Metal-alkyl complexes Stability

Liu, B., Ctii, D.M., Ma, 1. et al. (2007) Synthesis and reactivity of rare earth metal alkyl complexes stabilized by aniUdo phosphinimine and amino phosphine ligands. Chemistry —A European Journal, 13, 834. 

Many complexes that contain alkyl ligands bearing -hydrogens readily decompose to form olefins and metal-hydride complexes. Such p-hydrogen elimination is the most common process that limits the stability of met -alkyl complexes, although other elimination processes noted beloTv can occur. Kochi summarized early available information on the mechanism of such P-eliminations, and there is also considerable information in early reviews of the chemistry of alkyl ligands. More recent reviews survey all of the decomposition modes available to transition metal alkyl complexes, but emphasize the work of individual authors. ... 

There are still some factors that stabilize metal-alkyl complexes ... 

The phenyl complexes often decompose as metal-alkyl complexes according to the P-H-elimination mechanism. The benzyne intermediate can sometimes be stabilized as shown by Schrock with a tantalum complex ... 

There are several reports of rare earth NHC complexes with additional stabilization by cyclometallation of one of the ligand side chains. In most cases a C-H activation to form a metal-alkyl complex is required. For example, at the same time as Yb-NHC adduct 13 was reported, Takats observed the cyclometallation of N-Me substituent of IMe as a result of the reaction of Yb(CH2SiMe3)(THF)] with an excess of free carbene to form [(Tp " )( IMe) Yb(CH2 MeiMeXTHF)] 43. ... 

An important development over the last two years has been the confirmation that cationic Group IV metal alkyl complexes are the catalytically active species in olefin polymerizations with homogeneous Ziegler-Natta catalysts. A number of complexes of the type [Cp2MR(L)] (M = Ti, Zr, Hf) have now been prepared, particularly when stabilized by donor ligands such as L = THF. The chemistry of zirconium complexes of this type has been reviewed. Some cationic benzyl complexes with stereorigid ligands (29)... 

For a number of metal-alkyl complexes (M—R) the thermal stability decreases in the order R = Me > Et > Pr. In particular, the methyl complexes are often markedly more stable than the higher homologues. The lower stability of the ethyl and propyl complexes may be due in part to inductive effects and in part to steric weakening of the M-C bond. Another explanation, which is more plausible in some cases, is that the interaction discussed above gives rise to a transition state of lower energy than required for unaided homolytic dissociation, for example ... 

Many complexes are known which contain one or more hydrogens directiy attached to a transition metal by an essentially covalent bond [2,3]. The occurrence and stability of these complexes frequently parallels that of metal-alkyl complexes. Thus thermally stable transition metal-hydrides are found when the complexes are kinetically staUe, that is, they often... 

Transition metal alkyls are often relatively unstable earlier views had attributed this either to an inherently weak M—C bond and/or to the ready homolysis of this bond to produce free radicals. Furthermore, the presence of stabilizing ir-acceptor ligands such as Cp , CO, or RjP was regarded as almost obligatory. However, (1) the M—C bond is not particularly weak compared say to the M—N bond, and (2) the presence of the new type of ligand on the metal could make the complex kinetically stable thus, even isoleptic complexes, i.e., compounds of the form MR , might be accessible 78, 239). These predictions have largely been borne out (see Table VII). 

The 7r-back donation stabilizes the alkene-metal 7c-bonding and therefore this is the reason why alkene complexes of the low-valent early transition metals so far isolated did not catalyze any polymerization. Some of them catalyze the oligomerization of olefins via metallocyclic mechanism [25,30,37-39]. For example, a zirconium-alkyl complex, CpZrn(CH2CH3)(7/4-butadiene)(dmpe) (dmpe = l,2-bis(dimethylphosphino)ethane) (24), catalyzed the selective dimerization of ethylene to 1-butene (Scheme I) [37, 38]. 

The active species of the metallocene/MAO catalyst system have now been established as being three-coordinated cationic alkyl complexes [Cp2MR] + (14-electron species). A number of cationic alkyl metallocene complexes have been synthesized with various anionic components. Some structurally characterized complexes are presented in Table 4 [75,76], These cationic Group 4 complexes are coordinatively unsaturated and often stabilized by weak interactions, such as agostic interactions, as well as by cation-anion interactions. Under polymerization conditions such weak interactions smoothly provide the metal sites for monomers. 

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