Half-pseudoferrocene cations from nucleophilic addition of o-carboranyl anions to the [(η6-mesitylene)2Fe]2+ dication

Abstract

Reactions between the mesitylene (mes) dication [(η⁶-mes)₂Fe]²⁺ (1a) [(PF₆⁻)₂ salt] and lithium o-carboranes Li[1-R-1,2-C₂B₁₀H₁₁] (2) (R = H, 2a; Me, 2b; Ph, 2c) at low temperature (−60 °C, 1 h, followed by stirring for 2 h at r.t.) in THF resulted in a clean addition of the corresponding carborane anions to one of the unsubstituted arene sites in 1a, forming a series of orange monocations of general structure [(η⁵-mes-exo-6-{2-R-1,2-C₂B₁₀H₁₁})Fe(η⁶-mes)]⁺ (3) (R = H, 3a; Me, 3b; Ph, 3c) which were isolated as PF₆⁻ salts (3PF₆) in yields ranging 50–75%. Individual complexes were obtained on purification by LC or preparative TLC on a silica gel substrate, using MeCN–CH₂Cl₂ mixtures as the mobile phase. Interestingly, the room-temperature reaction between 2a (threefold excess) and 1a(PF₆)₂ with a reverse order of addition of the reaction components yielded an orange salt [(η⁵-mes-exo-6-{1,2-C₂B₁₀H₁₁})Fe(η⁶-mes)]⁺[closo–nido-H₁₁B₁₀C₂-C₂B₁₀H₁₂]⁻ (3acCA) (cCA = conjucto-carborane anion = [closo-nido-H₁₁B₁₀C₂-C₂B₁₀H₁₂]⁻) as a sole product in 71% yield. The formation of this conjucto anion can be taken as a strong support for the participation of a radical-chain mechanism in the ostensible nucleophilic addition which we suppose to be initiated by the formation of the [(mes)₂Fe⁺]˙ radical cation. The structures of both 3PF₆ and 3acCA have been established by X-ray diffraction and the constitution of all compounds isolated is in agreement with elemental analyses, multinuclear NMR data, and MS spectra.