Ruthenium binuclear complexes

Bernhard and coworkers [58] have addressed the discovery of ionic iridium(III) and ruthenium(II) complexes by combinatorial luminophore synthesis and screening (Scheme 5.9). Starting from iridium trichloride, cyclometalation with (hetero)arylpyridyl ligands 45 (Fig. 5.17) gives rise to the formation of binuclear iridium complexes 46. Upon complexation with bidentate N,N- or P,P-ligands 47 (Fig. 5.17), the cationic complex 48 is formed, which upon anion metathesis with hexafluorophosphate is transformed into the target complex 49. In this sequence, the step from 46 to 48 was performed in a traditional and a parallel manner, the latter leading to a library of 100 iridium and 10 ruthenium complexes. 

The reaction of an aqueous solution of la with an excess of NaOH or Na2C03 gives two cationic products 24 and 25. Complex 25 is the minor product, but it can be completely and irreversibly formed by recrystallization of 24 in acetone (35). The structure of 25, initially proposed as the binuclear tris-hydroxo-bridged ruthenium complex 25A (Scheme 2), has... 

Binuclear [RuX2(arene)]2 (1) and mononuclear RuX2(L) (arene) (3) derivatives have been shown to be useful precursors for access to alkyl-or hydrido(arene)ruthenium complexes. The latter are key compounds for the formation of arene ruthenium(O) intermediates capable of C—H bond activation leading to new hydrido and cyclometallated ruthenium arene derivatives. Arene ruthenium carboxylates appear to be useful derivatives of alkyl-ruthenium as precursors of hydrido-ruthenium complexes their access is examined first. 

Table 6 lists data for binuclear mixed valence pentaammine ruthenium complexes. 

Backvall et al. [10] have recently reported substantial improvements in the described process. A range of 1 -phenylethanol derivatives can be synthesized from the raeemates in excellent yields and >99 % ee by using a binuclear ruthenium complex combined with an immobilized lipase and a specifically designed acyl donor (4-Cl-PhOAc). Even aliphatic alcohols and diols... 

TABLE 13. Formal electrode potentials (vs. SCE) and relative separations for the sequential oxidations and reductions of the binuclear ruthenium complexes [112] in CH2CI2 solution as a function of the spacer... 

Deprotonation of a binuclear complex with (bpy)2ClRu moieties and 1,5-dipyridyl-l,3-pentadiene as bridging ligand yielded the cyanine-bridged complex, contrarily to the unsuccessful case of the bis(pentaammine)ruthenium complex (cf. (NH3)5Ru(py-) above) [14a], However, cyclic voltammetry and spectroelectro-chemical studies revealed that oxidation occurred at the cyanine bridging segment rather than on ruthenium atoms, thus precluding the preparation of a mixed-valence species. 

Electrochemical studies of the tetranuclear ruthenium complex of ligand (62) indicate two reversible two-electron redox waves in the anodic region attributed to oxidation of the Ru11 metal centers. This complex is said to act as two binuclear subunits with little or no electronic interaction between subunits. The first two-electron oxidation process results in a mixed valence complex with each subunit having a Ru11 and Ru111 metal center. 

Water oxidation is a thermodynamically unfavorable process which involves the transfer of four electrons. Several catalysts have been developed and simultaneously several mechanisms for these types of chemical transformations have been proposed [13]. Binuclear ruthenium complexes have drawn considerable attention in the context of water oxidation. The anthracene-bridged binuclear Ru bis-hydroxide bis-quinone... 

The importance of hydroxycarbonyl intermediates is well illustrated in a recent study of the stepwise oxidation of CO to CO2 by binuclear ruthenium complexes". Deprotonation of a diruthenium(I) aquo species yields a hydroxy intermediate which rearranges to an isolable hydroxycarbonyl complex, equation (h). Deprotonation of the hydroxycarbonyl with NEts in dichloromethane results in a formally diruthenium(O) ii-CO complex, equation (i). 

The use of Me3NO to induce substitution of dppm (bis(diphenylphosphino)-methane) for CO molecules on dinuclear iron complexes led to insertion of CO into C-C bonds of alkyne-derived metallacycles. Similar behavior was observed when [PPNJCl salts were used to favor the formation of alkyne-substituted triruthenium dppm-containing clusters.I This behavior should be compared with the insertion of CO into allenylidene and phosphido-bridging ligands occurring when dppm coordinates to binuclear ruthenium complexes as shown in Fig. 3. This reaction is a nucleophilic attack of the coordinated allenylidene and phosphido groups on a coordinated CO (see Section 2.8.2.2). 

Of particular interest are the cases where, by a suitable design of the complex, the factors (a) to (d) above can be eliminated so that the departure of the electrochemical response from the purely statistical behaviour arises solely from cause (e). Two examples of such complexes which have been well studied are binuclear ruthenium complexes bridged by 4,4 -bipyridyl type ligands, and binuclear complexes where L, the binucleating macrocyclic ligand, is a symmetric... 

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