How Does the Redox State of Polyoxovanadates Influence the

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

How Does the Redox State of Polyoxovanadates Influence the Collective Behavior in Solution? A Case Study with [I@V18O42]q− (q = 3, 5, 7, 11, and 13) Albert Solé-Daura,† Almudena Notario-Estévez,† Jorge J. Carbó,† Josep M. Poblet,† Coen de Graaf,†,‡ Kirill Yu. Monakhov,§ and Xavier López*,† †

Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, Marcel·lí Domingo 1, 43007 Tarragona, Spain Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain § Leibniz Institute of Surface Engineering (IOM), Permoserstraße 15, 04318 Leipzig, Germany

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF TEXAS AT DALLAS on 03/05/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: A series of stable reduction−oxidation states of the cagelike [I@VIVxVV18−xO42]5−x polyoxovanadate (POV) with x = 8, 10, 12, 16, and 18 were studied with density functional theory and molecular dynamics to gain insight into the structural and electron distribution characteristics of these metal−oxo clusters and to analyze the charge/redox-dependent assemblage processes in water and acetonitrile (MeCN) solutions. The calculations show that the interplay between the POV redox state (molecular charge) and the solvent polarity, countercation size, and hydrophilicity (or hydrophobicity) controls the POV agglomeration phenomena, which substantially differ between aqueous and MeCN media. In MeCN, agglomeration is more pronounced for intermediate-charged POVs, whereas in water, the lowest-charged POVs and organic countercations tend to agglomerate into a microphase. Tests made on wet MeCN show diminished agglomeration with respect to pure MeCN. Simulations with alkali countercations in water show that only the highest-charged POV can form agglomerates. The herein presented theoretical investigation aims to support experimental studies of POVs in the field of functional nanomaterials and surfaces, where controlled molecular deposition from the liquid phase onto solid substrates requires knowledge about the features of these metal−oxo clusters in discrete solutions.

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INTRODUCTION Polyoxovanadates (POVs) are a subclass of polyoxometalates (POMs) that feature outstanding redox activity. Combined with the fact that most POVs are paramagnetic qualifies these types of compounds as potential candidates for integration into new nanoelectronic and/or nanospintronic devices.1 To this end, gaining knowledge on their electronic structure and collective behavior in solution is of paramount interest. In a recently published contribution, we probed the ability of the mixed-valence host−guest [I@VIV10VV8O42]5− structure (represented in Figure 1, labeled as V18(10:8) or simply (10:8) for short) to form agglomerates made of two or more [I@ V18O42]q− (V18) units, both in aqueous and in acetonitrile (MeCN) solutions in the presence of tetraethylammonium (TEA) countercations.2 The formation of agglomerates in MeCN was found to be mainly governed by countercationmediated interactions between anions, which are enhanced by the low polarity of the solvent. In fact, Liu and co-workers had already proposed cation-mediated interactions between POM units to be the main driving factor for the formation of giant blackberry-like structures starting from isolated POMs in solution.3 Also, some of us recently reported that tetrabutylammonium (TBA) cations mediate the contacts between POMs in micellar structures grown in polar/nonpolar solvent mixtures.4 We also found that, conversely to their behavior in © XXXX American Chemical Society

Figure 1. Ball-and-stick representation of the ideal D4d arrangement of V18. The different regions are indicated. Color code: V, gray; O, red; I, magenta.

MeCN, V18(10:8) anions agglomerate via water-mediated interactions in aqueous media, which can be occasionally assisted by cations acting as linkers. This was fully supported by other molecular dynamics (MD) studies performed on Received: December 17, 2018

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DOI: 10.1021/acs.inorgchem.8b03508 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry different polyoxotungstates (POTs).5−10 The observed dependence on the solvent nature suggests that the solution properties of POMs are strongly sensitive to their surroundings. Related to this, it was demonstrated that the nature of the solvent and countercation has a strong influence on the synthesis and stability of POM clusters and POM-based supramolecular assemblies,11 as well as on other aspects such as their activity as water oxidation catalysts.12 Furthermore, the charge, size, and shape of the POM have been recognized to be determinant in self-assembly processes13 or in their interaction with biological systems.14 Therefore, understanding how the behavior of POMs in condensed phases is impacted by the aforementioned factors might grant access to a modulation of their speciation in solution by a rational modification of the experimental conditions. For this reason, several computational groups have attached their attention to the dynamic properties of POMs14f,15−28 and, more specifically, to understanding their collective behavior.4−10,29 For instance, Wipff and co-workers analyzed three Keggin POMs with different overall charges, [PW12O40]3−, [SiW12O40]4−, and [AlW12O40]5− (PW12, SiW12, and AlW12, respectively, for short), by means of MD simulations. They determined that the formation of agglomerates in a water medium increases in the order SiW12 < PW12 < AlW12.7 This initially indicated no direct relationship between the POT charge and the agglomeration behavior. Three years later, Antonio and co-workers revisited the collective behavior of these polyoxoanions and found that, in contrast to the previously reported trend, the ability to agglomerate is inversely proportional to the negative charge of the anion.8 In fact, only PW12 anions were capable of forming contact pairs in solution, presumably for carrying the lowest charge among the series. On the other hand, the stronger, long-range repulsion between their higher-charged congeners SiW12 and AlW12 was considered to hinder anion···anion association. This conclusion was supported not only by MD simulations but also by experimental small-angle X-ray scattering (SAXS) measurements. In line with this finding, Yang and co-workers reported the ability of PW12 anions to form agglomerates in the presence of ionic liquids,10 and Chaumont and Wipff probed that they also tend to agglomerate at the interface between water and organic solvents or even on a substrate surface such as graphite.29 Recently, Serapian and Bo analyzed the collective behavior of lithium POT salts in water and showed that stronger agglomeration occurs for the higher negatively charged POTs.9 Although the main interacting modes between POMs in solution have been identified, the different parameters influencing their dynamic properties and how they do it still remain poorly understood. Moreover, most of the MD studies on POMs performed so far are devoted to analysis of the behavior of tungsten-based systems, rendering the dynamic properties of POVs a highly unexplored area for computational chemistry. Herein we present a comprehensive study combining density functional theory (DFT), Car−Parrinello molecular dynamics (CPMD), and MD methodologies, aiming to get a complete picture of the redox-active V18-type polyoxoanions and their interactions with other species in solution, with a focus on agglomeration phenomena. In the present work, we widen our previous work2 by showing how agglomeration is affected by several intrinsic parameters such as the POV charge/redox state, type of solvent, and countercation. TEA and alkali-metal salts of five different [I@VIVxVV18−xO42]5−x polyoxoanions

were analyzed. On the basis of the work of Müller et al.,30 we considered redox states characterized by x = 8, 10, 12, 16, and 18 with overall charges q = 3−, 5−, 7−, 11−, and 13−, respectively. These states are labeled (VIV:VV), that is, (8:10), (10:8), (12:6), (16:2), and (18:0). Two different experimentally tested environments were explored, namely, water and MeCN. Considering that V18, like most cagelike POVs, can easily undergo redox processes without significantly changing its geometric structure, we suspect that it can be used as a realistic molecular model to address fundamentally important goals, in which a wide variety of experimental situations need to be analyzed. Because highly charged polyoxoanions have a high affinity to H+ in protic solvents, we also investigated the propensity of the “fully reduced” V18(18:0) species to be protonated in neutral aqueous solution by means of CPMD simulations.

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COMPUTATIONAL DETAILS

DFT. DFT31 calculations, as implemented in the ADF 2016.0132 package, were carried out for geometry optimizations of all compounds by means of the BP8633 functional. The total energies and electron densities and the properties derived from them were obtained from single-point calculations with the B3LYP34 hybrid functional. Grimme’s dispersion corrections35 and the relativistic effects via the zeroth-order regular approximation (ZORA)36 were included. We used all-electron triple-ζ-quality basis sets with double polarization functions (TZ2P) for all atoms. The solvent effects were treated by the conductor-like screening model (COSMO)37 with ε = 78.4 for water and ε = 37.5 for MeCN. Point-group symmetries were applied as discussed in the text. Open-shell electronic configurations were treated with the spin-unrestricted formalism. Atomic spin densities (ASDs) were obtained from Bader’s electron density partition.38 A data set collection of computational results is available in the ioChem-BD repository39 and can be accessed via https://doi. org/10.19061/iochem-bd-2-32. In order to determine protonation energies, we used the experimental value of 1139 kJ mol−1 (272.2 kcal mol−1) for the standard free energy of a proton in aqueous solution,40 as has been successfully employed by some of us to explain experimental evidence related to protonation phenomena on a POM framework.41 Also, we used the value of −437 kJ mol−1 (−104.5 kcal mol−1) reported by Dixon and co-workers for the hydration free energy of a hydroxide ion.42 CPMD. Simulations were performed with the CPMD program package43 at the DFT level, adopting the generalized-gradientcorrected BLYP exchange-correlation functional.34a,c The electronic structure was described by expansion of the valence electronic wave functions into a plane-wave basis set, which is limited by an energy cutoff of 80 Ry. The interaction between the valence electrons and the ionic cores was treated using the pseudopotential (PP) approximation. Norm-conserving Troullier−Martins PPs were used for the O and K centers44 whereas for V a semicore PP was used. A Bachelet−Hamann−Schlüter (BHS) PP used for I45 and H atoms was described by a norm-conserving Goedecker-type PP.46 During the MD simulations, the wave functions are propagated in the Car− Parrinello scheme, by integrating the equations of motion derived from the Car−Parrinello Lagrangian.47 We used a time step of 0.144 fs. A fictitious electronic mass of 900 au was employed, and H atoms were substituted by D atoms. The Nosé−Hoover thermostat48 for the nuclear degrees of freedom was used to maintain the temperature at 300 K. The simulated system contains a (18:0) anion in a cubic supercell of 203 Å3 filled with 200 H2O molecules and 13 K+ cations to neutralize the charge. This box size ensures a minimal distance between the POV’s centers of mass of 10 Å, avoiding possible interactions between anions. The initial geometry was obtained from a short classical MD simulation performed to start from a realistic distribution of species around the POV. Previous to the production run of 5 ps, 1 ps of equilibration was performed. B


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Inorganic Chemistry MD. Atomistic MD simulations were performed using the GROMACS 4.5.4 code49 and AMBER99 force field.50 This methodology already allowed the study of other aggregative and ion-pairing processes involving polynuclear metal−oxo clusters.2,5−10,14f,26,29 The potential energy of the system is empirically described by the sum of the bonding terms that include the bond, angle, and dihedral deformation energies and the nonbonding terms, which consist of pairwise additive 1−6−12 electrostatic and van der Waals potentials. The latter serve to describe interactions between atoms in different molecules or which are separated by more than three bonds within a molecule. Specific force-field parameters for nonstandard fragments such as POV anions were obtained following the procedure developed by Bonet-Á valos et al.19,20 Initially, the structures of all of the polyoxoanions were fully optimized without symmetry constraints using the Gaussian09 package51 at the DFT level using the BP86 functional33 and a LANL2DZ atomic basis set52 for all of the atoms. In all cases, POV anions were computed as high-spin complexes. The solvent effects were taken into account in the geometry optimizations by means of the dielectric IEF-PCM model,53 as implemented in Gaussian09.51 Then, CHELPG atomic charges were obtained from single-point calculations in vacuo of the optimized structures. The convergence of the wave function with an energy criterion of 10−7 atomic units (au) was not achieved for anions bearing charges of 11− and 13−. However, because the atomic charges computed in the condensed phase can be fitted to a linear regression model, we assumed that those in the gas phase can be fitted as well. Thus, we obtained the charges for (16:2) and (18:0) in the gas phase by extrapolation from those computed for less-charged anions. The set of Lennard-Jones parameters for POVs and atomic charges for Et4N+ (namely, TEA) cations were taken from ref 2. These parameters were able to reproduce classically the POM···solvent and POM···cation radial distribution functions (RDFs) computed by means of CPMD simulations (see Figure S1). MD simulations of the POV salts were performed in two different solvents, water and MeCN. The TIP3P water model54 was used to represent solvent H2O molecules, and MeCN was described by the full-atom model provided by van der Spoel et al.55 In all cases, 15 POV anions and the number of cations required to neutralize the system were embedded in cubic solvent boxes of 69.33 Å3 in order to achieve an overall POV concentration of ca. 75 mM. The simulations were performed with 3D periodic boundary conditions using an atom cutoff of 14 Å for van der Waals interactions and of 10 Å for Coulombic interactions, correcting the long-range electrostatics with the particle−particle mesh Ewald (PME) summation method.56 Newton equations of motion were integrated using the leapfrog algorithm57 with a time step of 1 fs. Bonds involving H atoms were restrained by the LINCS algorithm.58 All of the simulations were performed at 300 K, starting with random velocities. Thus, the temperature was controlled by coupling the system to a thermal bath using the Berendsen algorithm59 with a relaxation time of 0.1 ps. In simulations within an isothermal−isobaric (NPT) ensemble, the system was also coupled to a Berendsen barostat59 with a relaxation time of 5 ps to keep the pressure at 1 bar. Before the production runs, all systems were equilibrated by 1000 steps of energy minimization, followed by an initial 250 ps run (500 ps in the case of wet MeCN solutions) at constant volume and temperature (NVT) fixing the solute. Then, a 250 ps NVT run with the solute relaxed and a 500 ps simulation at constant pressure (NPT) to readjust the box size (and thus the density), followed by a final 250 ps run at constant volume (NVT). Finally, all systems were simulated for 40 ns within a canonical (NVT) ensemble, collecting data from the trajectories every 2 ps.

a V5 hemisphere about an axis coincident with a S4 element of the Td form, as demonstrated by Müller et al.30,60 The 18 V atoms follow the arrangement V1:V4:V8:V4:V1, with three structural types of V, that is apical (2 × V1), outer ring (2 × V4), and inner ring (1 × V8), as illustrated in Figure 1 for the D4d arrangement. Calculations show that the fully oxidized empty cage [V18O42]6+ with the D4d arrangement is 1.27 eV (121 kJ mol−1) more stable that the Td one, a value that diminishes to 0.95 eV (92 kJ mol−1) for the 10-electronreduced mixed-valence form. With iodine encapsulated in the (10:8) form, [I@V18O42]5−, the D4d structure is the most stable by 1.03 eV (100 kJ mol−1) and the Td isomer does not compete with the other isomer at room temperature unless the encapsulated guest is a more voluminous species such as VO43− (see structure 8 in ref 30). In the following sections, we therefore limit our discussion to the D4d arrangement. As was recently shown for the mixed-valence {V22O54} assemblies,61 large imaginary vibrational frequencies were encountered in the symmetric structural minimum. In the present case, the real minimum of C1 symmetry was found to be 21 kJ mol−1 more stable than the D4d one, as a consequence of the pseudo Jahn−Teller effect.61,62 The distorted minimum has averaged geometrical parameters for V−Oterminal, I−V, and V−V distances of 1.622, 3.785, and 3.149 Å, respectively, rather close to those determined by X-ray diffraction (1.605, 3.788, and 2.910 Å). Herein the main focus is placed on the distorted form of the most stable arrangement. In the (10:8) distorted system, the HOMO−LUMO63 gap is rather large, 1.12 eV, and the singly occupied orbitals containing the 10 metallic electrons are found in the range −6.44 to −5.77 eV. Bader ASDs derived from our optimized C1 structure indicate that these 10 electrons are largely spread over 16 V atoms [see values for (10:8) in Table S1], suggesting minor differences between the V centers. For comparison, we recomputed the electronic structure, with 10 unpaired electrons taking the crude, disordered X-ray atom positions for the same compound. For this structure, contrarily, notable differences are observed in the ASDs because one of the apical V atoms carries 1 electron, whereas the remaining 9 electrons are mainly shared by 9 other metal atoms. The different electron distribution is attributed to the packing effects existing in the crystalline phase, which force formally identical positions to behave differently. This implies that in the crystallographic structure electrons tend to be more localized than in the more regular optimized geometries, which are representative of the liquid phase or vacuum. Importantly, these results may also indicate that a small structural reorganization might induce notable changes in the V valence d-electron density. We also deduce that if the polyoxoanion is found in an isotropic environment, the average electron distribution may resemble that of the highest-symmetry structure. Regarding the low-symmetry structure, the electrons are highly dispersed over the whole system, as observed in other POVs.61 Changing the number of electrons in (10:8) by oxidation or reduction entails an associated molecular negative charge variation, producing a shift of all of the orbital energies. For the 2-electron-oxidized species (8:10), the LUMO is placed at −5.01 eV with a HOMO−LUMO gap of 1.14 eV in a water solvent. Adding 2 electrons to (10:8), we obtain (12:6) with a LUMO energy of −3.95 eV and a HOMO−LUMO gap of 1.36 eV. In MeCN, the frontier orbitals of (10:8) are 0.13 eV higher than those in water but only 0.03 eV in (8:10), whereas for

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RESULTS AND DISCUSSION DFT Calculations. Structural and Electronic Characteristics of V18. The crystallographically determined [I@ V18O42]5− structureas the typical representative of the V18 nuclearity seriespresents two isomers with ideal Td and D4d point group symmetries, which are related by a 45° rotation of C


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Inorganic Chemistry Scheme 1. Frontier Molecular Orbital Energies (in eV) and Occupations for V18

Figure 2. (A) MEP map projected on a density isosurface (ρ = 0.003) for each (VIV:VV) state of V18. The scale (shown on the right) is chosen to include the whole range of potentials. (B) MEP for the (18:0) species, for which the electrostatic potential scale has been adjusted to a narrower range to discern regions of different nucleophilicities. (C) 2D cut of the electrostatic potential plotted inside the cavity of V18(10:8), on a plane containing the geometrical center. The potential gets more negative from blue to red. Note the large spacing between the isopotential lines in the central region.

each of the 8 V centers of the outer ring, 0.56 for each of the 8 V in the inner ring, and a residual amount (around 0.12) in the apical sites. The 2 additional electrons are thereby distributed over the outer ring V atoms, whereas the apical positions remain formally VV. Further reductions to generate (16:2) and (18:0) are thermodynamically less favored. For (16:2), the outer and inner rings have 8 electrons (saturation), whereas the apical V atoms have no valence electrons. Finally, (18:0) has all VIV and full electron localization in this redox variant is expected. Table S1 provides an overview of the ASDs computed for the discussed forms. Concerning oxidation, (10:8) → (8:10) + 2e costs 11.03 eV due to the deep-lying orbitals from which the electrons are removed. Depopulation takes place in the outer rings, which retain 2 electrons each.

(12:6), the upshift amounts to 0.24 eV. The orbital upshift when water is replaced by MeCN is thus more pronounced for the more anionic POV species (see Scheme 1 for the species discussed). As expected, the electron distribution among the V centers and the HOMO−LUMO gaps remains unaltered. The energy associated with the addition/removal of electrons in V18(10:8) is crucial for further applications. In water, the (10:8) + 2e → (12:6) process gives ΔE = −10.41 eV, a highly exothermic one related to the very negative LUMO of (10:8). In conditions of no protonation, the increase of charge upshifts all of the orbitals, the highest doubly occupied one going from −7.43 to −6.88 eV upon 2electron reduction. In the (12:6) state, the highest electron lies at −5.31 eV and E(LUMO) = −3.95 eV with ASDs = 0.95 for D


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Inorganic Chemistry These DFT results indicate that (10:8) is naturally the central electronic state for the [I@V18O42] systems with partially oxidized apical V atoms and partially reduced outer and inner rings, with 6 and 4 electrons each. The relationship between the electron population, (de)localization, and conductivity is not straightforward. From the present DFT calculations, we obtained the number of electrons present in each region of the V18 shell at each stage of reduction−oxidation and how many on average can be attributed to each V center. If MeCN is the solvent, the (10:8) + 2e → (12:6) reduction gives ΔE = −10.00 eV (0.4 eV less favored than in water). Oxidation (10:8) → (8:10) + 2e gives ΔE = 10.24 eV, which is 0.24 eV less positive than in water. Such a lower endothermicity responds to the comparatively more stable species carrying lower charge in MeCN than in water. Both aqueous and MeCN solutions of (10:8) make it the most stable form. Changes in this state of reduction are differently accessible in both solvents: reduction is easier in water (a more exothermic process), whereas oxidation is easier in MeCN (a less endothermic process). This trend is explained by the anionic nature of the V18 assembly. The electron density function, ρ(r), allows one to compute and represent the electrostatic potential at any point of space. By mapping the molecular electrostatic potential64 (MEP) for each (VIV:VV) state, we can see changes in the surface properties of V18 (see Figure 2A). The MEPs evolve from blue to red, indicating more negative electrostatic potentials (more cation-attractive and dipole-orienting) at the surface as the molecular charge gets more negative. Approximately, the average surface potential for (8:10), (10:8), (12:6), (16:2), and (18:0) is −0.21, −0.38, −0.56, −0.93 and −1.10 au, respectively. This increment of the surface nucleophilicity parallels the gradual energy increase of all of the occupied molecular orbitals shown in Scheme 1. Taking (18:0), for instance, and adapting the electrostatic potential range represented, detailed information about the electro- and nucleophilicity of different regions of its surface can be traced (Figure 2B). Common to most classical POMs, bridging O atoms are more nucleophilic than terminal ones. We expect that, for the most negatively charged POVs, the red regions are strongly solvated in the presence of water. On the other hand, anions with low q− might be poorly solvated. The consequences that these facts have in the solution behavior of V18 will be discussed in the MD section. Figure 2C shows the electrostatic potential in an arbitrary plane of the interior of the V18(10:8) cage. In the center of the cavity, the potential is fairly constant and takes intermediate values (green lines), and then it goes to less negative potentials as one approaches the inner side of the cage (blue lines). Effects of the Encapsulated Guest Species. To explore the preference of the {V18O42} cage for particular guest species, the energy of the hypothetical encapsulation process in solution was estimated:

conditions inside the {V18O42} cage are such that the encapsulation of anionic guests is largely favorable. From our analysis, we conclude that solvation plays a key role in this process; that is, the interaction between K+ and its solvation shell is strong enough to prevent the cation to be encapsulated. In contrast, the weaker solvation of I− or H2O species pushes the balance in favor of encapsulation. Interestingly, the POV host cage in its resting electron population state (10:8) features a negative electrostatic potential in the internal region, close to −0.23 au, whereas the most negative region is found in the external surface, close to bridging O atoms, with a value of −0.35 au. Additionally, the local dipole moments (μ) located at each square-pyramidal VO5 building unit point to the interior of the cage, a fact that might explain the larger stabilization of encapsulated anionic moieties. The values of μ were estimated computationally from a smaller model to be μ = 82 D for VVO5 and 100 D for VIVO5 building units. CPMD. In general, POMs possessing a high charge density are susceptible to protonation at the typically most basic bridging O atoms.65 This may lead one to think that the “fully reduced” V18(18:0) with an overall charge of 13− should be protonated in the presence of protic solvents. This situation was simulated with a box containing one V18(18:0), 200 H2O molecules, and 13 K+ to ensure electroneutrality. Because of computational limitations, the POV was modeled in a closedshell electronic configuration because the basicity of the metal−oxo cluster might depend more on its electrostatic potential and not much on its electronic structure. However, we have benchmarked our simulation with a shorter run with spin polarization (vide infra). The OPOV···H RDF averaged over the whole simulation time (Figure 3) shows no peaks at

Figure 3. (Left) RDF between the POV O atoms and all of the H atoms, gOPOM···H(r), obtained from a CPMD simulation. (Right) Representative snapshot of the maximum of the peak.

OPOM−H bonding distance, with the first observable peak appearing at 1.85 Å, within the range of a moderate hydrogen bond.66 Thus, during 5 ps of simulation, the bare V18(18:0) cluster is stable and not prone to tearing a proton from a H2O molecule. In agreement, static DFT calculations predicted that the reaction POVq− + H2O → HPOV(q−1)− + OH− is endothermic by 54 kJ mol−1 (12.8 kcal mol−1). Nevertheless, we cannot discard that protonation can occur in a slightly acidic medium because the process POVq− + H+(aq) → HPOV(q−1)− was computed to be exothermic by 123 kJ mol−1 (29.4 kcal mol−1). The CPMD simulation also served to analyze different structural parameters of this polyoxanion surrounded by explicit H2O molecules and countercations, and how they vary along the time. Figure 4A shows a histogram compiling all of the V−O distances within the POV framework extracted from the trajectory. The bell-shaped distribution centered at 1.65 Å corresponds to the terminal VO distances, a parameter that oscillates within a range of ca. 0.2 Å. The

guest + V18O42 → guest@V18O42

where guest = I−, K+, and H2O. The DFT results show that the encapsulation of I is highly favorable, with ΔE = −186 kJ mol−1 (−44.5 kcal mol−1) at the present level of calculation. If a H2O molecule is taken as the guest, ΔE is remarkably decreased to −51 kJ mol−1 (−12.1 kcal mol−1). The process becomes energetically unfavored if a K+ ion is incorporated into the central polyoxoanion cavity with ΔE = 11 kJ mol−1 (2.6 kcal mol−1). The electrostatic E


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Inorganic Chemistry

excellent agreement compared with simulations performed with open- and closed-shell formalisms. MD Simulations. Simulations of TEA-Counterbalanced POVs in Water. We first simulated the series of POVs chargebalanced by TEA ions, hydrophobic in nature, systematically exploring the molecular charge characteristics. Visual analysis of the computed trajectories revealed that the V18···V18 agglomeration phenomenon is lessened as the charge of the polyoxoanion increases. While the highest-charged V18(12:6), (16:2), and (18:0) mostly remain as monomers during the whole simulation, V18(10:8) and (8:10) are rather prone to agglomerate. The least-charged anion, (8:10) with q = 3−, is the one that tends to agglomerate the most, forming a hydrophobic microphase with the organic TEA countercations. In fact, all of the (8:10) anions are merged into one single supramolecular domain after ca. 10 ns of the production run (see Figure 5, left). Chaumont and Wipff also reported the

Figure 4. Dynamic behavior of the {I@V18O42} structure in aqueous solution. (A) Histogram of the V−O distances sampled from the 5 ps CPMD simulation (blue bars) and from the static DFT-optimized structure (red bars). All distances are rounded to the nearest 0.01 Å. (B) Evolution of the distance between the I atom and COM along the simulation (left). The COM was recalculated at every step. The superposition of all of the I positions spanned inside the cage during the simulation (right). Data from the CPMD simulation was sampled every 7.2 fs. Figure 5. Snapshot of the periodic simulation box at the end of the run (t = 40 ns) for TEA salts of V18(8:10) (q = 3−) and V18(18:0) (q = 13−) in water.

distribution centered at 1.94 Å includes a wider array of distances (from 1.80 to 2.20 Å) and is assigned to the weaker V−O bonds involving bridging O atoms. Note that the distances obtained from static DFT with an implicit solvation model are in excellent agreement with the CPMD ones, with the most likely CPMD distances being only ∼0.02 Å shorter than those in the DFT structure. Furthermore, we evaluated the mobility of the I atom inside the “fully reduced” {V18O42} cage. The motion of the encapsulated I atom is quite limited, typically 0.1−0.2 Å (up to 0.3 Å) away from the center of mass (COM) of the metal−oxo cluster, which is consistent with the fact that the host is anionic as well (see Figure 4B). Static DFT calculations agree with these results, predicting that putting the I atom at 0.2 Å from its equilibrium position entails an energy penalty of ca. 17 kJ mol−1. In a 5 ps simulation at 25 °C, it is only expected to see processes with an energetic cost of ca. 8 kJ mol−1. Although if one also accounts for distortion of the POV cage to adapt to the I displacement, it can easily reach positions more than 0.2 Å away from the COM of the V18 host (Figure 4B). Last, we carried out a shorter simulation (2 ps) with spin polarization and observed that the mobility of the I is enhanced (reaching dI···COM > 0.4 Å) in the equatorial plane. This might be caused by the fact that, in the open-shell situation, the electrons are better distributed all over the structure and less localized in the inner-ring region. A reduction of the repulsion between the POV cage and the I guest ion allows the latter to move more freely. Apart from that, variations in the V−O distances relative to the closedshell system are smaller than 0.01 Å and, as expected, protonation was not observed in this case either. Figure S2 shows that both the structural parameters of the V18 moiety and the distribution of other species surrounding it are in

formation of a hydrophobic microphase when studying the TBA3[PW12O40] compound in water both at 60 and 150 mM.5 The former lies in a concentration regime that makes them comparable with the results of the herein-described simulations performed with a TEA−POV concentration of 75 mM. Hence, although the phosphotungstate ion simulated by Chaumont et al. is not as hydrophobic as V18(8:10) (with solvation energy67 Esolv = −823 kJ mol−1 for PW12 and Esolv = −677 kJ mol−1 for V18), their solution behaviors are similar, likely because of the higher hydrophobic character of the TBA countercations compared to the TEA ones that we use in our simulations. Therefore, it can be safely assumed that these results are consistent and support each other. The V18(10:8) anion (q = 5−), which is more hydrophilic (Esolv = −1313 kJ mol−1) than [PW12O40]3− and certainly more than its less-charged congener V18(8:10), also forms hydrophobic agglomerates with TEA cations. However, in this case, the largest agglomerate does not gather all of the polyoxoanions into a hydrophobic microphase but up to 8 of them (out of 15) in the last simulation steps. The other anions were found to follow a Brownian motion in water as dimers or trimers in which the TEA countercations mediate the contact between the V18 units. In line with these observations, Figure 6A represents the I−I RDFs calculated for the differently charged POVs, indicating the existence of sharp peaks for V18(8:10) and V18(10:8), which are indicative of the preferred distances between close anions. The I···I RDF for the V18(8:10) form, which evolves toward a hydrophobic microphase during the simulation, shows two peaks centered at 13.9 and 24.1 Å. The first peak at F


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Inorganic Chemistry

Figure 6. (A) RDFs between I centers of POV anions, gI···I(r), for simulations of V18(8:10), (10:8), and (12:6) in water with TEA countercations. Those corresponding to V18(16:2) and (18:0) are omitted because they do not provide valuable information but can be found in Figure S4. Red curves represent the coordination number, N(r), obtained from integrating g(r), which gives information about the number of species on average at a given distance from the reference. Curves are averaged over the last 10 ns of the simulation, and data are sampled every 2 ps. (B) Representative snapshot of a direct contact between V18(8:10) anions, in which several H2O molecules act as linkers, forming hydrogen bonds between POVs. Only H2O molecules participating directly in the interaction are displayed. (C) Representative snapshot of a cation-mediated contact between V18(10:8) anions. Distances are in angstroms.

13.9 Å integrates to N = 6.7, almost 7, anions per V18 unit within the structure of the microphase. Remarkably, this peak covers a wide range of distances from 10.9 to 20.5 Å and, therefore, an array of configurations that include direct V18··· V18 contacts and cation-mediated ones (V18···TEA···V18). These types of contacts between POVs are illustrated in parts B and C of Figure 6, respectively. The peak at 24.1 Å corresponds to the distance between a reference V18 and the next neighbor of the anion directly attached to it (see Figure S3 for a schematic representation of these contacts). Although cation-mediated contacts between POMs are widely accepted and supposedly participate in self-assembly processes, giving nanometric structures,3,4 direct contacts between them are less intuitive considering the fact that polyanions may repel each other in solution. However, direct POM···POM contacts were already probed by means of SAXS measurements and MD simulations.5−10 Furthermore, Antonio and co-workers showed that the weak short-range attraction between monomers, which is the main cause for the stabilization of “randomly percolated associated monomers” of PW12O403− (that is, chainlike agglomerates), arises from hydrogen bonding with H2O molecules and/or hydronium cations. 8 The I···I RDF associated with V18(10:8) reveals a single sharp peak centered at 15.1 Å that corresponds to a TEA-mediated contact between anions (Figure 6C). In agreement with the observed lower agglomeration compared to the least-charged polyoxoanion, this peak integrates to N = 3.1, revealing that fewer POV units surround each V18(10:8). In our previous study concerning the collective behavior of V18(10:8),2 we found that this species mainly interacts through direct contacts in dilute solutions (10 mM). However, at a 7 times greater concentration of salt, the POV···TEA ion pairing is enhanced and strongly affects the agglomeration. The average number of TEA cations in close contact with each V18 unit at 75 mM was determined to be 6.7 (see Figure S5 and Table S2) compared to 0.3 at a concentration of 10 mM.2 Thus, in a situation in which ion

pairing is significant, cation-mediated contacts predominate over the direct ones because TEA cations can shield the POV···POV repulsion better than water. In turn, the overall number of neighbors per POV unit was found to be much higher when increasing the concentration and moving from water- to cation-mediated contacts (from 0.9 anions at 10 mM to 3.1 anions at 75 mM, as stated above). Upon going from V18(10:8) with charge 5− to V18(12:6) with charge 7−, gI···I(r) does not show any peak (Figures 6A and S2), indicating that agglomeration of the POVs is not favored under these conditions. As anticipated, the ion pairing plays a key role in the agglomeration phenomenon, mediating contacts between V18 units. Figure 7A compares the ion pairing along the series by plotting the V18···TEA RDF for the different anions, revealing that the higher the negative charge of the polyoxoanion, the lower the ion pairing. In principle, a higher negative charge of the anion should provide stronger V18···TEA interactions because of the stronger electrostatic potentials between charged species. However, increasing the negative charge of the anion also increases its hydrophilicity, a fact evidenced in Figure 7B. The V18···H2O interactions become stronger as the POV increases its charge, and, concomitantly, the V18···TEA ion pairing decreases. It can also be seen that a new peak at shorter I···Owater distances appears for species (16:2) and (18:0). In Figure S5B, this fact is illustrated with the dependence of the individual V18···water interaction energy as a function of the POV charge, which resembles our previous results relating the charge density and hydrophilicity.14 To gain a better understanding of the results obtained, we compared the evolution of V18···water and V 18···TEA interactions along the series. In order to allow a comparison, we used, instead of the total V18···water interaction energy represented in Figure S5B, that associated with the part of the V18 surface involved in a V18···TEA contact. This procedure allowed one to unveil whether a V18 anion is more prone to G


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Inorganic Chemistry

low POV charges. Interestingly, the two functions representing the direct POV···TEA and POV···water interactions cross at a charge of −6.5 (Figure 8), which corresponds to the agglomeration limit observed in the simulations. This indicates that the difference between the direct interactions is the main factor, among others, governing the agglomeration phenomena. Furthermore, in contrast with what we observed for V18(10:8) at low concentration,2 water-mediated contacts were not observed for (12:6), (16:2), or (18:0), despite the fact that they present low ion pairing with TEA. In this case, the lack of direct contacts was ascribed to a too strong anion··· anion repulsion, which is analogous to what Antonio and coworkers observed for Keggin-like polyoxoanions bearing high charge densities.8 Simulations of TEA-Counterbalanced POVs in Pure MeCN. To gain insight into how the nature of the solvent influences POV agglomeration, we replaced water by MeCN to analyze the same series of compounds. Unlike what was observed in a water solvent, the MD results show no clear dependence of agglomeration on the POV charge. Intermediate-charge anions (10:8) and (12:6) with q = 5− and 7−, respectively, tend to agglomerate the most with clearly preferred intermolecular distances, having on average N = 3.5 and 3.2 neighbors at a distance of 15.1 and 16.3 Å from each V18 unit, respectively (Table 1, entries 2 and 3). The shorter distance in the q = 5− case can be attributed to the weaker V18···V18 repulsion. These intermolecular distances correspond to a TEA-mediated interacting mode and are, indeed, close to those computed for TEA-mediated contacts in water. On the other hand, the least-charged (8:10) system (q = 3−) does not show well-defined V18···V18 RDF peaks (see Figure 9A for the V18···V18 RDFs of the series). Table 1 compiles the main features of the computed RDFs for these simulations, and Figures S8 and S9 display them separately. When moving to higher-charged anions, one may observe that the peaks are broadened and shifted to longer intermolecular distances. This results in a distribution of distances ranging from 17.5 to 23.5 Å, which represent V18··· V18 contacts mediated by two TEA cations. Also, the number of neighboring POV units decreases significantly with the POV charge until V18(18:0) for which N = 2 anions closer than 23.5 Å, with this being the smallest in the series. This V18···TEA··· TEA···V18 interaction has, to the best of our knowledge, not been reported before and might arise from the strong ion pairing that this salt presents in a less polar solvent such as MeCN, with 12.9 TEA cations surrounding each V18(18:0) anion on average. However, according to the potential of mean force (PMF) computed from the RDF,68 which is represented in Figure 9B, the stabilizing character of these interactions is much smaller (ca. kBT ≈ 2.5 kJ mol−1) than the value computed for the single-cation-mediated V18(10:8)···TEA··· V18(10:8) agglomerate (ca. 3kBT; Figure S10). In the absence of hydrophobic forces that make POVs and TEA merge together, POV···TEA ion pairing in MeCN mainly depends on their electrostatic charges. Thus, considering that interactions between charged species are less shielded in MeCN because of its lower dielectric constant, one can assume that ion pairing will be stronger for more-charged POVs. Indeed, the V18···TEA RDFs all show a sharp peak that roughly integrates to the number of cations needed to neutralize the anion charge (see Table 1 and Figure S9). Exceptionally, those anions forming cation-mediated agglomerates display a higher coordination number of cations because they share them

Figure 7. Superposition of (A) V18···TEA and (B) V18···Owater RDFs for the five different anions, from (8:10) to the “fully reduced” (18:0), taking as a reference the I centers of POV and N centers of the TEA cations, gI···N(r), or the water O atom, gI···Owater(r), respectively. The red arrow in part B indicates the new peak at shorter I···Owater distances appearing for (16:2) and (18:0) species. For further details, see Figure S5 and Table S2. In part A, curves are averaged over the last 10 ns of the simulation, and in part B, they are averaged over 250 ps of the simulation in which the POVs are frozen and fully surrounded by water. Data sampled every 2 ps.

letting a region of its surface interact with TEA or keeping it solvated. Figure S6 shows a conceptual scheme that helps interpret the nonbonding potentials compared. Figure 8

Figure 8. Evolution of V18···TEA (green) and 2.6% of V18···water (red) nonbonding interaction energy per POV and TEA unit as a function of the charge of the anion. Because of the scarcity of POV··· TEA contacts involving the two anions with the highest charge, their POV···TEA interaction energies were obtained from extrapolation on the linear regression obtained from the remaining three (see Figure S7 for details). Values were averaged over the last 10 ns of the simulations, sampling data every 2 ps.

compares the V18···TEA interaction energies with 2.6% of the POV···water one, with 2.6% being the percent of POV surface that is estimated to become fully desolvated, thus allowing a direct POV···TEA contact. The interaction of POV with TEA grows with the charge at much a smaller rate than that with water (see Figure 8). Thus, at high POV charges, the interaction with water dominates and the POV···TEA ionpairing contacts are not observed, whereas they are present at H


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Inorganic Chemistry

Table 1. Main Characteristics of the V18···V18 and V18···TEA RDFs Computed for Different TEA Salts in MeCN: Position of the First Peak (in Å), Coordination Number (N) Obtained by Integration of the First Peak, and Integration Distance (in Å)a V18···V18 (VIV:VV)

charge

(8:10) (10:8) (12:6) (16:2)

3− 5− 7− 11−

(18:0)

13−

first peak position 15.05 16.25 19.25 21.65 21.65

N

V18···TEA integration distance

no peak 3.49 3.22 1.03 2.07 2.04

18.05 18.65 20.45 23.45 23.45

first peak position

N

integration distance

9.05 8.45 8.45 7.85

2.48 8.10 9.90 11.12

12.05 10.85 10.85 10.25

8.45

12.88

9.65

a

Values were averaged over the last 10 ns of the simulation.

purity, it is not always possible to ensure a 100% water-free solvent because it could take moisture from the air in the course of the analyses. We therefore evaluated the impact of including different amounts of explicit H2O molecules (1, 5, and 15% volume of water in the MeCN solution) on the collective behavior of POVs. These simulations were carried out on V18(10:8) and V18(12:6), which are those presenting agglomerative behavior in pure MeCN. Figure S11 shows the V18···V18 RDFs obtained for both anions. The peaks between ca. 15 and 16 Å, associated with cation-mediated V18···V18 contacts, become broader and less defined upon an increase in the percent of water contained in the solution. The area under these peaks also decreases with the addition of water, as reflected in Table 2, which compares the V18···V18 coordination number (NPOV) for the TEA salts in pure and wet MeCN. As seen in Figure 8, H2O molecules (if present in the medium) compete with TEA cations to interact with the surface of V18. Therefore, it is reasonable that POV···TEA ion pairing decreases for higher percent of water (Table 2, column 4), diminishing agglomeration. It is also noteworthy that NPOV = 3.11 computed for V18(10:8) in pure water is comparable with that in pure MeCN (3.49), indicating that the agglomeration of this anion is maximal at both extreme situations (0 or 100% of water). This suggests that there is a critical percentage of water that minimizes agglomeration. This percentage would be enough to decrease POV···TEA ion pairing by solvating the polyoxoanions but not enough to activate the agglomeration induced by hydrophobic forces, as discussed previously for the POVs solvated in water. However, in the particular case of the TEA salt of V18(12:6) (and, consequently, for higher-charged V18 anions), the lack of agglomeration in pure water suggests that, when the water amount is increased, NPOV steadily decreases because of the higher hydrophilic character of the POV, in agreement with the more negative EPOV···total values in (12:6) (Table 2). This finding fully agrees with recently published SAXS experiments on the (10:8) species.2 In a concentration regime of 5 mM, with the main species in solution being the nonassociated monomer, the addition of small amounts of water led to a slight increase of the average particle size. According to the MD results, this enlargement might be caused by the fact that H2O molecules in wet MeCN tend to concentrate at the surface of the anions, increasing their effective hydrodynamic radii. In fact, although the addition of water in our simulations goes along with a moderate decrease in agglomeration, it is also accompanied by a decrease in the mobility of the anions. The average diffusion coefficient (D) measured for POV anions in pure MeCN is higher than that computed in a solution of wet MeCN with 15% water, namely, (2.86 ± 0.41) × 10−6 versus (1.33 ± 0.65) × 10−6 cm2 s−1, in

Figure 9. (A) Superposition of V18···V18 RDFs, gI···I(r), for the series of polyoxoanions. Curves averaged over the last 10 ns of the simulation. Details on these RDFs can be found in Table 1. (B) Pairwise PMF between “fully reduced” V18(18:0) anions in MeCN. The PMF was calculated from RDF as W(r)/kBT = −ln[g(r)]. (C) Representative snapshot of a V18(18:0)···V18(18:0) contact mediated by two TEA units. The POV···POV intermolecular distance (in Å) corresponds to the minimum in the free-energy curve shown in part B.

(Table 1, entries 2 and 3). Overall, there is an optimal POV surface coverage that favors the agglomeration through TEAmediated contacts. Polyoxoanions bearing very low negative charges are not capable of removing the solvation shell from TEA cations, which are obviously better solvated in MeCN than in water, thus leading to no agglomeration. For a different reason, but with a similar result, too highly charged anions are not prone to agglomeration either; in this case, the TEAmediated POV···POV contacts are hampered because of the generation of a dense layer of positive charges around each POV. For this reason, moderately charged polyoxoanions present the most pronounced agglomeration, as summarized in Figure 10. Simulations of TEA-Counterbalanced POVs in Wet MeCN. Although the MeCN used in the experiments is usually of high I


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Inorganic Chemistry

Figure 10. Relationship between increasing the molecular charge of the POV, from top to bottom, and ion pairing with TEA (green positive spheres) and agglomeration in pure MeCN.

are and, consequently, the more stable the species in solution is. Simulations of Alkali-Metal-Counterbalanced POVs in Water. To complete this study, we assessed the behavior of the POV series in water with alkali countercations (Li+, K+, and Cs+). For simulations with K+, the main characteristics are compiled in Figures S13−S17 and Table S3. K+ is a hydrophilic cation, and no significant agglomeration is observed within the POV series (Figure S13), revealing the important role of hydrophobic cations (as TEA) to induce this collective phenomenon. Only an incipient cation-mediated agglomeration is observed (last RDF of Figure S13) for the most-charged POV in the series, (18:0). A snapshot of this simulation is shown in Figure 11A. Further details on the study with K+ can be found in the section MD simulations with K+ countercations in the Supporting Information. Prompted by the results obtained for the V18(18:0) case, we investigated in more detail whether POV agglomeration is induced by the presence of Li+ or Cs+ cations. The sizes of the different cations (e.g., the charge density) strongly affect the interactions with their environment. The computed nonbonding interaction energy between a single cation and its aqueous solvation shell decreases with the cation size as −77, −124, and −282 for Cs+, K+, and Li+, respectively. As well as having a compact solvation shell, we expect that the most hydrophilic cations can also feature longer-lived interactions with highly anionic POVs. The increase of the gI···I(r) peaks from Cs+ to Li+ in the RDFs of Figure 11B reveals a stronger cation-mediated V18(18:0) agglomeration in the presence of a smaller cation. As a consequence of the strong agglomeration, the V18···Li+ RDF does not converge to 1, as would be expected for a homogeneous distribution of the species at long distances. This effect was also observed in other simulations (see Figure S18). V18(18:0) is sufficiently charged to remove the solvation shell from all of the tested cations, even from the highly hydrophilic Li+, opening the possibility for the bare cation to facilitate agglomeration of the POV, as indeed occurs for Li+ (Figure S19). It is worth noting that the volumetric densities of the V18···Cs+ and V18···K+ contacts are

Table 2. Comparison between Simulations of TEA Salts of V18(10:8) and V18(12:6) in Pure MeCN, Pure Water, and Solutions of Wet MeCN with Different Percentages of Watera (VIV:VV)

% water volume

NPOV

NTEA

EPOV···totalc

(10:8), q = 5−

0 1 5 15 100b 0 1 5 15 100b

3.49 2.31 2.30 1.34 3.11 3.22 2.59 2.88 2.37

8.10 6.84 6.51 5.06 6.70 9.90 9.58 8.55 7.08 3.80

−760 ± 9 −800 ± 81 −900 ± 48 −1000 ± 52 −1100 ± 39 −1100 ± 15 −1200 ± 26 −1700 ± 55 −1900 ± 69 −2100 ± 52

(12:6), q = 7−

a

Values averaged over the last 10 ns of simulation. Data sampled every 2 ps. The V18···V18 and V18···TEA coordination numbers, N(r), were measured in all cases at the distance corresponding to the first minimum of the respective RDF (see Table S2 and Figure S9). Energies are given in kJ mol−1 and per molecule. bValues taken from Table S2. cEPOV‑total represents the overall nonbonding interaction energies between a POV and the rest of the system and has been averaged over all of the POVs.

line with an increase of the average particle size of the monomers caused by water addition. The present results agree with the fact that the hydrodynamic radius of V18 increases by a more effective solvation shell made of H2O molecules when more than 1% water is present. Reinforcing this interpretation, the interaction energy between the POV and other species in solution (Figure S12) indicates that the stability of the POV is largely increased upon water addition and that this trend is associated with the POV···H2O stabilizing interactions. The last column of Table 2 compares the nonbonding interaction energy between a single POV molecule and the whole system in the studied cases. The more negative the value is, the more attractive the forces between the molecule and its environment J


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Inorganic Chemistry

Figure 11. (A) Representative snapshot of a V18(18:0)···V18(18:0) contact mediated by K+ cations. Distances are given in angstroms. (B) Comparison of the V18···V18 and V18···cation RDFs for alkali salts of the (18:0) anion in water (pink, green, and purple lines for Li+, K+, and Cs+), taking as a reference the I centers of V18. (C) Volumetric densities of the different cations surrounding V18(18:0) during the last 10 ns of simulation in water (for distances from I < 11.5 Å).

anions, which remain as disperse monomers. In solutions of wet MeCN, agglomeration is hampered as the percentage of water increases. Considering the highly hydrophilic K+ countercation, we only observed some agglomeration for the “fully reduced” V18(18:0) derivative, which is charged enough to pull away the solvent shell from K+ and, thus, yield cationmediated interactions between POVs. Smaller atomic cations like Li+, despite being more strongly solvated, mediate stronger V18···M+···V18 interactions, inducing a larger extent of agglomeration. Overall, our calculations have provided useful guidelines to fine-tune speciation of a typical, fully inorganic POV in solution.

similar and limited to bridging O sites of the POV. Conversely, the smaller Li+ presents a different interaction pattern, mostly involving terminal O sites (Figure 11C), which are those participating more actively in agglomeration.

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CONCLUSIONS We described the results of a combined DFT, CPMD, and MD theoretical analysis on the POV species [I@V18O42]q− in solution, with the redox state of V centers (IV/V) altered depending on q. The goal of this study was to advance the area of soluble POMs toward the rational design of new inorganic vanadium−oxo clusters. DFT calculations show that, in the absence of crystal packing effects, electrons tend to be distributed evenly over virtually all of the V atoms. Combined DFT and CPMD data suggest certain mobility (at least 0.2 Å away from the center of mass) of the encapsulated I at room temperature. From a methodological point of view, a structural analysis along the simulation reveals that the geometrical parameters with explicit solvent molecules and countercations are well reproduced by static DFT calculations using a continuum solvent model. Furthermore, CPMD shows that the “fully reduced” [I@V18O42]13− species (18:0) does not behave as a base in aqueous solution. Also, maps of the MEP show that the surface of the anion reflects the variations in the molecular charge. The collective dynamic behavior of V18 solutions, with special emphasis on agglomeration, is analyzed by MD simulations. It depends on the total charge (q−) of the POV, solvent, and countercation. We observed that V18 agglomeration is mediated by TEA countercations; that is, TEA units act as linkers between V18 units. In aqueous solution, the least hydrophilic V18 units (less negative q−) agglomerate into a microphase owing to the hydrophobicity of TEA, whereas those with more negative q− interact more strongly with water solvent molecules than with TEA cations, reducing both ion pairing and agglomeration. In MeCN, V18··· TEA ion pairing and agglomeration are more effective and can be controlled by tuning the redox state (q−) of V18. Simulations strongly suggest a strong effect of the solvent in the collective behavior of TEA-counterbalanced V18 anions. In this regard, moderately charged polyoxoanions (q = −5 and −7) feature the right ion-pairing balance to form long-lived agglomerates, at variance with the lowest- and highest-charged

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03508. Tables of ASDs obtained from DFT calculations, RDF graphics, schemes of agglomeration modes, numerical tables, snapshots of simulations, and structural information obtained from MD simulations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Albert Solé-Daura: 0000-0002-3781-3107 Jorge J. Carbó: 0000-0002-3945-6721 Josep M. Poblet: 0000-0002-4533-0623 Coen de Graaf: 0000-0001-8114-6658 Kirill Yu. Monakhov: 0000-0002-1013-0680 Xavier López: 0000-0003-0322-6796 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Spanish government (Projects CTQ2017-87269-P and CTQ2017-83566-P) and the Generalitat de Catalunya (Grant 2017-SGR629). J.M.P. thanks the ICREA foundation for an ICREA ACADEMIA award. K.Y.M. K


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Inorganic Chemistry is thankful for the financial support of the Emmy Noether program of the Deutsche Forschungsgemeinschaft.

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