Syntheses and reactivities of polymethylated ferrocenes: The effect of methylation upon electrophilic addition

Abstract

A series of ferrocene derivatives with three, four, five, six and seven methyl group substituents has been prepared and characterized by multinuclear NMR, high resolution mass spectrometry and elemental analyses. Chemical oxidation of heptamethylferrocene with silver tetrafluoroborate produced paramagnetic heptamethylferricenium tetrafluorborate and the structure of one isomer of [C5Me4HFeC5Me3H2][BF4] was determined by single crystal X-ray diffraction. Electrochemical studies of the entire series of iron sandwich compounds from ferrocene to decamethylferrocene reveal a highly linear shift of the reversible half-wave potential (E1/2) for the FeIII/FeII couple such that each additional methyl group lowers E1/2 by an average of 54 mV. A new triple-decker complex has been synthesized by the addition of a ruthenium electrophile, [Cp*Ru(NCMe)3][PF6], (Cp* = C5Me5) to heptamethylferrocene, Me7FcH. While previous work established that reactions between [Cp*Ru(NCMe)3][PF6] and ferrocene derivatives with eight or more methyl group substituents readily form bimetallic complexes, reactions with ferrocene derivatives with six or fewer methyl groups brought about ring abstraction and afforded only monometallic products. Thus it may be concluded that there is a “tipping point” in the series of polymethylated ferrocenes such that the electron donating ability and/or steric influence of seven methyl groups is just sufficient to stabilize a ruthenium-iron tripledecker complex.