Robust, Alkali-Stable, Triscarbonyl Metal Derivatives of Hexametalate Anions, [M6O19{M‘(CO)3}n](8-n)- (M = Nb, Ta; M‘ = Mn, Re; n = 1, 2)

Two binuclear Ni-inserted polyoxotantalates based on {NiTa10O32} units with catalytic activity

Two Ni-inserted polyoxotantalates, K5.5Na2H0.5[Ni(H2O)2{NiTa10O30(OH)2}]·21H2O (1) and K6Na4[Ni(en){NiTa10O32}]·22H2O (2, en = ethanediamine), were synthesized in this work. Crystallographic data analyses reveal that 1 and 2 have similar configurations. A minor difference between these two structures is that the {Ni(H2O)2} unit in 1 is replaced by {Ni(en)} unit in 2. Notably, the other Ni in 1 and 2 is located as a heteroatom at the center of the {Ta10} unit, which is reported in POTas for the first time. Moreover, 2 exhibits excellent catalytic performance in transesterification reactions in a preliminary exploration of the catalytic ability of the synthesized POTas.

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.

Hexameric polyoxotantalate with proton conduction properties

The first Fe-implanted polyoxotantalate (POTa), K12Na14H7.4[Fe10.7Ta1.3O8(OH)8(H2O)2(Ta6O19)6]·114.5H2O (1), has been obtained by self-assembly in alkaline solution. The polyanion consists of six Lindqvist-type {Ta6} units linked together by {Fe10.7Ta1.3}. The compound not only possesses the highest nuclearity transition metal-oxygen cluster, but also has the highest degree of polymerization in the POTa field to date. And 1 possesses remarkable proton conduction.

All-inorganic ferric wheel based on hexaniobate-anion linkers

Utilizing the inherent ability of Lindquist-type hexaniobate cluster-anions, [Nb₆O₁₉]^8-, to serve as oxo-donor ligands in complexes with transition-metal cations, we report the synthesis and characterization of the first all-inorganic "ferric" wheel, Li₄₈[(Nb₆O₁₉)₈Fe₈(OH)₈]·88H₂O, comprised of eight Fe atoms linked by eight hexaniobate cluster-anion ligands. Bond valence sum analysis of the X-ray structure and the synthesis conditions themselves indicate that the Fe atoms are in the +3 oxidation state. This is confirmed by magnetic susceptibility and electron paramagnetic resonance (EPR) measurements which indicate the presence of high spin (S = 5/2) Fe(III) ions. In addition, magnetic susceptibility measurements reveal long-range superexchange antiferromagnetic interactions between the hexaniobate-ligand separated Fe³⁺ ions (J = -0.22 cm^-1). More generally, the results suggest the use of hexaniobate cluster-anions as linkers in the synthesis of other two- or three-dimensional polyoxometalate framework structures.

Synthesis, crystal structure and selected properties of K2[Ni(dien)2]{[Ni(dien)]2Ta6O19}·11 H2O

The new compound K2[Ni(dien)2]{[Ni(dien)]2Ta6O19}·11 H2O crystallized at room temperature applying a diffusion based reaction in a H2O/DMSO mixture using K8{Ta6O19}·16 H2O, Ni(NO3)2·6H2O and dien (diethylenetriamine). In the crystal structure, the Lindqvist-type anion [Ta6O19]8– is structurally expanded by two octahedrally Ni2+-centered complexes via three Ni–µ 2-O–Ta bonds thus generating the new {[Ni(dien)]2Ta6O19}4– anion. Two KO8 polyhedra share a common edge to form a K2O14 moiety, which connects the {[Ni(dien)]2Ta6O19}4– cluster shells into chains. The isolated [Ni(dien)2]2+ complexes are located in voids generated by the structural arrangement of the chains. An extended hydrogen bonding network between the different constituents generates a 3D network. The crystal water molecules can be thermally removed to form a highly crystalline dehydrated compound. Partial water uptake leads to the formation of a crystalline intermediate with a reduced unit cell volume compared to the fully hydrated sample. Water sorption experiments demonstrate that the fully dehydrated sample can be fully reconverted to the hydrated compound. The crystal field splitting parameters for the octahedrally coordinated Ni2+-centered complexes have been evaluated from an UV/Vis spectrum yielding D q = 1056 cm−1 and B = 887 cm−1.

Base Catalysis of Sodium Salts of [Ta6−xNbxO19]8− Mixed-Oxide Clusters

The solid base catalysis of sodium salts of Lindqvist-type metal oxide clusters was investigated using a Knoevenagel condensation reaction. We successfully synthesized the sodium salts of Ta and Nb mixed-oxide clusters Na8−nHn[(Ta6−xNbx)O19]·15H2O (Na-Ta6−xNbx, n = 0, 1, x = 0–6) and found them to exhibit activity for proton abstraction from nitrile substrates with a pKa value of 23.8, which is comparable to that of the conventional solid base MgO. The Ta-rich Na-Ta6 and Na-Ta4Nb2 exhibited high activity among Ta and Nb mixed-oxide clusters. Synchrotron X-ray diffraction (SXRD) measurements, Fourier-transform infrared (FT-IR) spectroscopy, and X-ray absorption spectroscopy (XAS) revealed the structure of Na-Ta6−xNbx: (1) The crystal structure changed from Na7H[M6O19]·15H2O to Na8[M6O19]·15H2O (M = Ta or Nb) by the anisotropic expansion of the unit cell with an increase in Ta content; (2) Highly symmetrical Lindqvist [Ta6−xNbxO19]8− was generated in Na-Ta4Nb2 and Na-Ta6 because of the symmetrical association of Na+ ions with [Ta6−xNbxO19]8− in the structure. DFT calculation revealed that the Lindqvist structures with high symmetry have large NBO charges on surface oxygen species, which are strongly related to base catalytic activity, whereas the composition hardly affects the NBO charges. The above results showed that the Brønsted base catalysis was sensitive to the symmetry of the Lindqvist [Ta6−xNbxO19]8− structure. These findings contribute to the design of solid base catalysts composed of anionic metal oxide clusters with alkaline-metal cations.

3d Transition-Metal-Mediated Columbite Nanocatalysts for Decentralized Electrosynthesis of Hydrogen Peroxide

Decentralized electrosynthesis of hydrogen peroxide (H₂ O₂ ) via oxygen reduction reaction (ORR) can enable applications in disinfection control, pulping and textile bleaching, wastewater treatment, and renewable energy storage. Transition metal oxides are usually not efficient catalysts because they are more selective to produce H₂ O. Here, it is shown that divalent 3d transition metal cations (Mn, Fe, Co, Ni, and Cu) can control the catalytic activity and selectivity of columbite nanoparticles. They are synthesized using polyoxoniobate (K₇ HNb₆ O₁₉ ·13H₂ O) and divalent metal cations by a hydrothermal method. The optimal NiNb₂ O₆ holds an H₂ O₂ selectivity of 96% with the corresponding H₂ O₂ Faradaic efficiency of 92% in a wide potential window from 0.2 to 0.6 V in alkaline electrolyte, superior to other transition metal oxide catalysts. Ex situ X-ray photoelectron and operando Fourier-transformed infrared spectroscopic studies, together with density functional theory calculations, reveal that 3d transition metals shift the d-band center of catalytically active surface Nb atoms and change their interactions with ORR intermediates. In an application demonstration, NiNb₂ O₆ delivers H₂ O₂ productivity up to 1 mol_H2O2 g_cat ^-1 h^-1 in an H-shaped electrolyzer and can yield catholytes containing 300 × 10^-3 m H₂ O₂ to efficiently decomposing several organic dyes. The low-cost 3d transition-metal-mediated columbite catalysts show excellent application potentials.

Coordination capacity of Keggin anions as polytopic ligands: case study of [VNb12O40]15

Reaction of Na9H4[VNb12O40{NbO(CO3)}2] with [(C6H6)RuCl2]2 (molar ratio {VNb12} : {(C6H6)Ru} = 1 : 4) in aqueous solution gives a mixture of [α-{(C6H6)Ru}4VNb12O40]7- and [α-{(C6H6)Ru}3VNb12O40]9-. Direct acetone diffusion into mother liquor leads to crystallization of Na6H[α-{(C6H6)Ru}4VNb12O40]·41.25H2O (1), characterized by single crystal X-ray diffraction (SCXRD). This anion has four organometallic fragments coordinated to the α-Keggin type [VNb12O40]15- backbone in different manner. Three {(C6H6)Ru}2+ groups cap triangular faces and one group a rectangular face of [VNb12O40]15-. Equilibrated mixture of [α-{(C6H6)Ru}4VNb12O40]7- and [α-{(C6H6)Ru}3VNb12O40]9- was studied by 1H DOSY NMR, HPLC-ICP-AES and HPLC-ESI-MS combined techniques. Direct chromatographic separation of these complexes results in unexpected transformation of both species into [α-{(C6H6)Ru}5VNb12O40]5-, isolated and characterized as Na5[α-{(C6H6)Ru}5VNb12O40]·16H2O (2). This anion contains five coordinated organometallic groups occupying both triangular and rectangular faces.

A 1D Helical Chain Heterpolyniobate Templated by AsO33

A novel polyoxoniobate cluster directed by AsO₃^3-, {[Cu(phen)]₅[Nb₆O₁₉][As₂Nb₆O₂₂]}₂ (1a), has been successfully synthesized by a "two-pot" strategy, namely, the conventional aqueous solution and diffusional methods. It incorporates two {[Cu(phen)]₂Nb₆O₁₉} and two {[Cu(phen)]₃As₂Nb₆O₂₂} subunits. Surprisingly, by changing the reaction conditions, one of the subunits {[Cu(phen)]₂Nb₆O₁₉} was isolated as a guanidinium salt. These architectures were structurally characterized by single crystal X-ray diffraction, TGA, IR, elemental analysis and PXRD, etc. Furthermore, the behavior of 1 in aqueous solution was measured by electrospray ionization mass spectrometry, and the magnetic property in the low temperature range indicates the presence of antiferromagnetic interactions.

Self-assembly of polyoxovanadate-capped polyoxoniobates and their catalytic decontamination of sulfur mustard simulants

Two novel polyoxovanadate-capped polyoxoniobates with non-classical structural motifs have been successfully assembled, which are active heterogeneous catalysts for the oxidative detoxification of sulfur mustard simulants.

Discovery of the selenotantalate building block and its lanthanide derivatives: design, synthesis, and RhB decolorization properties

Herein we present the first Se/Ta polyoxometalate and its lanthanide derivatives with good photocatalytic activity for the decolorization of RhB.

A new phosphotungstate-supported rhenium carbonyl derivative: synthesis, characterization and catalytic selective oxidation of thiophenes

We synthesized a new phosphotungstate-supported rhenium carbonyl derivative, [N(CH₃)₄]₆H₄[P₂W₁₇O₆₂{Re(CO)₃}₂]·25H₂O, existing the first {Re₂} fragment. Moreover, it also exhibits high efficiency for the selective oxidation of thiophenes.

Synthesis, characterization and catalytic epoxidation properties of a new tellurotungstate(iv)-supported rhenium carbonyl derivative

We synthesized a new tellurotungstate(iv)-supported rhenium carbonyl derivative, Na₂H₂[(CH₃)₄N]₆[Te₂W₂₀O₇₀{Re(CO)₃}₂]·20H₂O (1). Additionally, compound 1 showed excellent catalytic activity in the selective epoxidation of alkenes under comparatively mild reaction conditions.

Inorganic–organic hybrid high-dimensional polyoxotantalates and their structural transformations triggered by water

In this work, novel dimeric polyoxotantalate (POTa) clusters {Cu(en)(Ta₆O₁₉)}₂/{Cu(enMe)(Ta₆O₁₉)}₂ were introduced as SBUs to construct a new family of extended POTa materials, including the first two 3D POTa frameworks and two 2D POTa layers.

Polyoxometalate-supported metal carbonyl derivatives: from synthetic strategies to structural diversity and applications

This review aims to give an overview of the POM-supported metal carbonyl complexes obtained so far, focusing on their structural diversity and potential photochemical and catalytic properties.

A novel Ta/W mixed-addendum polyoxometalate with photocatalytic properties

An unprecedented polytantalotungstate (POTT), Cs_12.5K_4.5H[Ta₁₂Si₄W₃₇O₁₅₈]·25H₂O (1), based on the {SiW₉Ta₃O₄₀}^7- cluster was hydrothermally synthesized. A photocatalytic study revealed that 1 exhibits significant photocatalytic water splitting activity.

Immobilization of carbonyl rhenium tripods on the surface of a trinickel-substituted Dawson-type polyoxotungstate

A monomeric trinickel(ii)-substituted phosphotungstate [H₉P₂W₁₅O₆₂Ni₃]⁹⁻ ion is stabilized by grafting three {Re(CO)₃} organometallic groups on the surface of a trinickel cluster for the first time. The resulting polyanions [P₂W₁₅O₅₆Ni₃(H₂O)₃(μ₃-OH)₃(Re(CO)₃)₃]⁶⁻ stacked along the a-axis to form a wavy line.

An isotetramolybdate-supported rhenium carbonyl derivative: synthesis, characterization, and use as a catalyst for sulfoxidation

We successfully synthesized an isotetramolybdate-supported rhenium carbonyl derivative, [(CH₃)₄N]₄[{Re(CO)₃}₄(Mo₄O₁₆)]·H₂O, which showed excellent catalytic activity in the selective oxidation of sulfides under comparatively mild reaction conditions.

Biomolecule-mediated hydrothermal synthesis of polyoxoniobate-CdS nanohybrids with enhanced photocatalytic performance for hydrogen production and RhB degradation

Using a biomolecule of l-cystine as the sulfur source and coordinating agent, polyoxoniobate-CdS nanohybrids were successfully synthesized under mild hydrothermal conditions. The adsorption of ammonium group (-NH₂) in l-cystine molecular structure on the surface of CdS renders the amine-anchored CdS positively charged, which readily combines with the negatively charged polyoxoniobate clusters in terms of the electrostatic interaction. The as-obtained polyoxoniobate-CdS nanohybrids exhibit much superior activity for H₂ evolution and RhB degradation under visible light as compared to the unhybridized CdS and polyoxoniobate. After co-loading Nb₆ and NiS as cocatalyst, the H₂-evolution activity of the nanohybrids is further increased up to 39 times as high as that of naked CdS, which can be attributed to an enhanced electron-transfer by adopting polyoxoniobate as electron-acceptor to retard the electron-hole recombination. The work may open an avenue for the green synthesis of cost-effective POMs-CdS nanohybrid photocatalysts for solar energy applications.

Discovery of Heteropolytantalate: Synthesis and Structure of Two 6-Peroxotantalo-4-phosphate Clusters

Polyoxometalates (POMs) of Nb and Ta are greatly different from those of Mo, W, and V that have been studied extensively and developed well. The latter can be formed simply by acidification of their aqueous monomeric oxoanions and has found application areas from catalysis to magnetism, materials science, medicine, and nanotechnology. Even now the polyoxoniobate (PONb) chemistry has accelerated dramatically over the last 15 years, and a vast expansion of available PONbs has been reported. However, after nearly 200 years of POM research, Ta-based POM chemistry is still at its infant stage and only dominated by the isopolyoxotantalate ions (Ta₆ and Ta₁₀) and transition-metal-capped Ta₆ species, along with two Ti-substituted polyoxotantalates [Ti₂Ta₈O₂₈]^8- and [Ti₁₂Ta₆O₄₄]^10- reported very recently. In this study, we discover two novel peroxotantalophosphate clusters [P₄(TaO₂)₆O₂₅]^12- (1) and [P₄(TaO₂)₆O₂₄]^10- (2) by incorporating phosphorus heteroatom into Ta-oxo framework, which represent the first two examples of heteropolytantalate. Interestingly, two P₂Ta₃ half-units are cis- and trans-condensed in 1 and 2, leading to "open" and "closed" configurations, respectively. These two chemically and structurally related clusters can be isolated in a controlled manner, and the yields are relatively high. Both compounds were characterized in the solid state by single-crystal X-ray diffraction, ³¹P MAS NMR, FT-IR, TGA, and elemental analysis as well as by ³¹P NMR in solution. The results presented here provide a strategy to be applicable to other heteroatom-incorporated polyoxotantalates and further expand the phase space for polyoxotantalate chemistry.

Lanthanide derivatives of Ta/W mixed-addendum POMs as proton-conducting materials

Four lanthanide derivatives were isolated by the self-assembly of {P₂W₁₅Ta₃O₆₂} and lanthanide ions, which showed good proton conduction performance.

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.

Isopentatungstate-supported metal carbonyl derivative: synthesis, characterization, and catalytic properties for alkene epoxidation

An isopentatungstate-supported metal carbonyl derivative has been synthesized and characterized, and can efficiently catalyze alkene epoxidation with high activity.

Synthesis and characterization of a series of novel polyoxometalate-supported carbonyl manganese derivatives

A series of novel heteropolytungstates-supported carbonyl manganese derivatives have been synthesized, which are the first examples of grafting [Mn(CO)₃]⁺ moieties and Mn²⁺ on the tungsto-antimonates/bismutates.

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.

Antimony-dependent expansion for the Keggin heteropolyniobate family

Antimony-dependent syntheses leading to the expansion of the Keggin heteropolyoxoniobate family involved nine new members.

A nona-vacant Keggin-type tricarbonyl rhenium derivative {[PMo3O16][Re(CO)3]4}5− and its catalytic performance for CO2 cycloaddition reactions

A nona-vacant Keggin-type tricarbonyl rhenium derivative {[PMo₃O₁₆][Re(CO)₃]₄}⁵⁻ was synthesized and its frontier orbitals were computed. Interestingly, it showed good catalytic activity for the CO₂ cycloaddition reaction.