Selective Methane Oxidation by Heterogenized Iridium Catalysts.

Oxidative methane (CH4) carbonylation promises a direct route to the synthesis of value-added oxygenates such as acetic acid (CH3COOH). Here, we report a strategy to realize oxidative CH4 carbonylation through immobilized Ir complexes on an oxide support. Our immobilization approach not only enables direct CH4 activation but also allows for easy separation and reutilization of the catalyst. Furthermore, we show that a key step, methyl migration, that forms a C-C bond, is sensitive to the electrophilicity of carbonyl, which can be tuned by a gentle reduction to the Ir centers. While the as-prepared catalyst that mainly featured Ir(IV) preferred CH3COOH production, a reduced catalyst featuring predominantly Ir(III) led to a significant increase of CH3OH production at the expense of the reduced yield of CH3COOH.

Electrocatalytic, Homogeneous Ammonia Oxidation in Water to Nitrate and Nitrite with a Copper Complex.

Electrocatalytic ammonia oxidation at room temperature and pressure allows energy-economical and environmentally friendly production of nitrites and nitrates. Few molecular catalysts, however, have been developed for this six- or eight-electron oxidation process. We now report [Cu(bipyalk)]+, a homogeneous electrocatalyst that realizes the title reaction in water at 94% Faradaic efficiency. The catalyst exhibits high selectivity against water oxidation in aqueous media, as [Cu(bipyalk)]+ is not competent for water oxidation.

Accessing Molecular Dimeric Ir Water Oxidation Catalysts from Coordination Precursors.

One ongoing challenge in the field of iridium-based water oxidation catalysts is to develop a molecular precatalyst affording well-defined homogeneous active species for catalysis. Our previous work by using organometallic precatalysts Cp*Ir(pyalk)OH and Ir(pyalk)(CO)2 (pyalk = (2-pyridyl)-2-propanolate) suggested a µ-oxo-bridged Ir dimer as the probable resting state, although the structure of the active species remained elusive. During the activation, the ligands Cp* and CO were found to oxidatively degrade into acetic acid or other products, which coordinate to Ir centers and affect the catalytic reaction. Two related dimers bearing two pyalk ligands on each iridium were crystallized for structural analysis. However, preliminary results indicated that these crystallographically characterized dimers are not active catalysts. In this work, we accessed a mixture of dinuclear iridium species from a coordination precursor, Na[Ir(pyalk)Cl4], and assayed their catalytic activity for oxygen evolution by using NaIO4 as the oxidant. This catalyst showed comparable oxygen-evolution activity to the ones previously reported from organometallic precursors without demanding oxidative activation to remove sacrificial ligands. Future research along this direction is expected to provide insights and design principles toward a well-defined active species.

Optimization of Surface Loading of the Silatrane Anchoring Group on TiO2.

Anchoring groups are usually needed for the attachment of small molecules to metal oxide surfaces such as in water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs). Here, we optimize the surface loading onto titanium dioxide surfaces of the silatrane anchoring group, a triethanolamine-protected trialkoxysilane. This anchoring group is not yet widely used because prior protocols afforded low surface coverage, but it has the advantage of high stability over a wide pH range and at both oxidizing and reducing potentials when bound. A new and improved method for estimating surface coverage is described here and used to determine that loading using previously reported binding protocols is very low. However, we were able to uncover several factors contributing to this low loading, which has allowed us to develop methods to greatly improve surface coverage for a variety of silatranes. Most notably, we were able to increase the loading of a model arylsilatrane by 145% through use of a benzoic acid additive. This is not general acid catalysis because alkylsilatranes are not similarly affected and 4-t-butylbenzoic acid, having a similar pKa to benzoic acid, is not effective. Because the bulky t-butyl group of the latter additive is not expected to pi-stack with our arylsilatrane, we have tentatively assigned this enhancement to aromatic stacking between the aromatic additive and the arylsilatrane.

Diazo coupling for surface attachment of small molecules to TiO2 nanoparticles.

Robust surface attachment of molecular species to metal oxide semiconductors is desirable for many applications. Here, we report the interfacial diazo coupling of surface-bound amines with aromatics to bind them to the surface of TiO2 nanoparticles via a siloxane anchor and a diazo linker. The technique shows potential for the inexpensive, stable, modular and tunable attachment of molecules to metal oxide surfaces.

Strongly Coupled Phenazine-Porphyrin Dyads: Light-Harvesting Molecular Assemblies with Broad Absorption Coverage.

The development of light-harvesting architectures with broad absorption coverage in the visible region continues to be an important research area in the field of artificial photosynthesis. Here, we introduce a new class of ethynyl-linked panchromatic dyads composed of dibenzophenazines coupled ortho and meta to tetrapyrroles with an anchoring group that can be grafted onto metal oxide surfaces. Quantum chemical calculations and photophysical measurements of the synthesized materials reveal that both of the dibenzophenazine dyads absorb broadly from 300 to 636 nm and exhibit absorption bands different from those of the constituent chromophore units. Moreover, the different points of attachment of dibenzophenazines to tetrapyrroles give different absorption profiles which computations suggest result from differences in the planarity of the two dyads. Applicability of the dyads in artificial photosynthesis systems was assessed by their incorporation and characterization of their performance in dye-sensitized solar cells.