Organometallic radicals stable species

Acyclic Tricoordinate Radicals As one can easily note, the above examples of the cyclic radicals of Si and Ge atoms have rather particular structures featuring the cyclic delocalization of the odd electrons. The simple tricoordinate acyclic radicals of the type RsE (E = Si, Ge, Sn, Pb), lacking the stabilization effects of the cyclic Jt-delocalization, constitute another, more general and even more challenging, class of stable organometallic radicals. " Consequently, the search for such highly symmetrical species appeared to be of primary importance for synthetic chemists. 

Some of these organometallic radicals are sufficiently stable to be isolable - in particular, many stable 17-electron complexes are known. As organic radicals, organometallic radicals are indeed stabilized by bulk around the metal center that inhibits radical reactions. When they are not sterically stabilized, their main property, beside the redox ones, is the very fast intra- or intermolecular interconversion between the 17-electron and 19-electron forms. The combination of a 17-electron complex with a 2-electron L ligand to give a 19-electron species is schematized below ... 

Chemical or electrochemical reduction of 18-electron precursors is, in principle, the most straightforward route to 19-electron radicals. The electrochemical reduction of stable organometallics has been studied for many years.89 By reference to Scheme 1, one can see that ligand dissociation may accompany reduction of M—L to M—L , and to generate the 19-electron complex as the predominant species it is necessary that Ksq for the 19e <-> 17e interconversion be small. This is more likely to occur as E° becomes less negative, i.e., an easily reduced (electron-poor) M—L is less prone to dissociate ligand L after reduction. Even if these conditions are met, the formation of 18-electron M2- may still be thermodynamically favored, and radical species may not be seen. Equations (11) and (15) are... 

A frequently used indirect method involves cyclizable (cf. (7)) or other mechanistic probes which should provide evidence for free radical intermediates and thus for SET [19,37,59]. However, Newcomb and Curran have pointed out the pitfalls of such an approach especially if iodide precursors are used [17]. The supposedly radical-indicative reaction may come about albeit slower by a different, nonradical mechanism or the radical formation may occur via a secondary process which is not directly related to the first reaction step. A similar side-route can be made responsible for the appearance of stable radical compounds which may arise via a comproportionation reaction between non-reduced starting material and the doubly reduced species which can be formed from a hydro form (the normal product, Eq. (5)) and the usually strongly basic organometallic or hydridic reagents (Eq. (9)) [58]. The ability of strong bases to produce reduced radical species via complicated electron/proton transfer processes has been known for some time in the chemistry of quinones and quaternary salts [60,61]. 

This popular approach, illustrated by Eq. 3 [32] for an organometallic example, must be carefully checked with respect to the question whether the resulting stable and thus ESR-observable species (e.g. a nitroxide radical complex) is perhaps just a product from a preformed charge-transfer complex between the substrate and the spin trap. 

Control via a reversible homolytic cleavage of a weak covalent bond leading to a propagating radical and a stable free radical. The latter should only react with the propagating radical and can be a nitroxide [37,38], an N-based radical [39], or an organometallic species [40,41]. They are generally called sta-... 

The Rh and Ir species in oxidation state +IV are usually very reactive (see Section IV.B), and only a few stable, unequivocal organometallic Ir(IV) radicals are known to date. The first reported stable example is the neutral tetrakis-mesityl Ir(IV) compound [Ir (mes)4)] (39). This unusual species was obtained in 20% yield by reaction of partially dehydrated IrCl3.nH20 with mesityl-lithium at room temperature (RT). The route to this species is rather unclear, but a disproportionation reaction, 211 ° Ir° + Ir , was suggested as one of the... 

Although some of the species desribed in this section are stable in certain solvents or under certain conditions, they reveal a remarkably high reactivity toward a variety of substrates. This section focuses on the diverse radical-type reactivity of open-shell organometallic species of the groups 9(V111B) and lO(VIII) transition metals. 

Again the organometallic must be carefully chosen to be compatible with the deposition chemistries and temperatures used in the process. This cannot be overemphasized as more complex structures are deposited. The complexity of this task increases when all the reactants are organometallics because the carrier gas (hydrogen) also generally participates directly in the reaction to effectively convert initial radical reaction co-products into neutral, more stable ultimate species. An overview of precursors is provided by Jones [19]. 

In addition to the studies on reduction and oxidation of metalloporphyrins, radiolytic methods have been used to investigate reactions of radicals with metalloporphyrins that lead to formation of metal-carbon bonds. Formation of metal-carbon bonds has been implicated in various catalytic reactions and in biological systems. Therefore, numerous studies have been carried out on the formation and decomposition of such bonds involving porphyrin complexes of Pe 38.s3,62,68-70co, ° Rh, and other metals, as well as complexes of related macrocycles, such as Co-phthalocyanine and Co-B,2. Certain oxidation states of transition metal ions react with free radicals by attachment to form organometallic products, some of which are stable but others are short-lived. Pulse radiolysis has been used to investigate the formation and decay of such species. 

One and two electron oxidative addition processes that involve electron transfer between alkyl radicals and transition metal species have been exploited in organic synthesis for many years. These reactions can ultimately result in the formation of stable metal-alkyl complexes. The formation of such organometallic species during ATRP would have several implications on the role of the catalyst. The relative bond dissociation energies of the the Mt-R, Mt-X, and R-X bonds would ultimately dictate whether polymerization would be inhibited by the formation of a Mt-R bond, whether initiation efficiency might just be reduced, or whether the entire polymerization could be mediated through the reversible formation of such a Mt-R bond (as in stable free radical polymerization, or SFRP).[ ]... 

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