Anion Coordination Chemistry

Kristin Bowman-James, Antonio Bianchi, Enrique García-Espana

• Publisher: Wiley-VCH
• Year: 2012
• Print ISBN:
  9783527323708
• eBook ISBN:
  9783527639519
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Organic Chemistry

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.

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 pK_a 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...

Table of contents

• Cover
• Related Titles
• Title Page
• Copyright
• Preface
• List of Contributors
• Chapter 1: Aspects of Anion Coordination from Historical Perspectives
• Chapter 2: Thermodynamic Aspects of Anion Coordination
• Chapter 3: Structural Aspects of Anion Coordination Chemistry
• Chapter 4: Synthetic Strategies
• Chapter 5: Template Synthesis
• Chapter 6: Anion–π Interactions in Molecular Recognition
• Chapter 7: Receptors for Biologically Relevant Anions
• Chapter 8: Synthetic Amphiphilic Peptides that Self-Assemble to Membrane-Active Anion Transporters
• Chapter 9: Anion Sensing by Fluorescence Quenching or Revival
• Index