Large anion binding in water

Abstract

Large water-soluble anions with chaotropic character display surprisingly strong supramolecular interactions in water, for example, with macrocyclic receptors, polymers, biomembranes, and other hydrophobic cavities and interfaces. The high affinity is traced back to a hitherto underestimated driving force, the chaotropic effect, which is orthogonal to the common hydrophobic effect. This review focuses on the binding of large anions with water-soluble macrocyclic hosts, including cyclodextrins, cucurbiturils, bambusurils, biotinurils, and other organic receptors. The high affinity of large anions to molecular receptors has been implemented in several lines of new applications, which are highlighted herein.

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Khaleel I. Assaf

Khaleel I. Assaf was born in 1985 in Amman, Jordan. He started his academic journey at the Hashemite University. In 2010, he received a DAAD fellowship, which allowed him to pursue his Ph.D. study at Constructor University, Germany, working under the direction of Werner M. Nau. Subsequently, he continued his academic pursuits as a postdoctoral fellow. Currently, he serves as an Associate Professor of organic chemistry at Al-Balqa Applied University in Jordan. His main research interests fall into the fields of physical-organic and supramolecular chemistry, with a passion for unraveling the complexities of molecular recognition, leading to the design of functional materials and applications.

Werner M. Nau

Werner M. Nau was born in Fulda, Germany, in 1968. Following his Bachelor's and Master's studies in chemistry in Germany and Canada, he obtained his PhD from the University of Würzburg in 1994 in spectroscopy and organic chemistry. Following his postdoc at the University of Ottawa, Canada, with J.C. Scaiano in photochemistry, he completed his habilitation in physical chemistry with J. Wirz at the University of Basel, Switzerland, where he became Assistant Professor in 2000. Since 2002, he has been Professor of Chemistry at Constructor University, Germany, and since 2012 he has been Dean of Science. His research combines supramolecular chemistry with organic photochemistry and focuses on applications in enzyme and membrane transport assays.

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Introduction

The binding of “mineral water” anions such as F⁻, Cl⁻, CO₃²⁻, SO₄²⁻, PO₄³⁻, and NO₃⁻ in water has been in the focus of anion binding by supramolecular receptors, among others, due to their biological and environmental importance. These common anions are strongly solvated in aqueous solution, which presents a challenge for the supramolecular design of the associated receptors.^1–4 In contrast, the binding of large anions such as I⁻, SCN⁻, ClO₄⁻, and PF₆⁻, which have a lower charge density and are weakly hydrated, has received comparably less attention, among others, due to the fact that they play, with the notable exception of I⁻, less critical natural roles.^5–9 On the other hand, the diametrically opposed effects that the two groups of anions can exert on biological systems have been recognized since long by the Hofmeister series, which conventionally classifies the character of ions as being salting-out (kosmotropic) or salting-in (chaotropic) in nature.¹⁰ Recently, the supramolecular chemistry of long-known synthetic cluster anions of the borate-^11,12 and polyoxometalate-type¹³ has moved into the focus, driven by the observation that their affinity with hydrophobic binding sites does exceed,¹⁴ against expectation, those of hydrophobic guest molecules.^15–17 These large anions were recognized to expand the traditional Hofmeister series as so-called superchaotropic anions,^14,18–34 and their peculiar supramolecular chemistry has revived the interest in large anion binding in water.^{14,25,26,35–49} The present review provides a short historical background and a summary of the most recent developments in this emerging field. Amphiphilic anions, which are composed of long hydrophobic chains and kosmotropic head groups, such as sodium dodecyl sulphate, are also large anions in the metric sense, but display entirely different properties and are not the subject of this review. Also excluded from further discussion are hydrophobic anions, such as Ph₄B⁻, which in contrast to superchaotropic anions display positive hydration free energies.¹⁸

Historical developments in large anion binding and initial empirical observations (up to 2015)

Cyclodextrins^17,50,51 (CDs, Fig. 1) are the oldest and most intensively studied class of macrocyclic hosts, and they are also the first ones that were found to display sizable affinity towards large anions, despite their known propensity to preferentially complex hydrophobic guests inside their cavity.^{5–8,52–54} The first qualitative observation for the binding of a large anion, namely I⁻, by α-CD was made by Schlenk and Sand 60 years back.⁵ Soon after that, the first binding constant (K_a) between ClO₄⁻ and α-CD was determined by Cramer et al., who found that this large anion competed with the binding of 4-nitrophenolate.⁵⁵ This binding could be readily followed by optical titration through the color change of the organic guest, incidentally also setting up the putative first example for an indicator displacement assay in host–guest chemistry. The binding was sizable for an inorganic anion (K_a = 29 M⁻¹), but insufficiently high to draw attention at that time. Cramer also established the binding of NO₃⁻ with α-CD, while no binding was observed for SO₄²⁻ and PO₄³⁻.⁵⁵ Several subsequent studies showed that α-CD also binds many other anions, such as PF₆⁻, I₃⁻, and SCN⁻, with a sizable binding affinity,^56–59 while for anions such as F⁻, Cl⁻, and Br⁻, the affinity was very low, if at all detectable.^60,61 In addition, the next larger homologous host, β-CD, also displayed a sizable but lower affinity to large anions.^6,8 The study by Wojcik and Rohrbach from 1975 reported a range of 10–29 M⁻¹ for the binding of SCN⁻, I⁻, and ClO₄⁻ to α-CD. Matsui et al. used NMR spectroscopy to study the complexation-induced chemical shifts of the CD protons (including α-, β-, and γ-CD) for different anions,⁵² which again revealed a preferential binding of certain anions (N₃⁻, I⁻, SCN⁻, and ClO₄⁻), with the most stable inclusion complexes being observed with α-CD. In contrast, anions such as F⁻, HCO₃⁻, H₂PO₄⁻, HPO₄²⁻, and SO₄²⁻ showed no or negligible changes in the chemical shifts. Similarly, Taraszewska and Wójcik studied systematically the complexation of inorganic anions with β-CD.⁸ Although the reported association constants of the anions were lower (<9 M⁻¹) than for α-CD (up to 29 M⁻¹), the weakly hydrated anions showed stronger binding than strongly hydrated ones (such as PO₄³⁻), Table 1.

Fig. 1 Chemical structures of macrocyclic receptors with sizable affinity to large anions in aqueous solution.

Table 1 Association constants (K_a) of selected large anions to water-soluble macrocycles in aqueous solution, sorted according to the anion size and grouped according to the type of macrocycle; see Fig. 1

Host	Anion	Radius^a/pm	K a × 10^3/M^−1	Technique^b	Year^c	Ref.
a Taken from ref. 106 and 107. b The applied techniques for the determination of association constants are abbreviated as follows: cond. stands for conductometric titration; NMR stands for ^1H NMR titration; calor. stands for calorimetry titration; optical stands for competitive optical titration (UV-Vis or fluorescence spectroscopy); electr. stands for electrochemical (potentiometric) titration; and ITC stands for isothermal titration calorimetry. c Year refers to the first report. d Given as an averaged value from different reports. e Approximate values based on the calculated molecular volume using HyperChem(TM) Professional 8.0 software,^108 with the assumption of a spherical shape from which the radius was obtained. f From ref. 18. g Calculated from the diameters obtained from XRD structures. h Very weak binding; Ka was not determined. i Hexakis(6-deoxy-6-amino-)-α-CD. j Mono[6-(1-pyridinio)-6-deoxy]-α-CD. k Values correspond to the cluster core; calculated from the diameters obtained from the geometry-optimized structures. l Mono[6-(1-pyridinio)-6-deoxy]-β-CD. m Heptakis(6-deoxy-6-amino-)-β-CD. n 1 : 2 host–guest complexation was reported. o Mono[6-(1-pyridinio)-6-deoxy]-γ-CD. p Octakis(6-deoxy-6-amino)-γ-CD. q Functionalized bambusuril with R = –CH2Ph-COO^−, see Fig. 1. r Functionalized bambusuril with R = –(CH2CH2O)3CH3, see Fig. 1.
α-CD^j	Br^−	196	0.01	NMR	1993	98
α-CD^j	NO3^−	200	0.02	NMR	1993	98
α-CD	SCN^−	213	0.03^d	NMR, cond., calor.	1975	7, 52, 53, 56 and 58
α-CD^j	SCN^−	213	0.36	NMR	1993	98
α-CD	I^−	220	0.02^d	NMR, cond., calor.	1967	7, 52, 53, 55, 56, 58 and 60
α-CD^j	I^−	220	0.20	NMR	1993	98
α-CD	ClO4^−	240	0.04^d	NMR, cond., calor.	1967	7, 52, 53, 55–58, 61 and 82
α-CD^j	ClO4^−	240	0.33	NMR	1993	98
α-CD	CF3SO3^−	255^e	0.04	Calor. and optical	1996	57
α-CD	I3^−	285	270^d	Electr.	1947	64 and 99–101
α-CD	PF6^−	295	0.04	Calor. and optical	1996	57
α-CD^i	Fe(CN)6^4−	343	300	Electr.	1996	102
α-CD^i	Fe(CN)6^3−	380	2	Electr.	1996	102
α-CD	B10H10^2−	393^f	0.04	NMR	2019	37
α-CD	B12H12^2−	400^f	0.1	NMR	2015	14
α-CD	Re6S8(CN)6^4−	580^g	0.73	ITC	2019	81
α-CD	Re6Se8(CN)6^4−	590^g	0.03	ITC	2019	81
α-CD	Re6Te8(CN)6^4−	600^g	—^h	ITC	2019	81
β-CD	SCN^−	213	0.01^d	Cond., NMR, calor.,	1975	6–8, 52, 56 and 58
β-CD	I^−	220	0.01^d	Cond., NMR, optical	1977	7, 52, 56 and 103
β-CD^l	I^−	220	0.04	NMR	1993	98
β-CD	ClO4^−	240	0.01^d	NMR, calor., optical	1977	6–8, 52, 56, 57 and 103
β-CD	CF3SO3^−	255	0.06	Calor.	1996	57
β-CD	I3^−	285	3.03	Electr.	1947	99, 100 and 64
β-CD	PF6^−	295	0.09	Calor.	1996	57
β-CD^m	Fe(CN)6^4−	343	800	Electr.	1996	102
β-CD^m	Fe(CN)6^3−	380	4	Electr.	1996	102
β-CD	B10H10^2−	393^f	0.11	NMR	2019	37
β-CD	B12H12^2−	400^f	0.2	NMR	2015	14
β-CD	Ph4B^−	425^e	0.01	Electrophoresis	2001	104
β-CD	B21H18^−	425^k	130	ITC	2020	85
β-CD	Re6S8(CN)6^4−	580^g	0.88	ITC	2019	81
β-CD	COSAN-1	583^k	26	ITC	2019	35
β-CD	COSAN-2	583^k	710	ITC	2019	35
β-CD	COSAN-3	583^k	21	ITC	2019	35
β-CD	COSAN-4	583^k	19	ITC	2019	35
β-CD	COSAN-5	583^k	24	ITC	2019	35
β-CD	COSAN-6	583^k	38	ITC	2019	35
β-CD	COSAN-7	583^k	180	ITC	2019	35
β-CD	COSAN-8	583^k	55	ITC	2019	35
β-CD	COSAN-9	583^k	59	ITC	2019	35
β-CD	Re6Se8(CN)6^4−	590^g	0.75	ITC	2019	81
β-CD	Re6Te8(CN)6^4−	600^g	0.08	ITC	2019	81
β-CD	PMO12O40^3−	692^f	2	ITC	2015	46
γ-CD	SCN^−	213	4.1	NMR	1997	52
γ-CD	I^−	220	4.9	NMR	1997	52
γ-CD^o	I^−	220	0.02	NMR	1993	98
γ-CD	I3^−	285	0.5	Electr.	1985	64
γ-CD^p	Fe(CN)6^4−	343	1000	Electr.	1996	102
γ-CD^p	Fe(CN)6^3−	380	5	Electr.	1996	102
γ-CD	B10H10^2−	393^f	0.06	NMR	2019	37
γ-CD	B12H12S^2−	400^f	9.2	ITC	2015	14
γ-CD	Ph4B^−	425^e	108	Electrophoresis	2001	104
γ-CD	B21H18^−	425^k	1800	ITC	2020	85
γ-CD	H3NB10Cl9^−	500^k	5.9	ITC	2019	36
γ-CD	Me3NB10Cl9^−	500^k	77	ITC	2019	36
γ-CD	B12Cl12^2−	525	17	ITC	2015	14
γ-CD	H3NB10Br9^−	530^k	2.4	ITC	2019	36
γ-CD	Mr3NB10Br9^−	530^k	430	ITC	2019	36
γ-CD	Me3NB10I9^−	558^k	78	ITC	2019	36
γ-CD	H3NB10I9^−	558^k	3.8	ITC	2019	36
γ-CD	B12Br12^2−	560^f	960	ITC	2015	14
γ-CD	Re6S8(CN)6^4−	580^g	0.9	ITC	2018	83
γ-CD	COSAN-1	583^k	191	ITC	2019	35
γ-CD	COSAN-2	583^k	3000	ITC	2019	35
γ-CD	COSAN-3	583^k	300	ITC	2019	35
γ-CD	COSAN-4	583^k	3600, 90^n	ITC	2019	35
γ-CD	COSAN-5	583^k	1300	ITC	2019	35
γ-CD	COSAN-6	583^k	6, 8000^n	ITC	2019	35
γ-CD	COSAN-7	583^k	72	ITC	2019	35
γ-CD	COSAN-8	583^k	12	ITC	2019	35
γ-CD	COSAN-9	583^k	720	ITC	2019	35
γ-CD	H2W12O40^6−	590^g	0.02	NMR	2021	93
γ-CD	BW12O40^5−	590^g	1.03	ITC	2021	93
γ-CD	SiW12O40^4−	590^g	17.2, 0.56^n	ITC	2021	93
γ-CD	PW12O40^3−	590^g	2.92	ITC	2021	93
γ-CD	SiW11MoO40^4−	590^g	17.7, 0.371^n	ITC	2021	94
γ-CD	PW11VO40^4−	590^g	8.5, 0.22^n	ITC	2021	94
γ-CD	PW11VO40^5−	590^g	0.164	ITC	2021	94
γ-CD	SiW11MoO40^5−	590^g	0.478	ITC	2021	94
γ-CD	SiW11VO40^5−	590^g	0.618	ITC	2021	94
γ-CD	SiW11VO40^6−	590^g	—^h	ITC	2021	94
γ-CD	SiW11MoO40^6−	590^g	—^h	ITC	2021	94
γ-CD	Re6Se8(CN)6^4−	590^g	1.5, 13^n	ITC	2018	83
γ-CD	B12I12^2−	590^f	67	ITC	2015	14
γ-CD	Re6Te8(CN)6^4−	600^g	38, 13^n	ITC	2018	83
γ-CD	PMO12O40^3−	692^f	40, 1^n	ITC	2015	46
γ-CD	P2W18O62^6−	701^f	3.2, 0.37^n	ITC	2017	25
δ-CD	B12Cl12^2−	525^f	2500	ITC	2016	44
δ-CD	B12Br12^2−	560^f	2600	ITC	2016	44
δ-CD	B12I12^2−	590^f	67	ITC	2016	44
ε-CD	B12Cl12^2−	525^f	29	ITC	2016	44
ε-CD	B12Br12^2−	560^f	140	ITC	2016	44
ε-CD	B12I12^2−	590^f	2100	ITC	2016	44
ζ-CD	B12Cl12^2−	525^f	2	ITC	2016	44
ζ-CD	B12Br12^2−	560^f	6	ITC	2016	44
ζ-CD	B12I12^2−	590^f	8	ITC	2016	44
Bambusuril^q	F^−	133	0.1	NMR	2015	45
Bambusuril^q	F^−	133	0.01	ITC	2015	45
Bambusuril^q	Cl^−	181	0.91	NMR	2015	45
Bambusuril^q	Cl^−	181	0.4	ITC	2015	45
Bambusuril^q	CN^−	191	1.1	NMR	2015	45
Bambusuril^q	Br^−	196	140	NMR	2015	45
Bambusuril^q	Br^−	196	33	ITC	2015	45
Bambusuril^q	NO3^−	200	480	NMR	2015	45
Bambusuril^q	I^−	220	10 000	NMR	2015	45
Bambusuril^q	I^−	220	2000	ITC	2015	45
Bambusuril^q	BF4^−	230	4300	NMR	2015	45
Bambusuril^q	ClO4^−	240	55 000	NMR	2015	45
Bambusuril^q	IO4^−	250	6.5	NMR	2015	45
Bambusuril^q	ReO4^−	260	30	NMR	2015	45
Bambusuril^q	PF6^−	295	2200	NMR	2015	45
Bambusuril^r	Cl^−	181	1.2	ITC	2022	77
Bambusuril^r	CN^−	191	21	NMR	2022	77
Bambusuril^r	N3^−	195	21	ITC	2022	77
Bambusuril^r	Br^−	196	120	ITC	2022	77
Bambusuril^r	NO3^−	200	330	ITC	2022	77
Bambusuril^r	SCN^−	213	1200	ITC	2022	77
Bambusuril^r	I^−	220	32 000	ITC	2022	77
Bambusuril^r	SeCN^−	229^e	2800	ITC	2022	77
Bambusuril^r	BF4^−	230	1300	ITC	2022	77
Bambusuril^r	ClO4^−	240	47 000	ITC	2022	77
Bambusuril^r	IO4^−	250	56	ITC	2022	77
Bambusuril^r	ReO4^−	260	470	ITC	2022	77
Bambusuril^r	PF6^−	295	1600	ITC	2022	77
Bambusuril^r	[Au(CN)2]^−	320	1300	ITC	2022	77
Bambusuril^r	[Ag(CN)2]^−	330	13	ITC	2022	77
Biotin[6]uril	Cl^−	181	0.06	NMR	2015	47
Biotin[6]uril	Cl^−	181	0.03	ITC	2015	47
Biotin[6]uril	N3^−	195	0.79	NMR	2015	47
Biotin[6]uril	N3^−	195	0.4	ITC	2015	47
Biotin[6]uril	Br^−	196	1	NMR	2015	47
Biotin[6]uril	Br^−	196	0.5	ITC	2015	47
Biotin[6]uril	NO3^−	200	0.08	NMR	2015	47
Biotin[6]uril	NO3^−	200	0.05	ITC	2015	47
Biotin[6]uril	CNO^−	203	0.01	NMR	2015	47
Biotin[6]uril	CNO^−	203	0.06	ITC	2015	47
Biotin[6]uril	SCN^−	213	32	NMR	2015	47
Biotin[6]uril	SCN^−	213	13	ITC	2015	47
Biotin[6]uril	I^−	220	4^d	NMR	2015	47
Biotin[6]uril	I^−	220	2.5	ITC	2015	47
Biotin[6]uril	SeCN^−	229^e	20	NMR	2015	47
Biotin[6]uril	SeCN^−	229^e	10	ITC	2015	47
Biotin[6]uril	ClO4^−	240	0.5	NMR	2015	47
Biotin[6]uril	ClO4^−	240	0.25	ITC	2015	47
Octa acid	NO3^−	200	<0.01	NMR	2011	66
Octa acid	SCN^−	213	0.03	NMR	2011	66
Octa acid	SCN^−	213	0.04	ITC	2016	105
Octa acid	ClO3^−	220^e	<0.01	NMR	2011	66
Octa acid	I^−	220	0.01	NMR	2011	66
Octa acid	I^−	220	0.02	ITC	2016	105
Octa acid	BH3CN^−	225^e	0.15	ITC	2016	105
Octa acid	ClO4^−	240	0.10	NMR	2011	66
Octa acid	ClO4^−	240	0.1	ITC	2016	105
Octa acid	N(CN)2^−	240^e	0.04	ITC	2016	105
Octa acid	IO4^−	250	0.24	ITC	2016	105
Octa acid	CF3SO3^−	255^e	0.31	ITC	2016	105
Octa acid	CH3SO2S^−	260^e	0.66	ITC	2016	105
Octa acid	ReO4^−	260	0.37	ITC	2016	105
Octa acid	Cl2CHCO2^−	270^e	0.05	ITC	2016	105
Octa acid	Cl3CCO2^−	283^e	6.34	ITC	2016	105
Octa acid	PF6^−	295	0.79	ITC	2016	105

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The original study by Cramer et al. did not allude to the factors responsible for the large anion binding by CDs. Subsequently, through the investigation of a series of different anions, an empirical relationship between their binding affinities and their chaotropic (water-structure breaking) character emerged.^8,52,58 Wojcik and Rohrbach related the trend in binding constants of anions to their B⁻ viscosity coefficient,⁵⁸ a coefficient of the linear term of the concentration-dependence of proton relaxation rates, which has later become an accepted parameter for the water-structural entropic properties (chaotropicity) of ions.^62,63 A linear correlation between the binding constants⁵⁸ of the anions and their B⁻ coefficient was established. Matsui et al. explicitly interpreted the differential binding of anions to CDs in terms of their chaotropic or “anti-chaotropic” character,⁵² a qualification which has later been replaced by the term “kosmotropic”, but a detailed interpretation in terms of the underlying intermolecular interactions remained elusive. Similarly, Taraszewska and Wójcik studied systematically the complexation of inorganic anions with β-CD⁸ and related the magnitude of their affinities to their position in the Hofmeister series as well.

The higher binding affinity of the anions to CDs is also related to the size/shape complementarity, which is reflected in the higher affinity of the large anions (I⁻, SCN⁻, and ClO₄⁻),⁵² because only those are able to fill the hydrophobic cavity to a significant extent. The binding constant of I₃⁻ decreased from α- to β- to γ-CD.⁶⁴ Due to the large cavity size of γ-CD, only few examples of the binding of classical anions by this host have been reported.^52,64 A summary of anion binding to CDs is shown in Table 1, which clearly indicates that the binding affinity of anions to CDs increases with the chaotropicity/size of the anions. However, even though independent correlations between the binding affinities of inorganic anions with CDs and their chaotropicity became apparent, the unusual affinity pattern received little attention, presumably because the absolute binding affinities were too small to have major practical implications.

While studying Hofmeister effects on the binding of an organic guest (adamantanecarboxylate) with an aromatic host (octa acid, Fig. 1),^65,66 it eventually emerged that large anions, e.g. ClO₄⁻, showed also sizable binding to this macrocycle (95 M⁻¹).^66,67 The chaotropic anion competed with the binding of adamantanecarboxylate in the same manner as Cramer had previously observed for the 4-nitrophenolate-α-CD couple.⁵⁵ The binding affinity of adamantane carboxylic acid to octa acid was found to slightly increase in the presence of kosmotropic anions such as F⁻, an observation which was linked to the enhanced hydrophobic effect caused by the presence of such anions. In contrast, chaotropic anions such as SCN⁻ and ClO₄⁻ lowered the binding affinity of adamantane carboxylic acid, which was attributed to the ability of chaotropic anions to weaken the hydrophobic effect and, thereby, the competition with the hydrophobic guest for the hydrophobic cavity of octa acid.⁶⁶ Although the driving force for ClO₄⁻ binding was not discussed, it transpired that the phenomenon of large anion binding to hydrophobic pockets was more general and not limited to CDs.⁶⁸ One tentative explanation involved the presence of high-energy water inside these hosts, which could account for an “intrinsic” driving force that would drive the encapsulation process regardless of the precise chemical nature or the charge status of a suitably sized spherical guest.^69,70 In 2010, upon successful synthesis of a new macrocyclic host derived from a glycoluril derivative through an acid condensation reaction with formaldehyde, Sindelar and co-workers noticed by X-ray diffraction of the obtained hexamer receptor (bambusuril, Fig. 1) that the cavity of this molecular container accommodated a Cl⁻ ion.⁷¹ The macrocycle was later intensively investigated for its binding to halides with high affinity and selectivity (see below).^{38,39,72–77}

Recent developments in large anion binding and mechanistic interpretations (after 2015)

The tentative idea that the release of high-energy water from the macrocycles presents the major driving force for the complexation of large anions needed to be abandoned in 2015 when an exceptionally strong binding of several large anions to different macrocycles (Fig. 1) was established by several research groups.^14,45–47 The affinities reached micromolar values and rivaled, e.g. for CDs,¹⁴ the affinities of hydrophobic guests, which until then had been considered as the tightest binding motifs.^15,78,79

Besides classical large anions with chaotropic character such as I⁻, PF₆⁻, and ClO₄⁻, which showed much stronger binding to bambusuril and biotinuril than to CDs,^45,47 two classes of very large, highly charged, synthetic anions, namely boron clusters and polyoxometalates (Fig. 2), were found to complex very strongly to γ-CD^{14,25,26,35–37,40,46,80} as well as to the larger homologues (ε-CD, δ-CD, and ζ-CD,⁴⁴ see Fig. 1). In the course of two independent investigations, it turned out, by employing classical Hofmeister salting-in¹⁴ and cloud point measurements,^21,23 that the large cluster anions behave as superchaotropic anions which expand the Hofmeister series to the chaotropic side and which outperform classical chaotropic anions such as ClO₄⁻ in regard to their salting-in properties.²¹

Fig. 2 Molecular structures and size comparison of large (chaotropic) anions with sizable binding affinity to macrocyclic receptors in water.

While it became apparent that the binding strength to the hydrophobic cavity increased with the chaotropicity of the anion (up to the turning point where the anion size largely exceeded the cavity size),^14,44,81 the underlying driving forces remained more difficult to pinpoint. A first hint came from the thermochemical signature of the binding process, which showed that the binding of the superchaotropic anions is enthalpically driven.¹⁴ In retrospect, one of the first studies that pointed to an enthalpic driving force for the complexation of large inorganic anions by macrocycles was published in 1973, in which the binding of ClO₄⁻ to α-CD had shown a favorable enthalpic contribution of −9.7 kcal mol⁻¹ and an unfavorable entropy.⁸² Similar signatures were reported on several occasions.^52,56,57

The effect became very pronounced when large anions were investigated.^14,25,44,83 For example, the thermochemical trend was also observed for the complexation of dodecaborate anions with CDs, and it was for the first time linked to their chaotropic character; the generic driving force was accordingly referred to as the chaotropic effect, to differentiate from the entropically driven (classical) hydrophobic effect.^14,44 Following the theory of Marcus for the aqueous solvation of ions, the large favorable enthalpy and unfavorable entropy were related to water structure recovery upon the relocation of the anion into the cavity as well as to the dispersion interaction between the highly polarizable guests and the cavity walls. Roughly speaking, the larger and more polarizable an anion, the lower its charge density, and the better delocalized its charge, the higher its chaotropicity (Fig. 2), until a turning point, when an ion adopts more hydrophobic ionic character (such as BPh₄⁻) on a continuous scale of aqueous solvation.¹⁸

Cyclodextrins as receptors for boron cluster anions

Dodecaborate anions of the type B₁₂X₁₂²⁻ (X: H, Cl, Br, and I, Fig. 2) form stable complexes with CDs, in particular with γ-CD, in aqueous solution,¹⁴ as reflected in their high binding affinities (e.g., ca. 10⁶ M⁻¹ for B₁₂Br₁₂²⁻ with γ-CD, see Table 1 for the binding affinity of other clusters).¹⁴ The single-crystal X-ray structure of the B₁₂Br₁₂²⁻/γ-CD complex is shown in Fig. 3a. Although the chaotropic effect has in common with the hydrophobic effect that the involved guests are weakly hydrated, they are conceptually distinct in that chaotropic anions interfere differently with the solvation-shell water structure than the hydrophobic species do. The contrasting solvation behavior is borne out by the diametrically opposed thermodynamic fingerprints of boron cluster anions versus hydrophobic residues.¹⁸ Since chaotropic agents are thought to “break” the water structure in the solvation shell (as assessed through large positive water-structural contributions to the hydration entropy), one may argue that their relocation into the hydrophobic cavity is not characterized by the positive entropic dehydration component characteristic of the hydrophobic effect, but rather by a strongly negative enthalpic one. High binding affinities of boron cluster anions have also been obtained for large-ring CDs (δ-, ε-, and ζ-CD).⁸⁴ For example, the binding affinity of B₁₂Br₁₂²⁻ with δ-CD (2.60 × 10⁶versus 0.96 × 10⁶ M⁻¹) even exceeded that with γ-CD, while B₁₂I₁₂²⁻ was found to strongly associate with the large ζ-CD cavity, with K_a = 2.1 × 10⁶ M⁻¹, Table 1.⁴⁴ A weaker binding affinity was obtained for the smaller B₁₀H₁₀²⁻ cluster with CDs,³⁷ while the binding affinity of the larger B₂₁H₁₈⁻ cluster to CDs exceeded that of B₁₂Br₁₂²⁻.⁸⁵ Other boron cluster anions of the type [H₃NB₁₀X₉]⁻ and [Me₃NB₁₀X₉]⁻ also showed sizable binding affinity to γ-CD. The complexation of the mono-anionic ammonium-substituted decaborate derivatives with γ-CD was, however, in general weaker than that of the di-anionic dodecaborate clusters (Table 1).³⁶ The variation of binding constants pointed to an interesting structure–activity relationship.

Fig. 3 Single-crystal X-ray structures of different superchaotropic anion complexes with CDs. (a) γ-CD/B₁₂Br₁₂²⁻, (b) γ-CD/PMO₁₂O₄₀³⁻, (c) β-CD/PMO₁₂O₄₀³⁻, (d) γ-CD/Re₆Te₈(CN)₆⁴⁻, (e) γ-CD/M₆O₁₉²⁻, (f) γ-CD/P₂W₁₈O₆₂⁶⁻/[Ta₆Br₁₂(H₂O)₆]²⁺, and (g) γ-CD/[Mo₁₅₄O₄₆₂H₁₄(H₂O)₇₀]¹⁴⁻/P₂W₁₈O₆₂⁶⁻.

Very recently, metallacarboranes, another class of sandwich inorganic boron clusters, have been found to form stable complexes with CDs in aqueous solution.³⁵ Cobalt bis(1,2-dicarbollide) and its derivatives (COSANs, Fig. 4), as mono-anionic clusters, associate strongly with the cavity of β- and γ-CD, Table 1.³⁵ The complexation of COSANs with CDs was investigated by NMR and UV−visible spectroscopy, as well as mass spectrometry, all of which confirmed the strong binding. Isothermal titration calorimetry (ITC) revealed high association constants up to 10⁶ M⁻¹.³⁵ The interaction between COSAN and the glucose moiety has also been studied in the context of biopolymeric interactions,⁸⁶ and they were found to bind to cationic peptides, which enables their use as membrane carriers.⁸⁷ Recently, the ability of COSANs to solubilize hydrophobic substances in water has been reported.⁸⁸ At first sight, this mimics the “salting-in” feature known for chaotropic and superchaotropic ions,¹⁴ but the mechanism turned out to be more involved in this case.

Fig. 4 Chemical structures of cobalt bis(1,2-dicarbollide) anions, which form stable complexes with β- and γ-CD in aqueous solution; valencies are saturated with hydrogens (B–H or C–H) unless substituents are shown explicitly.

Cyclodextrins as receptors for polyoxometalates and related inorganic cluster anions

In 2015, Stoddart and co-workers investigated the complexation of polyoxometalates with CDs.⁴⁶ The binding of PMO₁₂O₄₀³⁻ with β- and γ-CD was established in solution and in the solid state. ¹H NMR experiments confirmed the formation of inclusion complexes, in 1 : 1 and 2 : 1 host–guest stoichiometry with β- and γ-CD, respectively. The association constants were measured by ITC. The obtained binding affinity with β-CD was found to be 2 × 10³ M⁻¹, while the two-step sequential binding fitting for the complexation with γ-CD resulted in K_a1 = 4 × 10⁴ and K_a2 = 1 × 10³ M⁻¹.⁴⁶ The crystal structures of the PMO₁₂O₄₀³⁻ complexes with γ-CD and β-CD are shown in Fig. 3b and 3c. The complexation of PMO₁₂O₄₀³⁻ with (chiral) CDs was recognized to induce chirality of the encapsulated cluster in aqueous solution.⁸⁹

Three-component organic–inorganic systems were constructed based on the supramolecular interaction between γ-CD and the Dawson-type polyoxometalate, P₂W₁₈O₆₂⁶⁻, and a cationic cluster [Ta₆Br₁₂(H₂O)₆]²⁺.²⁵ X-ray analysis (Fig. 3f) revealed that the cationic cluster is trapped between two CD units, while the anionic cluster facilitates the assembly by linking the CD units from the narrow rim.²⁵ A two-site binding model was used to analyse the ITC data, Table 1. The P₂W₁₈O₆₂⁶⁻ anion afforded association constants of 3.2 × 10³ and 3.7 × 10² M⁻¹ for the 1 : 1 and 2 : 1 host–guest stoichiometries, respectively. Higher affinity values were obtained for the inclusion complexes of the cationic cluster, 1.5 × 10⁵ M⁻¹ for the 1 : 1 and 1.3 × 10⁵ M⁻¹ for the 2 : 1 host–guest stoichiometry. In a similar manner, hierarchical architectures were prepared by using γ-CD as a host in combination with the metallic cationic cluster [{Re₆Se₈}(H₂O)₆]²⁺ as well as the anionic ones [{Re₆Se₈}(CN)₆]⁴⁻ and P₂W₁₈O₆₂⁶⁻.⁹⁰ The solid-state structures of the ternary assemblies indicated that two γ-CD units interact with P₂W₁₈O₆₂⁶⁻ through the primary rim (partial inclusion), while the [{Re₆Se₈}(H₂O)₆]²⁺ cluster is positioned at the secondary rim. For the anionic [{Re₆Se₈}(CN)₆]⁴⁻ cluster, a 2 : 1 host–guest inclusion complex is formed, in which the cluster remains entrapped within two γ-CD units facing each other from the wider rims, while the polyoxometalate acts as a linker that connects the CD units through the association with the outer surface.⁹⁰ Another three-component hybrid assembly of the [Mo₁₅₄O₄₆₂H₁₄(H₂O)₇₀]¹⁴⁻ molecular wheel (Mo₁₅₄) encapsulating a sandwich-type complex between P₂W₁₈O₆₂⁶⁻ and two γ-CDs was reported (Fig. 3g), which revealed the association of these large anions with CDs in aqueous solution by NMR and solid-state structural analysis.²⁶

Water-soluble salts of anionic chalcogenide octahedral rhenium clusters (Re₆X₈(CN)₆⁴⁻, X = S, Se, Te) form stable inclusion complexes with CDs in aqueous solution.^81,83,91 A partial inclusion of the large anions was obtained for Re₆X₈(CN)₆⁴⁻ with α- and β-CD, while deeper inclusion complexes were observed with γ-CD.^81,83 In the case of α-CD, only the Re₆S₈(CN)₆⁴⁻ anion was found to be complexed between two CD units near the secondary rim. The Re₆X₈(CN)₆⁴⁻ anions are partially embedded between the β-CD rings through their primary rims. Similar to the single-crystal structure of B₁₂Br₁₂²⁻/γ-CD, the Re₆Se₈(CN)₆⁴⁻ and Re₆Te₈(CN)₆⁴⁻ clusters were entrapped between two γ-CD rings (Fig. 3d), while the smallest anion, Re₆S₈(CN)₆⁴⁻, formed a partial inclusion complex with γ-CD, in which the anion interacts with the secondary rims of γ-CD. In ¹H NMR aqueous-solution spectra, the complexation-induced chemical shifts confirmed the formation of inclusion complexes, as indicated by the large downfield shifts of the inner protons of γ-CD. In addition, ⁷⁷Se and ¹²⁵Te NMR experiments supported the complexation pattern in solution. The binding affinities of the anions to γ-CD were determined using ITC experiments, which showed that the complexation is enthalpically driven, in agreement with the chaotropic effect.⁸³ With the smallest CD homologue, α-CD, only the smallest cluster, Re₆S₈(CN)₆⁴⁻, showed a sizable 1 : 1 binding affinity of 730 M⁻¹. The larger clusters showed much weaker binding affinity to α-CD. The complexation of the three clusters to β-CD showed size complementarity, that is, the binding constants decreased with increasing size of the cluster, Table 1.⁸¹ The affinity of Re₆S₈(CN)₆⁴⁻ to β-CD (K_a = 900 M⁻¹) was slightly higher than that to α-CD. Interestingly, the oxidized form of the rhenium cluster showed higher affinity to β-CD (K_a1:1 = 2.3 × 10⁵ M⁻¹) compared to the parent one.⁹¹ This difference in binding affinity can be attributed to the higher chaotropicity of the oxidized form (Re₆S₈(CN)₆³⁻), induced by its lower charge density.⁹¹ Most recently, Leclerc et al. reported the formation of a 1 : 1 exclusion complex between γ-CD and a Preyssler-type anion, [NaP₅W₃₀O₁₁₀]¹⁴⁻.⁹² The large and highly charged anion interacts with the primary face of the γ-CD as revealed by single-crystal X-ray diffraction and ¹H NMR experiments.

Structural analysis in the solid-state indicated the formation of 1 : 1 inclusion complexes between Lindqvist M₆O₁₉²⁻ ions and γ-CD (Fig. 3f), in which the anion is encapsulated inside the cavity. ¹H NMR analysis also confirmed the complexation.⁸⁰ Very recently, Yao et al. studied the complexation of Keggin-type polyoxotungstate anions with γ-CD in aqueous solution.⁹³ Despite the fact that the investigated anions have similar structure and size, their binding affinity to γ-CD was found to decrease with increasing charge density on the anions, following the order H₂W₁₂O₄₀⁶⁻ < BW₁₂O₄₀⁵⁻ < SiW₁₂O₄₀⁴⁻ < PW₁₂O₄₀³⁻.⁹³ Similarly, the host–guest complexation of [XW₁₁MO₄₀]ⁿ⁻ Keggin-type anions (X = P or Si and M = Mo^V/VI or V^IV/V) by γ-CD, in which the anionic net charge can vary from 3− to 6−, indicated that the binding affinity is highest for the anion with lower charge density.⁹⁴ Such trends can be directly related to the variation of their chaotropicity, which decreases with increasing charge.¹⁸ γ-CD was also found to form host–guest complexes with metal chloride clusters ([M₆Cl₁₄]²⁻, M = Mo and W).⁹⁵

The cationic cluster, [Nb₆Cl₁₂(H₂O)₆]²⁺, binds to γ-CD with an association constant of 2.2 × 10³ M⁻¹ in water.⁹⁶ Despite its cationic nature, the complexation thermodynamics showed a large favorable enthalpic energy gain counterbalanced by a strong entropic penalty, a trend that is similar to the binding of superchaotropic anions, and which is consistent with the chaotropic effect. Cationic palladium(II)-oxo clusters, [Pd^II₆O₁₂M₈{(CH₃)₂AsO₂}₁₆(H₂O)₈]⁴⁺ (M = Ce^IV and Th^IV), have been recently synthesized and showed strong binding affinity (K_a > 10⁵ M⁻¹) to negatively charged macrocyclic hosts, namely 4-sulfonatocalix[n]arenes (n = 4, 6, and 8).⁹⁷ The binding events were again enthalpically driven. This observation may allow the classification of such large cations (e.g. [{Re₆Se₈}(H₂O)₆]²⁺, [Nb₆Cl₁₂(H₂O)₆]²⁺, and [Ta₆Br₁₂(H₂O)₆]²⁺) as being superchaotropic in nature, thereby constituting the first examples of the area of “large cation binding in water” that is not further scrutinized here.

Cucurbiturils, bambusurils, and biotinurils

Cucurbit[n]urils, Fig. 1, are water-soluble macrocyclic host molecules that are known for their high binding affinity towards cationic and neutral guest molecules, Fig. 1.^109–111 Their cavity is classified as hydrophobic and it is accessible through two identical carbonyl rims. Examples of the encapsulation of anions inside CBs are rare,^112–119 which can be attributed to the high barrier required to cross the negatively charged openings. Although there is no evidence for the inclusion of anions inside CBs in solution, single-crystal X-ray structures revealed several cases.¹¹⁸ For example, chloride and nitrate ions were found inside the cavity of CB5 and its permethylated derivatives, in which the complexation is assisted by the docking of the counter cations at the rims.^113–115

In contrast to CBn, bambusurils (Fig. 1) have been recognized as anion receptors in both organic and aqueous solutions.^{38,45,71–73,120} The X-ray structure of bambus[6]uril revealed anions inside its cavity, such as Cl⁻, which indeed acts as a template in the synthesis of bambusurils.⁷² Cl⁻ can be displaced by other halides, such as I⁻, in a mixture of methanol and chloroform, as confirmed by ¹H NMR experiments.⁷¹ The recognition propensity of a water-soluble bambusuril was tested with various anions (F⁻, Cl⁻, Br⁻, I⁻, CN⁻, IO₄⁻, ReO₄⁻, NO₃⁻, PF₆⁻, BF₄⁻, and ClO₄−⁻) in water, see Table 1. The measured association constants were surprisingly high (K_a > 10² M⁻¹) even for highly solvated ions, such as F⁻, and reached values of 10⁷ M⁻¹ for I⁻ and ClO₄⁻.⁴⁵ ITC data in water verified a 1 : 1 binding mode in solution and showed that the complexation was associated with large favourable enthalpy values compensated by entropic penalties.⁴⁵ Systematic analysis of the binding of halides to a bambusuril derivative, soluble in various solvents, was also performed.¹²⁰ The obtained results indicated that halide binding in water and chloroform is invariably driven by favorable enthalpic effects, modulated by an entropic penalty, while in alcohols and nonpolar solvents both a favorable enthalpy and entropy contribute to anion binding, most pronounced for large anions.¹²⁰

The acid-mediated reaction between biotin and formaldehyde in hydrochloric acid led to the selective formation of a 6 + 6 macrocycle (biotin[6]uril, Fig. 1).¹²¹ A templating effect of the halide anions was assigned as the driving force for hexamer formation in H₂SO₄. In analogy to bambusuril, biotin[6]uril binds a variety of inorganic anions in water.^47,121 The affinity followed the order Cl⁻ < NO₃⁻ < ClO₄⁻ < N₃⁻ < Br⁻ < I⁻ < SeCN⁻ < SCN⁻, Table 1. With the exception of ClO₄⁻, which may be too large, this ordering follows again the chaotropicity of the anions. The thermodynamics of complexation was found to be enthalpically favorable and entropically unfavourable.⁴⁷ The single-crystal X-ray structure of I⁻ entrapped inside the cavity of biotin[6]uril is shown in Fig. 5.

Fig. 5 Single-crystal X-ray structure of biotin[6]uril encapsulating an iodide ion.¹²¹

Hemicucurbit[n]urils are another congener of the cucurbituril family.^122–124 Their formation (n = 6 and 8) during the synthesis was found to be controlled by the presence of anions (templating effect), such that the formation of cyclohexanohemicucurbit[6]uril (cycHC[6]) is predominant with halides, while carboxylates and PF₆⁻ selectively facilitate the formation of cycHC[8].¹²⁵ The chiral (all-R)-cyclohexanohemicucurbit[8]uril was found to selectively bind large anions in methanol.¹²³ The highest association constant was measured for SbF₆⁻ (K_a = 2.5 × 10⁵ M⁻¹). In the gas phase, the order SbF₆⁻ ≈ PF₆⁻ > ReO₄⁻ > ClO₄⁻ > SCN⁻ > BF₄⁻ > HSO₄⁻ > CF₃SO₃⁻ for anion complexation was obtained, Fig. 6.

Fig. 6 (a) Crystal structure of the cyclohexanohemicucurbit[8]uril·SbF₆⁻ complex. (b) Binding affinities (log K_a) of large anions to cyclohexanohemicucurbit[8]uril as a function of the anion volume (V_anion). Note the clear trend of increasing affinity with the anion size when the two perfluorinated organic ions at the bottom are excluded. Reproduced from ref. 123 with permission from the Royal Society of Chemistry.

It is worth mentioning that the association of chaotropic anions with macrocyclic hosts is not limited to the interaction with macrocyclic cavities and the formation of inclusion-type complexes, but it applies similarly to the interaction with the outer walls, to form exclusion-type of complexes.^{118,126–140} For example, superchaotropic anions, such as polyoxometalates and dodecaborate clusters, associate with the hydrophobic exterior of CBn macrocycles (Fig. 7), which results in the formation of hybrid aggregates.^134–136

Fig. 7 Single-crystal X-ray structure of the exclusion complexes of superchaotropic anions with CB7. (a) CB7-B₁₂Cl₁₂²⁻ and (b) CB8-V₁₈O₄₂¹²⁻.

Octa acid and other macrocyclic hosts

Gibb and co-workers found that chaotropic anions have sizable affinity to the hydrophobic concavity of the octa acid host (Fig. 1).^65–67,105 An extended series of anions (as sodium salts) was studied (F⁻, SO₄²⁻, AcO⁻, Cl⁻, Br⁻, NO₃⁻, ClO₃⁻, I⁻, SCN⁻, and ClO₄⁻).^{67,68,105,1411}H NMR displacement experiments provided evidence for the actual inclusion of large anions into octa acid, while ITC experiments revealed association constants (1–100 M⁻¹, Table 1) in the order NO₃⁻ < ClO₃⁻ < I⁻ < SCN⁻ ≪ ClO₄⁻. The binding of other anions to octa acid was further investigated by ¹H NMR and ITC.¹⁰⁵ The largest affinities were measured for Cl₃CCO₂⁻ (K_a = 6340 M⁻¹) and PF₆⁻ (K_a = 790 M⁻¹). The thermodynamic signature of the binding of these anions is in line with the chaotropic effect established for CDs.

Examples of the binding of large anions to other molecular receptors and cages have also been documented.^142–148 The stability of dodecaborate anions with CDs and tetrathiafulvalene (Fig. 8) was established in the gas phase.¹⁴⁹ Nitschke and co-workers reported on the encapsulation of dianionic clusters, Mo₆O₁₉²⁻ and B₁₂F₁₂²⁻, inside a metal–organic cage in acetonitrile with relatively high binding constants of 2.9 × 10³ and 2.2 × 10³ M⁻¹, respectively, which were one order of magnitude higher than those measured for monoanionic guests, such as BPh₄⁻, CB₁₁H₁₂⁻, and B(C₆F₅)₄⁻, which pointed to a dominant electrostatic effect.¹⁵⁰ The Mo₆O₁₉²⁻ cluster was found to form an inclusion complex with a coordination cage in organic solvents as supported by NMR and ESI-MS results.¹⁵¹ The association of conventional chaotropic anions with other molecular receptors was also observed, in non-aqueous solution. The weakly coordinating anion PF₆⁻ showed very high affinity to the tricarbazolo triazolophane macrocycle (Fig. 8) in organic solvents.¹⁵² The C5-symmetric penta-t-butylpentacyanopentabenzo[25]annulene macrocycle, known as cyanostar, showed also high association constants for large anions (e.g., PF₆⁻ and ClO₄⁻).¹⁵³ Evidently, since the aqueous desolvation effect does not play a role here, dispersion interactions of the large anions must also be important.

Fig. 8 Example of other molecular receptors with established affinity to large anions.

Potential applications of large anion binding

The earliest application of a supramolecular host–guest complex of a large anion is the famous color reaction of I⁻ (I₃⁻) with starch that has been discovered more than 200 years ago.¹⁵⁴ Although not macrocyclic but helical in structure, starch is the polymeric equivalent of the oligomeric CDs, such that the driving force for large anion binding is the same: the chaotropic effect.

With large superchaotropic anions a new high-affinity binding motif or anchoring unit for CDs has been identified, which opens several lines of applications.^{41,43,49,96,155,156} For example, the dodecaborate anion B₁₂H₁₁SH²⁻, as an auxiliary anchor residue, has been tethered to an organic moiety to synthesize water-soluble anchor dyes with high affinity to CDs.⁴³ Upon complexation with CDs, dodecaborate-substituted dyes showed marked changes in their UV-visible and fluorescence properties, which enabled their application for indicator displacement assays.⁴³ Another carboxyfluorescein dye of this type, with B₁₂Br₁₁OR²⁻ as the anchoring unit, has also been reported.¹⁵⁷ Other hybrid organic–inorganic derivatives of dodecaborate clusters were synthesized by tethering nitroanilines to the B₁₂H₁₂²⁻ anion.¹⁵⁸ These new aniline-substituted dodecaborates showed higher basicity than the parent nitroanilines and were able to form stable inclusion complexes with β- and γ-CD.¹⁵⁸

Larsen and Beeren have recently exploited this affinity in biotechnology, namely, in a templated directed synthesis of CDs (Fig. 9).^49,159 For example, each CD, namely, α-, β-, and γ-CD, can be selectively obtained in high yields in the presence of sodium dodecyl sulfate, 1-adamantane carboxylic acid, and NaBPh₄, respectively, as templating guest molecules of the amphiphilic or hydrophobic ionic type. Interestingly, large ring CDs (δ-CD and ε-CD) could also be synthesized by treating smaller CD homologues, such as α-CD with cyclodextrin glycosyl transferase (CGTase) in the presence of dodecaborate anions (e.g. Cs₂B₁₂I₁₂), where they serve as a template (Fig. 9), owing to their high affinity to large-ring CDs.⁴⁴ In this manner, halogenated dodecaborate cluster anions serve as templates for the practical synthesis of these precious large-ring CD homologues.

Fig. 9 Template-selective synthesis of CDs in the presence of differently sized anionic guest molecules. Reproduced from ref. 49 with permission from the Royal Society of Chemistry.

The largest dodecaborate anion, B₁₂I₁₂²⁻, is also a promising X-ray contrast agent for medical diagnostics (owing to the heavy-atom content) but it is known to induce hemolysis at high mM concentrations.^160,161 Hollow and Johnstone have recently reported that the complexation of the iodinated cluster with a permethylated γ-CD derivative prevents hemolysis, due to complexation.¹⁶⁰ The authors also reported the XRD structure of the B₁₂I₁₂²⁻·γ-CD complex which bears close similarity to that of B₁₂Br₁₂²⁻·γ-CD (Fig. 3a).

CDs have also been used as templates for the synthesis of polyoxometalates. The Lindqvist M₆O₁₉²⁻ ions (M = Mo or W) were found to be formed in the presence of γ-CD, despite their low aqueous solubility.⁸⁰ Recently, Falaise et al. have shown that the Mo₁₅₄ polyoxometalate can readily be made in the presence of γ-CD as a templating agent.²⁷ Host–guest complexation between the host and molybdenum-halide anionic clusters in aqueous solution was successfully applied to generate luminescent and redox-active materials.¹⁶² In detail, [Mo₆Xⁱ₈Cl₆]²⁻ ions with Xⁱ = Cl, Br or I are known to undergo hydrolysis in aqueous solution, and consequently form precipitates. This, however, could be suppressed, in particular for the brominated and iodinated clusters, upon complexation with γ-CD due to the formation of stable host–guest inclusion complexes, as indicated by ¹H NMR experiments.¹⁶² Furthermore, the host–guest complexes were assembled in the presence of Al³⁺-based polycations, which did not affect the structure of the γ-CD/[Mo₆Xⁱ₈Cl₆]²⁻ complex, and showed no effect on the optical and electrochemical properties of the [Mo₆Xⁱ₈Cl₆]²⁻ clusters.¹⁶² Similarly, the hydrolytic stability of [W₆X₈Cl₆]²⁻ (X = Br and I) was enhanced upon encapsulation inside two γ-CD rings.¹⁶³

Another application has recently been reported using bambusuril functionalized with an aza-crown ether, which can be utilized for the extraction of simple inorganic salts from aqueous solution to organic solvents (chloroform).¹⁶⁴ Dodecaborate anions were found to facilitate the preparation of stable and dispersed gold nanoparticles (AuNPs), which allowed for the further fabrication of the nanoparticle surface through supramolecular host–guest complexation.⁴¹ For example, gold nanoparticles stabilized by B₁₂H₁₁SH²⁻ anions were functionalized by an amphiphilic calixarene macrocycle (Fig. 8).⁴¹ The cationic calixarene derivative forms a stable complex with B₁₂H₁₁SH²⁻ clusters, which allowed for the assembly of mono- and bilayers of calixarene on the surface of AuNPs in a controlled manner (Fig. 10a). Zhang and co-workers have also reported on the ability of the CB7-B₁₂H₁₂²⁻ assembly to reduce noble metals, for example Pt⁴⁺, to the zerovalent form, and subsequently the formation of nanoparticles which can be used for applications in catalysis.¹⁶⁵ Similarly, Qi et al. prepared metal nanomaterials based on the assembly of B₁₂H₁₂²⁻-capped nanoparticles with CD-functionalized Fe₃O₄ (Fig. 10b). The new nano-composites showed remarkable catalytic activity and recyclability in the selective reduction of nitroaromatic compounds.¹⁶⁶

Fig. 10 (a) Assembly of a calixarene derivative on B₁₂H₁₁SH²⁻ coated AuNPs.⁴¹ (b) The preparation of B₁₂H₁₂-capped gold nanoparticles and CD-functionalized Fe₃O₄ and their assembly as a catalyst for the reduction of nitroaromatics.¹⁶⁶

The propensity of large anions to associate with the exterior of macrocyclic hosts (see above) leads to the formation of higher-order aggregates (precipitation) and, thus, enables applications in the direction of separation and isolation. CBn/dodecaborate assemblies have been used as adsorbent materials for the preferential removal of organic dyes from aqueous solution.¹⁶⁷ Qi et al. used the CBn/B₁₂H₁₂²⁻ assembly to prepare metal-cluster supramolecular composites as catalysts for Suzuki reactions.¹⁶⁸ Heptamolybdate (Mo₇O₂₄⁶⁻) showed a preferential association with the outer surface of CBn.¹³⁵ In detail, the Mo₇O₂₄⁶⁻ anion forms an exclusion complex with CB7 in aqueous solution, which precipitates out. The finding has led to a new strategy for the separation of ^99mTc from ⁹⁹Mo solutions.

The propensity of superchaotropic anions to form precipitates with CBn macrocycles by outer-wall binding has been initially described for dodecaborate clusters as a manifestation of the chaotropic effect.¹³⁴ Since then, this macrocycle-induced precipitation has been exploited for the separation of different precious metal complexes.^169–171 For example, gold can be separated through the high binding of [AuBr₄]⁻ and [AuCl₄]⁻ to CB6 as well as α-CD, while PtCl₆²⁻ forms also an exclusion complex with CB6. The resulting complexes immediately precipitate, allowing their separation. The superchaotropic anions thereby act as a “supramolecular glue”,¹³⁴ made possible by their large size and external binding. Interestingly, the binding shows high selectivity, because PtCl₆²⁻ can be separated from PdCl₄²⁻ and RhCl₆³⁻.¹⁷⁰ Very likely, this is related to the fact that PtCl₆²⁻ is the anion with the highest chaotropicity in this large-anion set, because PdCl₄²⁻ is less chaotropic on account of its smaller size, while RhCl₆³⁻ is less chaotropic on account of its higher charge. These structure–activity relationships in large anion binding in water are emerging only now, and will help design refined applications in the future.

Recently, Wang et al. have explored the supramolecular assembly of CB8 with a dodecaborate derivative for image-guided photodynamic therapy applications (Fig. 11).¹⁷² The B₁₂H₁₂²⁻ cluster was monofunctionalized with an RGD cell-binding sequence through polyethylene glycol units for targeted therapy purposes. The modified cluster can assemble at the exterior of CB8, which is prefilled with methylene blue, in aqueous solution, to form nanoparticles. Methylene blue served as a photosensitizer dye. The nanoparticles were found to accumulate in the tumor region when injected in mice through the tail vein. The release of methylene blue from the cavity could then be triggered through competitive binding using a peptide with an aromatic N-terminus.

Fig. 11 Schematic illustration for the implementation of the chaotropic effect for the design of nano-carriers for image-guided photodynamic therapy application. (a) the association of the chaotropic B₁₂H₁₂²⁻ anion to CB8, (b) molecular assembly of the functionalized B₁₂H₁₂²⁻ cluster and CB8, (c) activation of supramolecular photosensitizer, and (d) the working principle of the assembly as smart drug carrier for image-guided photodynamic therapy in vivo. Reproduced with permission from ref. 172 Copyright 2020 John Wiley and Sons, Inc.

The self-assembly of β-CD with SiW₁₂O₄₀⁴⁻ provides a versatile tool to prepare metal–organic frameworks (MOFs).¹⁷³ The structure of the (KCl₄)Na₇[(β-CD)₃(SiW₁₂O₄₀)]·9H₂O composite indicated that the anionic polyoxometalate is captured by three β-CD units. The new hybrid-MOF possesses high catalytic activity for the chemical oxidation of alcohols, alkenes, and thiophene to their corresponding products in the presence of H₂O₂.¹⁷³ MOFs have also been synthesized based on the interaction between the polyoxometalate PW₁₂O₄₀³⁻ and α-CD as well as a polyoxopalladate (P₁₀Pd_15.5O₅₀¹⁹⁻) and γ-CD.¹⁷⁴ Although the large size of the anionic clusters does not allow the formation of inclusion-type complexes, the propensity of the large ions to associate with the outer walls of CDs drives the assembly into MOFs.¹⁷⁴ Other examples have recently been reported.¹⁷⁵ The strong interaction between the native CDs and Anderson–Evans type polyoxometalates, namely [AlMo₆O₁₈(OH)₆]³⁻, was used to design infinite CD-channels linked through a polyoxometalate.¹⁷⁵

While the development of chemosensors for kosmotropic anions presents an important challenge in supramolecular chemistry,^2,176–182 sensing applications for large chaotropic and superchaotropic anions are of comparably lesser interest, because most of them are purely synthetic in nature (BF₄⁻, PF₆⁻, boron cluster anions, polyoxometalates), and therefore of limited environmental or biological concern. An exception is the perchlorate anion, ClO₄⁻, which is formed as a by-product from oxidative processes of chloride ions or chlorinated hydrocarbons, and for which several specific receptors have been reported.¹⁸³

Conclusions

The research field of anion recognition has experienced considerable developments in the past two decades. Despite a large number of reports on the binding of natural (small) inorganic anions by synthetic receptors, their relatively weak binding properties frequently limit the application potential to areas where the ions themselves serve either as analytes (ion sensing) or as cargos (ion transport) rather than as supramolecular building blocks on their own. The emergence of large cluster anions as tight binders for several macrocycles in 2015 has allowed their utilization as new assembly motifs in aqueous solution that have afforded, during the past 10 years, a wide spectrum of complex supramolecular architectures and several applications, ranging from the design of functional materials for catalysis to separation agents. Large anions, in particular those of the boron cluster and polyoxometalate type, display superchaotropic properties, which allow them to interact with extraordinary affinity with hydrophobic surfaces, and in particular with the hydrophobic cavity of macrocyclic hosts, which can be traced back to a combination of desolvation and dispersion effects. Since the two types of superchaotropic anions are emerging key ingredients for solid-state batteries^184,185 and photocatalytic water-splitting,¹⁸⁶ large anion binding could also become relevant for future energy-related applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work has been supported by the DFG through projects NA-868/14 (chaotropic effect of boron cluster anions) and NA-868/17 (chaotropic effect of polyoxometalates).

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