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Coordination Chemistry Reviews 257 (2013) 1716– 1727

Contents lists available at SciVerse ScienceDirect

Coordination Chemistry Reviews

jo u r n al hom ep age: www.elsev ier .com/ locate /ccr

eview

ncapsulation of anions: Macrocyclic receptors based on metaloordination and anion–� interactions

ntonio Frontera ∗

epartament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, Palma de Mallorca, Baleares, E-07122, Spain

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17162. Theoretical studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1717

2.1. Physical nature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17172.2. The influence of metal coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17172.3. Condensed polycyclic aromatics as multicenter receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1718

3. Experimental studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17193.1. “Umpolung” via �6 coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17193.2. �-pockets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17203.3. �-Cages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1722

4. Recent achievements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17245. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1726

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1726References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1726

r t i c l e i n f o

rticle history:eceived 23 November 2012ccepted 31 January 2013vailable online xxx

a b s t r a c t

Supramolecular chemistry of anions is a topic of continuous interest due to vital role of anions in manykey chemical and biological processes. The dominant functional groups for constructing anion bind-ing receptors are amides and amines/ammonium cations, where the key interaction to bind the anionis the hydrogen bond. This review covers systems where the binding of the anion with the receptoris through anion–� interactions, i.e., the non-covalent forces between electron-deficient aromatic sys-

eywords:nionson-covalent interactionsnion–� interactionseteroaromatic ligandsolecular architecture

tems and anions. Moreover, this review is focused to those receptors where coordination metals playa prominent role thanks to two very important characteristics: first, they are crucial for the synthesisand assembly of the receptor and, secondly, they strongly change the �-binding ability of the aromaticligands, increasing significantly their �-acidity and, consequently, enhancing the ability of the ring tointeract favorably with anions.

upramolecular chemistry

. Introduction

Supramolecular chemistry is a multidisciplinary field that isrowing incredibly fast as indicated by the large number of arti-les, reviews, and books published every year. The emergence ofupramolecular chemistry has a profound effect nowadays on thefficiency of preparation of structures of different sizes, shapes and

unctionalities. It is the chemistry of the non-covalent bond, whichorms the basis of highly specific recognition, transport, and reg-lation events. Non-covalent interactions have a prominent role

∗ Corresponding author. Tel.: +34 971173498; fax: +34 971173426.E-mail addresses: toni.frontera@uib.es, antonio frontera@yahoo.es

010-8545/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.ccr.2013.01.032

© 2013 Elsevier B.V. All rights reserved.

in many chemical and biological processes, often orchestrated byan intricate combination of non-covalent interactions [1,2]. Forinstance, these interactions are the foundation of the life processitself, the definitive expression of function. In synthetic chemistry,interactions between rationally designed molecular subunits drivethe assembly of aggregates of different sizes with targeted guests[3–6]. The accurate description of interactions between moleculesis essential for the understanding and development of this field.The supramolecular chemistry usually relies on strong, directionalinteractions like hydrogen bonding and halogen bonding, and less

directional forces like ion pairing. In addition, non-covalent inter-actions involving aromatic rings are extremely important in thisfield [7,8]. They play a key role in chemistry and biology [9], in par-ticular drug–receptor interactions, crystal engineering and protein

try Reviews 257 (2013) 1716– 1727 1717

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A. Frontera / Coordination Chemis

olding [10]. A clear example is the role of stacking interactionsn DNA, RNA [11,12] and protein–DNA aromatic interactions [13].nother important and widely recognized non-covalent interac-

ion that involves aromatic rings is the cation–� bonding [14,15] ofreat importance in biology [16–18] and supramolecular chemistry19–22]. In recent years, the anion–� interaction [23–25], i.e. theon-covalent force between electron-deficient aromatic systemsnd anions [26,27], has also been recognized as a non-covalentonding interaction. Its nature has been described by numerousheoretical studies [28–31] in addition to several experimentalnvestigations [32–36]. Anion–� interactions may be prominentlayers in fields as diverse as medicine, environmental chem-

stry, chemical and biological processes [23,37–39]. Moreover, theirpplication to the design of highly selective anion receptors andhannels definitively confirms their significance in the field ofupramolecular chemistry [40,41].

The synthesis of molecular receptors designed to bind anions is aopic of continuous attention [42–44]. The main reason is the essen-ial function played by anions, which are ubiquitous throughoutiological systems [45,46]. In addition, some anions are environ-entally very relevant due to their toxicity. For example, the

veruse of anionic pollutants (phosphate and nitrate) as fertilizersauses eutrophication of lakes and rivers [47]. Other anions, suchs pertechnetate are generated from the re-processing of nuclearuel [48] and their release to the sea is strictly controlled or pro-ibited. Consequently, the design and synthesis of selective anioneceptors and sensors is a relevant area of interest [49]. To thisespect, the anion–� interaction is gaining interest among scien-ist, since a new family of anion receptors based on this interactions emerging in the literature [23,50]. A clear example that com-ines hydrogen bonding and anion–� interaction for the binding ofnions with neutral �-acceptors in solution and the solid state haseen recently published by Albrecht et al. [51].

The aim of this review is to highlight the convenient use of metaloordination for the synthesis of structures of different size andhape using different metals. This coordination has a double effect.irstly, the metal coordination facilitates the assembly of the cagerom smaller molecular components (ligands) in most cases withemarkable high yields. Secondly, the metal coordination stronglyffects the �-acidity of the aromatic ligand, thus increasing thetrength of the anion–� interactions. This favors the ability of theeceptor to encapsulate the anion in its interior. Therefore, thiseview is focused on the bonding relationship between anions and

systems coordinated to transition metals, describing work at thexperimental and theoretical forefront. It is developed under threeeadings. First, key aspects of the physical nature of the interactionnd the influence of metal coordination are presented for assim-lation by a non-expert readership. Next, key examples from theiterature are described where the molecular recognition is driveny anion–� bonding. Finally, very recent advances in this field areeviewed.

. Theoretical studies

.1. Physical nature

The physical nature of the anion–� interaction has been exten-ively analyzed using high level ab initio calculations and severalartition energy schemes in order to decompose the total inter-ction energy into individual components [52,53]. From them, theeneral agreement conclusion is that electrostatic forces and ion-

nduced polarization are the main energetic contributors to thenion–� interaction [54,55]. The electrostatic term is explainedy means of the permanent quadrupole moment (Q) of the arene,hich is the first non-zero multipole moment in symmetric arenes.

Fig. 1. Schematic representation of the quadrupole moments of benzene, hexaflu-orobenzene and pyridine coordinated to a transition metal (TR, L = ligand) (left) andthe anion-induced dipole (right).

Moreover, in most asymmetric arenes, where the dipole moment(�) is non-zero, the �z component is approximately zero and,equally to symmetric arenes, the electrostatic attraction is basicallydue to the existence of the adequate component (perpendicular tothe ring plane) of the quadrupole moment (Qzz). The Qzz of ben-zene is negative, but can be turned positive by attaching electronwithdrawing substituents to the ring (Fig. 1). Similarly, the Qzz

of pyridine is negative but can be turned positive by attaching acoordination metal to the nitrogen atom. Therefore, the counterintuitive interaction between an anion and an aromatic ring canbecome attractive. The ion-induced polarization of the �-electronsystem by the anion is significant, inducing a dipole. Therefore, apolarization contribution to the total interaction energy is derivedfrom the interaction of the anion with the induced dipole [52–55].

This understanding of the physical nature (vide supra) has beenused in a predictive way. The dual bonding mode exhibited byarenes with negligible quadrupole moments is a clear example[56,57]. Since electrostatic and polarization terms dominate bothanion–� and cation–� interactions, molecules with negligible Qzz

values such as 1,3,5-trifluorobenzene (Qzz = 0.57 B) and s-triazine(Qzz = 0.90 B) must be able to interact with both anions and cationssince the sign of the induced dipole is always opposite to the signof the interacting ion. A second example involves the interaction ofanions with electron-rich aromatic rings such as benzene, which isexpected to be strongly repulsive. However, due to a compensat-ing effect between the electrostatic (unfavorable) and ion-inducedpolarization (favorable) forces roughly cancel each other out. Forthis reason, the anion–� interaction energy of benzene with chlo-ride is very small, but favorable [58,59].

From the aforementioned considerations, it is clear that todesign a strong anion–� interaction, the aromatic ring should havea large and positive quadrupole moment and a large molecularpolarizability. For the first condition, a problem is the limited num-ber of electron withdrawing groups available for constructing thebinding blocks of the future anion receptor. To have a large valueof Qzz, the use of NO2 and CN groups is required. However, itis synthetically very complicated to attach more than three strongelectron withdrawing groups and the spacer to the aromatic ringto construct the receptor. This limitation is very difficult to over-come and other strategies should be followed, as described in thefollowing section.

2.2. The influence of metal coordination

The influence of metal coordination to heteroaromatic rings onthe energetics of anion–� interactions has been analyzed in the lit-

erature [60,61]. In the first one it has been demonstrated that thecoordination of both pyridine and pyrazine with AgI dramaticallyenhances their ability to establish anion–� binding [60]. Theoreti-cal calculations clearly show that the binding energies of anion–�

1718 A. Frontera / Coordination Chemistry Reviews 257 (2013) 1716– 1727

razine

csouetacinh

tdtbttEcciofaF

tioHtiTttama

Fic

Fig. 2. Interaction energies of pyridine and py

omplexes of pyridine and pyrazine with Cl− are significantlytronger when AgI is coordinated to the nitrogen atom. As a matterf fact, the interaction of pyridine itself with chloride is slightlynfavorable (2 kcal/mol) indicating that the ring is not �-acidicnough to form a stable Py···Cl−anion–� complex (Fig. 2). However,he interaction becomes favorable (–14.8 kcal/mol) if the nitrogentom of pyridine is coordinated to the transition metal (AgI). Thishange is even more pronounced in pyrazine complexes, where thenteraction energy becomes 33.2 kcal/mol more favorable if bothitrogen atoms are coordinated to AgI. A more recent investigationas also demonstrated similar results for s-triazine [62].

While the first work is completely a theoretical investiga-ion, the second work [61] combines theory and experiment toemonstrate that s-tetrazine coordinated to four AgI atoms ishe most powerful anion–� acceptor reported to date. It haseen demonstrated theoretically that the interaction energy of s-etrazine with nitrate ion strengthens from −9.6 kcal/mol (Fig. 3)o −62.4 kcal/mol when the arene is tetracoordinated to AgI.xperimentally, several X-ray crystal structures of s-tetrazine �4-oordinated to AgI have been reported, exhibiting very closeontacts between the anion and the s-tetrazine ring (see Fig. 3)ndicating strong anion–� interactions, in agreement with the the-retical predictions. Similar structures have been also reportedor dimethyltetrazine tetracoordinated to AgI. Interestingly thenion–� distance for the perchlorate anion in YOYTAI (2.61 A, seeig. 3, left) is the shortest reported to date.

As mentioned above, the coordination of the heteroaryl ring tohe metal has a strong influence and significance on the anion bind-ng capability of the arene. In general the anion–� binding abilityf the receptor increases as the oxidation state of the metal does.owever, the electronic nature of the other ligands coordinated

o the metal also influences the interaction strength [23,50]. Thiss a rather unexplored topic that deserves further investigation.he coordination of the heteroaromatic ring is somehow similaro the effect of protonation. For instance, azines can be easily pro-

onated by simply adjusting the pH of the medium increasing thenion binding ability of the ring (anion–�+ interactions). The geo-etric and energetic features of anion–�+ complexes of several

romatic cations (tropylium, quinolizinylium) and various anions

ig. 3. Fragments of the X-ray crystal structures containing �4-coordination of 1,2,4,5-nteractions are indicated by dashed lines (distances in Å). The CSD reference codes arelarity.

anion–� complexes from references [60,61].

have been reported along with crystallographic structures [63,64].Moreover, the anion–�+ interaction participates in the formationof a robust recognition motif in the transition metal malonate com-plexes using protonated 2-amino-4-picoline and 2-aminopyridineas the auxiliary ligands [65,66]. Lastly, the importance of this inter-action in protonated purine and pyrimidine bases has been recentlyreviewed [67]. As expected, these complexes exhibit very large(>80 kcal/mol) binding energies due to strong electrostatic effectsthat dominate the interaction.

2.3. Condensed polycyclic aromatics as multicenter receptors

The annelation of electron deficient heteroaryl rings has a com-parable effect for the electronic structure as the introduction ofstrong electron withdrawing groups. This fact extends existingstrategy for constructing anion receptors. This approach has beenused by Kim Dunbar et al. to construct a neutral anion-receptorbased on 1,4,5,8,9,12-hexaazatriphenylene (HAT) substituted withcyano groups that is able to interact with three anions on one sideof the polycyclic ring and with one anion on the opposite side [68].In addition, Ballester and coworkers [69] have studied the anion–�binding ability of HAT(CN)6 both theoretical and experimentally.The formation of typical anion–� non-covalent complexes, charge-transfer complexes or electron transfer adducts strongly dependson the type of anion.

A specially illustrative example of this particular topic is thebicyclic aromatic arene pyridazino[4,5-d]pyridazine ring. It pre-serves the donor ability of pyridazine and has stronger �-acidiccharacter. Moreover, it is readily accessible by a one-pot synthe-sis involving inverse electron demand Diels–Alder cycloaddition[70]. The anion–� interaction ability of this ligand has been ana-lyzed theoretically [71,72] and it has been demonstrated that theinteraction energies with several anions are more favorable thanthose of pyrazine. The theoretical study has been extended to largerpolycyclic arenes based on pyridazino[4,5-d]pyridazine and the

interaction energy increases with the number of fused ring (seeFig. 4).

Experimentally, the pyridazino[4,5-d]pyridazine ligand (Lpp)has been used by Gural’skiy et al. [70] to demonstrate the ability of

tetrazine and 3,6-dimethyl-1,2,4,5-tetrazine from ref [61]. The relevant anion–� indicated. In the YOYTOW structure, the hydrogen atoms have been omitted for

A. Frontera / Coordination Chemistry Reviews 257 (2013) 1716– 1727 1719

ridaz

tsnrmpaOitopa3

lbopbecsodcbppdw

Fi

Fig. 4. Interaction energies of Cl−–� complexes of py

his ring to establish anion–� interactions providing an attractivetructural prototype and a unique �-bifunctional building block forovel polymeric and molecular anion receptors. Two examples areepresented in Fig. 5. In one of them, [Cu(H2O)2(Lpp)2](ClO4)2, theetal–organic portion adopts a 2D square grid structure where a

air of anions is encapsulated into each square, establishing closenion–� interactions. The other one is a hydroxopolymer {[Cu(�-H)(Lpp)](H2NSO3)•H2O}n and reveals a very remarkable anion–�

nteraction mode that occurs between the ligand as a double recep-or for oxygen and nitrogen sites of the anion. The dimensionsf the sulfamate anion and pyridazino[4,5-d]pyridazine liganderfectly match facilitating an effective double binding (anion–�nd lp–�), in which the corresponding distances are 3.14 and.10 A.

Further experimental evidence for the enhanced ability of thisigand to participate in anion–� interactions has been reportedy Domasevitch et al. [73,74]. Their examination of the behaviorf pyridazino[4,5-d]pyridazine toward silver(I) reveals a specialotential for the design of 3D coordination frameworks. This fusedicyclic pyridazine is an extraordinary system that combines veryfficiently its �N-donor ability and pronounced �-acidity uponoordination. The reported coordination frameworks [73] exhibittrong anion–� interactions, including unprecedented examplesf double anion–� binding that occur between pyridazino[4,5-]pyridazine and SiF6

2− anions and the structure of a mixed-anionomplex where the ligand established anion–� interactions withoth BF4

− and SiF62− anions (see Fig. 6). This unique combination of

ronounced �-acidity and �N-donor ability puts forward its widerotential for functionalization of hydrophobic crystal cavities andevelopment of molecular and polymeric metal–organic speciesith special anion binding properties.

ig. 5. Fragments of the X-ray crystal structures containing �2- and �4-coordination ofndicated by dashed lines (distances in Å). The CSD reference codes are indicated.

ino[4,5-d]pyridazine and related rings from ref [71].

3. Experimental studies

To date, a great deal of experimental work has provided strongevidence of the importance of the anion–� interaction in the solidstate. In the following sections, we present selected experimen-tal examples where the combination of metal coordination andanion–� interactions play a key role in the anion recognition pro-cess. Therefore, the potential use and importance of this synergeticcombination in the supramolecular chemistry of anions is stronglydemonstrated.

3.1. “Umpolung” via �6 coordination

As mentioned earlier, strong anion–� interactions are possibleusing aromatic rings with large and positive quadrupole moments.However, it is also possible to establish a strong anion-� interactionbetween anions and electron-rich arenes with negative Qzz values.This can be achieved in the situation where the aromatic systeminteracts with an anion and a transition metal on opposing sidesof the ring [75–77]. Theoretical studies have demonstrated thatthese ternary complexes A−···�···TR-Ln (A = anion, TR = transitionmetal, L = ligand) the interaction energies are large and negativeand the equilibrium distances are shorter than the correspond-ing distances of the binary complexes, indicating a reinforcementof both interactions. Experimental studies published by Atwoodand co-workers [78–80] using X-ray crystallography and 1H NMRtitration experiments, have shown that the host–guest behavior

of calixarenes and cyclotriveratrylenes can be inverted by com-plexation of the arene rings with transition metals (Ru, Ir, Rh),thus allowing anionic guest species (instead of cationic) to beincluded within the molecular cavity. Two examples involving two

pyridazino[4,5-d]pyridazine from ref. [70]. The relevant anion–� interactions are

1720 A. Frontera / Coordination Chemistry Reviews 257 (2013) 1716– 1727

F azinod

dbimohbahir

omoteibst

ig. 6. Fragments of the X-ray crystal structures containing Ag(I) complexes of pyridashed lines (distances in Å). The CSD reference codes are indicated.

ifferent metals and anions are shown in Fig. 7. The ability toind anions in a size- and shape-selective fashion is of extreme

mportance in the detection and removal of unwanted environ-ental contaminants and is also of interest in the development

f synthetic enzyme mimics. These studies demonstrate that theost–guest properties of the calixarenes are significantly modifiedy capping of the calixarene faces with transition metal centersnd constitute clear examples of organometallic anion-bindingosts where complementarity between anion and cavity size is

mportant allowing the anion to interact equally with all aromaticings.

More recently, it has been demonstrated that the metalationf the exterior arene faces of a molecular capsule with [Cp × Ru]+

oieties [81,82] (see Fig. 8) results in a �-acidic cavity capablef encapsulating anions. The anion complexes have been crys-allographically characterized and the encapsulation has beenstablished by NMR spectroscopy. These are nice examples thatllustrate how the combination of the inverse of polarity provoked

y the metal coordination together with the establishment of sixtrong anion–� contacts determines the encapsulation of anions inhe �-acidic cavity.

Fig. 7. X-ray crystal structures of organometallic anion-binding

[4,5-d]pyridazine from ref [73]. The relevant anion–� interactions are indicated by

3.2. �-pockets

While much research has looked into pyridazine–, triazine–and tetrazine–anion interactions, reports of five-memberedheterocycle–anion interactions are rarely found in the literature[26,83]. However anion–� interactions frequently occur in this typeof ring. In addition, anions usually occupy �-pockets in many crystalstructures where five-membered heterocycle rings are coordinatedto transition metals. Some examples are described in this section.

Brooker’s group [84,85] has published the preparation and X-ray characterization of several mononuclear nickel(II) complexesusing 4-pyrrolyl-3,5-di(2-pyridyl)-1,2,4-triazole ligand with theanions ClO4

−, BF4−, PF6

−and SbF6−. Strong anion–� interactions

are observed in the solid state, with one interesting motif presentin all four structures. Independently of size and shape, the anionsoccupy a “�-pocket” formed by two coordinated triazole rings andone coordinated pyridine ring. Two examples are shown in Fig. 9,where either ClO4

− or SbF6−occupy the same position in the crystal,

establishing two anion–� interactions with five membered triaz-ole rings and one anion–� interaction with the pyridine ring, allcoordinated to NiII metal centers.

hosts NAYRIO (BF4−) and NAYREK (I−) from refs. [78–80].

A. Frontera / Coordination Chemistry Reviews 257 (2013) 1716– 1727 1721

Fig. 8. X-ray crystal structures of organometallic anion-binding cages RAYFEX (SbF6−) and RAYFAZ (CF3SO3

−) from ref. [81].

F solid

i for cla

pRputfttrei

Fi

ig. 9. Crystal structures of NiII complexes showing evidence of �-pockets in the

nteractions are indicated by dashed lines. The hydrogen atoms have been omitted

In 2007, Li et al. published unprecedented coordination com-lexes of terdentate 2,6-bis(1,2,3-triazol-4-yl)pyridine ligand [86].emarkably, they were the first structurally characterized exam-les of the 1,2,3-triazole motif employed in a terdentate ligand andsing FeIII as metal center. This ligand provides stable coordina-ion environments resembling terpyridine but with a distinctiveavorable difference, which is the reduced steric bulk provided byhe N2 nitrogen atom of 1,2,3-triazole. Moreover, this ligand facili-

ates the construction of supramolecular systems. One structure isepresented in Fig. 10 (CIGJOS, left) where an acidic �-pocket is gen-rated in the solid state structure. The anion is well-accommodatedn the pocket, establishing four anion–� interactions with the

ig. 10. Examples of anion–� contact in coordinated triazole based ligands from refs. [86ndicated by dashed lines (distances in Å). The hydrogen atoms have been omitted for cla

state from ref. [84]. The CSD reference codes are indicated. The relevant anion–�rity.

triazole rings. Another interesting example is the self-assemblyof rigid bis(triazol-ylidene) ligands with linear coordinated Ag(I)or Au(I) metal centers leading to nano-scale, cylindrical foldamerstructures, which finally evolve into well-defined nanofiberscapable of gelating solvents [87]. Interestingly, the generatedmetallosupramolecular container is able to encapsulate twocounteranions into the cavity (see KALYED in Fig. 10). EachBF4

−establishes three anion–� contacts with the triazole rings.I

Remarkably, this structure can be transmetalated with Au with

retention of the geometry of the supramolecular structure.Finally, Domasevitch’s group has reported multiple anion–�

interactions in the accommodation of NCSe− anions inside

,87]. The CSD reference codes are indicated. The relevant anion–� interactions arerity.

1722 A. Frontera / Coordination Chemistry Reviews 257 (2013) 1716– 1727

Fig. 11. Fragment of the X-ray structure showing the concerted triple anion–� andCH···Se interactions that dominate the non-covalent bonding of selenocyanate fromref. [83]. The CSD reference code is indicated. The relevant anion–� interactions areio

hfitsoadCatdaatul

dhaeptopTcpor

tsAhs

inant in the presence of SbF − ion, while smaller anions (BF −

ndicated by dashed lines (distances in Å). The adamantane molecules have beenmitted for clarity.

ydrophobic pockets of adamantane/1,2,4-triazole coordinationramework [88]. The interactions involve the soft Se site of the amb-functional NCSe− anion and three triazole groups arranged on aricadmium matrix. The framework generates hydrophobic pocketserving for hosting the uncoordinated anions by a concerted actionf Se−–� and CH···Se interactions (see Fig. 11). The Se––� inter-ctions are probably very weak since the corresponding Se–C(N)istances are close to the sum of van der Waals radii of Se and

or N atoms. Nevertheless, in all anion–� contacts the seleniumtom is located almost exactly above the corresponding ring cen-roid, which suggests a directional interaction. This tri-cadmiumodeca-triazole core is a multi-center receptor of anions providing

flexible approach for the developing of metal-organic arrays fornion recognition. The adamantane molecular platform facilitateshe formation of multiconnected coordination frameworks and it isseful for the developing of geometrically rigid aliphatic bridging

igands.Many examples of interactions of anions with pyridines,

iazines, triazines and tetrazines coordinated to metal centersave been described in the literature. Some relevant examples arelso described in this section. For instance, one of the pioneeringxperimental studies devoted to the anion–� interactions wasublished by Gamez and co-workers [89], who described the syn-hesis and X-ray characterization of a tetranuclear copper complexf the octadentate ligand N,N′,N′′,N′ ′ ′-tetrakis{2,4-bis(di-2-yridylamino)-1,3,5-triazinyl}-1,4,8,11-tetraazacyclotetradecane.he key feature of this assembly is the encapsulation of twohloride anions into two equivalent host cavities formed by fouryridine rings of the ligand (Fig. 12). The anion–� interactionsbserved here are enabled by the coordination of the pyridineings to copper, which enhances their �-acidic character.

Two further examples that illustrate the ability of triazines andetrazines coordinated to AgI to establish anion–� interactions arehown in Fig. 13. The first example (KEZHIH) is a self-assembled

gI coordination network directed by anion–� interactions, whichas been reported by Zhou et al. [90]. Interestingly they demon-trate that the self-assembly of AgI metal complexes with

Fig. 12. Fragment of the X-ray structure FAQYOM showing the encapsulation of twochloride ions into the �-pockets from ref. [89]. The CSD reference code is indicated.The hydrogen atoms have been omitted for clarity.

2,4,6-tri(2-pyridyl)-1,3,5-triazine, is directed and controlled bypolyatomic anions (ClO4

−, BF4−, and PF6

−) through anion–� inter-actions. The binding mode illustrated in Fig. 13 (left), where theanion is sandwiched by two triazine rings coordinated to threeAg metal centers, is repeated in the crystal structure forminginfinite 1D columns. Notably, the formation of highly symmetric,isostructural coordination networks was observed for all anionsin good yield and in all cases the anions were located on the C3-axis above the central triazine rings of the ligands. The secondexample (FEPCOT; see Fig. 13, right) was published by Dunbarand co-workers during their investigation on the role of anion–�interactions in the assembly of AgI complexes of the 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine ligand [91]. This work was the firstexample of a comprehensive investigation of anion–� interactionsas controlling elements in self-assembly reactions. The combina-tion of the ligand with AgX salts (X = PF6

−, AsF6−, SbF6

−) gavecomplexes of different structural types, depending on the experi-mental conditions and the anion used. A selected example is shownin Fig. 13, which is the compound obtained by reaction of AgI andthe ligand in a 1:1 ratio in the presence of AsF6

− ion that favorsthe formation of propeller-type compounds. Apart from the anionsize, which plays a role in the packing of the resulting structures,anion–� interactions are a major factor in determining the pre-ferred structural motif [91].

3.3. �-Cages

Dunbar’s group has performed extensive experimental workto rationalize and understand the interplay between anionsand electron-deficient aromatic ligands that mediate in theself-assembly formation and properties of supramolecular archi-tectures. In particular, they have discovered that the nuclearityof several metallocyclophanes is governed by the identity ofthe encapsulated anion [92]. The comprehensive investigationof an anion-templated self-assembly reaction between first rowtransition metal ions MII (M = Ni, Zn, Mn, Fe, Cu) and the 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine ligand. The formation of polygonalcomplexes was observed in the presence of certain anions (BF4

−,ClO4

−, SbF6−), which selectively determine the aggregation state.

The formation of an unprecedented molecular pentagon is dom-

6 4

and ClO4− ions) template molecular squares. The competing

influence of the anions in stabilizing the different cyclic enti-ties was confirmed by using both mass spectrometric and X-ray

A. Frontera / Coordination Chemistry Reviews 257 (2013) 1716– 1727 1723

Fig. 13. Left: anion–� assembled coordination complex of 2,4,6-tri(2-pyridyl)-1,3,5-trazine with ClO4− from ref. [90]. Right: Propeller-type complex of 3,6-bis(2-pyridyl)-

1,2,4,5-tetrazine with AsF6− from ref. [91]. The CSD reference codes are indicated. The relevant anion–� interactions are indicated by dashed lines (distances in Å). The

hydrogen atoms in the FEPCOT structure have been omitted for clarity.

Fig. 14. X-ray geometries of the NiII-pentagon and NiII-square units complexed to several anions and their scheme of interconversion from ref. [92]. The CSD reference codesare indicated. The hydrogen atoms have been omitted for clarity.

1724 A. Frontera / Coordination Chemistry Reviews 257 (2013) 1716– 1727

F ethyl)sulfide from ref. [93]. The CSD reference codes are indicated. The hydrogen atomsh

cwnetNatPatsip

Bsatmlgaessdiomuu

cbmwpXaac

[yi

ig. 15. X-ray fragments of the isomorphous structures based on bis(4-pyrimidylmave been omitted for clarity.

rystallographic determinations. For example, the NiII-pentagonas less thermodynamically stable than the NiII-square. Ion sig-als corresponding to the square complex begin to appear in thelectrospray ionization mass spectrum after the correspondingetrabutylammonium salts of BF4

−, I−or ClO4− to a solution of the

iII-pentagon. A complete conversion is accomplished by addingn excess of tetrahedral or I−anion (Fig. 14). Anion-exchange reac-ions of NiII-square···ClO4

−indicate larger anion such as IO4−and

F6−cannot replace ClO4

− inside the cavity, but that the linear Br3−

nion is capable of doing so. The nuclearity of the cages is dic-ated by the identity of the anion present in solution during theelf-assembly process, which is attributed to favorable anion–�nteractions between the anion inside the cavity and the 3,6-bis(2-yridyl)-1,2,4,5-tetrazine ligands.

A uniform mode of anion–� binding has been described bylack et al. [93] in a series of coordination polymers that encap-ulate BF4

−, ClO4−, and PF6

− anions. They are formed from AgI ionsnd bis(4-pyrimidylmethyl)sulfide ligand, which is a flexible mul-imodal ligand able to provide both strong N-donor sites and, upon

etal coordination, �-acidic ring centers. The combination of thisigand with AgX (X− = BF4

−, ClO4−, and PF6

−) in a 1:1 molar ratioave complexes with two-dimensional structures characterized by

(4,4) topology. The cavities formed by this network are able toncapsulate anions by means of four complementary �–anion–�andwich interactions with two pyrimidine rings (Fig. 15). All threetructures were isomorphous, regardless of involving anions ofifferent sizes. This important investigation provides further exper-

mental evidence for the usefulness of N-heterocycles in the designf anion receptors upon coordination to transition metals throughultiple anion–� interactions, demonstrating the potential for the

se of anion–� bonding in the molecular recognition of anionssing �-cages.

Lippert and collaborators have described [94] an exceptionalase of SO4

2− trapping in a cationic host that is not dominatedy hydrogen bonding, instead it is the high positive charge of theetal-containing host and the possibility of the anion to retain aater molecule from its hydration shell. Consequently, the energyenalty of complete desolvation is significantly reduced [95,96].-ray crystallography suggests that anion–� and lone pair–� inter-ctions between oxygen atoms of sulfate and water, respectively,nd electron-poor bipirazine rings are important for the host–guestomplexation (see Fig. 16).

Interesting �-acidic cages have been also observed in a set of2 × 2] copper(I) grids coordinated to 2-amino-4,6-bis(pyrazol-1-l)pyrimidine ligand and differing in the accompanying counteran-on, PF6−, BF4

−, ClO4−, p-Me-C6H4SO3

−, or CF3SO3− [97]. In this

Fig. 16. X-ray structure of the cationic host complexed simultaneously to sulfateanion and water published by Lippert and collaborators [94]. Distances in Å. Thehydrogen atoms have been omitted for clarity. The CSD reference code is indicated.

study the influence of the amino group in the ligand upon the anionencapsulation properties have been analyzed. More specifically, theeffects of this group over the strength of the anion interactionswith the walls of the cavities and the orientation adopted by theencapsulated anions in the solid state have been studied. Interest-ingly, the stability of the grids in solution allows comparing theselectivity of the different anions. Moreover, experiments of anioninterchange in the solid–liquid interface have demonstrated thatthe anion can be exchanged and the crystallinity of the samples ismostly preserved after the exchange process. The cavities formedby this grid encapsulate two anions, each one by means of two�–anion–� sandwich interactions with two pyrazine rings (Fig. 17).

4. Recent achievements

In this part of the review, recent advances in this field aredescribed. They demonstrate that there is a continuous and increas-ing interest for investigating and developing anion-binding hostsbased on both aromatic ligands and coordination metals.

One of these investigations reports a new porous coordina-

tion polymer forming a two-dimensional layered network [98].It is generated from Cu(BF4)2

•6H2O, bipyridine ligand and for-mate as co-ligand. Single-crystal X-ray diffraction analysis revealsa (4,4) 2D coordination network exhibiting voids that contain the

A. Frontera / Coordination Chemistry Reviews 257 (2013) 1716– 1727 1725

F left) ad e CSD

cbswaaifi

((tcptFcApeviiosmc

e1pwqaTs

Fvc

excited states. Remarkably, the simple crystallization of the AuI–CuI

complex yields multiple crystals, apparently quite similar but withdifferent solid-state photophysical properties. Since the anion (or

ig. 17. X-ray structures of the isomorphous �-acidic cages complexed to ClO4−(

ashed lines (distances in Å). The hydrogen atoms have been omitted for clarity. Th

ounter-ions and water guest molecules. The anions are stabilizedy hydrogen bonds and anion–� interactions to form an intricateupramolecular framework. The coordination compound showseak catalytic activity in the cyanosilylation of aldehydes. The �-

cidic catalysts is able to generate 2-(methylsiloxy)propionitrile,lbeit in low yield (24% of conversion after 24 h, see Fig. 18). Thiss the first example of a reaction catalyzed by a metal organicramework (MOF) whose catalytic activity is related to anion–�nteractions.

The electroactive ligands (2,4-bis-tetrathiafulvalene[6-dipyridin-2′-ylamino)]-1,3,5-triazine) and2-tetrathiafulvalene[4,6-bis-(dipyridin-2′-ylamino)]-1,3,5-riazine) have been recently synthesized and used for metaloordination experiments with Zn(II) [99]. The resulting com-lexes exhibit anion–� supramolecular interactions between theriazine ring and either Cl− or ClO4

− (one structure is shown inig. 19). These complexes are useful for the preparation of radicalation salts as demonstrated using electrochemical experiments.lthough numerous tetrathiafulvalene based ligands and com-lexes have been described to date [100,101], the presence of thelectron poor triazine ring bonded for the first time to tetrathiaful-alene makes possible the establishment of anion–� interactionsn the solid state and their interplay with other supramolecularnteractions, as well as their tuning with the tetrathiafulvalenexidation state. Future research in these systems includes theynthesis and property analyses of metal complexes containingagnetic ions to associate conductivity and magnetism in radical

ation salts.Fluorescent carbazole-based dipyrazole ligands have been

mployed [102] to coordinate with dipalladium corners using,10-phenanthroline as coligand, in aqueous solution to affordositively charged [M8L4]8+ or [M4L2]4+ multimetallomacrocyclesith remarkable water solubility. They were obtained in nearly

uantitative yield based on fluorescent carbazole–dipyrazole lig-nds and dipalladium clips via a directed self-assembly process.heir structures were characterized by a combination of spectro-copic techniques, including 1H NMR spectroscopy, electrospray

H

O+ Me3SiCN

CH2Cl2

CN

OSiMe3 12 %, blank24 %, MOF24 h

ig. 18. Cyanosilylation reaction to test the catalytic ability of MOF is shown. Con-ersions after 24 h are indicated for the reaction without (blank) and with theatalyst [98].

nd BF4−(right) from ref. [97]. The relevant anion–� interactions are indicated by

reference codes are indicated.

ionization mass spectrometry, and elemental analysis. Inter-estingly, X-ray diffraction analysis shows that the counterions(BF4

−anions) are trapped in the dipalladium clips through anion–�interactions (see Fig. 20). The luminescence properties and inter-action toward anions of these metallomacrocycles were discussedand used to demonstrate that one of them selectively interacts withSCN−anions in aqueous solution.

Chen et al. [103] have published the modulation of lumines-cence by anion–� interactions in a dinuclear AuI–CuI complex.It is trigonally coordinated to three diphenylpyridinephosphineligands and crystallizes in two polymorphs and a pseudopoly-morph with short AuI–CuI separations. Strikingly, these crystalsluminesce different colors ranging from blue to yellow under UVillumination. The structures are almost equivalent and the maindifference resides in the relative location of the anions and sol-vate molecules (see Fig. 21). These interactions are responsible forthe different emission properties, as demonstrated by theoreti-cal calculations. These results illustrate the importance anion–�non-covalent interactions on the energies of both the ground and

Fig. 19. X-ray structure of the triazine based complex interacting with twoClO4

−anions from ref. [99]. Distances given in Å. The hydrogen atoms have beenomitted for clarity.

1726 A. Frontera / Coordination Chemistry Reviews 257 (2013) 1716– 1727

Fig. 20. X-ray structures of the dipalladium clips complexed to two BF4−anions from ref. [102]. Distances given in Å. The CSD reference code is indicated.

F onallh

sss

5

tsnoTcim(osbmbbf

A

v

ig. 21. X-ray structures of two polymorphs obtained from crystallization of the trigave been omitted for clarity.

olvent molecule) influences the electronic properties easily, theolid-state emission can be modulated by simple anion (or solvent)ubstitution.

. Concluding remarks

The main intention of this review is to highlight the utility ofhe anion–� interaction in the molecular recognition of anions,pecially combined with the utilization of aromatic ligands coordi-ated to transition metals. This combination allows the generationf �-pockets and �-cages that are useful for trapping anions.he experimental and theoretical studies described in this reviewlearly demonstrate the exceptional power of this interaction tompact the field of supramolecular chemistry. The scientific com-

unity is being awakened to the potential use of this strategyanion–� interaction and coordination chemistry) for the designf novel sensors, hosts, catalysts, and materials. The use of bowlhaped cavitands decorated with different functional groups maye suitable for applications in drug delivery systems in the phar-aceutical industry. Non-covalent interactions such as hydrogen

onding, halogen bonding, C H/�, anion–� being incorporatedetween the cavitand and active pharmaceutical ingredient mightacilitate its release under physicochemical conditions [104].

cknowledgments

Financial support from the Ministerio de Ciencia e Inno-ación (MICINN) of Spain (projects CTQ2011-27512/BQU and

y coordinated AuI–CuI complex from ref. [103]. Distances in Å. The hydrogen atoms

CONSOLIDER INGENIO 2010, CSD2010-00065), FEDER funds andDirecció General de Recerca i Innovació del Govern Balear (project23/2011, FEDER funds) is gratefully acknowledged.

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