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Review

Structural and Electronic Properties of Polyoxovanadoborates Containing the [V12B18O60] Core in Different Mixed Valence States

by
Patricio Hermosilla-Ibáñez
1,2,
Karina Muñoz-Becerra
1,2,
Verónica Paredes-García
2,3,
Eric Le Fur
4,
Evgenia Spodine
2,5,* and
Diego Venegas-Yazigi
1,2,*
1
Facultad de Química y Biología, Universidad de Santiago de Chile, USACH, Av. Libertador Bernardo O’Higgins 3363, 9170022, Santiago, Chile
2
Centro para el Desarrollo de la Nanociencia y Nanotecnología, CEDENNA, 9170022, Santiago, Chile
3
Departamento de Ciencias Químicas, Universidad Andres Bello, Republica 275, 8370146, Santiago, Chile
4
ENSCR, UMR 6226, 11, allée de Beaulieu - CS 50837-35708 Rennes Cedex 07, France
5
Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile, Sergio Livingstone 1007, Independencia, 8380492, Santiago, Chile
*
Authors to whom correspondence should be addressed.
Inorganics 2015, 3(3), 309-331; https://doi.org/10.3390/inorganics3030309
Submission received: 15 April 2015 / Revised: 29 May 2015 / Accepted: 5 June 2015 / Published: 3 July 2015
(This article belongs to the Special Issue Polyoxometalates)
Browse Figures
Versions Notes

Abstract

:
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.

1. 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.
The polyoxovanadates (VO) constitute an appealing family of polyanions, due principally to the structural plasticity that vanadium atoms present in their different oxidation states. The variable topology of the polynuclear VO systems is given by the {VxOy} entities with diverse connectivities. The literature reports polyoxovanadates containing from five to 34 metal centers: [V5O14]3− [20], [V10O28]6− [21], [V12O32]4− [22], [V13O34]3− [23], [V15O42]9− [24], [V15O36]5− [25], [V18O42]12− [26], [V19O49]9− [27], [V22O54]6− [28], and [V34O82]10− [29]. When a heteroatom such as boron is condensed in these systems, the structural richness of the VO cluster is increased; this new family of polyoxometalates is termed polyoxovanadoborates (VBO). To date the known polyanions included in the VBO family are [V6B20] [30,31,32,33], [V6B22] [34], [V10B28] [35,36,37], [V12B16] [38,39,40,41], [V12B17] [42], [V12B18] [31,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59], and [V12B32] [60,61]. The first VBO polyanions were reported at the end of the 1990s by Rijssenbeek et al. [42], and in 2000 Williams et al. published the first review on vanadoborate clusters as a chapter in Contemporary Boron Chemistry [62].
As a result of VV and VIV 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.

2. Polyoxovanadoborates: Vanadate and Borate Fragments

The structures of all the VBO compounds reported in the literature are formed by the condensation of vanadate and borate fragments. The vanadate fragments always contain five-coordinated vanadium atoms (VO5) in a [4+1] square base pyramidal coordination geometry, while the borate fragments include trigonal (BO3) and tetrahedral (BO4) borate units. The different connectivity between the (VO5), (BO3), and (BO4) units leads to varied VBO cores with open barrel-like structures ([V10B28] and [V12B32]), and closed spherical-like structures ([V6B20], [V6B22], [V12B16], [V12B17], and [V12B18]). Among all the abovementioned clusters, [V12B18] is the one most studied from a structural point of view.

2.1. Vanadate Fragments

Some selected examples of clusters have been analyzed in order to summarize the bond lengths given for the following fragments.
[V6O18]-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].
[V6O18]-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].
[V10O30]: 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].
[V12O36]-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].
[V12O36]-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.
Inorganics 03 00309 g001 1024
Figure 1. Polyhedral representation of the vanadate fragments.
Figure 1. Polyhedral representation of the vanadate fragments.
Inorganics 03 00309 g001

2.2. 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.
[B10O22]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 1.349-1.391 Å, while the tetrahedral ones are between 1.425 and 1.545 Å [30,33].
[B11O24]15−: This undecaborate can be described as constituted by the abovementioned [B10O22]14− fragment with an extra (BO3) unit linked to the apical oxygen atom of the central tetrahedral (BO4) group (Figure 2b) 11[4:∆+7T]. This polyborate fragment has been described in compounds formed by the [V6B22] core. The trigonal and tetrahedral B-O bond distances range from 1.359 to 1.376 Å, and 1.430 to 1.546 Å, respectively [34].
[B14O32]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].
[B8O21]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].
[B18O42]3°−: This polyborate ring forms part of the compounds containing the [V12B18] core. It is made of six (B3O7)5− units, each one constituted by one trigonal and two tetrahedral boron atoms. Each (B3O7)5− building-block contains one terminal (BO3) group, and therefore each [B18O42]3°− fragment has six peripheral (BO3) units, 18[6: ∆+12T] (Figure 2e). The trigonal B-O bonds range from 1.340 to 1.382 Å, while the tetrahedral ones are between 1.420 and 1.525 Å [43,52].
[B16O36]24−: This polyborate ring is formed of four pairs of (BO4) units linked by four (BO3) trigonal entities; each pair is additionally condensed to a terminal trigonal BO3, 16[8:∆+8T] (Figure 2f). This fragment is part of the [V12B32] family. The B-O bond distances are between 1.310 and 1.450 Å for the trigonal units, and between 1.410 and 1.530 Å for the tetrahedral units [60,61].
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].
Inorganics 03 00309 g002 1024
Figure 2. Polyhedral representation of the borate fragments.
Figure 2. Polyhedral representation of the borate fragments.
Inorganics 03 00309 g002
Figure 3 shows the different polyoxovanadoborate clusters [VxByOz] generated by the condensation of the [ViOj] and [BhOk] fragments described above.
Inorganics 03 00309 g003 1024
Figure 3. Structural representation of the different polyoxovanadoborate cores.
Figure 3. Structural representation of the different polyoxovanadoborate cores.
Inorganics 03 00309 g003

3. Structural Description of the [V12B18O60] 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.

4. [V12B18O60] 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 VIV to VV of 10/2.
Lattices (1), (3), (4), and (5) present one crystallographic site for the protonated diamine, while (2) presents three crystallographically different sites for the (1,3-diapH2)2+ cations. The (1,3-diapH2)2+ ions in lattices (2) and (5) adopt a “W”-type conformation [66]. In lattices (2) and (3), the diammonium cations do not adopt a preferential order in the lattice, whereas the (H3teta)3+ molecules in framework (4) are defined by the authors as forming pseudo-hexagonal channels [34]. In framework (5), the (1,3-diapH2)2+ cations are described as all oriented along the c axis [57]. Within all the crystalline lattices, each protonated diamine connects four [V12B18O60H6]1°− clusters through unidirectional, bifurcated, and trifurcated hydrogen bonds, thus generating supramolecular structures [59].
Table
Table 1. List of the lattices with protonated amines as counterbalancing ions. The mixed valence VIV/VV ratio is indicated for each lattice.
Table 1. List of the lattices with protonated amines as counterbalancing ions. The mixed valence VIV/VV ratio is indicated for each lattice.
CompoundFormulaVIV/VV RatioRef.
1(enH2)5{(VO)12O6[B3O6(OH)]6} H2O10/2[42]
2(1,3-diapH2)5{(VO)12O6[B3O6(OH)]6} 6H2O10/2[42]
3(H2dap)2H6{(VO)12O6[B3O6(OH)]6(H2O)} 13H2O10/2[49]
4[H3teta]3[V12B18O54(OH)6(H2O)] (H3O) 5H2O10/2[34]
5(NH4)8(1,3-diapH2)[V12B18O60H6] 5H2O10/2[57]

5. [V12B18O60] 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 CdII ions and crystallizes in the cubic centrosymmetric space group Pn-3. The asymmetric unit consists of a half of one CdII 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 CdII ions is completed with oxygen atoms of water molecules. The CdII-(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. The first system of this kind is compound (6), reported by Zhang et al. in 1999 [43]. This compound includes five-coordinated Zn(en)22+ cations, whose coordination sphere is completed by coordination to the VBO polyanion through one oxygen atom from the polyborate fragment, forming a Zn-(μ3-O)-B2 covalent bond (Zn-O; 2.042(2) Å) (Figure 4a). Lin et al. [45] reported compound (8), which has six-coordinated Ni(en)22+ complexes. These complete their coordination sphere with one VBO cluster bonded through two oxygen atoms, thus forming two Ni-(μ3-O)-B2 bonds (Ni-O; 2.086(4) to 2.224(4) Å) (Figure 4b). Zn(teta)22+ complexes are part of the crystalline system of (9) (Figure 4c) reported by Liu and Zhou [48], while Zn(dien)22+ and [Zn(dien)2(H2O)]2+ complexes are introduced in the lattice of compound (10) (Figure 4d), reported by Liu et al. [49]. The M-O distances of the M-(μ3-O)-B2 bonds have values of 1.979(2) Å for (9) and 2.001(4) to 2.436(5) Å for (10).
Table
Table 2. List of the lattices with transition metal ions and coordination compounds as counterbalancing ions. The mixed valence VIV/VV ratio is indicated for each lattice.
Table 2. List of the lattices with transition metal ions and coordination compounds as counterbalancing ions. The mixed valence VIV/VV ratio is indicated for each lattice.
CompoundFormulaVIV/VV RatioRef.
6[Zn(en)2]6[(VO)12O6B18O39(OH)3] 13H2O9/3[43]
7H3{[Cu(en)2]5[(VO)12O6B18O42]}[B(OH)3]2 16H2O7/5 †[44]
8[Ni(en)2]6H2[(VO)12O6B18O42] 15H2O8/4[45]
9[Zn(teta)]6[(VO)12O6B18O36(OH)6](H2O) 8H2O12/0[48]
10{[Zn(dien)]2[Zn(dien)(H2O)]4(VO)12O6[B3O6(OH)]6(H2O)}2 15H2O12/0[49]
11{[Cu(dien)(H2O)]3V12B18O54(OH)6(H2O)} 4H3O 5.5H2O10/2[55]
12{[Cd(H2O)2]3V12B18O54(OH)6(H2O)} 4H3O 9.5H2O10/2[55]
13[Zn(H2teta)2V12B18O54(OH)6] 4H3O10/2[63]
† Calculated by us according to the stoichiometric formula given by the authors.
Six M(en)22+ moieties (M = Zn, Ni) are coordinated to each VBO polyanion in (6) and (8), while six Zn(teta)2+ units are bonded in (9). In (10) there are two Zn(dien)2+ and four [Zn(dien)(H2O)]2+ entities coordinated to the same [V12B18O60] core (Figure 4). The fact that (10) presents six different crystallographic positions for the zinc complexes is concordant with its low symmetry, the triclinic crystalline system (P-1). Crystalline system (6) is rhombohedral, whereas (8) and (9) both crystallize in a trigonal system presenting one crystallographic site for the metal-amine entities.
Inorganics 03 00309 g004 1024
Figure 4. The [V12B18O60] core with the complexes: (a) Zn(en)22+ for (6), (b) Ni(en)22+ for (8), (c) Zn(teta)2+ for (9), and (d) Zn(dien)2+ and [Zn(dien)(H2O)]2+ for (10).
Figure 4. The [V12B18O60] core with the complexes: (a) Zn(en)22+ for (6), (b) Ni(en)22+ for (8), (c) Zn(teta)2+ for (9), and (d) Zn(dien)2+ and [Zn(dien)(H2O)]2+ for (10).
Inorganics 03 00309 g004
Complexes Cu(en)22+, [Cu(dien)(H2O)]2+, and Zn(H2teta)22+ are part of the crystalline lattices of (7), (11), and (13), respectively, each of them coordinated to the [V12B18O60] clusters. However, in these lattices the metal-complexes are linking adjacent polyoxovanadoborate cores though V-(μ2-O)-MII-(μ2-O)-V bonds. In (7) and (11) four and six [V12B18O60] polyanions, respectively, are connected through this type of bonds, while in (13) only two adjacent clusters are linked by the above-mentioned type of bond. The M-O distances range from 2.464(4) to 2.536(4) Å in (7), and 2.033(8) to 2.166(7) Å in (13), while for (11) all distances are 2.292(5) Å.

6. [V12B18O60] 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. To the best of our knowledge, five of the eight reported inorganic frameworks included in this classification, and listed in Table 3, have been published by our research group. Brown et al. reported in 2011 the first framework that includes only Na+ acting as charge-compensating ions and coordinated to the [V12B18O60] core (14) [51], while one year later Zhou et al. published the same compound (14), as (15) [54]. The crystalline lattices (18), (19), (20), and (21) were obtained using the same hydrothermal synthesis conditions as for (14). In comparison with the aforementioned systems, compounds (15), (16), and (17) were obtained by adding auxiliary reducing agents to the reaction mixtures (Na2SO3, K2SO3, Ni, Co). Despite this fact, all the studied crystalline lattices included in this section present the same 10VIV/2VV mixed valence ratio. Sodium ions from the isostructural crystalline lattices (14) and (15) are coordinated to six oxygen atoms in a distorted octahedral coordination environment, with Na-O bond lengths ranging from 2.218(5) to 3.040(4) Å for (14) and 2.265(3) to 2.843(1) Å for (15). Zhou et al. mentioned that the alternating -Na-O-Na- connectivities in (15) generate 14-ring channels in the (100) direction, where the [V12B18O60] clusters are found, thus permitting a 3D growth of the inorganic framework [54]. Lattices (16) to (19) contain only potassium cations coordinated to oxygen atoms from the VBO polyanonic ligands and/or from water molecules. These ions are found in the form of [KOx] units in different crystallographic sites and with different coordination geometries. [KOx] units with x = 6, 7, 8, 9, and 10 are found in (16) and (17), with K-O distances ranging from 2.610(2) to 3.420(6) Å. In compounds (18) and (19), as reported by Hermosilla-Ibáñez et al. [57], the K+ ions are coordinated to six and seven oxygen atoms, with K-O bonds ranging from 2.468(3) to 3.060(2) Å and 2.625(5) to 3.064(3) Å, respectively. (20) has two different alkaline ions (K+ and Cs+) in its crystalline lattice. The potassium cations are found to be six- and seven-coordinated (K-O; 2.662(5) to 3.408(4) Å), while cesium cations are always eight-coordinated (Cs-O; 2.970(3) to 3.516(4) Å).
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 five-coordinated ions range from 1.921(2) to 2.976(4) Å, and have values of 3.142(3) Å for the six-coordinated 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
Table 3. List of the lattices with alkaline ions as counterbalancing cations. The mixed valence VIV/VV ratio is indicated for each lattice.
Table 3. List of the lattices with alkaline ions as counterbalancing cations. The mixed valence VIV/VV ratio is indicated for each lattice.
CompoundFormulaVIV/VV RatioRef.
14(Na)10[(H2O)V12B18O60H6] 18H2O10/2[51]
15{Na2B18V12O54(OH)6(H2O)[Na8(H2O)16]} 2H2O10/2[54]
16{K2V12B18O54(OH)6(H2O)[K8(H2O)16]} 3H2O10/2[67]
17{K10V12B18O54(OH)6(H2O)} 14H2O10/2[67]
18K8(NH4)2[V12B18O60H6] 18H2O10/2[57]
19K10[V12B18O60H6] 10H2O10/2[57]
20K8Cs2[V12B18O60H6] 10H2O10/2[57]
21Li8(NH4)2[V12B18O60H6] 8.02H2O10/2[58]

7. [V12B18O60] Cores with Organic Ammonium, Alkaline, and/or Transition Metal Ions as Counterbalancing Cations: The Mixed Family

Table
Table 4. List of the lattices with organic ammonium, alkaline, and/or transition metal ions as counterbalancing cations. The mixed valence VIV/VV ratio is indicated for each lattice.
Table 4. List of the lattices with organic ammonium, alkaline, and/or transition metal ions as counterbalancing cations. The mixed valence VIV/VV ratio is indicated for each lattice.
CompoundFormulaVIV/VV RatioRef.
22(enH2)4Na4H3[(V12O6B18O42] 8H2O9/3[46]
23K3Na5(H2NCH2CH2NH3)2{(VO)12O6[B3O6(OH)]6}(H2O) 12H2O10/2[47]
24Na8[Cu(en)2]2[V12B18O60H6](NO3)2 14.7H2O8/4[51]
25Na7[Cu(en)2]2[V12B18O60H6](NO3) 15.5H2O8/4[51]
26[Na(H2O)]2[Na(H2O)2]2[Cu(en)2][V12B18O54(OH)6] (H3O)2 (H2O)188/4[50]
27{[Na(H2O)4]3[V12B18O54(OH)6(H2O)]2}(H4tren)4 (H3O) 41H2O10/2[53]
28Na8(H3O){[Ni(H2O)5][V12B18O60H6]} 12.5H2O11/1[52]
29Na5(H3O)4{[Ni(H2O)3(en)][V12B18O60H6]} 9H2O11/1[52]
30Na9(H3O){Zn0.5[V12B18O60H6]} 11H2O11/1[52]
31[Hen][H2en]{[Zn(en)2]3[V12B18O60H6]} 3H2O9/3[52]
32{[Na(H2O)3]4Na2V12B18O56(OH)4(H2O)}(H3dien)210/2[55]
33{V12B18O54(OH)6(H2O)[K6(H2O)12]} 2(H2dien) 3H2O10/2[67]
34K6(CH3NH3)4[V12B18O54(OH)6(H2O)] 2en 12H2O10/2[68]
35K(H3O)(enH2)4[V12B18O60H6] 9.60H2O10/2[58]
36K5(H3O)2(1,3-diapH2)2[V12B18O60H6] 10.8H2O11/1[59]
37K2(H3O)7(enH2)[V12B18O60H6] 9.0H2O11/1[59]
38[Zn(H3tepa)V12B18O54(OH)6][H2en]2 H3O 3H2O10/2[63]
39[V12B18Zn3O63H12] 3(C4N3H16) 10NH4 5H2O-[56]
40[V12B18Mn3O63H12] 3(C4N3H16) 10NH4 5H2O-[56]
41[V12B18Ni3O63H12] 3(C4N3H16) 10NH4 5H2O-[56]
The systems that include organic ammonium ions along with alkaline and/or transition metal ions in their frameworks are listed in Table 4. (22), (23), (27), (32), (33), (34), (35), (36), and (37) contain alkaline ions and protonated amine ions with different degrees of protonation. Sodium cations are present in the crystalline lattices of (22), (27), and (32) together with protonated ethylenediamine (en), tris(2-aminoethyl)amine (tren), and diethylenetriamine (dien) molecules, respectively. Lin et al. classified framework (22) as a two-dimensional lattice built of Na+ coordinated to the VBO clusters [46], while (27) is described by Zhou et al. as a 1D crystalline system considering the Na-POM connectivities [53]. The coordination of Na+ to oxygen atoms of the polyanonic clusters, together with the hydrogen bond interactions between the protonated amines and the POM, stabilize the three-dimensional lattice (32). Additionally to the Na+, (23) includes K+ in its 3D crystalline lattice along with monoprotonated ethylenediamine (Hen). In this compound, sodium ions are seven-coordinated (Na-O: 2.335 to 3.500 Å), whereas potassium cations are nine-coordinated (K-O: 2.640 to 3.302 Å). Systems (33), (34), (35), (36), and (37) contain K+ along with protonated amine ions in their frameworks. Diprotonated dien is found in (33) while diprotonated en is present in (35) and (37); lattice (36) contains 1,3-propanediammonium cations. Methylammonium cations are present in framework (34) due to the decomposition of the ethylenediamine added to the reaction mixture [68]. Potassium cations are coordinated to different oxygen atoms of the vanadyl groups and borate fragments (terminal and bridging groups), and from water molecules, being six-, seven-, eight-, and nine-coordinated, with K-O distances ranging from 2.507(2) to 3.430(7) Å in (33), 2.729(3) to 3.326(3) Å in (34), 2.774(5) to 2.865(5) Å for (35), 2.552(6) to 3.108(6) Å for (36), and 2.140(2) to 3.090(7) Å for (37). The crystalline lattice of the abovementioned systems ((33)–(37)) is stabilized by the coordination of the potassium cations to the polyoxometalate anions, and by the different modes of hydrogen bonds (unidirectional, bifurcated, and trifurcated) between the protonated amine molecules and the VBO clusters. In the crystalline packing of (35), (36), and (37), additional auxiliary cations of hydronium ions also stabilize the lattices. The bond valence sum calculation indicated that the VBO clusters in (33), (34), and (35) have a mixed valence ratio of 10VIV/2VV, while in (36) and (37) the ratio is 11VIV/1VV.
Inorganics 03 00309 g005 1024
Figure 5. The [V12B18O60] core bonded to: (a) Ni(H2O)52+ in (28) and (b) [Ni(H2O)3(en)]2+ in (29).
Figure 5. The [V12B18O60] core bonded to: (a) Ni(H2O)52+ in (28) and (b) [Ni(H2O)3(en)]2+ in (29).
Inorganics 03 00309 g005
Unlike the crystalline systems previously mentioned within this section that contain protonated amine and alkaline cations, (28), (29), (30), (31), and (38) include coordination complexes of a secondary transition metal ion. Hermosilla-Ibáñez et al. in 2012 reported four new compounds based on the [V12B18O60] core, whose negative charge is stabilized by Ni(H2O)52+, Na+ and H3O+ in (28), by [Ni(H2O)3(en)]2+, Na+ and H3O+ in (29), by ZnII, Na+ and H3O+ in (30), and by Zn(en)22+, Hen+ and H2en2+ in (31) [52]. Each of the Ni(H2O)52+ and [Ni(H2O)3(en)]2+ complexes in (28) and (29), respectively, are coordinated to the [V12B18O60] cluster through only one oxygen atom from a vanadyl group (Figure 5). This connection type differs from system (8) described above and reported by Lin et al. [45], in which six Ni(en)22+ complexes are connected to two oxygen atoms from the borate fragment (Ni-(μ3-O)-B2) of the VBO polyanion (Figure 4b). Each zinc cation with partial occupancy (0.5) in (30) is four-coordinated to two vanadyl oxygen atoms and to two B-O-B oxygen bridges from two adjacent polyoxovanadoborates (Zn-O distances of 2.337(7) and 2.457(7) Å). Lattices (31) and (38) contain six-coordinated zinc complexes bonded to the [V12B18O60] clusters. Two en molecules and two oxygen-bridging atoms (B-O-B) from the same polyanion are bonded to the ZnII in (31), while two tetraethylenepentamine (tepa) molecules, in addition to two vanadyl oxygen atoms from adjacent polyanions, are coordinated to the ZnII in (38). The electroneutrality of both compounds is attained by protonated ethylenediamine ions, which allow the stabilization of the crystalline frameworks by hydrogen bonds. Additionally, (31) can be compared with compound (6) due to the fact that the latter contains six Zn(en)22+ complexes connected to one [V12B18O60] core, while (31) contains only three Zn(en)22+ complexes coordinated to the same polyoxometalate.

8. 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 MnII, NiII, CuII, and ZnII, 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 NiII ((8), (28), (29) and (41)), CuII ((7), (11), (24), (25), and (26)) and ZnII ((9), (10), (13), (31), and (38)) share an octahedral geometry (OC-6). The square planar geometry mode (SP-4) is only found for CuII included in lattices (24) and (25). ZnII presents the highest plasticity among the other transition metal ions, with coordination numbers four, five, and six. The four-coordinated ZnII ions are in a tetrahedral geometry (T-4), whereas the five-coordinated ZnII ions are found to be in a square pyramidal geometry (SPY-5) and vacant octahedral (vOC-5) geometries.
Table
Table 5. List of the best geometries estimated for the alkaline and transition metal ions, using SHAPE 2.1.
Table 5. List of the best geometries estimated for the alkaline and transition metal ions, using SHAPE 2.1.
CationGeometryGeometry Symbol
LiSquare Pyramid(SPY-5)
NaVacant Octahedron(vOC-5)
Trigonal Bipyramid(TBPY-5)
Square Pyramid(SPY-5)
Pentagonal Pyramid(PPY-6)
Octahedron(OC-6)
Trigonal Prism(TPR-6)
KPentagonal Pyramid(PPY-6)
Octahedron(OC-6)
Trigonal Prism(TPR-6)
Capped Octahedron(COC-7)
Capped Trigonal Prism(CTPR-7)
Square Antiprism(SAPR-8)
Triangular Dodecahedron(TDD-8)
Tricaped Trigonal Prism(TCTPR-9)
CsHexagonal Bipyramid(HBPY-8)
MnTrigonal Prism(TPR-6)
NiOctahedron(OC-6)
CuSquare(SP-4)
Octahedron(OC-6)
ZnTetrahedron(T-4)
Square Pyramid(SPY-5)
Vacant Octahedron(vOC-5)
Octahedron(OC-6)

9. 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.

10. 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 10VIV/2VV, 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 VIV plus three uncoupled CuII centers (g = 2.00 for both atoms) [55]. However, when the χT value of the three uncoupled CuII 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.
Table
Table 6. List of the reported compounds with magnetic property studies.
Table 6. List of the reported compounds with magnetic property studies.
CompoundAuxiliary cationsVIV/VV RatioχT (emu K mol−1) (300 K)χT (emu K mol−1) (2 K)Ref.
5NH4+, (1,3-diapH2)2+10/23.340.33[57]
11[Cu(dien)(H2O)]2+10/24.81 (3.68) *0.56[55]
12Cd(H2O)22+10/23.600.10[55]
15Na+10/23.530.23[54]
18K+, NH4+10/23.570.40[57]
19K+10/23.580.38[57]
27[Na(H2O)4]+, (H4tren)4+10/23.830.15[53]
38Zn(H3tepa)2+, (enH2)2+10/21.540.11[63]
* The χT value in parentheses is the χT value of the polyanion, which was calculated by subtracting the χT value for the three uncoupled CuII centers, considering a g = 2 (χT = 1.13 emu K mol−1).
Inorganics 03 00309 g006 1024
Figure 6. χT values for each lattice.
Figure 6. χT values for each lattice.
Inorganics 03 00309 g006

11. 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 10VIV/2VV 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.

Acknowledgments

The authors acknowledge financial support from projects FONDECYT 1120004 and Financiamiento Basal FB0807 (CEDENNA). This work was done under the international collaborative project LIA-MIF 836. Powered@ NLHPC: This research was partially supported by the super-computing infrastructure of the NLHPC (ECM-02), Centre for Mathematical Modelling CMM, Universidad de Chile. KMB thanks 21100772 and AT-24121391 doctoral scholarships and the USACH-French Embassy agreement for a doctoral mobility grant. PHI thanks Vicerrectoría de Investigación, Desarrollo e Innovación (USACH) for the POSTDOC_DICYT 001316 scholarship.

Author Contributions

The manuscript was written with equal contributions from all authors. DVY owns the general idea and plan of the publication. Most of the presented results are part of the Doctoral thesis of PHI and KMB. ES and VPG were part of the discussion and analysis of the presented data. ELF collaborated with the structural determinations.

Conflicts of Interest

The authors declare no conflict of interest.

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MDPI and ACS Style

Hermosilla-Ibáñez, P.; Muñoz-Becerra, K.; Paredes-García, V.; Fur, E.L.; Spodine, E.; Venegas-Yazigi, D. Structural and Electronic Properties of Polyoxovanadoborates Containing the [V12B18O60] Core in Different Mixed Valence States. Inorganics 2015, 3, 309-331. https://doi.org/10.3390/inorganics3030309

AMA Style

Hermosilla-Ibáñez P, Muñoz-Becerra K, Paredes-García V, Fur EL, Spodine E, Venegas-Yazigi D. Structural and Electronic Properties of Polyoxovanadoborates Containing the [V12B18O60] Core in Different Mixed Valence States. Inorganics. 2015; 3(3):309-331. https://doi.org/10.3390/inorganics3030309

Chicago/Turabian Style

Hermosilla-Ibáñez, Patricio, Karina Muñoz-Becerra, Verónica Paredes-García, Eric Le Fur, Evgenia Spodine, and Diego Venegas-Yazigi. 2015. "Structural and Electronic Properties of Polyoxovanadoborates Containing the [V12B18O60] Core in Different Mixed Valence States" Inorganics 3, no. 3: 309-331. https://doi.org/10.3390/inorganics3030309

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