Structural and Electronic Properties of Polyoxovanadoborates Containing the [ V 12 B 18 O 60 ] Core in Different Mixed Valence States

This review summarizes all published data until April 2015 related to crystalline lattices formed by the [V12B18O60] core, which generates polyanionic clusters with different degrees of protonation and mixed-valence ratios. The negative charge of this cluster is counterbalanced by different cations such as protonated amines, hydronium, and alkaline, and transition metal ions. The cluster is shown to form extended 1D, 2D, or 3D frameworks by forming covalent bonds or presenting hydrogen bond interactions with the present secondary cations. These cations have little influence on the solid state reflectance UV-visible spectra of the polyanionic cluster, but are shown to modify the FT-IR spectra and the magnetic behavior of the different reported species. OPEN ACCESS Inorganics 2015, 3 310


Introduction
Polyoxometalates (POMs) have been systematically studied during the last decades due to their structural variety and physico-chemical properties [1][2][3][4][5][6][7][8][9][10][11][12][13].The structural variety of these polyanionic clusters is generated by their interaction with or bonding to different cationic species such as protonated amines, ammonium and hydronium ions, alkaline, transition metal, and lanthanide ions.These cationic species can exist in the lattice as charge-compensating agents, or linked to the external oxygen atoms of the polyanionic species, and thus serve to increase the dimensionality of these systems [14][15][16][17][18][19].The crystalline packing is stabilized by hydrogen bonds, in the case of ammonium cations, or by the formation of covalent bonds through the interaction of the external oxygen atoms of the polyanionic species with different metal cations, such as alkaline or transition metal ions.
As a result of V V and V IV sharing similar geometries, the redox processes between both states can be accessed by keeping the initial structure of the polyanion constant, thus generating several mixed valence systems.This feature is directly related to the magnetic and electronic properties shown by these compounds.However, these properties have been studied less extensively than the structural ones.In this review we report and discuss the structural and electronic properties of several polyanions derived from the [V12B18O60] core and their corresponding crystalline lattices.

Vanadate Fragments
Some selected examples of clusters have been analyzed in order to summarize the bond lengths given for the following fragments.
[V 6 O 18 ]-type A: The [V6O18]-type A hexanuclear fragment shown in Figure 1a consists of six (VO5) units connected by two equatorial oxygen atoms sharing two opposite edges to form a ring-like structure.In this moiety, the vanadium atoms are coplanar and the V=O groups are on the periphery.This hexanuclear ring is found in the structures formed by the [V6B20] and [V6B22] cores.The V=O bond lengths range from 1.605 to 1.625 Å, while the V-O distances are between 1.944 and 1.970 Å [30,33].
[V 6 O 18 ]-type B: The topology of this fragment changes from the above mentioned hexanuclear ring to a triangular array (Figure 1b), due to the different condensation of the (VO5) units, which are arranged in an alternated way of adjacent and opposite edges.This fragment can be found solely in the [V12B18] core.The V=O and V-O bonds lengths range from 1.599 to 1.638 Å, and between 1.906 and 2.023 Å, respectively [43,52].
[V 10 O 30 ]: This decanuclear moiety is similar to the [V6O18]-type A, where the (VO5) polyhedra share their opposite edges, forming a coplanar toroidal ring (Figure 1c).The compounds constituted by the [V10B28] core present this fragment in their structures.The V=O bond distances range from 1.599 to 1.640 Å; the V-O bond distances range from 1.894 to 1.989 Å [35,63].
[V 12 O 36 ]-type A: This dodecanuclear vanadate fragment can be seen as the union of two mutually perpendicular semicircles, each one formed by five (VO5) polyhedra linked by opposite edges.Both semicircles are connected by two additional (VO5) polyhedral, thus forming a continuous ring as shown in Figure 1d.This moiety is found in the compounds containing the [V12B16] and [V12B17] cores.The V=O bond distances are between 1.609 and 1.632 Å, while the V-O bond distances range from 1.919 to 2.019 Å [40,41].
[V 12 O 36 ]-type B: This dodecanuclear fragment is described as a planar ring, whose topology is similar to the above-mentioned [V6O18]-type A and [V10O30] rings (Figure 1e).This fragment can be found in the structures of compounds with the [V12B32] core.The V=O and V-O bonds lengths range from 1.569 to 1.649 Å, and between 1.890 and 1.990 Å, respectively [60,61].
As expected, the shorter distances are observed for the vanadyl groups (V=O), as compared to the single bonds (V-O).However, the analysis of the different vanadium-oxygen distances for all the vanadate fragments will depend on the mixed valence ratio, which is not the case for the borate fragments.Therefore a comparative analysis is not possible.

Borate Fragments
In order to describe the different borate units, the topological classification proposed by Christ and Clark will be used [64].Some selected examples of clusters have been considered in order to analyze the bond lengths given for the following fragments.
[B 10 O 22 ] 14− : This unit is composed by three (B3O8) 7− building-blocks connected to an additional (BO4) entity, located in the middle of the fragment, as shown in Figure 2a.The triangular decaborate unit has three trigonal boron atoms at the corner of the triangle, and seven tetrahedral boron atoms 10[3: ∆+7T].This decaborate is present in compounds with the [V6B20] core.The trigonal B-O bonds range from 1.349-1.391Å, while the tetrahedral ones are between 1.425 and 1.545 Å [30,33].
[B 14 O 32 ] 22− : The structure of this ring-like fragment is composed of eight tetrahedral and six trigonal boron atoms.Four (BO4) units form two pairs and these are linked by two (BO3) group forming a ring (Figure 2c); four additional (BO3) entities are terminal groups bridging each pair of (BO4) groups 14:2[2(3:∆+2T)+(3:∆)].The compounds containing the [V10B28] core have this tetradecaborate unit.The B-O bond distances of the trigonal units range from 1.338 and 1.391 Å, while the B-O bond distances of the tetrahedral units are between 1.422 and 1.500 Å [35,63].
[B 8 O 21 ] 21− : This polyanionic entity is the lowest borate nuclearity fragment described to date.This structure is formed by a chain of six tetrahedral (BO4) units.Two trigonal (BO3) units bridge the second/third and fourth/fifth tetrahedral units of the chain (Figure 2d) 8:2[4:∆+3T].This fragment is found in clusters with the [V12B16] core.The B-O bond distances of the trigonal and tetrahedral units are between 1.348 and 1.389 Å, and 1.416 and 1.530 Å, respectively [40,41].
Based on the abovementioned data, it is possible to infer that all the trigonal B-O bond distances are shorter than those corresponding to the tetrahedral ones.An exception is that of the [B16O36] 24− fragment, which has been reported in only two studies [60].

[V 12 B 18 O 60 ] Cores with Protonated Amines as Counterbalancing Ions
In this section lattices based on the [V12B18O60] core including protonated diamines, ammonium, and hydronium as counterbalancing ions will be described, and are listed in Table 1.Rijssenbeek et al. [42], in 1997, obtained by hydrothermal synthesis the first two VBO crystalline systems based on the [V12B18O60] core, with 1,2-ethylenediammonium (enH2) 2+ (1) and 1,3-propanediammonium (1,3-diapH2) 2+ (2) ions to compensate for the negative charge.The authors mentioned that the organic molecule is included in the synthetic medium as a structure-directing agent of the framework.In both ( 1) and ( 2), the diammonium ions occupy the intercluster space along with water solvation molecules.In 2011 Liu et al. [49] reported another lattice having 1,2-propanediammonium (1,2-dapH2) 2+ or (H2dap) 2+ (3), along with hydronium ions as charge counterbalance cations.(3) was also obtained by hydrothermal synthesis including additionally Cu(CH3COO)2 2H2O in the reaction mixture, a species that was not included in the final crystalline packing.However, the authors did not mention if the same lattice is formed leaving out the copper source.Bigger ammonium cations derived from triethylenetetramine (H3teta) 3+ together with hydronium ions were found in the crystalline system (4) based on the [V12B18O60] core, studied by Liu et al. in 2013 [34].The hydrothermal synthetic procedure used also included a secondary metal source, metallic cobalt powder.In this case, the authors pointed out that the presence of the transition metal is indispensable to obtain (4).In 2014 we reported a new crystalline system (5) containing (1,3-diapH2) 2+ and ammonium as counterbalancing ions.The synthesis was also carried out using the hydrothermal method, in which the ammonium ions were included in the lattice by adding (NH4)2HPO4 to the reaction mixture [57].
All the abovementioned compounds were described as having the same degree of protonation of the [V12B18O60] core, thus being based on the [V12B18O60H6] 1 °− polyanion.The five studied clusters have the same mixed valence ratio of V IV to V V of 10/2.
Table 1.List of the lattices with protonated amines as counterbalancing ions.The mixed valence V IV /V V ratio is indicated for each lattice.

Compound
Formula V IV /V V Ratio Ref.

[V 12 B 18 O 60 ] Cores with Transition Metal Ions and Coordination Compounds as Counterbalancing Cations
The lattices that include transition metal ions and coordination compounds, together with the [V12B18O60] core in their crystallographic packing, are listed in Table 2.In some cases the metal cations are found coordinated to the clusters through the oxygen atoms of the polyanions and to water molecules, while in other cases coordination complexes with organic molecules are bonded to the VBO clusters.Thus, this class of systems can be considered as functionalized polyoxovanadoborates.
Compound ( 12) has a pure inorganic framework that contains six-coordinated Cd II ions and crystallizes in the cubic centrosymmetric space group Pn-3.The asymmetric unit consists of a half of one Cd II ion, two vanadium atoms, and three boron atoms.The divalent cations are connected to the polyanions sharing μ3-bridge-oxygen atoms from the [B18O42] 30− and [V6O18] n− fragments of the [V12B18O60] clusters, leading to a porous 3D lattice.The coordination sphere of the Cd II ions is completed with oxygen atoms of water molecules.The Cd II -(μ3-O)-B2 distances range from 2.184(3) to 2.552(3) Å.Despite the fact that diethylenetriamine (dien) was added to the reaction mixture, this amine is not present in the crystalline system (12).
Very appealing crystalline structures are formed when metal cations do not act as bridges between clusters, but are only bonded to one [V12B18O60] cluster, thus decorating it as coordination complexes.

[V 12 B 18 O 60 ] Cores with Alkaline Ions as Counterbalancing Cations
Lattices that contain exclusively alkaline cations coordinated to the [V12B18O60] multi-dentate ligands are scarce, in comparison with the lattices that include transition metal ions and organic ammonium ions.
Hermosilla-Ibáñez et al. reported in 2014 the first framework of the VBO family ( 21) that contains the alkaline ions with the smallest ionic radius (Li + ).To the best of our knowledge, this is the sole example of a lattice with lithium counterions [58].This compound crystallizes in the centrosymmetric cubic space group Pn-3, being the first example of such high symmetry.The literature reports examples of crystalline systems with lower symmetries [51,57,67].Two of the three different crystallographic types of lithium cations are five-coordinated, and one is six-coordinated.The Li-O distances for the fivecoordinated ions range from 1.921(2) to 2.976(4) Å, and have values of 3.142(3) Å for the sixcoordinated Li + centers.Due to the long Li-O distances of the six-coordinated centers, the authors classified the observed distances as pseudo-coordinative interactions [58].
Table 3. List of the lattices with alkaline ions as counterbalancing cations.The mixed valence V IV /V V ratio is indicated for each lattice.

Coordination Geometry Analysis of the Counterbalancing Alkaline and Secondary Transition Metal Ions
On the basis of the crystallographic data included in the literature, we have calculated the best geometry for the alkaline and transition metal ions included in the corresponding crystalline lattices based on the [V12B18O60] core, using the SHAPE 2.1 program [69].To carry out this study, the maximum M-O distance used to defined the coordination sphere for M = Na + and K + is 3.1 Å.In (16) Zhou et al. considered a longer K-O coordination distance of 3.4 Å, thus defining a 10-coordinate mode for some of the potassium ions, which is not included in our analysis [67].
Among all the counterbalancing alkaline ions of the [V12B18O60] polyanions, Li + is found to be five-coordinated, with a square pyramidal geometry (SPY-5) [58], while Cs + is eight-coordinated in a hexagonal bipyramidal geometry (HBPY-8) [57].On the other hand, Na + and K + are found with more than one coordination number.As we reported earlier for (14), hexa-coordinated Na + ions are found with octahedral (OC-6) and trigonal prismatic (TPR-6) geometries [58].Nevertheless, extra geometries for the [NaOx] are determined by the different coordination numbers (x = 5 and 6) in the other systems based on the [V12B18O60] cluster that contain sodium ions in their crystalline packing.As expected, potassium ions with a bigger ionic radius than sodium ions present coordination numbers from six to nine, as has been previously found in the literature [70][71][72].Three different geometries are found when K + are six-coordinated, two when seven-coordinated, two when eight-coordinated, and one when nine-coordinated.The corresponding geometries are listed in Table 5.In the case of Mn II , Ni II , Cu II , and Zn II , four-and six-coordination is found, as can be seen in Table 5.Only the six-coordinated manganese ions, which occupy only one crystallographic site in framework (40), adopt the trigonal prismatic geometry (TPR-6).The six-coordinated Ni II (( 8), ( 28), ( 29) and ( 41)), Cu II (( 7), (11), (24), (25), and ( 26)) and Zn II (( 9), ( 10), ( 13), (31), and (38)) share an octahedral geometry (OC-6).The square planar geometry mode (SP-4) is only found for Cu II included in lattices (24) and (25).Zn II presents the highest plasticity among the other transition metal ions, with coordination numbers four, five, and six.The four-coordinated Zn II ions are in a tetrahedral geometry (T-4), whereas the five-coordinated Zn II ions are found to be in a square pyramidal geometry (SPY-5) and vacant octahedral (vOC-5) geometries.

Spectroscopic Properties
The FT-IR fingerprint region characteristic of the [V12B18O60] core is observed between ca.640 and 1420 cm −1 .The asymmetric and symmetric V-O-V stretching vibrations appear in the low energy region between 640 and 880 cm −1 , whereas the bands observed in the range of 900 and 960 cm −1 are assigned to the terminal V-O stretching vibrations of the vanadyl group.On the other hand, the borate fragments are characterized by B-O asymmetrical stretching vibrations for both the [BO3] and [BO4] units, appearing between 1020 and 1150 cm −1 for the trigonal and between 1300 and 1420 cm −1 for the tetrahedral units [34,43,44,[48][49][50][52][53][54][55]57,58,63,67,68].Müller et al. reported that the energy and shape of the vanadyl stretching bands depend on the oxidation state and on the existing interactions of the vanadyl groups of the polyoxovanadate anions in the crystalline packing [26].In the IR spectra reported for all the studied systems, only (11) presents a sharp stretching vibration of the terminal V-O group.This fact can be rationalized considering that in this framework the polyanion has all the vanadyl groups equally connected to [Cu(dien)(H2O)] 2+ complexes.
The optical properties of compounds ( 4), ( 5), ( 7), ( 9), ( 10), ( 12), ( 15), ( 16), ( 18), ( 19), ( 20), ( 21), ( 27), ( 29), (30), (35), and (38) have been studied by solid-state diffuse reflectance spectroscopy in the UV-visible region, since they are all insoluble in most common organic solvents and water.Three bands in the studied UV-visible region are reported for almost all the investigated systems: two bands in the high energy region, between 243 and 230 nm, and 344 and 310 nm, depending on the species, and one band that appears between 590 to 517 nm.In general, the two absorption bands in the high energy region are assigned as O→V and O→B charge transfer transitions, respectively.The less intense band in the low energy region has been assigned to Intervalence Charge Transfer Transitions (IVCT) and to d-d transitions [34,48,[52][53][54][55]57,58,63,67].With respect to the low energy absorption bands (ca.500 nm), most authors consider that these arise from "presumably d-d electronic transitions" [34,53,54,63].With respect to the bands in this same visible region of the polymetallic vanadium species, Robin and Day consider that they should be assigned to mixed valence absorptions [73].However, the real meaning of these bands should become apparent once a more complete electronic description has been attained from quantum mechanical calculations.We are currently calculating the electronic spectra of these species by DFT methods.
Considering the similarity of the UV-visible spectra of these systems even when the polyanions are functionalized with secondary transition metal atoms [52], we can deduce that the crystalline lattices have a negligible effect on the electronic properties of the [V12B18O60] core.

Magnetic Properties
Among all the studied compounds included in this review, the magnetic properties of only ( 5), ( 11), ( 12), ( 15), ( 18), ( 19), (27), and (38) have been reported [53][54][55]57,63].All of these systems have a mixed valence ratio of 10V IV /2V V , and present a bulk antiferromagnetic behavior.The χT values at 300 and 2 K for the abovementioned compounds are listed in Table 6.(11) presents the highest χT value of 4.81 emu K mol −1 , which was explained by Zhou et al., assuming that this value is very close to the theoretical χT value of 4.88 emu K mol −1 , considering 10 uncoupled V IV plus three uncoupled Cu II centers (g = 2.00 for both atoms) [55].However, when the χT value of the three uncoupled Cu II centers is subtracted (g = 2.00), the resultant χT value for the [V12B18O60] polyanion of ( 11) is 3.68 emu K mol −1 , thus presenting the same trend followed by ( 5), ( 12), ( 15), ( 18), (19), and (27) (Figure 6).From the χT(T) graph reported by Zhou et al. [53] it is possible to infer that ( 27) is almost magnetically uncoupled at room temperature.On the other hand, the χT values of the rest of compounds, (5), ( 12), ( 15), (18), and (19) show that all of them are magnetically coupled at room temperature.Within the observed tendency of 3.34 to 3.83 emu K mol −1 , there is no clear correlation between the magnetic properties and the nature of the frameworks, which include different cations interacting with the polyanions, even though it is clear that the interactions between the different cations and the polyanion in the lattice affect the magnitude of the exchange phenomena in the cluster.
It is interesting to point out that Hermosilla-Ibáñez et al. reported in 2014 that it is possible to show by DFT calculations that the alkaline ions in compounds ( 18) and ( 19) quench the intracluster antiferromagnetic coupling, in comparison with compound (5) [57].In this study, the results indicated that the presence of alkaline ions perturbs the extent of the spin density of the magnetic orbitals (dxy); this perturbation is dependent on the distance between the alkaline cation and the oxygen of the vanadyl groups.Thus, the obtained modification of the orbital overlap due to the presence of the alkaline cations influences the magnitude of the antiferromagnetic interactions.Nevertheless, the existence of additional hydrogen bonds and/or covalent bonds should also influence the global magnetic properties.
As can be seen, the most coupled system is (38), which includes Zn(H3tepa) 2+ and (enH2) 2+ as counterbalancing ions.In this system the Zn(H3tepa) 2+ complexes are coordinated to the [V12B18O60] polyanions through two oxygen atoms of vanadyl groups from adjacent polyoxovanadoborates, i.e., acting as bridges between two polyanions.As discussed above, the coordination of the vanadyl groups with a cation influences the electronic properties, i.e. stretching vibrations and exchange interactions.In (38) the presence of the zinc(II) cations bridging the polyanion clearly increases the antiferromagnetic behavior of the material.* The χT value in parentheses is the χT value of the polyanion, which was calculated by subtracting the χT value for the three uncoupled Cu II centers, considering a g = 2 (χT = 1.13 emu K mol −1 ).

Final Remarks
The structural stability of the [V12B18O60] core allows the formation of polyoxometalate species with different crystalline lattices, depending on the cations present in the synthesis.This polyanion is potentially able to share bridging B-O-B oxygen atoms, both vanadyl and bridging B-O-B oxygen atoms, and in some cases, the 12 oxygen atoms from the vanadyl groups, thus permitting one-, two-, or three-dimensional frameworks to be obtained.The presence of auxiliary cations may be responsible for the alignment of the organic amines in only one direction, since they can fill hindered nucleophilic sites around the polyanions.
Hermosilla-Ibáñez et al. demonstrated that the organic diamines act as reducing agents in the reactions, as the presence of nitrate ions in the final mother liquors was detected by ionic liquid chromatography [57].The FT-IR results show that the coordination of a cation to each of the existing vanadyl groups of the polyanion produces a single and sharp stretching band for the vanadyl group.Thus, the coordination of the vanadyl groups with cations influences the electronic properties, i.e., stretching vibrations and exchange interactions.However, the similarity of the solid state reflectance spectra indicates that the crystalline lattices have a negligible effect on the electronic spectra of the [V12B18O60] core.
From the reported magnetic data it is clear to conclude that the [V12B18O60H6] cluster with a 10V IV /2V V mixed-valence ratio presents a global antiferromagnetic exchange among the 10 spin carriers.It can also be concluded that the interactions of the cations in the crystal packing with the polyanion can modify the global antiferromagnetic interaction in the polyanion.Further studies must be done in order to reach a deeper understanding of the magnetic behavior in these compounds.At this time our group is working on the rationalization of the magnetic properties of the [V12B18O60] family.

Figure 3
Figure 3 shows the different polyoxovanadoborate clusters [VxByOz] generated by the condensation of the [ViOj] and [BhOk] fragments described above.

Figure 3 .
Figure 3. Structural representation of the different polyoxovanadoborate cores.

3 .
Structural Description of the [V 12 B 18 O 60 ] Core The [V12B18O60] polyoxovanadoborate core consists of two vanadate [V6O18]-type B and one [B18O42] 3 °− borate fragments.Each vanadate fragment has six five-coordinated (VO5) vanadium centers adopting a [4+1] square base pyramidal coordination geometry.The vanadium atom is displaced from the best mean plane formed by the four equatorial oxygen atoms towards the axial vanadyl group, by ca.0.7 Å.The angles formed between the V=O bond and the four equatorial V-O bonds are from 100 to 110°.All the V=O distances are in the range of 1.57 Å to 1.68 Å [65].The borate fragment [B18O42] 30− is condensed to the two [V6O18]-type B moieties, thus remaining in a sandwich-type configuration in the middle of the [V12B18O60] polyanion (Figure 3f).A water molecule is always found (with partial occupancy most of the time) within the cavity of the [V12B18O60] polyanion.

Table 2 .
List of the lattices with transition metal ions and coordination compounds as counterbalancing ions.The mixed valence V IV /V V ratio is indicated for each lattice.Calculated by us according to the stoichiometric formula given by the authors.

V 12 B 18 O 60 ] Cores with Organic Ammonium, Alkaline, and/or Transition Metal Ions as Counterbalancing Cations: The Mixed FamilyTable 4 .
List of the lattices with organic ammonium, alkaline, and/or transition metal ions as counterbalancing cations.The mixed valence V IV /V V ratio is indicated for each lattice.

Table 5 .
List of the best geometries estimated for the alkaline and transition metal ions, using SHAPE 2.1.

Table 6 .
List of the reported compounds with magnetic property studies.