The mechanism of the copolymerization of CO(2) and epoxides to afford the corresponding polycarbonates catalyzed by a highly active and thermally stable cobalt(III) complex with 1,5,7-triabicyclo[4,4,0] dec-5-ene (designated as TBD, a sterically hindered organic base) anchored on the ligand framework has been studied by means of electrospray ionization mass spectrometry (ESI-MS) and Fourier transform infrared spectroscopy (FTIR). The single-site, cobalt-based catalyst exhibited excellent activity and selectivity for polymer formation during CO(2)/propylene oxide (PO) copolymerization even at temperatures up to 100 degrees C and high [epoxide]/[catalyst] ratios, and/or low CO(2) pressures. The anchored TBD on the ligand framework plays an important role in maintaining thermal stability and high activity of the catalyst. ESI-MS and FTIR studies, in combination with some control experiments, confirmed the formation of the carboxylate intermediate with regard to the anchored TBD on the catalyst ligand framework. This analysis demonstrated that the formed carboxylate intermediate helped to stabilize the active Co(III) species against decomposition to inactive Co(II) by reversibly intramolecular Co-O bond formation and dissociation. Previous studies of binary catalyst systems based on Co(III)-Salen complexes did not address the role of these nucleophilic cocatalysts in stabilizing active Co(III) species during the copolymerization. The present study provides a new mechanistic understanding of these binary catalyst systems in which alternating chain-growth and dissociation of propagating carboxylate species derived from the nucleophilic axial anion and the nucleophilic cocatalyst take turns at both sides of the Co(III)-Salen center. This significantly increases the reaction rate and also helps to stabilize the active SalenCo(III) against decomposition to inactive SalenCo(II) even at low CO(2) pressures and/or relatively high temperatures.
DMSO methylates a broad range of amines in the presence of formic acid, providing a novel, green and practical method for amine methylation. The protocol also allows the one-pot transformation of aromatic nitro compounds into dimethylated amines in the presence of a simple iron catalyst.
Reduced graphite oxide-NiFe 2 O 4 (RGO-NiFe 2 O 4 ) composites were synthesized by adding different amounts of NH 3 $H 2 O into a mixed aqueous solution of graphite oxide, Ni(NO 3 ) 2 and Fe(NO 3 ) 3 at room temperature. NH 3 $H 2 O was used to adjust the synthesis system's pH value. The morphology and the microstructure of the as-prepared composites were characterized by X-ray diffraction (XRD), BrunauerEmmett-Teller (BET) and transmission electron microscope (TEM) techniques. The structure characterizations indicate that NiFe 2 O 4 successfully deposited on the surface of the RGO and the morphologies of RGO-NiFe 2 O 4 show a transparent structure with NiFe 2 O 4 homogeneously distributed on the RGO surfaces. Capacitive properties of the synthesized electrodes were studied using cyclic voltammetry and electrochemical impedance spectroscopy in a three-electrode experimental setup using 1 M Na 2 SO 4 aqueous solution as electrolyte. It is found that the pH value plays an important role in controlling the electrochemical properties of these electrodes. Among the synthesized electrodes, RGONiFe 10 (pH ¼ 10) shows the best capacitive properties because of its suitable particle size and good dispersion property. It could be anticipated that the synthesized electrodes will gain promising applications as novel electrode materials in supercapacitors and other devices by virtue of their outstanding characteristics of controllable capacitance and facile synthesis.
An unprecedented Rh-catalyzed direct methylation of ketones with N,N-dimethylformamide (DMF) is disclosed. The reaction shows a broad substrate scope, tolerating both aryl and alkyl ketones with various substituents. Mechanistic studies suggest that DMF delivers a methylene fragment followed by a hydride in the methylation process.
A new
design for photoresponsive shape memory hydrogels and their
possible applications are demonstrated in the present study. We show
that the photodissociable Fe3+-carboxylate coordination
can be utilized as a molecular switch to realize photocontrol of shape
memory on both macroscopic and microscopic scales and enable a number
of functions. Indeed, Fe3+-carboxylate coordination can
fix a large tensile strain (up to 680%) of the sodium alginate/polyacrylamide
hydrogel through cross-linking of sodium alginate chains, and subsequent
UV irradiation allows strain energy release in spatially selected
regions through reduction of Fe3+ to Fe2+. By
manipulating light irradiation, complex 3D structures are obtained
from 2D hydrogel sheets, and they exhibit complex solvent-driven actuation
behaviors due to a light-changeable modulus and cross-linking density
in the hydrogel. Based on the same approach, micropatterns can be
inscribed on the hydrogel surface using mask-assisted irradiation,
and they exhibit chain orientation-mediated anisotropic topography
change upon solvent exchange. Moreover, light-controlled strain energy
release also enables changing hydrogel surface wettability by solvent
replacement. The demonstrated mechanism for photoresponsive hydrogels
is highly efficient and applicable to many systems, which offers new
perspectives in developing hydrogels with multiple photoresponsive
functions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.