Redox-induced isomerizations

Most redox-induced isomerizations are initiated by oxidation. Reductions that lead to isomerization are much less common. Two examples are Eqs. (38) and (39), both of which are accomplished by initial reduction with sodium amalgam followed by stoichiometric oxidation.165,165 The neu-... 

Scheme 6.12 Redox-induced isomerization of a ruthenium vinylidene and alkyne complex.

Scheme 13 Generalized square scheme for redox-induced isomerizations.

Scheme 14 Square scheme for redox-induced isomerization of ( -C5Ph5)Rh(77 -C8H8).

As far as the fac/mer isomerization is concerned, the typical redox-induced conversion is that illustrated in Scheme 3 for complex [MnI(CO)3(dppm)Cl] (dppm = Ph2PCH2PPh2).10... 

In order to illustrate some redox-induced molecular rearrangements a little more complicated than simple isomerizations we will consider... 

There are many examples of redox-induced structural rearrangements in organometallic complexes the reader is referred to a review by Connelly. The simplest rearrangement is cis/trans or fac/mer isomerization, usually induced by oxidation. Thus, cw-[Mn(CO)2(dppe)2] slowly converts to the trans isomer with a rate constant of 10 s at room temperature. Upon oxidation, the cis — trans isomerization increases in rate by 7 powers of 10. The process is not catalytic, so that stoichiometric oxidation followed by reduction is required to synthetically utilize the increased reactivity of the radicals in the conversion of 18-electron cis-[Mn(CO)2(dppe)2l to frani-[Mn(CO)2(dppe)2]. An example of oxidatively induced fac mer isomerization is given in Scheme 10. The fac mer reaction for the neutral 18-electron isomer is slow, with 2 = 2 X 10 s , K2 = 4. The reaction fac mer is much faster and... 

Kochi s book from 1978 [la] helped to establish electron-transfer and radical reactions as a crucial part of mainstream organometallic chemistry. The importance of such reactions is evident from Astruc s book [lb], still the most comprehensive and authoritative book in the area, and from several reviews and review collections [2] on aspects of organometallic electron-transfer reactivity. This chapter will be fully devoted to the use of electrochemical techniques to obtain bond-energy data for organometallic complexes, a topic that has not been previously reviewed. Aspects of the energetics of redox-induced structural changes and isomerizations, a thoroughly pursued topic, has been reviewed [2o] and will not be included here. 

CN(Me)CH=CHNMe] involves a complex series of isomerizations (52). The interconversion of cis- and lrans-[Mo(CO)4(carbene)2] is more straightforward but differs from that of [Mo(CO)2(dppe)2] (Section II,C,6). The /rans-tetracarbonyl is oxidized at 0.11 V to the cis cation which is reduced at this potential to ci s-[Mo(CO)4(carbene)2] (E° = 0.34 V). Thus, redox-induced trans-cis isomerization can occur with no net current flow (53). 

Ti -vinyl [Ti2-C(CH2Ph)CH2] or hydride complexes is often followed by chemical steps, and examples of redox induced proton or hydrogen elimination (involving heterolytic or homolytic N-H, C-H or metal-H bond cleavage), nucleophilic attack or isomerization reactions have been presented. 

The bis-benzo-15-crown-5 ferrocene compound [7] containing two vinylic linkages was formed in a mixture of three isomeric components, the cis-cis, cis-trans and trans-trans isomers, which proved inseparable. However, the precedent of insignificant differences found between the magnitudes of the metal cation-induced anodic shifts in the ferrocenyl redox potentials of the respective separated cis and trans isomers [2a] and [2b] led us to use the same isomeric mixture of [7] throughout the subsequent FABMS and electrochemical group 1 and 2 metal cation complexation experiments,... 

Experiments were carried out to determine if during the ac electrolysis the ligand isomerization requires the formation of the reduced and oxidized form (59). This would indicate an excited state mechanism. If the intermediate formation of the reduced or oxidized complex is sufficient to induce the isomerization, excited states are not required. First support in favor of a true epc was obtained by the results of the ac electrolysis of Re(trans-SP)2(CO)jCl in the presence of redox buffers. Tetramethyl-p-phenylenediamine (TMPD) was used as reductant and the paraquat cation (PQ +) served as oxidant. 

Excitation to inner-sphere charge transfer (ISCT) transitions induces radial charge shift within the coordination entity, which may result in redox reactions including the central atom and ligands. The charge redistribution increases the complex susceptibility towards protonation, isomerization, redox processes, or nucleophilic or electrophilic attack. 

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