Ruthenium complexes donors

The emission can also be quenched by electron donors causing reduction of the ruthenium complex ion. 

Let us now examine sample sets of data. We shall consider two reactions, the formation of a biradical1 [Eq. (7-10)] and an electron transfer reaction between two ruthenium complexes [Eq. (7-11)], in which LN represent nitrogen-donor ligands specified in the original reference.2 The chemical equations are... 

The research group of Backvall employed the Shvo s ruthenium complex (1) [21] for the racemization. This complex is activated by heat. For the KR they used p-chlorophenyl acetate as the acyl donor in combination with thermostable enzymes, such as CALB [20] (Figure 4.7). This was the first practical chemoenzymatic DKR affording acetylated sec-alcohols in high yields and excellent enantioselectivities. In the best case 100% conversion (92% isolated yield) with 99% ee was obtained. This method was subsequently applied to a variety of different substrates and it is employed (with a different ruthenium complex) by the Dutch company DSM for the large-scale production of (R)-phenylethanol [22]. 

Ruthenium Complexes Having an OH Proton Donor and a RuH as Hydride Donor... 

In the transition metal-catalyzed reactions described above, the addition of a small quantity of base dramatically increases the reaction rate [17-21]. A more elegant approach is to include a basic site into the catalysts, as is depicted in Scheme 20.13. Noyori and others proposed a mechanism for reactions catalyzed with these 16-electron ruthenium complexes (30) that involves a six-membered transition state (31) [48-50]. The basic nitrogen atom of the ligand abstracts the hydroxyl proton from the hydrogen donor (16) and, in a concerted manner, a hydride shift takes place from the a-position of the alcohol to ruthenium (a), re-... 

The dual role of these ruthenium complexes, as both an NO donor and scavenger, appears to lie strongly in the binding constant of NO to Ru(II) and the ligands present, which can facilitate NO dissociation. Clearly the investigative approach into these systems has just touched the surface and the future appears very promising in these potential NO source/scavenger systems. 

Aromatics occur as ligands in ruthenium complexes that are used for hydrogen transfer reaction, i.e. two hydrogen atoms are transferred from a donor molecule, e.g. an alcohol, to a ketone, producing another alcohol. Especially the enantiospecific variant has become important, see Chapter 4.4. The substitution pattern of the aromatic compound influences the enantioselectivity of the reaction. 

As shown earlier, A,A-dimethylaniline acts as an electron donor toward the electronically excited Ru(ll) tris(dipyridyl)complex (Bock et al. 1979). Nocera s group studied the effect of salt formation on the redox interaction between the ruthenium complex and the A/,At-dimethylaniline moiety. Two different salts, depicted in Scheme 5.26, were prepared and studied (Deng et al. 1997, Kirby et al. 1997, Roberts et al. 1997). 

DKR of secondary alcohol is achieved by coupling enzyme-catalyzed resolution with metal-catalyzed racemization. For efficient DKR, these catalyhc reactions must be compatible with each other. In the case of DKR of secondary alcohol with the lipase-ruthenium combinahon, the use of a proper acyl donor (required for enzymatic reaction) is parhcularly crucial because metal catalyst can react with the acyl donor or its deacylated form. Popular vinyl acetate is incompatible with all the ruthenium complexes, while isopropenyl acetate can be used with most monomeric ruthenium complexes. p-Chlorophenyl acetate (PCPA) is the best acyl donor for use with dimeric ruthenium complex 1. On the other hand, reaction temperature is another crucial factor. Many enzymes lose their activities at elevated temperatures. Thus, the racemizahon catalyst should show good catalytic efficiency at room temperature to be combined with these enzymes. One representative example is subtilisin. This enzyme rapidly loses catalytic activities at elevated temperatures and gradually even at ambient temperature. It therefore is compatible with the racemization catalysts 6-9, showing good activities at ambient temperature. In case the racemization catalyst requires an elevated temperature, CALB is the best counterpart. 

Although the majority of ruthenium complexes contain tertiary phosphines as co-ligands, N-donor ligands are present in complexes obtained with Ru(tmeda) Cp [45], RuCl(Me2bpy)(PPh3)2 ]46], RuQ(L) [L = (dmpz)2-acetate [47], 2,6-(dmpz)2-... 

In contrast to ferrocenes, osmium and ruthenium complexes are capable of forming coordinative bonds with donor centers of GO including histidine imidazoles. There are therefore two ways of bringing coordinated transition metals onto enzyme surfaces, i.e., via natural and artificial donor sites. Artificial centers are commonly made of functionalized pyridines or imidazoles, which must be covalently attached to GO followed by the complexation of an osmium or... 

Transfer of an electron from a photoexcited donor to an acceptor has also been studied. Ballard and Mauzerall [219] photoexcited zinc octaethylporphyrin and found both the triplet—triplet rate coefficient and ion yields indicate a reaction radius of 2.0 0.1 nm, some 0.6 nm larger than twice the radius of this metal ligand. However, electron transfer from pyridine—ruthenium complexes does not appear to be facilitated by electron tunnelling (see Chap. 3, Sect. 2.1). 

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