Remotely Bonded Bridging Dioxygen Ligands Enhance Hydrogen Transfer in a Silica-Supported Tetrairidium Cluster Catalyst.

A longstanding challenge in catalysis by noble metals has been to understand the origin of enhancements of rates of hydrogen transfer that result from the bonding of oxygen near metal sites. We investigated structurally well-defined catalysts consisting of supported tetrairidium carbonyl clusters with single-atom (apical iridium) catalytic sites for ethylene hydrogenation. Reaction of the clusters with ethylene and H2 followed by O2 led to the onset of catalytic activity as a terminal CO ligand at each apical Ir atom was removed and bridging dioxygen ligands replaced CO ligands at neighboring (basal-plane) sites. The presence of the dioxygen ligands caused a 6-fold increase in the catalytic reaction rate, which is explained by the electron-withdrawing capability induced by the bridging dioxygen ligands, consistent with the inference that reductive elimination is rate-determining. Electronic-structure calculations demonstrate an additional role of the dioxygen ligands, changing the mechanism of hydrogen transfer from one involving equatorial hydride ligands to that involving bridging hydride ligands. This mechanism is made evident by an inverse kinetic isotope effect observed in ethylene hydrogenation reactions with H2 and, alternatively, with D2 on the cluster incorporating the dioxygen ligands and is a consequence of quasi-equilibrated hydrogen transfer in this catalyst. The same mechanism accounts for rate enhancements induced by the bridging dioxygen ligands for the catalytic reaction of H2 with D2 to give HD. We posit that the mechanism involving bridging hydride ligands facilitated by oxygen ligands remote from the catalytic site may have some generality in catalysis by oxide-supported noble metals.

Wettability Reversal of Hydrophobic Pigment Particles Comprising Nanoscale Organosilane Shells: Concentrated Aqueous Dispersions and Corrosion-Resistant Waterborne Coatings.

We demonstrate the synthesis of polysiloxane-modified inorganic-oxide nanoparticles comprising a TiO2-based pigment (Ti-Pure R-706), which undergo drastic wettability reversal from a hydrophilic wet state to a hydrophobic state upon drying. Furthermore, the dry hydrophobic pigment particles can be reversibly converted back to a hydrophilic form by the application of high shear aqueous milling. Our synthetic approach involves first condensing the cross-linking monomer CH3Si(OH)3 onto the surface of Ti-Pure R-706 at pH 9.5 ± 0.2 in an aqueous suspension. After drying this surface-modified material in the presence of a polyanionic dispersant so as to preserve the primary particle size via dynamic light scattering, it is trimethylsilyl-capped with (CH3)3SiOH, which consumes some residual Si-OH functionalities, and washed to remove all dispersant and excess reagents. Transmission electron microscopy demonstrates a ∼6 nm polysiloxane coating uniformly surrounding the surface of the pigment particle. A 70 wt % (37 vol %) concentrated aqueous slurry of the hydrophobically modified pigment particles prepared in the absence of dispersant exhibits rheological characteristics that are nearly the same as an aqueous dispersion of native unmodified hydrophilic Ti-Pure R-706 comprising an optimal amount of the organic anionic dispersant. It is also possible to synthesize dispersions without the use of an added surfactant and/or dispersant at even higher solid concentrations of up to 75 wt % (43 vol %) in water, conditions at which even the hydrophilic native Ti-Pure R-706 oxide pigment yields a gel-like paste in the absence of a dispersant. Films prepared by drying an aqueous suspension of these pigment particles exhibited a hydrophobic contact angle of ∼125°. When acrylic-based waterborne coatings were prepared comprising these surface-modified Ti Pure R-706 pigments, they showed excellent corrosion protection of a mild steel substrate. These data point to a wettability reversal in which the particles change from hydrophobic to hydrophilic upon high-shear aqueous milling and vice versa upon drying. 29Si CP/MAS NMR spectroscopy highlights the importance of flexibility of the polysiloxane coating for achieving this wettability reversal, a result that emphasizes the importance of surface reconstruction.

Spectroscopic Characterization of µ-η1:η1-Peroxo Ligands Formed by Reaction of Dioxygen with Electron-Rich Iridium Clusters.

Although oxygen is a common ligand in supported metal catalysts, its coordination has been challenging to elucidate. We now characterize a diiridium complex that has been previously shown by X-ray diffraction crystallography to incorporate a µ-η1:η1-peroxo ligand. We observe markedly enhanced intensity at 788 cm-1 in the Raman spectrum of this complex, which is a consequence of bonding of the peroxo ligand but does not shift upon 18O labeling. Electronic structure calculations at the density functional theory level suggest that this increase in Raman intensity results from bands associated with rocking of CH2 substituents directly attached to P(Ph)2 groups coupling with the O-O band. These results provide part of the foundation for understanding oxygen ligands on a silica-supported tetrairidium carbonyl cluster stabilized with bulky electron-donating phosphine ligands [p-tert-butyl-calix[4]arene(OPr)3(OCH2PPh2) (Ph = phenyl; Pr = propyl)]. Reaction of the cluster with O2 also led to the growing in of a Raman band at 788 cm-1, similar to that in the diiridium complex and also assigned to the bonding of a bridging peroxo ligand. Infrared spectra recorded as the supported cluster reacted in sequential exposures to (i) H2, (ii) O2, (iii) H2, and (iv) CO indicate that two bridging peroxo ligands were bonded irreversibly per tetrairidium cluster, replacing bridging carbonyl ligands without altering either the cluster frame or the phosphine ligands. X-ray absorption near edge and infrared spectra include isosbestic points signifying a stoichiometrically simple reaction of the cluster with O2, and mass spectra of the effluent gas show that CO2 formed by oxidation of one terminal CO ligand per cluster as H2 (and not H2O) formed, evidence that hydride ligands had been present on the cluster following treatment (i). The understanding of how O2 reacts with the metal polyhedron provides a foundation for understanding of how oxidation catalysis may proceed on the surfaces of noble metals.

Synthesis and Electrochemical Stability of Ultrahigh Aspect Ratio Nanoporous Gold after Calixarene-Phosphine Ligand Removal.

Leveraging our previous report on the synthesis of calixarene-capped ultrahigh aspect-ratio nanoporous gold, we now report a new class of nanoporous gold comprising removed calixarene-phosphine ligands using UV/ozone treatment. The removal of the calixarene ligands by this treatment is supported by XPS measurements. TEM further shows the extraordinary stability of the ∼1 nm building blocks comprising the nanoporous gold wall after UV/ozone treatment and subsequent strongly reducing electrochemical environments. Sensing of nitrobenzene is used as a method of characterization to show that the surface chemistry of the nanoporous gold assemblies has radically changed after the UV/ozone treatment.

Bulky Calixarene Ligands Stabilize Supported Iridium Pair-Site Catalysts.

Although essentially molecular noble metal species provide active sites and highly tunable platforms for the design of supported catalysts, the susceptibility of the metals to reduction and aggregation and the consequent loss of catalytic activity and selectivity limit opportunities for their application. Here, we demonstrate a new construct to stabilize supported molecular noble-metal catalysts, taking advantage of sterically bulky ligands on the metal that serve as surrogate supports and isolate the active sites under conditions involving steady-state catalytic turnover in a reducing environment. The result is demonstrated with an iridium pair-site catalyst incorporating P-bridging calix[4]arene ligands dispersed on siliceous supports, chosen as prototypes because they offer weakly interacting surfaces on which metal aggregation is prone to occur. This catalyst was used for the hydrogenation of ethylene in a flow reactor. Atomic-resolution imaging of the Ir centers and spectra of the catalyst before and after use show that the metals resisted aggregation and deactivation, remaining atomically dispersed and accessible for catalysis. This strategy thus allows the stabilization of the catalysts even when they are weakly anchored to supports.

SSZ-70 borosilicate delamination without sonication: effect of framework topology on olefin epoxidation catalysis.

We report a scalable delamination procedure for a SSZ-70-framework layered-zeolite precursor, which for the first time does not involve either sonication or long-chain surfactants. Our approach instead relies on the mild heating of layered zeolite precursor B-SSZ-70(P) in an aqueous solution containing Zn(NO3)2 and tetrabutylammonium fluoride. Powder X-ray diffraction data are consistent with a loss of long-range order along the z-direction, while 29Si MAS NMR spectroscopy demonstrates preservation of the zeolite framework crystallinity during delamination. The resulting delaminated material, DZ-2, possesses 1.4-fold higher external surface area relative to the nondelaminated three-dimensional zeolite B-SSZ-70, based on N2 physisorption data at 77 K. DZ-2 was functionalized with cationic Ti heteroatoms to synthesize Ti-DZ-2 via exchange with framework B. Ti-DZ-2 contains isolated titanium centers in its crystalline framework, as shown by UV-Vis spectroscopy. The generality of the synthetic delamination approach and catalyst synthesis is demonstrated with the synthesis of delaminated material DZ-3, which is derived from layered zeolite precursor ERB-1(P) with MWW framework topology. Upon catalytic testing for the epoxidation of 1-octene with ethylbenzene hydroperoxide as oxidant, under harsh tail-end conditions that deactivate amorphous Ti-silica-based catalysts, Ti-DZ-2 exhibits the highest per-Ti-site activity, selectivity, and stability for 1-octene epoxidation of all catalysts investigated. This testing includes the prior benchmark delaminated zeolite catalyst in this area, Ti-UCB-4, which possesses similar external surface area to Ti-DZ-2 but requires sonication and long-chain surfactants for its synthesis. The synthesis of DZ-2 is the first example of an economical delamination of layered zeolite precursor SSZ-70(P) and opens up new doors to the development of delaminated zeolites as commercial catalysts.

Outer-Sphere Control of Catalysis on Surfaces: A Comparative Study of Ti(IV) Single-Sites Grafted on Amorphous versus Crystalline Silicates for Alkene Epoxidation.

The effect of outer-sphere environment on alkene epoxidation catalysis using an organic hydroperoxide oxidant is demonstrated for calix[4]arene-TiIV single-sites grafted on amorphous vs crystalline delaminated zeotype (UCB-4) silicates as supports. A chelating calix[4]arene macrocyclic ligand helps enforce a constant TiIV inner-sphere, as characterized by UV-visible and X-ray absorption spectroscopies, thus enabling the rigorous comparison of outer-sphere environments across different siliceous supports. These outer-sphere environments are characterized by solid-state 1H NMR spectroscopy to comprise proximally organized silanols confined within 12 membered-ring cups in crystalline UCB-4, and are responsible for up to 5-fold enhancements in rates of epoxidation by TiIV centers.

Nanoporous gold assemblies of calixarene-phosphine-capped colloids.

The synthesis of high surface-area colloidal assemblies of calixarene-phosphine-capped nanoporous gold is reported under reductive electrochemical conditions. These materials uniquely exhibit a remarkably thin wall thickness down to 10 nm, while possessing pore sizes on the order of up to hundreds of nanometers, which can be controlled via choice of organic ligand.

Nanosized Gadolinium and Uranium-Two Representatives of High-Reactivity Lanthanide and Actinide Metal Nanoparticles.

Gadolinium (Gd0) and uranium (U0) nanoparticles are prepared via lithium naphthalenide ([LiNaph])-driven reduction in tetrahydrofuran (THF) using GdCl3 and UCl4, respectively, as low-cost starting materials. The as-prepared Gd0 and U0 suspensions are colloidally stable and contain metal nanoparticles with diameters of 2.5 ± 0.7 nm (Gd0) and 2.0 ± 0.5 nm (U0). Whereas THF suspensions are chemically stable under inert conditions (Ar and vacuum), nanoparticulate powder samples show high reactivity in contact with, for example, oxygen, moisture, alcohols, or halogens. Such small and highly reactive Gd0 and U0 nanoparticles are first prepared via a dependable liquid-phase synthesis and stand as representatives for further nanosized lanthanides and actinides.

Ti(0) nanoparticles via lithium-naphthalenide-driven reduction.

Metallic titanium (Ti(0)) nanoparticles, 1.5 ± 0.4 nm in diameter, are obtained via lithium naphthalenide ([LiNaph])-driven reduction of TiCl4× 2THF in tetrahydrofuran (THF). HRTEM, fast Fourier transformation (FFT), optical spectra and X-ray absorption near edge structure (XANES) confirm their chemical composition. Besides their pyrophoric properties, their high reactivity is validated by direct transformation of Ti(0) into TiC maintaining the size.

Sodium-Naphthalenide-Driven Synthesis of Base-Metal Nanoparticles and Follow-up Reactions.

Mo(0), W(0), Fe(0), Ru(0), Re(0), and Zn(0) nanoparticles­essentially base metals­are prepared as a general strategy by a sodium naphthalenide ([NaNaph])-driven reduction of simple metal chlorides in ethers (1,2-dimethoxyethane (DME), tetrahydrofuran (THF)). All the nanoparticles have diameters ≤10 nm, and they can be obtained either as powder samples or long-term stable suspensions. Direct follow-up reactions (e.g., Mo(0)+S8, FeCl3+AsCl3, ReCl5+MoCl5), moreover, allow the preparation of MoS2, FeAs2, or Re4Mo nanoparticles of similar size as the pristine metals (≤10 nm).

Tungsten nanoparticles from liquid-ammonia-based synthesis.

Tungsten nanoparticles were obtained from liquid-ammonia-based synthesis via reduction of WCl6 with dissolved sodium. The W(0) nanoparticles exhibit a diameter of 1-2 nm and can be dispersed in alkanes, showing a grayish-orange color due to red-shifted plasmon resonance absorption.