The Ruthenocenylmethylium Cation:  Isolation and Structures of η ⁵ -Cyclopentadienyl- η ⁶ -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 η 5 -cyclopentadienyl- η 6 -fulvene-ruthenium(II). The UV−vis spectrum of 1 + shows a d−d transition at an energy similar to those of ruthenocene and the η 5 -cyclopentadienyl- η 6 -benzeneruthenium(II) cation, but with increased absorptivity. Cyclic voltammetry indicates that 1 + is reduced at considerably less negative potential than its isomer, the η 5 -cyclopentadienyl- η 6 -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.