The Ruthenocenylmethylium Cation:  Isolation and Structures of η⁵-Cyclopentadienyl-η⁶-fulvene-ruthenium(II) Salts

Salts of the ruthenocenylmethylium cation, 1⁺, can be synthesized from the reaction of ruthenocenylmethanol with either Brønsted or Lewis acids. The X-ray crystal structures of the tetrakis{3,5-bis(trifluoromethyl)phenyl}borate and trifluoromethanesulfonate salts of 1⁺ reveal that the methylium carbon is bound to the ruthenium with Ru−C bond lengths in the range 2.251(9)−2.40(1) Å and confirm the description of the cation structure as η⁵-cyclopentadienyl-η⁶-fulvene-ruthenium(II). The UV−vis spectrum of 1⁺ shows a d−d transition at an energy similar to those of ruthenocene and the η⁵-cyclopentadienyl-η⁶-benzeneruthenium(II) cation, but with increased absorptivity. Cyclic voltammetry indicates that 1⁺ is reduced at considerably less negative potential than its isomer, the η⁵-cyclopentadienyl-η⁶-benzene-ruthenium(II) cation. Chemical reduction with sodium amalgam in tetrahydrofuran leads to the formation of methylruthenocene, 1,2-bis(ruthenocenyl)ethane, and bis(ruthenocenylmethyl)ether. Reaction of 1⁺ with triphenylphosphine affords the (ruthenocenylmethyl)triphenylphosphonium cation; the crystal structure of the dichloromethane solvate of its tetrafluoroborate salt has been determined. Density functional calculations closely reproduce the crystallographically determined geometry of 1⁺ and allow rationalization of some characteristics of its structure, spectroscopy, and reactivity. The calculations suggest that the ferrocenylmethylium cation, 3⁺, has a geometry similar to 1⁺ with similar orbital structure, albeit with considerably more d-character to the occupied frontier orbitals.