Square-redox scheme

Square-redox scheme for the CO-induced reductive elimination of hydride from an iron-hydride complex... 

A complete analysis of the square scheme is complex since disproportionation and/or other second-order cross-redox reactions have to be taken into consideration. However, the limiting cases of the square scheme are much more tractable. An interesting aspect of the square reaction scheme is that, in principle, it applies to all one-electron processes with reaction steps A+ B+ and A B coupled to the heterogeneous charge transfer. For example, the redox-induced hapticity change, which accompanies the reduction of Ru( j - CeMee), has been proposed [113] to be responsible for the apparently slow rate of electron transfer. That is, the limiting case of an apparent overall Einev process is observed for what in reality is a square scheme mechanism. 

Figure 7. The square equilibrium scheme describing the solution behaviour of a ligand L, displaying coordinating tendencies towards a metal M and having an appended redox subunit.

One of the earliest series of metal complexes which showed strong, redox-dependent near-IR absorptions is the well-known set of square-planar bis-dithiolene complexes of Ni, Pd, and Pt (Scheme 4). Extensive delocalization between metal and ligand orbitals in these non-innocent systems means that assignment of oxidation states is problematic, but does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic, and dianionic species. 

Scheme 2 Different redox states of square-planar nickel bis(edt) complexes...

Studies in the area of electrochemical molecular recognition deal with bifunctional receptor molecules that contain not only binding sites but also one or more redox-active centres whose electron transfer reaction is coupled to the receptor s complexation. Such systems can be described by the scheme of squares as shown in Scheme 1. 

For more complex mechanisms, picturesque names such as square, ladder, fence [18] or cubic schemes [20] have been selected. In redox polymer films, additional transport of counterions, solvation, and polymer reconfiguration are important and four-dimensional hyper-cubes are needed to describe the reactions [21]. 

If for an oxidation step, the chemical reaction of B leads to the oxidized form of the second redox couple B (and not the reduced one as in the earlier case) and a second chemical transformation from A leads back to A [reaction (14)], we arrive at a square scheme (Figure 11), which forms the basis for many important redox systems [18, 58]. Again SET steps... 

When both components of the redox couple adsorb on the electrode surface, the mechanism (2.172)-(2.174) transforms into the following square scheme [128] ... 

Scheme 3 Terminal i (CO) frequencies (squares) versus redox potential for [Pt24(CO)3o]" the circles show the corresponding response of i>(CO) on poly crystalline platinum (from Ref 37 with permission).

Scheme 1 Scheme of squares for cis-trans redox isomerism. 

Figure 2.1 A square scheme illustrating a redox-driven intramolecular motion. Species with an asterisk (Ox and Red ) are metastable and tend to rearrange to their stable topological isomer (Ox and Red).

The pioneering papers by Stoddart and Sauvage have stimulated the design of a variety of movable rotaxanes and catenanes, whose controlled motion is promoted by a redox change. In all cases, the process of the redox-driven intramolecular motion can be described by a square scheme, as illustrated in Fig. 2.1. 

There exist other types of redox-driven intramolecular motions that can be interpreted on the basis of the square scheme of Fig. 2.1 and are promoted by... 

Figure 2.6 A square scheme illustrating the pendular motion of an iron center, driven hy the Fe Fe1" redox couple. As judged from voltammetric experiment carried out at varying potential scan rate, the lifetime for both translocation processes is <10 ms.

Here, the / conformers each of the oxidized and the reduced forms are related by the 2 (/ - 1) equilibrium constants Kt and K, respectively, and by the i redox potentials E. A quantitative analysis of the redox potential in the square scheme of Eq. 10.3 re-... 

The following square scheme (Figure 1) gives a simplified view of the redox-triggered folding-defolding process 181... 

Reaction of Cytochrome cIinn with Bis(ferrozine)copper(II) Knowledge of the redox properties of cytochrome c was an encouragement to initiate a kinetics investigation of the reduction of an unusual copper(II) complex species by cyt c11. Ferrozine (5,6-bis(4-sulphonatophenyl)-3-(2-pyridyl)-1,2.4-triazine)286 (see Scheme 7.1), a ligand that had come to prominence as a sensitive spectrophotometric probe for the presence of aqua-Fe(II),19c,287 forms a bis complex with Cu(II) that is square pyramidal, with a water molecule in a fifth axial position, whereas the bis-ferrozine complex of Cu(I) is tetrahedral.286 These geometries are based primarily upon analysis of the UV/visible spectrum. Both complexes are anionic, as for the tris-oxalato complex of cobalt in reaction with cytochrome c (Section 7.3.3.4), the expectation is that the two partners will bind sufficiently strongly in the precursor complex to allow separation of the precursor formation constant from the electron transfer rate constant, from the empirical kinetic data. 

The dichotomy of CV behavior between strongly and weakly interacting anions had been rationalized in the seminal article by Echegoyen s and Kaifer s groups with the square Scheme 1 [61]. When the strength of the interaction between the anion and the reduced redox form (here ferrocenyl) is significant, a new wave appears, and the variation of ferrocenyl potential between the free and bound forms of Scheme 1 is related to the ratio of apparent association constants F e -F ound = A °(V) = 0.059 log (K+/K0) at 25 °C. ound corresponds to the addition of one equiv. anion per ferrocenyl branch or the stoichiometric amount determined from the break points, for instance in Fig. 1. 

Scheme 1 Square scheme in the cases where (i) the host-guest interaction is strong even in the reduced redox form of the host (top) and (ii) this interaction is negligible (bottom) [101]...

Here, the i conformers each of the oxidized and the reduced forms are related by the 2 (i-1) equilibrium constants Kt and K, respectively, and by the i redox potentials Ef. A quantitative analysis of the redox potential in the square scheme of Eq. 11.4 requires a knowledge of all equilibrium constants. For labile systems this is only possible when theoretical methods can be applied. Molecular mechanics has been used in this context to calculate the conformational equilibria and then to predict the electrochemical behavior of [Co(sep)]3+/2+11511, [Co(dien)2]3+/2+11511 and [Co (S)-pn 3]3+/2+13451 (sep is defined in Table 11.1, dien in Table 8.1, pn in Table 8.2). 

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