Half-reactions model complexes

With regard to eqn. (2), which represents the metal deposition half reaction in electroless deposition, in a simplistic sense we see that it is analogous to an electrodeposition process. With respect to the reducing agent reaction, organic [20, 21] and relatively complex inorganic oxidation reactions [22] have similarly been widely studied electrochemically. It is therefore reasonable to think that electroless deposition could be described, or modeled, using an electrochemical approach. 

The above considered reactions model the reductive half cycle of GO where a primary alcohol is oxidized to an aldehyde with concomitant reduction of a (phe-noxyl)copper(II) complex to the reduced (phenol)copper(I) species. In the first two cases, reoxidation of the reduced catalyst was achieved by an external oxidant such as tris(4-bromophenyl)aminium or an electrode but not dioxygen. 

An advantage of the mediator model (Equation 9) is that it can be used to simplify the problem of describing contaminant reduction reactions if the mediator is characterized more easily than the bulk donor. In this case, the bulk donor is best neglected and the problem reduced to the mediator and contaminant half-reactions. The advantage is greatest when a complex microbiological transformation process can be reduced to a reaction with a well defined biogenic mediators, such as quinones (98, 99), porphyrins, or corronoids (100-102). 

With an arbitrary definition of KNaX as equal to unity, thus establishing a reference half reaction, the equilibrium constant for any other half reaction can be determined from measured selectivity coefficients. The Gapon equation can be readily implemented in this manner. Implementation of the Vanselow equation, however, requires modification of the general equilibrium models to account for the more complex dependence of mole fractions on the molar concentrations. An example ion-exchange calculation using the half reaction approach to represent the Gapon equation is presented in Appendix 2. 

The mechanism for the hydroxylation of aromatic substrates by flavoprotein monooxygenases has been the subject of signiflcant research interest and controversy over the past decade. These enzymes (p-hydroxybenzoate hydroxylase, phenol hydroxylase, and melilotate hydroxylase) catalyze the initial step in the )8-ketoadipic acid pathway, the hydroxylation of substituted phenols into catechols (Scheme 55). Oxygen is required as cosubstrate, which is activated by the reduced FAD cofactor. The complex mechanism for the oxidative half-reaction is thought to consist of at least four steps and three intermediates 239-242) and to involve a controversial 4a,5-ring-opened flavin 242, 249, 250) (Scheme 56). The flavin C4a-hydroperoxy intermediate 64 and flavin C4a-hydroxy intermediate 65 have been assigned the structures shown in Scheme 56 based on the UV absorbance spectra of various model compounds compared with that of the modified enzyme cofactor alkylated at N(5) 243). However, evidence for the intermediacy of various ring-opened flavin species has been tentative at best, as model compounds and model reactions do not support such an intermediate 242). 

We explored the hydroformylation reaction for monometal model complexes that represent one half of the bimetal catalysts 1 or 5. These tests give us an idea about whether each metal centre is functioning as a conventional mononuclear catalyst or whether there is some cooperativity. Thus the catalytic activity and selectivity of the complexes [RuCl(j/ -Ph2PCH2PPh2)Cp], [RuCl -HC(PPh2)3)Cp] and [ RhCl(CO)2 2] were studied under the same conditions as described above. 

GOase model compounds were lately prepared by Stack and Pratt from two Schiff base derivatives, namely ( )-A A/ -bis(3,5-di- cr -butylsalicylidene)- m s-cyclohexane-l,2-diamine and ( )-AT,A7-bis(3,5-di- cr -butylsalicyl)- m s-cyclohexane-l,2-diamine (H2L ) as shown in Fig. 13 (77). Reaction of Cu(OAc)2 H2O and sodium hydroxide with H2L (Fig- 13a) or H2L (Fig. 13b) in methanol yielded two monocopper complexes, which showed spectroscopic features similar to those of GOase upon oxidation with silver hexafiuoroantimonate. In addition, these two radical compounds could perform the (stoichiometric) oxidation of benzyl alcohol to benzaldehyde, thereby mimicking the oxidizing half-reaction of GOase. 

Molybdenum and tungsten complexes as models for oxygen atom transfer enzymes have been deployed in the full catalytic cycle from Scheme 4.3 predominantly in the early days of this field of research. A selection of the respective determined Michaelis-Menten parameters were expertly reviewed by Holm et al. Since in some cases both forms of model complexes (M and M mimicking the fully reduced or fully oxidized active sites, respectively) are isolable and available in a sufficient amount, the isolated half-reactions are much more often investigated than the whole catalytic cycle. This means that either the reduced form of the enzyme model is oxidized by an oxygen donor substrate like TMAO or the oxidized form is reduced by an oxygen acceptor substrate like triphenylphosphine (PhgP). The observed kinetic behaviour is in some cases described to be of a saturation type. An observation which... 

An additional aspect in studying the half-reactions arises when a reaction is not treated as pseudo first order reaction. This is the case when the substrate is in approximately the same concentration as the metal complex. Caradonna et al. for instance studied the required treatment for such a case. Due to the accumulation of the product in the course of the reaction, the back reaction is not negligible anymore. To the best of our knowledge, analyses of the back reaction have never been mentioned in the literature with respect to oxygen atom transfer reactions of model complexes and this is due to several reasons. Either the reaction rate is too small compared to the forward reaction so that experimental difficulties in determining the reaction rate constant emerged. Or, as was emphasized by Enemark et No bis(dithi-olene) molybdenum or tungsten eomplex was observed to reduce PhjPO . ... 

Studies in this laboratory (69) of the water soluble ferri-heme model Fem(TPPS) in aqueous solution have shown that this species also undergoes reductive nitrosylation in solutions that are moderately acidic (pH 4-6) (Eq. (32)). The rate of this reaction includes a buffer dependent term indicating that the reaction of the Fem(TPPS)(NO) complex with H20 is subject to general base catalysis. The reaction depicted in Eq. (33) is not observable at pH values < 3, since the half-cell reduction potential for the nitrite anion (Eq. (1)) is pH dependent, and Eq. (33) is no longer thermodynamically favorable. 

The half-order of the rate with respect to [02] and the two-term rate law were taken as evidence for a chain mechanism which involves one-electron transfer steps and proceeds via two different reaction paths. The formation of the dimer f(RS)2Cu(p-O2)Cu(RS)2] complex in the initiation phase is the core of the model, as asymmetric dissociation of this species produces two chain carriers. Earlier literature results were contested by rejecting the feasibility of a free-radical mechanism which would imply a redox shuttle between Cu(II) and Cu(I). It was assumed that the substrate remains bonded to the metal center throughout the whole process and the free thiyl radical, RS, does not form during the reaction. It was argued that if free RS radicals formed they would certainly be involved in an almost diffusion-controlled reaction with dioxygen, and the intermediate peroxo species would open alternative reaction paths to generate products other than cystine. This would clearly contradict the noted high selectivity of the autoxidation reaction. 

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