Ferrocene units

Networks with labeled branch points have also been synthesized by anionic techniques. The crosslinks contain either ferrocene units 107) or lead, originating respectively from vinylferrocene or tetrakis[4(l-phenylvinyl)phenyl]plumbane 94). 

Fig. 2. Electronic structure of biferrocene (2) (a) energy diagram (b) illustration of large electronic interaction between dx2 >2 orbitals (e2 ) of ferrocene units through pz orbitals of Cp rings (54).

The first systematic study of the effects of IT bands on the number of nuclei and the oxidation number was carried out for oligo(dihexylferro-cenylene)s, 132 13s (83,84). All the oxidation states of 132-136 were generated by a quantitative chemical oxidation method. Table III shows the dependence of umax, emax, and Av1j2 values of the IT band on the oxidation state and the number of ferrocene units. The Ai/j/2 values... 

The electrochemistry of some single-atom bridged ferrocene dimers gives a separation of two le waves. The carbon atoms in CROH and C = 0 can communicate between ferrocene units in 32 and 33 (101). Diferrocenyl sulfide, 34, shows significant AE° values, 0.29-0.33 V, indicating the presence of strong internuclear interactions (102,103). 

Cationic ferrocene complexes with one, two, and four cationic [B(R)bpy] (bpy = 2,2,-bipyridine) acceptors such as 66 show absorption at Amax = 496-540 nm with the contribution of charge transfer between the ferrocene unit and the B(R)bpy substituent(s) (165). This is confirmed by the EPR spectrum of the monoreduced neutral species, which features a line shape indicating a considerable admixture of the ligand and metal orbitals. Preparation and physical properties of the related polymer, 67, have also been reported (166). 

Segmented Poly(ether urethane) Filins Containing Ferrocene Units in the Hard Segments... 

Mesomorphic dendrimers containing electroactive units have potential for construction of dendrimer based molecular switches. Deschenaux et al. reported [154] the synthesis and liquid-crystalline properties of a novel dendrimer containing six mesomorphic ferrocene units. Apart from exhibiting a broad enantiotropic smectic A phase as determined by polarized optical microscopy, DSC, and XRD studies, thermogravimetry revealed the excellent thermal stability of the macromolecule. 

There are, of course, metal-containing dendrimers that belong to more than one of the above-mentioned categories. Examples are the heptametallic dendrimer made of a central Fe(Cp)(C6Me6)+ core and coated with 6 ferrocene moieties [ 12], and the heterometallic dendrimers made of an organic core, containing up to 6 Pt(IV)-based organometallic species in the branches, and coated with up to 12 ferrocene units [13]. 

Mixed-metal dendrimers containing up to 6 Pt(IV)-based organometallic species in the branches and 12 peripheral ferrocene units (8) have recently been synthesized and their electrochemical behavior investigated [13]. As in the previously discussed examples, multi-electron reversible oxidation processes, assigned to the equivalent, non-interacting ferrocene units, have been observed. The authors point out that cyclic voltammetry is a powerful tool to support the structure of the dendrimers containing ferrocene units. 

Fig. 2. A supramolecular array (9 a) obtained enclosing the eight ferrocene units of dendrimer 9 into (3-CD molecules... 

U. H. F. Bunz, G. Roidl, M. Altmann, V. Enkelmann, K. D. Shimizu, Synthesis and Structural Characterization of Novel Organometallic Dehydroannulenes with Fused CpCo-Cyclobutadiene and Ferrocene Units Including a Cydic Fullerenyne Segment , J. Am Chem Soc 1999,121, 10719-10726. 

In 1990 we reported the synthesis of new redox-responsive crown ether molecules that contain a conjugated link between the crown ether unit and a ferrocene redox-active centre (Beer et al., 1990a). Examples of some of the species synthesized are shown in Fig. 5. The electrochemical behaviour of these species was investigated and also the electrochemical behaviour of their analogues with a saturated link between the ferrocene unit and the crown ether. The changes in the CVs of [2a] upon addition of magnesium cations are shown in Fig. 6. The metal cation-induced anodic shifts of [2a], [2b] and also their saturated analogue [3] and vinyl derivatives [4a], [4b] are shown in Table 1. 

On occasions one may observe only two oxidation processes, a one-electron and a two-electron event, respectively, if there is an electronic interaction between only two of the three sites. This is, for example, the case of the triferrocene molecule in which selenium atoms bridge the three ferrocene units, Figure 20.33... 

Finally, to prove how in some cases the mutual interactions among ferrocene units in multiferrocenes can be difficult to predict, consider... 

The appearance of two separate oxidation processes in these polymers is attributed to the existence of electronic communication between the ferrocene units, so that the first oxidation is centred on alternating ferrocene sites, whereas the second oxidation is centred on the remaining sites. For example, for polyferrocenyl-persulfide, the electron removal path reported in Scheme 3 has been proposed (the oxidation state of the iron atoms is represented).42... 

Organometallic chemistry, in particular the chemistry of ferrocenes, is affording significant contributions to the field. As illustrated in Figure 28, ferrocene-based dendrimers can assume compositions ranging from a single ferrocene unit properly functionalized45 to multiferrocene units placed in the outer zone of the surface.46... 

Figure 28 A schematic representation of the possible composition of ferrocene-based dendrimers. (a) Dendrimers containing a single ferrocene unit (b) dendrimers containing multiple ferrocene units...

The dendrimers containing more ferrocene units are more interesting from the electrochemical viewpoint. Scheme 6 shows the 1st and 2nd generation members of a silicon-centred series containing four and eight ferrocenyl units, respectively.48... 

As seen, these complexes also exhibit a single oxidation process involving a number of electrons equal to the number of ferrocene units. 

Figure 33 shows the cyclic voltammetric response of the 8-Fc derivative. It gives rise to two separate reversible oxidations (each one involving four electrons) separated by 0.19 V. This means that the overall oxidation proceeds through two separated steps each one involving four alternate ferrocene units. 

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