Anion Binding - Supramolecular Chemistry 2nd Edn PDF

Anion Binding 4 ‘A man ought warily to begin [deficit] charges which, once begun, will continue.’ Francis Bacon (1561–1626), Essays (Of Expenses), 1625 ...

Anion Binding 4 ‘A man ought warily to begin [deficit] charges which, once begun, will continue.’ Francis Bacon (1561–1626), Essays (Of Expenses), 1625  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 224 Anion Binding 4.1 Introduction 4.1.1 Scope Sessler, J. L., Gale, P. A. and Cho, W.-S., Anion Receptor Chemistry, Royal Society of Chemistry: Cambridge, 2006. The field of non-covalent anion coordination chemistry as we know it today may be traced back to a report by C. H. Park and H. E. Simmons of the du Pont de Nemours Company in 1968, concerning the halide complexation properties of a series of macrobicyclic hosts termed katapinands (4.1).1 The katapinands (named from the Greek katapino, meaning to swallow up or engulf) are able to bind halide ions within their macrobicyclic cavity when protonated at the bridgehead nitrogen atoms. This encapsulation behaviour (Figure 4.1), later confirmed by X-ray crystal structure determination, was the first example of anion binding by a macrocyclic host, and preceded the discovery of the cryptates (Section 3.4) by several years. In fact, Simmons and Park’s paper, submitted on 13 November 1967, was the second major contribution to the then unborn field of supramolecular chemistry to come out of the laboratories of the du Pont Company. Seven months earlier, on 13 April, Charles Pedersen had submitted his landmark work on the cation-binding behaviour of dibenzocrown-6, which marked the beginning of modern supramolecular chemistry. Interest and developments in non-covalent anion coordination chemistry continued sporadically throughout the 1970s and early 1980s, with the synthesis of several hosts of crucial conceptual impor- tance (mainly of cryptand type, analogous to the katapinands) by the groups of Franz P. Schmidtchen of the Technical University of München, Germany, and Jean-Marie Lehn.2 It was not until the late 1980s, however, that anion complexation became a popular topic as a new generation of chemists began to address this relatively unconquered frontier; even the 1996 Comprehensive Supramolecular Chemistry devotes only one chapter fully to anion binding.3 Perhaps with some degree of prophecy, Lehn in 1992 described the field of supramolecular anion coordination chemistry as ‘a full member of the field of supramolecular chemistry’. In 2006 Sessler, Gale and Cho published a major review that forms the key reference to this chapter. The huge amount of activity they detail in their book would seem to bear him out. Indeed, the challenge of anion complexation chemistry has spawned an enormous variety of imaginative host molecules, which exercise the skill of the chemist both in their synthesis, the scale of their architecture and in their modes of anion binding. The applications of cation-binding ligands are legion; from mimics of biological ion transport to mining and the extraction of metals and as selective catalysts. Similarly it is worth spending a moment to look at real world issues of interest to anion supramolecular chemists. Simple inorganic anions are ubiquitous in the natural world; chloride is a major component of the oceans and it is the dominant anion in biological extra-cellular fluid. Nitrate (from N2 oxidation) and sulfate from burning organo sulfur compound containing fossil fuels) are key components in acid rain and roadside particulate matter. Hydrogen carbonate and carboxylates are also key biological anions, while carbonates, phos- phates and silicates are the major anions in biomineralised materials such as the exoskeletons of radi- olarian, and in bone. Phosphates and nitrates in fertilisers are beneficial to agriculture but also major pollution hazards since such bioavailable sources of phosphorus and nitrogen are often biolimiting, i.e. rate of microorganism growth is limited by the amount of these elements that are present. Excess fertilisers, for example in fresh water lakes from agricultural runoff, causes a process termed eutroph- ication; the uncontrolled growth of large floating masses of algae that deplete local dissolved oxygen Supramolecular Chemistry, 2nd edition J. W. Steed and J. L. Atwood © 2009 John Wiley & Sons, Ltd ISBN: 978-0-470-51233-3  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Introduction 225 Figure 4.1 Minimised structure of the chloride complex of 1,11-diazabicyclo[9.9.9]nonacosane; katapinand 4.1b. levels, killing fish and damaging the aquatic ecosystem. Other anthropogenic anions are also major pollutants, e.g. the highly soluble and mobile 99TcO4
and ClO4
. Technetium-99 is a β-emitter with a half-life of 213,000 years and is a product of the nuclear fuel cycle, formed in ca. 6 % fission yield and can leach from nuclear waste storage facilities. Perchlorate was used extensively as an explosive and rocket propellant. Recent concerns revolve around its extensive contamination of the Colorado river in the USA, affecting the drinking water supplies of ca. 15 million people. Its toxicity data effects are still under debate. Naturally occurring polluting anions such as arsenate are also a problem, contami- nating wells in developing countries such as Bangadesh. Anions are crucial in biological systems – perhaps this is why imbalances in their concentration have such serious effects. Between 70 and 75 per cent of enzyme substrates and cofactors are anions, very often phosphate residues (as in ATP and ADP) or as inorganic phosphate (H2PO4
). Chloride anion is the major extracellular anion, and it is responsible for the maintenance of ionic strength. A protein called the cystic fibrosis transmembrane conductance regulator (CFTR) protein acts both as a transmembrane chloride channel and as a regu- lator of other ion channels (Section 2.2). Mutations in the genes that code for this protein result in impaired chloride transport and chloride ion transmembrane balance, giving rise to the lethal genetic disease cystic fibrosis. 4.1.2 Challenges in Anion Receptor Chemistry Despite the early discovery of the katapinands, non-covalent anion coordination chemistry was relatively slow to develop in comparison with the development of hosts for cations and even neutral molecules. While it is generally true that anion hosts obey the same rules that govern the magnitude of binding con- stants and host selectivity in cation hosts (primarily based on preorganisation, complementarity, solvation and size and shape effects), their application is made much more difficult because of some of the intrinsic properties of anions, listed below. Anions are relatively large and therefore require receptors of considerably greater size than cations. For example, one of the smallest anions, F
, is comparable in ionic radius to K (1.33Å versus 1.38Å). Other selected anion radii are shown in Table 4.1. Even simple inorganic anions occur in a range of shapes and geometries, e.g. spherical (halides), linear (SCN
, N3
), planar (NO3
, PtCl42
), tetrahedral (PO43
, SO42
), octahedral (PF6
, Fe(CN) 63
) as well as more complicated examples as in the case of biologically important oligophosphate anions.  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 226 Anion Binding Table 4.1 Properties of some common anions and cations (ionic radii in part from Reference 4). Ion Radius (Å) ∆Ghydration (kJ mol
1) pKa (298K)
F (6 coord.) 1.33
465 3.3
Cl (6 coord.) 1.81
340 Low
Br (6 coord.) 1.96
315 Low I
(6 coord.) 2.20
275 Low ClO4
2.50
430
NO3
1.79
300
1.4 CO32
1.78
1315 6.4, 10.3 SO42
2.30
1080 Low, 2.0 PO43
2.38
2765 2.1, 6.2, 12.4 H2PO4
2.00
465 2.1, 6.2, 12.4 PdCl62
3.19
695
Na
2.2 n/a
Cs
3.5 n/a
 Li (6 coord.) 0.76
475
 Na (6 coord.) 1.02
365
 K (6 coord.) 1.38
295
Cs (6 coord.) 1.67
250
Ca 2 (6 coord.) 1.00
505
Zn 2 (6 coord.) 0.74
1955
Al 3 (6 coord.) 0.54
4525
La3 (6 coord.) 1.03
3145
NH4 1.48
285 9.3 In comparison to cations of similar size, anions have high free energies of solvation and hence anion hosts must compete more effectively with the surrounding medium, e.g. ∆Ghydration(F
) 
465 kJ mol
1, ∆Ghydration(K) 
295 kJ mol
1. Other solvation free energies are given in Table 4.1. Many anions exist only in a relatively narrow pH window, which can cause problems especially in the case of receptors based upon polyammonium salts where the host may not be fully protonated in the pH region in which the anion is present in the desired form. Anions are usually coordinatively saturated and therefore bind only via weak forces such as hydro- gen bonding and van der Waals interactions, although they can form dative bonds. In considering the differential or selective binding of one anion or another, the intrinsic properties of anions mean that we are not on a ‘level playing field’; it is considerably easier to bind some anions than others. In general, in the absence of specific chemical recognition between anion and host, anion bind- ing selectivity, particularly in solvent extraction experiments or in the detection of anions by membrane- based ion selective electrodes, follows the order of anion hydrophobicity. This order is termed the Hofmeister series, or lyotropic series and was first outlined in 1888 from experiments based on the  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Biological Anion Receptors 227 Table 4.2 The Hofmeister Series Weakly hydrated (hydrophobic) Strongly hydrated (hydrophilic) Anions: organic anions  ClO4
 I

SCN  NO3
 ClO3
…  Br  Cl
 F
, IO3
 CH3CO2
, CO32
 HPO42
, SO42
 citrate3

Cations: N(CH3) 4  NH4  Cs  Rb  K  Na  H  Ca2  Mg2, Al3 ability of various ions to ‘salt out’ proteins from water. The Hofmeister series is shown in Table 4.2, where the left-hand side represents anions that dissolve and denature proteins in concentrated salt solu- tions, and the right-hand side represents precipitation of proteins. The series is thought to be related to the ordering of the water solvent by the various species and is also correlated with the anion hydration energy.5 It is, of course, impossible to accurately consider the binding of an anion or a cation in isolation. Electrostatic forces are so strong that ions are never far away from an oppositely charged counter-ion and hence any neutral molecule capable of acting as a host for anions also acts as a host for the counter- cation. Any cationic molecule that acts as a host for anions is in competition with the counter-cation, particularly in non-polar solvents where ion pairing can be very significant. In this chapter we will deal with both neutral and cationic anion binding systems in which relatively little attention is paid to the counter-ion and it is generally assumed that counter ion effects are, if not negligible, at least constant within the system under study. In Chapter 5 we will consider hosts that explicitly and selectively bind both ions of an ion pair. 4.2 Biological Anion Receptors Mangani, S. and Ferraroni, M., ‘Natural anion receptors: anion recognition by proteins’, in Supramolecular Chemistry of Anions, Bianchi, A., Bowman-James, K. and Garcia-España, E. (eds), Wiley: New York, 1997, 63–78. Before embarking upon a discussion of the design and preparation of artificial anion hosts it is worthwhile looking briefly at the way in which Nature carries out the task of anion complexation and transport. At least 14 mitochondrial anion transport systems have been identified to date including systems involving flux of ADP, ATP, citrate, phosphates, glutamate, fumarate, maleate, oxaloacetate and halides. Glutamate (Chapter 2, Figure 2.29) in particular plays the central role in mammalian nitrogen flow, serving as both a nitrogen donor and nitrogen acceptor, and is a key ingredient in aminotransferase-catalysed amino acid synthesis. A key result, the structure of a chloride channel protein, was reported in 2002 and, along with earlier work on potassium channels, led to the award of a share of the 2003 Nobel Prize in Chemistry to Roderick MacKinnon (Section 2.2). In biochemical anion binding, the enzyme or protein host is always part of a functioning biological system, e.g. in biocatalysis or anion transport. Thus natural anion binding systems must not only have high affin- ity for their target anion and low affinity for other species present in the cell or extracellular fluid (thermodynamic selectivity), but must also complex and release their substrates rapidly and at the appropriate time (kinetic selectivity, Section 1.7). This has the result that anion binding proteins tend not to be rigidly preorganised macrocyclic or macrobicyclic systems, but much more flexible thread- like species, relying on tertiary interactions to assemble them into anion bonding conformations. The lack of preorganisation is made up for by a large number of enthalpically stabilising protein–anion interactions.  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 228 Anion Binding 4.2.1 Anion Binding Proteins Work by Florante Quiocho at Rice University, Texas, USA, has resulted in the crystallographic char- acterisation of two bacterial periplasmic anion transport proteins termed phosphate binding protein (PBP) and sulfate binding protein (SBP).6 The function of the proteins is to bind tightly to the anion once it has crossed the bacterial cell membrane by passive diffusion. The structures of the two pro- teins are remarkably similar to one another. In each case, the anions are bound within a cleft some 8 Å deep, formed by the intersection of two protein globular domains, folded in a similar way to one another. The crucial difference between the two structures, which gives rise to their almost complete selectivity for their respective substrates (selectivity factor of about 104), is the arrangement of hydro- gen bonding residues at the protein binding site. In the case of PBP, the protein responds to the fact that both HPO42
and H2PO4
are capable of acting as hydrogen bond donors as well as acceptors. The crystal structure determinations of this protein all include well-resolved, bound HPO42
, which is held in place by a total of 12 hydrogen bonding interactions with N/O … O distances between 2.62 and 2.92Å. Seven are from NH groups from the protein main chain or arginine side chain residues, four are from OH groups (two serine and two threonine), and one involves an oxygen atom from a carboxylate anion (Asp56), which acts as a hydrogen bond acceptor. It is this hydrogen bond accept- ing group that is the key to the selectivity of the protein; this group is absent in SBP. The sulfate anion in the SBP structure is held in place by a total of seven hydrogen bonds from backbone NH, serine OH and tryptophan NH groups, all of which act as hydrogen bond donors (Figure 4.2). Replacement of the hydrogen bond donor serine130 with cysteine (SH instead of OH), alanine (CH3 substituent) or glycine (no substituent) by site-directed mutagenesis reduces the affinity of the protein for sulfate. In the case of cysteine, the reduction in affinity is by a factor of about 3200 as a result of unfavour- able steric interactions. The other replacements (which result in the loss of one of the seven hydrogen bonds) both reduce affinity by 100 and 15 times, respectively, corresponding to the loss of a hydrogen bond of about 7 kJ mol
1. More recently the high resolution X-ray crystal structure of the sulfate complex of DNA helicase RepA has also been reported. This study reveals a total of six hydrogen bonding interactions between anion and protein with a seventh interaction to a water molecule. The sulfate anions are occupying the active sites where the product phosphate residues usually bind. Sulfate binding induces significant conformational changes in the enzyme suggesting a structural change to an ‘open’ form upon binding and hydrolysis of the nucleotide 5′-triphosphate substrate that could be essential for DNA duplex-unwinding activity.7 Figure 4.2 (a) Stereoview of the SBP–sulfate interactions in Salmonella typhimurium (reproduced from with permission from Elsevier); (b) Structures of serine, cysteine, alanine and glycine.  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Biological Anion Receptors 229 The biological recognition of phosphate and sulfate has also been the subject of a survey of the Cambridge Structural Database (Section 8.4). This study reveals that the average hydrogen bonding S  O … H angle of 127.9º is some 9 degrees larger than in the analogous phosphate interaction. Also, sulfonyl hydrogen bonding interactions are clustered much more densely about this value than are phosphoryl analogues. The former also tend towards a nearly eclipsed geometry in contrast to the phosphoryl preference for gauche interactions. 4.2.2 Arginine as an Anion Binding Site Of particular importance in anion binding proteins and enzymes is the arginine residue, which con- tains a guanidine group. Guanidinium, the protonated form of guanidine, is an excellent anion binding site because it remains protonated over an extremely wide pH range (pKa  13.5 for the parent CN3H6) and can participate in double hydrogen bonds with carboxylates, phosphate, sulfate etc., as well as a unique interaction with two anions termed the arginine fork motif (Figure 4.3). Important arginine-containing biological systems include superoxide dismutase (a Cu, Zn enzyme that catalyses the transformation of superoxide (O2
) into hydrogen peroxide and dioxygen), and citrate synthase. It has been proposed that argentine-based proteins are able to use the argentine fork motif to recognise particular loops and bulges in RNA and indeed RNA-binding regions in proteins such as the human immuno-deficiency (HIV) virus tat protein exhibit argentine-rich regions. The 2.5-Å resolution crystal structure of another arginine-based system, Yersinia protein tyrosine phosphatase (PTPase), has been determined in the presence of tungstate anion. Protein tyrosine phosphatases, along with kinases, regulate phosphorylation levels necessary for cell growth, signalling and differentiation. Yersinia is the bacterium responsible for the bubonic plague, and secretes a PTPase that suppresses host immune cells. The protein structure shows the sequestra- tion of the WO 42
guest within a network of 12 hydrogen bonds ranging in length from 2.7 to 3.4 Å. Notably, four of these hydrogen bonds arise from the guanidinium moieties of arginine residues within the enzyme, highlighting the importance of these functionalities in biological systems. Tungstate is often used as a model for other tetrahedral anions, and the presence of a heavy atom like tungsten also makes the process of solving protein structures much easier. The structure of the tungstate complex differs from that of the native protein in that a 10-residue loop has moved an average of 3.3Å in order to encapsulate the bound anion. If such a conformational change is also observed on binding a phosphotyrosine substrate, the structures give an excellent insight into the mechanism of proton transfer during the proposed hydrolysis steps undergone by the enzyme– substrate complex. O O P O R +H N O R NH2 H H C H HN N N N CO2H NH2 H H two sets of double hydrogen bonds in arginine the arginine fork motif O O guanidine group P R O O Figure 4.3 The amino acid arginine and the ‘arginine fork’ binding mode with phosphate anion residues.  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 230 Anion Binding Tyr 248 Tyr 248 Arg 145 Arg 145 L-OPhe L-OPhe Wat 29 Asn 144 Wat 29 Asn 144 Glu 270 Zn Glu 270 Arg 127 Agr 127 His 196 His 196 His 69 His 69 Glu 72 Glu 72 Figure 4.4 Stereoview of the binding of L-phenyllactate to CPA. (© Wiley-VCH Verlag GmbH & Co. Reproduced by permission). Another enzyme that uses an arginine residue to bind its anionic substrate is carboxypeptidase A (CPA). The role of this zinc-containing enzyme is the hydrolytic cleavage of the terminal pep- tide or ester bond at the carboxylate end of polypeptide or ester substrates bearing β-aromatic side chains on the last residue. The binding of the carboxylate anion portion of a substrate is carried out by a positively charged arginine residue (Arg145) forming a ‘salt bridge’ of two identical, charge- assisted hydrogen bonds, as shown in Figure 4.3. There also exists additional hydrogen bonding to other residues including a tyrosine (Tyr248), giving a total of two hydrogen bonds per carboxylate oxygen atom. The participation of the tyrosine represents a good example of induced fit, since this residue moves from its location 14 Å away in the native enzyme upon substrate binding. The β-aromatic ring of the substrate is bound within a carefully tailored hydrophobic pocket, and is a necessary part of the enzyme selectivity. The crystal structure of the active site of the enzyme with bound L-phenyllactate (which acts as an inhibitor to the enzyme function) is shown in Figure 4.4. In the binding of the real polymeric substrate, further binding sub-sites closer to the enzyme surface also play a role. 4.2.3 Main Chain Anion Binding Sites in Proteins: Nests Watson, J. D., Milner-White, E. J., ‘A novel main-chain anion-binding site in proteins: the nest. A particular combination of φ and ψ values in successive residues gives rise to anion-binding sites that occur commonly and are found often at functionally important regions’, J. Mol. Biol. 2002, 315, 171–182. Protein backbones are made up of polyamide residues from polymerisation of amino acids. Amides are good hydrogen bond donors and hence interact strongly with anions and polar functionality with a partial negative charge such as the oxygen atoms of carbonyls. Anions are bound strongly by am- ides in proteins at the nitrogen terminus of an α-helix where usually two or three free amide NH groups occur that are not involved in holding the helix together. In addition to this common feature, systematic analysis of protein crystal structures by James Milner-White of the University of Glasgow, UK, has revealed another common anion-binding site, namely the ‘nest’ motif. The nest involves alternating three consecutive residues with particular main chain dihedral angles such as to give a three amide NH pocket in which especially the first and third amide groups can hydrogen bond to  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Biological Anion Receptors 231 Figure 4.5 (a) the positions of the three amide NH atoms in the anion-binding nest motif found in a wide range of functional anion binding proteins, (b) the five-amide compound nest in the P-loop of p21 ras – a nucleotide triphosphate binding protein and (c) a six-amide compound nest surrounding the iron sulfur core in ferredoxin. (Reproduced from Section Key Reference with permission from Elsevier). the anion (or negatively polarised) guest. Typical nest geometries are shown in Figure 4.5. A search of 67 well defined and mutually different protein structures revealed a total of 323 nests showing that the motif is a very common one. The nest motif can expand to give compound nests comprising four or more residues with alternating torsional characteristics that organise the NH groups into a pocket surrounding a central binding site. Among many functionally important nests is the P-loop; a glycine-rich sequence that binds to triphosphate anions in many ATP and GTP binding proteins. The P-loop comprises five amide NH units, shown in spacefilling mode in Figure 4.5b, forming a feature described as a ‘giant anion hole’. Nests are also features of the oxyanion hole found in serine prote- ases, they form the second coordination sphere of Ca2 in the trigger protein calmodulin, and some of the longest extended nest features bind to iron-sulfur anions in iron sulfur proteins such as ferredoxin. The geometries of some functional anion-binding nests are shown in Figure 4.6. 4.2.4 Pyrrole-Based Biomolecules In 1992 the X-ray crystal structure of porphobilinogen deaminase was solved, an enzyme that is involved with the biosynthesis of a linear tetrapyrrole precursor to protoporphyrin IX, found in haemoglobin (a) (b) (c) (d) O O O O O O O O O N N N O N P N O N N H H H H O H H O O H HN HN HN NH HN O NH O - O NH S NH O O O O Fe S P O – O HN NH O O NH NH S N O– S O O O Ca 2+ Fe H P O S O S Figure 4.6 Hydrogen bonding in functional nests (a) oxyanion hole in serine protease binding the substrate tetrahedral intermediate, (b) P-loop showing the β-phosphate (middle phosphate residue) of GTP bound to a compound nest, (c) Ca2 binding in calmodulin. The calcium is bound to three carboxylate oxygen atoms which are in turn bound to bound to a five-amide compound nest (d) the cysteine-bound Fe2 unit in spinach ferredoxin.  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 232 Anion Binding O O O O O N N O Me O H H O O O O O N + NH3 NH HN HN R' R (a) (b) Figure 4.7 (a) interaction of a dipyrromethane-based cofactor in porphobilinogen deaminase with an aspartic acid-derived side chain. The carboxylate residues of the co-factor are stabilised by protein arginine residues. (b) general structure of the prodigiosins (R, R′  alkyl). (Section 2.5).10 The key aspect of this structure is the interaction of a dipyrromethane unit in the enzyme’s co-factor through the pyrrolic NH groups to a carboxylate side chain of an aspartic acid residue (Figure 4.7a). The co-factor carboxylate side chains also binds to enzyme arginine groups. If the aspartic acid residue is replaced by glutamte the enzyme loses essentially all activity suggesting that this interaction is particularly important. The dipyrromethane moiety is also a core component of the prodigiosins, a family of naturally oc- curring tripyrrolic red pigments (Figure 4.7b). Prodigiosins have very promising immunosuppressive and anti-cancer properties and have proven to be an inspiration for recent research into a range of arti- ficial analogues with extensive anion-binding properties but more flexibility and versatility than cyclic analogues such as porphyrins (and their expanded homologues) and calixpyrroles (Section 4.6.4). 4.3 Concepts in Anion Host Design Schmidtchen, F. P., ‘Reflections on the construction of anion receptors - Is there a sign to resign from design?’, Coord. Chem. Rev. 2006, 250, 2918–2928. 4.3.1 Preorganisation In Chapter 1, we defined a host as a molecule possessing convergent binding sites, and a guest as one pos- sessing divergent binding sites. We would also like a host to exhibit selectivity or discrimination between one guest and another. In the case of anion coordination chemistry, this definition is still helpful, but we run into some trouble on two accounts. Firstly, the large size and high polarisability of anions means that non-directional forces such as dispersion interactions play a significant role in anion binding. Also, as a charged particle, an anion intrinsically experiences an electrostatic attraction to molecules, even if they are not positively charged, simply because there is a charge difference between the negatively charged anion and an electrically neutral molecule. This means that a significant contribution to anion binding comes from dispersion and electrostatic interactions, which are non-directional, and so in some sense the whole host is a binding site, although some regions of the host will interact more strongly with the anion than others. The second problem concerns the definition of binding sites on the anion itself. The coordinatively saturated nature of anions and in some cases the lack (with the exception of anions such  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Concepts in Anion Host Design 233 + + H N N H NH + + HN anion out, out in, in Scheme 4.1 Conformational change in the katapinands on anion binding. as H2PO4
) of specific hydrogen bonding functionality, means that species, such as halides especially, behave approximately like spherical charges and have a highly versatile coordination geometry without specific binding sites. The generalised definition of a host as a convergent entity allows us to rule out metal ions as anion hosts, even though they display a great deal of selectivity in their interactions with anions (cf. the HSAB theory, Section 3.1), simply because there is generally only one binding site for an individual anion per metal and the metal ion sites are divergent (nobody would seriously propose the Na ion in NaCl as an anion binding host, even though Na is selective for Cl
over I
with a binding constant of over 10300 based solely on Coulombic considerations, and in the cubic NaCl lattice a Cl
ion is oc- tahedrally surrounded by six Na ions, Figure 1.9). Clearly, however, the bicyclic katapinands (4.1) are classifiable as anion hosts. They bind their anionic guest species via two converging N—H … X
hydrogen bonds (the NH groups are the binding sites) and display a size-based selectivity according to the dimensions of the host (as determined by the chain length, n). The [8.8.8]-bridged katapinand (4.1a) exhibits no significant anion binding; the analogous [9.9.9] host (4.1b) exhibits binding con- stants of the order of 102 M
1 for Cl
in water/trifluoroacetic acid solvent, with a modest selectivity for chloride over bromide of a factor of about 8, while [10.10.10] (4.1c) binds Cl
, Br
and I
with little selectivity. The katapinands are not preorganised hosts, however, and undergo a major conformational change on anion binding, corresponding to a rearrangement from an out, out geometry, to an in, in conformer (Scheme 4.1). This is, in effect, the same sort of process undergone by the crown ethers on cation binding in aqueous solution. Given these observations, it is clearly important to know to what degree the bicyclic nature of the katapinand contributes to its anion binding ability. Does the three-dimensional host structure help to organise the two binding sites, or does the driving force for the organisation of the binding in,in conformation come from the interaction with the Cl
anion? This is tantamount to asking how impor- tant is preorganisation in anion complexation chemistry. The low binding constants observed for the katapinands suggests that as with cation binding, preorganisation (or the lack of it in the katapinands) it is of significant importance and should be incorporated into host design. 4.3.2 Entropic Considerations As with all host-guest systems, in designing an anion-binding host both preorganisation, and comple- mentarity are of crucial importance, particularly with regard to maximising the enthalpically favour- able interaction between anion and receptor. Entropic effects are also of fundamental importance and go beyond the mere ‘release of high energy solvent’ paradigm. For example isothermal titration calorimetric measurements (Section 1.4.2) show that the binding of oxoanions to polyammonium hosts in water essentially enthalpically neutral and all of the driving force is entropic. In the key reference to Section 4.3 Schmidtchen breaks down the entropic contribution to anion binding into  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 234 Anion Binding two components, ∆Sobs  ∆Sintrinsic  ∆Ssolv. The solvation effects (∆Ssolv) are essentially not subject to molecular design; however, the chemist may have some hope to control ∆Sintrinsic which may itself be broken down into a number of factors for easier understanding: ∆Sintrinsic  ∆Stransrot  ∆Svibration  ∆Sconformation  ∆Sconfiguration. The ∆Stransrot component refers to the change in translational and rota- tional entropy of the host and guest upon association. While a matter of debate, this term is essentially constant for the kinds of artificial host-guest systems we are concerned with and so is not subject to design. The remaining terms are highly important in design terms. The ∆Svibration term refers to the generation of vibrational entropy on association, ∆Sconformation refers to the freezing of internal rota- tions upon association and ∆Sconfiguration is related to the different possible geometrical arrangements of the binding partners. The vibrational and conformational terms are closely related. If a flexible host interacts loosely with a flexible guest such that internal rotations are relatively unimpeded and vibrations associated with the bonds between host and guest have low force constants then the entropic cost of binding is small, resulting in a favourable situation – the generation of low frequency motions increases entropy and hence augments affinity. Conversely, in the case of very tight binding internal rota- tions are severely restricted (and in fact become high frequency vibrations) and bonding vibrations increase in frequency and decrease in amplitude. This situation results in a decrease in entropy and lowered affinity. Thus strong but relatively unselective binding (e.g. for sequestration or extraction) will correspond to a favourable association entropy, while tight, structure-specific binding will give a less favourable association entropy more appropriate in situations involving anion catalysis, region-/ stereoselective binding or self-assembly. These considerations are subject to interference from the additional ∆Sconfiguration component, however. It is possible for a given host-anion complex to bring in a number of different, distinct orientations with significant barrier to interconversion yet of comparable relative energy even though many techniques such as NMR spectroscopy mask this possibility by rapid averaging under ambient conditions. A full discussion of entropic considerations in design is given in the key reference. 4.3.3 Considerations Particular to Anions The design of preorganised, complementary anion binding hosts exhibiting maximum complexation free energy and selectivity must follow from an appreciation of some of the fundamental characteris- tics of anions. Listed below are some of the issues that are important to anion binding in particular. Negative Charge The negative charge suggests that both neutral, and especially positively charged, hosts will bind an- ions. Unfortunately electrostatic interactions are non-directional and so all anions will be attracted to hosts on an electrostatic basis, forming a solvated ion pair in solution. Affinity would be expected to increase with anion net charge. If a positively charged host is chosen, there will be a significant degree of competition with the existing counter anions, thus any observed binding constant really represents a relative selectivity factor for the binding of one anion over the other. Commonly counter cations such as NBu4 that are thought to interact only weakly with anions are chosen, however even these exhibit significant ion pairing. For example, a 1 mM solution of tetrabutylammonium chloride in CH2Cl2 is less that 20 % dissociated at 22 oC.11 Moreover many tetrabutylammonium salts are very hydroscopic and extremely difficult to dry and can even decompose in the absence of water. One possible solution is the use of cryptand complexes of potassium salts of target anions, however this is not widespread and has its own purity concerns.  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Concepts in Anion Host Design 235 Lewis Basicity The vast majority of anions are Lewis bases, although there are some exceptions that do not have lone pairs (e.g. AlH4
, B(C6H 5) 4
, closo-B12H122
) or are only very weak bases (e.g. B(C6F5) 4
). This property suggests that hosts containing Lewis acidic atoms, such as organo boron, mer- cury or tin compounds, or metal cations in general, might make the basis of a suitable host by formation of coordinate bonds. This is the basis of the anticrown hosts, which resemble the crown ethers with Lewis basic oxygen replaced by Lewis acidic binding sites (Section 4.7.4). Lewis acid–base coordinate interactions have a high degree of directionality and so make a good basis for the design of selective hosts. The property of Lewis basicity also makes anions suitable as hydrogen bond acceptors, as in halide binding by katapinands, in which charge- assisted N—H … Cl hydrogen bonds are responsible for a large amount of the complexation ability of the hosts. High Polarisability Anions are highly polarisable and so van der Waals interactions will be significant. While non-directional, these are related to the contact surface area of host and anion, and so three- dimensional encapsulation of the anion should enhance binding of all anions capable of fitting within the host. Solvation Anions generally have high solvation energies and so, to an even greater degree than cation binding, the medium in which solution–anion complexation experiments are carried out will strongly influence binding constant measurements. Binding constants for monovalent inorganic anions of about 102–103 in water represent strong binding, whereas strong binding is achieved much more readily in non-polar solvents, such as chloroform, because of the anions’ solvophobic properties. Binding constants are expected to increase on changing solvent in the order H2O  DMSO  MeCN  CHCl3  CCl4, although interesting enthalpy–entropy compensation effects may be observed in the complexation of all kinds of guests (see, for example, Section 1.9). Coordination Number Another question that is of potential importance concerns the coordination number and geometry of a bound anion. In the search for complementary hosts for metal cations, metal binding is much stronger in cases where the coordination number and geometry of the metal matches that of the ligand (octahedral, tetrahedral, square planar etc.). The importance is diminished significantly in cases such as alkali metal cations, in which there is no strong coordination preference. In crystalline NaCl, the chloride is in a regular, octahedral six-coordinate environment. In contrast, in the case of the katapi- nands the encapsulated chloride may be thought of as two-coordinate since it engages in two N—H … Cl hydrogen bonds. In fact, a wide variety of anion coordination geometries are known, with the number of interactions an anion is able to form increasing with its size. Artificial halide hosts may bind via two to six interactions in acyclic and cyclic structures, while larger, multi-atom anions may form anything from zero to three hydrogen-bonded interactions per terminal atom. The structure of the protein PBP (Section 4.2.1) with its 12 hydrogen bonds, three to each phosphate oxygen atom, thus represents an excellent example of the maximisation of coordination number in order to obtain the maximum stabilisation.  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 236 Anion Binding 4.4 From Cation Hosts to Anion Hosts – a Simple Change in pH Garcia-España, E., Diaz, P., Llinares, J. M. and Bianchi, A., ‘Anion coordination chemistry in aqueous solution of polyammonium receptors’, Coord. Chem. Rev. 2006, 250, 2952–2986. 4.4.1 Tetrahedral Receptors By far the most obvious way in which to address the binding properties of anions is to combine electrostatic attraction for a positively charged host with the anions’ Lewis basic character, which should enable then to act as hydrogen bond acceptors. This is exactly the strategy that led to Park and Simmons’ original success with the katapinands. There exists an enormous variety of cryptands designed for complexation of metal ions by virtue of the Lewis basic nature of their tertiary nitrogen bridgeheads and polyether or secondary amine chains. A simple change in solu- tion pH should result in the protonation of these amine functionalities, according to their rela- tive basicities (Section 3.11), to give a katapinand-like host for anions. The only factors of real importance are that the cryptand should be large enough to incorporate the anion (i.e. as large as [2.2.2]cryptand or bigger), and that there are not so many repulsions between the protonated nitrogen atoms that the host is incapable of converging, or is rendered too weakly basic and does not protonate at all. This simple approach is beautifully illustrated by the macrotricyclic cryptand 3.29, which may be regarded as arising from four fused triazacrown-6 rings. Compound 3.29 has been termed a ‘soccer ball’ molecule because of its nearly perfect spherical shape. The pres- ence of the four nitrogen bridgeheads on this molecule make it a highly versatile example of a tetrahedral receptor. In addition to its Lewis basic properties, which enable it to bind strongly to cations, in its tetraprotonated form it is also capable of binding anions such as Cl
, acting via the formation of four hydrogen bonds supported by electrostatic interactions with the ether oxygen atoms. Thus, depending on the pH of the medium, the neutral host may bind cations, especially ammonium (NH4); in its diprotonated state, it binds neutral molecules, particularly water; and in its tetraprotonated state, it binds anions such as Cl
(Figure 4.8). Unlike the katapinands, the protonated soccer ball 3.29-4H has an extremely high affi nity for Cl
(log K11  4, methanol/ water) and the selectivity for Cl
over Br
is a factor of about 50.12 Such affinities and selectivities represent significant achievements in anion complexation chemistry (especially of monovalent anions), where host–guest affi nities are dramatically less than those observed for cation binding. The X-ray crystal structure of 3.29-4H ⊂ Cl
(Figure 4.9) shows that the N—H … Cl
hydrogen bonded distances are 3.09 Å, comparable to the katapinand structure (3.10 Å), clearly indicating that both hosts are geometrically complementary to chloride. The greater affi nity of 3.29-4H Me+ N N O O X X O + + N O N Me N XX N Me O O X X N + N 3.29 Me X = O, 4.2 X = (CH2)2, 4.3  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License From Cation Hosts to Anion Hosts – a Simple Change in pH 237 + + N N N O O O H H H O O O O + O O N O Cl H H H + N H N +N H H H N H N H N O O O O O + O N O N O N O + Figure 4.8 Tetrahedral recognition: the ‘soccer ball’ cryptand as a receptor for ammonium ion, water or chloride, depending on the pH of the medium. therefore must rest with its much more preorganised nature, its greater positive charge and the greater number of binding sites. The lower affi nity for Br
is related to the non-optimal match between the longer N—H … Br hydrogen bond distances and the cavity dimensions. The soccer ball cryptate is clearly an excellent match for its target anion in terms of preorganisation, size, charge and hydrogen bonding functionality. It is not clear, however, from a comparison of soccer ball and katapinand hosts, which of these factors is the most important in determining anion binding affinity and selectivity. As early as 1977, some evidence was gained on the influence of hydrogen bonding by Schmidtchen by the preparation of 4.2, the N-methyl analogue of 3.29, and a closely related hydrocarbon- bridged species (4.3). The addition of the methyl group in both cases causes the hosts to adopt an outward conformation (otherwise the methyl groups would be forced into close proximity with one another at the centre of the molecule). This makes the tetracationic cavity much larger than in 3.29-4H and as a result the highest binding constants are observed for bromide and iodide (log K  2.25 in water). The X-ray crystal structure of the iodide complex is shown in Figure 4.9(b) and reveals that the iodide anion is situated centrally in the tetrahedral macrotricyclic cage, some 4.54Å from the cationic nitrogen centres. Figure 4.9 (a) X-ray crystal structure of the ‘soccer ball’ chloride cryptate 3.29-4H ⊂ Cl
and (b) structure of the iodide cryptate of 4.3. (Reproduced by permission of The Royal Society of Chemistry).  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 238 Anion Binding 4.4.2 Shape Selectivity Dietrich, B. Guilhem, J. Lehn, J.-M., Pascard, C. and Sonveaux, E., ‘Molecular recognition in anion coordination chemistry’, Helv. Chim. Acta., 1984, 67, 91–104. In the case of the tetrahedral receptors discussed in Section 4.4.1, the halide coordination geometries (as determined by their shortest intermolecular contacts or hydrogen bonded distances) may be de- scribed as tetrahedral. In the preceding sections, we have already mentioned halide coordination num- bers ranging from two to six. So, is there any evidence for preferred halide coordination geometries, or are coordination environments imposed on the halides by the preorganisation and topology of the host? Huge insight into this question and other factors affecting anion binding selectivity is obtained by the N O N N N H H H H N H H N N H H N N O N N N H H H N HN N N O bis(tren) 4.4 4.5 binding characteristics of the cylindrical macrobicycle bis(tren) (4.4) and the smaller analogue (4.5). The X-ray crystal structures of the complexes of hexaprotonated bis(tren) (4.4-6H) with F
, Cl
, Br
and N3
(azide) are shown in Figure 4.10, along with the structure of the fluoride cryptate of (4.5-6H). Note that the tertiary bridgehead nitrogen atom is not protonated because its basicity is reduced dra- matically as a consequence of its proximity to the protonated secondary amines in each case. Figure 4.10 X-ray structures of the bis(tren) (4.4-6H) cryptates of (a) F
, (b) Cl
, (c) Br
and (d) N3
; (e) structure of the F
complex of 4.5-6H.  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License From Cation Hosts to Anion Hosts – a Simple Change in pH 239 The solution binding constants in water for halide binding by protonated bis(tren) are (log K) 4.19, 3.0, 2.6 and 2.15 for F
, Cl
, Br
and I
, respectively. In addition to fluoride, azide is also bound very strongly (log K  4.30). Surprisingly, the order of halide binding constants follows their hydration ener- gies with the most highly solvated (fluoride) also bound the most strongly (an anti-Hofmeister selectivity order). This is even more remarkable given the crystal structure of the compound (Figure 4.10a), which shows that F
is a very poor fit for the long, cylindrical bis(tren) cavity. The F
anion is situated far to one side of the macrocycle, in an approximately tetrahedral arrangement, with mean N … F hydrogen bonding distances of 2.72Å. The larger chloride and bromide ions (Figure 4.10b and c) are situated much more symmetrically in the cavity, interacting with six NH groups, with mean N … Cl
distances of 3.19–3.39Å and N…Br
distances of 3.33–3.47Å. Note that these distances are much longer than the optimum N … Cl
contacts found for 4.1b and 3.29-6H of about 3.10Å. Consistent with this ob- servation, the binding of chloride, bromide and iodide is much weaker than fluoride. The best fit to the cylindrical cavity in terms of shape and topological match is the cylindrical azide anion (Figure 4.10d), which is bound the most strongly. The N … N3
distances are of optimum length and fall into a narrow range of 2.81–3.02Å, indicating symmetrical binding arising from good complementarity. The overall selectivity sequence of 4.4 for monovalent anions is in the order ClO4
, I
, Br
, Cl
 CH3CO2
 HCO2
 NO3
, NO2
 F
, N3
. This trend does not appear to show significant correlation with anion size or hydration energy series, or with the Hofmeister series (Section 4.1.2). In fact, the binding in 4.4 is composed of a combination of electrostatic contributions, in which anions with a relatively high charge density such as F
are bound strongly, and a topological (i.e. shape as opposed to size) complementarity favouring azide (which also has a high negative charge density). The electrostatic factors become ap- parent as soon as the affinity for multiply charged species such as P2O74
(log K  10.30), ATP 4
and ADP3
are examined. All of these anions are bound very strongly, the 4
species more so than 3
. When all of these factors come together (size and shape complementarity, strong electrostatic interac- tions and multiple hydrogen bonds), extremely strong binding is observed. The small host 4.5-6H is a perfect match for fluoride (Figure 4.10e), binding in a distorted trigonal prismatic geometry with N … F
distances in the range 2.76–2.86Å and a binding constant in water, log K, of 11.2.15 4.4.3 Ammonium-Based Podands Hossain, A., Liljegren, J. A., Powell, D., Bowman-James, K., ‘Anion binding with a tripodal amine’, Inorg. Chem. 2004, 43, 3751–3755. Even simple protonated tripodal amines such as tren (2,2′,2″-tris(aminoethyl)amine, 4.6) itself exhibit interesting anion-binding geometries. The X-ray crystal structure of tren·HBr (Figure 4.11a) shows the bromide anion interacting with all three terminal amine groups, one of which is protonated.16 H2N H 2N NH2 H N HN N H N N N NH2 NH2 tren NH 4.6 4.7 4.8  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 240 Anion Binding Figure 4.11 X-ray crystal structures of (a) tren · HBr and (b) the trischelate unit in 4.7-4H · 4H2PO4
. Similar tripodal complexes are formed by triprotonated tren with HPO42
and SO42
.17 Work by Kristin Bowman-James at the University of Kansas, USA, has resulted in a tren-derived solution phase host 4.7. The tetraprotonated form of 4.7 exhibits a trischelate interaction with H2PO4
(Figure 4.11b). Despite the lack of preorganisation, the binding constants of the triprotonated form (as the triflate salt) for H2PO4
and HSO4
(in non-competitive chloroform solvent) are significant (log K  3) while other anions are bound with long K  2. It is suggested that the residual acidic protons on the HSO4
and H2PO4
anions can protonate the bridgehead nitrogen atom resulting in ion-pairing. These interesting results with 4.7 build on earlier work on 4.8 which acts as a fluorescent sensor for anions. The receptor is triprotonated at pH 6 and addition of HPO42
results in a 145 % increase in the fluorescence intensity of the anthracenyl chromophore, a phenomenon also seen in the sensing of metal complexes (Section 11.3.2) termed chelation enhanced fluorescence (CHEF).18 4.4.4 Two-Dimensional Hosts Boudon, S., Decian, A., Fischer, J., Hosseini, M. W., Lehn, J.-M. and Wipff, G., ‘Structural and anion coordination features of macrocyclic polyammonium cations in the solid, solution and computational phases’, J. Coord. Chem., 1991, 23, 113–135. The anion coordination behaviour of a large range of azacorand type macrocycles such as 4.9–4.14 (nitrogen analogues of the crown ethers) has been studied in aqueous solution. Binding and protonation constant measurements may be made readily by potentiometric titration (Section 1.4.2). Frequently, a variety of differently protonated species are present and the anion binding experiment is a summation of the contributions from a variety of different solution equilibria. As a result, the titration data must be deconvoluted by regression calculation into each of the different components present, often resulting in large errors. Because water is such a strongly solvating medium with a high dielectric constant, gener- ally only interactions between multiply charged species are significant. As formal charge increases, however, extremely thermodynamically stable complexes are formed. Hexacyclen (4.9), aneN6, the aza analogue of crown-6, has been shown by potentiometric titration to be an extremely strong diprotic acid in its hexaprotonated form, meaning that for most prac- tical purposes in all but very acidic solutions, it exists as the tetraprotonated form 4.9-4H. This is a general kind of observation for macrocycles of type 4.14 in which the nitrogen atoms are separated by only two carbon atoms. Protonation of all of the nitrogen atoms results in severe electrostatic repulsions  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License From Cation Hosts to Anion Hosts – a Simple Change in pH 241 between the NH2 groups and dramatically lowers pKa values compared to acyclic analogues, which almost invariably adopt a linear, all-anti conformation when fully protonated to minimise repulsion between the cationic groups. In solution, hexacyclen binds nitrate in preference to halide anions, but X-ray crystallographic results indicate that none of these anions is included within the macrocycle. Halides adopt a perching geometry with N … Cl
distance of 3.07–3.28Å, while in the X-ray crystal structure of 4.9-4H · 2Cl
· 2NO3
, the nitrate anions interact with the host indirectly via included water molecules. Crystals of 4.9-6H ·2Cl
·4NO3
do show direct interaction with the nitrate anions via one hydrogen bond each. Clearly the hexacyclen cavity, similar in size to crown-6, but partially filled by the NH protons, is much too small to include anions. Antonio Bianchi (University of Florence, Italy) and Enrique Garcia-España (University of Valencia, Spain) have investigated much larger rings up to aneN12. Such large-ring species are able to act as hosts for large inorganic anions such as [PdCl4] 2
and [Fe(CN) 6] 4
, which can be included within the macrocyclic cavity, as exemplified by the X-ray crystal structure of H10 aneN1010 with [PdCl4] 2
guest (Figure 4.12). The term ‘supercomplexes’ was adopted for these complexes of metal complexes. The elegant simplicity of this structure is belied by the complexity of the solution behaviour of the multiple protonation states of the amine. N N N NH H HN N H H NH H HN NH HN NH H HN N NH H HN NH HN N H H N N hexacyclen 4.9 4.10 4.11 O O NH N NH N N H H NH H HN NH HN NH HN H NH H HN N HN O O N O H H n N N 4.12 4.13 4.14 n=2-7 These workers have also studied the complexation of [Fe(CN) 6] 4
by a range of large-ring azac- orands. The speciation diagram (solution composition as a function of pH) for the behaviour of this anion with aneN9 is shown in Figure 4.13. As this diagram reveals, species all the way from tetraprotonated to octaprotonated macrocycle are present and indeed the macrocycle may retain four protons even at a fairly basic pH of 9 or higher. The hexacyanoferrate(II) anion is interesting because, in addition to potentiometry, its complexation may be monitored by cyclic voltammetry (Box 4.1). The Fe(II)/Fe(III) redox couple in this species is highly reversible and sensitive to the iron atom’s second coordination sphere. The shifts in redox potential as a function of pH are also plotted in Figure 4.13. The diagram indicates clearly that the anion becomes progressively more difficult to oxidise as the state of protonation of the host–increases. It is more difficult to pull an electron away from the more positively charged host-guest complexes. Also, the potential does not increase smoothly, but changes  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 242 Anion Binding Figure 4.12 X-ray crystal structure of the [PdCl4] 2
complex of H10 aneN1010, showing the anion inclusion.19 in a stepped fashion in accordance with the peaks in the concentrations of the various solution species. Stepwise binding constants for the various solution species are given in Table 4.3. The table shows firstly that binding constants for all of the complexes formed are very high in aque- ous solution (compared to complexes of univalent anions), consistent with the highly charged nature of the interacting species. Secondly, there is a distinct increasing stability as the charge on the macrocycle increases, consistent with the electrostatic nature of the interactions. Finally, it is also noteworthy that the most stable compounds are formed with the smallest macrocycle, where the positive charge is more concentrated (fewer intervening non-protonated NH groups). Host–anion interactions are of the charge assisted N—H … NCFe
type, and no complexes are formed with macrocycles with fewer than four protons, the minimum required for charge neutralisation. In order to avoid the problem of increasing difficulty in fully protonating macrocycles like hexacy- clen and higher analogues, which have two carbon bridges between the nitrogen atoms, a number of Fe(CN)64– 100.5 % H7L1Fe(CN)63+ H5L1Fe(CN)6+ H6L1Fe(CN)62+.4.3 E½(V) 50 H 8L 1Fe(.2 CN) 6 6 ) CN 4+ 4 1 Fe(.1 HL 0 4 5 6 7 8 9 10 11 pH Figure 4.13 Distribution (speciation) diagram ( ) for the system H/[Fe(CN) 6]4
/aneN9 and the Fe(II)/Fe(III) redox potential (E/V) ( ) versus pH. (Reproduced with permission from © 1987 American Chemical Society).  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License From Cation Hosts to Anion Hosts – a Simple Change in pH 243 Table 4.3 Binding constants (log K) for the stepwise formation of ‘supercomplexes’ between 4.14 (n  3–5) and [Fe(CN) 6] 4
according to the reactions shown.20 Log K Reaction n3 n4 n5 (4.14)-4H  [Fe(CN) 6] 4
 [H4 (4.14)Fe(CN6)] 4.06 3.69 3.61 (4.14)-5H  [Fe(CN) 6] 4
 [H5(4.14)Fe(CN6)] 5.63 4.78 4.66  (4.14)-6H  [Fe(CN) 6] 4
 [H6 (4.14)Fe(CN6)] 2 7.60 6.23 5.72  (4.14)-7H  [Fe(CN) 6] 4
 [H7(4.14)Fe(CN6)] 3 9.33 7.92 6.93  (4.14)-8H  [Fe(CN) 6] 4
 [H8 (4.14)Fe(CN6)] 4 — 9.03 8.07 Table 4.4 Variation of pKa with spacer length in macrocycles of type Hm [4n]aneN4m (n  2–4; m  3 and 4). Host n pKa3 pKa4 4.15a 2 1.7 1 4.15b 3 6.9 5.4 4.15c 4 10.6 8.9 alternatives have been prepared. Macrocycles such as 4.10 and 4.11 allow protonated nitrogen atoms to separate further by use of propenyl spacers, while 4.12 and 4.13, which are less conformationally flexible, use ether groups to space out positive charge. This strategy proved successful in that the fully protonated species all exhibit pKa values above 7. The dependence of pKa upon spacer length is exem- plified by the series [4n]aneN4 (n  2–4, 4.15), which shows the pKa variation given in Table 4.4. For 4.10–4.13, anion binding behaviour is similar in type to that observed for compounds of type 4.14, however, with binding constants increasing with anion charge. For anions of the same charge, struc- tural effects were observed in which anions that matched the symmetry of the macrocycles were bound most tightly (e.g. three-fold axis of SO42
with 4.13-6H; four-fold symmetry of squarate, C4O42
, with 4.11-8H). Also, larger anions tend to complex more strongly with the larger macrocycle, 4.11-8H. Interestingly, binding constants for multiply charged anions such as ATP are significantly higher than analogous acyclic reference compounds such as spermine, H2N(CH2)3NH(CH2) 4NH(CH2)3NH2, indi- cating a significant anion binding macrocyclic effect, analogous to that found for the crown ethers as cation hosts. The ATP and ADP binding ability of macrocycles such as 4.12 has been used as the basis for an artificial ATP producing system (Section 12.3). C + + H2 n NH2 (CH2) NH2 N N n n n H H O O + + CH2 CH2 NH2 (CH2) H2N H H m O O N N NH2 (CH2) NH C + n + 2 H2 n 4.16 (n = 7 and 10) 4.15 with α,ω -dicarboxylic acid guest  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 244 Anion Binding Figure 4.14 (a) Length-based recognition of α ,ω -dicarboxylic acids by monocyclic hosts 4.16. (b) Binding constants for hosts 4.16 as a function of dicarboxylate guest spacer length, m. The ability of monocyclic azacrown type hosts to recognise anions on a size and shape fit basis has been investigated further by the preparation of hosts of type 4.16 that contain two-binding-domains. It was anticipated that the ditopic hosts 4.16, which possess a cavity of varying size, dependent on the length of the spacer, (CH2) n, would exhibit selectivity for α,ω-dicarboxylic acids
O2C(CH2) mCO2
according to the length of the acid (determined by the length of the (CH2) m group). Those dianionic guests that best fit the cavity should be bound the strongest. Observed binding constants for compounds 4.16 with n  7 and 10 as a function of guest length, m, are shown in Figure 4.14. Modest peak selectivities are observed, signifying the dimensional matching of the host and guest, despite the relative flexibility of both partners.21 Another interesting class of anion receptors based upon protonated nitrogen atoms are the expanded porphyrin macrocycles such as 4.17 (diprotonated sapphyrin) and compound 4.18. The tetrapyrrole porphyrin macrocycles are excellent hosts for metal cations such as Fe2 and Mg2 (e.g. haemoglobin and chlorophylls, Sections 2.3–2.5); however, their cavity dimensions are too small to accommodate anions. Conversely, expanded porphyrins such as 4.17 comprising five or more pyrrole residues present a rigid macrocyclic cavity about 5.5Å in diameter, in which (particularly when protonated) the NH R + + + N N N N N N H N H H H H + H H N H H N + N H H N NH HN H H H H N H N N + N N 4.17a R = H O H 4.18 4.19 4.17b R = N N N O NH2  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License From Cation Hosts to Anion Hosts – a Simple Change in pH 245 Figure 4.15 (a) X-ray crystal structure of the diprotonated expanded porphyrin host sapphyrin (4.17) with encapsulated fluoride. The fluoride anion lies entirely within the plane of the macrocycle.22 (b) SO42
binding by diprotonated octaphrin.23 groups are suitable for anion complexation. Diprotonated sapphyrin (4.17) forms an extremely stable complex with fluoride (log K  5.0) in methanol solution. The excellent fit of the anion within the sapphyrin cavity is demonstrated by X-ray crystallography (Figure 4.15a) with five short, symmetrical N … F distances in the range 2.67–2.79 Å.22 The fact that this very large macrocycle is ideally suited to the encapsulation of the smallest anion is a clear indication of the problems faced in preparing hosts large enough to bind effectively to anions. In contrast, the analogous structure with chloride shows binding of two chloride anions above and below the plane of the macrocycle, due to poor fit. The bind- ing constant for chloride is lower by a factor of more than 103. Phosphate residues of organophosphates and polyphosphates are also bound with a single P—O
bond perching just above the plane of the macrocycle, and sapphyrin is useful in phosphate transport. In order to prepare chloride-selective macrocycles based on the polypyrrole motif, non-aromatic expanded porphyrins have been designed which possess a much larger cavity. Compound 4.18 has been shown by X-ray crystallography to exhibit a good fit to chloride, binding through four N—H … Cl
hydrogen bonds with N … Cl distances of 3.11–3.25Å. The host–chloride complex may be detected by fast atom bombardment (FAB) mass spectrometry, suggesting that it is associated in the gas phase. Solution binding constants in CH2Cl2 (note the much less polar solvent, which is required because of the lower solubility of the macrocycle in more polar media) are 2  105 M
1 for Cl
and 1.4  104 for F
. The expanded porphyrin systems were also studied by anion transport experiments from one aqueous layer to another via a CH2Cl2 liquid membrane (Figure 4.16). These experiments show that 4.18 is a much better fluoride carrier than 4.17, but does not carry Cl
nearly as effectively. This is consistent with the stronger binding to Cl
(and stronger binding of F
by 4.17), which means that the anion is not readily released after transport (cf. cation receptors and cation carriers, Sections 3.8.2 and 3.14.2). The initial anion flux, Φ, (defined as the number of moles of anion transported per unit time) is 5.03 µmol h
1 for F
and 1.56 µmol h
1 for Cl
. Furthermore, addition of Cl
was found to inhibit F
transport. In the absence of the macrocycle, the flux is negligible. A gradual neutralisation of the pH on both sides of the experiment was also noted, consistent with a symport mechanism in which H and halide anion are carried simultaneously in the same direction (cf. transport of Na and K across cell membranes via antiport, Section 2.2; see also Section 5.1.5).  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 246 Anion Binding Figure 4.16 Simultaneous transport of H and Cl
(symport) through a liquid CH2Cl2 membrane (X  F, Cl). In order to recognise and transport nucleotide anions sapphyrin has been derivatised with a lariat- type side arm bearing a cytosine derivative, complementary to guanosine monophosphate (5′-GMP) but not to its cytosine or adenosine analogues (5′-CMP and 5′-AMP). Simultaneous recognition of the phosphate anion group and the guanosine nucleobase resulted in the selective transport of 5′-GMP by a factor of 8 – 100.24 Expanding the ring size still further gives cyclo[n]pyrroles (n  6–8); pyrrole-based macrocycles with 6-8 pyrrole units linked by a single bond, and able to encapsulate increasingly larger anions. Thus in the neutral form of cyclopyrrole (4.19) binds hydrogen sulfate in DMSO with log K  5.8  103 M
1. This binding probably reflects protonation of the neutral macrocycle by the acidic anion and indeed an X-ray crystal structure has been obtained of diprotonated octaphyin binding SO42
, Figure 4.15b.23 4.4.5 Cyclophane Hosts A range of cyclophanes (compounds containing a bridged aromatic ring – for a full discussion of cyclophane chemistry see Section 6.5) with protonated nitrogen functionalities have been used with great success as anion binding hosts. In particular, the diphenylmethane moiety is commonly used  10.1002/9780470740880.ch4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/9780470740880.ch4 by Uva Universiteitsbibliotheek, Wiley Online Library on [16/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License From Cation Hosts to Anion Hosts – a Simple Change in pH 247 as a spacer in the construction of cyclophane hosts for both anions and neutral molecules in order to impart curvature and increase the size of the host walls while retaining rigidity. Macrocycle 4.20 binds anions such as 1-anilino-8-naphthalenesulfonate (ANS), which is used as a fluorescent probe in order to measure host–ANS affinity spectrophotometrically. Binding of ANS relies on hydrophobic and π–π stacking interactions, as well as electrostatic interactions and hydrogen bonds, since neutral molecules are also strongly bound. H N H N