Ruthenocene, metallocen ligands

The common metathesis reactions for the preparation of metallocenes, treating a metal salt MX2 with NaCp, are hampered in the case of ruthenium by the lack of suitable Ru salts. (Rul2 is commercially available, but is still not commonly used in the synthesis of rathenocene.) Thus, ruthenocene has been obtained from Ru(acac)3 and NaCp in very low yield and later from RuCb and NaCp in 50-60% yield. It has now become apparent that alkene polymers, in particular [Ru(nbd)Cl2]x, but also [Ru(cod)Cl2]x and hydrazine derivatives (Section 3.1), can serve as Ru precursors. Equally successful in many cases is reductive complexation of cyclopentadiene in ethanol in the presence of Zn (Section 3.2), which furnishes the metallocene in about 80% yield. Decamethylruthenocene (82) was first obtained by the Zn reduction route in 20% yield, but can now be prepared conveniently from halide complexes [Cp RuCl2]2 or [Cp RuCl]4, a common method for the preparation of symmetrical and unsymmetrical sandwich compounds of ruthenium featuring one alkyl-substituted ligand. 

Very recently, Gmeiner and co-workers prepared metallocene-derived receptor ligands for G-protein-coupled receptors (GPGRs) such as dopamine and serotonin receptor subtypes. They used ruthenocene and ferrocene derivatives, in which the metallocenes replaced cyclophanes as so-called fancy bioisosters. In particular, compound 31 (Scheme 32) showed sub-nanomolar affinity and high specificity for the dopamine D4 and serotonin HTia receptor subtypes, and may thus be a suitable lead structure for the further development of selective organometallic GPCR ligands. 

The density functional approach has been used in an investigation of the protonation of ferrocene, ruthenocene, and osmocene in the gas phase, and the general conclusion is that the addition of a proton to the carbon atoms in the cyclopentadienyl ring is favored in ferrocene, whereas metal protonation is favored with ruthenocene and osmocene. The results obtained from these calculations were used in the interpretation of electrophilic substitution reactions of metallocenes. The basicity of the ligand group l,l -bis(diphenylphosphino)metallocene has also been examined. This is an important aspect for catalysis because it has proved difficult to obtain results which... 

An important part of metallocene chemistry remains the use of the metallocene group as a ligand. Thus both ruthenocene and ferrocene cyclophosphazene complexes have been reported. Direct tungsten-carbon bonds can also be generated to give... 

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