Oxidative activation microscopic reversibility

The question of which pathway is preferred was very recently addressed for several diimine-chelated platinum complexes (93). It was convincingly shown for dimethyl complexes chelated by a variety of diimines that the metal is the kinetic site of protonation. In the system under investigation, acetonitrile was used as the trapping ligand L (see Fig. 1) which reacted with the methane complex B to form the elimination product C and also reacted with the five-coordinate alkyl hydride species D to form the stable six-coordinate complex E (93). An increase in the concentration of acetonitrile led to increased yields of the methyl (hydrido)platinum(IV) complex E relative to the platinum(II) product C. It was concluded that the equilibration between the species D and B and the irreversible and associative1 reactions of these species with acetonitrile occur at comparable rates such that the kinetic product of the protonation is more efficiently trapped at higher acetonitrile concentrations. Thus, in these systems protonation occurs preferentially at platinum and, by the principle of microscopic reversibility, this indicates that C-H activation with these systems occurs preferentially via oxidative addition (93). 

The authors point out that the dependence of the site of electrophilic attack on the ligand trans to the hydride in the model systems may be important with respect to alkane activation. If the information is transferable to Pt-alkyls, protonation at the metal rather than the alkyl should be favored with weak (and hard ) a-donor ligands like Cl- and H20. These are the ligands involved in Shilov chemistry and so by the principle of microscopic reversibility, C-H oxidative addition may be favored over electrophilic activation for these related complexes. 

An understanding of N—H activation via oxidative addition to Ir(I) fragments - and of its microscopic reverse, reductive elimination - is of fundamental importance... 

In closely related experiments it was shown that sp C—H activation takes place reversibly within the coordinahon sphere of the electron-rich Ir(I)-diphosphine complex 58 (Scheme 6.9) to form an alkyl-amino-hydrido derivative 57 reminiscent of the CCM intermediate 24 the solid-state structure of 57 is shown in Figure 6.13 [40]. It appears that C—H activation only takes place after coordination of the amine function to the Ir(I) center (complex 58, NMR characterized). Amine coordination allows to break the chloro bridge of 59 and to augment the electron density of the metal center, thus favoring oxidative addihon of the C—H bond. Most importantly, the microscopic reverse of this C—H activation process (i.e. C—H reductive elimination) models the final step of the CCM cycle (see Scheme 6.1) indeed, the reaction of Scheme 6.10 is cleanly reversible at 373 K. 

The product elimination step proceeds with cleavage of the catalyst-substrate bonds. This may occur by dissociation, solvolysis, or a coupling of substrate moieties to form the product. The last of these involves covalent bond formation within the product, and corresponds to the microscopic reverse of oxidative addition. Upon reductive elimination both the coordination number and formal oxidation state of the metal complex decrease. In most homogeneous catalytic processes, the product elimination step, while essential, is usually not rate determining. The larger kinetic barriers are more frequently encountered in substrate activation and/or transformation. 

Levels of volatility that would lead to unacceptable rates of vapor transport-driven sintering, attrition of catalytically-active materials, or corrosion of catalytic materials or support oxides by transport from contaminants or substrate materials can be estimated given equilibrium vapor pressures and a few assumptions about evaporation rates and mass transport. In particular, the rate of condensation of a vapor species on its source solid phase at high temperatures is almost certainly non-activated and may show little configurational restriction. Therefore, using the principle of microscopic reversibility, we can take the rate constant for condensation to be approximately equal to the collision frequency. 

Chart 11.4 Nomenclature of C—H activation via metal insertion into C—H bond and the microscopic reverse reaction. (Note The term oxidative addition is used for the single step of metal insertion into a coordinated C—H bond as well as the overall two-step process of C—H coordination followed by metal insertion into the C—H bond.)... 

Oxidative addition and the microscopic reverse, reductive elimination, involving formal Pt(0)/Pt(II) as well as Pt(II)/Pt(IV) redox couples, have been of long-standing interest.44-47 Using bisphosphine platinum systems, ab initio calculations provided insight into the thermodynamics and activation barriers for oxidative addition reactions as a function of the substrate being activated (Scheme 11.20). The calculated... 

Even though C—H oxidative addition from Pt(II) to give Pt(IV) is viable, additional distinctions can be made for these transformations. For example, beginning from four-coordinate Pt(II) complexes of the type [L3Pt(R)J+, C—H activation could proceed directly via C—H coordination and oxidative addition or by initial loss of ligand L (Scheme 11.24). To probe these reactions, several groups have studied the reductive elimination of C—H bonds from octahedral Pt(IV) complexes, which by the principle of microscopic reversibility can confer information about the oxidative addition reactions.52-55... 

Five-coordinate Pt(TV) species with silyl ligands are poised to perform Si-C or Si-H reductive elimination from Pt(IV). Note that the microscopic reverse of the latter reaction, Si-H oxidative addition, was used to synthesize the first five-coordinate Pt(TV) complexes with silyl ligands (2a-c) [84]. Complex 6, which has Pt-Me groups and a Pt-SiMe3 group, was observed to react over time at room temperamre to form tetramethylsilane, the product of Si-C reductive elimination, and intractable Pt products [91]. The five-coordinate complex ( pypyr)Pt(H>2SiEt3, 7a, was found to react with HSiMeaEt to form product 7b. Study of this reaction showed that Si-H reductive eliminaticm from 7a was rate-determining and it occurred directly from the five-coordinate complex [97]. Reaction of 7a with phosphines at room temperature led to the formation of a Pt(II)H(PR3) complex and free silane, the product of Si-H reductive elimination. Complex 7a was observed to be an active catalyst for the hydrosilylation of ethylene, tert-butylethylene, and alkynes [97]. 

Reductive elimination which produces C—C bonds is of considerable importance, representing the microscopic reverse for little known C—C activation reactions. [CpaMRj] complexes (M = Ti, Zr R = alkyl) have recently been shown to eliminate R2 upon one-electron oxidation. 

In the very early stages of oxidation the oxide layer is discontinuous both kinetic and electron microscope" studies have shown that oxidation commences by the lateral extension of discrete oxide nuclei. It is only once these interlace that the direction of mass transport becomes of importance. In the majority of cases the metal then diffuses across the oxide layer in the form of cations and electrons (cationic diffusion), or as with the heavy metal oxides, oxygen may diffuse as ions with a flow of electrons in the reverse direction (anionic diffusion). The number of metals oxidising by both cationic and anionic diffusion is believed to be small, since a favourable energy of activation for one ion generally means an unfavourable value for the other... 

The microscopic mechanisms responsible for the electrochemical behavior of metallic and oxide electrodes were exhaustively analyzed in the literature [17-22, 24, 60, 62-69] and are outside of the main scope of this chapter. One should only mention that the solid-electrolyte additions into the electrode composition make it possible to increase reversibility and to enlarge the domain of temperatures and chemical potentials where the electrode can be safely used, a result of the TPB expansion and microstructural stabilization. Although the mixed-conducting and catalytically active additives such as doped ceria might also be useful from the cell impedance point of view, their use for the reference electrodes is limited if oxygen nonstoichiometry changes may occur under the cell operation conditions. 

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