| Pentasubstituted cycltienyl ligands The body of kledge concerning compounds with penta-substituted cycltienyl ligands is extensive, with organometallic complexes of the pentamethylcycltienyl and pentaphenylcycltienyl ligands being well-kn. Substitutions on the cycltienyl rings of rhodocenes and rhodocenium salts produce compounds of higher stability as they allow for the increased delocalisation of positive charge or electron density and also provide steric hindrance against other species approaching the metal centre. Various mono- and di-substituted rhodocenium species are kn, but substantial stabilisation is not achieved without greater substitutions. Kn highly substituted rhodocenium salts include decamethylrhodocenium hexafluorophosphate PF6, decaisopropylrhodocenium hexafluorophosphate PF6, and octaphenylrhodocenium hexafluorophosphate .[Note 5] Decamethylrhodocenium tetrafluoroborate can be synthesised from the tris(acetone) complex (BF4)2 by reaction with pentamethylcycltiene, and the analogous iridium synthesis is also kn. Decaisopropylrhodicnium hexafluorophosphate was synthesised in 1,2-dimethoxyethane (solvent) in an unusual one-pot synthesis that involves the ation of 20 carbon–carbon bondsIn a similar reaction, pentaisopropylrhodocenium hexafluorophosphate PF6 can be synthesised from pentamethylrhodocenium hexafluorophosphate PF6 in 80% yield. These reactions demonstrate that the acidity of the methyl hydrogens in a pentamethylcycltienyl complex can be considerably increased by the presence of the metal centre. Mechanistiy, the reaction proceeds with potassium hydroxide deprotonating a methyl group and the resulting carbanion undergoing nucleophilic substitution with methyl iodide to a new carbon–carbon bond. The compounds pentaphenylrhodocenium tetrafluoroborate BF4, and pentamethylpentaphenylrhodocenium tetrafluoroborate BF4 have also been reported. They demonstrate that rhodium sandwich compounds can be prepared from half-sandwich precursors. For example, in an approach broly similar to the tris(acetone) synthesis of decamethylrhodocenium tetrafluoroborate, pentaphenylrhodocenium tetrafluoroborate has been synthesised from the tris(acetonitrile) salt (BF4)2 by reaction with sodium cycltienide: The staggered conation of ferrocene, D5d symmetry (left), and the eclipsed conation of ruthenocene, D5h symmetry (right) Octaphenylrhodocene, , is the first rhodocene derivative to be isolated at room temperature. Its olive-green crystals decompose rapidly in , and within minutes in air, demonstrating a dramatiy greater air sensitivity than the analogous cobalt complex, although it is significantly more stable than rhodocene itself. This difference is attributed to the relatively lower stability of the rhodium(II) state as d to the cobalt(II) state. The reduction potential for the + cation (measured in dimethylamide relative the ferrocenium / ferrocene couple) is −1.44 V, consistent with the greater thermodynamic stabilisation of the rhodocene by the C5HPh4 ligand d with the C5H5 or C5Me5 ligands. Cobaltocene is a useful one-electron reducing agent in the research laboratory as it is soluble in non-polar organic solvents, and its redox couple is sufficiently well behaved that it may be used as an internal standard in cyclic voltammetry. No substituted rhodocene yet prepared has demonstrated sufficient stability to be used in a similar way. |
