Ethylene insertion into agostic

Theoretical calculations at DFT level for ethylene insertion into Ti-Me bonds of cationic alkylamidinato complexes [TiMe(R1NGRNR1)2]+ (R = H, Ph R1 = H, SiMe3) have been performed,320 as have calculations for a bis(/5-diketonato)titanium model system (Scheme 137) in the presence of ethylene. Special attention is paid to the possible occurrence of agostic alkyl complexes and to the mechanism of ethylene uptake, chain propagation, and termination.321... 

While the migratory insertion/BHE of the [CpM(L)(H)(C2H4)]" complexes is characterized by the five stationary points depicted in Scheme 1.1, additional isomers may be relevant for the insertion into a metal-alkyl bond. For example, for M = Cr it is known that the initial insertion product of the ethylene-alkyl complex I is a y-agostic complex Ily, which subsequently isomerizes to p- and a-agostic species Iip and Ila [26], as shown in Scheme 1.2. 

The key assmnption in the scheme is that the Ti-CH CH species is unique in that it is relatively stable. This stability may arise from a strong P-agostic interaction between the hydrogen atom of the methyl group and the Ti atom. Other Ti-alkyl species proposed in the scheme do not possess this stability. In this scheme, formation of the Ti-CH CH species occurs as a result of an ethylene insertion reaction into the T-H bond or after a chain transfer reaction with ethylene. 

Table 3.5 Hsts energy differences of ethylene coordination and insertion for metahacycle expansion and 1-hexene hberation via agostic-assisted P-hydrogen transfer (TS[6X—8X]) in each model. The role of DME on the transformation of ethylene polymerization into ethylene trimerization can be understood readily. Before adding DME, the coordination of extra ethylene molecules occurred spontaneously with exoergic effect (cationic models L, O, and P) or with a neghgible endoergic effect for both neutral models M and N. As a result, the formation of metahacycle was determined...

Many theoretical studies of insertion concern the very important case of insertion of ethylene (or a-alkenes) into a growing polymer, following the Cossee-Arlman direct insertion mechanism (Scheme 6.2) [6], In this particular case, the resting states of the catalyst before and after each propagation step are similar, and contain an M-alkyl bond stabilized by an agostic interaction. The interactions with the solvent or external ligands must be also rather similar for both resting states. Thus, the values calculated must correspond only to the coordination and insertion of the alkene. 

The catalytic cycle shown at the top of Figure 1.1 consists of four intermediates, which differ by the kind of agostic interaction displayed (none, a-, P-, or y-agostic). The insertion step always leads to the y-agostic isomer as the initial product, except for the insertion of ethylene into the Cr-H bond, as there is no y-H atom in the resulting complex [39]. Furthermore, two alternative mechanisms for the termination of the polymerization process exist ... 

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