Lanthanum-Promoted Electrocatalyst for the Oxygen Evolution Reaction: Unique Catalyst or Oxide Deconstruction?

A conventional performance metric for electrocatalysts that promote the oxygen evolution reaction (OER) is the current density at a given overpotential. However, the assumption that increased current density at lower overpotentials indicates superior catalyst design is precarious for OER catalysts in the working environment, as the crystalline lattice is prone to deconstruction and amorphization, thus greatly increasing the concentration of catalytic active sites. We show this to be the case for La incorporation into CoO.

The role of metal accessibility on carbon dioxide electroreduction in atomically precise nanoclusters.

Atomically precise nanoclusters (NCs) can be designed with high faradaic efficiency for the electrochemical reduction of CO to CO (FE) and provide useful model systems for studying the metal-catalysed CO reduction reaction (CORR). While size-dependent trends are commonly evoked, the effect of NC size on catalytic activity is often convoluted by other factors such as changes to surface structure, ligand density, and electronic structure, which makes it challenging to establish rigorous structure-property relationships. Herein, we report a detailed investigation of a series of NCs [AuAg(C[triple bond, length as m-dash]CR)Cl(PPh), AuAg(C[triple bond, length as m-dash]CR)Cl, and Au(C[triple bond, length as m-dash]CR)/AuAg(C[triple bond, length as m-dash]CR)] with similar sizes and core structures but different ligand packing densities to investigate how the number of accessible metal sites impacts CORR activity and selectivity.

Oxidation Chemistry of Bicarbonate and Peroxybicarbonate: Implications for Carbonate Management in Energy Storage.

Carbonate formation presents a major challenge to energy storage applications based on low-temperature CO electrolysis and recyclable metal-air batteries. While direct electrochemical oxidation of (bi)carbonate represents a straightforward route for carbonate management, knowledge of the feasibility and mechanisms of direct oxidation is presently lacking. Herein, we report the isolation and characterization of the bis(triphenylphosphine)iminium salts of bicarbonate and peroxybicarbonate, thus enabling the examination of their oxidation chemistry.

Self-healing oxygen evolution catalysts.

Electrochemical and photoelectrochemical water splitting offers a scalable approach to producing hydrogen from renewable sources for sustainable energy storage. Depending on the applications, oxygen evolution catalysts (OECs) may perform water splitting under a variety of conditions. However, low stability and/or activity present challenges to the design of OECs, prompting the design of self-healing OECs composed of earth-abundant first-row transition metal oxides.

Insensitivity of Magnetic Coupling to Ligand Substitution in a Series of Tetraoxolene Radical-Bridged Fe Complexes.

The elucidation of magnetostructural correlations between bridging ligand substitution and strength of magnetic coupling is essential to the development of high-temperature molecule-based magnetic materials. Toward this end, we report the series of tetraoxolene-bridged Fe complexes [(MeTPyA)Fe(L)] (MeTPyA = tris(6-methyl-2-pyridylmethyl)amine; = 2: LH = 3,6-dimethoxy-2,5-dihydroxo-1,4-benzoquinone, LH = 3,6-dichloro-2,5-dihydroxo-1,4-benzoquinone, Na[L] = sodium 3,6-dinitro-2,5-dihydroxo-1,4-benzoquinone; = 4: L = 3,6-bis(dimethylsulfonium)-2,5-dihydroxo-1,4-benzoquinone diylide) and their one-electron-reduced analogues. Variable-temperature dc magnetic susceptibility data reveal the presence of weak ferromagnetic superexchange between Fe centers in the oxidized species, with exchange constants of = +1.

Metal-Organic Framework Magnets.

Metal-organic frameworks represent the ultimate chemical platform on which to develop a new generation of designer magnets. In contrast to the inorganic solids that have dominated permanent magnet technology for decades, metal-organic frameworks offer numerous advantages, most notably the nearly infinite chemical space through which to synthesize predesigned and tunable structures with controllable properties. Moreover, the presence of a rigid, crystalline structure based on organic linkers enables the potential for permanent porosity and postsynthetic chemical modification of the inorganic and organic components.

Strong π-Backbonding Enables Record Magnetic Exchange Coupling Through Cyanide.

The paramagnetic cyano-bridged complex PhB(BuIm)Fe-NC-Mo(NBuAr) (Ar = 3,5-MeCH) is readily assembled from a new four-coordinate, high-spin ( = 2) iron(II) monocyanide complex and the three-coordinate molybdenum(III) complex Mo(NBuAr). X-ray diffraction and IR spectroscopy reveal that delocalization of unpaired electron density into the cyanide π* orbitals leads to a reduction of the C-N bond order. Direct current (dc) magnetic susceptibility measurements, supported by electronic structure calculations, demonstrate the presence of strong antiferromagnetic exchange between spin centers, with a coupling constant of = -122(2) cm.

Selective Binding and Quantitation of Calcium with a Cobalt-Based Magnetic Resonance Probe.

We report a cobalt-based paramagnetic chemical exchange saturation transfer (PARACEST) magnetic resonance (MR) probe that is able to selectively bind and quantitate the concentration of Ca ions under physiological conditions. The parent LCo complex features CEST-active carboxamide groups and an uncoordinated crown ether moiety in close proximity to a high-spin pseudo-octahedral Co center. Addition of Na, Mg, K, and Ca leads to binding of these metal ions within the crown ether.

Dramatic enhancement in pH sensitivity and signal intensity through ligand modification of a dicobalt PARACEST probe.

The employment of an ancillary amine-substituted bisphosphonate ligand affords a dicobalt complex able to quantitate pH with a remarkably high sensitivity of 8.8(5) pH unit-1 at 37 °C through a ratiometric paramagnetic chemical exchange saturation transfer (PARACEST) approach, where the different pH dependences of amine and amide CEST peak intensities are utilized.

Electronic Effects of Ligand Substitution in a Family of Co PARACEST pH Probes.

We report three new Co-based paramagnetic chemical exchange saturation transfer (PARACEST) probes with the ability to ratiometrically quantitate pH. A Co complex, [LCo(etidronate)], featuring tetra(carboxamide) and OH-substituted etidronate ligands with opposing pH-dependent CEST peak intensities, was previously shown to exhibit a linear correlation between log(CEST/CEST) and pH in the pH range 6.5-7.

pH-Dependent spin state population and F NMR chemical shift via remote ligand protonation in an iron(ii) complex.

An Fe complex that features a pH-dependent spin state population, by virtue of a variable ligand protonation state, is described. This behavior leads to a highly pH-dependent F NMR chemical shift with a sensitivity of 13.9(5) ppm per pH unit at 37 °C, thereby demonstrating the potential utility of the complex as a F chemical shift-based pH sensor.

Ratiometric pH Imaging with a Co MRI Probe via CEST Effects of Opposing pH Dependences.

We report a Co-based magnetic resonance (MR) probe that enables the ratiometric quantitation and imaging of pH through chemical exchange saturation transfer (CEST). This approach is illustrated in a series of air- and water-stable Co complexes featuring CEST-active tetra(carboxamide) and/or hydroxyl-substituted bisphosphonate ligands. For the complex bearing both ligands, variable-pH CEST and NMR analyses reveal highly shifted carboxamide and hydroxyl peaks with intensities that increase and decrease with increasing pH, respectively.

Spin-crossover and high-spin iron(ii) complexes as chemical shift F magnetic resonance thermometers.

The potential utility of paramagnetic transition metal complexes as chemical shift F magnetic resonance (MR) thermometers is demonstrated. Further, spin-crossover Fe complexes are shown to provide much higher temperature sensitivity than do the high-spin analogues, owing to the variation of spin state with temperature in the former complexes. This approach is illustrated through a series of Fe complexes supported by symmetrically and asymmetrically substituted 1,4,7-triazacyclononane ligand scaffolds bearing 3-fluoro-2-picolyl derivatives as pendent groups (L ).