The structure and mechanism of the formation of sites which initiate ethylene polymerization in the atomically dispersed Phillips catalyst (Cr/ SiO 2 ) are two of the great unsolved mysteries of heterogeneous catalysis. After CO or C 2 H 4 reduction of silica-supported Cr VI ions to Cr II ions in the precatalyst, exposure to ethylene results in the formation of organoCr III sites that are capable of initiating polymerization without recourse to an external alkylating cocatalyst. In this work, a Phillips catalyst prepared, via sol−gel chemistry, as a mesoporous, optically transparent monolith was reduced with CO to the spectroscopically determined Cr II end point. Ethylene causes rapid reoxidation of these Cr II sites to Cr III , even at low temperatures. Solid-state 13 C CP-MAS NMR, IR, and Raman spectroscopies reveal that the resulting sites contain a vinyl ligand, described as (≡SiO) 2 Cr III −CHCH 2 although likely with a higher coordination number, which are capable of initiating polymerization. The formation of these vinyl sites is an incommensurate redox reaction involving one-electron oxidation of Cr II via ethylene disproportionation. The accompanying formation of organic radical intermediates and their characteristic reaction products suggest that the key step is homolysis of a Cr−ethyl bond. Plausible pathways for the initiation mechanism are suggested.

Complexes with two and three coordinate chromium(i).

Hydrogen is considered as an ideal and sustainable energy carrier because of its high energy density and carbon-free combustion. Electrochemical water splitting is the only solution for uninterrupted, scalable, and sustainable production of hydrogen without carbon emission. However, a large-scale hydrogen production through electrochemical water splitting depends on the availability of earth-abundant electrocatalysts and a suitable electrolyte medium. In this article, we demonstrate that hydrogen evolution reaction (HER) performance of electrocatalytic materials can be controlled by their surface functionalization and selection of a suitable electrolyte solution. Here, we report syntheses of few-layered MoS2 nanosheets, NiO nanoparticles (NPs), and multiwalled carbon nanotubes (MWCNTs) using scalable production methods from earth-abundant materials. Magnetic measurements of as-produced electrocatalyst materials demonstrate that MoS2 nanoflakes are diamagnetic, whereas surface-functionalized MoS2 and its composite with carbon nanotubes have strong ferromagnetism. The HER performance of the few-layered pristine MoS2 nanoflakes, MoS2/NiO NPs, and MoS2/NiO NPs/MWCNT nanocomposite electrocatalysts are studied in acidic and alkaline media. For bare MoS2, the values of overpotential (η10) in alkaline and acidic media are 0.45 and 0.54 V, respectively. Similarly, the values of current density at 0.5 V overpotential are 27 and 6.2 mA/cm2 in alkaline and acidic media, respectively. The surface functionalization acts adversely in the both alkaline and acidic media. MoS2 nanosheets functionalized with NiO NPs also demonstrated excellent performance for oxygen evolution reaction with anodic current of ~60 mA/cm2 and Tafel slope of 78 mVdec−1 in alkaline medium.

Magnetic properties of the series of three linear, trimetallic chain compounds Cr2Cr(dpa)4Cl2, 1, Mo2Cr(dpa)4Cl2, 2, and W2Cr(dpa)4Cl2, 3 (dpa = 2,2'-dipyridylamido), have been studied using variable-temperature dc and ac magnetometry and high-frequency EPR spectroscopy. All three compounds possess an S = 2 electronic ground state arising from the terminal Cr(2+) ion, which exhibits slow magnetic relaxation under an applied magnetic field, as evidenced by ac magnetic susceptibility and magnetization measurements. The slow relaxation stems from the existence of an easy-axis magnetic anisotropy, which is bolstered by the axial symmetry of the compounds and has been quantified through rigorous high-frequency EPR measurements. The magnitude of D in these compounds increases when heavier ions are substituted into the trimetallic chain; thus D = -1.640, -2.187, and -3.617 cm(-1) for Cr2Cr(dpa)4Cl2, Mo2Cr(dpa)4Cl2, and W2Cr(dpa)4Cl2, respectively. Additionally, the D value measured for W2Cr(dpa)4Cl2 is the largest yet reported for a high-spin Cr(2+) system. While earlier studies have demonstrated that ligands containing heavy atoms can enhance magnetic anisotropy, this is the first report of this phenomenon using heavy metal atoms as "ligands".

Traditionally computational methods have been employed to explain the observation of novel properties in materials. The use of computational models to anticipate the onset of such properties in quantum dots (QDs) a priori of their synthetic preparation would facilitate the rapid development of new materials. We demonstrate that the use of computational modeling can allow the design of magnetic semiconductor QDs based on iron doped ZnSe prior to the preparation of the sample. DFT modeling predicts the formation of multinuclear Fe clusters within the 10% Fe doped ZnSe QD to relieve lattice strain leading to the onset of competing ferromagnetic (FM)–antiferromagnetic (AFM) interactions, or in effect spin frustration, between the local spins. The magnetic properties when iron is incorporated into a 1.8 nm ZnSe QD are computationally analyzed using standard density functional theory (DFT) simulations, and the resultant spin and Fe localization models are experimentally evaluated using SQUID, 57Fe Mössbauer, and electron paramagnetic resonance (EPR) spectroscopy. The observation that the experimental results agree with the DFT predicted behavior demonstrates the value of using modeling when targeting a desired material property.

The 48-Fe III-containing 96-tungsto-16-phosphate, [Fe III 48 (OH) 76 (H 2 O) 16 (HP 2 W 12 O 48) 8 ] 36À (Fe 48), has been synthesized and structurally characterized. This polyanion comprises eight equivalent {Fe III 6 P 2 W 12 }u nits that are linked in an end-on fashion forming am acrocyclic assembly that contains more iron centers than any other polyoxometalate (POM) known to date. Then ovel Fe 48 was synthesized by asimple one-pot reactionofa n{Fe 22 }c oordinationc omplex with the hexalacunary {P 2 W 12 }P OM precursor in water. Thet itle polyanion was characterizedb y single-crystal XRD, FTIR, TGA, magnetic and electrochemical studies.

Dilute magnetic semiconductor quantum dots (DMSQDs) have emerged as potential materials for future spin-based technologies. A critical parameter for the development of this technology is a relatively long electron spin phase memory time, T 2 , which could be gleaned from relaxation measurements via pulsed EPR (pEPR). T 2 must be long enough for the data storage and manipulation processes. Thus, pEPR studies of lightly doped quantum dots (QDs) have recently yielded important spin dynamical data on DMSQDs. The earlier studies have, however, been carried out mostly at 3.5 T and 9.5 GHz. Since relaxation dynamics depend sensitively on the Zeeman field and temperature, this work reports the use of higher microwave frequencies up to 240 GHz, in continuous wave (CW) and pEPR modes. The samples studied are ∼2 nm 0.8, 1.6, 3.2% Mn-doped ZnSe QDs. Unlike at 9.5 GHz, the use of higher frequencies enables us to resolve EPR peaks from surface and core sites. A detailed examination of the temperature and frequency dependence of the spin−lattice relaxation time T 1 across a wide range of temperatures (1.8−20 K) and frequencies (9.7−240 GHz) revealed that the relaxation mechanism involves the direct processes. The T 1 decay is found to be bimodal, in contrast to the earlier low-frequency data. Additionally, the observation of large T 2 times and Rabi oscillations indicates that Mn spins in the doped ZnSe QDs can be successfully manipulated and thus be promising components in quantum computation devices.

The 15-copper(ii)-containing 36-tungsto-4-silicates [Cu15O2(OH)10X(A-α-SiW9O34)4]25- (X = Cl, 1; Br, 2) have been prepared in 70% yield by reaction of the trilacunary 9-tungstosilicate precursor [A-α-SiW9O34]10- with Cu2+ ions in aqueous pH 8 medium. Both polyanions 1 and 2 were isolated as hydrated mixed potassium/sodium salts and characterized in the solid state by FT-IR, TGA, single-crystal XRD, and elemental analysis. DC magnetic susceptibility measurements from 1.8-300 K established the ground state to be paramagnetic with a magnetic moment corresponding to 15 uncoupled Cu2+ (S = 1/2) ions. EPR measurements and simulations were consistent with this analysis. Electrochemical studies were performed for polyanions 1 and 2 dissolved in solution to elucidate the electroactivity of both copper and tungstate sites. Using 2 as a representative example, the electrocatalytic activity towards CO2 reduction upon deposition on a glassy carbon electrode surface, while retaining selectivity relative to hydrogen evolution, was demonstrated.