Outer-sphere mechanisms catalysts

Most of the kinetic models predict that the sulfite ion radical is easily oxidized by 02 and/or the oxidized form of the catalyst, but this species was rarely considered as a potential oxidant. In a recent pulse radiolysis study, the oxidation of Ni(II and I) and Cu(II and I) macrocyclic complexes by SO was studied under anaerobic conditions (117). In the reactions with Ni(I) and Cu(I) complexes intermediates could not be detected, and the electron transfer was interpreted in terms of a simple outer-sphere mechanism. In contrast, time resolved spectra confirmed the formation of intermediates with a ligand-radical nature in the reactions of the M(II) ions. The formation of a product with a sulfonated macrocycle and another with an additional double bond in the macrocycle were isolated in the reaction with [NiCR]2+. These results may require the refinement of the kinetic model proposed by Lepentsiotis for the [NiCR]2+ SO/ 02 system (116). 

Thus, in hydrogen-transfer reactions, most of the catalysts do prefer the outer-sphere mechanism instead of the MPV or the insertion mechanisms. For instance, the high stability of the intermediate formed, alkoxide in the case of carbonyl hydrogenation, is a major drawback for the inner-sphere mechanism. Nevertheless, in some particular cases, the inner-sphere mechanism may be competitive with the outer-sphere one. In these cases, some requirements must be accomplished, such as the high lability of one of the metal ligands in order to allow easily the substrate coordination or the formation of not very stable intermediates. 

Fig. 4. Energy profiles in THF for both concerted pathways at B3LYP level for the hydrogenation of ketones by the Shvo s catalyst. Inner-sphere mechanism dashed fine outer-sphere mechanism solid line.

Scheme 20. Concerted outer-sphere mechanism for carbonyl hydrogenation by the Shvo s catalyst.

Fig. 5. Transition-states of the concerted outer-sphere mechanism for the hydrogenation of ketones in both the model (left) and complete (right) Shvo s catalysts.

In the present chapter, a classification of the hydrogenation reaction mechanisms according to the necessity (or not) of the coordination of the substrate to the catalyst is presented. These mechanisms are mainly classified between inner-sphere and outer-sphere mechanisms. In turns, the inner-sphere mechanisms can be divided in insertion and Meerweein-Ponndorf-Verley (MPV) mechanisms. Most of the hydrogenation reactions are classified within the insertion mechanism. The outer-sphere mechanisms are divided in bifunctional and ionic mechanisms. Their common characteristic is that the hydrogenation takes place by the addition of H+ and H- counterparts. The main difference is that for the former the transfer takes place simultaneously, whereas for the latter the hydrogen transfer is stepwise. 

There has been some exploration of the mechanism of reduction of d transition metal complexes by M2+(aq) (M = Eu, Yb, Sm). Both inner- and outer-sphere mechanisms are believed to operate. Thus the ready reduction of [Co(en)3]3+ by Eu2+(aq) is necessarily outer-sphere. 2 However, the strong rate dependence on the nature of X when [Co(NH3)5X]2+ or [Cr(H20)5X]2+ (X = F, Cl, Br or I) are reduced by Eu2+(aq) possibly suggests an inner-sphere mechanism.653 The more vigorous reducing agent Yb2+ reacts with [Co(NH3)6]3+ and [Co(en)3]3+ by an outer-sphere route but with [Cr(H20)5X]2+ (X = halide) by the inner-sphere mechanism.654 Outer-sphere redox reactions are catalyzed by electron-transfer catalysts such as derivatives of isonicotinic acid, one of the most efficient of which is iV-phenyl-methylisonicotinate, as the free radical intermediate does not suffer attenuation through disproportionation. Using this catalyst, the outer-sphere reaction between Eu2+(aq) and [Co(py)(NH3)5]3+ proceeds as in reactions (18) and (19). Values found were ki = 5.8 x KFM-1 s 1 and k kx = 16.655... 

In electron transfer between metal ions, a metal-ion catalyst normally reacts by nonassociative activation, in which the species do not form long-lived binuclear intermediates. The catalytic process often can be rationalized by reactivity patterns e.g., the Cu catalyses of the oxidation "of V(III) by Fe(III). This catalysis by Cu occurs by outer-sphere mechanisms as in ... 

The activity of complex [lT2(CH3CN)(H)3(p-H)(P Pr3)2(p-Pz)2] as a catalyst for the hydrogenation of diphenylacetylene and ethylene contrasts with its inactivity when employed in the hydrogenation of A -benzylideneaniline. However, when transformed into its protonated derivative, for example, [lr2(CH3CN)(H)2(H2) ( 4-H)(P Pr3)2(p-Pz)2]BF4 by reaction with HBF4, the new complex becomes a very active catalyst for C=N hydrogenation [111]. The catalytic cycle involves fast elementary steps of hydride and proton transfer according to an ionic outer sphere mechanism that takes place at one of the iridium centers of the binuclear complex (Scheme 27). 

In the ionic mechanism, the proton and the hydride are sequentially transferred to a substrate, whereas in the metal-ligand bifunctional mechanism, the transfer occurs simultaneously. In both cases, the source of the H is a transition metal hydride, but the source of the proton can be a metal hydride or N—H or 0—H bonds. It seems likely that some previously reported catalysts could follow nonclassical outer-sphere mechanisms... 

The term electrocatalysis is, however, more commonly applied to systems where the oxidation or reduction requires bond formation, or at least a strong interaction of the reactant, intermediates, or the product with the electrode surface. The catalyst is the electrode material itself or a species adsorbed from solution. This chapter will discuss this more limited definition of electrocatalysis (note also that simple electron transfer reactions which are pictured as occurring by an outer sphere mechanism and may have very high exchange current densities, are not normally considered within electrocatalysis — in this book they are dealt with in Chapter 3). 

The electron-transfer step may follow an inner-sphere or outer-sphere mechanism, as discussed in the next chapter. The main interest here has been to find more economical oxidants than Pt(IV). The trick is to find a species which will oxidize Pt(II))— R but not the Pt(II) catalyst itself. 

Mechanistically, it is widely accepted that these procedures involve the coordination of the electrophilic agent by the metal species with the subsequent nucleophilic attack by the less hindered enantiotopic face (outer sphere mechanism). In some cases, a two-site binding mode of the catalyst has been also postulated with both partners of the process (inner sphere mechanism), simultaneously interacting with the catalytic unit. The latter arrangement will concur to define a positive proximity effect during the C—C bond-forming step. 

Bifunctional rhenium complexes related to the Shvo catalyst have been used in TH reactions, including tests on three non-prochiral imines, with TOFs up to 79 h obtained for imines. In common with the Shvo catalysts, DFT calculations have indicated the operation of an outer-sphere mechanism for the reaction [118]. 

There has been considerable recent interest in the reductions of [Fe(CN)6]. The electron exchange with A -propyl-l,4-dihydronicotinamide is catalyzed by alkali metal ions. The increase in reaction rate is attributed to the polarizability of M and the observed linear free energy relationship is discussed. An outer-sphere mechanism is postulated in the oxidation of phenothiazines. A free radical mechanism involving the alcohol anion is invoked in the reaction of 1-and 2-propanol in aqueous alkaline media, the kinetic order being unity for [Fe(CN)6], OH, and alcohol concentrations. Catalysis by metal ions has also been observed in the presence of copper(II) and ruthenium(III) complexes. In the oxidation of a-hydroxypropionic acid in alkaline media,a Cu(II)-ligand complex is formed which is oxidized slowly to a copper(III) species. Alkaline ferricyanide oxidizes butanol, the process being catalyzed by chlororuthenium complexes.The rate law is consistent with oxidation of the alcohol by the Ru(III) followed by reoxidation of the catalyst by [Fe(CN)6]. The rate law is of the form ... 

Asymmetric hydrogenation (Section 9.3) of C=C, C=0, and C=N bonds is widely employed with numerous catalysts. The example shown in Eq. 14.20 uses a Noyori catalyst that is believed to operate by an outer-sphere mechanism of Section 9.3 with transfer of H from the metal to carbonyl carbon and H+ from the amino hgand to the carbonyl oxygen, the carbonyl substrate not being directly coordinated to the metal. 

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