The Ruthenocenylmethylium Cation:  Isolation and Structures of <i>η</i>⁵-Cyclopentadienyl-<i>η</i>⁶-fulvene-ruthenium(II) Salts

Salts of the ruthenocenylmethylium cation, <b>1</b>⁺, 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 <b>1</b><b>⁺</b> 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 <i>η</i>⁵-cyclopentadienyl-<i>η</i>⁶-fulvene-ruthenium(II). The UV−vis spectrum of <b>1</b><b>⁺</b> shows a d−d transition at an energy similar to those of ruthenocene and the <i>η</i>⁵-cyclopentadienyl-<i>η</i>⁶-benzeneruthenium(II) cation, but with increased absorptivity. Cyclic voltammetry indicates that <b>1</b><b>⁺</b> is reduced at considerably less negative potential than its isomer, the <i>η</i>⁵-cyclopentadienyl-<i>η</i>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 <b>1</b><b>⁺</b> 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 <b>1</b><b>⁺</b> and allow rationalization of some characteristics of its structure, spectroscopy, and reactivity. The calculations suggest that the ferrocenylmethylium cation, <b>3</b>⁺, has a geometry similar to <b>1</b><b>⁺</b> with similar orbital structure, albeit with considerably more d-character to the occupied frontier orbitals.