Assembly of triangular trimetallic complexes by triamidophosphine ligands: spin-frustrated Mn2+ plaquettes and diamagnetic Mg2+ analogues with a combined through-space, through-bond pathway for 31P-31P spin-spin coupling

The reactions of 2 equiv of the ligand precursor P(CH2NHPh)3 or P[CH2NH-3,5-(CF3)2C6H3]3 with 3 equiv of Mn[N(SiMe3)2]2 provide high-yielding routes to the triangular trinuclear Mn(II) complexes [P(CH2NPh)3]2Mn3(THF)3.1.5THF and [P(CH2N-3,5-(CF3)2C6H3)3]2Mn3(THF)3. The solid-state structures of these paramagnetic complexes have approximate C3 symmetry. The magnetic moments from 300 to 1.8 K could be fit as a magnetic Jahn-Teller distorted isosceles triangle. These complexes exhibit spin frustration and possess an S = 1/2 ground state, as revealed by a plot of magnetization versus field at 1.8 K; at fields above 3.8 T, the occupation of an excited state with S = 3/2 becomes significant. The diamagnetic magnesium analogues were prepared by the reaction of the ligand precursor P(CH2NHPh)3, P[CH2NH-3,5-(CF3)2C6H3]3, or P(CH2NH-3,5-Me2C6H3)3 with nBu2Mg. The solid-state structures of [P(CH2NPh)3]2Mg3(THF)3.1.5THF and [P(CH2N-3,5-(CF3)2C6H3)3]2Mg3(THF)3 were determined. Solution 1H NMR spectroscopy was used to demonstrate that the solid-state structures are maintained in solution. The aryl group of the terminal amido donor exhibits slow rotation on the NMR time scale, and this was found to be an electronic effect. Solution 31P{1H} NMR spectroscopy revealed an unexpected 15 Hz coupling between phosphorus nuclei in these complexes. Calculations on a model complex using density functional theory demonstrates that this coupling occurs via a combined through-space, through-bond pathway.     

Robust partial least squares regression—part III,outlier analysis and application studies

This paper is the third part of the work on robust partial least squares (RPLS) regression. The paper focuses on implementation issues for outlier detection and diagnosis. Furthermore, the paper introduces a numerically more efficient algorithm for determining the Stahel–Donoho estimator (SDE). This has been identified as a potential drawback of the new proposed RPLS algorithm, detailed in Part II of this work. Finally, a total of three application studies are presented which involve data recorded from (i) a calibration experiment (similar number of variables/observations), (ii) a distillation process for purifying benzene (considerably more observations than variables) and (iii) an experiment of a multi‐component concentration determination using Raman spectroscopy (considerably more variables than observations). Copyright © 2008 John Wiley & Sons, Ltd.     

Size-dependent phase transformation kinetics in nanocrystalline ZnS

Nanocrystalline ZnS was coarsened under hydrothermal conditions to investigate the effect of particle size on phase transformation kinetics. Although bulk wurtzite is metastable relative to sphalerite below 1020 degrees C at low pressure, sphalerite transforms to wurtzite at 225 degrees C in the hydrothermal experiments. This indicates that nanocrystalline wurtzite is stable at low temperature. High-resolution transmission electron microscope data indicate there are no pure wurtzite particles in the coarsened samples and that wurtzite only grows on the surface of coarsened sphalerite particles. Crystal growth of wurtzite stops when the diameter of the sphalerite-wurtzite interface reaches approximately 22 nm. We infer that crystal growth of wurtzite is kinetically controlled by the radius of the sphalerite-wurtzite interface. A new phase transformation kinetic model based on collective movement of atoms across the interface is developed to explain the experimental results.     

Molecular chemistry of consequence to renewable energy

Energy conversion cycles are aimed at driving unfavorable, small-molecule activation reactions with a photon harnessed directly by a transition-metal catalyst or indirectly by a transition-metal catalyst at the surface of a photovoltaic cell. The construction of such cycles confronts daunting challenges because they rely on chemical transformations not understood at the most basic levels. These transformations include multielectron transfer, proton-coupled electron transfer, and bond-breaking and -making reactions of energy-poor substrates. We have begun to explore these poorly understood areas of molecular science with transition-metal complexes that promote hydrogen production and oxygen bond-breaking and -making chemistry of consequence to water splitting.     

Palladium-catalyzed asymmetric phosphination: enantioselective synthesis of a p-chirogenic phosphine

The racemic secondary phosphine PH(Me)(Is) (1, Is = 2,4,6-(i-Pr)3C6H2) was coupled with PhI in the presence of NaOSiMe3 and the catalyst Pd((R,R)-Me-Duphos)(Ph)(I) (3) to give P(Ph)(Me)(Is) (2) in up to 78% ee. The intermediate phosphido complex Pd((R,R)-Me-Duphos)(Ph)(P(Me)(Is)) (5a,b) was observed as a mixture of diastereomers by low-temperature 31P NMR. The rate of interconversion of 5a,b by phosphorus inversion is greater than or equal to that of reductive elimination, which suggests that the enantiodetermining step occurs after Pd-P bond formation.     

Optical sensing of amine vapors with a series of tin compounds

Certain common Sn(2+) salts exhibit bright luminescence of various optical wavelengths upon excitation with 254 nm UV irradiation at room temperature. Light emission can be selectively quenched by exposure to low concentrations of amines, and can in some cases be regenerated with either a nitrogen purge or exposure to acid vapors, rendering the compounds useful for application in optical sensing arrays.     

Reversible, surface-controlled structure transformation in nanoparticles induced by an aggregation state

The structure of 3 nm ZnS nanoparticles differs from that of bulk ZnS and is shown to vary with the particle aggregation state. Dispersed or weakly aggregated nanoparticles in suspension have a more distorted internal structure than strongly aggregated nanoparticles. Reversible switching between distorted and crystalline structures can be induced by changing the aggregation state via slow drying and ultrasonic agitation. The transformation was analyzed using pair distribution function data from wide angle x-ray diffraction and the aggregation state monitored via small angle x-ray scattering. Molecular modeling provides insight into particle-particle interactions that induce the structural changes. The reversible nature also implies a low activation energy of nanoparticle transformation and indicates that distorted nanoparticles are not trapped in a metastable state.     

Photoluminescence of porous silicon surfaces stabilized through Lewis acid mediated hydrosilylation

Lewis acid mediated hydrosilylation on porous silicon surfaces permits facile incorporation of a wide variety of functionalities through stable silicon–carbon bonds. The surfaces demonstrate high chemical stability with respect to hydrofluoric acid and aqueous base. The effects of the covalently bound surface groups on photoluminescence have been investigated and it was noted that alkyl and alkenyl termination induced only small decreases in photoluminescence efficiency. Aromatic substituents conjugated through a vinyl group, however, bring about almost complete quenching of the observed photoluminescence, regardless of substitution with either electron withdrawing (chloride) or donating (methyl) functionalities. The photoluminescence fatigue of dodecyl terminated surfaces in air for 12–16 h periods has been monitored and compared to unfunctionalized porous silicon. In air, the photoluminescence of dodecyl terminated surfaces degrades faster than the unfunctionalized porous silicon but under inert atmosphere (nitrogen), the rate of photoluminescence fatigue is slow in both cases and approximately equivalent.