Synthesis, crystal structures, and redox behavior of some pentamethylcyclopentadienyl arene ruthenium salts

• Crystal structures of three RuCp*(arene)+ salts.
• Comparison of electrochemical data with any other group 8 MCp*(arene) cations.
• Chemical reduction of one of these cations to give the first example of a group 8 MCp(η4-arene)-type anion.

Hexafluorophosphates of (η5-pentamethylcyclopentadienyl)(η6-arene)ruthenium cations (arene = 1,3,5-triethylbenzene, 1,3,5-tris(trimethylsilylmethyl)benzene, and pentamethylbenzene) have been obtained from the reaction of [Ru(η5-C5Me5)(NCCH3)3]+PF6−[Ru(η5-C5Me5)(NCCH3)3]+PF6− with the appropriate arene; their X-ray single-crystal structures have been determined and compared to those of related compounds. The electrochemistry of these cations is compared to that of other [Ru(η5-C5Me5)(η6-arene)]+ and [Fe(η5-C5Me5)(η6-arene)]+ species; the 1,3,5-C6H3(CH2SiMe3)3 derivative is reduced at the most cathodic potential (Epc = −2.96 V vs. ferrocenium/ferrocene). Reduction with sodium amalgam gives a dimer, [Ru(η5-C5Me5)]2[μ-η5:η5-(arene)2], in the case of arene = 1,3,5-C6H3Et3, analogous to what has been previously reported for the mesitylene complex, and a mixture of products when arene = C6HMe5. The 1,3,5-C6H3(CH2SiMe3)3 derivative is comparatively unreactive to sodium amalgam, although long reaction times lead to desilylation and formation of [Ru(η5-C5Me5)]2[μ-η5:η5-C6H3Me3C6H3Me3]. Use of sodium–potassium alloy, on the other hand, leads to formation of a 18-electron anionic Ru0 complex: K+[Ru(η5-C5Me5){η4-1,3,5-C6H3(CH2SiMe3)3}]–.

Three ruthenium pentamethylcyclopentadienyl arene hexafluorophosphate salts have been synthesized and structurally characterized. The 1,3,5-tris(trimethylsilylmethyl)benzene example is reduced at a more cathodic potential than its trimethyl- and triethylbenzene analogs. Chemical reduction with sodium–potassium alloy leads to the formation of an anionic Ru0 η4-arene complex.Figure optionsDownload as PowerPoint slide