Alkene complexes structures

A number of X-ray crystal structures of nickel(O) complexes containing both alkenes and phosphines have been reported to date with the aim of gaining more information on the bonding in metal alkene complexes. Structural data for the most relevant nickel(O) phosphine alkene complexes are reported in Table 9. 

Studies employing Ru(II) complexes, such as [(C6H6)Ru(H20)3]tos2 (tos =p-tolu-enesulfonate), revealed similar effects on recycling, although they were initially more active than their Ru(III) counterparts. For example, in aqueous polymerizations of 1 catalyzed by Ru(H20)6tos2, induction periods were initially as short as 50 s. An important step in the identification of the active species in this polymerization was made when a ruthenium-alkene complex (Structure 2) was observed after polymerization of 1 initiated by Ru(H20)6tos2 [25-27]. 

Over the last decade, the chemistry of the carbon-carbon triple bond has experienced a vigorous resurgence [1]. Whereas construction of alkyne-con-taining systems had previously been a laborious process, the advent of new synthetic methodology based on organotransition metal complexes has revolutionized the field [2]. Specifically, palladium-catalyzed cross-coupling reactions between alkyne sp-carbon atoms and sp -carbon atoms of arenes and alkenes have allowed for rapid assembly of relatively complex structures [3]. In particular, the preparation of alkyne-rich macrocycles, the subject of this report, has benefited enormously from these recent advances. For the purpose of this review, we Emit the discussion to cychc systems which contain benzene and acetylene moieties only, henceforth referred to as phenylacetylene and phenyldiacetylene macrocycles (PAMs and PDMs, respectively). Not only have a wide... 

Figure 28 X-ray structures of the copper-alkene complexes 67 and 68, and geometry of the molecules of 71. 67 and 68 reproduced with permission from the Royal society of chemistry.

Figure 29 X-ray structures of the copper-alkene complexes 69, 70, and 72. 69 and 70 reproduced with permission from ACS publications. 72 reproduced with permission from Elsevier.

As shown in Table 1, a remarkable variety of alkene complexes bearing metal centers in a low oxidation state have been isolated and their structures have been determined by X-ray analysis. All the C-C bond distances in olefins coordinated to early transition metals at low oxidation states are more or less elongated compared to free ethylene. These structural data, together with those from NMR studies [14], indicate a major contribution of the metallacyclo-propane structure (2), a fact which is also supported by calculation studies [15]. In the case of ethylene bridging two metal centers such as [ Cp2ZrX 2(iu-f/-C2H4)] (3), the M-C bond could be characterized as a er-bond and there is a little contribution from the / -ethylene canonical structure [16-18]. 

A large number of (mostly zero-valent) nickel-alkene complexes has been reported. Although these complexes have not been recently reviewed, their general properties and structures were expertly described in 1982 [21]. A complete overview of the reported nickel-alkene and nickel-alkyl complexes is beyond the scope of this section, in which a selection of nickel-alkene and nickel-alkyl complexes is described, mostly related to possible intermediates in hydrogenation catalysis. 

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

In this situation, it is evident that it is not crucial to determine whether the reaction proceeds via an early or a late transition state, since the outcome would be the same in both cases. This property is a result of the close similarity between the initial and final structures indeed, the allyl moiety undergoes a rotation of only 30° from its idealised initial geometry to form the -coordinated alkene complex. 

The mechanism is illustrated in Figure 13.9. Nucleophilic attack leads to rotation and formation of the 7t-alkene complex. The left-hand structure will undergo a counter clockwise rotation and experience large steric hindrance. The clockwise rotation on the right leads to a much more favourable situation. 

Having generated suitable (partially) cationic, Lewis acidic metal centers, several factors need to be considered to understand the progress of the alkene polymerisation reaction the coordination of the monomer, and the role (if any) of the counteranion on catalyst activity and, possibly, on the stereoselectivity of monomer enchainment. Since in d° metal systems there is no back-bonding, the formation of alkene complexes relies entirely on the rather weak donor properties of these ligands. In catalytic systems complexes of the type [L2M(R) (alkene)] cannot be detected and constitute structures more closely related to the transition state rather than intermediates or resting states. Information about metal-alkene interactions, bond distances and energetics comes from model studies and a combination of spectroscopic and kinetic techniques. 

When secondary Grignard reagents are used, the coupling product sometimes is derived from the corresponding primary alkyl group.169 This transformation can occur by reversible formation of a nickel-alkene complex from the cr-bonded alkyl group. Reformation of the cr-bonded structure will be preferred at the less hindered primary position. 

From the encouraging results obtained in the reactions of a series of gold(III) oxo complexes with olefins [56], Cinellu et al. tried to achieve the supposed oxametalla-cyclic intermediate, which had never been isolated before [25]. In the reaction of 8 and norbornene 56, if the - atoms were considered to be equivalents of coordinated water, and it was therefore possible to talk about the gold-catalyzed addition of water to an alkene. The metallaoxetane 58 was separated from the gold-alkene complex 57 and characterized by X-ray crystal structure analysis. The subsequent stoichiometric reaction yielded epoxide 59 (Scheme 8.5). 

As far as the fullerene internal structure is concerned, there is little change on metal complexation. The metal bound transannular [6,6] bond is elongated relative to the remaining fullerene C=C bonds. It often attains a length (=1.5 A) comparable with that of other C—C bonds such as the transannular [6,5] bond or those for an analogous alkene complex. The structure often is described as metallacyclopro-... 

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