Polynuclear Ruthenium Complexes

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

Organometallic Compounds. Ruthenium, predominately in the oxidation states 0 and +2, forms numerous mononuclear and polynuclear organometalhc compounds. A few examples of compounds in both higher and lower oxidation states also exist. The chemistry of polynuclear ruthenium complexes is extensive and has been reviewed (53—59). 

Polynuclear metal complexes are more suited for water oxidation catalyst because of their nature to act as multielectron transfer reagents in addition to the fact that charge delocalization can lead to stabilization of the catalyst rather than decomposition during the process. The trinuclear ruthenium complexes Ru-red and Ru-brown, [(NH3)sRu-0-Ru(NH3)4-0-Ru(NH3)j] - (Ru "-Ru" -Ru" ) and [(NH3)sRu-0-Ru(NH3)4-0-Ru(NH3)5] + (Ru -Ru" -Ru ), respectively, have been shown to be efficient water oxidation catalysts for oxygen evolution with high turnover numbers When Ru-red was dissolved in an acidic aqueous solution, it underwent one-electron oxidation with the formation of Ru-brown. When Ru-brown was dissolved in a basic solution, the complex underwent reduction to produce Ru-red. The one-electron oxidation and reduction of the Ru-red and Ru-brown has already been well established (Eq. 11) 6 65-6 )... 

This section surveys the use of various di-, tri-, and polynuclear ruthenium complexes as precursors for the homogeneous hydroformylation of alkenes. Several arbitrary assumptions have been made so as to include dinuclear starting complexes which are strictly not cluster compounds. Moreover, no distinction is made between neutral and anionic precursors. Also, in several cases, particularly in the patents, information is lacking concerning the intermediate species involved in the catalytic cycles. Interestingly, half of the described systems come from patents, and there are few fundamental studies which clearly establish the implication of cluster species during the catalysis. 

The recently characterized ruthenium complex (Ph3P)2(Ph2PC6H4)RuH2 K CioHs-(Et20) in THF catalyses the hydrogenation of polynuclear aromatics, predominantly to their tetrahydro-derivatives, at 100 °C and 620 kPa gauge of hydrogen. ... 

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