Electronic interactions in oligoferrocenes with cationic, neutral and anionic four-coordinate boron bridges

Dinuclear and trinuclear ferrocene complexes {[Fc(2)BMe(2)] Li, [Fc-BMe2-fc-BMe2-Fc] Li-2, Fc(2)B(pyind), [Fc(2)B(bipy)] PF6, [Fc-B(bipy)-fc-B(bipy)-Fc](PF6)(2)} bearing anionic, uncharged, and cationic four-coordinate boron bridges have been synthesized (Fc: ferrocenyl; fc: 1,1'-ferrocenylene; pyind: 5-fluoro-2-(2'-pyridyl) indolyl; bipy: 2,2'-bipyridyl). The molecular structures of [Fc(2)BMe(2)] Li(12-crown-4)(2), [Fc-BMe2-fc-BMe2-Fc](Li(12-crown-4)(2))(2), Fc(2)B(pyind), and [Fc(2)B(bipy)] PF6 were determined by X-ray crystallography. The anionic aggregates [Fc(2)BMe(2)](-) and [Fc-BMe2-fc-BMe2-Fc](2-) are very sensitive to air and moisture whereas bromide salts of their cationic counterparts [Fc(2)B(bipy)](+) and [Fc-B(bipy)-fc-B(bipy)-Fc](2+) may be dissolved in water without decomposition. Cyclic voltammograms of the diferrocene species show two well-resolved one-electron transitions separated by 0.21 V ([Fc(2)BMe(2)] Li; Edegrees =- 0.43 V, - 0.64 V; vs. FcH/FcH(+)), 0.18 V (Fc(2)B(pyind); Edegrees' =- 0.03 V, - 0.21 V), and 0.16 V ([Fc(2)B(bipy)] PF6; Edegrees' =+ 0.23 V, + 0.07 V), which indicates electronic interactions between the two ferrocenyl substituents. Two redox waves with an intensity ratio of 1 : 2 are observed in the cyclic voltammograms of the trinuclear derivatives [Fc-BMe2-fc-BMe2-Fc] Li-2 and [Fc-B(bipy)-fc-B(bipy)-Fc](PF6)(2). In the case of the BMe2-bridged species, the electrochemically unique central ferrocenylene unit is oxidized at a much more cathodic potential value (Edegrees' =- 1.21 V) than the two terminal ferrocenyl substituents (Edegrees' =- 0.51 V). The opposite is true in the case of the B(bipy)-bridged trimer where oxidation of the terminal ferrocenyl groups (Edegrees' =+ 0.03 V) precedes oxidation of the internal iron atom (Edegrees' =+ 0.26 V). The Fe(II)/ Fe(III) redox potentials of the mono- and dianionic species differ to a much larger extent from the redox potential of parent ferrocene (Edegrees' = 0 V) than the Edegrees' values of the corresponding mono- and dicationic derivatives. Apart from electrostatic interactions, the electrochemical properties of BMe2- and B(bipy)-bridged oligoferrocenes are determined by the pronounced positive inductive effect of triorganoborate substituents together with positive sigma/pi* hyperconjugation on the one hand and ferrocene-to-B(bipy) charge transfer on the other.