Proton-Controlled Regioselective Synthesis of [Cp*(dppe)Fe–C≡C-1-(η6-C10H7)Ru(η5-Cp](PF6) and Electron-Driven Haptotropic Rearrangement of the (η5-Cp)Ru+Arenophile

Abstract

Treatment of [Cp*(dppe)Fe–C≡C-1-naphthyl] (3) with [CpRu(NCCH3)3](PF6) (2(PF6)) in CH2Cl2 provides the heterobinuclear complex [Cp*(dppe)Fe–C≡C-1-(η6-C10H7)RuCp](PF6) in 80% isolated yield (Cp = η5-C5H5, Cp* = η5-C5Me5, dppe = 1,2-bis(diphenylphosphino)ethane). Complexation of the CpRu+ arenophile fragment specifically takes place onto either the substituted naphthyl ring A or on the unsubstituted ring B, providing isomers 1A(PF6) and 1B(PF6) in a 70/30 ratio. Under the same conditions, complexation of the cationic vinylidene 3H(PF6) or the iron(III) complex 3(PF6) affords 1A(PF6) and 1B(PF6) in a 30/70 ratio upon deprotonation of the dicationic vinylidenes 1AH(PF6)2 and 1BH(PF6)2 and monoelectronic reduction of the dicationic acetylides 1A(PF6)2 and 1B(PF6)2, respectively. The new compounds were characterized by NMR, IR, cyclic voltammetry, and UV–vis methods. The X-ray crystal structures show that the (η6-arene)Ru distances lengthen according to the following sequence of compounds: 1B(PF6), 1B(PF6)2, and 1A(PF6). It is found that the isomerization of 1A(PF6) into 1B(PF6) can be achieved at 20 °C upon activation of the (η6-arene)Ru bond by one-electron oxidation of the remote iron center and assistance from a coordinating solvent molecule. The experimental data have been analyzed with the support of theoretical calculations performed at the density functional theory (DFT) level. It is proposed that the reaction pathway involves a transition state in which the CpRu+ entity is η3-coordinated to the naphthyl rings in an exocyclic manner in a position opposite to the alkynyl iron moiety. Theoretical results also reflect the ability of the transition state to accommodate coordinating solvent molecules such as acetonitrile to lower the activation energy barrier of the haptotropic rearrangement.