Two Compartmentalized Inner Receptors for the ... - ACS Publications

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

Article pubs.acs.org/IC

Two Compartmentalized Inner Receptors for the Tetramethylammonium Guest within a Keplerate-Type Capsule Nancy Watfa,†,‡ Mohamed Haouas,*,† Sébastien Floquet,*,† Akram Hijazi,‡ Daoud Naoufal,‡ Francis Taulelle,† and Emmanuel Cadot† †

Institut Lavoisier de Versailles, UMR CNRS 8180, University of Versailles Saint Quentin en Yvelines, University Paris-Saclay, 45 avenue des Etats-Unis, 78035 Versailles, France ‡ Laboratoire de Chimie de Coordination Inorganique et Organométallique, Université Libanaise, Faculté des Sciences I, Hadath, Lebanon S Supporting Information *

ABSTRACT: The host−guest interactions between the spherical porous Keplerate anion, [Mo132O372(CH3CO2)30(H2O)72]42− (abbreviated {Mo132}) and the tetramethylammonium cation have been investigated extensively by one- and two-dimensional (EXSY, ROESY, and DOSY) and variable-temperature NMR. Evidence of two inner receptor sites specific for a NMe4+ guest appears consistent with a quite striking compartmentalization phenomenon. ROESY NMR analyses showed that both sites exhibit a close spatial proximity with the hanging inner acetate groups, while a quantitative EXSY study revealed that these two sites are differentiated by their exchange rates. These NMR data support the hypothesis that these two inner sites could be delimited by the hanging inner acetate groups forming triangular (S1) or pentagonal (S2) hydrophobic pockets on the inner side of the capsule wall. Furthermore, the stability constants associated with the trapping process of the NMe4+ guest on both the S1 and S2 sites have been determined, showing that the stability constant of the S1 sites decreases significantly as the concentration of the capsule increases gradually, while that of the S2 sites remains nearly unaffected. Such an observation has been interpreted as a result of the plugging process of the {Mo9O9} pores by the counterions NH4+, which causes unfavorable electrostatic interactions for the NMe4+ coordination on the proximal S1 site. Finally, the thermodynamic parameters of the NMe4+ transfer from the solvated situation to the interior of the capsule were estimated from variable-temperature NMR experiments that provide the split of the global process into two successive events corresponding to the plugging and transfer across the inorganic shell.

■

1995 by Müller and co-workers.14 The Keplerate-type POMs belong to this class of compounds and correspond probably to one of the most fascinating and aesthetic arrangements in these “mesoscopic” species (see Figure 1). Resulting from a symmetry-driven self-assembling process, the Keplerate-type structure exhibits a high idealized Ih symmetry combining 12 archetypical pentagonal motifs {M(M)5O21} with M = Mo or W, held together through 30 {Mo2E2O2} linkers with E = O15,16 or S in a spheroidal topology.17−19 In addition to the tunable composition of the inorganic skeleton, the nature of the 30 inner ligands can be changed from acetate (for the most classical inner ligand) to specific ligands, thus giving the possibility of tuning the inner functionalities of the capsule, such as hydrophobicity/hydrophilicity,20,21 charge density,21−24 or coordination properties.22,23,25−27 Such chemical and structural features influence the transfer capacities of specific substrates across the 20 channels.28−30 This scenario has been

INTRODUCTION Polyoxometalates (POMs) represent a class of discrete metal− oxo polynuclear assemblies usually formed by successive polycondensation reactions of basic oxoanion [MO4]n−, with M corresponding to the early-transition-metal group (M = W, Mo, V, Nb, or Ta).1−4 Under specific and controlled conditions, outstandingly sophisticated POM species can be obtained in the restricted chemical area ranging from the discrete monomer to the infinite metal oxide. The numerous practical options accessible to the chemist to vary the POM architectures allowed the discovery of a rich, wide, and diverse class of compounds unique in their combined functionalities such as structural, redox, acidic, magnetic, electronic, or optic. So, POM compounds are found in many disciplines including magnetism,5 catalysis,6−8 biology,9,10 medicine,11 and materials science.4,12,13 Among the archetypical metal−oxo cores as integral building blocks of the POM assembly, the pentagonal motif {M6O21} with M = Mo or W constitutes the structural invariant component found within a series of very large POM species initiated by the discovery of the “big wheel” {Mo154} in © XXXX American Chemical Society

Received: June 28, 2016

A

DOI: 10.1021/acs.inorgchem.6b01516 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

identified receptors arises from a unique process across the pores and driven mainly by attractive electrostatic forces strengthened by an additional contribution based on hydrophobic interactions.

■

EXPERIMENTAL SECTION

Sample Preparations. All reagents were used as received from commercial suppliers. The {Mo132}−Keplerate precursor was prepared as reported by Müller et al.37 NMR Experiments. 1H NMR spectra were recorded in D2O. 1H NMR chemical shifts were referenced to tetramethylsilane as an external standard (δ = 0 ppm). NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer at room temperature (300 K) when it is not specified. For variable-temperature studies, 1H NMR spectra were acquired in the 300−345 K range. Translational diffusion measurements were performed using Bruker’s “ledbpgs2s” stimulatedecho DOSY pulse sequence including bipolar and spoil gradients. Apparent diffusion coefficients were obtained using an adapted algorithm based on the inverse Laplace transform and maximum entropy. Collection of the 2D phase-sensitive EXSY data included 16 transients per increment with a total of 1024 increments. A standard phase-sensitive NOESY pulse sequence was used, preparation−90°− t1−90°−tm−90°−acquisition. The mixing time (tm) was ranging from 0 to 500 ms, and the recycle time was 1 s. The ROESY experiments were obtained using a standard phase-sensitive pulse sequence, 16 scans per t1 increment, 3 s recycle time, 1024 t1 increments, and a mixing time ranging from 30 to 300 ms. The power level was 23 dB corresponding to 17 mW. Variable-Concentration NMR Study. A stock aqueous solution containing both (NH4)52{Mo132} at 5 mM and NMe4Br at 15 mM was first prepared. From these mother solutions, a series of diluted solutions were prepared with a constant ratio of NMe4+/{Mo132} = 3 and concentrations ranging from 0.2 to 5 mM in {Mo132}. The number of equivalents of NMe4+ was determined accurately from the 1 H NMR integrals of the NMe4+ and acetate species. Variable NMe4+/{Mo132} Ratio Study. In a glass vial, 560 mg of (NH4)52{Mo132} was dissolved in 4 mL of D2O (5 mM). Another stock solution of 60 mM NMe4Br in D2O was prepared. For each titration point, different volumes of this NMe4Br stock solution (0.25, 0.5, 1, and 1.5 mL) were added to 0.5 mL of the (NH4)52{Mo132} solution, and the final volume was completed to 2 mL with D2O.

Figure 1. Mixed representation (polyhedral, wire, and space filling) of the {Mo132} Keplerate-type anion highlighting the 30 acetate ligands (red and green spheres) coordinated to the 30 {Mo2O4} linkers (gray sticks), which connect the 12 pentagonal motifs (gray polyhedra).

nicely described by Müller and co-workers, who demonstrated that the active pores {Mo9O9L3}, where L = sulfate or phosphate, are able to trap specific cations in specific positions31 or recently that the large hydrophobic inner cavity is accessible for large organic guests and sequestration of hydrophobic molecules.32−34 Structural characterizations of mixed salts of {Mo132} revealed that well-defined sites located in the vicinity of the sulfate-type pores {Mo9O9(SO4)3} coordinate specifically a variety of cations such as alkali (Na+, Rb+, and Cs+) or organic (guadininium or protonated urea) cations.31 Recently, we pointed out the unusual and intriguing behavior of the apolar tetramethylammonium ions (NMe4+) in the presence of large metal−oxo species, described as the striking capability of the NMe4+ ion to act as a guest for hydrophilic pockets of the POM.18,35,36 The resulting host−guest systems were structurally characterized in the solid state by X-ray diffraction18,36 or in solution by NMR methods.35 From these results, distinct scenarios were identified, where the NMe4+ ions appear (i) firmly trapped within hollow species like in clathrate compounds,18 (ii) in slow exchange between well-adapted pockets and solvent,18 and (iii) in a fast exchange regime in the “crown-ether”-like {Mo9O9} pores of {Mo132}-type Keplerate ions.18,35 A reliable set of quantitative data obtained by 1H DOSY NMR spectroscopy in liquid provided new insight about the “plugging” processes of the 20 {Mo9O9} pores of the capsule. While the uptake of the alkylammonium cations by the hydrophilic Keplerate pores is predominantly based on electrostatic interactions, analysis of the results obtained from a large series of alkylammonium ions leads to the conclusion that the plugging process is also enhanced by a significant hydrophobic contribution. However, the interactions between the NMe4+ and Keplerate ions are not restricted to the exclusive snapshot describing the NMe4+ ion plugged within the {Mo9O9} pores. Herein, we report the second part of the scenario showing the NMe4+ guest inside the {Mo132} capsule. Investigations were carried out by multitechnique NMR methods [one- and two-dimensional (1D and 2D) EXSY, ROESY, and DOSY and variable-temperature (VT)-NMR], highlighting inward NMe4+ ions distributed over two distinct receptors close to the inner acetate ligands. Determination of the thermodynamic and exchange parameters by VT-NMR experiments showed that NMe4+ transfer onto the two

■

RESULTS AND DISCUSSION Evidence of Encapsulated NMe4+ Ions. Figure 2a shows the typical 1H NMR spectrum of an aqueous solution of {Mo132} at a concentration of 5 mM in the presence of 3 equiv of NMe4+. As is usually observed, the set of resonances in the 0.7−0.9 ppm range is assigned to the methyl groups of the acetate ligands in different coordination modes inside the capsule,15 while the narrow signal at 1.9 ppm is due to solvated acetates/acetic acid outside the Keplerate capsule. Besides, the signal at 3.2−3.3 ppm was assigned to NMe4+ cations in fast exchange between the solvent and the 20 {Mo9O9} pores present at the surface of the {Mo132} capsule.35 Surprisingly, two additional signals are observed at 2.9 and 3.0 ppm, denoted as S1 and S2, respectively. They could be assigned to NMe4+ cations encapsulated in the {Mo132} cavity, according to the typical shielding effect expected by the insertion of a guest in a hollow host.15,18 This interpretation was plentifully justified by complementary NMR experiments (see below), which suggest a singular behavior of the NMe4+ cations because such NMR features were not observed in the presence of other alkylammonium cations such as MeNH 3 + , Me 2 NH 2 + , Me3NH+, or Me2NEt2+ and Me3NPr+. It is worth noting that such behavior is also specific to acetate−type Keplerates because no such NMR signatures were observed with sulfateB


Article

Inorganic Chemistry

Figure 3. Variation of the self-diffusion coefficients of NMe4+ (▲) and acetate (○) ions with concentration in {Mo132}: (a) data related to the encapsulated acetate (0.7−0.9 ppm) and NMe4+ species (2.9 and 3.0 ppm); (b) data corresponding to the NMe4+ species (3.3 ppm) in a fast exchange process involving solvated and plugged situations at the {Mo132} surface; (c) self-diffusion coefficient of the weakly interacting solvated acetate ions. The 1H DOSY NMR experiments have been carried out at a fixed ratio of NMe4+/{Mo132} = 3.

and S2 NMe4+ species (δ = 2.9 and 3.0 ppm) remain nearly constant over the concentration domain even though the S1 and S2 signals became hardly detectable for concentration lower than 0.7 mmol/L (see Figure 3a). This behavior features the formation of a stable inclusion complex involved in a slow host−guest exchange process. While the inner coordination of the acetate ligands on the 30 {Mo2O4} linkers is wellestablished, similar behavior observed for the apolar NMe4+ ions provides interesting and unexpected insight into the properties of {Mo132} as a molecular host. In stark contrast, {Mo132} exerts a strong influence on the D value of the outward NMe4+ species (δ ≈ 3.15 ppm), which undergoes a 60% decrease in a narrow concentration range (see Figure 3b). Such a variation has been analyzed previously as the result of labile interactions involving the solvated NMe4+ guest species and the 20 {Mo9O9} pores of the ionic capsule.18,35 Quantitative analyses evidenced a competition process between the apolar NMe4+ guests and the 52 NH4+ present in the {Mo 132 } formula 37 and allowed determination of the thermodynamic parameters associated with the plugging process.35 Finally, the self-diffusion coefficient of the free acetate ions (δ = 1.9 ppm) displays a weak concentration dependence (see Figure 3c) probably because of ion pairing with NH4+ cations rather than interactions with the {Mo132} capsule. In the end, the variable-concentration 1H DOSY NMR of the NMe4/{Mo132} system highlights nicely two types of supramolecular host−guest complexes both involving NMe4+ ions either firmly trapped in the cavity or plugged at the {Mo9O9} pores on the outer surface. Identification of the Inner Receptors. Confirmation of the NMR assignments and further insight about the relationships between the observed resonances were brought by 2D NMR experiments including EXSY and ROESY. The 2D 1H EXSY NMR spectrum of NMe4/{Mo132} solutions is shown in Figure 4. Analysis of the EXSY spectrum (recorded with 500 ms mixing time) reveals the typical exchange pathways between solvated/outer (1.9 ppm) and linked/inner acetates (0.7 and 0.9 ppm), which have been previously reported by Müller et al.41 However, in this present study, similar cross peaks are also

Figure 2. 1H NMR spectra of [Mo132O372(H2O)72(CH3COO)30]42− in a D2O solution containing 3 equiv of NMe4+ ion per {Mo132}: (a) 1D spectrum showing the signals (in red frame) attributed to trapped NMe4+ species; (b) DOSY 2D spectrum highlighting similar selfdiffusion coefficient D = 110 μm2/s for encapsulated species (acetate linkers and these specific NMe4+ cations).

type Keplerates when the inner acetate ligands are fully replaced by sulfates. Furthermore, diffusion-ordered NMR spectroscopy has been employed to discriminate the NMR signals through their related self-diffusion coefficient.38 Such a method has been successfully applied to gain new insight about the characterization and speciation of cyclic or hollow polyoxo(thio)metalates including Keplerate-type compounds.39,40 The 2D 1H DOSY NMR spectrum of a NMe4/{Mo132} solution, shown in Figure 2b, reveals that the acetate linkers (0.7−0.9 ppm) and the NMe4+ cations identified as S1 (2.9 ppm) and S2 (3.0 ppm) exhibit similar self-diffusion coefficients with a significant low value of D ≈ 110 μm2/s, while at the same time, higher D values were found for the corresponding outer species such as D ≈ 760 μm2/s for acetates and D ≈ 350 μm2/s for NMe4+. These observations support the encapsulation of NMe4+ species, which exhibit self-diffusion coefficients related to the large dimension of the Keplerate ion. Using the Stokes− Einstein equation, which describes diffusion of spherical objects, the calculated hydrodynamic radius is RH ≈ 17 Å, consistent with the crystallographic dimension of the {Mo132} capsule (about 32 Å in diameter).40 Furthermore, the selfdiffusion coefficients of the interacting species have been measured for variable concentration in {Mo132} ranging from 0 to 5 mmol/L (see Figure 3). The D variations appear highly illustrative of the nature and strength of respective interactions with the Keplerate capsule. The low D values of the coordinated acetates (δ ≈ 0.8 ppm) and those of the two S1 C


Article

Inorganic Chemistry

Figure 4. (a) 1H EXSY NMR spectrum (500 ms mixing time) of a D2O solution of [Mo132O372(H2O)72(CH3COO)30]42− in the presence of NMe4Br, with C({Mo132}) = 5 mmol/L and NMe4+/{Mo132} = 1.5 at room temperature. (b) Expansion of the region containing the NMe4+ resonances around 3 ppm.

detected between the resonance of the outer NMe4+ at ca. 3.3 ppm and the inner ones at 2.9 (S1) and 3.0 ppm (S2), confirming definitively the assignment of these signals to encapsulated NMe4+ species. Similar to that observed with acetates, an exchange pathway from the solvent to the internal cavity is available for transfer of the NMe4+ cations across the inorganic shell. Interestingly, the intensity of the cross peak involving the signal S1 appears much more intense than that attached to the signal S2. This may be the result of two significantly different exchange regimes. For instance, decreasing the mixing time to 50 ms leads to the vanishing of the EXSY correlations associated with the S2 signal, while those involving the S1 resonance are retained (see Figure S5). This indicates that the NMe4+ substrate shows a higher exchange rate on the S1-type receptor compared to the S2 one. Detailed quantitative studies of the exchange processes are presented and discussed in the next sections. Exchange between two sites, here S1 and S2, taking place during the mixing time is limited by their relaxation T1 values. The exchange between S1 and S2 is most probably promoted by the exchange of acetates, providing the necessary fluctuation for NMe4+ to move between the S1 and S2 sites. The probability of the exchange on these sites involving acetates and the much lower mole fraction of NMe4+ is indeed much lower than that of the acetates exchanging alone. This results in an almost undetectable S1−S2 exchange in EXSY compared to the faster acetate exchange, as was previously observed.41 Then, the presence of two distinct sites revealed by the well-resolved S1 and S2 NMR signals describes a typical compartmentalization of receptors able to trap the NMe4+ species. Importantly, the degree of compartmentalization of these receptors should depend (i) on their spatial separation, (ii) on their relative binding constant, and also (iii) on the energy barrier, which characterizes the chemical rate of exchange between S1 and S2. Additional 1H NMR experiments based on dipolar interactions such as ROESY have been carried out to get further complementary information about these two inner NMe4 receptors.20,42 The 2D 1H ROESY NMR spectrum of NMe4/{Mo132}, shown in Figure 5, reveals clear on-phase correlations with respect to the diagonal signals (blue cross-peaks), which correspond to the slow exchange regime relative to the NMR time scale and similarly observed previously in the EXSY

Figure 5. 1H ROESY NMR spectrum of a D2O solution of [Mo132O372(H2O)72(CH3COO)30]42− in the presence of NMe4Br, with C({Mo132}) = 5 mmol/L and NMe4+/{Mo132} = 1.5. Negative ROE cross-peaks are shown as red contours, relative to the positive diagonal peaks in blue. The red arrows indicate the peaks involved in the ROESY correlations.

spectrum (Figure 4). More importantly, antiphase correlations (red cross-peaks) corresponding to dipolar contact were observed. These correlations involve the S1- and S2-type NMe4+ and the inner acetate ligands at 0.7−0.8 ppm, thus confirming unambiguously the spatial proximity of the trapped NMe4+ species with the acetate ligands bound to the {Mo2O4} linkers. Furthermore, the ROESY spectrum does not show any detectable dipolar contact between the 0.9 ppm line and the S1 or S2 resonances. While the 0.9 ppm line has been previously attributed to acetate ligands coordinated on the inner side of the pentagonal unit,41 these results suggest that the binding of S1 and S2 receptors should be located on the internal wall of the cavity, surrounded by the acetates coordinated at the {Mo2O4} linkers in the vicinity of the {Mo9O9} pore and/or close to the pentagonal motif {Mo6O21} (when acetates are not coordinated on this motif). Schematic representations (given in Figure 6) show the two possible dispositions for the NMe4+ species within the pockets lined by three or five inner coordinated acetates. Considering the EXSY results for which the signal S1 exhibits a faster exchange regime with the exterior than the site S2, the S1 receptor should be assigned to the inward triangular cavity of the pores (Figure 6b), while the S2 receptor should be located in the pockets delimited by the five D


Article

Inorganic Chemistry

their occupancy factor is concentration-dependent. Indeed, at low concentration, S1 is prominent, but as the concentration increases, S2 becomes progressively dominant, reaching up to an optimal ratio of 40:60 at 5 mmol/L. These results are shown in Figure 7b, plotting the number of encapsulated NMe4+ per {Mo132} as a function of the concentration. They reveal that the number of encrypted NMe4+ cations does not exceed 0.5 per {Mo132} capsule in these experimental conditions. Then, we can assume that the global encapsulation process could be expressed by eq 3 and the related equilibrium constant of transfer Kt is given by eq 4. Csolv, Cin, and CKep correspond to solvated NMe 4 + [NMe 4 + (aq)], total encrypted NMe 4 + (NMe4@{Mo132}), and NMe4-free {Mo132} capsules, respectively. Figure 6. Mixed representation (wire and space filling) of the Keplerate-type anion {Mo132}. The 30 inner acetate ligands (green and red spheres) attached to the inorganic framework (gray stick) define 12 pentagonal sites (related to the S2 signal) and 20 trigonal sites (related to the S1 signal). These two types of inner hydrophobic pockets should be involved as receptors for the inner immobilization of the NMe4+ ion (blue sphere).

(1)

{NMe4 +}S1 → {NMe4 +}S2

(2)

(3)

K t = C in /CsolvC Kep

(4)

The concentrations of the different species involved in eqs 3 and 4 were determined from the integrated lines of the 1H NMR spectra given in Figure 7 (see the Supporting Information for more details). Nonetheless, in the outward environment, two limit situations have to be considered for the NMe4+ species, being either fully solvated in an aqueous solution or plugged within the {Mo9O9} pores of the capsules.35 The proportion between solvated and guest situations can be calculated from the stability constants KNH4 and KNMe4 of the plugging process involving NH4+ and NMe4+ both in competition and the 20 {Mo9O9} pores. The details of these calculations have been fully described previously35 and are given in the Supporting Information (SI-1). The consistency of eqs 3 and 4 has been verified by graphically plotting Cin versus CsolvCKep (see Figures S1 and S2). A linear correlation has been established for the S2 site over the concentration range leading to a constant of transfer Kt(S2) = 130 ± 10 M−1 at room temperature. Similar behavior is not observed for the S1 site, and graphical analysis shows that the conditional constant of transfer Kt(S1) decreases significantly from about Kt(S1) = 150 ± 10 M−1 at infinite dilution to about 20 at 5 mmol/L (see Figure 8). This could be explained by the external pore plugging by cations. Actually, using the plugging

methyl groups of the acetates surrounding each pentagonal motifs (see Figure 6a). In view of such assignments and considering that the encapsulation process should take place preferentially through the 20 {Mo9O9} pores, a simple model describing NMe4+ transfer within the cavity of the Keplerate capsule can be given as two successive steps represented by eqs 1 and 2: NMe4 +(aq) → {NMe4 +}S1

NMe4 +(aq) + {Mo132} ⇌ NMe4@{Mo132}

Determination of the Equilibrium Constants of the Transfer Reactions. The 1H NMR spectra of an equilibrated aqueous solution at various concentrations of NMe4/{Mo132} are shown in Figure 7a. At low concentration, the S1 and S2 signals corresponding to encrypted NMe4+ cations are nearly unobserved. However, as the concentration increases, the intensity ratio of these two resonances significantly grows. Interestingly, the proportion between the S1 and S2 signals varies with the concentration of the Keplerate, meaning that

Figure 7. (a) Variable-concentration 1H NMR spectra of {Mo132} in a D2O solution containing 3 equiv of NMe4+ ions highlighting the NMe4+ region. Inset: Focus on the two resonances S1 (2.9 ppm) and S2 (3.0 ppm) assigned to the encrypted NMe4+ ions. (b) Variation of the encrypted NMe4+ per {Mo132} capsule (corresponding to the S1 and S2 signals) versus the concentration.

Figure 8. Dependence of conditional equilibrium constants Kt of NMe4+ transfer, with the Keplerate concentration highlighting the change of the inclusion ability of NMe4+ on the S1 sites, while the S2 receptor affinity remains quite unaffected. E


Article

Inorganic Chemistry constants (KNMe4 = 1550 M−1 and KNH4 = 370 M−1) determined in our previous study35 reveals that the 20 pores were found to be fully plugged by a NH4+ cation for C° > 1.5 mM (see Figure S1). In this case, the presence of cations on the outer side of the pore should result from electrostatic repulsions of the NMe4+ ions located in the inner side of the pore (S1 receptor). In contrast, the S2 site is not affected by the cationic plugging, which is consistent with the proposed assignment for S2 at the inner side of the pentagonal sites and then far enough from the external plugging sites (see Figure 6). This highlights the specific decrease of the inclusion ability of the {Mo132} capsule on the S1 sites with the plugging of the 20 pores by cationic species (here NH4+), while the S2 receptor remains quite unaffected. Furthermore, the Kt values determined for the transfer process (Kt = 20−150 M−1) are significantly lower than that found for NMe4+ plugging at the pore (KNMe4 = 1550 M−1). This is perhaps the result of nonspecific competitive effects involving the ammonium cations present in large excess compared to NMe4+. Indeed, dynamic calculations [molecular dynamics (MD) simulation] carried out by Bo and co-workers showed that ammonium cations enter in the capsule through the {Mo9O9} gates.35 They found that for a 7 × 7 × 7 nm3 box modeling a 4.8 mmol/L aqueous solution of Keplerate-type {Mo132} associated with its 42 NH4+ ions, the capsule can accept about 12 NH4+ ions, while in similar experimental conditions, about 0.5 NMe4+ ion was found for a presence of 3 per capsule. Additional experiments were conducted by varying the NMe4/Keplerate ratio up to 9 for a fixed concentration at 5 mmol/L (ratios higher than 9 provoke precipitation of the Keplerate ion). Such conditions allowed one to reach one NMe4+ per {Mo132} (see Figure S3 in section SI-2). Moreover, the relative populations of NMe4+ in S1 and S2 receptors were found to be nearly constant to ca. 40:60, respectively (see Figure S4). This means that the S1/S2 ratio does not vary with the NMe4+/{Mo132} ratio but does sensitively vary with the overall concentration, confirming that the main factor affecting the relative S1/S2 balance is the external pore plugging effect involving the NH4+ ion. Thermodynamic Parameters for the Transfer Process. The dynamic character and quantification of the transfer processes have been investigated by variable-temperature NMR (Figure 9). As the temperature increases, NMR signals broaden because of increasing exchange rates involving either acetates or

NMe4+, but in both cases, coalescence was not observed in this range of temperature probably because of the high energy barriers, as was already reported by Müller et al. for acetates.41 Interestingly, the relative proportion of encapsulated NMe4+ was found to decrease significantly from 15% to about 4% as the temperature increases from RT to 342 K. This thermal change was found to be perfectly reversible by subsequent increasing−decreasing variable-temperature experiments. Thermodynamic parameters of NMe4+ transfer from solvent to inner sites have been determined through an usual procedure (Van’t Hoff plot shown in Figure 9b). For each temperature, the relative populations between S1 and S2 remain nearly unchanged, allowing calculation of the averaged equilibrium constant Kt related to the global transfer of NMe4+ toward the S1 and S2 sites together. Plots of lnKt versus T−1 (Figure 9b) then revealed the standard enthalpy (ΔrH* = −67 kJ/mol) and entropy (ΔrS* = −175 J/K·mol) changes. While the entropic contribution appears strongly unfavorable, the enthalpy compensation operates (ΔrH* < TΔrS*) to give an enthalpically driven process of transfer. However, the sign of the entropy and enthalpy values can be discussed simply by considering transfer of the NMe4+ ions from the solvated situation to the inner S1 and S2 sites. The negative entropy change should arise from neat immobilization of the NMe4+ cation into the confined sites S1 and S2 of the large Keplerate anion. Such immobilization should be associated with the observed self-diffusion coefficient of the S1 and S2 NMe4+ (D = 110 μm2/s), a low value fitting with the size of the Keplerate ion. Nevertheless, the main contribution to the negative enthalpy change should correspond to the local electrostatic attraction between the acetate groups and cationic guests. These results and their related interpretations appear to be fairly complementary with those corresponding to the plugging process of the NMe4+ ion at the {Mo9O9} pores reported previously.35 Conjugation of both processes gives a complete thermodynamic description of the host−guest events from the solvated situation to immobilization onto S1 or S2 receptors. Actually, transfer of the NMe4+ ion within the capsule can be described either from the solvent (global transfer) or from the plugging situation (specific transfer), as shown in Figure 10. Then, determination of the thermodynamic parameters associated with the specific transfer can be easily calculated from those related to the plugging (previously reported) and

Figure 9. (a) Variable-temperature (heating and cooling cycles) 1H NMR spectra of a D2O solution of {Mo132} in the presence of NMe4Br with C° = 5 mmol/L and NMe4+/{Mo132} = 3. Inset: Vertical expansion (×16) showing the S1 and S2 signals. (b) Van’t Hoff plot related to the equilibrium process of NMe4+ transfer from the solvent to the Keplerate cavity.

Figure 10. Schematic decomposition of the global transfer process of the NMe4+ ion (blue spheres) from the solvated situation to encapsulation in the capsule (gray stick). The thermodynamic parameters have been determined experimentally for the plugging (see ref 35) and for the global transfer processes (this work), allowing calculations of ΔrS* and ΔrH* for the specific transfer. F


Article

Inorganic Chemistry

(see Table S1 in section SI-3). This suggests that the S1 site should be closer or more accessible to the exterior. Small differences in ΔG⧧ of both processes (70−80 kJ/mol) reflect, however, the same range of magnitude for the rate constants k of exchange motions (see Tables 1 and S1 in section SI-3). Such an observation includes even the exchange process involving acetate (ΔG⧧298 K = 75 kJ/mol) and should be consistent with a common pathway of transfer across the {Mo9O9} ether-crown-like pores. Because the magnitudes of the activation parameters ΔH⧧ and ΔS⧧ fall in the same order for the NMe4+ guest, those associated with the acetate transfer appear to differ significantly probably because of the anionic nature of the acetate ions. Although at this stage it is not easy to elucidate the origin of such variations, one reasonable hypothesis can be expressed as follows. In the vicinity of the {Mo9O9} pores, ion pairing could occur between the acetate and ammonium ions. The formation of such a transition state could contribute to the large change in entropy penalty observed for acetates (ΔS⧧ = −97 J/mol·K). Furthermore, the ion-pairing formation should also reduce unfavorable electrostatic interactions between the anionic guest and oxygen atoms lining the {Mo9O9} pore that could be reflected in the smaller activation enthalpy.

those of the specific transfer processes (see above). The two successive steps appear to be driven by a neat enthalpy decrease, explained mainly by the predominant role of the electrostatic interactions between the cationic guest and anionic capsule. Besides, the entropy penalties appear significantly different between both steps, weaker for the plugging process (ΔrS* = −17.5 J/mol·K) and larger for the specific transfer (ΔrS* = −157.5 J/mol·K). Such an observation can be tentatively understood by considering desolvation of the apolar cationic guest, which occurs through the plugging process. In this context, the release of water molecules is expected to provide an entropy gain. On the contrary, the specific transfer should not require any significant desolvation of the guest, which cannot balance the large entropy penalty because of the strong immobilization of NMe4+ within the S1 and S2 receptors. Bo and co-workers showed by MD simulations that the plugged situation corresponds to wriggling NMe4+ ions facing the {Mo9O9} pores,35 a situation that should differ significantly from that corresponding to the interior of the capsule described as being a confined environment.43,44 Analysis of the present results should give some indication about the description of this confined environment, which is probably characterized by a weak molecular mobility and restricted degrees of freedom. These results are consistent with a favorable transfer process based on electrostatic attractive interactions (ΔrH* < 0). Nevertheless, the enthalpy change appears more or less compensated by the entropic contribution, which depends on the nature of the host−guest events, i.e., plugging on the pores (moderate effect of restricted mobility) or immobilization in the inner S1 and S2 receptors (major effect due to confinement). Activation Parameters for the Transfer Process. The exchange processes between the solvated/plugged and trapped NMe4+ ions in the S1 and S2 sites were directly investigated by variable-temperature EXSY experiments (see Figures S5−S7 in section SI-3). Quantitative analysis of diagonal peaks and crosspeaks as a function of the mixing time provides exchange rates between the outer and inner guest situations. From Eyring plots (see Figures S8 and S9 in section SI-3), the activation enthalpy ΔH⧧ and entropy ΔS⧧ were derived for transfer processes related in eqs 5 and 6 and summarized in Table 1. NMe4 +out ⇌ S1

(5)

NMe4 +out ⇌ S2

(6)

■

CONCLUSION The interactions of the NMe4+ cation with the Keplerate capsule in an aqueous solution have been studied by 2D correlation techniques including EXSY, ROESY, and DOSY, which highlighted the short- and long-range dipolar proximities, dynamics, and diffusive properties of guest species such as NMe4+ or acetate ions. A quantitative study showed a specific uptake for the NMe4+ substrate based on the establishment of two distinct receptors (denoted as S1 and S2) able to compartmentalize the NMe4+ guest within the capsule. NMR structural analysis of both sites is consistent with close spatial proximities between the inner acetates and the trapped NMe4+ guests. Interestingly, variation of the relative occupancy factor on S1 and S2 with the concentration of the {Mo132} system has been interpreted as induced by an electrostatic repulsion effect, which mainly influences the affinity of the S1 site, while that of the S2 site remains quite unchanged. This effect could arise from the plugging of the {Mo9O9} pores by the NMe4+ ion, which occurs in close proximity to the S1 receptors. The full description of the encapsulation process of the NMe4+ species considers, in a first step, a fast exchange between the NMe4+ solvated cations and the 20 available {Mo9O9} pores distributed over the external surface of the capsule. When the concentration is high enough, uptake of NMe4+ via transfer across the pore occurs in a second step. These two successive events describe a global NMe4+ transfer from a full solvated situation to a confined environment in the interior of the capsule. The global transfer process was found to be entropically unfavorable, which could be explained by the different environments of the cationic substrate until the confined conditions within the capsule. In conclusion, the current study gives new insight about the structural, thermodynamic, and dynamics properties of the host−guest chemistry involving the apolar NMe4+ guest and the large Keplerate ions. The diversity and flexibility of the depicted interactions (hydrophobic, electrostatic, steric, etc.) offer unique opportunities to develop innovative supramolecular chemistry by finely tuning the properties of the versatile capsules such as the hydrophobic/hydrophilic character or

In addition, the activation parameters related to the exchange of acetates were also determined from the same experiments and are included in Table 1.41 The rate constant of the exchange process involving the inner S1 site and outer NMe4+, i.e., kS1 = 2−7 s−1 for the temperature range 27−46 °C, is about 5 times faster than that involving S2 Table 1. Activation Parameters (ΔH⧧, ΔS⧧, and ΔG⧧) Obtained for Exchange Processes between Solvated and Linked Species of Either NMe4+ Ions on the S1 and S2 Sites or Acetate Ligands exchange process

ΔH⧧ (kJ/mol)

ΔS⧧ (J/mol·K)

ΔG⧧298 K (kJ/mol)

NMe4+out ⇌ S1 NMe4+out ⇌ S2 AcOout ⇌ AcOlink

60 ± 5 71 ± 7 46 ± 2

−40 ± 17 −23 ± 25 −97 ± 7

72 ± 10 77 ± 15 75 ± 5 G


Article

Inorganic Chemistry

(6) Sartorel, A.; Bonchio, M.; Campagna, S.; Scandola, F. Tetrametallic Molecular Catalysts for Photochemical Water Oxidation. Chem. Soc. Rev. 2013, 42, 2262−2280. (7) Lv, H. J.; Geletii, Y. V.; Zhao, C. C.; Vickers, J. W.; Zhu, G. B.; Luo, Z.; Song, J.; Lian, T. Q.; Musaev, D. G.; Hill, C. L. Polyoxometalate Water Oxidation Catalysts and the Production of Green Fuel. Chem. Soc. Rev. 2012, 41, 7572−7589. (8) Jimenez-Lozano, P.; Ivanchikova, I. D.; Kholdeeva, O. A.; Poblet, J. M.; Carbo, J. J. Alkene Oxidation by Ti-Containing Polyoxometalates. Unambiguous Characterization of the Role of the Protonation State. Chem. Commun. 2012, 48, 9266−9268. (9) Yamase, T. Anti-Tumor, -Viral, and -Bacterial Activities of Polyoxometalates for Realizing an Inorganic Drug. J. Mater. Chem. 2005, 15, 4773−4782. (10) Absillis, G.; Parac-Vogt, T. N. Peptide Bond Hydrolysis Catalyzed by the Wells−Dawson Zr(α2-P2W17O61)2 Polyoxometalate. Inorg. Chem. 2012, 51, 9902−9910. (11) Hasenknopf, B. Polyoxometalates: Introduction to a Class of Inorganic Compounds and their Biomedical Applications. Front. Biosci., Landmark Ed. 2005, 10, 275−287. (12) Miras, H. N.; Vila-Nadal, L.; Cronin, L. Polyoxometalate Based Open-Frameworks (POM-OFs). Chem. Soc. Rev. 2014, 43, 5679− 5699. (13) Wang, Y. F.; Weinstock, I. A. Polyoxometalate-Decorated Nanoparticles. Chem. Soc. Rev. 2012, 41, 7479−7496. (14) Müller, A.; Krickemeyer, E.; Meyer, J.; Bögge, H.; Peters, F.; Plass, W.; Diemann, E.; Dillinger, S.; Nonnenbruch, F.; Randerath, M.; Menke, C. Mo154(NO)14O420(OH)28(H2O)70(25±5)‑: a Water Soluble Big Wheel with More than 700 Atoms and a Relative Molecular-Mass of about 24000. Angew. Chem., Int. Ed. Engl. 1995, 34, 2122−2124. (15) Müller, A.; Gouzerh, P. From Linking of Metal-Oxide Building Blocks in a Dynamic Library to Giant Clusters with Unique Properties and Towards Adaptive Chemistry. Chem. Soc. Rev. 2012, 41, 7431− 7463. (16) Müller, A.; Gouzerh, P. Capsules with Highly Active Pores and Interiors: Versatile Platforms at the Nanoscale. Chem. - Eur. J. 2014, 20, 4862−4873. (17) Bannani, F.; Floquet, S.; Leclerc-Laronze, N.; Haouas, M.; Taulelle, F.; Marrot, J.; Kögerler, P.; Cadot, E. Cubic Box versus Spheroidal Capsule Built from Defect and Intact Pentagonal Units. J. Am. Chem. Soc. 2012, 134, 19342−19345. (18) Korenev, V. S.; Boulay, A. G.; Haouas, M.; Bannani, F.; Fedin, V. P.; Sokolov, M. N.; Terazzi, E.; Garai, S.; Müller, A.; Taulelle, F.; Marrot, J.; Leclerc, N.; Floquet, S.; Cadot, E. Tracking ″Apolar″ NMe4+ Ions within Two Polyoxothiomolybdates that Have the Same Pores: Smaller Clathrate and Larger Highly Porous Clusters in Action. Chem. - Eur. J. 2014, 20, 3097−3105. (19) Schäffer, C.; Todea, A. M.; Bögge, H.; Cadot, E.; Gouzerh, P.; Kopilevich, S.; Weinstock, I. A.; Müller, A. Softening of Pore and Interior Properties of a Metal-Oxide-Based Capsule: Substituting 60 Oxide by 60 Sulfide Ligands. Angew. Chem., Int. Ed. 2011, 50, 12326− 12329. (20) Schäffer, C.; Todea, A. M.; Bögge, H.; Petina, O. A.; Rehder, D.; Haupt, E. T. K.; Müller, A. Hydrophobic Interactions and Clustering in a Porous Capsule: Option to Remove Hydrophobic Materials from Water. Chem. - Eur. J. 2011, 17, 9634−9639. (21) Schäffer, C.; Bögge, H.; Merca, A.; Weinstock, I. A.; Rehder, D.; Haupt, E. T. K.; Müller, A. A Spherical 24 Butyrate Aggregate with a Hydrophobic Cavity in a Capsule with Flexible Pores: Confinement Effects and Uptake-Release Equilibria at Elevated Temperatures. Angew. Chem., Int. Ed. 2009, 48, 8051−8056. (22) Merca, A.; Haupt, E. T. K.; Mitra, T.; Bögge, H.; Rehder, D.; Müller, A. Mimicking Biological Cation-Transport Based on SphereSurface Supramolecular Chemistry: Simultaneous Interaction of Porous Capsules with Molecular Plugs and Passing Cations. Chem. Eur. J. 2007, 13, 7650−7658. (23) Müller, A.; Rehder, D.; Haupt, E. T. K.; Merca, A.; Bögge, H.; Schmidtmann, M.; Heinze-Bruckner, G. Artificial Cells: TemperatureDependent, Reversible Li+-ion Uptake/Release Equilibrium at Metal

steric hindrance of the receptors that composed the inner cavity. Besides, the diameter of the {Mo9O3E3} pores can be adjusted by the nature of the E components (E = O or S) or the nature of the countercations of the anionic capsule able to modify the transfer capability through the 20 pores of the capsule. Of special interest is the use of these spherical porous nanoobjects as selective adsorbents, catalyst supports, or reagent carriers in potential applications emerging from modern concepts in recognition and clustering chemistry.15,45−47

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01516. Analytical treatment of the variable-concentration 1H NMR data and calculation of the equilibrium constant Kt of NMe4+ cation transfer into the Keplerate capsule (section SI-1), titration experiment and especially variable NMe4+/Keplerate ratio 1H NMR dependence (section SI-2), and variable-temperature 1H EXSY NMR experiments (section SI-3) (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: sebastien.fl[email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the University of Versailles Saint Quentin (France), the CNRS (France), the Institut Universitaire de France (IUF), and the COST Action PoCheMoN for their financial support. Prof. Carles Bo and Dr. Dolores Melgar (ICIQ, Tarragona, Spain) are also warmly acknowledged for fruitful discussions.

■

REFERENCES

(1) Cadot, E.; Sokolov, M. N.; Fedin, V. P.; Simonnet-Jegat, C.; Floquet, S.; Sécheresse, F. A Building Block Strategy to Access SulfurFunctionalized Polyoxometalate Based Systems Using {Mo2S2O2} and {Mo3S4} as Constitutional Units, Linkers or Templates. Chem. Soc. Rev. 2012, 41, 7335−7353. (2) Miras, H. N.; Yan, J.; Long, D. L.; Cronin, L. Engineering Polyoxometalates with Emergent Properties. Chem. Soc. Rev. 2012, 41, 7403−7430. (3) Nomiya, K.; Sakai, Y.; Matsunaga, S. Chemistry of Group IV Metal Ion-Containing Polyoxometalates. Eur. J. Inorg. Chem. 2011, 2011, 179−196. (4) Proust, A.; Matt, B.; Villanneau, R.; Guillemot, G.; Gouzerh, P.; Izzet, G. Functionalization and Post-Functionalization: a Step Towards Polyoxometalate-Based Materials. Chem. Soc. Rev. 2012, 41, 7605− 7622. (5) Clemente-Juan, J. M.; Coronado, E.; Gaita-Arino, A. Magnetic Polyoxometalates: from Molecular Magnetism to Molecular Spintronics and Quantum Computing. Chem. Soc. Rev. 2012, 41, 7464− 7478. H


Article

Inorganic Chemistry Oxide Nanocontainer Pores. Angew. Chem., Int. Ed. 2004, 43, 4466− 4470. (24) Lai, T.-L.; Awada, M.; Floquet, S.; Roch-Marchal, C.; Watfa, N.; Marrot, J.; Haouas, M.; Taulelle, F.; Cadot, E. Tunable Keplerate Type-Cluster ″Mo132″ Cavity with Dicarboxylate Anions. Chem. - Eur. J. 2015, 21, 13311−13320. (25) Gilles, A.; Mihai, S.; Nasr, G.; Mahon, E.; Garai, S.; Müller, A.; Barboiu, M. Highly Selective Li + Ion Transport by Porous Molybdenum-Oxide Keplerate-Type Nanocapsules Integrated in a Supported Liquid Membrane. Isr. J. Chem. 2014, 54, 423−423. (26) Müller, A.; Zhou, Y. S.; Zhang, L. J.; Bögge, H.; Schmidtmann, M.; Dressel, M.; van Slageren, J. En Route to Coordination Chemistry under Confined Conditions in a Porous Capsule: Pr3+ with Different Coordination Shells. Chem. Commun. 2004, 2038−2039. (27) Garai, S.; Rubčić, M.; Bögge, H.; Gouzerh, P.; Müller, A. Porous Capsules with a Large Number of Active Sites: Nucleation/Growth under Confined Conditions. Chem. - Eur. J. 2015, 21, 4321−4325. (28) Müller, A.; Toma, L.; Bögge, H.; Schäffer, C.; Stammler, A. Porous Capsules Allow Pore Opening and Closing that Results in Cation Uptake. Angew. Chem., Int. Ed. 2005, 44, 7757−7761. (29) Müller, A.; Bögge, H.; Henry, M. Coordination Chemistry under Confined Conditions: a Simplified Illustrative View. C. R. Chim. 2005, 8, 47−56. (30) Henry, M.; Bögge, H.; Diemann, E.; Müller, A. Chameleon Water: Assemblies Confined in Nanocapsules. J. Mol. Liq. 2005, 118, 155−162. (31) Müller, A.; Das, S. K.; Talismanov, S.; Roy, S.; Beckmann, E.; Bögge, H.; Schmidtmann, M.; Merca, A.; Berkle, A.; Allouche, L.; Zhou, Y. S.; Zhang, L. J. Trapping Cations in Specific Positions in Tuneable ″Artificial Cell″ Channels: New Nanochemistry Perspectives. Angew. Chem., Int. Ed. 2003, 42, 5039−5044. (32) Garai, S.; Bögge, H.; Merca, A.; Petina, O.; Grego, A.; Gouzerh, P.; Haupt, E. T. K.; Weinstock, I. A.; Müller, A. Densely Packed Hydrophobic Clustering: Encapsulated Valerates Form a HighTemperature-Stable {Mo132} Capsule System. Angew. Chem., Int. Ed. 2016, 55, 6634−6637. (33) Kopilevich, S.; Gottlieb, H.; Keinan-Adamsky, K.; Müller, A.; Weinstock, I. A. The Uptake and Assembly of Alkanes within a Porous Nanocapsule in Water: New Information about Hydrophobic Confinement. Angew. Chem. 2016, 128, 4552−4557. (34) Ziv, A.; Grego, A.; Kopilevich, S.; Zeiri, L.; Miro, P.; Bo, C.; Müller, A.; Weinstock, I. A. Flexible Pores of a Metal Oxide-Based Capsule Permit Entry of Comparatively Larger Organic Guests. J. Am. Chem. Soc. 2009, 131, 6380−6382. (35) Watfa, N.; Melgar, D.; Haouas, M.; Taulelle, F.; Hijazi, A.; Naoufal, D.; Avalos, J. B.; Floquet, S.; Bo, C.; Cadot, E. Hydrophobic Effect as a Driving Force for Host-Guest Chemistry of a MultiReceptor Keplerate-Type Capsule. J. Am. Chem. Soc. 2015, 137, 5845− 5851. (36) Pilette, M. A.; Marrot, J.; Duval, S.; Bannani, F.; Floquet, S.; Sécheresse, F.; Cadot, E. {Mo2O2S2}2+-Directed Synthesis of a Polyoxotungstoarsenate(III). Structure and 183W NMR Studies of the [(α-AsW9O33)3(WO(OH2))3(Mo2O2S2(H2O)4)]13‑ Anion. C. R. Chim. 2012, 15, 124−129. (37) Müller, A.; Krickemeyer, E.; Bögge, H.; Schmidtmann, M.; Peters, F. Organizational Forms of Matter: An Inorganic Super Fullerene and Keplerate Based on Molybdenum Oxide. Angew. Chem., Int. Ed. 1998, 37, 3359−3363. (38) Bonhomme, C.; Gervais, C.; Laurencin, D. Recent NMR Developments Applied to Organic-Inorganic Materials. Prog. Nucl. Magn. Reson. Spectrosc. 2014, 77, 1−48. (39) Duval, S.; Floquet, S.; Simonnet-Jegat, C.; Marrot, J.; Biboum, R. N.; Keita, B.; Nadjo, L.; Haouas, M.; Taulelle, F.; Cadot, E. Capture of the Mo3S44+ Cluster within a {Mo18} Macrocycle Yielding a Supramolecular Assembly Stabilized by a Dynamic H-Bond Network. J. Am. Chem. Soc. 2010, 132, 2069−2077. (40) Floquet, S.; Brun, S.; Lemonnier, J. F.; Henry, M.; Delsuc, M. A.; Prigent, Y.; Cadot, E.; Taulelle, F. Molecular Weights of Cyclic and

Hollow Clusters Measured by DOSY NMR Spectroscopy. J. Am. Chem. Soc. 2009, 131, 17254−17259. (41) Petina, O.; Rehder, D.; Haupt, E. T. K.; Grego, A.; Weinstock, I. A.; Merca, A.; Bögge, H.; Szakacs, J.; Müller, A. Guests on Different Internal Capsule Sites Exchange with Each Other and with the Outside. Angew. Chem., Int. Ed. 2011, 50, 410−414. (42) Grego, A.; Müller, A.; Weinstock, I. A. Stepwise-Resolved Thermodynamics of Hydrophobic Self-Assembly. Angew. Chem., Int. Ed. 2013, 52, 8358−8362. (43) Garcia-Rates, M.; Miro, P.; Müller, A.; Bo, C.; Avalos, J. B. Encapsulated Water Inside Mo-132 Capsules: The Role of LongRange Correlations of about 1 nm. J. Phys. Chem. C 2014, 118, 5545− 5555. (44) Mitra, T.; Miro, P.; Tomsa, A. R.; Merca, A.; Bögge, H.; Avalos, J. B.; Poblet, J. M.; Bo, C.; Müller, A. Gated and Differently Functionalized (New) Porous Capsules Direct Encapsulates’ Structures: Higher and Lower Density Water. Chem. - Eur. J. 2009, 15, 1844−1852. (45) Müller, A.; Merca, A.; Al-Karawi, A. J. M.; Garai, S.; Bögge, H.; Hou, G. F.; Wu, L. X.; Haupt, E. T. K.; Rehder, D.; Haso, F.; Liu, T. B. Chemical Adaptability: The Integration of Different Kinds of Matter into Giant Molecular Metal Oxides. Chem. - Eur. J. 2012, 18, 16310− 16318. (46) Inokuma, Y.; Kawano, M.; Fujita, M. Crystalline Molecular Flasks. Nat. Chem. 2011, 3, 349−358. (47) Raymond, K. N.; Allred, B. E.; Sia, A. K. Coordination Chemistry of Microbial Iron Transport. Acc. Chem. Res. 2015, 48, 2496−2505.

I


Two Compartmentalized Inner Receptors for the ... - ACS Publications

Recommend Documents