A Coordination Network That Catalyzes O₂-Based Oxidations

Reaction of Tb(III) and two bridging ligands, a redox-active benzoic acid-terminated hexavandate ([V6O13{(OCH2)3C(4-CONHC6H4CO2H)}2]2- (1) and 4,4‘-bis(pyridine-N-dioxide) (bpdo) produces a catalytically active open-framework pillared layer-type coordination polymer, Tb1. The network material catalyzes aerobic oxidation of PrSH to PrSSPr and the oxidation tetrahydrothiophene (THT) to tetrahydrothiophene oxide (THTO) by tert-butylhydroperoxide under ambient conditions. Tb(III) ions and bpdo units form two-dimensional (2D) coordination layers, and the 2D layers are connected by 1 to produce a three-dimensional coordination network. IR and powder X-ray diffraction of Tb1 before and after catalysis indicate the catalyst maintains an open framework structure during the catalytic reactions

A Coordination Network That Catalyzes O₂-Based Oxidations

Reaction of Tb(III) and two bridging ligands, a redox-active benzoic acid-terminated hexavandate ([V6O13{(OCH2)3C(4-CONHC6H4CO2H)}2]2- (1) and 4,4‘-bis(pyridine-N-dioxide) (bpdo) produces a catalytically active open-framework pillared layer-type coordination polymer, Tb1. The network material catalyzes aerobic oxidation of PrSH to PrSSPr and the oxidation tetrahydrothiophene (THT) to tetrahydrothiophene oxide (THTO) by tert-butylhydroperoxide under ambient conditions. Tb(III) ions and bpdo units form two-dimensional (2D) coordination layers, and the 2D layers are connected by 1 to produce a three-dimensional coordination network. IR and powder X-ray diffraction of Tb1 before and after catalysis indicate the catalyst maintains an open framework structure during the catalytic reactions

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-

A Coordination Network That Catalyzes O₂-Based Oxidations

Reaction of Tb(III) and two bridging ligands, a redox-active benzoic acid-terminated hexavandate ([V6O13{(OCH2)3C(4-CONHC6H4CO2H)}2]2- (1) and 4,4‘-bis(pyridine-N-dioxide) (bpdo) produces a catalytically active open-framework pillared layer-type coordination polymer, Tb1. The network material catalyzes aerobic oxidation of PrSH to PrSSPr and the oxidation tetrahydrothiophene (THT) to tetrahydrothiophene oxide (THTO) by tert-butylhydroperoxide under ambient conditions. Tb(III) ions and bpdo units form two-dimensional (2D) coordination layers, and the 2D layers are connected by 1 to produce a three-dimensional coordination network. IR and powder X-ray diffraction of Tb1 before and after catalysis indicate the catalyst maintains an open framework structure during the catalytic reactions

Mechanism of Reaction of Reduced Polyoxometalates with O₂ Evaluated by ¹⁷O NMR

Mechanism of Reaction of Reduced Polyoxometalates with O2 Evaluated by 17O NM

Mechanism in Polyoxometalate-Catalyzed Homogeneous Hydrocarbon Oxo Transfer Oxidation. The [Co₄(H₂O)₂P₂W₁₈O₆₈]^10-/<i>p</i>-Cyano-<i>N</i>,<i>N</i>-dimethylaniline <i>N</i>-Oxide Selective Catalytic Epoxidation System

The Co-substituted heteropolyanions [Co4P2W18O68]10- and [CoPW11O39]5- catalyze the highly selective epoxidation of disubstituted alkenes and stilbenes by p-cyano-N,N-dimethylaniline N-oxide (CDMANO). Terminal alkenes are not readily epoxidized. The following d-electron transition metal-substituted (TMSP) complexes are less selective and two orders of magnitude less reactive than the Co complexes:  [MnIIIPW11O39]4-, [MnIIPW11O39]5-, [FeIIIPW11O39]4-, and [NiIIPW11O39]5-. The system (TBA)8H2[Co4P2W18O68] (TBA = n-Bu4N+) (TBA1)/CDMANO/alkene/CH3CN solvent is homogeneous throughout. The values for K1 (constant for 1:1 association of the following ligands with 1 under the catalytic conditions in these studies = acetonitrile solution, 25 or 50 °C) are 275 ± 13 (N-methylimidazole), 4.3 ± 0.1 (pyridine), 59 ± 3 (4-picoline N-oxide), 22 ± 1 (4-cyanopyridine N-oxide), and 57 ± 5 (N-methylmorpholine N-oxide, MMNO, a model for CDMANO). Comparisons of the electronic absorption spectra of 1 under catalytic turnover and several other conditions indicate formation of the 1:1 CDMANO adduct, a result also consistent with thermodynamic binding and kinetic data. Chromatographic separation and spectral (UV−visible, NMR) evidence indicate that the brown color in the epoxidation reactions evident after many turnovers results from condensed heterocyclic structures from oxidation of the principal product derived from CDMANO during catalysis, p-cyano-N,N-dimethylaniline (CDMA). Evaluation of the kinetics of cyclohexene epoxidation by CDMANO over a wide range of conditions affords the following empirical rate law:  +{d[epoxide]/dt}initial = k‘[cyclohexene]i[CDMANO]i[1]total)/(k‘‘[CDMANO]i + k‘‘‘[cyclohexene]i + k‘‘‘‘[cyclohexene]i[CDMANO]i + k‘‘‘‘‘). This is inconsistent with several common catalytic oxygenation mechanisms but consistent with a three-step mechanism:  an initial pre-equilibrium association of 1 and CDMANO; loss of CDMA and formation of a reactive high-valent cobalt intermediate; and then transfer of oxygen from the intermediate to alkene

Tetrairon and Hexairon Hydroxo/Acetato Clusters Stabilized by Multiple Polyoxometalate Scaffolds. Structures, Magnetic Properties, and Chemistry of a Dimer and a Trimer

Investigation of the catalytically relevant γ-diiron(III) Keggin complexes in aqueous acetate buffer leads to a dimeric C2v-symmetric polyanion, [{Fe(OH)(OAc)}4(γ-SiW10O36)2]12- (3) and a trimeric C2-symmetric polyanion, [{Fe6(OH)9(H2O)2(OAc)2}(γ-SiW10O36)3]17- (4). Polyanion 3 incorporates a hydroxo/acetato-bridged tetrairon(III) core, while 4 incorporates a trigonal prismatic hydroxo/acetato-bridged hexairon(III) core. The monomeric building unit of 3 and 4, {γ-SiW10Fe2}, adopts the “out-of-pocket” structural motif (with two corner-sharing FeO6 coordination polyhedra no longer connected to the internal SiO4 tetrahedron of the Keggin unit) also observed in the {γ-SiW10Fe2}-type structures isolated from nonbuffered aqueous solutions. Following hydrolysis, 3 is converted to 4 as confirmed by 29Si NMR. Magnetic measurements establish that in both 3 and 4 all exchange interactions are antiferromagnetic

A Nanoring−Nanosphere Molecule, {Mo₂₁₄V₃₀}: Pushing the Boundaries of Controllable Inorganic Structural Organization at the Molecular Level

A controlled, Raman-monitored chemical reduction of a molybdate and vanadate mixture affords a new type of molybdenum-oxide-based cluster showing an unprecedented level of inorganic structural organization. The cluster incorporates two nanosized substructures (a ring and a sphere) in an open clam-like assembly. Multiple methods indicate that the nanoring contains delocalized electrons and the nanosphere contains localized but interacting electrons

Tetrairon and Hexairon Hydroxo/Acetato Clusters Stabilized by Multiple Polyoxometalate Scaffolds. Structures, Magnetic Properties, and Chemistry of a Dimer and a Trimer

Investigation of the catalytically relevant γ-diiron(III) Keggin complexes in aqueous acetate buffer leads to a dimeric C2v-symmetric polyanion, [{Fe(OH)(OAc)}4(γ-SiW10O36)2]12- (3) and a trimeric C2-symmetric polyanion, [{Fe6(OH)9(H2O)2(OAc)2}(γ-SiW10O36)3]17- (4). Polyanion 3 incorporates a hydroxo/acetato-bridged tetrairon(III) core, while 4 incorporates a trigonal prismatic hydroxo/acetato-bridged hexairon(III) core. The monomeric building unit of 3 and 4, {γ-SiW10Fe2}, adopts the “out-of-pocket” structural motif (with two corner-sharing FeO6 coordination polyhedra no longer connected to the internal SiO4 tetrahedron of the Keggin unit) also observed in the {γ-SiW10Fe2}-type structures isolated from nonbuffered aqueous solutions. Following hydrolysis, 3 is converted to 4 as confirmed by 29Si NMR. Magnetic measurements establish that in both 3 and 4 all exchange interactions are antiferromagnetic

A Nanoring−Nanosphere Molecule, {Mo₂₁₄V₃₀}: Pushing the Boundaries of Controllable Inorganic Structural Organization at the Molecular Level

A controlled, Raman-monitored chemical reduction of a molybdate and vanadate mixture affords a new type of molybdenum-oxide-based cluster showing an unprecedented level of inorganic structural organization. The cluster incorporates two nanosized substructures (a ring and a sphere) in an open clam-like assembly. Multiple methods indicate that the nanoring contains delocalized electrons and the nanosphere contains localized but interacting electrons