Latency for All: Enabling Latency of Hoveyda-Grubbs Second-Generation Catalysts by Adding Phosphite Ligands.

The Hoveyda-Grubbs catalyst (HG2, M720 Umicore) is among the most widely used catalysts in olefin metathesis reactions. Given the usefulness of HG2 and the great interest in developing latent olefin metathesis catalysts for numerous applications, we developed a method to introduce phosphite molecules as ancillary ligands into the precatalyst framework. This modification alters the geometry of the complex from an active trans-dichloro form to a latent cis-dichloro species. Most unusually, the ligands coordinate to HG2 only in solidified solutions (most likely due to entropic factors), providing latent catalysts that can be activated on demand by heat or light by regenerating the original HG2 catalyst. Of particular interest is the use of these latent catalysts in ring-opening metathesis polymerization (ROMP) reactions and 3D printing methods. Indeed, the novel complexes displayed the required latency toward ROMP monomers, even the most reactive dicyclopentadiene. Irradiation with 405 nm light readily results in the expedited formation of the desired polymers. This novel approach provides a general and straightforward way to access efficient and well-defined latent olefin metathesis catalysts.

Entrapment of metastable nanocrystals by polyoxometalates.

Entirely inorganic polyoxometalate cluster-anion ligands are herein used to entrap and impart heat resistance to metastable metal-oxide based NCs. This is demonstrated by trapping 6-line ferrihydrite NCs that form as intermediates in the spontaneous conversion of ß-FeOOH to hematite (α-Fe2O3). The results suggest that polyoxometalate ligation may prove to be a general method for stabilizing metastable phases as the cores of entirely inorganic macro-anion like complexes.

Polyoxometalate-Complexed Indium Hydroxide: Atomically Homogeneous Impregnation via Countercation Exchange.

Metal hydroxides catalyze organic transformations and photochemical processes and serve as precursors for the oxide layers of functional multicomponent devices. However, no general methods are available for the preparation of stable water-soluble complexes of metal hydroxide nanocrystals (NCs) that might be more effective in catalysis and serve as versatile precursors for the reproducible fabrication of multicomponent devices. We now report that InIII-substituted monodefect Wells-Dawson (WD) polyoxometalate (POM) cluster anions, [α2-P2W17O61InIIIOH)]8-, serve as ligands for stable, water-soluble complexes, 1, of platelike, predominantly cubic-phase (dzhalindite) In(OH)3 NCs that after optimization contain ca. 10% InOOH. Images from cryogenic tranmsission electron microscopy reveal numerous WD ligands at the surfaces of platelike NCs, with average dimensions of 17 × 28 × 2 nm, each complexed by an average of ca. 450 InIII-substituted WD cluster anions and charge-balanced by 3600 Na+ countercations. Facilitated by the water solubility of 1, countercation exchange is used to stoichiometrically disperse ca. 1800 Cu2+ ions in an atomically homogeneous fashion around the surfaces of each NC core. The utility of this impregnation method is illustrated by using the ion-exchanged material as an electrocatalyst that reduces CO2 to CO 15 times faster per milligram of Cu than does K6Cu[P2CuII(H2O)W17O61] (control) alone. More generally, the findings point to POM complexation as a promising method for stabilizing and solubilizing reactive d-, p-, and f-block metal hydroxide NCs and for enabling their utilization as versatile components in the fabrication of functional multicomponent materials.

Emergence of Visible-Light Water Oxidation Upon Hexaniobate-Ligand Entrapment of Quantum-Confined Copper-Oxide Cores.

The formation of small 1 to 3 nm organic-ligand free metal-oxide nanocrystals (NCs) is essential to utilization of their attractive size-dependent properties in electronic devices and catalysis. We now report that hexaniobate cluster-anions, [Nb6 O19 ]8- , can arrest the growth of metal-oxide NCs and stabilize them as water-soluble complexes. This is exemplified by formation of hexaniobate-complexed 2.4-nm monoclinic-phase CuO NCs (1), whose ca. 350 Cu-atom cores feature quantum-confinement effects that impart an unprecedented ability to catalyze visible-light water oxidation with no added photosensitizers or applied potentials, and at rates exceeding those of hematite NCs. The findings point to polyoxoniobate-ligand entrapment as a potentially general method for harnessing the size-dependent properties of very small semiconductor NCs as the cores of versatile, entirely-inorganic complexes.

Complexed Semiconductor Cores Activate Hexaniobate Ligands as Nucleophilic Sites for Solar-Light Reduction of CO2 by Water.

Although pure and functionalized solid-state polyniobates such as layered perovskites and niobate nanosheets are photocatalysts for renewable-energy processes, analogous reactions by molecular polyoxoniobate cluster-anions are nearly absent from the literature. We now report that under simulated solar light, hexaniobate cluster-anion encapsulated 30-NiII -ion "fragments" of surface-protonated cubic-phase-like NiO cores activate the hexaniobate ligands towards CO2 reduction by water. Photoexcitation of the NiO cores promotes charge-transfer reduction of NbV to NbIV , increasing electron density at bridging oxo atoms of Nb-µ-O-Nb linkages that bind and convert CO2 to CO. Photogenerated NiO "holes" simultaneously oxidize water to dioxygen. The findings point to molecular complexation of suitable semiconductor "fragments" as a general method for utilizing electron-dense polyoxoniobate anions as nucleophilic photocatalysts for solar-light driven activation and reduction of small molecules.

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.

Soluble Complexes of Cobalt Oxide Fragments Bring the Unique CO2 Photoreduction Activity of a Bulk Material into the Flexible Domain of Molecular Science.

The deposition of metal oxides is essential to the fabrication of numerous multicomponent solid-state devices and catalysts. However, the reproducible formation of homogeneous metal oxide films or of nanoparticle dispersions at solid interfaces remains an ongoing challenge. Here we report that molecular hexaniobate cluster anion complexes of structurally and electronically distinct fragments of cubic-spinel and monoclinic Co3O4 can serve as tractable yet well-defined functional analogues of bulk cobalt oxide. Notably, the energies of the highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) of the molecular complexes, 1, closely match the valence- and conduction-band (VB and CB) energies of the parent bulk oxides. Use of 1 as a molecular analogue of the parent oxides is demonstrated by its remarkably simple deployment as a cocatalyst for direct Z-scheme reduction of CO2 by solar light and water. Namely, evaporation of an aqueous solution of 1 on TiO2-coated fluorinated tin oxide windows (TiO2/FTO), immersion in wet acetonitrile, and irradiation by simulated solar light under an atmosphere of CO2 give H2, CO, and CH4 in ratios nearly identical to those obtained using 20 nm spinel-Co3O4 nanocrystals, but 15 times more rapidly on a Co basis and more rapidly overall than other reported systems. Detailed investigation of the photocatalytic properties of 1 on TiO2/FTO includes confirmation of a direct Z-scheme charge-carrier migration pathway by in situ irradiated X-ray photoelectron spectroscopy. More generally, the findings point to a potentially important new role for coordination chemistry that bridges the conceptual divide between molecular and solid-state science.

Self-Assembly and Ionic-Lattice-like Secondary Structure of a Flexible Linear Polymer of Highly Charged Inorganic Building Blocks.

Among molecular building blocks, metal oxide cluster anions and their countercations provide multiple options for the self-assembly of functional materials. Currently, however, rational design concepts are limited to electrostatic interactions with metal or organic countercations or to the attachment and subsequent reactions of functionalized organic ligands. We now demonstrate that bridging µ-oxo linkages can be used to string together a bifunctional Keggin anion building block, [PNb2Mo10O40]5- (1), the diniobium(V) analogue of [PV2Mo10O40]5- (2). Induction of µ-oxo ligation between the NbV═O moieties of 1 in acetonitrile via step-growth polymerization gives linear polymers with entirely inorganic backbones, some comprising over 140 000 repeating units, each with a 3- charge, exceeding that of previously reported organic or inorganic polyelectrolytes. As the chain grows, its flexible µ-oxo-linked backbone, with associated countercations, coils into a compact 270 nm diameter spherical secondary structure as a result of electrostatic interactions not unlike those within ionic lattices. More generally, the findings point to new options for the rational design of multidimensional structures based on µ-oxo linkages between NbV═O-functionalized building blocks.

Selective Oxidation by H5[PV2Mo10O40] in a Highly Acidic Medium.

Dissolution of the polyoxometalate (POM) cluster anion H5[PV2Mo10O40] (1; a mixture of positional isomers) in 50% aq H2SO4 dramatically enhances its ability to oxidize methylarenes, while fully retaining the high selectivities typical of this versatile oxidant. To better understand this impressive reactivity, we now provide new information regarding the nature of 1 (115 mM) in 50% (9.4 M) H2SO4. Data from 51V NMR spectroscopy and cyclic voltammetry reveal that as the volume of H2SO4 in water is incrementally increased to 50%, V(V) ions are stoichiometrically released from 1, generating two reactive pervanadyl, VO2+, ions, each with a one-electron reduction potential of ca. 0.95 V (versus Ag/AgCl), compared to 0.46 V for 1 in 1.0 M aq H2SO4. Phosphorus-31 NMR spectra obtained in parallel reveal the presence of PO43-, which at 50% H2SO4 accounts for all the P(V) initially present in 1. Addition of (NH4)2SO4 leads to the formation of crystalline [NH4]6[Mo2O5(SO4)4] (34% yield based on Mo), whose structure (from single-crystal X-ray diffraction) features a corner-shared, permolybdenyl [Mo2O5]2+ core, conceptually derived by acid condensation of two MoO3 moieties. While 1 in 50% aq H2SO4 oxidizes p-xylene to p-methylbenzaldehyde with conversion and selectivity both greater than 90%, reaction with VO2+ alone gives the same high conversion, but at a significantly lower selectivity. Importantly, selectivity is fully restored by adding [NH4]6[Mo2O5(SO4)4], suggesting a central role for Mo(VI) in attenuating the (generally) poor selectivity achievable using VO2+ alone. Finally, 31P and 51V NMR spectra show that intact 1 is fully restored upon dilution to 1 M H2SO4.

Alcohols as Latent Hydrophobes: Entropically Driven Uptake of 1,2-Diol Functionalized Ligands by a Porous Capsule in Water.

Alcohols, with hydroxyl groups compositionally identical to water itself, are consummate hydrophiles, whose high solubilities preclude spontaneous self-assembly in water. Nevertheless, the solute-solvent interactions associated with their highly favorable solvation enthalpies impose substantial entropic costs, similar in magnitude to those that drive the hydrophobic assembly of alkanes. We now show that under nanoconfined conditions this normally dormant "hydrophobicity" can emerge as the driving force for alcohol encapsulation. Using a porous molecular capsule, the displacement of endohedrally coordinated formate ligands (HCO2-) by 1,2-hydroxyl-functionalized l-glycerate (l-gly, l-HOCH2(HO)CHCO2-) was investigated by van't Hoff analysis of variable-temperature 1H NMR in D2O. At pD 5.8, l-gly uptake is enthalpically inhibited. Upon attenuation of this unfavorable change in enthalpy by cosequestration of protons within the alcoholic environment provided by encapsulated diol-functionalized ligands, - TΔ S° dominates over Δ H°, spontaneously filling the capsule to its host capacity of 24 l-gly ligands via an entropically driven hydrophobic response.

A Simple Coulombic Model for 31P NMR Spectra of Cluster-Encapsulated Phosphorus Atoms.

While sophisticated computational methods can predict 31P NMR spectra of phosphorus atoms encapsulated within Keggin-derived heteropoly tungstate and molybdate cluster anions, calculated and experimental chemical shift values typically deviate considerably from one another. Motivated by the observation that experimentally determined 31P chemical shift values within a series of water-soluble plenary and metal-cation substituted lacunary Keggin anions, [PM nW11O39](7- n)- (M n = Ag+, Zn2+, Nb5+, W6+) and [(PW11O39)2M n](14- n)- (M n = Y3+, Zr4+), varied as a linear function of the oxidation states, n, of the complexed M n cations, a linear correlation was sought between observed chemical shift values and the net Coulombic forces experienced by the encapsulated phosphorus atoms. The Coulombic model based on Shannon radii, published electronegativity values, and bond angles from X-ray crystallographic data remarkably accounted for the relative 31P chemical shift values of phosphorus atoms in over 50 metal-oxide cluster anions, including large structures comprised of up to four Keggin-derived fragments with an overall R2 value of 0.974. With the model being applied here to three cluster anions whose 31P chemical shift values are reported here for the first time, predicted and experimental values differed by only ±0.4 ppm.

Influence of Anionic Ligand Exchange in Latent Sulfur-Chelated Ruthenium Precatalysts.

Four new cis-dianionic S-chelated ruthenium benzylidenes were synthesized by chloride ligand exchange. The special cis-dianionic conformation of these complexes contributed to a particularly facile anion exchange process, producing room-temperature-latent precatalysts. Their catalytic activity was strongly influenced by the solvent used. The latent iodide complex very efficiently promoted ring-closing metathesis by heating in toluene. Conversely, carboxylate ligands produced quite poor catalysts, but could abstract chlorides from chlorinated solvents to transform into active precatalysts. In tetrahydrofuran (THF), the S-chelated dichloro complex was shown to promote cycloisomerization instead of metathesis; however, the metathesis activity in THF could be recovered in the presence of phenylacetylene as a cocatalyst. Under the same conditions, all the other complexes required addition of LiCl to mimic this dichotomous behavior.

Crystal structure determination of rac-11'-(1-acetyl-1H-indazol-3-yl)-11',11a'-di-hydro-10'H,17'H-spiro-[indene-2,18'-[5a,16b]methano-tri-indeno[1,2-b:1',2'-d:2'',1''-g]oxocine]-1,3,10',12',17'(10a'H)-penta-one aceto-nitrile 1.5-solvate.

The title compound, C46H26N2O7·1.5CH3CN, is the aldol condensation product of bindone with indazole-3-carbaldehyde followed by double inter-molecular cyclization. The asymmetric unit, which has monoclinic P21/c symmetry, contains two independent mol-ecules of the title compound and three aceto-nitrile mol-ecules. The title mol-ecule comprises a central eight-membered ring, which contains an enol-ester, from which five arms extend. The arms exhibit inter-molecular inter-actions within the crystal lattice between mol-ecules of the title compound and with co-crystallized solvent mol-ecules (aceto-nitrile).