Oxidation-Triggered Ring-Opening Metathesis Polymerization

Valence-variable Catalysts for Redox-controlled Switchable Ring-opening Polymerization

As a representative class of sustainable polymer materials, biodegradable polymers have attracted increasing interest in recent years. Despite significant advance of related polymerization techniques, realizing high sequence-control and easy-handling in ring-opening (co)polymerizations still remains a central challenge. To this end, a promising solution is the development of valence-variable metal-based catalysts for redox-induced switchable polymerization of cyclic esters, cyclic ethers, epoxides, and CO₂ . Through a valence-determined electron effect, the switch between different catalytically active states as well as dormant state contributes to convenient formation of polymer products with desired microstructures and various practical performances. This redox-controlled switchable strategy for controlled synthesis of polymers is overviewed in this Review with a focus on potential applications and challenges for further studies.

Forever young: the first seventy years of ferrocene

The discovery of ferrocene, [Fe(η5-C5H5)2], seventy years ago has significantly influenced chemical research and provided a key impetus for establishing and rapidly expanding organometallic chemistry, which has continued at a...

Half-sandwich manganese complexes Cp(CO)2Mn(NHC) as redox-active organometallic fragments

Oxidation of the half-sandwich Mn^I complexes Cp(CO)₂Mn(NHC) bearing dialkyl-, arylalkyl- and diarylsubstituted N-heterocyclic carbene ligands (NHC = IMe, IMeMes, IMes) affords the corresponding stable Mn^II radical cations [Cp(CO)₂Mn(NHC)](BF₄) isolated in 92-95% yield. Systematic X-ray diffraction studies of the series of Mn^I and Mn^II NHC complexes revealed the expected characteristic structural changes upon oxidation, namely the elongation of the Mn-CO and Mn-NHC bonds as well as the diminution of the OC-Mn-CO angle. ESR spectra of [Cp(CO)₂Mn(IMes)](BF₄) in frozen solution (CH₂Cl₂/toluene 1 : 1, 70 K) allowed the identification of two conformers for this complex and their structural assignment using DFT calculations. The stability of these NHC complexes in both metal oxidation states, moderate oxidation potentials and the ease of detection of Mn^II species by a variety of spectroscopic techniques (UV-Vis, IR, paramagnetic ¹H NMR, and ESR) make these compounds promising objects for applications as redox-active organometallic fragments.

Role of External Field in Polymerization: Mechanism and Kinetics

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

Gold(ii) in redox-switchable gold(i) catalysis

Gold(ii) species catalyse the cyclisation of N(2-propyn-1-yl)benzamide to 2-phenyl-5-vinylidene-2-oxazoline without halide abstraction while the saturated gold(i) complex is inactive. Redox-switching between gold(ii) and gold(i) turns catalytic turnover on and off.

A Ring-Opening Metathesis Polymerization Catalyst That Exhibits Redox-Switchable Monomer Selectivities

A ring-opening metathesis polymerization catalyst supported by a redox-active N-heterocyclic carbene was synthesized and found to undergo reversible reduction. In its neutral form, the catalyst polymerized 1,5-cis,cis-cyclooctadiene at a higher rate than that of a norbornene derivative; however, upon reduction, the selectivity was found to reverse. Utilizing this oxidation state dependent selectivity, a series of copolymers with controlled compositions, microstructures, and physical properties were prepared by redox-switching the catalyst over the course of a series of polymerization reactions.

Hoveyda–Grubbs catalyst analogues bearing the derivatives of N-phenylpyrrol in the carbene ligand – structure, stability, activity and unique ruthenium–phenyl interactions

N-Phenylpyrrole-2,6-diisopropylphenyl ruthenium complex and its perbrominated derivative are active in ring-closing metathesis at 80 °C, but inactive at room temperature.

A redox-switchable ring-closing metathesis catalyst

A ring-closing metathesis catalyst was arrested upon reduction of a redox-active ligand; subsequent oxidation restored catalytic activity.

Multistate-Mediated Rearrangements and FeCl2 Elimination in Dinuclear FePd Complexes

Mass spectrometric, spectroscopic, and computational characterization of a novel bifunctional iron-palladium complex proves a change of coordination upon solvation. Collisional excitation reveals FeCl2 and HCl elimination in a solvent-modulated competition. Hereby, syn and anti isomers, identified by theoretical calculations, favor and disfavor FeCl2 elimination, respectively. The FeCl2 elimination likely proceeds by chlorido and Cp ligand exchange among the metallic centers in a concerted, ballet-like manner. A multitude of stationary points were identified along the computed multistep reaction coordinates of the three conceivable spin states. The quintet state shows a static Jahn-Teller type relaxation by a tilt away of the Cp ligand at the iron center. The direct singlet-quintet spin crossover is an unprecedented assumption, leaving behind the triplet state as a spectator without involvement. The FeCl2 elimination would decrease catalytic activity. It is kinetically hindered within a range of applicable temperatures in conceivable technical applications.