Alkyls decomposition pathways

There has been little insight into potential decomposition pathways for the Ni(II) system due to sparse experimental evidence. Polymerization results with catalysts bearing different alkyl and fluorinated substituents have suggested that a C-H activation process analogous to that occuring with the Pd(II) catalysts is unlikely with Ni(TT) [28], Instead, side reactions between Ni and the aluminum coactivator, present as it is in such large excess, have been implicated. The formation of nickel dialkyl species and their subsequent reductive elimination to Ni(0) is one possible deactivation mechanism [68]. 

The primary reason for the thermal stability or instability of the alkyls and aryls of the lanthanides and actinides is kinetic in nature. As in all kinetic processes, the rate of reaction is dependent upon the activation energy between reactants and products. By considering the various decomposition pathways and factors which enhance or inhibit these pathways one can rationalize the observed sta-bihties of the various complexes. 

The discovery of hexamethyltungsten by Shortland and Wilkinson (259, 260) has prompted an interest in the synthesis and physical properties of alkylated transition metal compounds. The relatively high thermal stability of these derivatives is plausibly ascribed to the lack of a low-energy decomposition pathway. This is particularly true in the case of ligands such as Me3SiCH2 — and Me3 CCH2 for which /3-hydrogen transfer is precluded (43, 282). 

Finally, in the context of these results, the well recognized, apparently lower thermal (and photochemical) stability of secondary cobalt-alkyls, relative to primary ones, may reflect the greater accessibility of irreversible decomposition pathways involving olefin elimination (i.e., through schemes such as that in Reactions 25-26), in addition to some probable lowering of the metal-alkyl bond-dissociation energy. 

The resulting extraordinary stability of NHC-metal complexes has been utilized in many challenging applications. However, an increasing number of publications report that the metal-carbene bond is not inert [30-38]. For example, the migratory insertion of an NHC into a ruthenium-carbon double bond [30], the reductive elimination of alkylimidazolium salts from NHC alkyl complexes [37] or the ligand substitution of NHC ligands by phosphines [36,38] was described. In addition, the formation of palladium black is frequently observed in applications of palladium NHC complexes, also pointing at decomposition pathways. 

The most studied alkyl and aryl compounds are of the type Cp3AnR, An = Th, U, Np, and their Cp analogues. Unlike the d-block alkyls, decomposition by /3-H transfer and alkene elimination is not a major route but both thermal and photochemical pathways are multiple and complex. 

This decomposition pathway leads to equimolar amounts of alkenes and alkanes. Complexes with /3-agostically bonded alkyl groups are likely intermediates structures of this kind may sometimes be the ground state, as in [(P—P)Pt(r72-C2H5)]+., 5... 

A second, very important decomposition pathway involves the activation of C-H bonds on the N-alkyl [142-147] or N-aryl [148-151] sidechains. OccassionaUy, even C-C activation in the sidechain is observed [152]. Similar C-H activation is observed in transition metal phosphides, especially when the phosphorus ligand has a SMes substituent [153]. 

A recent study on the stability of various indium alkyl derivatives has been performed using differential scanning calorimetry (DSC), which provides a comprehensive thermal fingerprint of the compounds. In addition, when this method of thermal analyses is used in conjunction with thermogravimetric analysis coupled to FTIR and/or GCMS evolved gas analysis, it can provide a complete mechanism for the decomposition pathway of prospective compounds. ... 

Pure alkyl transition metal complexes are rare, owing to the -Elimination decay mechanism. An effective strategy to prepare such compounds must therefore aim at blocking possible decomposition pathways. Neopentyl is void of hydrogen in the jS-position, and neopentyl... 

Pulse radiolysis has been used to study the transient formation and decomposition of cobalt-alkyl bonds in aqueous solution in the same manner as it has been used for chromium alkyls. And as for chromium alkyls, bond homolysis is a major decomposition pathway (28). For bond formation reactions, pulse radiolysis shows that they are assisted by increases in pressure. This feature results from the homolysis having a larger activation volume than the bond formation reaction, resulting in a significantly negative overall reaction volume for the process (29). In general for all of these metal-alkyl bond homolysis reactions of the aquo complexes, steric hindrance facilitates the reaction. Ligand effects also play a role, but the factors involved are more subtle. 

There are three main decomposition pathways for 7 elimination of (i) RX, (ii) R X or (iii) R"CHO shown in Scheme 8. The route via (i) has been termed a retro-Arbuzov reaction. The decomposition pathway is structure-sensitive, route (i) being characteristic for R = alkyl. When R = aryl, route (ii) is favoured especially if the volatile halide is... 

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