Elena Antonova

Organic Functionalization of Polyoxovanadates: SbN Bonds and Charge Control

Numerous approaches towards the utilization of polyoxometalates (POMs) require their direct covalent attachment to organic groups. This ranges from immobilization of POM catalysts on polymer matrices and integration of POMs into porous metal–organic frameworks (MOFs) to electrochromic hybrid systems. The synthesis strategies developed to date are predominantly not based on the formation of metal– organic bonds, but utilize other main-group elements as bonding mediators to link the organic functions to the POMs. These coupling reactions formally correspond to condensation of the POM cluster with alcohols or carboxylates (M O R), or amines (M=N R), but also with organodiazenido, organophosphato, organosilyl, or organotin groups. However, these strategies primarily target polyoxomolybdates and -tungstates. Various types of subsequent reactions of the organic functions attached to the POM (for example, 1,3dipolar cycloadditions (“click” chemistry), Sonogashira and Heck coupling reactions, or Diels–Alder reactions) emphasize the potential for the integration of such hybrid systems as building blocks into other structures. Polyoxovanadates, such as decavanadate, also exhibit interesting reactivities towards biomolecules, as exemplified by hydrolytic DNA cleavage, inhibition of oxygen consumption in membranes, or the inhibition of 6-phosphogluconate hydrogenase by tetravanadate. Organopolyoxovanadates, however, are relatively rare. Their synthesis usually is limited to a formal exchange of terminal oxido sites with alkoxides or phosphonates, the integration of which strongly affects, or directs, the resulting structure of the vanadate framework. Herein we present two examples that demonstrate how organic amines or ammonium groups can be added to existing polyoxovanadates through the formation of Sb N bonds, whereby the framework structure is retained and the negative charge on it can be largely or completely compensated, which can increase the solubility the polyoxovanadates in organic solvents. Our approach is based on the recently discovered class of antimonate polyoxovanadates, such as the spherical cluster anions [V16Sb4O42(H2O)] 8 , [V15Sb6O42(H2O)] 6 , and [V14Sb8O42(H2O)] 4 . Following systematic adjustments to the synthesis, which is strongly dependent on the pH value, we were able to isolate the organically functionalized analogues [V14Sb III 8(C6H15N3)4O42(H2O)]·4H2O (1) and (C6H17N3)2[V IV 15Sb III 6 (C6H15N3)2O42(H2O)]·2.5H2O (2), in which protonated amines bind to the spherical polyoxovanadate clusters through Sb N bonds. The structure of the two clusters in 1 and 2 can formally be derived from the archetypal spherical isopolyoxovanadate(IV) {V18O42} [11] by replacing four (in 1) or three (in 2) O4V= O square-based pyramids with an equal number of O2Sb(m-O)SbO2 groups (1: Sb-m-O: 1.930(6)–2.031(5) , 2 : Sb-mO: 1.930(5)–2.040(4) ). Abstraction of four VO5 pyramids in [V18O42] 12 results in the formation of two perpendicular, intersecting (O4V=O)8 rings of edge-sharing VO5 pyramids (Figure 1a). Both the geometric parameters of the Sb2O5 groups in 1 and 2 as well as the V-m-O (1.912(6)–1.981(6) ) and V-Oterminal (1.584(7)–1.637(6) ) bond lengths and V···V distances (2.818(2)–3.1011(19) ) are in line with other antimony polyoxovanadates.

One-step hydrothermal synthesis of hierarchical Ag/Bi2WO6 composites: In situ growth monitoring and photocatalytic activity studies

Hierarchical Ag/Bi2WO6 nanomaterials were prepared by a facile one-step hydrothermal method in mixed acetic acid and ethylene glycol (EG) medium. EG is employed as mild reducing agent for the formation of metallic Ag from Ag+ precursors. In situ energy dispersive X-ray diffraction (EDXRD) monitoring showed that the hydrothermal formation kinetics of Bi2WO6 in the presence of EG was significantly slowed down due to its very high viscosity. The photocatalytic activities of Ag/Bi2WO6 composites were evaluated by the photodegradation of methylene blue (MB) under visible light irradiation. The photocatalytic activity of Bi2WO6 is strongly influenced by the Ag loading. The enhanced catalytic activity of the composites is based on the cooperative effects of plasmon absorption band and separation of photogenerated electron-hole pairs.

Expansion of Antimonato Polyoxovanadates with Transition Metal Complexes: (Co(N3C5H15)2)2[{Co(N3C5H15)2}V15Sb6O42(H2O)]·5H2O and (Ni(N3C5H15)2)2[{Ni(N3C5H15)2}V15Sb6O42(H2O)]·8H2O

Two new polyoxovanadates (Co(N(3)C(5)H(15))(2))(2)[{Co(N(3)C(5)H(15))(2)}V(15)Sb(6)O(42)(H(2)O)]·5H(2)O (1) and (Ni(N(3)C(5)H(15))(2))(2)[{Ni(N(3)C(5)H(15))(2)}V(15)Sb(6)O(42)(H(2)O)]·8H(2)O (2) (N(3)C(5)H(15) = N-(2-aminoethyl)-1,3-propanediamine) were synthesized under solvothermal conditions and structurally characterized. In both structures the [V(15)Sb(6)O(42)(H(2)O)](6-) shell displays the main structural motif, which is strongly related to the {V(18)O(42)} archetype cluster. Both compounds crystallize in the triclinic space group P1 with a = 14.3438(4), b = 16.6471(6), c = 18.9186(6) Å, α = 87.291(3)°, β = 83.340(3)°, γ = 78.890(3)°, and V = 4401.4(2) Å(3) (1) and a = 14.5697(13), b = 15.8523(16), c = 20.2411(18) Å, α = 86.702(11)°, β = 84.957(11)°, γ = 76.941(11)°, and V = 4533.0(7) Å(3) (2). In the structure of 1 the [V(15)Sb(6)O(42)(H(2)O)](6-) cluster anion is bound to a [Co(N(3)C(5)H(15))(2)](2+) complex via a terminal oxygen atom. In the Co(2+)-centered complex, one of the amine ligands coordinates in tridentate mode and the second one in bidentate mode to form a strongly distorted CoN(5)O octahedron. Similarly, in compound 2 an analogous NiN(5)O complex is joined to the [V(15)Sb(6)O(42)(H(2)O)](6-) anion via the same attachment mode. A remarkable difference between the two compounds is the orientation of the noncoordinated propylamine group leading to intermolecular Sb···O contacts in 1 and to Sb···N interactions in 2. In the solid-state lattices of 1 and 2, two additional [M(N(3)C(5)H(15))(2)](2+) complexes act as countercations and are located between the [{M(N(3)C(5)H(15))(2)}V(15)Sb(6)O(42)(H(2)O)](4-) anions. Between the anions and cations strong N-H···O hydrogen bonds are observed. In both compounds the clusters are stacked along the b axis in an ABAB fashion with cations and water molecules occupying the space between the clusters. Magnetic characterization demonstrates that the Ni(2+) and Co(2+) cations do not significantly couple with the S = 1/2 vanadyl groups. The susceptibility data can be successfully reproduced assuming a distorted ligand field for the Co(2+) ions (1) and an O(h)-symmetric Ni(2+) ligand field (2).

Controlling Nucleation and Crystal Growth of a Distinct Polyoxovanadate Cluster: An In Situ Energy Dispersive X‐ray Diffraction Study under Solvothermal Conditions

The formation of the antimonato polyoxovanadates [V(14)Sb(8)(C(6)H(15)N(3))(4)O(42)(H(2)O)]·4H(2)O (1), (C(6)H(17)N(3))(2)[V(15)Sb(6)(C(6)H(15)N(3))(2)O(42)(H(2)O)]·2.5H(2)O (2), {C(6)H(15)N(3)}(4)[V(16)Sb(4)O(42)]2H(2)O (3) (C(6)H(15)N(3)=1-(2-aminoethyl)piperazine, AEP) has been studied under solvothermal conditions by using in situ energy dispersive X-ray diffraction (EDRXD). The syntheses were performed with an identical ratio for Sb(2)O(3) and NH(4)VO(3). If the reactions slurries are not stirred during the solvothermal reaction and by applying 70-75% amine concentration, the products contain all three compounds, whereas 3 is observed at 80%. Under stirring conditions, variation of the concentration of AEP led to crystallization of the three different compounds at distinct concentrations, that is, 1 is formed at 75%, 1 and 2 between 75 and 80% and 3 at 80%. At an amine concentration of 77.5%, first reflections of 2 occurred and at later stages, compound 1 started to crystallize. The sample with the lowest number of V(IV) species was formed at the lowest amine concentration, whereas crystallization of 3 required the highest concentration. The formation of the compounds occurred without crystalline intermediates and/or precursors. With increasing reaction temperature, the incubation time was significantly reduced.

A C2-symmetric antimonato polyoxovanadate cluster [V16Sb4O42(H2O)](8-) derived from the {V18O42} archetype.

The new antimonato polyoxovanadate [V(IV)(16)Sb(III)(4)O(42)(H(2)O)](8-) cluster (1a) is the main structural motif of the solvothermally obtained compound {(trenH(2))Zn(tren)}(2)[V(16)Sb(4)O(42)(H(2)O)]·xH(2)O (x = 6-10) (1) (tren = tris(2-aminoethyl)amine). The C(2)-symmetric cluster structure is closely related to the {V(18)O(42)} archetype. 1 crystallizes in the monoclinic space group C2/c with a = 30.7070(19) Å, b = 13.9512(5) Å, c = 23.1435(14) Å, β = 128.076(6)°, and V = 7804.8(7) Å(3). The orientation of the [Sb(III)(2)O(5)](4-) groups in each cluster leads to intermolecular Sb···O contacts and the formation of channels between the clusters. [Zn(tren)(trenH(2))] complexes with trigonal bipyramidal coordination environments are located between the [V(16)Sb(4)O(42)(H(2)O)](8-) anions, and form a three dimensional network with them via strong N-H···O hydrogen bonds. Up to 250 °C crystal water molecules are emitted, which are reversibly incorporated in humid air.

Assembly of [V15Sb6O42(H2O)]6− cluster shells into higher dimensional aggregates via weak Sb⋯N/Sb⋯O intercluster interactions and a new polyoxovanadate with a discrete [V16Sb4O42(H2O)]8− cluster shell

Systematic variation of reaction parameters such as reaction temperature and concentration of the amine under solvothermal conditions using NH4VO3, Sb2O3, CoCl2·6H2O and tris(2-amino)ethylamine (tren) led to the formation of three new antimonato polyoxovanadates: {Co(tren)(H2O)}3[V15Sb6O42(H2O)]·H2O (1), {Co2(tren)3}2{Co(tren)(en)}[{V15Sb6O42(H2O)(Co(tren)2)}V15Sb6O42(H2O)]·xH2O (x ≈ 11) (2) and {Co(tren)(trenH2)}2[V16Sb4O42(H2O)]·6H2O (3). The structures of 1 and 2 are composed of [V15Sb6O42(H2O)]6− cluster anions, which weakly interact with each other through relatively short Sb⋯O bonds. The transition metal complex {Co(tren)(H2O)}2+ acts as a counter ion in 1. In 2 charge neutrality is achieved by the two cations {Co2(tren)3}4+ and {Co(tren)(en)}2+ which are isolated and by {Co(tren)2}2+ being bound to one cluster anion. The main structural motifs in compound 3 are the [V16Sb4O42]8− cluster shell and isolated {Co(tren)(trenH2)}4+ cations. The structures of all three cluster anions can be derived from the {V18O42} archetype by replacing three VO5 moieties by three Sb2O3 units in 1 and 2 and by substituting two VO5 square pyramids by two Sb2O3 groups in 3.