Alkane complexes dissociative substitutions

Dissociative substitution reactions, even in alkane solvents, occur through intermediates that typically contain coordinated solvent. For this reason and because of the intermediacy of alkane complexes in C-H activation, a number of studies have been conducted on the mechanism of the displacement of alkanes and even noble gases from the coordination sphere of a metal. A few of these studies are summarized in Scheme 5.5. 

These Rh complexes have been the subject of intense interest due to their propensity for C-H activation of alkanes (Section 3.3.2.7). The noble gas complexes [CpRh(CO)L] and [Cp Rh(CO)L] (L = Kr, Xe) have also been studied in supercritical fluid solution at room temperature [120]. For both Kr and Xe, the Cp complex is ca. 20-30 times more reactive towards CO than the Cp analogue. Kinetic data and activation parameters indicated an associative mechanism for substitution of Xe by CO, in contrast to Group 7 complexes, [CpM(CO)2Xe] for which evidence supports a dissociative mechanism. 

The kinetics of formation in liquid Kr are inconsistent with a dissociative mechanism but are rationalized by an equilibrium between Kr and cyclohexane complexes prior to OA. vco for Cp Rh(CO)(CyH) is unobservable, probably because of overlap with that of Cp Rh(CO)(Kr).135 Use of cyclohexane- 2 to examine isotopic substitution effects shows an inverse (Kh/Kd = ca 0.1) for the interconversion of Kr and alkane adducts, i.e C6D12 binds more strongly to M than C6H12.116 Although initially contrary to expectations based on simple zero-point energy arguments, this is explainable by the increase in the number of vibrational modes for bound alkane versus free alkane (see Section 7.5.2). As observed for the formation of H2 complexes, the inverse effect is perfectly consistent with formation of an alkane [Pg.400]

Alkyl anions have been implicated as intermediates stabilized by a neutral molecule. Alkoxide ions when photolysed in a pulsed ICR spectrometer dissociate into alkanes and enolate anions The intermediate 19 in equation 25 can be represented by two possible extremes. In 19a the alkyl anion R is solvated by a ketone and inl9b the radical anion of the ketone is solvated by the radical R. The structure of this intermediate will then depend on the relative electron affinities of the alkyl group R and the ketone. Brauman and collaborators photolysed a series of 2-substituted-2-propoxides (18 with R = CH3, R" = H and R varied). For substituents R = CF3, H, Ph and H2C=CH, the C—R bond dissociation energies for homolytic fission are larger than the C—CH3 bond energy, i.e. if the intermediate complex has the structure 19b then methane would be expected to be produced. Conversely, since these R groups form more stable anions than CH3, decomposition via 19a should result in RH. The experimental observation that only RH is formed led to the conclusion that 19 is best described by the solvated alkyl anion structure 19a. 

To illustrate the propensity of these complexes to react by associative processes, consider the exchange of alkanes and arenes in complexes of the t5rpe [PtL2(R)(R-H)] by associative or dissociative mechanisms. Although it is difficult to conceive of a more weakly bound ligand than the alkane in a a-complex, the rate of associative substitution of acetonitrile for the bound alkanes is faster than simple dissociation of the alkane (Equation 5.16). ... 

Although additives to induce radical chemistry have allowed ligand substitutions of 18-electron complexes to be conducted under mild conditions, photochemical reactions provide a common and practical alternative. Photochemically induced dissociation of carbonyl ligands is most common, but photochemical dissociations of other dative ligands are known. Several examples are shown in Equations 5.36-5.40. These examples illustrate the dissociation of CO from homoleptic carbonyl compounds of iron - and chromium, the dissociation of CO from piano-stool carbonyl compounds, " ttie dissociation of N, and the dissociation of a carbodiimide to generate an intermediate that coordinates and cleaves the C-H bonds of alkanes. In some cases, like the formation of the two THE complexes, the products of the photochemical process are not isolated instead, they are treated in situ with a ligand, such as a phosphine, to form monosubstitution products selectively. 

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