Ethene complexes

Examine the sequence of structures corresponding to Ziegler-Natta polymerization of ethene, or more specifically, one addition step starting from a zirconocene-ethene complex where R=CH3. Plot energy (vertical axis) vs. frame number (horizontal axis). Sketch Lewis structures for the initial complex, the final adduct and the transition state. Indicate weak or partial bonding by using dotted lines. 

For unsymmetrical zirconacyclopentadienes, Cp2ZrEt2, which we developed as an equivalent to the zirconocene—ethene complex (3), is a very useful reagent [13]. Two different alkynes couple selectively via zirconacyclopentenes (4) (Eq. 2.3). 

When the ethene pressure is raised the number of branches decreases, while the productivity of the catalyst remains the same (the reaction is zero order in ethene pressure). Chain walking requires an open site at the metal and obviously the competition between ethene complexation and chain walking determines the number of branches formed. 

Insertion of CO is therefore always kinetically controlled. When an alkyl palladium species has formed, the open site will be occupied by a coordinating CO molecule. Carbon monoxide coordinates more strongly to palladium than ethene, even when the palladium centre is cationic. The reason for this is steric the cone angle of ethene is much larger than that of CO and the steric hindrance in the ethene complex is therefore much larger. If the barriers of activation for the insertion processes of ethene and CO are of the same order of... 

The cw-l,2-bis(diphenylphosphino)ethene complexes [Fe(Ph2PCH=CHPPh2)2X2], X = C1 or Br, undergo pressure-induced spin-state transitions at about 8kbar for the chloro complex, about 60 kbar for the bromo complex. The difference is ascribed to the difference in ligand field strengths and lattice cooperativity. ... 

In contrast to theoretical results reported by Morokuma [29] and Ziegler [30], as well as previous studies with Pd"-phen model compounds [26], the lowest experimental energy barrier was found for the migratory insertion of the acyl (ethene) complex (Eq. (10)). The relative rates of alkyl to CO and alkyl to ethene migratory insertion reactions allowed one to estimate that sequential ethene insertions occur once for every ca. 10 insertions of CO into the Pd-alkyl bond [18]. 

Calculations confirm the Cossee mechanism for olefin insertion [7, 26-28]. For the simple Me2AlEt model, the ethene complexation energy is only a few kcal/mol. The activation energy calculated at the highest theoretical level [7] agrees well with the experimental estimate. 

Chiral platinum (and palladium) ethene complexes derived from Diop [2,2-dimethyl-4,5-bis-(diphenylphosphinomethyl)-l,3-dioxolane] act as chiral derivatising agents for the 31P-based NMR assay of the enantiomeric purity of certain chiral alkenes and allenes80. 

Reaction of the platinum complex with a molar excess of racemic acrylamide 7 leads to four distinct AB quartets in the 31P-NMR spectrum (with associated platinum satellites) due to the four possible diastereomeric species. These are formed by nonselective binding of the Re-and 57-face of both enantiomers. Such behaviour may be expected when the stereogenic center under examination is remote from the metal center. After reaction of the ethene complex with carvone 8 the metal is bound with complete stereoselectivity to the less hindered Si- 5/-face of the endocyclic, electron-poor double bond. Spectra of optically pure and racemic carvone complexes are shown in Figure 14. 

Stable ethene complexes of Rh similar to 17 have been isolated and shown to have the rr-complex structure. Formation of 17 must be an equilibrium process because addition of extra L reduces the rate of hydrogenation by shifting the equilibrium to the left. 

The 32-electron dinuclear species MRh(/t-CO)2Cpf [114, M = Co (166) 115, M = Rh (166,167) and 116, M = Ir (110,168) ] contain short M-Rh separations, consistent with the presence of a formal double bond. Theoretical studies on Rh2( -CO)2Cp2 (112) reveal that it possesses an acceptor 2b2 and a donor 3a similar in character to the n and n orbitals of ethene. Complexes 114-116 react readily with a variety of ML fragments that are iso-labal with CH2, to afford trinuclear clusters (27,110,166), and several Rh-Pt... 

The main conclusions are that (i) Fivefold coordinated Cr3+ sites exposed on the (0112) faces, although reactive in the formation of weak molecular Cr3+-ethene complexes, are not active in catalytic polymerization, and (ii) polymerization (and oligomerization) activity is attributed only to Cr2+ centers, located at structural defects (such as edges, steps, and corners). This last conclusion strongly suggests that a highly coordinatively unsaturated state is a necessary prerequisite for the polymerization activity of Crx+ centers. 

The Dewar-Chatt-Duncanson model of bonding in ethene complexes is shown in figure below. And is analogous to the... 

An isolobal relationship exists between 5 and ethene. The logic goes that it should therefore be possible to construct molecules containing the Cp2-Rh2(CO)2 unit that are formally analogous to ethene complexes. In practice, 5 is used because of the essential nonavailability of the Cp species. Many compounds have been prepared by using this approach (195,236). 

Figure 6.3 Selected syntheses of ethene complexes acac = propane-2,4-dionate (acetylacetonate)...

An easier route to the same unsymmetric zirconacyclopentadiene 20 is the use of Cp2ZrEt2 as reagent (easily prepared from Cp2ZrCl2 with two equivalents of EtMgBr). This reagent (Scheme 9), equivalent to a zirconocene-ethene complex 24, reacts similarly to the Negishi-type reagent, which was prepared in the... 

Hydride attack on (5b) occurs, unlike ( a), not only at the 6-vinlynic carbon but also at the a-carbon of the p-vinyl, giving a very low yield of the ethene complex i... 

In a simplified picture, the mechanism of the Zr-catalyzed ethylmagnesation can be rationalized as shown in Scheme 1 [8]. At first, the zirconocene-ethene complex 12 is generated from the catalyst precursor Cp2ZrCl2. Complex 12 can also be regarded as a metallacyclopropane 16. After coordination and insertion of the alkene 10, a metalla-cyclopentane 13 is formed, which subsequently reacts with the Grignard reagent regioselectively to the open-chain intermediate... 

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