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

Reactions between the mesitylene (mes) dication [(η(6)-mes)(2)Fe](2+) (1a) [(PF(6)(-))(2) salt] and lithium o-carboranes Li[1-R-1,2-C(2)B(10)H(11)] (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 [(η(5)-mes-exo-6-{2-R-1,2-C(2)B(10)H(11)})Fe(η(6)-mes)](+) (3) (R = H, 3a; Me, 3b; Ph, 3c) which were isolated as PF(6)(-) salts (3PF(6)) in yields ranging 50-75%. Individual complexes were obtained on purification by LC or preparative TLC on a silica gel substrate, using MeCN-CH(2)Cl(2) mixtures as the mobile phase. Interestingly, the room-temperature reaction between 2a (threefold excess) and 1a(PF(6))(2) with a reverse order of addition of the reaction components yielded an orange salt [(η(5)-mes-exo-6-{1,2-C(2)B(10)H(11)})Fe(η(6)-mes)](+)[closo-nido-H(11)B(10)C(2)-C(2)B(10)H(12)](-) (3acCA) (cCA = conjucto-carborane anion = [closo-nido-H(11)B(10)C(2)-C(2)B(10)H(12)](-)) 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)(2)Fe(+)]˙ radical cation. The structures of both 3PF(6) 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.