Ferrocene derivatives structures

It is typical that the photorearrangement 57 - 58 proceeds readily.37 This seems to rule out the triplet state of the rearranging molecule 57, reasoning that the ferrocene part of the molecule would very probably act as a triplet quencher,35 as is the case with other ferrocene derivatives structurally able to undergo triplet photoreactions.36 On the other hand, the reaction 55 - 56 cannot be sensitized with acetophenone33 (ET = 73.6 kcal mole-1 32ft). 

Harrowfield et al. [37-39] have described the structures of several dimethyl sulfoxide adducts of homo bimetallic complexes of rare earth metal cations with p-/e rt-butyl calix[8]arene and i /i-ferrocene derivatives of bridged calix[4]arenes. Ludwing et al. [40] described the solvent extraction behavior of three calixarene-type cyclophanes toward trivalent lanthanides La (Ln = La, Nd, Eu, Er, and Yb). By using p-tert-huty ca-lix[6Jarene hexacarboxylic acid, the lanthanides were extracted from the aqueous phase at pH 2-3.5. The ex-tractability is Nb, Eu > La > Er > Yb. 

Over the past decade a number of new covalently bonded TTF/ferrocene adducts have been reported [77, 78]. The crystal structure of the l,l -bis(l,3-dithiole-2-ylidine)-substituted ferrocene derivative has been published [77]. In this complex, ferrocene has essentially been incorporated as a molecular spacer between the two l,3-dithole-2-ylidene rings forming a stretched TTF molecule. This adduct, and its methyl-substituted derivative, have been combined with TCNQ to form charge-transfer complexes with room temperature powder conductivities of 0.2 S cm-1. Similar diferrocenyl complexes have been prepared with bis (dithiolene) metal complexes [79, 80]. 

Butacene SNPE, France has also developed a Butacene 800 [HTPB chemically reacted and grafted with a ferrocene derivative in the chain Structure (4.34)] binder which is free from such drawbacks (Figure 4.10). Some details of Butacene 800 are as under ... 

Figure 3.4 Structures of ferrocene derivatives investigated as guests for CB7.

The ubiquitous electrochemical behavior of ferrocene and its relative chemical stability have made this organometallic complex a useful group for the preparation of redox-active devices. The incorporation of ferrocene-modified amino acids into larger polypeptide structures can therefore lead to electrochemically active de novo designed proteins. In addition, the attachment of ferrocene derivatives to peptides make them electroactive and eligible to electrochemical detection. Hence, it is not surprising that the first synthesis of a ferrocene-modified amino acid dates back to the 1950s. 

Scheme 4.3 Iron as a structural and sensing element in 4.55. Fluoride binding to the boronic acid derived groups results in a lowering of the ferrocene-derived core s oxidation potential, allowing aerial oxidation of the Fe(II) centre to Fe (III) with concomitant colour change from orange to green.

Another way to assess ion channel conductance is to use artificial phospholipid vesicles (liposomes) as cell models. These structures (described in more detail in the next chapter) are commonly used to transport vaccines, drugs, enzymes, or other substances to target cells or organs. The vesicles, which are several hundred nanometres in diameter, do not suffer from interference from residual natural ion channel peptides or ionophores, unlike purified natural cells. A liposome model was used to test the ion transport behaviour of the redox-active hydraphile 12.36. The compound transports Na+ and the process can also be monitored using 23Na NMR spectroscopy.26 The presence of the ferrocene-derived group in the central relay allows the ion transport to be redox-controlled - oxidation to ferrocinium completely prevents Na+ transport for electrostatic reasons. Some representative data from a planar bilayer measurement is shown for hydraphile 12.36 in Figure 12.16. 

The accessible ferrocene/ferrocenium redox couple of ferrocene has led to its frequent use in electrochemical anion sensors. The chemical and structural similarity between ferrocene and cobaltocenium has meant that receptors based on these complexes often share the same design. The most relevant difference is that the ferrocene derivatives are neutral (until oxidised to ferrocenium),have no inherent electrostatic interaction with anions and therefore their complexes with anions exhibit lower stability constants. 

Figure 18.5 Control of mass transfer through an azobenzene-modified film, (a) Structure and photoisomerization of azobenzene (b) structures of ferrocene derivatives used to monitor anodic current at the film-electrode interface and (c) current as a function of light exposure for FDM and FDMDG redox probes.57 (Reprinted with permission from N. Liu et al., Nano Lett. 2004, 4, 551-554. Copyright 2004 American Chemical Society.)...

One redox species is chemically insensitive with respect to variation in E1/2, e.g. a ferrocene derivative, and serves as an internal reference in a linear sweep voltammogram. The second species is chemically sensitive, e.g. a pH sensitive quinone or a CO sensitive ferraazetine derivative, which has an E1/2 that varies with the changes in the chemical environment. A linear sweep voltammogram thus shows two waves, one for the reference molecule and one for the indicator molecule. The shift for the indicator wave along the potential or current axis provides a method for analyte detection. Surface derivitization, proof-of-structure, and proof-of-concept sensor functions are demonstrated. 

One structure that seemed attractive was ferrocinnamic acid, a ferrocene derivative carrying an acrylic acid chain. The substrate examined was the p-nitrophenyl ester of this acid (I) (12). It was clear from the known binding constants of ferrocene (4) and of p-nitrophenyl acetate that the ferrocene nucleus should be the one... 

Fluoride anion strongly interacts with various inorganic and organic boron and silicon compounds. These reactions are the basis for several fluoride sensors. Interaction of fluoride with boron compounds results in electron density redistribution and may also induce structural changes. Formation of fluoride complex by ferrocene derivative (Figure 16.19a) results in a decrease of oxidation potential by 200 mV... 

The difficulties of interpretation presented by some of these complexes are well illustrated by the sequence of ferrocene derivatives (VIII to X). The spectrum of VIII shows a near-infrared absorption as expected for a mixed valence complex, but the band has a shoulder, and at low temperatures is resolved into two bands. Complex IX has a well-defined band almost certainly ascribable to intervalence transfer, but complex X with a similar molecular structure has no such... 

Both metals form bis-cyclopentadienyl compounds and numerous substituted derivatives. None of these have anything approaching a ferrocene-like structure since the M2+ ions have closed d-shells. Cp2Zn is an infinite polymer while Cp2Cd is a monomer with tj -CsHs groups. 

We are already well accustomed with the historical relationship seen in the development of NHC ligands with special features. They often follow the example of successful phosphane ligands. Chiral NHC are no exception, although in key areas the very difference in shape between NHC and phosphanes prevents the development of NHC ligands as structural phosphane mimics. The axial (atropisomers with binaphthyl backbone [6,7]) and planar (ferrocene derivatives [8,9], [2,2]-paracyclophanes [10,11]) chiral examples, however, are styled on their phosphane predecessors [12-15]. We will first look at the unique options for NHC ligands before we turn to the more familiar phosphane mimics. 

Kinetic resolution of ferrocene derivatives, mainly alcohols, had an important place during the early stage of stereochemical investigations of ferrocene derivatives. The reaction of (partially) resolved ferrocenylalkyl alcohols and amines with racemic 2-phenylbutyric acid anhydride (Korean s method) was the basis for the configurational assignment before the establishment of structures by X-ray crystallography [41]. There has been some debate on the reliability of the method [62, 63], and additional chirality information seems necessary for certainty. Recently, the kinetic resolution of 1-ferrocenylethanol by transesterification with vinyl acetate, catalyzed by a lipase from PseudomonasJluorescens, led to an enantiomeric excess of 90—96% of both enantiomers [64], opening new preparative aspects. 

Ugi has coined the term stereorelating synthesis for the sequence lithiation/reac-tion with electrophiles [62,118], and used this technique as a method for the chemical correlation of the structure and for the determination of the enantiomeric purity of many 1,2-disubstituted ferrocene derivatives obtained either by resolution or by asymmetric synthesis (for a compilation, see [118]). It is important to note that all stereochemical features discussed above for central chiral compounds, such as retentive nucleophilic substitution, remain valid when more substituents are present at the ferrocene ring and the conversion of functional groups in planar chiral ferrocenes can be achieved by the same methods as described. 

A last example to be discussed in this section is particularly instructive from a structural point of view. In the case where the two substituents in the 1,1 position are (4-methylthio-phenyl)-2- -ethenyl, the corresponding ferrocene derivative (see Fig. 8-17) still acts as a donor and forms a 1 2 CT complex with TCNQ, but the degree of electron transfer is very weak [65]. This material is non-conducting (<7rt of pressed pellets < 10" S cm" ), shows very weak signals in both ESR spectra and magnetic susceptibility measurements, and displays a CN stretching frequency typical for neutral TCNQ. As illustrated in Fig. 8-17, the solid slate structure of this CT complex is characterized by DA2 stacks, whereby the donor in the DA2 unit assumes an antiperiplanar conformation, thus being able to accomodate the... 

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