Outer-sphere operating catalysts

Gauging catalysis by reference to an electrode where electrons are delivered (or eaten up) in an outer-sphere manner, redox catalysis is not expected to operate at a monolayer coated electrode (Figure 4.10), since, as discussed in Section 4.2.1, redox catalysis results from the three-dimensional dispersion of the catalyst. In contrast, there is no reason that chemical catalysis could not be operative at a monolayer coated electrode. For the same reasons, both redox catalysis and chemical catalysis are expected to function at multilayer electrode coatings (Figure 4.10). 

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... 

The identification and investigation of both processes by spectroscopic techniques is not straightforward. In the latter case, the investigation requires the detection of weak inner- and outer-sphere substrate-ligand interactions, which are difficult to interrogate by most spectroscopic techniques as the perturbation to the metal centre can be quite small. These interactions can, however, be investigated by EPR techniques. By probing these key stmcture-reactivity relationships, one can build an accurate model for enantiomer discrimination and ultimately provide a fundamental basis for improvement in the operation of enantioselective catalysts. 

A series of molybdenum and tungsten catalysts for the hydrogenation of ketones have been reported by Bullock (101,102). These catalysts operate by an outer-sphere ionic mechanism. The reaction occurs by proton transfer from a cationic metal dihydride (or a metal dihydrogen species), followed by hydride transfer from a neutral metal hydride to a ketone (Fig. 34). 

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]. 

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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