New As-Rich Arsenato-Polyoxovanadate Clusters: Solvothermal Synthesis and Selected Properties of [V 6IV As 8III O 26 ] 4 − Cluster-Containing Compounds

: Three new arsenato-polyoxovanadates with the composition [M(en) 3 ] 2 [V 6 As 8 O 26 ] (M = Co 2+ (I), Zn 2+ (II), and Cd 2+ (III)) were synthesized under solvothermal conditions in high yields, thus signiﬁcantly enhancing the knowledge of As-rich polyoxovanadate cluster chemistry. The compounds are isostructural and feature the very rare [V 6IV As 8III O 26 ] 4 − cluster anion. The cluster shell is constructed by interconnection of two trimeric {V 3 O 11 } groups consisting of three edge-sharing VO 5 polyhedra and four As 2 O 5 units, which are formed by two corner-sharing AsO 3 pyramids. While the As 2 O 5 group is a common structural feature in arsenato-polyoxovanadates, the {V 3 O 11 } unit is only observed in V-rich high-nuclear heteroatom-containing polyoxovanadates {V 14 E 8 } (E = As, Sb, Ge). The complexes adopt the Λ ( δδδ ) conformation, which is the most stable arrangement. Interestingly, the unit cell parameters do not scale with the volume of the [M(en) 3 ] 2+ complexes, assuming a constant volume of the anion. Only a very detailed Hirshfeld surface analysis revealed that the van der Waals volume of the {V 6 As 8 O 26 } moiety is the smallest for the Cd-containing compound, while the volumes of the anions in the other two compounds are very similar. Therefore, the observed trends of the lattice parameters can be explained on the basis of these ﬁndings. Furthermore, intermolecular interactions include As · · · H contacts in addition to O · · · H and H · · · H interactions. The electronic spectrum of I contains d–d transitions of the vanadyl group and of the Co 2+ cation. As expected only the d–d transitions of the VO 2+ unit occur for II and III.


Introduction
The synthesis and characterization of polyoxometalates (POMs) with the elements V, Mo and W [1][2][3][4][5][6] is a still growing research field because several POMs show promising properties for applications in technologically important areas such as catalysis [7][8][9][10][11][12], in biomedicine [13][14][15], sorption of molecules [16] or in molecular electronics [17]. The most commonly applied synthesis methods are reactions at room or at moderate temperatures (reflux) in polar media such as water applying small molecular moieties which form larger cluster anions by self-assembly and condensation reactions [18]. However, many compounds were also synthesized using large, preformed POMs and/or applying hydrothermal conditions. It needs to be mentioned that not only artificial POMs are known, but a large number of POMs were also found in minerals [19], with several of them having no counterpart in synthetically produced compounds.

Syntheses
All syntheses were performed in glass tubes (volume: 11 mL), heating the reaction mixtures at 150 • C for up to 4 d. Subsequently, the glass tubes were cooled down and the solids were filtered off. In all syntheses a green product was obtained containing needle-like crystals suitable for single crystal structure determination. The products were washed with water and ethanol and were stored in a desiccator. Upscaling experiments were performed in steel autoclaves with a 30 mL Teflon liner. The temperature of 150 • C was held for 4 d followed by cooling the autoclave to room temperature. The treatment of the reaction products was identical with that obtained in glass tubes.
For all syntheses in glass tubes, 1.341 mmol NH 4 VO 3 (156.9 mg) and 1.194 mmol As 2 O 3 (236.2 mg) were used. For I, 0.66 mmol (157 mg) CoCl 2 ·6H 2 O; for II, 0.66 mmol (145 mg) Zn(CH 3 COO) 2 ·2H 2 O; and for III, 0.66 mmol (150.8 mg) CdCl 2 ·2.5H 2 O were mixed with the other solid educts. The liquid was composed of 1.7 mL ethylenediamine, and 2.3 mL H 2 O was added to the solid and the slurry was shaken. The syntheses in the autoclaves were performed using a quadrupled amount of solids and liquid. The yields based on NH 4 VO 3 were 65% for I, 58% for II and 90% for III.

X-ray Powder Diffraction
The X-ray powder diffraction patterns were collected with a STOE Stadi-P diffractometer using a MYTHEN 1K detector (DECTRIS) and monochromatized Cu-K α1 radiation (λ = 1.540598 Å). The experimental and the calculated patterns match perfectly indicating phase purity of the samples ( Figure S1).

Spectroscopic Characterization and Chemical Composition
The infrared spectra were recorded at 295 K with a Bruker Vertex70 FT-IR spectrometer. The UV/vis diffuse reflectance spectra were measured using the UV/vis two-channel spectrometer Cary 5 from Carian Techtron Pty., Darmstadt and BaSO 4 served as white standard. The raw data were converted into the Kubelka-Munk factor.

Crystal Structure Determination
Single crystal data collections were performed with a STOE Imaging Plate Diffraction System (IPDS-2) using Cu-Kα radiation (λ = 1.54184 Å). Structure solution was performed with SHELXT [30] and structure refinements were performed against F2 using SHELXL-2016 [31]. All non-hydrogen atoms were refined with anisotropic displacement parameters. The C-H and N-H hydrogen atoms were positioned with idealized geometry and were refined isotropically with Uiso (H) = 1.2 Ueq (C) using a riding model. Selected crystal data and refinement results are listed in Table 1.   Figure S2). The complexes adopt the Λ configuration and the C-C bonds are nearly parallel to the C 3 axis of the molecules, i.e., the en ligands are in the δ conformation leading to Λ(δδδ) [32], which is the most stable conformation. The M 2+ -N bond lengths follow the ionic radii of the cations with the shortest for Co-N (average: 2.170 Å, Supplementary Materials, Table S1), slightly longer bonds for Zn-N (average: 2.190 Å, Table S1) and the longest bonds for Cd-N (average: 2.365 Å, Supplementary Materials, Table S1). The N-M 2+ -N angles clearly deviate from the ideal values of an octahedron and the three complexes exhibit different degrees of distortion (Supplementary Materials, Table S1). Using the minimum bounding ellipsoid (MBE) approach [33] the volumes of the polyhedra and the distortion expressed by the shape parameter S were calculated. The smallest volumes are obtained for the Co 2+ and Zn 2+ centered complexes (42. (Table 2). Only small differences can be seen demonstrating the rigidity of the VO 5 polyhedra. In all three compounds, the V1 centered polyhedron is axially compressed and that containing V2 is slightly axially stretched.   Tables S2-S4), and agree with those reported earlier [28,29]. Normally, this angle is larger in high-nuclear As-POVs and is in the range from 132 to 136°, indicating that the As-O-As angle is slightly strained.
The cluster [V6As8O26] 4− is composed of two symmetry-related trimeric units, {V3O11}, comprising three VO5 pyramids sharing common edges ( Figure 1). The two {V3O11} moieties are bridged by the As2O5 groups to form the cluster anion, which contains a cavity with dimensions 4.4 · 7 Å 2 (measured from coordinate-to-coordinate). The size of the cavity is too small for hosting a guest molecule as is often observed for the larger As-POV cluster anions. All geometric parameters are in good agreement with those published for compounds with the [V6As8O26] 4− cluster shell [28,29].
We note that the {V3O11} building unit is a common motif in high-nuclear heteroatomcontaining POVs {V14E8} (E = As, Sb, Ge) [36][37][38][39][40][41][42][43][44]. In the crystal structures of these compounds eight edge-sharing VO5 pyramids form a central ring which is capped on both sides by the {V3O11} groups, which are rotated by approximately 90° against each other. Removal of the central ring leads to the structure of the cluster anion in the title compounds. Using the bond valence sum (BVS) method, the valence states of V and As were calculated yielding +4 for the V centers and +3 for the As atoms justifying the formula [V6 IV As8 III O26] 4− .
The arrangement of the cations and anions in the structures is shown in Figure 2 , Tables S2-S4), and agree with those reported earlier [28,29]. Normally, this angle is larger in high-nuclear As-POVs and is in the range from 132 to 136 • , indicating that the As-O-As angle is slightly strained.
The cluster [V 6 As 8 O 26 ] 4− is composed of two symmetry-related trimeric units, {V 3 O 11 }, comprising three VO 5 pyramids sharing common edges ( Figure 1). The two {V 3 O 11 } moieties are bridged by the As 2 O 5 groups to form the cluster anion, which contains a cavity with dimensions 4.4 · 7 Å 2 (measured from coordinate-to-coordinate). The size of the cavity is too small for hosting a guest molecule as is often observed for the larger As-POV cluster anions. All geometric parameters are in good agreement with those published for compounds with the [V 6 As 8 O 26 ] 4− cluster shell [28,29].
We note that the {V 3 O 11 } building unit is a common motif in high-nuclear heteroatomcontaining POVs {V 14 E 8 } (E = As, Sb, Ge) [36][37][38][39][40][41][42][43][44]. In the crystal structures of these compounds eight edge-sharing VO 5 pyramids form a central ring which is capped on both sides by the {V 3 O 11 } groups, which are rotated by approximately 90 • against each other. Removal of the central ring leads to the structure of the cluster anion in the title compounds. Using the bond valence sum (BVS) method, the valence states of V and As were calculated yielding +4 for the V centers and +3 for the As atoms justifying the formula [

Hirshfeld Surface Analysis
Cations and anions are connected by N-H···O and C-H···O hydrogen bonding interactions (Supplementary Materials, Tables S5-S7) generating a three-dimensional network. For gaining more insight into the intermolecular interactions a Hirshfeld surface analysis was performed [45][46][47][48]. A Hirshfeld surface is constructed applying the equation (1) vdW = van der Waals radius; di represents the distance of a point on the surface to the nearest nucleus inside the surface; de is the distance from such a point to the next neighbor outside the surface. If an interatomic distance is shorter than the sum of the van der Waals radii, a red area occurs on the Hirshfeld surface, and all other colored regions indicate interatomic distances longer than the sum of the van der Waals radii. The analysis reveals that the differences between the three compounds are relatively small and the Hirshfeld surfaces of the anions and cations for the compounds are shown in Figure 3.

Hirshfeld Surface Analysis
Cations and anions are connected by N-H· · · O and C-H· · · O hydrogen bonding interactions (Supplementary Materials, Tables S5-S7) generating a three-dimensional network. For gaining more insight into the intermolecular interactions a Hirshfeld surface analysis was performed [45][46][47][48]. A Hirshfeld surface is constructed applying the equation vdW = van der Waals radius; d i represents the distance of a point on the surface to the nearest nucleus inside the surface; d e is the distance from such a point to the next neighbor outside the surface. If an interatomic distance is shorter than the sum of the van der Waals radii, a red area occurs on the Hirshfeld surface, and all other colored regions indicate interatomic distances longer than the sum of the van der Waals radii. The analysis reveals that the differences between the three compounds are relatively small and the Hirshfeld surfaces of the anions and cations for the compounds are shown in Figure 3.

Hirshfeld Surface Analysis
Cations and anions are connected by N-H···O and C-H···O hydrogen bonding interactions (Supplementary Materials, Tables S5-S7) generating a three-dimensional network. For gaining more insight into the intermolecular interactions a Hirshfeld surface analysis was performed [45][46][47][48]. A Hirshfeld surface is constructed applying the equation (1) vdW = van der Waals radius; di represents the distance of a point on the surface to the nearest nucleus inside the surface; de is the distance from such a point to the next neighbor outside the surface. If an interatomic distance is shorter than the sum of the van der Waals radii, a red area occurs on the Hirshfeld surface, and all other colored regions indicate interatomic distances longer than the sum of the van der Waals radii. The analysis reveals that the differences between the three compounds are relatively small and the Hirshfeld surfaces of the anions and cations for the compounds are shown in Figure 3.  All the red areas on the Hirshfeld surfaces are caused by intermolecular O···H interactions to NH2/CH2 groups of the en ligands (see also Supplementary Materials, Tables S5-S7 for the geometric parameters of the hydrogen bonds). A further interpretation of the 3D Hirshfeld surface is not straightforward; therefore, a fingerprint plot can be constructed representing a 2D view of the 3D surface ( Figure 4). Three different intermolecular interactions are identified for the cations: H···H (30.3 (Co, Zn) and 33.4% (Cd)), H···O (54.8 (Co, Zn) and 52.3% (Cd)) and H···As (15 (Co, Zn) and 14.4% (Cd)). Comparing the fingerprint plots of [M(en)3] 2+ complexes (M = Co, Zn) with that of [Cd(en)3] 2+ the sharp spike caused by H···O interactions appears at the same de-di position, but a second less sharp feature occurs for the Cd-containing complex at somewhat longer de-di values caused by H···H interactions, which are located at even longer de-di values in the Zn-resp. Co-containing complexes. For the anions the interactions are 74.4% for O···H and 26.6% for As···H, and they are virtually identical for the three compounds. For As···H interactions the shortest contact is at 2.77 (Co), 2.76 (Zn) and 2.85 Å (Cd), which are shorter than the sum of van der Waals radii of 2.95 Å [49]. The corresponding angles N-H···As are 153.4 (Co), 155 (Zn) and 160.3° (Cd) pointing towards significant interactions. A second As···H separation at 2.91 (Co), 2.89 (Zn) and 2.87 Å (Cd) with the C-H···As angles of 156.5 (Co), 154.6 (Zn) and 145.5° (Cd) indicates weaker intermolecular interactions.
(a) All the red areas on the Hirshfeld surfaces are caused by intermolecular O· · · H interactions to NH 2 /CH 2 groups of the en ligands (see also Supplementary Materials, Tables S5-S7 for the geometric parameters of the hydrogen bonds). A further interpretation of the 3D Hirshfeld surface is not straightforward; therefore, a fingerprint plot can be constructed representing a 2D view of the 3D surface ( Figure 4). Three different intermolecular interactions are identified for the cations: H· · · H (30.3 (Co, Zn) and 33.4% (Cd)), H· · · O (54.8 (Co, Zn) and 52.3% (Cd)) and H· · · As (15 (Co, Zn) and 14.4% (Cd)). Comparing the fingerprint plots of [M(en) 3 ] 2+ complexes (M = Co, Zn) with that of [Cd(en) 3 ] 2+ the sharp spike caused by H· · · O interactions appears at the same d e -d i position, but a second less sharp feature occurs for the Cd-containing complex at somewhat longer d e -d i values caused by H· · · H interactions, which are located at even longer d e -d i values in the Zn-resp. Co-containing complexes. For the anions the interactions are 74.4% for O· · · H and 26.6% for As· · · H, and they are virtually identical for the three compounds. For As· · · H interactions the shortest contact is at 2.77 (Co), 2.76 (Zn) and 2.85 Å (Cd), which are shorter than the sum of van der Waals radii of 2.95 Å [49]. The corresponding angles N-H· · · As are 153.4 (Co), 155 (Zn) and 160.3 • (Cd) pointing towards significant interactions. A second As· · · H separation at 2.91 (Co), 2.89 (Zn) and 2.87 Å (Cd) with the C-H· · · As angles of 156.5 (Co), 154.6 (Zn) and 145.5 • (Cd) indicates weaker intermolecular interactions.
The Hirshfeld surface calculations yield further quantities, such as the volumes and areas of the cations and anions. The volumes of the cations do not differ much: 290.87 for I, 289.28 for II and 290.73 Å 3 for III. However, for the anions clearly different volumes were calculated: 649.05 for I, 644.97 for II and 628.89 Å 3 for III. These differences may account for the non-regular changes of the lattice parameters across the series of compounds. the shortest contact is at 2.77 (Co), 2.76 (Zn) and 2.85 Å (Cd), which are shorter than the sum of van der Waals radii of 2.95 Å [49]. The corresponding angles N-H···As are 153.4 (Co), 155 (Zn) and 160.3° (Cd) pointing towards significant interactions. A second As···H separation at 2.91 (Co), 2.89 (Zn) and 2.87 Å (Cd) with the C-H···As angles of 156.5 (Co), 154.6 (Zn) and 145.5° (Cd) indicates weaker intermolecular interactions.  The Hirshfeld surface calculations yield further quantities, such as the volumes and areas of the cations and anions. The volumes of the cations do not differ much: 290.87 for I, 289.28 for II and 290.73 Å 3 for III. However, for the anions clearly different volumes were calculated: 649.05 for I, 644.97 for II and 628.89 Å 3 for III. These differences may account for the non-regular changes of the lattice parameters across the series of compounds.

Spectroscopic Investigations
The IR spectra of the three compounds in the fingerprint region are displayed in Fig [50,51]. The NH2 bending vibration occurs at 1593 cm −1 for III, and is split into two signals for I and II (1600 and 1580 cm −1 ). The CH2 scissor vibration is located at around 1465 cm −1 and the CH2 twist occurs at ≈1330 cm −1 for I-III. The strong absorption at ≈960 cm −1 is caused by the V=O stretching vibration and is typical for the VO 2+ vanadyl group. For the N-C-C-N skeleton six modes are expected which can be classified as three skeleton stretching vibrations, two N-C-C-N deformation modes and one torsional vibration around the C-C bond. The three stretching bands are between 900 and 1100 cm −1 , but the overlap with other absorptions makes a distinct assignment impossible. The remaining absorptions of the skeleton are located at much lower wavenumber around 500 cm −1 . Below 900 cm −1 the V-O-V, V-O-As, As-O-As, and As-O vibrations are located which cannot be unambiguously assigned. According to data in the literature, the M-N stretching modes are clearly located below 450 cm −1 .

Spectroscopic Investigations
The IR spectra of the three compounds in the fingerprint region are displayed in Figure 5. In the NH 2 and CH 2 regions (not shown here) all spectra contain absorptions of the asymmetric and symmetric N-H stretching vibrations: 3330, 3309 and 3286 cm −1 for I, 3340, 3315 and 3292 cm −1 for II, and 3353, 3331 and 3288 cm −1 for III. The bands of the CH 2 groups are at 2967, 2930, and 2885 cm −1 for I, at 2969, 2931 and 2887 cm −1 for II, and at 2969, 2924, and 2879 cm −1 for III. The occurrence of three bands in the N-H region is a typical sign for NH 2 groups involved in hydrogen boding interactions. All these signals observed for the title compounds are in agreement with literature values of tris(ethylenediamine) complexes of the cations in I-III [50,51]. The NH 2 bending vibration occurs at 1593 cm −1 for III, and is split into two signals for I and II (1600 and 1580 cm −1 ). The CH 2 scissor vibration is located at around 1465 cm −1 and the CH 2 twist occurs at ≈1330 cm −1 for I-III. The strong absorption at ≈960 cm −1 is caused by the V=O stretching vibration and is typical for the VO 2+ vanadyl group. For the N-C-C-N skeleton six modes are expected which can be classified as three skeleton stretching vibrations, two N-C-C-N deformation modes and one torsional vibration around the C-C bond. The three stretching bands are between 900 and 1100 cm −1 , but the overlap with other absorptions makes a distinct assignment impossible. The remaining absorptions of the skeleton are located at much lower wavenumber around 500 cm −1 . Below 900 cm −1 the V-O-V, V-O-As, As-O-As, and As-O vibrations are located which cannot be unambiguously assigned. According to data in the literature, the M-N stretching modes are clearly located below 450 cm −1 .
sified as three skeleton stretching vibrations, two N-C-C-N deformation modes and one torsional vibration around the C-C bond. The three stretching bands are between 900 and 1100 cm −1 , but the overlap with other absorptions makes a distinct assignment impossible. The remaining absorptions of the skeleton are located at much lower wavenumber around 500 cm −1 . Below 900 cm −1 the V-O-V, V-O-As, As-O-As, and As-O vibrations are located which cannot be unambiguously assigned. According to data in the literature, the M-N stretching modes are clearly located below 450 cm −1 . In the UV/vis spectra of the compounds (Figure 6), two absorptions are observed for II and III, which are located at ≈1.5 (12,098 cm −1 , 827 nm) and ≈2.0 eV (16,131 cm −1 , 620 nm), while for I an additional signal is present at ≈2.65 eV (21,374 cm −1 , 468 nm) and the first transition is broader than for II and III. In addition, the absorption maximum of the first signal of I is slightly shifted to lower energy (1.44 eV, 11,614 cm −1 , 861 nm). For compounds II and III, no d-d transitions are possible for the d 10 electronic configuration; therefore, the absorptions must be caused by the vanadyl groups. For this group the d-d transitions 2 B 2g → 2 Eg, 2 B 2g → 2 B 1g and 2 B 2g → 2 A 1g are expected [52]. Depending on the environment, i.e., packing in the structure, and on the actual distortion of the VO 5 pyramids, the ideal C 4v symmetry is reduced, and the degeneracy of the first band is removed. This effect may be the reason that the absorption at ≈1.5 is relatively broad and consists of two overlapping bands. For I the situation is different because for the Co 2+ cation the first d-d transition 4 T 1g (F) → 4 T 2g (F) occurs in the energetic region of the first absorption band of the vanadyl unit and this a further reason for the broadness of the first absorption band in the spectrum of I. The signal at ≈2.65 eV can be assigned to the 4 T 1g (F) → 4 A 2g (F) transition of the Co 2+ cation. The strong increase in the absorptions above 3 eV is most probable caused by charge-transfer processes. In the UV/vis spectra of the compounds (Figure 6), two absorptions are observed for II and III, which are located at ≈1.5 (12098 cm −1 , 827 nm) and ≈2.0 eV (16131 cm −1 , 620 nm), while for I an additional signal is present at ≈2.65 eV (21374 cm −1 , 468 nm) and the first transition is broader than for II and III. In addition, the absorption maximum of the first signal of I is slightly shifted to lower energy (1.44 eV, 11,614 cm −1 , 861 nm). For compounds II and III, no d-d transitions are possible for the d 10 electronic configuration; therefore, the absorptions must be caused by the vanadyl groups. For this group the d-d transitions 2 B2g → 2 Eg, 2 B2g → 2 B1g and 2 B2g → 2 A1g are expected [52]. Depending on the environment, i.e., packing in the structure, and on the actual distortion of the VO5 pyramids, the ideal C4v symmetry is reduced, and the degeneracy of the first band is removed. This effect may be the reason that the absorption at ≈1.5 is relatively broad and consists of two overlapping bands. For I the situation is different because for the Co 2+ cation the first d-d transition 4 T1g(F) → 4 T2g(F) occurs in the energetic region of the first absorption band of the vanadyl unit and this a further reason for the broadness of the first absorption band in the spectrum of I. The signal at ≈2.65 eV can be assigned to the 4 T1g(F) → 4 A2g(F) transition of the Co 2+ cation. The strong increase in the absorptions above 3 eV is most probable caused by charge-transfer processes.

Conclusions
In this manuscript, we have reported the solvothermal synthesis and selected properties of three new As-rich polyoxovanadates with composition [M(en)3]2[V6As8O26] (M = Co, Zn, Cd), a significant step forward in the chemistry of As-rich POV clusters. It is surprising that the number of compounds featuring this small As-POV cluster is compara-

Conclusions
In this manuscript, we have reported the solvothermal synthesis and selected properties of three new As-rich polyoxovanadates with composition [M(en) 3 ] 2 [V 6 As 8 O 26 ] (M = Co, Zn, Cd), a significant step forward in the chemistry of As-rich POV clusters. It is surprising that the number of compounds featuring this small As-POV cluster is comparatively low compared with other high-nuclearity As-POVs, especially keeping in mind that the primary building units such as the As 2 O 5 handle and the trimeric {V 3 O 11 } group frequently occur in the structures of other As-POVs. From a synthetic point of view, the occurrence of three isostructural compounds prepared under solvothermal conditions is surprising because, in most cases, synthesis recipes cannot be directly transferred from, e.g., one transition metal cation to another. While the overall structural features of the three isostructural compounds are very similar, the Hirshfeld surface analysis reveals some subtle differences in the intermolecular interactions between I/II and III. The analysis identified the reason for the non-continuous variation of the lattice parameters, because one would on a first glance expect a steady change with the ionic radii of the cations. Surprisingly, the anion in III has the smallest van der Waals volume, allowing a denser packing of the constituents along [1 0 0].

Supplementary Materials:
The following are available online at https://www.mdpi.com/article/ 10.3390/cryst12101473/s1, Figure S1: Experimental and calculated X-ray powder patterns of I-III; Figure S2: ORTEP plots of the [M(en) 3 ] 2+ complexes in the structures of I, II, and III with labelling. Table S1. Selected bond lengths (Å) and angles ( • ) for the M 2+ coordination in I, II, and III.