Decaniobate: The Fruit Fly of Niobium Polyoxometalate Chemistry

ConspectusPolyoxometalates (POMs, metals = V4/5+, Nb5+, Ta5+, Mo5/6+, and W5/6+) can be described as molecular metal oxides. The V, Mo, and W-POMs (classic POMs) exhibit rich structural diversity with interesting redox properties, acid catalysis, inorganic ligands, and colorimetric properties and behavior. Nb and Ta POMs, while structurally similar, are generally stable only in base and redox behavior is rare, and they are synthetically far less accessible. The V, Mo, and W-POMs have been studied for well over a century, Nb-POM chemistry has emerged in the last 20 years, and Ta-POM chemistry is yet to see consistent and significant advances. Early and current success in Nb-POM chemistry is owed mainly to hydrothermal synthesis, which is wholly unsatisfying, given the black box nature of this technique.For the last 5 years and as summarized in this Account, we have exploited decaniobate, [Nb10O28]6- (Nb10), as a foundation to perform room-temperature, nearly pH-neutral manipulations of Nb-POM solutions. Nb10, with a rare neutral self-buffering pH, responds to any interactions with electrolytes (specifically oxoanions and metal cations) by undergoing transformations, leading to new topologies. The ease of Nb10 transformation yielding new generations of Nb-POMs, akin to an inorganic analogue of biological model organisms such as the fruit fly, inspired the title of this Account. The common building unit born from the disassembly of Nb10 is [Nb7O20(OH, H2O)2](5-7)-, and the hydroxyl/aqua ligands provide reactivity for linking via condensation reactions, ligand exchange, heterometals, or oxoanions. We can coax these newly assembled Nb-POMs (detected by small-angle X-ray scattering, SAXS) to crystallize via the usual methods of vapor diffusion, salting out, and reduced temperature, and the single-crystal X-ray diffraction structures are valuable for understanding reaction mechanisms to fine-tune control and yield a landscape of topologies and compositions. Beyond providing an opportunity to comprehend and diversify POM chemistry, the reactivity of Nb10 yields highly soluble (i.e., >2 M Nb), nearly neutral aqueous solutions of niobium, ideal for the solution-phase deposition of thin films, demonstrated with LiNbO3, (Na,K)NbO3, Nb2O5, and heterometal-doped Nb2O5. The obtained films are cohesive and smooth, enabled by the tendency of these solutions to gel if simply evaporated quickly.Per our current endeavors, this gelation behavior provides an opportunity to develop new soft, flexible materials including inorganic networks, organic-inorganic networks, and porous solids and explore their material properties including base catalysis and sorption (i.e., CO2). Nb-POM (and Ta-POM) discovery and implementation of properties is far from complete. While heterometal (d and f element) substitution is easy with classic POMs, imparting a whole host of functions (tuned luminescence, catalysis, electroactivity, etc.), it remains a challenge with Nb-POMs due to pH incompatibility with most heterometals. This grand challenge that defies fundamental aqueous behavior of metal cations requires the creation of liquid mixtures that include polymer and/or ionic liquid components, and the creation of such reaction media can impact synthesis beyond POM chemistry. The goal of this Account is to describe the recent advances in Nb-POM chemistry, afforded by the Nb10 "fruit fly", and to also provide insight into the next large steps needed to advance Nb-POM chemistry.

A Three-Dimensional (3D) Indium-Containing Polyoxoniobate Framework Based on {In5Nb71}n Helical Pillars

A rare 3D Indium-containing polyoxoniobate framework {H₉[Cu(en)₂(H₂O)₂][Cu(en)₂]₁₂[In(en)]₅[Nb₂₃-O₆₅(OH)₃(H₂O)₂]{Nb₂₄O₆₇(OH)₂(H₂O)₃]₂}·68H₂O(1), based on the In-containing polyoxoniobate cluster, {[In(en)]₅[Nb₂₃O₆₅(OH)₃(H₂O)₂][Nb₂₄O₆₇(OH)₂(H₂O)₃]₂}^35- ({In₅Nb₇₁}) and [Cu(en)₂]²⁺ linkers has been successfully synthesized. The nest-like cluster {In₅Nb₇₁} is constructed from one brand-new V-shaped {Nb₂₃O₇₀}, two triangle-shaped {Nb₂₄O₇₂} and five [In(en)]³⁺. The [In(en)] fragments link {Nb₂₄O₇₂} and {Nb₂₃O₇₀} units into unique {In₅Nb₇₁}_n helical pillars. The copper-amine complexes connect the {In₅Nb₇₁}_n helical pillars into a three-dimensional (3D) inorganic-organic hybrid In-Cu-containing framework. This material also exhibits good ionic conductivity and vapor adsorption capacity properties.

Polyoxoniobates as molecular building blocks in thin films

Niobium oxide thin films have been prepared by spin-coating aqueous solutions of tetramethylammonium salts of the isostructural polyoxometalate clusters [Nb₁₀O₂₈]^6-, [TiNb₉O₂₈]^7- and [Ti₂Nb₈O₂₈]^8- onto silicon wafers, and annealing them. The [Nb₁₀O₂₈]^6- cluster yields films of Nb₂O₅ in the orthorhombic and monoclinic crystal phases when annealed at 800 °C and 1000 °C, respectively, whereas the [TiNb₉O₂₈]^7- and [Ti₂Nb₈O₂₈]^8- clusters yield the monoclinic crystal phases of Ti₂Nb₁₂O₂₉ and TiNb₂O₇ (titanium-niobium oxides) in different ratios. We also demonstrate a protocol for depositing successive layers of metal oxide films. Finally, we explore factors affecting the roughness of the films.

Computational exploration of heterometal substitution into the decaniobate framework, [Nb10O28]6

The factors governing the substitution of group 4B-12B metals into the decaniobate framework are explored using density functional theory in order to ascertain whether (1) recently isolated [MNb9O28]x- clusters are kinetic or thermodynamic products, (2) density functional theory is a sufficient level of theory to accurately predict substitution patterns in polyoxometalates where ion pairing and other effects may operate, and (3) it can be used to guide future synthetic efforts. Computations using restricted, unrestricted and open-shell density functional theory at PBE0/def2-tzvp were found to correctly predict substitution patterns in known clusters, and were subsequently used to calculate the relative energies of a large series of [MNb9O28]x- clusters, to reveal trends and suggest potential synthetic approaches. OPBE/def2-tzvp correctly predicted favoured spin states of known substituted decametalates.

An inorganic Co-containing heteropolyoxoniobate: reversible chemochromism and H2O-dependent proton conductivity properties

A novel pure inorganic cobalt-containing heteropolyoxoniobate is constructed from crescent-shaped [SiNb18O54]14− units. It exhibits reversible chemochromism, H2O-dependent proton conductivity, water vapor adsorption and magnetic properties.

Soot Combustion over Niobium-Doped Cryptomelane (K-OMS-2) Nanorods—Redox State of Manganese and the Lattice Strain Control the Catalysts Performance

A series of Nb-doped (0–23 wt%) cryptomelane catalyst (Nb-K-OMS-2) was synthesized and thoroughly characterized by XRD, TEM/EDX, XRF, XPS, XAS, UV-Vis, and Raman techniques corroborated by the work function measurements. The obtained catalysts were tested for soot oxidation (Printex U) in model tight and loose contact modes. It was shown that the catalytic properties of the Nb-K-OMS-2 are controlled by the amount of Nb dopant in a strongly non-monotonous way. The introduction of niobium gives rise to the strain in the cryptomelane lattice, accompanied by significant Mn+3/Mn+4 ratio variations and concomitant work function changes. The isotopic exchange experiments revealed that the catalytic activity of the Nb-OMS-2 catalysts in soot combustion proceeds via the pathways, where both the activated suprafacial 18O and the surface 16O2− species participate together in the reaction. The niobium doping level controls the non-monotonous changes of the catalyst work function and the lattice strain, and variations of these parameters correlate well with the observed deSoot activity. To our best knowledge, the role of the lattice strain of the cryptomelane catalysts was documented for the first time in this study.

Syntheses, characterization and properties of two new dodeca-niobates presenting unprecedented features

Two new dodeca-niobate materials, K3[Nb2O2][H7SiNb12O40]·16H2O (1) and [Cu(en)2]9[(VNb12V1.69Nb0.31O42en0.31)2]·20.69H2O (2) (en = ethylenediamine), have been obtained under hydrothermal conditions and characterized by IR, UV-Vis, XRD, and single crystal X-ray diffraction analysis. Single crystal X-ray analysis reveals that compound 1 presents a reasonable bridging layer structure model and that K plays a great role in the three-dimensional structure. Compound 2 is the first capped Keggin POM with organic ligand chelating the capping atom. The photocatalytic hydrogen evolution and styrene epoxidation catalytic properties of the two compounds were also assessed. Additionally, compound 1 is also a good photocatalyst to degrade RhB.

Visible-light-driven hydrogen evolution using a polyoxometalate-based copper molecular catalyst

[Cu₅(OH)₄(H₂O)₂(A-α-SiW₉O₃₃)₂]¹⁰⁻ (1) was tested as a molecular catalyst for visible-light-driven H₂ evolution and exhibited a high TON of 718.9. Many stability studies showed that 1 could maintain its structure intact during the catalytic process.

A novel peroxopolyoxoniobate incorporating mixed heteroatoms: [P2Se2Nb6(O2)6O22]8-

A novel hetero selenato-phosphato-peroxopolyoxoniobate with the formula Cs₄H₄[P₂Se₂Nb₆(O₂)₆O₂₂]·10H₂O has been successfully isolated in an acidified aqueous hydrogen peroxide solution. The synthesized cluster represents the first example of a selenium-containing polyoxoniobate. Furthermore, the ESI-MS spectra show that the polyoxoanion structural unit [P₂Se₂Nb₆(O₂)₆O₂₂]^8- remains intact in aqueous solution.

Niobium-substituted octahedral molecular sieve (OMS-2) materials in selective oxidation of methanol to dimethoxymethane

The prepared bifunctional Nb-OMS-2 catalysts are promising candidate in partial oxidation of methanol into dimethoxymethane at a lower temperature suggesting that there is an important temperature regime when forming active sites for DMM production.

Titanium-Substituted Polyoxotantalate Clusters Exhibiting Wide pH Stabilities: [Ti2 Ta8 O28 ](8-) and [Ti12 Ta6 O44 ](10.)

Two new substituted polyoxotantalate clusters, [Ti2 Ta8 O28 ](8-) and [Ti12 Ta6 O44 ](10-) , considerably expand the pH range where tantalates persist in aqueous solution. The structures of [Ti2 Ta8 O28 ](8-) and [Ti12 Ta6 O44 ](10-) are reported as tetramethylammonium salts after synthesis at hydrothermal conditions in aqueous solution. These Ti-substituted polyoxotantalate clusters have analogues among recently discovered niobates, but are slightly larger and more persistent in solution. Most importantly, they exhibit a much wider range of pH stability than the familiar hexatantalate cluster, which is the only other tantalate known to be stable at highly basic pH conditions. These molecules are kinetically stable to near-neutral pH, making them excellent synthons for further development into materials and catalysts, and an significant advance in adapting tantalates for use in aqueous solutions.

Synthesis and Characterization of [(OH)TeNb5O18](6-) in Water Solution, Comparison with [Nb6O19](8-)

Reaction of [Nb6O19](8-) with H6TeO6 in water gives telluropentaniobate [(OH)TeNb5O18](6-) (1) as single product, which was isolated as Na(+) and mixed Na(+)/K(+) salts. Crystal structures were determined for Na6[(OH)TeNb5O18]·15H2O (Na6-1) and K6Na[Nb(5.5){Te(OH)}(0.5)O(18.5)]·26H2O (K6Na-1). Formation of 1 was monitored with electrospray ionization mass spectrometry (ESI-MS) and (125)Te NMR techniques. Capillary electrophoresis was used to calculate electrophoretic mobilities and radii of the anionic species in solutions of [(OH)TeNb5O18](6-) and [Nb6O19](8-) in borate buffer. No condensation or degradation products were detected. Reactions of 1 with {Cp*Rh}(2+) sources gives 1:1 and 2:1 hybrid polyoxometalate, which are present in solution as a mixture of isomers, as detected by (125)Te NMR. The isomerism is related to various possibilities of coordination of {Cp*Rh}(2+) to different {M3O3} faces, relative to the unique Te atom. According to ESI-MS experiments in water and methanol, rapid redistribution of the organometallic fragments between the 1:1 and 2:1 complexes takes place. The 1:1 complexes are more stable in water, while 2:1 complexes dominate in methanol. X-ray structural analysis of the crystals isolated from 2:1 reaction mixture allowed identification of Na3[{Cp*Rh}2TeNb5O19]·24H2O (Cp*2Rh2-1) with two {Cp*Rh}(2+) fragments capping the opposing faces of the Lindqvist anion.

Unique solubility of polyoxoniobate salts in methanol: coordination to cations and POM methylation

In methanolic solutions of A₄[{LM′}₂M₆O₁₉] ({LM′} = {Cp*Rh}²⁺, {Cp*Ir}²⁺, {(C₆H₆)Ru}²⁺; M = Nb, Ta; A = Na, K) formation of methoxo complexes [{LM′}₂M₆O_19−n(OMe)n] (n = 1–3) and coordination of CH₃OH to A⁺ take place.

Two Rh(III)-substituted polyoxoniobates and their base-induced transformation: [H2RhNb9O28](6-) and [Rh2(OH)4Nb10O30](8-)

Two new rhodium-substituted polyoxoniobates, [H2RhNb9O28](6-) (RhNb9) and [Rh2(OH)4Nb10O30](8-) (Rh2Nb10) are reported. The two distinct Rh(III)-substituted niobate clusters behave differently when the pH is raised with TMAOH: the Rh2Nb10 is stable until pH ∼ 12.7, but RhNb9 dissociates to form RhNb5 and RhNb10, similar to some of our other metal-substituted niobates, such as the MNb9 ions (M = Cr or Mn), which transform to MNb10 when the solution pH is raised.

A new Keggin-like niobium-phosphate cluster that reacts reversibly with hydrogen peroxide

Solid niobium phosphate (NbOPO₄·xH₂O) is well known as a biomass-conversion catalyst, yet the chemical pathways are unclear. Here we report solution chemistry of a new niobophosphate cluster [H₃Nb₉P₅O₄₁]⁹⁻ that reacts with H₂O₂ to form peroxo-Nb₄P₂, and this reaction can be reversed by hydrothermal treatment. We suggest that this cluster is a useful molecular model for the solid.