Ruthenium carboxylate ligands

Ruthenium has a considerable propensity to form polynuclear complexes, particularly with carboxylate ligands which as bridging ligands span the Ru centres, sometimes accompanied by a bridging 0x0 ligand. Preparation and properties of bi- and tri-nuclear acetato complexes of Ru have been reviewed [552]. 

Planar M30 units occur in the basic carboxylates of such metals as iron, ruthenium, manganese, vanadium and chromium the chromium compound has been known since 1919. The metal atoms are also linked by pairs of carboxylate ligands (often acetates), and have terminal ligands (generally pyridine or water). Mixed metal units, e.g. Fe2CrO,124 and mixed oxidation states, e.g. Ci CifO,1 ... 

Partial structure of [(S)-BINAP)] Ru(OjC-fBu)jj with carboxylate ligands omitted for clarity. The pseudo-equatorial phenyl rings are thrust forward, while the pseudo-axial phenyls are oriented away from the ruthenium center. 

A number of examples of oligomers and polymers containing metal-metal multiple bonds also have been reported. " Chisohn has synthesized polymers with metal-metal single and multiple bonds from bimetallic carboxylates and coordinating ligands. Scheme 24 illustrates the synfliesis of a ruthenimn coordination polymer that contains Ru-Ru double bonds. Polymer 81 was prepared from reaction of the ruthenium carboxylate monomer (79) wifli pyrazine (80). The polymer dissociated in coordinating solvents such as THF, and the MW was dependent on its concentration in noncoordinating solvents such as benzene. The polymer was thermally stable only to 140°C, at which point pyrazine was evolved. 

The bonding in these Ru30 carboxylates can be explained by the usual MO scheme for these systems. A cr-bonding framework involves using six orbitals from each ruthenium (one s, three p, two d) to form bonds to the central O, four carboxylate oxygens and the terminal ligand (PPh3H20, etc.). 

Ruthenium complexes containing this ligand are able to reduce a variety of double bonds with e.e. above 95%. In order to achieve high enantioselectivity, the reactant must show a strong preference for a specific orientation when complexed with the catalyst. This ordinarily requires the presence of a functional group that can coordinate with the metal. The ruthenium-BINAP catalyst has been used successfully with unsaturated amides,23 allylic and homoallylic alcohols,24 and unsaturated carboxylic acids.25... 

Ruthenium catalysts have also been used in this context.200,201 In particular, the cationic ruthenium complex, CpRu(CH3CN)3PF6, in conjunction with carboxylic acid ligand 3, has been used to achieve the remarkably chemoselective allylation of a variety of alcohols via dehydrative condensation with allyl alcohol (Equation (50)).202 It is worth noting that this transformation proceeds with 0.05 mol% catalyst loading and does not require the use of excess allyl alcohol. 

Ruthenium complexes B are stable in the presence of alcohols, amines, or water, even at 60 °C. Olefin metathesis can be realized even in water as solvent, either using ruthenium carbene complexes with water-soluble phosphine ligands [815], or in emulsions. These complexes are also stable in air [584]. No olefination of aldehydes, ketones, or derivatives of carboxylic acids has been observed [582]. During catalysis of olefin metathesis replacement of one phosphine ligand by an olefin can occur [598,809]. 

This first section on Ru(Vl) dioxo complexes complexes concentrates on those with other 0-donor ligands periodate, tellurate and carboxylates. There is a review on ruthenium acetato complexes [552],... 

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