, was known from the early times of the development of speciation techniques in solution, when it was noted that protonation constants were strongly influenced by the background salt used to keep the ionic strength constant [63]. Following these initial developments, Sanmartano and coworkers did extensive work on the determination of protonation constants in water with and without using ionic strength. In this way, this research group was able to measure interaction constants of polyammonium receptors with different anionic species [64, 65]. Along this line, Martell, Lehn, and coworkers reported an interesting study in which the basicity constants of the polyaza tricycle (5) were determined by pH-metric titrations using different salts to keep the ionic strength constant [66]. The authors observed that while the use of KClO4 did not produce significant differences in the constants with respect to the supposedly innocent trimethylbenzene sulfonate anion (TMBS), the use of KNO3 and KCl led to higher pKa values, particularly as more acidic conditions were reached. From these titrations, binding constants of nitrate and chloride with hexaprotonated 5 were determined to be 2.93 and 2.26 logarithmic units, respectively.
Similar events were observed in the biological world many years ago. The well-known Hofmeister series or lyotropic series [67] was postulated at the end of the nineteenth century to rank the relative influence of ions on the physical behavior of a wide variety of processes ranging from colloidal assembly to protein folding. The Hofmeister series, which is more pronounced for anions than for cations, orders anions in the way shown in Figure 1.2. The species to the left of Cl− are called kosmotropes, “water structure makers,” and those to the right of chloride are termed chaotropes, “water structure breakers.” While the kosmotropes are strongly hydrated and have stabilizing and salting-out effects on proteins and macromolecules, the chaotropes destabilize folded proteins and have a salting-in behavior.
Figure 1.2 Representation of the Hofmeister series.
Although originally these ion effects were attributed to making or breaking bulk water structure, more recent spectroscopic and thermodynamic studies pointed out that water structure is not central to the Hofmeister series and that macromolecule–anion interactions as well as interactions with water molecules in the first hydration shell seem to be the key point for explaining this behavior [68–72].
In this respect, as early as in the 1940s and 1950s, researchers sought to address the evidence and interpret the nature of the binding of anions to proteins [73]. Colvin, in 1952 [74], studying the interaction of a number of anions with the lysozyme, calf thymus histone sulfate, and protamine sulfate proteins using equilibrium dialysis techniques, concluded that although electrostatic charge–charge interactions may be chiefly responsible for the negative free energy of binding, there were other contributions such as van der Waals and solvation energies that can equal or even exceed the charge to charge component.
Figure 1.3 Scheme of the active sites of PBP (a) and SBP (b).
Another bacterial protein whose crystal structure has revealed interesting binding motifs to anions is haloalkane dehydrogenase, which converts 1-haloalkanes or α, ω-haloalkanes into primary alcohols and a halide ion by hydrolytic cleavage of the carbon–halogen bond with water as a cosubstrate and without any need for oxygen or cofactors [77]. The crystal structure of the dehalogenase with chloride as the product of the reaction shows that the halide is bound in the active site through four hydrogen bonds involving the N ε of the indole moieties of two tryptophan residues, the Cα of a proline, and a water molecule (Figure 1.4).
Figure 1.4 Schematic view of the interactions occurring in the active site of dehalogenase: (a) with the substrate before the start of the reaction, (b) with the alkyl intermediate and the chloride ion during the reaction, and (c) with the chloride ion and water molecules after hydrolysis.
One of the most important characteristics of anions is their Lewis base character. Therefore, compounds possessing suitable Lewis acid centers can be appropriate anion receptors. Several families of boranes, organotin, organogermanium, mercuroborands, acidic silica macrocycles, and a number of metallomacrocycles have been shown to display interesting binding properties with anions. Examples of this chemistry are included in Figures 1.5 and 1.6 and Refs [78–94].
Figure 1.5 ORTEP diagram of the fluoride complex of a boron–silicon receptor.
Taken from Ref. [85].
Figure 1.6 Reaction of FeCl2 and a tris-bipyridine ligand gives rise to a double helix with the chloride as a template [94].
Anion coordination chemistry and classical metal coordination chemistry have an interface in mixed metal complexes with exogen anionic ligands. Indeed, most of the ligands are anionic species belonging to groups 15–17 of the periodic table. Metal complexes can express their Lewis acid characteristics if they are coordinatively unsaturated or if they have coordination positions occupied by labile ligands that can be easily replaced. If this occurs, metal complexes are well suited for interacting with additional Lewis bases, which are very often anionic in nature, giving rise to mixed complexes. Mixed complexes in which the anionic ligand bridges between two or more different metal centers have been termed, in the new times of supramolecular chemistry, “cascade complexes” [95].
Formation of mixed complexes is the strategy of choice of many metalloenzymes dealing with the fixation and activation of small substrates. A classic example is the family of enzymes called carbonic anhydrases [96–98]. Carbonic anhydrases are ubiquitous enzymes that catalyze the hydration reaction of carbon dioxide and play roles in processes such as photosynthesis, respiration, calcification and decalcification, and pH buffering of fluids. Human carbonic anhydrase II (HCA II) is located in the erythrocytes and is the fastest isoenzyme accelerating CO2 hydrolysis by a factor of 107. Therefore, it is considered to be a perfectly evolved system, its rate being controlled just by diffusion. The active site of HCA II is formed by a Zn2+ cation coordinated to three nitrogen atoms from histidine residues and to a water molecule that is hydrogen bonded to a threonine residue and to a “relay” of water molecules that interconnects the coordination site with histidine 64 (Figure 1.7). The pKa of the coordinated water molecule in this environment is circa 7, so that at this pH, 50% is hydroxylated as Zn-OH−, thus generating a nucleophile that will attack CO2 to give the HCO3− form.
Figure 1.7 Schematic representation of the active site of HCA II showing the tetrahedral arrangement of three histidine residues and a water molecule.
The rate-determining step is precisely the deprotonation of the coordinated water molecule and the transfer of the proton through the chain of water molecules to His64, which assists the process.
Phosphatases are the enzymes in charge of the hydrolysis of phosphate monoesters. Metallophosphatases contain either Zn2+ or Fe3+ or both; one of their characteristics is the presence of at least two metal ions in the active site. Escherichia coli alkaline phosphatase contains two Zn2+ and one Mg2+ metal ions in the active center. In the first step of the catalytic mechanism, the phosphate group of the substrate interacts as a bridging η,η′-bis(monodentate) ligand through two of its oxygen atoms with the two Zn2+ ions, while its other two oxygen atoms form hydrogen bonds with an arginine residue rightly disposed in the polypeptide chain (Figure 1.8).
Figure 1.8 Active site of alkaline phosphatases.
Adapted from Ref. [99, 100].
A last example that we would like to recall is ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), which is the most abundant enzyme in nature [101]. Rubisco is a magnesium protein that is present in all the photosynthetic organisms participating in the first stage of the Calvin cycle. A lysine residue interacts with CO2, forming an elusive carbamate bond, which is stabilized by interaction with the Mg2+ ion and by a hydrogen bond network with other groups of the polypeptidic chain (Figure 1.9). The ternary complex formed interacts with the substrate, which is subsequently carboxylated.
Figure 1.9 Active site of the enzyme rubisco.
Adapted from Ref. [100].
In all these examples, anionic substrates bind (coordinate) to a metal ion in key steps of their catalytic cycles, which assists the process as a Lewis acid.
1.2 Halide and Pseudohalide Anions
Having all these points in mind, there is no doubt that the birth of supramolecular anion coordination chemistry as an organized scientific discipline can be traced back to the work started by Lehn and coworkers in the mid-1970s. The first seminal paper of Lehn's group dealt with the encapsulation of halide anions within tricyclic macrocycles 5–7 [56]. The parent compound of the series 5, already mentioned in the previous section, which is known as the soccer ball ligand in the jargon of the field, had been synthesized one year in advance by the same authors [102].
The authors started this paper stating that “Whereas very many metal cation complexes are known, stable anion complexes of organic ligands are very rare indeed.” By means of 13C NMR, the authors proved the inclusion of F−, Cl−, and Br− within the macrotricyclic cavity at the time when they found a remarkable Cl−/Br− selectivity in water of circa 1000.
No interaction was observed with the larger I− and with the monovalent anions NO3−, CF3COO−, and ClO4−. The crystal structure of [Cl− ⊂ H4(1)4+], where the mathematical symbol ⊂ stands for inclusive binding, shows that chloride was held within the tetraprotonated macrocycle by an array of four hydrogen bonds with the ammonium groups [103]. Years later, Lehn and Kintzinger, in collaboration with Dye and other scientists from the Michigan State University, used 35Cl NMR to study the interaction of halide anions with 5, 6, and several related polycycles [61].
This premier study on spherical anion recognition was followed by the work performed in Munich by Schmidtchen, who described the synthesis of a quaternized analog of 5 (receptor 8) [104]. In the same paper, similar macrocycles with hexamethylene and octamethylene bridges connecting the quaternary ammonium groups placed at the corners of the polycycles were also reported (9 and 10).
These azamacropolycycles, whose binding ability does not depend on pH, show modest affinity for halide anions in water. In the case of 9 and 10, selectivity for bromide and iodide over chloride was found. However, binding is clearly weaker than when auxiliary hydrogen bonding can occur. The crystal structure of an iodide complex with 9, having hexamethylene bridges, confirmed the inclusion of the anion in the macrotricyclic cavity [105] (Figure 1.10). This series expanded over a wide range of studies illustrating the conceptual utility of these systems for understanding the kinds of binding forces involved in anion coordination [106–118].
Figure 1.10 Views of the inclusion complex of I− into the cavity of 9.
Recognition of fluoride came up a little bit later, probably because of the higher difficulties in binding this anion in aqueous solution, which are associated with its high hydration energy in comparison to the other halides. In this respect, it has to be emphasized that most of the pioneering studies in anion coordination were carried out in water. The first stable fluoride complex was obtained with the bicyclic cage nicknamed O-BISTREN (11) [119].
However, as illustrated in Figure 1.11 [120], the fitting of fluoride within the cavity was not very snug. The anion sits off-center, forming hydrogen bonds with just four of the six ammonium groups of the macrocycle. Consequently, although higher constants were found for the interaction of fluoride with [H6(11)]6+, the selectivity over the other halides, Cl−, Br−, and I−, was not very large (log K 4.1, 3.0, 2.6, and 2.1 for F−, Cl−, Br−, and I−, respectively). In Figure 1.12, it can be seen that chloride fits more tightly into the cavity of 11. In this case, hydrogen bonds are formed between the encapsulated anion and all six ammonium groups of the cryptand, although some of them are relatively weak.
Figure 1.11 Views of F− included in the molecular cavity of hexaprotonated 11 showing the mismatch in size. Hydrogen atoms have been omitted.
Figure 1.12 View of Cl− included in the molecular cavity of hexaprotonated 11. Hydrogen atoms have been omitted.
With respect to fluoride binding, it is worth mentioning that, in 1984, a report by Suet and Handel appeared, describing the ability of different monocyclic tetraazamacrocycles with propylenic and butylenic chains (12–14) to bind this anion in aqueous solution [121]. The stability constants found for the interaction of fluoride with the tetraprotonated forms of 12, 13, and 14 were 1.9, 2.0, and 2.8 logarithmic units, respectively.
In order to obtain a more selective F− binder, the macrocycle C2-BISTREN was prepared by Lehn and coworkers [122] several years later (15) [123].
Figure 1.13 Views of the F− anion included in C2-BISTREN (15).
The success in obtaining a good fluoride-selective receptor led to the modification of the structure of 15 to obtain receptors that could match the size of the larger halides, Cl−, Br−, and I− (compounds 16 and 17) [123, 125, 126]. However, the results obtained, although pointing in the desired direction, did not show any particularly relevant selectivity. Receptor 18 (C5-BISTREN) prepared by Lehn's group was studied along with 11 by potentiometry in collaboration with Martell and coworkers. Such studies, and the crystal structure of the azide complex [57], gave the first indications of the possibility of the formation of binuclear or higher nuclear anionic complexes with two encapsulated anions or, even better, with the hydrogen bifluoride anion (HF2−) (see below for further developments) [127, 128]. The constants for the equilibrium between the hexaprotonated receptor and HF2− were 6.4 and 5.2 logarithmic units for 11 and 18, respectively.
O-BISTREN (11) was also the first synthetic receptor for which a crystal structure with an included N3− was solved by X-ray crystallography. The azide anion fits perfectly along the internal cavity of the receptor, forming each of its terminal nitrogen hydrogen bonds with the three ammonium groups of each of the two tren polyamine subunits of the cage (Figure 1.14) [57].
Figure 1.14 Azide anion included in the cavity of receptor 11.
Fortunately and curiously, this structure and the previously discussed one for fluoride, which have proved to be crucial for the development of the field of anion coordination, were accepted for publication in spite of having R factors of 16.2 and 19.8, respectively.
Since these initial findings, many efforts have been devoted to halide recognition with different types of receptors. Among polyammonium receptors, probably the most used have been cryptands obtained by 2 + 3 condensation of the tripodal polyamine tren and the corresponding aromatic dialdehydes followed by in situ reduction with an appropriate reducing species, often NaBH4 (19–23). One of the reasons for the large amount of work performed with these receptors is the readiness of its synthesis, which is much more straightforward than those required for preparing cages with aliphatic linkers between the tren subunits. These latter preps often require tedious protection and deprotection steps in addition to high dilution methods. Examples are receptors 19–23, which are abbreviated as MEACryp, pyridine azacryptand (PyEACryp), PEACryp, FuEACryp, and ThioEACryp following the short names proposed by the late Robert W. Hay from St. Andrews University [129].
Nelson [21], Bowman-James [20, 129], Fabbrizzi [19], and Ghosh [130], among others, have contributed extensively to this chemistry. Perhaps, one of the most interesting developments in this topic has been the crystallographic evidence that this kind of receptors can lodge two halide anions when they are extensively protonated. Figures 1.15 and 1.16 show the crystal structures of hexaprotonated 21 with two fluorides and a water molecule bridging between them forming an anion “cascade complex” [131], and presumably of a bichloride Cl–H–Cl anion included in hexaprotonated 19 [132].
Figure 1.15 View of the fluoride-water cascade complex of [H621]6+.
Figure 1.16 Presumed bichloride complex of hexaprotonated 19, bridging hydrogen not shown.
Also, in a rather early publication in the field, crystallography was used to prove the almost total inclusion of three bromide anions within the cavity of 19 MEACryp [133] (Figure 1.17).
Figure 1.17 View of three bromides partially included in receptor 19.
The polyazacryptands that impose rigidity are a corollary to 1,3,5 trisubstituted benzenes with bridges containing amines (24–26). This class of compounds, prepared by Lehn's group [134, 135] for the first time, form 1 : 1 halide:receptor complexes with a significant stability in water. Recently, crystal structures of anionic complexes have been obtained for a similar receptor 27 developed by Steed et al. [136] (Figure 1.18).
Figure 1.18 Halide anion included in hexaprotonated 27 (a) F−, (b) Cl−, (c) Br−, and (d) I−.
The initial work in the field was essentially performed with polyammonium receptors to take advantage of their charge. Indeed, the main characteristic distinguishing anions from all other guest species is precisely their negative charge. However, as advanced in Section 1.1, biological receptors, proteins, make use of a combination of binding motifs, which are provided in many instances by the side chains of amino acids and by the amide bonds of their backbones. The environment of protein clefts or pockets, where many binding sites reside, has a pronounced lipophilic character, and therefore, hydrogen bonds become stronger in this ambient condition with reduced water content. On the other hand, extraction strategies of pollutant anions from contaminated aqueous media often require hydrophobic receptors that can be kept soluble in a nonpolar solvent. Moreover, receptors can be grafted in resins or solid supports, making their solubility characteristics less critical.
On the basis of these important considerations, either charged or noncharged receptors containing a variety of hydrogen bonding donor groups came into play.
In this respect, Sessler, Ibers et al. [137] were, in 1990, the first to evidence a fluoride anion residing in the central hole of a sapphyrin, a 22-π-electron pentapyrrolic expanded porphyrin (28–31).
Treatment of sapphyrin (28) as its free base with aqueous HCl in dichloromethane followed by adding silver hexafluorophoshate and crystallizing by vapor diffusion led to the isolation of the diprotonated macrocycle with just one hexafluorophosphate counteranion and another anion located at the center of the macrocyclic hole. On the basis of independent synthesis and 19F-NMR studies, it was established that the central anion was fluoride. The anion is hydrogen bonded to all five pyrrolic nitrogens of the macrocycle (Figure 1.19).
Figure 1.19 Views of the structure of the fluoride complex of diprotonated 28, sapphyrin.
This first result on cyclic polypyrrole anion receptors gave rise to the evolution of these ligands and to the understanding of their chemistry and applications in a variety of fields [10, 138]. One of the first applications involved the capacity of these hydrophobic compounds for transporting fluoride anions across lipophilic membranes [139].
As commented above, another binding motif relevant to anion coordination in proteins is the amide groups constituting the protein backbone. The first time the amide functionality was introduced in the structure of an abiotic macrocyclic receptor probably dates back to 1986 when Pascal, Spergel, and Van Eggen published the synthesis of compound 32 [140].
The authors stated very enthusiastically that compound 32 “may be prepared from the easily accessible precursors 1,3,5-tris(bromomethyl)benzene and 1,3,5-benzenetriacetic acid in a short convergent synthesis, requiring no chromatographic steps, which may be completed in less than 24 h.” In the same paper, the authors indicated that 1H and 19F NMR studies carried out in DMSO-d6 suggested an association between the macrocycle and fluoride, although there was no certainty at that time about the inclusion of the anion. Since then, many amide-based receptors have been prepared [22, 23, 26, 29, 50, 53].
Kimura, Shiro and coworkers reported in 1989 amino-amide receptors 33, 34, along with a crystal structure of receptor 34, in which two azide anions were trapped between two macrocycles, forming a sort of hydrogen-bonded sandwich complex [141] (Figure 1.20).
Figure 1.20 Sandwich complex between receptor 34 and azide.
Recently, new cyclic receptors containing mixed amine–amide functions have been developed to take advantage of the charge of the potentially protonated amines and the hydrogen bond formation capabilities of both the amines and the amides [29]. A representative example of this chemistry is provided by the structure of bifluoride or azide anions encapsulated in the cavity of the tricyclic receptor 35 (Figure 1.21).
Figure 1.21 HF2− and N3− anions encapsulated in the tricyclic macrocycle 35.
Since anions behave as Lewis bases, cyclic or noncyclic receptors containing Lewis acid sites can serve for anion binding. Examples of this chemistry have been advanced in Section 1.1 [78, 81, 83, 85, 87–94], and some other examples are provided in Figure 1.22.
Figure 1.22 Crystal structure of an organotin compound binding fluoride [83] and an organoboron compound binding chloride [86], and of mecuracarborands binding chloride and iodide [78, 79].
As commented in Section 1.1, the interface between anion coordination chemistry and classical metal coordination chemistry is delimited by the so-called “cascade complexes” [95], a well-known class of multinuclear coordination compounds with bridging ligands. Relevant examples in the field of anion recognition can be found in the initial work of Martell and Lehn on O-BISTREN (11) and C5-BISTREN (18), in which, based on disquisitions about the stability constants, a hydroxide and a fluoride anion were postulated to be included within the metal centers [127, 128]. Fabbrizzi and coworkers, among others, have contributed remarkably to this chemistry with several studies and crystal structures. Figure 1.23 collects three representative structures [142–144]. Reviews dealing with this topic are collected in Refs [19, 31, 47, 145].
Figure 1.23 View of the “cascade complexes” formed by the binuclear Cu2+ complex of 22 with Cl− and Br− and between the Cu(II) complex of 23 with Br−.
1.3 Oxoanions
Oxoanions have triangular, tetrahedral, or more complex shapes resulting from the association of different triangles or tetrahedrons that can be also accompanied by organic residues as in mono- and polynucleotides. On the other hand, if anions are conjugated bases of protic acids, they will undergo protonation processes and their negative charge will depend on their basicity constants. A simple example is provided by phosphate, which displays in water stepwise constants of 11.5, 7.7, and 2.1 logarithmic units for its first, second, and third protonation steps, respectively [146]. Therefore, phosphate exists only as a trivalent anion in a very basic pH range, while at neutral pH it is present in aqueous solution as a mixture of the monovalent and divalent forms. This property can be advantageously used for discriminating between anions of different basicity.
We start this historical description with anions that are conjugated bases of strong acids and that do not change their formal charge with pH.
One of the first studies in this respect was performed by Gelb, Zompa et al. on the interaction of nitrate and halide anions with the monocyclic polyamine [18]aneN6 (36) [147]. Apart from deriving stability constants that were relatively low, only slightly above two logarithmic units for the interaction of the tetraprotonated macrocycle with nitrate and below two logarithmic units for its interaction with chloride, the authors described the crystal structure of the compound [(H4[18]aneN6)](NO3)2Cl2 · H2O (Figure 1.24). The nitrates and chlorides are placed outside the macrocyclic cavity, forming two different hydrogen bonding networks. One of them links the ammonium groups with the nitrate anions through relays of water molecules; in the other, the ammonium groups are directly bound to the chloride anions.
Figure 1.24 View of the hydrogen bonding network in [H4(36)](NO3)2Cl2 · H2O.
Two related crystal structures that have been more recently reported deserve to be mentioned since they illustrate an inclusive binding of nitrate anion in a monocyclic cavity. The first one is the crystal structure of the 24-membered dioxahexaazamacrocycle (37), usually known as O-BISDIEN, with nitrate [(H4(37)](NO3)4, reported by Bowman-James et al. [148], and the second one also corresponds to a 24-membered macrocycle with two meta-substituted pyridine spacers [(H4(38)](NO3)4, recently published by Valencia, García-España, and coworkers [149]. In both crystal structures, one of the nitrates is linked through two bifurcated hydrogen bonds to the four protonated amino groups of the macrocycle, which displays a boat-shaped conformation (Figure 1.25). In spite of this similarity, the situation of the nitrate anion with respect to the heteroatoms is different, being symmetrically placed between the two aromatic rings in the pyridine macrocycle.
Figure 1.25 Situation of the NO3− anion in tetraprotonated macrocycles (a) 36 and (b) 38.
Azacryptands (19–23) can encapsulate nitrate as it was observed crystallographically for 19 (MEACryp) in 1998 [150]. Two nitrate anions were included in the cavity, with a parallel orientation between them (Figure 1.26). Hydrogen bonds were formed between the six secondary amino groups of the cage and all the oxygen atoms of the nitrate anions.
Figure 1.26 Views of the two nitrates included in 19.
However, the first X-ray crystal structure solved for an oxoanion included in an azacryptand was for a perchlorate. Crystals of perchlorate anion included in the cavity of hexaprotonated 22 (FuEACryp) were obtained serendipitously in the course of an attempt to generate a binuclear manganese complex (Figure 1.27) [151]. Although some disorder obscured the hydrogen bond network of the included perchlorate, the participation of two types of hydrogen bonds, NH+–Operchlorate and NH+–Owater–Operchlorate, seemed clear. Since this first structure, a number of structures of azacryptands have appeared in the literature, in which two main coordination modes are observed that correspond either to inclusive anion binding or to facial binding of three anions similar to that shown in Figure 1.17 for 19.
Figure 1.27 View of the perchlorate anion included in the cavity of receptor 22 (FuEACryp) [151].
In this seminal paper, Nelson's group described another structure in which SiF62− was also included in the cavity of 20 (PyEACryp) (Figure 1.28).
Figure 1.28 View of the anionic complex formed between hexaprotonated 20 and SiF62−.
ReO4− is another anion belonging to this category whose study became relevant because its chemistry parallels that of radioactive 99mTcO4−. At the same time that 189ReO4− itself is of medical interest in connection with specific therapeutic and diagnostic applications [49, 152–154]. Cryptands of this series have also been shown to be capable of interacting with ReO4−, including the anion within its cavity as shown in Figure 1.29 for 19 (MEACryp) [155]. These studies were devoted to the extraction of pollutant anions from aqueous media, and to do so, Gloe and Nelson also employed a series of hydrophobic polyamines derived from tren, as those seen in 39–46. The extractabilities observed could not be explained solely on the basis of ligand lipophilicity since the level of protonation also played an important role.
Figure 1.29 ReO4− included in MeACryp (19).
Sulfate anion shows in aqueous solution a protonation step with a pKa around 1.7 and thus behaves essentially as a divalent anion over a wide pH range, differing from phosphate, which at neutral pH exists as a mixture of mono- and dihydrogenphosphate with formal charges of −2 and −1, respectively. Therefore, while sulfate can only accept hydrogen bonds, phosphate can both donate and accept hydrogen bonds. As mentioned earlier, this property is advantageously used by transport proteins to discriminate between these two anions.
The same kinds of receptors described for halides have also been traditionally used for binding sulfate. For instance, the monocycle [15]aneN5 (47) [156] has been proved to interact in water with several dianions including SO42−. Pyridinophane (48) interacts with SO42− and SeO42− among other anions [157] with log Ks values of around 3.5.
More recently, the Nelson and Bowman-James groups have published the sulfate crystal structures of FuEACryp (22) [159] and MEACryp (19) [160], respectively. In the same paper [159], Nelson and coworkers reported the crystal structures of thiosulfate and chromate encapsulated in the furan azacryptand (Figure 1.30). Stability constants for the interaction of the hexaprotonated FuEACryp with sulfate were reported to be over seven logarithmic units.
Figure 1.30 Sulfate anion included in the cavities of MEACryp and sulfate, thiosulfate, and chromate anions included in FuEACryp.
Receptors with amide functionalities can be appropriate for binding sulfate in less polar solvents than water [22, 29]. An interesting crystal structure of a sandwich sulfate complex was reported by Bowman-James' group in 2001 [161] (49, Figure 1.31). The sulfate anion accepts eight hydrogen bonds coming from the amide groups of both macrocycles.
Figure 1.31 Sandwich structure of sulfate anion in macrocycle 49.
1.4 Phosphate and Polyphosphate Anions
As previously commented, at neutral pH phosphate anions coexist as a mixture of the hydrogen and dihydrogenphosphate anionic forms, and therefore, phosphate can either donate or accept hydrogen bonds at this pH.
Some of the earliest research regarding phosphate recognition was carried out by Lehn's group and implied macrocycles 50–52, which incorporate guanidinium subunits in their framework [162]. Guanidinium groups are present in arginine side chains and are known to have important biological roles related to the maintenance of the tertiary structure of proteins through formation of salt bridges with carboxylate groups and to the binding and recognition of anionic substrates by enzyme receptor sites and antibodies. These roles are based on several important features of this moiety such as its permanent positive charge in aqueous solution at the pH values of biological interest (pKa ∼ 13.5) and formation of characteristic pairs of zwitterionic hydrogen bonds (Figure 1.32).
Figure 1.32 Zwitterionic hydrogen bonds formed by guanidinium groups.
The interaction of these guanidinium-complexone ligands with phosphate, pyrophosphate, and a series of carboxylate and polyacarboxylate anions was studied by pH-metric titration in pure water and in water:methanol mixtures. The largest constants in water for the interaction with PO43− were found for receptors 60 and 61, which displayed values of log Ks of about three logarithmic units, while in the case of P2O74−, constants of 4.3 and 4.1 logarithmic units were retrieved from the pH-metric data for the cases of receptors 57 and 59. In general, protonation of PO43− or P2O74− anions to give less negatively charged anionic forms led to stability decreases.
Regarding this kind of receptors, Hamilton et al. proposed in 1992 a couple of elegant systems (62 and 63) in which internal hydrogen bonding between carbonyl groups of the molecule and the guanidinium moiety induced favorable conformations in the receptors for their interaction with diphenylphosphate (Figure 1.33).
Figure 1.33 Internal C—O–H–N (guanidinium) bonds organizes the receptor for its interaction with phosphate.
Classical azamacrocycles are also appropriate ligands for binding phosphate and polyphosphate anions since the number of protonated and unprotonated amine groups they contain, and thus the overall charge and the number of hydrogen bond donors and acceptors, can be easily controlled by regulating the pH of the solution. Kimura et al. described in 1982 the interaction in water of the saturated macrocycles 36 and 64, in their triprotonated forms, with HPO42−, obtaining relatively low values of stability (2.04 and 1.1 logarithmic units, respectively) [99]. The same authors reported years later larger constants for the interaction of the monohydrogenphosphate anion with the tetra- and hexaprotonated forms of the ditopic azamacrocycle 65 (2.9 and 3.8 logarithmic units, respectively) [164].
Macrocycle 36 and larger congeners of the [3k]aneNk series with K between 7 and 12 were also shown to interact with anions derived from phosphate and pyrophosphate in aqueous solution (67–72) [165].
Finding crystal structures of phosphate or polyphosphate anions fully or partly included in aza monomacrocycles is not frequent. Early examples of such structures were reported by Martell et al. in 1995 and 1996 for azamacrocycles 73 and 74 (Figures 1.34 and 1.35) [166, 167]. The first structure reveals that H2P2O72− binds to pentaprotonated 73 through multiple hydrogen bonds, with one end of the substrate inserting into the macrocycle and the other one extending outside it. Three oxygen atoms of the inside PO3 unit and one oxygen atom of the outside PO3 unit form hydrogen bonds to the macrocycle, while the remaining two oxygen atoms of this outside PO3 unit are hydrogen bonded to oxygen atoms of a pyrophosphate belonging to another binary complex.
Figure 1.34 (a) View of H2P2O72− anion included in pentaprotonated [H5(73)]5+. (b) Hydrogen bond contacts of H2P2O72− in [H5(73) · (H2P2O7)]Cl3 · 5H2O. Parts of the connecting macrocycles are included in the drawing.
Figure 1.35 View of the H2P2O72− anion inserted in [H4(74)]4+.
Tetraprotonated 74 binds H2P2O72− anions inside the macrocyclic cavity, with each end of the anion hydrogen bonded to the nitrogen atoms of a m-xylyldiamine moiety through two of their oxygen atoms and the protonated third nitrogen atom of each end pointing away from the cavity and hydrogen bonded to nitrogen atoms of adjacent molecules. Other representative structural and/or solution studies regarding 2 + 2 azacyclophanes are included in Refs [168–171].
A study on the thermodynamic terms affecting the interaction of polyammonium receptors either of cyclic or acyclic nature with phosphate and pyrophosphate anions in aqueous solution indicated that there were five modes of hydrogen bond motifs in such systems [170, 171]; four of them involve ammonium or amine groups as donors (types I–IV), and just one involves amine groups as hydrogen bond acceptors (type V):
Binding mode I leading to hydrogen-bonded ion pair interactions should be of great importance in association processes occurring in solvents with high dielectric constant, such as water, since they provide synergetic hydrogen bonding and electrostatic attraction.
Type II bonds should be effective only in acidic enough conditions to permit extensive protonation of both the anion and the receptor. Type III bonds will be, however, favored in alkaline media, where both the anion and the receptor are extensively deprotonated. The entropic term associated with this charge transfer process should be favorable. Although types IV and V are possible hydrogen bond modes occurring between amines and compounds possessing –OH groups, mode V is known to be considerably stronger than mode IV. Hydrogen bonding modes I–IV imply a partial deprotonation of the amino group and a partial protonation of a phosphate oxygen. Since deprotonation of an amino group is a strongly endothermic reaction and protonation of HPO42− or H2PO4− or pyrophosphate anions is weakly endothermic or athermic, formation of hydrogen bonds I–IV should be endothermic, while different contributions depending mostly on the effect the process has on charge separation will be affecting the sign of the entropic term. Conversely, hydrogen bond V, which implies a partial protonation of an amino group and a partial deprotonation of a phosphate oxygen, would be accompanied by a negative enthalpy change and a favorable entropic term.
In the systems studied, I, II, and V are expected to be the principal hydrogen bonding modes with a relative importance directly connected with the extent of proton transfer, which in its turn depends on the N–O separation and the dielectric constant of the medium. Further discussion on this point is included in Chapter 2, devoted to energetics.
The possible use of polyammonium-based receptors as fluorescent chemosensors for phosphate anions was advanced by Czarnik and coworkers in 1989 [172], who proposed that tris(3-aminopropyl)amine derivatives appended with fluorophoric units could be useful for this purpose (75–77).
These authors proposed that monohydrogenphosphate could deliver a proton to triprotonated 75, blocking the photoinduced electron transfer from the amine to the excited fluorophore, producing a chelation enhancement of the fluorescence (CHEF) effect. In 1994, Czarnik reported that receptor 77 containing two tripodal polyamine units could operate as a pyrophosphate chemosensor by a mechanism similar to that just described for phosphate [173] (see Chapter 9).
1.5 Carboxylate Anions and Amino Acids
The study of carbonate and carboxylate anions emerged in the very early years of the supramolecular chemistry of anions. Guanidinium complexones 53–61 were checked for their capability to bind acetate, maleate, and fumarate in methanol:water 9 : 1 mixtures, obtaining relatively high stability values in some instances, as in 5.1 logarithmic units found for the interaction of 59 with maleate dianion [163].
On the basis of the guanidinium platform, Lehn, de Mendoza et al. reported receptor 78 for chiral recognition of aromatic carboxylate anions [174]. Sodium p-nitrobenzoate was quantitatively extracted from water by a chloroform solution of 78. Extraction experiments of sodium (S)-mandelate and (S)-naproxenate [(+)-6-methoxy-a-methyl-2-naphthaleneacetate] with 78-SS and 78-RR afforded the corresponding diastereomeric salts. Since free amino acids in zwitterionic form (valine, phenylvaline, and tryptophan) were not extracted from aqueous solution by 78, N-acetyl and N-tert-butoxycarbonyl derivatives of tryptophan were examined. It was observed that extraction of an excess of the racemic salts with 78-SS afforded in each case two diastereomeric excesses (de) of ∼17% for the L-tryptophan derivative.
On the basis of this scaffold, a few years later, de Mendoza et al. prepared the ditopic receptor 79 for amino acid recognition, including a naphthoyl moiety and a crown ether [175]. Competition liquid–liquid extraction experiments of aqueous solutions containing 13 amino acids showed good selectivity for L-phenylalanine. Chiral recognition was confirmed by NMR since the D-enantiomers were not extracted.
Figure 1.36 Schematic representation of the hydrogen bonding interaction of 81 and 82 with glutamate.
However, polyazamacrocycles were by far the most studied receptors in the early times of anion coordination chemistry. Lehn and coworkers, in their seminal JACS 1981 communication, reported the interaction of receptors 83–85 with several di- and tricarboxylate anions, along with sulfate, cyanometallate, and nucleotide anions [58]. Early cyanometallate and nucleotide anion-binding studies are presented in the next two sections.
The values obtained for the interaction of the hexaprotonated forms of 83 and 85 and the octaprotonated form of 84 with carboxylate anions indicated that electrostatic interactions played a major role in both strength and binding selectivity. In the same year, Kimura et al. published a polarographic study concerning the interaction of 36, the open-chain pentaamine 86, and the cyclic pentaamines (47), 87, and 88 with several mono- and dicarboxylate anions as well as with the tricarboxylate anion citrate [177].
The authors concluded that macromonocyclic pentaamines and hexaamines specifically interact at neutral pH with polyanions having the carboxylate functions at short distances, such as succinate, malate, citrate, malonate, and maleate, but fail to interact with the other dicarboxylates, fumarate, aspartate, and glutarate, and also with the monocarboxylates, acetate and lactate. In the same year, Kimura and coworkers also established electrophoretic protocols for analyzing polyamines using buffers containing di- or tricarboxylates at pH ≈ 8. Anomalous electrophoretic behavior of some macrocyclic polyamines migrating in the anode direction in citrate buffer solution at pH ≈ 6 was discovered [178]. The same group presented a polarographic study about the interaction in aqueous solution of the hexaazamacrocycle 36 and some of the pentaamines, 47, 48, 86–88, with carbonate anions. Such interaction persisted for some systems even at slightly acidic pH values [179].
Regarding selectivity aspects of dicarboxylate recognition, classic contributions were provided by Lehn's group [60, 180]. Hosseini and Lehn studied the interaction of macrocycles constituted by two dipropylenetriamine chains connected by propyl (17), heptyl (89), and decyl (90) hydrocarbon chains in their hexaprotonated forms with the series of dicarboxylate anions progressing from oxalate to sebacate, which has seven methylene units between the carboxylate groups. The authors concluded that there exists selectivity depending on the respective chain lengths of substrate and receptor; each receptor, [H6(89)]6+ and [H6(90)]6+, shows a marked selectivity peak for a given dicarboxylate (Figure 1.37).
Figure 1.37 Graphical representation of the stability constants (log Ks) of the complexes formed by [H6(89)]6+ and [H6(90)]6+ with dicarboxylates −O2C–(CH2)n–CO2− as a function of the number of carbon atoms (n) between the carboxylate functions.
Adapted from Ref. [60].
Shape selectivity was found years later by Bianchi, García-España, Luis et al. for the interaction of protonated 67 with the anions derived from the isomeric diacids and triacids 91–97 [181, 182] using citrate as a reference for a flexible substrate.
Since the basicities of the different anions explored are very different, a criterion balancing this point had to be adopted. The most appropriate way to compare the interaction of two different anions (anion 1 and anion anion 2) with receptor 67 is to calculate the distribution of complexed species as a function of pH for the mixed systems Anion 91 - Anion 92-97 and the overall percentages of formation [182, 183]. This method allows establishing selectivity ratios over the pH range studied and does not require any assumption of the location of protons in the interacting species, which is a frequent source of erroneous interpretation of selectivity. Using this approach, the following general selectivity order was found: 1,2,3-BTC > cis,cis-Kemp > 1,3,5-BTC > 1,2-BDC > 1,3-BDC > cis,trans-Kemp > citric acid. A more detailed discussion of this approach is included in Chapter 2, devoted to energetics of anion coordination.
Figure 1.38 Terephthalate dianion included in hexaprotonated 98.
