Ligand, additivity redox-active

In addition, Bryce and Chesney have developed chiral oxazolines linked to tetrathiafulvalene in order to use these ligands as redox-active ligands. When applied to the test reaction, these ligands gave only low enantioselec-tivities (<21% ee), as shown in Scheme 1.32. 

Nitrosylmetal Complexes Without Additional Redox-Active Ligands... 

NITROSYLMETAL COMPLEXES WITHOUT ADDITIONAL REDOX-ACTIVE LIGANDS... 

In addition, Bryce et al. have studied the binding of palladium to other S/N-ferrocenyloxazoline ligands by cyclic voltammetry and proved that it was reversible.These redox-active liganding systems were successfully used in the test reaction, providing the product in both high yield and enantioselectivity of up to 93% ee, as shown in Scheme 1.70. 

As briefly alluded to, there are different classes of redox-active ligands in addition to the above mentioned ones. For example, we have seen in Chapter 5, Section 8, that azo-groups (in particular, 2-(phenylazo)pyr-imidine) are able to undergo two separate one-electron reduction processes. Conjugated polynitriles (mnt, tcne, tcnq) also constitute an important class of redox-active molecules and the electrochemical behaviour of their metal complexes has been reviewed.107 The same holds as far as alkyldithiocarbamates (Rdtc) and their metal complexes are concerned,108 or nitrosyl complexes in their possible NO+[NO fNO redox sequence.109 Thus, we would like to conclude the present Chapter by discussing a few less known redox non-innocent ligands. 

More recent approaches to the effects of the ligands on the redox activity of metal complexes are based upon the assumption that the electrode potential of a redox change involving a metal complex is determined by the additivity of the electronic contribution of all the ligands linked to the metal centre, or to the overall balance between the c-donor and the 7r-acceptor capability of each ligand.3 In particular two ligand electrochemical parameters have gained popularity ... 

Key reactions include (a) substitution (b) electron transfer (c) activation of ligands (d) redox-catalyzed substitution (e) oxidative-addition reaction (f) insertion (g) redox-catalyzed insertion. 

As illustrated in Section IV.B.2.e, Rh(acac)3 exhibits an irreversible one-electron oxidation and an irreversible two-electron reduction in MeCN solution. The introduction of an anthrylmethyl group in the y-position of one of the acac ligands as in 130 modifies the redox activity of the entire complex . The irreversible two-electron reduction of Rh(acac)3 (E = —2.21s V, vs. SCE, thf) moves anodically by about 0.3 V (Ep = — 1.88 V) and is followed by a reversible, anthryl-centred reduction ( ° = —2.14 V). In addition, no rhodium-centred oxidation is detected. Since the anodic shift of the Rh(llt) Rh(I) step indicates that the anthrylmethyl group pushes electron density towards the rhodium(IIl) core, it does not seem possible that the lack of the Rh(III) Rh(IV) process might be due to its anodic shift beyond the solvent discharge. 

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