Gold complexes redox properties

Historically, OTEs were used to investigate complex redox reactions including mediated reactions of enzymes (-> mediators). Recently OTEs have been extensively applied to study spectroelectrochemical properties of thin films of electrochromic materials (- elec-trochromic devices, -> electrochromism), and various chromic deposits including bioelectrochemical materials (e.g. - enzymes) or network films of gold -> nanoparticles. They are often used in -> photoelectrochemistry as electrode materials. 

Several dinuclear units have been assembled to form linear Au5 + and Ane complexes. However, they were shown to be Au An mixed-valent compounds. The normal Au -Au distances are in the range 2.5-2.7A. The only binuclear gold(II) complex (15) with unsupported Au Au bond has been shown to possess interesting photophysical and thermal redox properties." ... 

An interesting feature concerning the redox properties of the complex, [Au(dddt)2] (dddt = 5,6-dihydro-l,4-dithiine-2,3-dithiolate, 3), is that the one-electron oxidized product, [Au(dddt)2]°, can be isolated14. An X-ray analysis of the neutral complex reveals a square planar gold structure stacked in dimeric units as a result of intermolecular S-----S contacts. Extended Hlickel calculations predict that the odd electron resides pri-... 

It has been reported that the electrical properties of single molecules incorporating redox groups (e.g. viologens [114, 119, 120, 123, 124], oligophenylene ethynylenes [122, 123], porphyrins [111, 126], oligo-anilines and thiophenes [116, 127], metal transition complexes [118,128-132], carotenes [133], ferrocenes [134,135],perylene tetracarboxylic bisimide [93, 136, 137] and redox-active proteins [138-143]), can be switched electrochemically. Such experiments, typically performed by STM on redox-active molecules tethered via Au-S bonds between a gold substrate and a tip under potential control, allow the possibility to examine directly the correlation between redox state and the conductance of individual molecules. 

Hexacyanoferrates were immobilized on Au covered with SAM of 3,3 -thiodipropionic acid [86]. It has been found from voltammetric studies that the surface coverage of hexacyanoferrate is close to one monolayer and such an electrode exhibits very good surface redox behavior. Cheng et al. [87] have described the formation of an extremely thin multilayer film of polybasic lanthanide heteropolytungstate-molybdate complex and cationic polymer of quaternary poly(4-vinylpyridine), partially complexed with osmium bis(2,2 -bipyridine) on a gold electrode precoated with a cysteamine SAM. Consequently, adsorption of inorganic species might also be related to the properties of SAMs. This problem will be discussed in detail in a separate section later. 

Fe2(III,III)2+ states. By considering that organometallic dendrimers based on conjugated ferrocene units are of special importance since mixed-valence states have interesting electrical, redox, and magnetic properties, recently three generations of polypropylene amine) dendrimers, decorated at their periphery with 4, 8, and 16 (compound 6, Fig. 6.6a) BFc units, respectively, have been synthesized and the electrochemical behavior of the dendrimers complexed with (3-cyclodextrins ([3-CD) and adsorbed at self-assembled monolayers (SAMs) of heptathioether-functionalized [3-CD on gold (molecular printboard) has been studied.40... 

Other work has demonstrated that it is possible to switch ON and OFF luminescence by reduction/oxidation, and it has been demonstrated that such switching is possible inside an OTTLE cell. Many alkynyl complexes, especially those of rhenium, platinum,copper, silver or gold " are highly luminescent from their excited MLCT or metal perturbed 71 states. This opens up the possibility to significantly influence their emissive properties by redox processes. An interesting example is found in recent work of Wong et Unlike other rhenium(I)-alkynyl complexes, heterobimetallic... 

Reduction of the gold and silver complexes of 15a,b was investigated by cyclic voltammetry at a Pt electrode . These data afford an interesting comparison of the electrochemical behavior of Au vs Ag and P vs As. The Au complexes are harder to reduce than the corresponding Ag complexes, which reflects the relative chemical stabUity of Au and Ag . The Ag complex, [Ag(15b)2](C104)3, decomposes readily in the presence of water, chloride and many organic solvents. A qualitative comparison of the reduction potential of [Au(15a)2] + (—0.45 V vs Ag/Ag+ or ca -1-0.15 V vs SCE) with the reversible redox couple for [Au(13a)2]+ (-1-0.46 V vs SCE) is also in line with the difference in electronic properties of methyl vs phenyl groups on P. 

The oxidative reactivities of silver(ii) and silver(m) have been compared. The first- and second-order terms in [Ag ] in various reactions have been assessed and an attempt has been made to resolve the apparent reverse of reactivity between the bi-and ter-valent silver ions, depending on the substrate. A reaction scheme for the oxidation of water has been presented and the data are consistent with a reactive dimeric form of Ag. The hydrolysis constant of Ag is also considered to be less than 31 mol . The properties of octaethylporphyrins show the Agi complex to be stable,and cyclic voltammetric studies show reversibility in the +3<- +2 redox steps. In the gold(m) (as AuCl4 ) oxidation of methionine, there is stereospecificity in the reaction, the product being the corresponding sulphoxide with no evidence for a sulphone. The reaction is biphasic, with an initial complex formation which is too rapid for study by conventional techniques followed by a slower intramolecular redox reaction leading to gold(i). The rate of this latter process is dependent on the methionine concentration. The oxidation also takes place when the substrate is part of a peptide chain. 

Triangular gold cluster complexes also show emissive properties. Thus, excitation of an acetonitrile solution of [Au3Cl HC(PPh3)2 2] leads to a photoluminescence centered at approximately 537 nm with a lifetime of 11 ps. The excited state redox potential for this [Au3] V[Au3] couple is -1.6V versus SSCE. ... 

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