Modeling Reactive Metal Oxides. Kinetics, Thermodynamics, and Mechanism of M₃ Cap Isomerization in Polyoxometalates

An investigation of M3O13 unit (“M3 cap”) isomerization in the classical polytungstodiphosphates α- and β-P2W18O626- has been undertaken because cap isomerism is an important and structurally well-studied phenomenon in many polyoxometalate families. The relative thermodynamic stabilities of the α (more stable) versus β isomers were established both in the solid state by differential scanning calorimetry (4.36 ± 0.64 kcal/mol) and in solution by 31P NMR (3.80 ± 0.57 kcal/mol). The isomerization of β-P2W18O626- to α-P2W18O626-, followed by 31P NMR, has a bimolecular rate constant k2 of 9.3 × 10-1 M-1 s-1 at 343 K in pH 4.24 acetate buffer. Several lines of evidence establish the validity of suggestions in the literature that isomerization goes through a lacunary (defect) intermediate. First, the rate is proportional to [OH-]. Second, isomerization increases at higher ionic strengths, and a Debye−Hückel plot is consistent with a rate-limiting reaction between β-P2W18O626- and OH- (two species with a charge product of 6). Third, alkali-metal cations stabilize the bimolecular transition state (K+ > Na+ > Li+), consistent with recent ion-pairing studies in polyoxometalate systems. Fourth, the monovanadium-substituted products α1- and α2-P2VW17O627- (51V NMR δ −554 ppm) form during isomerization in the presence of VO2+. The known lacunary compounds (α1- and α2-P2W17O6110-) also react rapidly with the same vanadium precursor. Fifth, solvent studies establish that isomerization does not occur when OH- is absent. A mechanism is proposed involving attack of OH- on β-P2W18O626-, loss of monomeric W(VI) from the M3 (M3O13) terminal cap, isomerization of the resulting lacunary compound to α-P2W17O6110-, and finally reaction of this species with monomeric W(VI) to form the thermodynamic and observed product, α-P2W18O626-

[(Fe^III(OH₂)₂)₃(A-α-PW₉O₃₄)₂]^9- on Cationic Silica Nanoparticles, a New Type of Material and Efficient Heterogeneous Catalyst for Aerobic Oxidations

Polyoxometalates (POMs) electrostatically bind to silica nanoparticles coated with cationic aluminum oxide “(Si/AlO2)n+” to form a new type of material (the anionic POMs replace Cl- counterions associated with the cationic surface sites). Association of a new ∼D3h POM of formula [(FeIII(OH2)2)3(A-α-PW9O34)2]9- (1) with the cationic nanoparticles (to form “K81/(Si/AlO2)”) was studied in detail. Elemental analysis, particle sizes from both laser light scattering and TEM before and after association of 1, the size of 1 from X-ray crystallography, and other methods provide mutually consistent data that indicate about 58 K8[(FeIII(OH2)2)3(A-α-PW9O34)2]- monoanions associate with the average nanoparticle (diameter of the K81/(Si/AlO2) product = ∼17 nm). While heterogeneity of the cationic sites and roughness of the (Si/AlO2)n+ surfaces make the associated POMs structurally nonuniform, the equivalent of ∼1 monolayer of 1 is present in K81/(Si/AlO2). Remarkably, while 1, the precursor (Si/AlO2)n+, and the components of 1, each alone, are inactive as catalysts for O2/air-based oxidation of sulfides or aldehydes in solution, K81/(Si/AlO2) is an active catalyst for both reactions (facile reaction with air at low temperature)

[(Fe^III(OH₂)₂)₃(A-α-PW₉O₃₄)₂]^9- on Cationic Silica Nanoparticles, a New Type of Material and Efficient Heterogeneous Catalyst for Aerobic Oxidations

Polyoxometalates (POMs) electrostatically bind to silica nanoparticles coated with cationic aluminum oxide “(Si/AlO2)n+” to form a new type of material (the anionic POMs replace Cl- counterions associated with the cationic surface sites). Association of a new ∼D3h POM of formula [(FeIII(OH2)2)3(A-α-PW9O34)2]9- (1) with the cationic nanoparticles (to form “K81/(Si/AlO2)”) was studied in detail. Elemental analysis, particle sizes from both laser light scattering and TEM before and after association of 1, the size of 1 from X-ray crystallography, and other methods provide mutually consistent data that indicate about 58 K8[(FeIII(OH2)2)3(A-α-PW9O34)2]- monoanions associate with the average nanoparticle (diameter of the K81/(Si/AlO2) product = ∼17 nm). While heterogeneity of the cationic sites and roughness of the (Si/AlO2)n+ surfaces make the associated POMs structurally nonuniform, the equivalent of ∼1 monolayer of 1 is present in K81/(Si/AlO2). Remarkably, while 1, the precursor (Si/AlO2)n+, and the components of 1, each alone, are inactive as catalysts for O2/air-based oxidation of sulfides or aldehydes in solution, K81/(Si/AlO2) is an active catalyst for both reactions (facile reaction with air at low temperature)

Two Structures Toward Understanding Evolution from Surfactant-Polyoxometalate Lamellae to Surfactant-Encapsulated Polyoxometalates

Surfactant-POM (polyoxometalate) phases are fascinating in both their self-assembly behavior and their utility as catalysts, probes, and photochromic, electrochromic, and magnetic devices. Well-ordered lamellar phases are formed when the surfactant:POM ratio is 4:1 or 2:1, and these have been described in great detail from single-crystal X-ray diffraction studies. However, the surfactant-encapsulated clusters (SECs) with much larger surfactant:POM ratios do not form single-crystals readily. Thus less is known about their structural detail, and the evolution from the well-ordered lamellar phases to the SECs with increasing surfactant:POM ratio has not been detailed. We present here two structures that have resulted from an investigation of understanding the evolution of the surfactant-POM lamellar phase as the surfactant:POM ratio increases. [H2SiMo12O40][CH3CN]2[C16H33N(CH3)3]4 (monoclinic #4, P21 a = 12.636(2) Å, b = 20.577(4) Å, c = 22.364(4) Å, β = 93.394(4)°) holds true to the preference of 4:1 surfactant:POM ratio in well-ordered crystalline phases, whereas [HxSiMo12O40][C16H33N(CH3)3]5[CH3CN]4 (triclinic No. 2, P1̅, a = 12.513(7) Å, b = 23.37(1) Å, c = 24.44(1) Å, α = 93.418(8)°, β = 92.046(8)°, γ = 99.113(7)°) provides the first example of a surfactant-POM phase with a surfactant:POM ratio >4. This structure provides a glimpse of the structural evolution from ordered lamellar POM-surfactant phases to more disordered phases such as the SECs

Two Structures Toward Understanding Evolution from Surfactant-Polyoxometalate Lamellae to Surfactant-Encapsulated Polyoxometalates

Surfactant-POM (polyoxometalate) phases are fascinating in both their self-assembly behavior and their utility as catalysts, probes, and photochromic, electrochromic, and magnetic devices. Well-ordered lamellar phases are formed when the surfactant:POM ratio is 4:1 or 2:1, and these have been described in great detail from single-crystal X-ray diffraction studies. However, the surfactant-encapsulated clusters (SECs) with much larger surfactant:POM ratios do not form single-crystals readily. Thus less is known about their structural detail, and the evolution from the well-ordered lamellar phases to the SECs with increasing surfactant:POM ratio has not been detailed. We present here two structures that have resulted from an investigation of understanding the evolution of the surfactant-POM lamellar phase as the surfactant:POM ratio increases. [H2SiMo12O40][CH3CN]2[C16H33N(CH3)3]4 (monoclinic #4, P21 a = 12.636(2) Å, b = 20.577(4) Å, c = 22.364(4) Å, β = 93.394(4)°) holds true to the preference of 4:1 surfactant:POM ratio in well-ordered crystalline phases, whereas [HxSiMo12O40][C16H33N(CH3)3]5[CH3CN]4 (triclinic No. 2, P1̅, a = 12.513(7) Å, b = 23.37(1) Å, c = 24.44(1) Å, α = 93.418(8)°, β = 92.046(8)°, γ = 99.113(7)°) provides the first example of a surfactant-POM phase with a surfactant:POM ratio >4. This structure provides a glimpse of the structural evolution from ordered lamellar POM-surfactant phases to more disordered phases such as the SECs

A Baker−Figgis Isomer of Conventional Sandwich Polyoxometalates. H₂Na₁₄[Fe^III₂(NaOH₂)₂(P₂W₁₅O₅₆)₂], a Diiron Catalyst for Catalytic H₂O₂-Based Epoxidation

A Baker−Figgis Isomer of Conventional Sandwich Polyoxometalates. H2Na14[FeIII2(NaOH2)2(P2W15O56)2], a Diiron Catalyst for Catalytic H2O2-Based Epoxidatio

Cupric Decamolybdodivanadophosphate. A Coordination Polymer Heterogeneous Catalyst for Rapid, High Conversion, High Selectivity Sulfoxidation Using the Ambient Environment

A new type of coordination network polymer involving the redox-active polyanion, PV2Mo10O405-, and bridging −CuII(OH2)4− units, {[(CuII(OH2)4)3(OH)]PV2Mo10O40}n (1), has been characterized by X-ray crystallography and several other methods. It is the first efficient heterogeneous (insoluble) catalyst for selective and rapid sulfoxidation using only the ambient environment (air at room temperature). Catalytic activity is enhanced by soluble nitrate in nontoxic perfluoropolyether (PFPE) media

Two Structures Toward Understanding Evolution from Surfactant-Polyoxometalate Lamellae to Surfactant-Encapsulated Polyoxometalates

Surfactant-POM (polyoxometalate) phases are fascinating in both their self-assembly behavior and their utility as catalysts, probes, and photochromic, electrochromic, and magnetic devices. Well-ordered lamellar phases are formed when the surfactant:POM ratio is 4:1 or 2:1, and these have been described in great detail from single-crystal X-ray diffraction studies. However, the surfactant-encapsulated clusters (SECs) with much larger surfactant:POM ratios do not form single-crystals readily. Thus less is known about their structural detail, and the evolution from the well-ordered lamellar phases to the SECs with increasing surfactant:POM ratio has not been detailed. We present here two structures that have resulted from an investigation of understanding the evolution of the surfactant-POM lamellar phase as the surfactant:POM ratio increases. [H2SiMo12O40][CH3CN]2[C16H33N(CH3)3]4 (monoclinic #4, P21 a = 12.636(2) Å, b = 20.577(4) Å, c = 22.364(4) Å, β = 93.394(4)°) holds true to the preference of 4:1 surfactant:POM ratio in well-ordered crystalline phases, whereas [HxSiMo12O40][C16H33N(CH3)3]5[CH3CN]4 (triclinic No. 2, P1̅, a = 12.513(7) Å, b = 23.37(1) Å, c = 24.44(1) Å, α = 93.418(8)°, β = 92.046(8)°, γ = 99.113(7)°) provides the first example of a surfactant-POM phase with a surfactant:POM ratio >4. This structure provides a glimpse of the structural evolution from ordered lamellar POM-surfactant phases to more disordered phases such as the SECs

Asymmetric Sandwich-Type Polyoxoanions. Synthesis, Characterization, and X-ray Crystal Structures of Diferric Complexes [TM^IIFe^III₂(P₂W₁₅O₅₆)(P₂TM^II₂W₁₃O₅₂)]^16-, TM = Cu or Co

Reaction of the diferric sandwich-type polyoxometalate (NaOH2)2FeIII2(P2W15O56)216- (1) with excess aqueous Cu(II) or Co(II) yields a new type of d-electron-metal substituted polyoxometalate, [TMIIFeIII2(P2W15O56) (P2TMII2W13O52)]16-, TM = Cu (2), Co (3), respectively. The structure of the sodium salt of 2 (Na2), determined by single-crystal X-ray diffraction analysis (a = 13.4413(9) Å, b = 21.2590(15) Å, c = 25.5207(18) Å, α = 80.475(2)°, β = 85.555(2)°, γ = 89.563(2)°, triclinic, P1̄, R1 = 5.42%, based on 43097 independent reflections), consists of a defect Fe2Cu central unit sandwiched between two different trivacant Wells−Dawson-type units, P2W15 and P2Cu2W13, where the latter unit has two octahedral Cu(II) ions substituted for two adjacent belt W(VI) atoms. The CuO5OH2 octahedron in the central unit shows pronounced Jahn−Teller distortion. A low-resolution X-ray structure of Na3 is included in the Supporting Information. UV−visible, infrared, 31P NMR, cyclic voltammetric, and elemental analysis data are all consistent with the structure determined from the X-ray analysis. Cyclic voltammograms of 2 and 3 exhibit multiple electron-transfer processes under ambient conditions, and copper or cobalt incorporation into the framework of 1 results in a substantial pertubation of the electrochemical properties of the polyoxotungstate framework. The tetra-n-butylammonium salts of 2 and 3 (readily prepared by metathesis) are stable and effective catalysts for the oxidation of some alkenes with high yields based on H2O2

Cupric Decamolybdodivanadophosphate. A Coordination Polymer Heterogeneous Catalyst for Rapid, High Conversion, High Selectivity Sulfoxidation Using the Ambient Environment

A new type of coordination network polymer involving the redox-active polyanion, PV2Mo10O405-, and bridging −CuII(OH2)4− units, {[(CuII(OH2)4)3(OH)]PV2Mo10O40}n (1), has been characterized by X-ray crystallography and several other methods. It is the first efficient heterogeneous (insoluble) catalyst for selective and rapid sulfoxidation using only the ambient environment (air at room temperature). Catalytic activity is enhanced by soluble nitrate in nontoxic perfluoropolyether (PFPE) media