Synthesis, characterization, and reactivity of half-sandwich Ru(II) complexes containing phosphine, arsine, stibine, and bismutine ligands

The synthesis and some reactions of the Ru(II) and Ru(IV) half-sandwich complexes RuCp(EPh₃)(CH₃CN)₂]⁺ (E=P, As, Sb, Bi) and RuCp(EPh₃)(η³-C₃H₅)Br]⁺ have been investigated. The chemistry of this class of compounds is characterized by a competitive coordination of EPh₃ either via a RuE or a η⁶-arene bond, where the latter is favored when the former is weaker, that is in going down the series. Thus in the case of Bi, the starting material RuCp(CH₃CN)₃]⁺ does not react with BiPh₃ to give RuCp(BiPh₃)(CH₃CN)₂]⁺ but instead gives only the η⁶-arene species RuCp(η⁶-PhBiPh₂)]⁺ and (RuCp)₂(μ-η⁶,η⁶-Ph₂BiPh)]²⁺. Similarly, the EPh₃ ligand can be replaced by an aromatic solvent or an arene substrate. Thus, the catalytic performance of RuCp(EPh₃)(CH₃CN)₂]⁺ for the isomerization of allyl-phenyl ethers to the corresponding 1-propenyl ethers is best with E=P, while the conversion drops significantly using the As and Sb derivatives. By the same token, only RuCp(PPh₃)(CH₃CN)₂]⁺ is stable in a non-aromatic solvent, whereas both RuCp(AsPh₃)(CH₃CN)₂]⁺ and RuCp(SbPh₃)(CH₃CN)₂]⁺ rearrange upon warming to RuCp(η⁶-PhEPh₂)]⁺ and related compounds. In addition, the potential of RuCp(EPh₃)(CH₃CN)₂]⁺ as precatalysts for the transfer hydrogenation of acetophenone and cyclohexanone has been investigated. Again aromatic substrates are clearly less suited than non-aromatic ones due to facile η⁶-arene coordination leading to catalyst's deactivation.