The degree of fragmentation of metallocomplexes with cleavage of the metal-ligand bond

A new method for estimating the degree of fragmentation of metallocomplexes under electron impact with cleavage of the metal-ligand bond was suggested. The method is based on mass spectrometric data. Using a representative selection of 67 organometallic compounds of different classes such as metal carbonyls, metallocenes, and cymantrene and bensenechrometricarbonyl derivatives, a correlation between the degree of fragmentation and the dissociation energy of metal-ligand bonds was developed. The correlation is evidence that the parameter can be used for quantitative estimation of the reactivity of metallocomplexes in the gas phase. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1374–1377, July, 1998.     

Polymer synthesis in the presence of bis(cyclopentadienyl) derivatives of Group IV–VI transition metal dichlorides: a quantum chemical study of particular reaction stages

Key pathways of interaction between growing radicals of vinyl monomers and bis(cyclopentadienyl) derivatives of Group IV–VI transition metal dichlorides were studied by the BP86/6-31G(d) and B3LYP/LanL2DZ quantum chemical methods. Prospects for use of these organometallic compounds as chain growth regulators in controlled polymerization were assessed. The character of the interaction of the compounds studied with the growing radicals is mainly determined by the metal atom. Dedicated to Academician G. A. Abakumov on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1690–1694, September, 2007.     

Complexing effect in the reaction of graft polymerization proceeding by the coordination-radical mechanism with the participation of copper ions

A new phenomenon in graft polymerization has been investigated: the complexing of growing radicals with copper ions. Graft polymerization of methacrylic acid to polycaproamide, initiated by the reversible K₂S₂O₈–Na₂S₂O₃ redox system in the presence of copper ions, is used as an illustration to show that the complexing of growing radicals with copper ions significantly enhances their activity. The concepts of the graft polymerization mechanism have been formulated.     

Strained heterocycles in radical chemistry

Over the last few decades the use of radicals in synthesis has witnessed an explosive growth through introduction of efficient chain and electron-transfer reactions. Strained heterocycles, in particular, have emerged as a highly versatile and readily available class of radical precursors. The generation of carbinyl radicals of heterocycles has resulted in many elegant applications of heteroatom-centered radicals, such as beta fragmentations, cyclizations, and intramolecular hydrogen atom abstractions. Direct electron transfer to strained heterocycles has been realized through the use of arene radical anions. The method combines the virtues of radical and organometallic chemistry to yield useful functionalized organolithium compounds. Epoxides have been opened with high regioselectivity by titanocene(III) reagents in either stoichiometric or catalytic quantities to yield beta-titanoxy radicals. This development has resulted in many new applications in natural product synthesis.     

On features of ionic coordination polymerization of vinyl cyclohexane

The work is devoted to the ionic coordination polymerization of vinyl cyclohexane. During the polymerization of vinyl cyclohexane, side reactions (viz. isomerization into unreactive isomers with internal double bond and transformation of monomer into ethylcyclohexane) occur on complex organometallic catalysts. A procedure has been developed for studying the kinetics of vinyl cyclohexane transformation by analytical gas-liquid chromatography. The effects of various catalytic systems and their individual components on the rates of these reactions have been investigated. From the experimental data, a reaction mechanism has been suggested. It is supposed that ethylcyclohexane is formed by catalytic hydrogenation of monomer with hydrogen evolved during formation of organometallic complex, as well as by the decomposition of the products of the interaction of vinyl cyclohexane with organo-aluminium compounds or titanium derivatives. Most of the isomers with internal double bond (exo- and endocyclic) are formed by a mechanism involving π-bonded intermediates between a monomer and a transition metal and the subsequent σ-alkyl or π-allylic intra- or intermolecular hydrogen transfer.     

Relationship between one-electron transition-metal reactivity and radical polymerization processes

Controlled radical polymerization has come along in leaps and bounds following the development of efficient transition-metal catalysts for atom-transfer radical polymerization. Another type of controlled radical polymerization process, namely organometallic radical polymerization, uses the reversible formation of metal-carbon bonds. Metals are also implicated in catalytic chain transfer, a process that involves the abstraction of hydrogen atoms. This Minireview discusses the importance of one-electron transition-metal reactivity in metal-mediated controlled radical polymerization processes.     

Temperature dependent reaction pathways in the addition of silyl, germyl, stannyl and plumbyl radicals to maleic anhydride and related compounds: an ESR study

The addition of Group IVB organometallic radicals to maleic anhydride, maleic thioanhydride, maleimide and N-methyl maleimide has been investigated by ESR spectroscopy. For Si and Ge centred radicals the reaction pathway has been found to be temp dependent, the addition of the organometallic radical occurring at the CC double bond at lower temps at either of the two CO groups at higher temps. For Sn and Pb centred radicals only the latter spin adducts could be detected even at low temp.     

Radical polymerization as an indicator reaction for determination of organic compounds

Polymerization of methyl methacrylate (MMA) and 4-vinylpyridine (VP) has been carried out in an aqueous solution in the presence of the initiating system persulfate-tetramethylethylenediamine. The reaction rate has been monitored by measuring the light absorbance of the suspension of the resulting polymer. The effect of 26 model organic compounds on the polymerization rate has been studied. It has been shown that the VP polymerization is inhibited by a smaller number compounds (9 compounds) than the MMA polymerization (22 compounds), which indicates that the former reaction has better selectivity, whereas the determination of model compounds using the MMA polymerization reaction is more sensitive. This is explained by the lower chain growth rate constant for VP vs. MMA and different stationary concentrations of radicals in the systems. The use of these indicator polymerization reactions makes it possible to distinguish some closely related compounds, e.g., 1,4-benzoquinone and 9,10-anthraquinone (MMA reaction) or dinitrophenol and 4-nitrophenol or phenol (VP reaction). Determination of ascorbic acid in a pharmaceutical formulation has been carried out.     

RAFT-mediated emulsion polymerization of vinyl acetate: a challenge towards producing high molecular weight poly(vinyl acetate)

The preparation of poly(vinyl acetate) with well-controlled structure has received a great deal of interest in recent years because of a large number of developments in living radical polymerization techniques. Among these techniques, the use of reversible addition–fragmentation chain transfer (RAFT)-mediated polymerization has been employed for the controlled polymerization of vinyl acetate due to the high susceptibility of this monomer towards chain transfer reactions. Here, a novel water-soluble N,N-dialkyl dithiocarbamate RAFT agent has been prepared and employed in the emulsion polymerization of vinyl acetate. The kinetic results reveal that the polymerization nucleation mechanism changes from homogeneous to micellar and RAFT-generated radicals can change the kinetic behavior from conventional emulsion polymerization to living radical polymerization. At higher concentrations of the modified RAFT agent, as a result of an aqueous phase reaction between RAFT and sulfate radicals, relatively more hydrophobic radicals are generated, which favors entry and propagation into micelles swollen with monomer. This observation was determined from the investigation of the polymerization rate and measurements of the average particle diameter and the number of particles per liter of the aqueous phase. Molecular weight analysis also demonstrated the participation of the RAFT agent in the polymerization in such a way as to restrict chain transfer reactions. This was determined by examining the evolution of polymer chain length and attaining higher molecular weights, even up to 50?% greater than the samples obtained from the conventional emulsion polymerization of vinyl acetate in the absence of the synthesized modified RAFT agent.     

New Phenomena in Organometallic‐Mediated Radical Polymerization (OMRP) and Perspectives for Control of Less Active Monomers

The impact of reversible bond formation between a growing radical chain and a metal complex (organometallic‐mediated radical polymerization (OMRP) equilibrium) to generate an organometallic intermediate/dormant species is analyzed with emphasis on the interplay between this and other one‐electron processes involving the metal complex, which include halogen transfer in atom transfer radical polymerization (ATRP), hydrogen‐atom transfer in catalytic chain transfer (CCT), and catalytic radical termination (CRT). The challenges facing the controlled polymerization of “less active monomers” (LAMs) are outlined and, after reviewing the recent achievements of OMRP in this area, the perspectives of this technique are analyzed.     

Synthesis and aromatizational rearrangements of new imino-, hydrazono-, and azino-2,5-cyclohexadienylidene systems as ligands for cascade type metallocomplexes

Three approaches to the synthesis ofN-substituted imino-, hydrazono-, and azino-2,5-cyclohexadienylidene systems based on reactions of 4-methyl-4-trichloromethylcyclohexa-2,5-dienone with aminophenols and hydrazones and condensation of hydrazones ofpara-semiquinoid ketones with carbonyl compounds, including that of the ferrocene series, were realized. The latter reaction, when applied to 3,6-dibromophenanthrene-9, 10-quinone, was accompanied by quantitative aromatizational molecular rearrangement with the elimination of the CCl₃ group. Using Rh¹ complexes as an example, it was shown that the heteroorganic ligands obtained can be used for the synthesis of mixed-ligand metallocomplexes with triple coordination of the metal atom including simultaneous metal-ligand interactions of the n-, π-, and σ-types. The principle of metal-ligand “cascade” appeared as a result of the generalization of two new phenomena of organometallic sereodynamics, which we have found recently^2,3 and have called oxidative and reductive redox-rotation. In the “cascade”, type1 (“metal-ligand-metal”) or type2 (“ligand-metal-ligand”) metallocomplexes, one or several coordinated metal ligand”) metallocomplexes, one or several coordinated metal atoms capable of concertedly and reversibly changing their valence in the course of intramolecular conformational transformation are in positions of mutual conjugation. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 363–367, February, 1997.     

光调控的活性自由基聚合的研究新进展

从引发和催化两个方面概述了光辐照在活性自由基聚合(LRP)中的应用,从机理上详细地分析了光辐照对氮氧调控自由基聚合(NMP)、原子转移自由基聚合(ATRP)、可逆加成-断裂链转移自由基聚合(RAFT)以及有机钴催化的可控自由基聚合反应(CMRP)的影响。与传统自由基聚合相比,光调控的活性自由基聚合方法可在温和的条件下生成自由基,能够克服传统LRP的一些缺陷,如降低催化反应活化能、提高聚合物末端官能度等。同时,本文对光调控反应的进一步应用以及新方法的产生也进行了展望。     

Photochromism of organometallic compounds with structural rearrangement

This article introduces photochromic properties together with structures of organometallic compounds that undergo photo-induced structural rearrangement. The aim of this review is to survey the research on photochromism by using organometallics which possess by their own nature the properties responsible for the photochromism such as bonding and structural fluxionality, electronic state fluctuation, and photochemically active characteristic in both solution and the solid state. Therefore, the organometallics which include the well-characterized organic photochromic moieties, considered to be derivatives of such kinds of organic photochromic compounds, are excluded in this article. Mono-, di-, and poly-nuclear organometallic compounds are presented based on the reaction types such as linkage isomerization, haptotropic rearrangement, and reorganization of metal–ligand and/or metal–metal bonds. Very recently, the crystalline-state photochromism is becoming an attractive field of photochromic chemistry. As a demonstrative example, the photochromism of organometallic rhodium dinuclear complexes having a dithionite ligand (μ-O₂SSO₂), which shows 100% reversible interconversion in the crystalline-state and have been developed in the authors’ laboratory, will be discussed.     

Ethylene polymerization initiated by metallocene catalysts (C5H5)4Mt-MAO (Mt = Ti, Zr) in the presence of organometallic modifiers

The effect of various organometallic compounds (Me₃Al, Et₃Al, i-Bu ₃Al, Et₂Zn, Me₄Sn, Et₄Pb) on the activity and productivity of catalytic systems based on (C₅H₅)₄Zr and (C₅H₅)₄Ti and the molecular- mass characteristics of polymers is investigated. The effect of additives on the activity of catalysts; on the shape of kinetic curves of polymerization; and on the molec ular mass, molecular-mass distribution, and poly- dispersity of the resulting polymers is associated with reversible chain transfer to the organometallic compounds.     

Bismuth in Dynamic Covalent Chemistry: Access to a Bowl-Type Macrocycle and a Barrel-Type Heptanuclear Complex Cation

Dynamic covalent chemistry (DCvC) is a powerful and widely applied tool in modern synthetic chemistry, which is based on the reversible cleavage and formation of covalent bonds. One of the inherent strengths of this approach is the perspective to reversibly generate in an operationally simple approach novel structural motifs that are difficult or impossible to access with more traditional methods and require multiple bond cleaving and bond forming steps. To date, these fundamentally important synthetic and conceptual challenges in the context of DCvC have predominantly been tackled by exploiting compounds of lighter p-block elements, even though heavier p-block elements show low bond dissociation energies and appear to be ideally suited for this approach. Here we show that a dinuclear organometallic bismuth compound, containing BiMe₂ groups that are connected by a thioxanthene linker, readily undergoes selective and reversible cleavage of its Bi−C bonds upon exposure to external stimuli. The exploitation of DCvC in the field of organometallic heavy p-block chemistry grants access to unprecedented macrocyclic and barrel-type oligonuclear compounds.     

Organometallic mediated radical polymerization of vinyl acetate with Fe(acac)2

The radical polymerization of vinyl acetate (VAc) is moderated by iron(II) acetylacetonate (Fe(acac)₂) by the organometallic route (OMRP), as well as by degenerative transfer polymerization (DTP) when in the presence of excess radicals, through the formation of thermally labile organometallic Fe^III dormant species. The poly(vinyl acetate) (PVAc)‐Fe^III(acac)₂ dormant species has been isolated in the form of an oligomer and characterized by ¹H NMR, EPR, and IR methods, and then used as a single‐component initiator for the OMRP of VAc. The degree of polymerization of this isolated oligomeric species demonstrates the limited ability of Fe(acac)₂, relative to the Co(acac)₂ congener, to rapidly trap the growing PVAc radical chain. Control under OMRP conditions is improved by the presence of Lewis bases, especially PMe₂Ph. On the other hand, iron(II) phthalocyanine inhibits the radical polymerization of VAc completely. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3494–3504     

Interaction of fullerene C60 with allylbenzene and allyl chloride

The reaction between allyl compounds and fullerene C₆₀ has been investigated via dilatometry under the conditions of free-radical polymerization. It has been shown that the rate of a variation in the volume of the reaction mixture plotted versus the concentration of fullerene C₆₀ is described by a curve with a minimum. It has been established that, in the presence of fullerene and the allyl monomer, the polymerization of methyl methacrylate proceeds without any induction period. It has been concluded that allyl radicals interact with fullerene C₆₀.     

Catalytic and coordination facets of single-site non-metallocene organometallic catalysts with N-heterocyclic scaffolds employed in olefin polymerization

This review discusses the principles underlying mononucleating N-heterocyclic ligand design, selectivity of metal centers, preparation of organometallic catalysts with a N-heterocyclic backbone, and their catalytic activity in olefin oligo/polymerization. A vast number of N-heterocyclic organometallic compounds have been applied for the polymerization on account of their modest cost, low toxicity, and the large availability of transition metals in stable and variable oxidation states, which makes them versatile precursors for these reactions. The main points of focus in this review are the key advances made over more the past 25 years in the design and development of non-metallocene single-site organometallic catalysts bearing different N-heterocyclic scaffolds as a backbone. These catalysts are applied as precursors for the transformation of ethylene, higher α-olefins, and cyclic olefins into oligo/polymers. Emphasis is placed on the architecture of ligand peripheries for tuning the formed polymer properties and the consequences on product formation of different alkyl or aryl substituents directly attached to the metal center in a N-heterocyclic ligand system.     

Controlled free-radical polymerization of methyl acrylate in the presence of a cyclic trithiocarbonate under γ-ray irradiation at low temperature

Controlled radical polymerization of MA has been achieved in the presence of a cyclic trithiocarbonate, 1,5-dihydro-2,4-benzodiehiepine-3-thione, under γ-ray irradiation (60 Gy/min) at low temperature. The narrow molecular weight distributions and the linear kinetics curve indicate that the polymerization is a controlled free-radical process at low temperature (especially at −76 °C). The structures of resultant polymers were characterized by matrix assisted laser desorption/ionization time-of-flight mass spectrometry, and the results show that cyclic polymers can be formed at −76 °C, which may result from the reduced diffusion rate and the suppressed chain-transfer reaction at the lower temperature. It is further evidenced that the good control of the polymerization at the lower temperature may be associated with the suppressed chain-transfer reaction, not like reversible addition-fragmentation transfer polymerization. The linear polymers probably result from the polymer chain radicals reacting with the radicals produced by the interaction of the irradiation and the monomer.     

Polymerization of Acrylonitrile Initiated by the Mn(III)/Ethane Thiol Redox System

Abstract

Kinetics of vinyl polymerization of acrylonitrile initiated by the redox system Mn(III)/ethane thiol were investigated in aqueous sulfuric acid in the temperature range of 30-45°C. The rate of polymerization, rate of manganic ion disappearance, etc. were measured. The effect of certain water-miscible organic solvents, neutral electrolytes, and organic nitrogen compounds on the rate has been investigated. A mechanism involving the formation of a complex between Mn(III) and the thiol, whose decomposition yields the initiating free radical with the polymerization being terminated by mutual combination of the growing radicals, has been suggested.