Redox behaviour pattern

In this chapter, for convenience of discussion, the redox behaviour of cyclic systems is divided into two categories. Initially, those reactions for which there is no marked alteration of the unsaturation pattern of the ligand are discussed. Subsequently, systems in which the final product involves an alteration in the ligand s unsaturation are treated. However, it is emphasized that a continuum exists between redox behaviour which is completely metal-centred and that which is solely ligand-based. 

The nitrogen atom in a-ferrocenylalkylamines generally shows the same reaction pattern as that in other amines alkylation and acylation do not provide synthetic problems. Due to the high stability of the a-ferrocenylalkyl carbocations, ammonium salts readily lose amine and are, therefore, important synthetic intermediates. Acylation of primary amines with esters of formic acid gives the formamides, which can be dehydrated to isocyanides by the standard POClj/diisopropylamine technique (Fig. 4-16) [92]. Chiral isocyanides are obtained from chiral amines without any racemization during the reaction sequence. The isocyanides undergo normal a-addition at the isocyanide carbon, but could not be deprotonated at the a-carbon by even strong bases. This deviation from the normal reactivity of isocyanides prompted us to study the electrochemistry of these compounds, but no abnormal redox behaviour, compared with that of other ferrocene derivatives, was detected [93]. The isocyanides form chromium pentacarbonyl complexes on treatment with Cr(CO)s(THF) (Fig. 4-16) and electrochemistry demonstrated that there is no electronic interaction between the two metal centres. 

One outcome of the migration of ions fi om one redox region into another should be a zonation of elements in relation to the reduced column. Element zonation is a reported feature of selective leach anomalies (Clark, 1996). Zonation could occur due to a variety of processes, the most important of which would be progressive deposition of redox-active species as they migrate into or out of the reduced column. The migration paths of reduced and oxidised ions are predictable provided the current flow patterns can be inferred and therefore, if the redox behaviour of a particular ion is known, the shape of anomalies can be inferred. 

The transition metal complexes shown in Fig. 8-8 thus display a pattern which follows consistently the concepts of two-step electrochemical tunneling and the theoretical formalism above. Mapping and working principles of redox switching and transistor -like behaviour close to the single-molecule level of interfacial electrochemical electron transfer have thus been achieved. This can be compared with biological macromolecules addressed below. 

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