Radicals metal-centered

Replacement of Labile Chlorines. When PVC is manufactured, competing reactions to the normal head-to-tail free-radical polymerization can sometimes take place. These side reactions are few ia number yet their presence ia the finished resin can be devastating. These abnormal stmctures have weakened carbon—chlorine bonds and are more susceptible to certain displacement reactions than are the normal PVC carbon—chlorine bonds. Carboxylate and mercaptide salts of certain metals, particularly organotin, zinc, cadmium, and antimony, attack these labile chlorine sites and replace them with a more thermally stable C—O or C—S bound ligand. These electrophilic metal centers can readily coordinate with the electronegative polarized chlorine atoms found at sites similar to stmctures (3—6). 

Dihydro-l,3-diborolenes are accessible as pentaorgano derivatives " and serve as ideal precursors of the diborolyl radieal. The formation of the radical and its interaction with a metal center is formally described as a metal oxidation. The resulting 2,3-dihydro-1,3-diborolyI ligand is either a three-electron donor or an anionic four-electron donor. l,3,4,5-Tetraethyl-2-methyl-2,3-dihydro-l,3-diborole reacts with [T -CpNi(CO)2] or (T7 -Cp>2Ni in hot mesitylene to yield a sandwich complex and a triple-decked complex ... 

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

The use of y-ray induced radical pol5unerization proved to be a successful alternative for the radical co-polymer-ization of metal complexes with ligands containing acrylic C—C double bonds [100-102,129,130]. In particular, the palladium(II) complex cw-[PdCl2(ICPA)2] (1, Scheme 4) was co-polymerized in DMF solution with DMA and MBAA (cross-linker, 4% mol), with no degradation of the metal center [100,101]. 

Le Mest Y, L Her M, Hendricks NH, Kim K, Collman JP. 1992. Electrochemical and spectroscopic properties of dimeric cofacial porphyrins with nonelectroactive metal centers. Delocalization processes in the porphyrin rr-cation-radical systems. Inorg Chem 31 835... 

It has been demonstrated that the MCR enzyme is active only if the metal center of coenzyme F430 is in the Ni1 form.1857 The natural substrate Me-CoM or simple methyl thioethers, however, do not react with Ni1 F430, which has lead to the proposal of a catalytic mechanism in which the addition of a thiyl radical to the S atom of the thioether giving a sulfuranyl radical intermediate is... 

Like all controlled radical polymerization processes, ATRP relies on a rapid equilibration between a very small concentration of active radical sites and a much larger concentration of dormant species, in order to reduce the potential for bimolecular termination (Scheme 3). The radicals are generated via a reversible process catalyzed by a transition metal complex with a suitable redox manifold. An organic initiator (many initiators have been used but halides are the most common), homolytically transfers its halogen atom to the metal center, thereby raising its oxidation state. The radical species thus formed may then undergo addition to one or more vinyl monomer units before the halide is transferred back from the metal. The reader is directed to several comprehensive reviews of this field for more detailed information. 

As exemplified above, among the various heteroleptic Cp M(dt)m complexes described so far, only a few series have been isolated in a radical state these are collected in Scheme 4 and finally concern only three classes, according to the formal electron count on the metal center ... 

Monomer, solvent, or radical coordination to the metal center... 

This chapter s discussion does not treat inorganic and organic free radicals and triplet states (such as dioxygen, O2), which produce EPR spectra. Rather, the focus here will be on EPR behavior of transition metal centers that occur in biological species. An excellent presentation of the subject, written by Graham Palmer, is found in Chapter 3 of reference l.16 The discussion here is summarized mostly from that source. 

This compound is not only of note because its spin crossover is centered near room temperature the TCNQ radical anions are also directly coordinated to the divalent metal center. In fact, TCNQ has strong electron affinity due to the electron-withdrawing capacity of the four cyano groups, hence TCNQ readily takes on an electron to form the radical anion TCNQ -. Coor-... 

The mechanism shown in Scheme 5 postulates the formation of a Fe(II)-semi-quinone intermediate. The attack of 02 on the substrate generates a peroxy radical which is reduced by the Fe(II) center to produce the Fe(III) peroxide complex. The semi-quinone character of the [FeL(DTBC)] complexes is clearly determined by the covalency of the iron(III)-catechol bond which is enhanced by increasing the Lewis acidity of the metal center. Thus, ultimately the non-participating ligand controls the extent of the Fe(II) - semi-quinone formation and the rate of the reaction provided that the rate-determining step is the reaction of 02 with the semiquinone intermediate. In the final stage, the substrate is oxygenated simultaneously with the release of the FemL complex. An alternative model, in which 02 attacks the Fe(II) center instead of the semi-quinone, cannot be excluded either. 

The half-order of the rate with respect to [02] and the two-term rate law were taken as evidence for a chain mechanism which involves one-electron transfer steps and proceeds via two different reaction paths. The formation of the dimer f(RS)2Cu(p-O2)Cu(RS)2] complex in the initiation phase is the core of the model, as asymmetric dissociation of this species produces two chain carriers. Earlier literature results were contested by rejecting the feasibility of a free-radical mechanism which would imply a redox shuttle between Cu(II) and Cu(I). It was assumed that the substrate remains bonded to the metal center throughout the whole process and the free thiyl radical, RS, does not form during the reaction. It was argued that if free RS radicals formed they would certainly be involved in an almost diffusion-controlled reaction with dioxygen, and the intermediate peroxo species would open alternative reaction paths to generate products other than cystine. This would clearly contradict the noted high selectivity of the autoxidation reaction. 

The third ligand was assumed to be coordinated to the metal center via the deprotonated 3-hydroxy and 4-carbonyl groups. This coordination mode allows delocalization of the electronic structure and intermolecu-lar electron transfer from the ligand to Cu(II). The Cu(I)-flavonoxy radical is in equilibrium with the precursor complex and formed at relatively low concentration levels. This species is attacked by dioxygen presumably at the C2 carbon atom of the flavonoxyl ligand. In principle, such an attack may also occur at the Cu(I) center, but because of the crowded coordination sphere of the metal ion it seems to be less favourable. The reaction is completed by the formation and fast rearrangement of a trioxametallocycle. 

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