Water Structure Recovery in Chaotropic Anion Recognition: High-Affinity Binding of Dodecaborate Clusters to γ-Cyclodextrin**

Dodecaborate anions of the type B12X122− and B12X11Y2− (X=H, Cl, Br, I and Y=OH, SH, NH3+, NR3+) form strong (Ka up to 106 L mol−1, for B12Br122−) inclusion complexes with γ-cyclodextrin (γ-CD). The micromolar affinities reached are the highest known for this native CD. The complexation exhibits highly negative enthalpies (up to −25 kcal mol−1) and entropies (TΔS up to −18.4 kcal mol−1, both for B12I122−), which position these guests at the bottom end of the well-known enthalpy-entropy correlation for CDs. The high driving force can be traced back to a chaotropic effect, according to which chaotropic anions have an intrinsic affinity to hydrophobic cavities in aqueous solution. In line with this argument, salting-in effects revealed dodecaborates as superchaotropic dianions.

Abstract: Dodecaborate anions of the type B 12 X 12 2À and B 12 X 11 Y 2À (X = H, Cl, Br,Iand Y = OH, SH, NH 3 + ,N R 3 + ) form strong (K a up to 10 6 Lmol À1 ,f or B 12 Br 12 2À )i nclusion complexes with g-cyclodextrin (g-CD). The micromolar affinities reached are the highest knownf or this native CD. The complexation exhibits highly negative enthalpies (up to À25 kcal mol À1 )a nd entropies (TDSu pt oÀ 18.4 kcal mol À1 , both for B 12 I 12 2À ), whichposition these guests at the bottom end of the well-known enthalpy-entropyc orrelation for CDs.T he high driving force can be traced backt oachaotropic effect, according to whichchaotropic anions have an intrinsic affinity to hydrophobic cavities in aqueous solution. In line with this argument, salting-in effects revealed dodecaborates as superchaotropic dianions.
Association phenomena in aqueous solution, whether between am acrocyclic host and an encapsulated guest or between abiological receptor and its corresponding substrate, are frequently accounted for in terms of ac onglomerate driving force,t he hydrophobic effect. Regardless of the precise description of the contributors to the hydrophobic effect, [1] it is intuitive that the tendencyo fas uitably sized guest molecule or residue to become encapsulated inside ah ydrophobic macrocyclic cavity scales with its own hydrophobicity,w hich in turn relates inversely to its water solubility.E xceptionally large affinities (picomolar and below for cucurbiturils as hosts) can thus be reached for highly hydrophobic adamantane,diamantane,ortriamantane residues as guests. [2] We now report that highly water-soluble dianionic dodecaborates can form surprisingly strong inclusion complexes with macrocyclic hosts, g-cyclodextrin in particular.W eh old ah itherto underestimated driving force, the "chaotropic effect", responsible for this affinity.
Borate clusters of the types B 12 X 12 2À and B 12 X 11 Y 2À (X = H, Cl, Br, Ia nd Y = OH, SH, NH 3 + ,N R 3 + ;F igure 1) are poorly coordinating and weakly basic inorganic anions with icosahedral structure and ap ermanent double negative charge of the core. [3] Their discovery in the 1960s led to numerous applications in medicinal chemistry and materials science, [4] among which their use in neutron capture therapy of cancer stands out as apractically relevant one. [5] Thehostguest chemistry of these hydrophilic cluster anions has not been previously described. [6] Cyclodextrins (CDs,F igure 1) are well-known for encapsulating aw ide range of hydrophobic organic [7] and organometallic compounds in their cavity, [8] with few examples of inorganic guests. [9] Thec omplexation of dodecaborates with different CD homologues and derivatives was investigated by 1 HNMR spectroscopy, [10] which was made possible by their high solubility (for example,5 0mm for Na 2 B 12 I 12 and more than 3 m for Na 2 B 12 H 11 SH). 1 HNMR titrations were conducted for all clusters (see the Supporting Information);t he largest spectral changes were observed for g-CD as host. In particular,wewitnessed apronounced complexation-induced shift of the H3 proton (Figure 2a,b), which is located inside the cavity near the secondary hydroxyl rim, signaling the formation of an inclusion complex. Some clusters (B 12 H 11 SH 2À ,s ee the Supporting Information) caused not only as ignificant down-field shift for H3 but an even larger one for H5 (for example,0 .17 versus 0.09 ppm), which confirmed that the dianions protruded deeply into the hydrophobic cavity (Figure 2c). Fort he B 12 H 11 NR 3 À clusters (with R = Me,E t, nPr, nBu), we observed selective 2D-ROESY cross-peaks between the aliphatic protons and the H-3p roton of g-CD,t hat is,t he functional groups Ya re positioned near the wider rim (see the Supporting Information).
TheN MR titration data for B 12 H 12 2À and the clusters of the type B 12 H 11 Y 2À (see the Supporting Information) could be well-fitted according to a1 :1 complexation model, also confirmed by Jobp lot analysis (Supporting Information). Ther esulting association constants for g-CD are shown in Table 1. Theg uest affinity trends can be largely rationalized in terms of established arguments from the toolbox of hostguest chemistry.F or example,w ith reference to the parent, B 12 H 12 2À ,t he more lipophilic SH substituent increases the affinity,w hile the more hydrophilic OH group decreases the affinity,w hich establishes the range of the respectable association constants (0.62-7.8 10 3 Lmol À1 )f or the nonhalogenated clusters.
Upon complexation of the perhalogenated dodecaborates (B 12 X 12 2À with X = Cl, Br, I) by g-CD,t he 1 HNMR spectra showed large down-field shifts for the inner H3 and H5 protons (Figure 2d-f and the Supporting Information) and up-field shifts of the outer H1, H2, and H4 ones,all indicative of deep inclusion. Thes hift was largest for H5 in the g-CD·B 12 Br 12 2À complex (0.7 ppm). All perhalogenated clusters showed very strong binding to g-CD,such that we needed to resort to isothermal titration calorimetry (ITC) to determine precise binding constants.T he highest affinity was obtained for B 12 Br 12 2À (9.6 10 5 Lmol À1 ,F igure 4a), followed by B 12 I 12 2À (6.7 10 4 Lmol À1 )a nd B 12 Cl 12 2À (1.7 10 4 Lmol À1 ). This up-and-down trend with increasing cluster size pointed to an ideal size matching for the intermediary brominated cluster (see the Supporting Information).
We obtained single crystals from g-CD/B 12 Br 12 2À mixtures and solved the interesting XRD structure (Figure 3a). [11] The CDs pack in the unit cell forming af ormal tubular crystal lattice (see the Supporting Information). Tw o g -CDs were observed to cap ad odecaborate cluster tightly (Br À H À C distances ca. 3), while the two wider CD rims were held together by intermolecular hydrogen bonds.I ts hould be noted that although the complexation stoichiometry in the solid phase (2:1) differs from that established in aqueous solution (predominantly 1:1, as established by the Jobp lot and ITC titrations;s ee the Supporting Information), the tendency for deep immersion is reflected in both phases.
Fort he highest-affinity clusters,I TC was used to analyze the complexation thermodynamics (Table 1). Invariably,t he binding is an enthalpically driven process.T here is ag ood correlation between enthalpy and guest size:t he enthalpy of ,a nd f) B 12 I 12

2À
,a ll as sodium salts.  2À cluster into the g-CD dimer. Forthe sake of clarity,the severely disordered B 12 Br 12 2À cluster is visualized by an ideal, but XRD-based, B 12 Br 12 2À cluster (see the SupportingInformation). b) Size comparison and DFT-computed electrostatic potential maps for B 12  ,and B 12 I 12 2À ; the red to blue surface color range spans À180.0 to + 180.0 kcal mol À1 .

Angewandte
Chemie complexation (DH8 8)increases from B 12 H 11 SH 2À to B 12 I 12 2À ,in line with increasing dispersion interactions.T his trend is counterbalanced by an increasing entropic penalty,t hat is, enthalpy-entropy compensation applies (Figure 4b), as is common for CDs. [7] Noteworthy,however, the binding of the highest-affinity perhalogenated clusters stands out in this correlation owing to their very exothermic binding and associated large entropic penalty,w hich exceed, for the B 12 I 12 2À cluster,the values known for any native CD complex. Table 1c ontains ar emarkable set of data for this native CD and introduces an ew and orthogonal host-guest anchor motif (g-CD·B 12 X 12 2À ). Until now,e ven association constants on the order of 10 3 Lmol À1 have been fairly difficult to achieve for g-CD, [2b,c,12] because its largecavity lacks the highenergy water content which assists the binding to smaller cavities. [1c] Thevalues observed for the halogenated dianions and g-CD even rival and exceed the values found for b-CD, the putative highest-affinity host among native CD homologues.
[2b] Fore xample,t he binding constants for the highly hydrophobic adamantyl or ferrocenyl residues (carboxylate or ammonium), which present well-known gold standards in the CD field, reach only ca. 310 4 Lmol À1 for b-CD, [2b,c, 7] av alue approached or surpassed across the entire dodecaborate series with g-CD.U ndoubtedly,t he spherical shape complementarity [13] of the purely inorganic guests and their high polarizabilities (see the Supporting Information), especially the halogenated ones,contribute to these high affinities through an optimization of dispersion interactions. [14] How-ever,ascan be seen from Figure 3b,the globular clusters vary tremendously in size (more than afactor of three) and also in their electrostatic potential, yet their binding constants with g-CD remain rather constant (only two orders of magnitude variation). It was exactly this relatively low "selectivity" which pointed to an additional (peculiar but generic) driving force for complexation.
Dodecaborate salts are not only highly water-soluble and display negative log(P OW )v alues, [15] but the dianions have very negative free energies and enthalpies of hydration (ca. À140 kcal mol À1 ). [16] They are evidently hydrophilic guests [17] such that ah ydrophobic effect cannot account for their high affinities.I ns earching for alternative explanations,w e recalled the nature of these clusters as (even if unconventional) anions and inspected precedents for anion binding to CDs. [18] Indeed, the binding of iodide to a-CD was already reported 50 years back [18a] and sizable affinities of perchlorate (up to 66 Lmol À1 )w ere measured later. [18d] Detailed studies ruled out ahydrophobic effect as cause of the inorganic anion binding, [18b] but showed that the affinities paralleled their position in the Hofmeister series: [18c] chaotropic anions (water structure breakers,s uch as ClO 4 À )s howed higher affinities than kosmotropic anions (water structure makers,s uch as HPO 4 2À ). We therefore followed the idea whether a" chaotropic effect" could be responsible for the high affinities of the dodecaborates.
We conducted classical salting-in experiments to assess the chaotropic nature of the clusters (see the Supporting Information), which had not been scrutinized before.Indeed, they cause al arge increase in the solubilty of adenine and riboflavin, two established standards. [19] Moreover,t he solubilizing power of dodecaborates exceeds even those of SCN À and PF 6 À ,t wo prototypal chaotropic anions.B ased on their salting-in propensity,d odecaborates can be classified as "superchaotropic" anions,t hat is,t hey reach beyond the traditional Hofmeister scale;t hey are also the first salting-in agents bearing two negative charges. [20] This finding has important implications for borate cluster chemistry as awhole,which will be subject to follow-up work. We mention here only that there exist numerous indications on their unusual water solvation, [15] strong interactions with lipid membranes or proteins, [21] as well as unusual affinities, especially of the halogenated dodecaborates,tocarbohydrate chromatography matrices, [22] all of which now appear in the new light of their superchaotropic character.T he superchaotropic nature of the B 12 X 12 2À clusters was independently confirmed by applying the semiempirical ionic solvation model developed by Marcus (see the Supporting Information). [23] Based on these new lines of evidence,w ec onclude that the complexation of dodecaborate dianions is driven by ac haotropic effect. [24] Although the chaotropic effect has in common with the hydrophobic effect that the involved guests are weakly hydrated (compared to kosmotropes or hydrogenbonding solutes), they are conceptually distinct in that chaotropic anions interfere qualitatively differently with the water structure than hydrophobic species do. [25] Thecontrasting hydration behavior is borne out by the diametrically opposed thermodynamic fingerprints of the borate clusters . . versus hydrophobic residues (triangular data points in Figure 4b).
In mechanistic detail, chaotropic ions decrease the water structure in their surrounding,with two immediately relevant consequences:T he water structural entropies for ionic hydration are positive,a nd there is an effective loss of hydrogen bonds around the anion. Both effects,w hich can also be modelled according to Marcus (see the Supporting Information), should be particularly pronounced for the dodecaborate cluster dianions.Upon relocation of chaotropic anions from the aqueous bulk into nonpolar binding pockets as ignificant recovery of the structure of the water network must take place,which should contribute apronounced loss in water structural entropy and ag ain in enthalpy as ac onsequence of the restoration of hydrogen bonds.
Theo bserved negative complexation entropies for the dodecaborate clusters (Table 1) are indeed on the order of what is expected from the water structural entropies estimated for chaotropic anions (see the Supporting Information). Thec orrelated large negative complexation enthalpies require energetic stabilizations,which are sufficiently large to overcome the concomitant decrease in ion-dipole interactions.T hey can be accounted for in terms of 1) the reformation of the broken hydrogen bonds in the aqueous bulk upon binding of the chaotropic anions and 2) increased dispersion interactions of the guests with the host than with water. That the recovery of hydrogen bonds presents an important supramolecular driving force is known from the release of high-energy water from small macroyclic cavities, [1c] while the importance of dispersion interactions can be deduced from the very high polarizabilities calculated for the borate clusters (see the Supporting Information). Since both enthalpic effects increase indirectly [26] or directly [1c, 27] with the polarizability of the anions,a nd because the chaotropic nature of anions increases with their size and polarizability,t he described chaotropic effect includes implicitly contributions from dispersion. [28] It transpires that the chaotropic effect pin-pointed here describes ag eneric driving force for the encapsulation of chaotropic anions into suitable sized organic cavities in aqueous solution, with the propensity:B 12 X 12 2À (new) @ PF 6 À > ClO 4 À > SCN À ,I À > Br À @ kosmotropes.C haotrope encapsulation results in an effective water structure recovery and is enthalpically driven, with an invariably negative entropic component as fingerprint. Thec haotropic effect accounts for previous (for CDs) [29] and very recent (for CDs and other macrocycles) [30] observations on the high-affinity binding of such anions,and it merges independent, consistent observations for the same anions to be driven to interfaces, [20, 26b,31] to penetrateinto lipid bilayers, [20,32] and to bind in protein binding pockets. [33] Keywords: boron clusters ·cyclodextrins ·H ofmeister series · host-guest complexes ·supramolecular chemistry