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Rotational Isomerism, Electronic Structures, and Basicity Properties of “Fully-Reduced” V14-type Heteropolyoxovanadates Aleksandar Kondinski,†,ζ Thomas Heine,†,‡ and Kirill Yu. Monakhov*,§ †

Department of Physics and Earth Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstraße 2, 04103 Leipzig, Germany § Institut für Anorganische Chemie, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany ‡

S Supporting Information *

ABSTRACT: We investigated computationally the α-, γ-, and β-isomeric structures, relative stabilities, and the electronic and basicity properties of magnetic [VIV14E8O50]12− (hereafter referred to as {V14E8}) heteropolyoxovanadates (heteroPOVs) and their heavier chalcogenide-substituted [VIV14E8O42X8]12− ({V14E8X8}) derivatives for E = SiIV, GeIV, and SnIV and X = S, Se, and Te. We used density functional theory (DFT) with scalar relativistic corrections in combination with the conductor-like screening model of solvation. The main purpose of this investigation is to introduce the structure− property relations in heteroPOVs as well as to assist the synthesis and molecular deposition of these molecular vanadium-oxide spin clusters on surfaces. “Fully-reduced” polyoxoanions {V14E8} and {V14E8X8} are virtually comprised of [VIV14O38]20− {V14} skeletons of different symmetries, that is, D2d for α-, D2 for γ-, and D4h for β-isomers, which are stabilized by the four {E2O3}2+ and four {E2OX2}2+ moieties, respectively. Our DFT calculations reveal stability trends α > γ > β for polyoxoanions {V14E8} and {V14E8X8}, based on relative energies and HOMO−LUMO energy gaps. The α-isomeric polyoxoanions {V14E8} and {V14E8X8} with the high negative net charges may easily pick up protons at the terminal E−Ot and E−Xt sites, respectively, which is evidenced by strongly negative enthalpies of monoprotonation. Energetically favorable sites on polyoxoanions α-{V14E8} and α-{V14E8X8} for electrostatic pairing with countercations were also determined. Among β and γ isomers, the hitherto unknown γ-[V14Sn8O50]12− and γ-[V14Sn8O42S8]12− seem to be the most viable targets for isolation. Furthermore, these Sn-substituted polyoxoanions are of high interest for electrochemical studies because of their capability to act as two-electron redox catalysts.

1. INTRODUCTION

isomers of the polyoxoaluminum(III) species [AlO4(Al(OH)2(H2O))12]7+.13 In the present work, we aim to shed light on rotational isomerism of POMs with an open-shell electronic character and bring polyoxovanadates (POVs), a subclass of POMs, into focus. POVs are classified as fully oxidized (VV), mixed-valence (VV/VIV or VIV/VIII), “fully-reduced” (VIV), and “highlyreduced” (VIII) species.1,5,14−18 To enhance their chemical reactivity toward different functionalization reactions and to alter their physical properties to a large extent, POVs can be furnished with the heavier elements of groups 14 (Si and Ge) and 15 (As and Sb). As a result, these semimetal-modified POVs19 (hereafter referred to as heteroPOVs) structurally expose the handle-like {E2O7} and {E2O5} groups, respectively, which are characterized by the different bonding and electronic situations. Among heteroPOVs, structures [V14E8O50]12− (E = SiIV and GeIV)20−23 and [V14E8O42]4− (E = AsIII and SbIII)24−27 composed exclusively of VIV ions attract considerable attention

Polyoxometalates (POMs) discrete molecular metal-oxide clustersexhibit a large number of nanosized architectures deriving from various sorts of structural isomerism. Typically, POMs feature isomerism based on a restricted rotation of one or several building units about an n-fold axis (commonly called rotational isomerism), which in particular cases can lead to isomers with very different redox and stability properties. Rotational isomerism has been the subject of numerous density functional theory (DFT) studies on diamagnetic (closed-shell) POMs. For instance, DFT has assessed the relative stability and electronic properties of many synthesized and hypothetical POM isomer families such as, for example, conventional Keggin [XM12O40]n− (M = Mo and W; X = AlIII, GaIII, SiIV, GeIV, PV, and AsV),6−8 reversed-Keggin [(MnO4) (CH3)12Sb12O24]6−9 and Wells-Dawson [(PO4)2M18O54]6− (M = Mo and W)10,11 as well as unconventional Wells-Dawson structures [W18O56(XO6)](4+p)− (X = W, Te, and I).12 Furthermore, DFT electronic structure calculations helped to establish relative stability trends between the five Keggin 1−5

© XXXX American Chemical Society

Received: November 14, 2015

A

DOI: 10.1021/acs.inorgchem.5b02636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (top) Three building units {V8O24}, {V3O11}, and {E2O7}/{E2O5X2} constitutive of polyoxoanions {V14E8} and {V14E8X8}. (middle) Vanadium-oxo skeletons {V14} of different symmetries. (bottom) Complete structures of the α-/γ-/β-isomeric heteroPOVs {V14E8} (or {V14E8X8}). The structures are shown in a combined ball-and-stick and polyhedral representation. Color code: O = red, E = green, V = blue, {E2O7}/{E2O5X2} = green, and {VO5} = gray.

[VIV14E8O42]4− (E = AsIII and SbIII) was also conducted. For this computational work we used the scalar-relativistic corrections as implemented by the ZORA formalism and the general gradient approximation (GGA) and hybrid functional levels combined with a continuum solvation model to include solvent effects (see Computational Details). Before we discuss structural properties of polyoxoanions {V14}, {V14E8}, and {V14E8X8}, we want to stress that quantum mechanical analysis of reduced/mixed-valence POVs was performed to date only in a small number of studies.35−39 One of the main reasons is that the complexity of the electronic and spin structures of these polyanionic molecules makes it difficult to tackle specific theoretical problems. The herein-studied robust polyoxoanions {V14E8} and {V14E8X8} are composed of 14 {VO5} square pyramids and eight {EO4}/{EO3X} tetrahedra (Figure 1). The 14 {VO5} square pyramids are linked through edges to form the virtual {V14} skeletons. These skeletons exhibit one {V8O24} ring that is interconnected to two {V3O11} triads. Depending on the respective orientation of the two {V3O11} triads around the S4 axis, one can construct three distinct rotational isomers. These isomers classified as α-/D2d, γ-/D2, and β-/D4h correspond to mutual triad orientations by 90°, 45°, and 0°, respectively. It is interesting to note that in contrast to the Baker−Figgis isomers of the Keggin polyoxoanions40 where the five possible rotational isomers follow an alphabetic order, that is, α, β, γ, δ, and ε in respect to the number of rotated triads, the alphabetical order in the studied heteroPOV family usually follows the chronology of the synthesis and structural characterization (see Table S1 in the Supporting Information).19

for the following reasons: (1) Their vanadium-oxo skeletons derive from the archetypal [VIV18O42]12− polyoxoanion28 with characteristic rhombicuboctahedral topology and exhibit different rotational isomers described as α, β, and γ structures. (2) The {E2O7} groups can be converted into {E2O5X2} groups by exchange of two terminal oxygen atoms coordinated at E for two heavier congeners (= X atoms), as exemplified by the experimentally characterized [V14Ge8O42S8]12− polyoxoanion with the handle-like {Ge2O5S2} heterogroups.29 Considering that specific macroscopic solid-state surfaces have a high affinity for S atoms,30−33 such a {E2O7} → {E2O5X2} transformation is likely to have significant relevance to studies of fundamental adsorption and related surface physics phenomena at the level of thin films as well as individual molecules. (3) The trivalent E atoms in the {E2O5} groups can be used directly for the covalent attachment of, for example, organic amine ligands that partly or fully compensate the negative net charge of heteroPOV. This has been demonstrated by Bensch and co-workers for compounds (H 2 aep) 2 [V I V 1 5 Sb I I I 6 (Haep) 2 O 4 2 (H 2 O)]·2.5H 2 O and [VIV14SbIII8(Haep)4O42(H2O)]·4H2O (aep = 1-(2-aminoethyl)piperazine = C6H15N3).34 These three points indicate the great functional versatility of heteroPOVs related to the incorporated {E2O7} and {E2O5} groups and prompted us to undertake a comprehensive DFT study of rotational isomerism, electronic structures, and basicity properties of the “fully-reduced” polyoxoanions [VIV14E8O50]12− ({V14E8}) and [VIV14E8O42X8]12− ({V14E8X8}) [E = SiIV, GeIV, and SnIV and X = S, Se, and Te] comprising isotropic quantum spin-1/2 vanadyl {VO}2+ moieties in their [VIV14O38]20− ({V14}) skeletons. A comparative study of protonation chemistry between heteroPOVs {V14E8}/{V14E8X8} and B


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Inorganic Chemistry Table 1. Characteristic Bond Lengths of the DFT-Optimized Structures of α-[V14Si8O50]12−, α-[V14Ge8O50]12−, and α[V14Ge8O42S8]12− at the Tabulated Computational Level and the Corresponding Bond Length Values, As Determined Crystallographically bond distances [Å] structure α-[V14Si8O50]12−

α-[V14Ge8O50]12−

α-[V14Ge8O42S8]12−

computational level and experimental source

V−Ob

V−Ot

Si/Ge−O

Ge−S

COSMO/ZORA-scalar-UBP86/TZP LDA/ZORA-scalar/TZP39 crystal structure20 COSMO/ZORA-scalar-UBP86/TZP LDA/ZORA-scalar/TZP39 crystal structure22 COSMO/ZORA-scalar-UBP86/TZP crystal structure29

1.942−1.996

1.627−1.644

1.891−2.029 1.949−1.992

1.587−1.605 1.629−1.645

1.924−2.005 1.948−1.997 1.919−1.998

1.590−1.610 1.627−1.645 1.590−1.630

1.576−1.697 1.601−1.716 1.553−1.674 1.704−1.839 1.733−1.858 1.712−1.771 1.814−1.834 1.778−1.809

2.139 2.096−2.122

Figure 2. (a) The inequivalent vanadium atoms in α-/γ-/β-[V14Si8O50]12−. (b) Structures α-/γ-/β-[V14O38]12− (colored) superimposed over the respective vanadium-oxo skeletons α-/γ-/β-[V14Si8O50]20− (gray). The structures are shown in a ball-and-stick representation. Color code: V = blue, O = red, and Si = green. Virtual modifications of {V14} skeletons upon incorporation of heterogroups are depicted by red arrows (compression), blue arrows (expansion) and curved violet arrows ({V8O24} ring bending).

To the best of our knowledge, the stepwise rotation of the {V3O11} triads around the S4 axis, determining the assignment α, β, or γ, has not been addressed in all heteroPOV chemistry so far. The four cavities in vanadium-oxo skeleton {V14} are closed with four {E2O7} or four {E2O5X2} groups to give polyoxoanions {V14E8} and {V14E8X8}, respectively. These handle-like groups are the result of the combination of every two {EO4} or {EO3X} tetrahedra via an oxo-bridge.

Notably, we did not perform studies of the lead(IV)substituted POVs because the plumbate heterogroups, {PbO4}4−, are usually strong oxidizing agents, and therefore, the existence of [VIV14PbIV8O50]12− does not hold a chemical rationale. In addition to this, the large Pb(IV) cations are found to coordinate to more than four O or S atoms in extended materials as PbO2 and PbS2 or in mononuclear solution species as [PbIV(OH)6]2−.41−44 C


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Inorganic Chemistry

in connectivity (Figure 2a). Therefore, each {V3O11} triad is constructed of one Va atom that bridges to two Vd atoms by the μ3-O atoms. The Vb atoms are involved in the bridging of the {V3O11} triads with the virtual {V8O24} rings. The Vc atoms participate in the construction of the {V8O24} rings and bridge to Vb atoms by the μ3-O atoms. Following this assignment, we note that while the four atoms Va, Vb, Vc, and Vd in α-/γ-{V14E8} and α-/γ-{V14E8X8} are distributed in a 2:4:4:4 fashion, in β-{V14E8} and β-{V14E8X8} species one distinguishes between three atoms Va, Vb, and Vd, distributed in a 4:2:8 fashion, due the higher symmetry of these isomers. The differences in the connectivity and the distribution of the structurally inequivalent VIV are obvious when inspecting the construction of the {V8O24} ring moieties in each of the three isomers. In α-{V14E8}/{V14E8X8} each Vb atom oxo-bridges to two Vc atoms, whereas in γ-{V14E8}/ {V14E8X8} each Vb atom oxo-bridges to one Vb and one Vc atom. In β-{V14E8}/{V14E8X8}, we do not distinguish between Vc and Vd atoms (that is, they are equivalent) because the Vb atoms can be viewed as connecting to four {V3O11} triads. The overall structural properties of polyoxoanions {V14E8} and {V14E8X8} can be described with respect to the interatomic distances of opposite V atoms in the same structural environment. Such interatomic parameters analyzed here are the distances between the opposite {V3O11} triads (that is, dVa1−Va2) and the distances defining the size of the {V8O24} rings in polyoxoanions {V14E8} and {V14E8X8} (that is, dVb1−Vb2 and dVc1−Va3 for α-/γ-{V14E8}/{V14E8X8} isomers and dVb1−Vb2 and dVd6−Vd7 for β-{V14E8}/{V14E8X8} isomers). To understand how E and X atoms affect topologies of polyoxoanions {V14E8} and {V14E8X8}, we separately examined the average interatomic distances and their respective standard deviations σ for SiIV-, GeIV-, and SnIV-substituted POVs {V14E8} and {V14E 8X8} and compared with those of α-/γ-/β-{V14} skeletons (see Table S6 in the Supporting Information). The α-/γ-/β-{V14} skeletons show calculated dVa1−Va2, dVb1−Vb2, and dVc1−Vc3 interatomic distances that lay outside the respective interatomic V−V distance ranges in {V14E8} and {V14E8X8}. This reveals that the {V14} skeletons are somewhat modified upon functionalization with {E2O7}/{E2O5X2} heterogroups. In α-{V14E8}/α-{V14E8X8} the interatomic Va1−Va2 distances increase with the increasing size of E atoms (that is, SiIV < GeIV < SnIV), while dVb1−Vb2 and dVc1−Vc3 are unaffected as illustrated by the very narrow V−V distance ranges in Table S6 of the Supporting Information. In γ-/β-{V14E8}/{V14E8X8} all three interatomic distances are affected by the covalent radii of E atoms (Si = 1.10 Å, Ge = 1.20 Å, and Sn = 1.39 Å).61 These virtual changes with respect to the {V14} isomers cause expansions and contractions along the {V8O24} rings (see α-/ γ-/β-[V14Si8O50]12− in Figure 2b). The small deviations in σ ranging from 0.2 to 1.8 pm in SiIV-, GeIV-, and SnIV-substituted POVs indicate that the interatomic V−V distances are unaffected by the covalent radii of X atoms (O = 0.66 Å; S = 1.05 Å; Se = 1.20 Å; and Te = 1.38 Å).61 This observation along with others described in Section 4.2 of the Supporting Information is not surprising because the terminal O or X atoms in {E2O7}/{E2O5X2} units bind only to the E atoms and do not have direct interactions with the vanadiumoxo skeleton {V14}. 3.2. Relative Stabilities and Electronic Properties. In this section we discuss UB3LYP calculated electronic properties of polyoxoanions {V14E8} and {V14E8X8} and examine how these properties depend on the rotational isomerism and the

From the coordination perspective, lead(IV) cations are not compatible with the topologies of polyoxoanions {V14E8} and {V14E8X8}.

2. COMPUTATIONAL DETAILS Density functional theory (DFT) calculations were performed with the ADF2013 program.45,46 Numerical integration was performed using Becke grid integration.47,48 Geometry optimization was conducted using GGA Becke exchange49 plus the Perdew 86 correlation50 (BP) functional and all-electron Slater basis sets of triple-ζ quality with one polarization function (that is, TZP).51 The spin-unrestricted formalism was used for all open-shell electronic systems. Scalar relativistic effects were accounted for using the zeroth-order regular approximation (ZORA).52−54 We also applied the COnductor-like Screening MOdel (COSMO) with the default parameters for water (ε = 78.39; solvent radius = 1.93 Å) where solvent-excluding surface correction was included.55,56 Next to geometry optimizations at COSMO/ZORA-scalar-UBP86/ TZP, we computed molecular properties (binding energies, atomic spin densities, frontier molecular orbitals, and molecular electrostatic potentials) of polyoxoanions {V14E8} and {V14E8X8} with the hybrid UB3LYP functional57 via single-point calculations (that is, at COSMO/ZORA-scalar-UB3LYP/TZP) using the minimum-energy structures obtained with the above-mentioned procedure. The choice of B3LYP functional for the description of the electronic structure of our heteroPOV systems was dictated by the fact that hybrid DFT functionals usually yield reliable atomic spin populations at the metal sites, while GGA functionals tend to overdelocalize the electron density.58,59 The optimized bare models of α-{V14E8} and α-{V14E8X8} were used in the study of the protonation affinities. Single H atoms were modeled in the vicinity of five different oxygen sites (vide infra). The obtained models of the type [HV14E8O50]11− and [HV14E8O42X8]11− were then fully optimized. Inclusion of spin−orbit coupling (SOC) relativistic effects has been shown to be important in the DFT calculations of 51V NMR chemical shifts.60 In the studied heteroPOVs, SOC slightly reduces the relative energy between isomers, as demonstrated on the example of α-/γ-/β[V14Si8O42Te8]12− (see Table S28 in the Supporting Information). Furthermore, SOC slightly lowers the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gap by ca. 200 meV as observed for α-/γ-/β-[V14Si8O42Te8]12− (see Table S29 and Figure S6 in the Supporting Information).

3. RESULTS AND DISCUSSION 3.1. Structural Models and Calculated Geometrical Parameters. To assess the accuracy of the level of theory employed in this study, we first conducted a comparative analysis of the optimized and experimental bond lengths of three polyoxoanions α-[V14Si8O50]12−,20,24 α[V14Ge8O50]12−,20,22,23 and α-[V14Ge8O42S8]12−.29 All UBP86 geometry optimizations were performed in the sufficient pentadecaplet states (that is, with all-parallel spins). The obtained DFT minimum-energy structures of these heteroPOVs show good agreement for V−Ob, V−Ot, Si−O, Ge−O, and Ge−S distances with the experimental ones determined by X-ray diffraction (Table 1). Next, the structures of the three isomeric, bare skeletons α-/γ-/β-{V14} (Figure 1) were studied to gain a comprehensive understanding of the individual contributions from the handle-like {E2O7} and {E2O5X2} heterogroups to the geometric and electronic structures of polyoxoanions {V14E8} and {V14E8X8}, respectively. The three rotational isomers have sets of structurally inequivalent VIV centers. For more facile comparison between these isomers, we introduced an alphabetical assignment of the V atoms and grouped them corresponding to their similarities D


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Inorganic Chemistry

Figure 3. Spin density isosurfaces indicating accumulation of α spins for the “fully-reduced” polyoxoanions α-[VIV14Si8O50]12−, γ-[VIV14Si8O50]12−, and β-[VIV14Si8O50]12−. The structures are shown in a ball-and-stick representation. Color code: V = blue, O = red, and Si = green.

Figure 4. (a) α-spin HOMO−LUMO energy gaps for α, γ, and β isomers of {V14Si8} and {V14Si8X8} in dependence on the respective {Si2O7} and {Si2O5X2} heterogroups (see Tables S9−S24 in the Supporting Information). (b) Molecular orbital diagram of [V14Si8O50]12−. (c) Molecular orbital diagram of [V14Si8O42Te8]12−. (d, e) Representation of frontier molecular orbitals, α- and β-spin HOMOs and LUMOs, for [V14Si8O50]12− and [V14Si8O42Te8]12−, respectively.

[V14Si8O50]12−. Figure 3 illustrates only the “fully-reduced” SiIV-substituted POVs with one unpaired electron per VIV center (ASD = 1.12−1.14), which occupies a d-atomic orbital. This fully agrees with the interpretation of the bond-valencesum calculations on vanadium atoms in heteroPOVs reported in previous studies.20−23,29 Note that the energies of the spin flip are commonly several orders of magnitude smaller than the other contributions (for example, orbital and Coulomb interactions) to the total bonding energy; therefore, the various spin orientations that

nature of E and X atoms in {E 2 O 7 } and {E 2 O 5 X 2 } heterogroups. We want to stress that these calculations were performed for the high-spin (all spins up, ↑) UBP86 structures of polyoxoanions {V14E8} and {V14E8X8}. To gain complementary insight into the electron distribution of metal electrons in heteroPOVs, we computed atomic spin densities (ASDs) for the optimized structures of the experimentally synthesized and characterized polyoxoanions α-[V 1 4 Si 8 O 5 0 ] 1 2 − , 2 0 α[V14Ge8O50]12−,22 and α-[V14Ge8O42S8]12−29 as well as the hypothetical polyoxoanions γ-[V 14 Si 8 O 50 ] 12− and βE


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Inorganic Chemistry

Figure 5. (a) α-spin HOMO−LUMO energy gaps for α, γ, and β isomers of {V14Ge8} and {V14Ge8X8} in dependence on the respective {Ge2O7} and {Ge2O5X2} heterogroups (see Tables S9−S24 in the Supporting Information). (b) Molecular orbital diagram of [V14Ge8O50]12−. (c) Molecular orbital diagram of [V14Ge8O42Te8]12−. (d, e) Representation of frontier molecular orbitals, α- and β-spin HOMOs and LUMOs, for [V14Ge8O50]12− and [V14Ge8O42Te8]12−, respectively.

the SiIV- and GeIV-substituted POVs {V14E8} and {V14E8X8}, the Sn-substituted ones show somewhat smaller energy differences between α and γ isomers, especially in the case of [V14Sn8O50]12− and [V14Sn8O42S8]12−, and this increases the chance for γ isomers of these polyoxoanions to be synthesized. The calculated α-spin HOMO−LUMO energy gaps of polyoxoanions {V14E8} and {V14E8X8} are in the range of 2.94−4.23 eV, reducing in the order of α > γ > β. This is in line with the relative stability trend derived from the relative energies ΔE summarized in Table S8 (Supporting Information). Interestingly, the electronic structures of polyoxoanions {V14E8} and {V14E8X8} with α- and β-spin HOMOs and LUMOs are tuned by the cumulative effects of rotational isomerism and bonding interactions of {E2O7} and {E2O5X2} heterogroups with the virtual vanadium-oxo skeletons {V14}, and this is demonstrated below. {V14Si8} and {V14Si8X8}. α-[V14Si8O50]12− exhibits α-spin HOMO−LUMO energy gap of 4.23 eV, which is the largest among polyoxoanions {V14E8} and {V14E8X8} (see Table S8 in the Supporting Information). This energy gap decreases by 0.2 and 0.4 eV in the case of corresponding γ and β isomers, respectively. This is associated with the α-HOMO energy level increasing in the order of α < γ < β, while the α-LUMO energy level remains largely unaffected (energy differences of ±0.04

must be considered when designing anti-ferromagnetic models of polyoxoanions {V14E8} and {V14E8X8} should not change the spatial distribution of the spin density over VIV centers. Taking this into account the structural, orbital, and electrostatic properties of the calculated ferromagnetic spin models of polyoxoanions {V14E8} and {V14E8X8} are unlikely to differ significantly from those of the conceptually more accurate antiferromagnetic models with lower total ground-state spins. It is noteworthy that a significant majority of POVs, such as, for example, polyoxoanions described herein, is characterized by anti-ferromagnetic coupling18,19 between the spin-1/2 vanadyl {VO}2+ moieties. A discussion on the determination of the true ground state of the representative polyoxoanions is beyond the scope of this paper. However, we currently tackle this very wellknown problem in POV magnetochemistry using a combination of DFT and semiempirical methods and model Hamiltonian calculations. Next, we conducted a comparative analysis of the relative energies calculated for polyoxoanions {V14E8} and {V14E8X8}. The α isomers have the lowest relative energies and, thus, are the most stable among the representative isomers investigated herein (see Table S8 in the Supporting Information). On the basis of these energies, the following trend in stability is established: α (highest) > γ > β (lowest). In comparison with F


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Inorganic Chemistry

Figure 6. (a) α-spin HOMO−LUMO energy gaps for α, γ, and β isomers of {V14Sn8} and {V14Sn8X8} in dependence on the respective {Sn2O7} and {Sn2O5X2} heterogroups (see Tables S9−S24 in the Supporting Information). (b) Molecular orbital diagram of [V14Sn8O50]12−. (c) Representation of frontier molecular orbitals, α- and β-spin HOMOs and LUMOs, for [V14Sn8O50]12−.

eV; Figure 4a,b). Analysis of the distribution of α-HOMO in α[V14Si8O50]12− (Figure 4d) shows that the highest frontier electron densities (ca. 80%) are located on the vicinal Vb,c,d atoms (and not on the apical Va centers situated between the two closely adjoined handle-like groups) and to a smaller degree on the bridging oxo ligands (see also Section 5.1 in the Supporting Information). For comparison, the rotational isomers of polyoxoanions [V14Si8O50]12− and [V14Si8O42S8]12− differ with respect to the degree of localization of their α-HOMOs. In α-[V14Si8O50]12− and α-[V14Si8O42S8]12− α-HOMO is delocalized over the {V8O24} ring (Vb and Vc atoms) and partially over the two {V3O11} triads (Vd atoms). In γ-[V14Si8O50]12− and γ[V14Si8O42S8]12− α-HOMO is localized over the four Vb centers, meaning that it covers only segments of the {V8O24} ring (see Figure S5 in the Supporting Information). In β[V14Si8O50]12− and β-[V14Si8O42S8]12− α-HOMO is predom-

inantly localized over the two Vb centers and with only small contributions over the neighboring Vc atoms (see Figure S5 in the Supporting Information). The high localization of αHOMOs in these γ and β isomers can be linked to distortions of the {V8O24} rings in respect of the relevant axes (vide supra). For the α, γ, and β isomeric polyoxoanions [V14Si8O50]12− and [V14Si8O42S8]12− we also observe that their β-HOMOs are predominantly localized over the terminal O and S atoms of the handle-like {Si2O7} and {Si2O5S2} groups, respectively, and they lie lower in energy than the corresponding α-HOMOs. In POM chemistry, β-HOMOs are typically distributed over the oxygen atoms,62 which makes the so-called “oxo band” (or in our particular case the “chalkoxide” band). Incorporation of heavier {Si2O5Te2} heterogroups into the vanadium-oxo skeleton {V14} to give [V14Si8O42Te8]12− results in a remarkable decrease in the energy difference between αHOMOs and β-HOMOs (Figure 4c,e). The similarity in the G


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Inorganic Chemistry

Figure 7. (a, left) A segment of polyoxoanions {V14E8} and {V14E8X8} in a combined ball-and-stick and polyhedral representation showing the possible five oxygen sites for protonation. Color code: V = blue, E = green, and O = red. (a, right) The color of the respective heteroatoms in polyoxoanions {V14E8} and {V14E8X8} corresponds to that of the energy diagrams. (b−d) Enthalpies of monoprotonation of SiIV-, GeIV-, and SnIVsubstituted POVs (α-{V14E8}/{V14E8X8}) as a function of protonation site, respectively.

systems. Their α-spin HOMOs are clearly separated from the β-spin HOMOs, and we find no significant effects on this energy separation introduced by the X heteroatoms (see Tables S9, S15, S17, and S21 in the Supporting Information). As seen in Figure 6c, α- and β-spin LUMOs in [V14Sn8O50]12− (same as in other heavier derivatives) are predominantly distributed over the Sn atoms (highest frontier electron densities of ca. 50%). This feature can be the starting point for further electrochemical studies of these yet hypothetical polyoxoanions because the first reduction is expected to occur primarily at SnIV metal centers. The composition of the LUMOs in the {Sn2O7}- and {Sn2O5X2}-modified heteroPOVs prompted us to briefly examine the electronic structures of “fully-reduced” group 15 (As, Sb) element-substituted POVs19 α-/γ-/β-[VIV14E8O42]4− (E = AsIII and SbIII). This comparison was further motivated by the synthetic accessibility of these α and β isomeric polyoxoanions (Table S1 and Figure S2 in the Supporting Information).24−27 The α-LUMOs of α-/γ-/β-[V14Sb8O42]4− are found to have a significant distribution over the Sb-centered atomic-like orbitals (see Figure S9 in the Supporting Information), which makes it comparable to the herein-reported LUMOs of [V14Sn8O50]12−. The unoccupied α-spin p-orbitals on Sb atoms in α-/γ-/β[V14Sb8O42]4− are likely to be involved in the formation of covalent Sb−N bonds, as exemplified by the experimentally characterized compound [VIV14Sb8(Haep)4O42(H2O)]·4H2O.34 3.3. Protonation Features and Energetics. For the reasons that POVs can exist in a variety of protonation states63 and potential in situ hydrogenation of these very sensitive molecular vanadium-oxide clusters deposited on surfaces might take place in the presence of H2 under ultrahigh vacuum conditions, it is of importance at this stage of the investigation to gain an understanding of which atomic positions are most likely to be prone to reaction with hydrogen ion H+. Studies on the reactivity of heteroPOVs toward molecular hydrogen are

orbital distribution and character is apparent from the fixed αHOMO−α-LUMO energy gaps (3.5, 3.47, and 3.43 eV for α, γ, and β isomers, respectively), which are nearly unaffected by the rotational isomerism of this polyoxoanion (see Table S8 in the Supporting Information). However, in the case of [V14Si8O42Se8]12− the rotational isomerism still gradually decreases the separation between α-HOMOs and β-HOMOs in the order of α > γ > β, which shifts the electron density distribution of the α-HOMO from Se to V atoms (see Tables S18−S20 in the Supporting Information). {V14Ge8} and {V14Ge8X8}. The α-spin HOMO−LUMO energy gaps of polyoxoanions α-[V14Ge8O50]12− and α[V14Ge8O42X8]12− (X = S and Se) are by ca. 0.2 and 0.4 eV larger than those of their corresponding γ and β isomers, respectively (see Table S8 in the Supporting Information). These heteroPOVs exhibit an α-spin energy gap (Figure 5a,b) as well as the distribution of α- and β-spin HOMOs (Figure 5d) similar to those of the SiIV-substituted POVs {V14E8} and {V14E8X8}. The frontier electron densities located on the terminal Te atoms in α-HOMOs of [V14Ge8O42Te8]12− are reduced in the order of α isomer (95%) > γ isomer (56%) > β isomer (0%) (Figure 5c,e), thus showing the electronic effect of the rotational isomerism. As with the SiIV-substituted POVs {V14E8} and {V14E8X8}, the α-LUMOs and β-LUMOs of the GeIV-substituted systems are mainly localized over the VIV atoms and the terminal O atoms of the {VO5} square pyramids. {V14Sn8} and {V14Sn8X8}. The α-spin HOMO−LUMO energy gaps of polyoxoanions [V 14 Sn 8 O 5 0 ] 12 − and [V14Sn8O42X8]12− (Figure 6a) are somewhat smaller than those of the SiIV- and GeIV-substituted POVs {V14E8} and {V14E8X8} (see Table S8 in the Supporting Information). For instance, the gap in α-[V14Sn8O50]12− is 3.42 eV (Figure 6a,b), which is 0.81 eV smaller than that of lighter α-[V14Si8O50]12− polyoxoanion. The rotational isomerism again determines the trend in the α-HOMO−α-LUMO gaps (typical order for heteroPOV isomers: α > γ > β) in these SnIV-substituted H


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Inorganic Chemistry

hydroxyl ligands64 (protonation of terminal VO group will destabilize the respective bonding region in heteroPOV); (ii) from a technical perspective, geometry optimizations of polyoxoanions α-{V14E8} and α-{V14E8X8} monoprotonated at site 5 were all-unsuccessful due to nonconverging selfconsistent field. Considering that protonation is affected by electrostatic as well as electronic and molecular orbital phenomena, we inspected the protonation profiles of α{V14E8} and α-{V14E8X8} for the remaining four sites with regard to the type of E atom (Figure 7b−d). Monoprotonation at sites 1−4 in α-{V14E8} and α{V14E8X8} is characterized by negative binding enthalpies, showing that these highly charged polyoxoanions have strong proton affinity (see Table S33 in the Supporting Information). How the E atoms influence the protonation enthalpies can be exemplified by analysis of monoprotonated α-{V14E8} polyoxoanions [heteroPOV(H)]11−. The α-isomeric SiIV-, GeIV-, and SnIV-substituted POVs exhibit for site 1 the highest protonation enthalpies of −188.7 kJ·mol−1 (α-[V14Si8O50]12−), −191.2 kJ·mol−1 (α-[V14Ge8O50]12−), and −201.8 kJ·mol−1 (α[V14Sn8O50]12−), respectively. As can be observed, the increase in the size of E atoms results in the elongation of the E−Ot bond distances (Si−Ot 1.576 Å, Ge−Ot 1.704 Å, and Sn−Ot 1.915 Å), and this presumably has an effect on the proton affinities of α-{V14E8} at site 1. Site 1 in the SiIV-substituted POVs α-{V14E8} and α{V14E8X8} is most favorable for H+ attack, as demonstrated by the large negative enthalpies ranging from −188.7 kJ·mol−1 for α-[HV14Si8O50]11− to −118 kJ·mol−1 for α-[HV14Si8O42Te8]11− (see Table S31 in the Supporting Information). Thus, the enthalpies of monoprotonation at this site become less negative as the terminal Ot atoms in {E2O7} are substituted for heavier Xt congeners (that is, S, Se, and Te) to give {E2O5X2}. This decrease in the magnitude of the protonation enthalpies can be related to the shrinking of the MEP at site 1 as the terminal chalcogen atoms in {E2O7}/{E2O5X2} become heavier or more metallic. Enthalpies of monoprotonation at sites 2, 3, and 4 have remarkably lower magnitudes than those at site 1 (Figure 7b). These enthalpies at sites 2 and 3 are in the ranges from −46.4 to −67.1 kJ·mol−1 and from −36.3 to −52.3 kJ·mol−1, respectively, reflecting the following trend: O/X = Te < Se < S < O. The relatively narrow ranges of protonation enthalpies predicted by DFT for sites 2 and 3 in α-{V14E8} and α{V14E8X8} indicate that these sites are not strongly affected by the type of heterogroups. The enthalpies at site 4 vary from −44.6 to −98.8 kJ·mol−1 and follow the trend O/X = O < Te < Se < S. This can be explained by the fact that HOMO of α[V14Si8O50]12− (see Figure 4) becomes largely destabilized once the polyoxoanion is monoprotonated. Similarly, the magnitude of the protonation enthalpies at site 1 in the GeIV- and SnIV-substituted POVs α-{V14E8} and α{V14E8X8} is remarkably decreasing as the atomic number of the terminal chalcogen atom in {E2O7}/{E2O5X2} increases (see Table S33 in the Supporting Information). Enthalpies of monoprotonation at site 1 in general remain the most negative among these α-{V14E8} and α-{V14E8X8}, however, with some exceptions. Three particular combinations of heavy E and X atoms within heteroPOV result in most negative enthalpies of monoprotonation at other sites, different than site 1. α[V14Ge8O42Te8]12− shows the most negative protonation enthalpies at site 4, while α-[V14Sn8O42X8]12− (X = Se and Te) is most negative at site 2. This illustrates that, although protonation in many polyoxoanions α-{V14E8} and α-

currently in progress and will be reported elsewhere. Also, the propensity of POVs to engage in intermolecular electrostatic interactions is worthy of analysis, because polyoxoanions may agglomerate in solution and on surfaces through countercations (at all events through hydrogen bonding). These two points are discussed below. HeteroPOVs {V14E8} and {V14E8X8} exhibit five types of O and X atoms (arbitrarily assigned as possible protonation sites 1 to 5), which can be grouped depending on their connectivity to E and V centers (Figure 7a). Protonation sites 1 are the terminal Ot and Xt atoms in {E2O7} and {E2O5X2} groups, respectively. Protonation sites 2 are the bridging μ2-O atoms in these handle-like groups. Protonation sites 3 consider protonation at the μ3-O atoms connecting the {E2O7}/ {E2O5X2} groups to the virtual vanadium-oxo skeleton {V14}. Protonation sites 4 are the μ3-O atoms bridging three VIV centers within heteroPOV. Finally, protonation sites 5 reflect the terminal Ot atoms in the square-pyramidal {VO5} units that are situated between the two closely adjoined {E2O7}/ {E2O5X2} handle-like groups. Whereas Mulliken and Hirshfeld charges differ between these indicated five sites in α-/γ-/β{V14E8} and α-/γ-/β-{V14E8X8}, no significant changes in the charges of a particular site were found when comparing between the different rotational isomers α, γ, and β (see the Supporting Information). This suggests that the rotational isomerism does not contribute to charge population differences in here studied heteroPOVs. The molecular electrostatic potential (MEP) of α-{V14E8} reveals that the terminal oxygen atoms at sites 1 and 5 possess highly negative electrostatic potentials. Thus, electrostatic pairing of the polyoxoanion component with countercations is likely to occur in their vicinity (see Figure S7 in the Supporting Information). The MEP of α-{V14E8X8} shows that electrostatic potential on the terminal X atoms gradually becomes less negative as the atomic number of X increases (that is, S < Se < Te). As a consequence, the most electronegative potentials in α-{V14E8X8} are found on the terminal Ot atoms in the square-pyramidal {VO5} units. The results obtained suggest the following order of decreasing the basicity of heteroPOVs α-{V14E8} and α-{V14E8X8} for O/X = O > S > Se > Te, and they give us an idea of how to somewhat inhibit possible spontaneous agglomeration of heteroPOVs in solution and, hence, their uncontrolled deposition on surfaces: lower basicity of heteroPOVs may imply poorer tendency to agglomerate. However, it remains open for further studies to gain deeper insights into potential agglomeration mechanisms of chalcogenide-substituted polyoxoanions [VIV14E8O42X8]12− ({V14E8X8}) in solution through weak ion pairing (for example, formation of nanoscale macroions) or covalent interaction between pairs of {V14E8X8} with the formation of X−X bonds (for example, S−S disulfide bridges as in proteins). Deeper insights into the protonation chemistry of these polyanionic molecules were gained through UBP86 calculations of polyoxoanions α-{V14E8} and α-{V14E8X8}. We thus analyzed the proton affinity trends at each of the five aforementioned oxygen sites in α-{V14E8} and α-{V14E8X8} by following the model reaction [heteroPOV]12− + H3O+ → [heteroPOV(H)]11− + H2O to find the preferable sites of H+ attack. Protonation at site 5 in α-{V14E8} and α-{V14E8X8} was excluded from consideration for the following reasons: (i) it is well-known that protonation of vanadyl moieties is unlikely to occur over the terminal oxo ligands (see terminal VO groups in heteroPOVs) that are far stronger π-donors than the I


Article

Inorganic Chemistry {V14E8X8} herein studied is expected to occur at site 1, in some particular cases mentioned above other nucleophilic sites in these heteroPOVs become more reactive or attractive for electrophilic attack of H+. As a result, our computational data are consistent with experimental data obtained for polyoxoanions α-[V14Si8O50]12− and α-[V14Ge8O50]12−, which demonstrate that sites 1 are most susceptible to protonation to give polyprotonated species.20,21 α-[V14Si8O42(OH)8]4− and α[V14Ge8O42(OH)8]4−. Furthermore, the crystal packing of α[V14Si8O50]12− and α-[V14Ge8O50]12− points out that these polyoxoanions interact with countercations in the close proximity of the most nucleophilic oxo sites 1.22,23 By contrast, mainly positive enthalpies of monoprotonation in the range from −2.9 to 94.2 kJ·mol−1 were observed for polyoxoanions α[VIV14E8O42]4− (E = AsIII and SbIII) characterized by the absence of terminal oxygen atoms at the pnictogen centers (see Table S37 in the Supporting Information). This observation is in agreement with the experimental findings showing that polyoxovanadatoarsenates and polyoxovanadatoantimonates of this type exhibit nonprotonated structures.19

5. Energetically most favorable sites for monoprotonation of α-isomeric polyoxoanions {V14E8} are the terminal oxygen atoms Ot of the {E2O7} heterogroups. While most of α-isomeric polyoxoanions {V14E8X8} exhibit highest proton affinities at the terminal Xt sites of {E2O5X2}, three congeners α-[V14Ge8O42Te8]12−, α[V14Sn8O42Se8]12−, and α-[V14Sn8O42Te8]12− were predicted to show somewhat different behavior. 6. Polyoxoanions α-[VIV14E8O42]4− with E = AsIII and SbIII are not prone to protonation because their pnictogen centers do not bear terminal oxygen atoms (sites 1) in the handle-like {E2O5} groups but have lone pairs of electrons, thus resulting in the low negative net charges of these heteroPOVs. Polyoxoanions α-/γ-/β[V14Sb8O42]4− are highly interesting for further computational studies because of the intriguing molecular orbital picture similar to that of SnIV-substituted POVs {V14E8} and {V14E8X8}. 7. Peripheral functionalization of POVs with heavier chalcogenides might inhibit self-agglomeration of highly charged structures [VIV14E8O42X8]12− ({V14E8X8}) in solution and on surfaces via hydrogen-bonded networks due to lowering the heteroPOV basicity for X = S > Se > Te. This work has broad implications for the understanding of rotational isomerism, electronic structures, and basicity properties of heteroPOVs. Certain “upgrades” of the current models such as considering anti-ferromagnetic coupling effects, pairing with multiple protons and countercations, and incorporation of guest molecules are indeed necessary to provide even more accurate insights into the solution stability and chemistry of these metal-oxide materials. In particular, the question whether relative stability of the α, γ, and β isomeric structures of “fullyreduced” V14-type heteroPOVs is conserved independently of or strongly depends on the total spin is yet to be answered in further theoretical studies. We believe that such model “upgrades” can be very helpful to design synthetic routes to some novel topologies (for example, the γ-isomers) and compositions presented herein. In our laboratories, we currently investigate the self-assembly mechanisms of transferable vanadium-oxo building blocks and the stability and properties of polyprotonated species in solution by electrochemical, spectroscopic, and mass spectrometric methods. The search for the “true” ground-state spin configuration by assessing the high-spin and low-spin solutions in these high-nuclearity magnetic heteroPOVs will imply the use of additional semiempirical calculations and model Hamiltonian calculations in combination with DFT studies. Furthermore, we perform numerical simulation of the hereinstudied heteroPOV structures on metallic surfaces to assess the effects of substrates on the polyoxoanion properties and thus explore the potential of these heteroPOVs in the condensed matter and surface physics as well as catalysis.

4. CONCLUSIONS AND OUTLOOK In summary, systematic DFT calculations employing scalarrelativistic effects and the COSMO as a continuum solvation model were performed to investigate the structural features, relative stabilities, electronic properties, and protonation site preferences of the “fully-reduced” polyoxoanions {V14E8} and {V14E8X8}. The following conclusions can be drawn from this study: 1. Analysis of the geometric parameters shows that the virtual, α-/γ-/β-isomeric vanadium-oxo skeletons {V14} undergo remarkable structural deformations to adopt the handle-like {E2O7} or {E2O5X2} heterogroups, which results in polyoxoanions {V14E8} and {V14E8X8}, respectively. Some of these deformations include elongation and compression along particular axes and out-of-plane distortions of the {V8O24} rings. The geometric parameters defining skeletons {V14} are influenced by the E atoms only. 2. The relative energies and the HOMO−LUMO energy gaps indicate that the stability of polyoxoanions {V14E8} and {V14E8X8} decreases in the order of α > γ > β. The energy differences between the α- and the γ-isomeric polyoxoanions {V14E8} and {V14E8X8} vary from 8 to 40 kJ·mol−1, while between the α and the β isomers they vary from 15 to 82 kJ·mol−1. 3. The nature of the frontier molecular orbitals in polyoxoanions {V14E8} and {V14E8X8} is directly affected by the composition of the {E2O7}/{E2O5X2} heterogroups as well as the rotational isomerism. The α-spin HOMO−LUMO energy gaps in {V14E8} and {V14E8X8} were calculated to be in the range of 2.9−4.2 eV. 4. The LUMOs in SiIV- and GeIV-substituted POVs {V14E8} and {V14E8X8} are predominantly located on the VIV centers, thus suggesting that vanadium ions can be further reduced (VIV → VIII) upon electrochemical reduction. By contrast, SnIV-substituted POVs {V14E8} and {V14E8X8} have LUMOs that are mainly distributed over the SnIV centers, therefore suggesting the possibility of two-electron redox reactions involving tin ions (SnIV− SnII). Conventional POMs are usually one-electron catalysts.65,66

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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.5b02636. Details of DFT calculations, including general overview, survey of reported V14E8-nuclearity heteroPOVs, theoretical studies of reduced polyoxometalates, geometric J


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(19) Monakhov, K. Y.; Bensch, W.; Kögerler, P. Chem. Soc. Rev. 2015, 44, 8443−8483. (20) Tripathi, A.; Hughbanks, T.; Clearfield, A. J. Am. Chem. Soc. 2003, 125, 10528−10529. (21) Wang, X.; Liu, L.; Jacobson, A. J. MRS Online Proc. Libr. 2011; Vol. 1309, http://dx.doi.org/10.1557/opl.2011.105. (22) Wang, J.; Näther, C.; Kögerler, P.; Bensch, W. Inorg. Chim. Acta 2010, 363, 4399−4404. (23) Whitfield, T.; Wang, X.; Jacobson, A. J. Inorg. Chem. 2003, 42, 3728−3733. (24) Huan, G.; Greaney, M. A.; Jacobson, A. J. J. Chem. Soc., Chem. Commun. 1991, 260−261. (25) Zheng, S.; Zhang, J.; Yang, G. Z. Anorg. Allg. Chem. 2005, 631, 170−173. (26) Zhang, L.; Zhao, X.; Xu, J.; Wang, T. J. Chem. Soc., Dalton Trans. 2002, 3275−3276. (27) Hu, X.; Xu, J.; Cui, X.; Song, J.; Wang, T. Inorg. Chem. Commun. 2004, 7, 264−267. (28) Johnson, G. K.; Schlemper, E. O. J. Am. Chem. Soc. 1978, 100, 3645−3646. (29) Pitzschke, D.; Wang, J.; Hoffmann, R.; Pöttgen, R.; Bensch, W. Angew. Chem., Int. Ed. 2006, 45, 1305−1308. (30) Parks, J. J.; Champagne, A. R.; Costi, T. A.; Shum, W. W.; Pasupathy, A. N.; Neuscamman, E.; Flores-Torres, S.; Cornaglia, P. S.; Aligia, A. A.; Balseiro, C. A.; Chan, G. K.; Abruna, H. D.; Ralph, D. C. Science 2010, 328, 1370−1373. (31) Dreiser, J.; Ako, A. M.; Wackerlin, C.; Heidler, J.; Anson, C. E.; Powell, A. K.; Piamonteze, C.; Nolting, F.; Rusponi, S.; Brune, H. J. Phys. Chem. C 2015, 119, 3550−3555. (32) Rodriguez-Douton, M. J.; Mannini, M.; Armelao, L.; Barra, A.; Tancini, E.; Sessoli, R.; Cornia, A. Chem. Commun. 2011, 47, 1467− 1469. (33) Heß, V.; Matthes, F.; Bürgler, D. E.; Monakhov, K. Y.; Besson, C.; Kögerler, P.; Ghisolfi, A.; Braunstein, P.; Schneider, C. M. Surf. Sci. 2015, 641, 210−215. (34) Antonova, E.; Näther, C.; Kögerler, P.; Bensch, W. Angew. Chem., Int. Ed. 2011, 50, 764−767. (35) Monakhov, K. Y.; Linnenberg, O.; Kozłowski, P.; van Leusen, J.; Besson, C.; Secker, T.; Ellern, A.; López, X.; Poblet, J. M.; Kögerler, P. Chem. - Eur. J. 2015, 21, 2387−2397. (36) (a) Zueva, E. M.; Borshch, S. A.; Petrova, M. M.; Chermette, H.; Kuznetsov, A. M. Eur. J. Inorg. Chem. 2007, 4317−4325. (b) Aronica, C.; Chastanet, G.; Zueva, E.; Borshch, S. A.; Clemente-Juan, J. M.; Luneau, D. J. Am. Chem. Soc. 2008, 130, 2365−2371. (37) (a) Cardona-Serra, S.; Clemente-Juan, J. M.; Coronado, E.; Gaita-Ariño, A.; Suaud, N.; Svoboda, O.; Bastardis, R.; Guihéry, N.; Palacios, J. J. Chem. - Eur. J. 2015, 21, 763−769. (b) Cardona-Serra, S.; Clemente-Juan, J. M.; Gaita-Ariño, A.; Suaud, N.; Svoboda, O.; Coronado, E. Chem. Commun. 2013, 49, 9621−9623. (c) Calzado, C. J.; Clemente-Juan, J. M.; Coronado, E.; Gaita-Ariño, A.; Suaud, N. Inorg. Chem. 2008, 47, 5889−5901. (38) Maslyuk, V. V.; Mertig, I.; Farberovich, O. V.; Tarantul, A.; Tsukerblat, B. Eur. J. Inorg. Chem. 2013, 1897−1902. (39) Janjua, M. R. S. A.; Su, Z.; Guan, W.; Irfan, A.; Muhammad, S.; Iqbal, M. Can. J. Chem. 2010, 88, 434−442. (40) Baker, L. C. W.; Figgis, J. S. J. Am. Chem. Soc. 1970, 92, 3794− 3797. (41) Taggart, J. E., Jr.; Foord, E. E.; Rosenzweig, A.; Hanson, T. Can. Mineral. 1988, 26, 905−910. (42) Silverman, M. S. Inorg. Chem. 1966, 5, 2067−2069. (43) Levy-Clement, C. Ann. Chim. 1975, 10, 105−114. (44) Becht, A.; Vogler, A. Inorg. Chem. 1993, 32, 2835−2837. (45) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; Van Gı ́sbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931−967. (46) ADF 2014, SCM, Theoretical Chemistry, Vrije Univesity, Amsterdam, The Netherlands http://www.scm.com/. (47) Becke, A. D. A J. Chem. Phys. 1988, 88, 2547−2553.

parameters, electronic properties, charge and protonation analysis, structural and electronic properties of isomers of [V14As8O42]4− and [V14Sb8O42]4−, Cartesian coordinates and total energies of the optimized structures; additional references. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

ζ Institut für Anorganische Chemie, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.K. thanks the Jacobs Univ. Bremen for providing financial support. K.Y.M. thanks the Deutsche Forschungsgemeinschaft for an Emmy Noether fellowship. The authors are also grateful to Dr. N. Vankova, Dr. L. Zhechkov, and Dr. P. Petkov for helpful discussions. This work was performed using the computational resources of the CLAMV (Computational Laboratories for Analysis, Modeling and Visualization) at Jacobs University Bremen.

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DEDICATION In memory of Prof. Alessandro Bagno. REFERENCES

(1) Long, D.; Tsunashima, R.; Cronin, L. Angew. Chem., Int. Ed. 2010, 49, 1736−1758. (2) Kortz, U.; Müller, A.; van Slageren, J.; Schnack, J.; Dalal, N. S.; Dressel, M. Coord. Chem. Rev. 2009, 253, 2315−2327. (3) Müller, A.; Peters, F.; Pope, M. T.; Gatteschi, D. Chem. Rev. 1998, 98, 239−271. (4) In Polyoxometalate Chemistry from Topology Via Self-Assembly to Applications. Pope, M. T.; Müller, A., Eds.; Springer: Netherlands, 2001; Vol. 7. (5) Cronin, L. High Nuclearity Clusters: Iso and Heteropolyoxoanions and Relatives. In Coordination Chemistry II; Elsevier, Ltd: Amsterdam, The Netherlands, 2003; Vol. 7, pp 1−56. (6) Guo, Y.; Pan, Q.; Wei, Y.; Li, Z.; Li, X. J. Mol. Struct.: THEOCHEM 2004, 676, 55−64. (7) López, X.; Maestre, J. M.; Bo, C.; Poblet, J. J. Am. Chem. Soc. 2001, 123, 9571−9576. (8) López, X.; Poblet, J. M. Inorg. Chem. 2004, 43, 6863−6865. (9) Zhang, F.; Guan, W.; Zhang, Y.; Xu, M.; Li, J.; Su, Z. Inorg. Chem. 2010, 49, 5472−5481. (10) López, X.; Bo, C.; Poblet, J.; Sarasa, J. P. Inorg. Chem. 2003, 42, 2634−2638. (11) Zhang, F.; Guan, W.; Yan, L.; Zhang, Y.; Xu, M.; HayfronBenjamin, E.; Su, Z. Inorg. Chem. 2011, 50, 4967−4977. (12) Vila-Nadal, L.; Mitchell, S. G.; Long, D.; Rodriguez-Fortea, A.; López, X.; Poblet, J. M.; Cronin, L. Dalton Trans. 2012, 41, 2264− 2271. (13) Andre Ohlin, C.; Rustad, J. R.; Casey, W. H. Dalton Trans. 2014, 43, 14533−14536. (14) Klemperer, W. G.; Marquart, T. A.; Yaghi, O. M. Angew. Chem. 1992, 104, 51−3. (See also Angew. Chem., Int. Ed. Engl., 1992, 31, 49− 51). (15) Livage, J. Coord. Chem. Rev. 1998, 178−180, 999−1018. (16) Hayashi, Y. Coord. Chem. Rev. 2011, 255, 2270−2280. (17) Gatteschi, D.; Pardi, L.; Barra, A. L.; Müller, A. Mol. Eng. 1993, 3, 157−169. (18) Gao, Y.; Chi, Y.; Hu, C. Polyhedron 2014, 83, 242−258. K


Article

Inorganic Chemistry (48) Franchini, M.; Philipsen, P. H. T.; Visscher, L. J. Comput. Chem. 2013, 34, 1819−1827. (49) Becke, A. D. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098− 3100. (50) Perdew, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33, 8822−8824. (51) van Lenthe, E.; Baerends, E. J. J. Comput. Chem. 2003, 24, 1142−1156. (52) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99, 4597−4610. (53) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1994, 101, 9783−9792. (54) van Lenthe, E.; Ehlers, A.; Baerends, E. J. Chem. Phys. 1999, 110, 8943−8953. (55) Klamt, A. J. Phys. Chem. 1995, 99, 2224−2235. (56) Pye, C. C.; Ziegler, T. Theor. Chem. Acc. 1999, 101, 396−408. (57) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623−11627. (58) Aparicio, P. A.; López, X.; Poblet, J. M. J. Mol. Eng. Mater. 2014, 2, 1440004. (59) Rodriguez-Fortea, A.; de Graaf, C.; Poblet, J. M. Chem. Phys. Lett. 2006, 428, 88−92. (60) Vankova, N.; Heine, T.; Kortz, U. Eur. J. Inorg. Chem. 2009, 5102−5108. (61) Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.; Echeverría, J.; Cremades, E.; Barragán, F.; Alvarez, S. Dalton Trans. 2008, 2832−2838. (62) Vila-Nadal, L.; Mitchell, S. G.; Markov, S.; Busche, C.; Georgiev, V.; Asenov, A.; Cronin, L. Chem. - Eur. J. 2013, 19, 16502−16511. (63) Keene, T. D.; D’Alessandro, D. M.; Kramer, K. W.; Price, J. R.; Price, D. J.; Decurtins, S.; Kepert, C. J. Inorg. Chem. 2012, 51, 9192− 9199. (64) Winkler, J. R.; Gray, H. B. Struct. Bonding (Berlin, Ger.) 2011, 142, 17−28. (65) Mbomekalle, I. M.; Cao, R.; Hardcastle, K. I.; Hill, C. L.; Ammam, M.; Keita, B.; Nadjo, L.; Anderson, T. M. C. R. Chim. 2005, 8, 1077−1086. (66) Keita, B.; Essaadi, K.; Nadjo, L.; Contant, R.; Justum, Y. J. Electroanal. Chem. 1996, 404, 271−279.

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