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

Abstract

Dinuclear and trinuclear ferrocene complexes {[Fc2BMe2]Li, [Fc-BMe2-fc-BMe2-Fc]Li2, Fc2B(pyind), [Fc2B(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 [Fc2BMe2]Li(12-crown-4)2, [Fc-BMe2-fc-BMe2-Fc](Li(12-crown-4)2)2, Fc2B(pyind), and [Fc2B(bipy)]PF6 were determined by X-ray crystallography. The anionic aggregates [Fc2BMe2]- 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 ([Fc2BMe2]Li; Eo' = -0.43 V, -0.64 V; vs. FcH/FcH+), 0.18 V (Fc2B(pyind); Eo' = -0.03 V, -0.21 V), and 0.16 V ([Fc2B(bipy)]PF6; Eo' = +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]Li2 and [Fc-B(bipy)-fc-B(bipy)-Fc](PF6)2. In the case of the BMe(2)-bridged species, the electrochemically unique central ferrocenylene unit is oxidized at a much more cathodic potential value (Eo' = -1.21 V) than the two terminal ferrocenyl substituents (Eo' = -0.51 V). The opposite is true in the case of the B(bipy)-bridged trimer where oxidation of the terminal ferrocenyl groups (Eo' = +0.03 V) precedes oxidation of the internal iron atom (Eo' = +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 (Eo' = 0 V) than the Eo' 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.

Reversible Expansion and Contraction of a 1,2‐Diborylated Ferrocene Dimer Promoted by Redox Chemistry and Nucleophile Binding

“…Lewis acid/Lewis base binding offers yet another opportunity to trigger conformational changes in 3 14. 26 Adducts 5 a and 5 b were obtained upon treatment of 3 with two equivalents of pyridine (py) and tert ‐butylpyridine ( t Bu‐py), respectively 21. Weak binding of pyridine to 3 is evident from partial dissociation into the monoadducts 4 and the free acid 3 in solution.…”

Examination of the Mixed‐Valence State of the Doubly Boron‐Bridged Diferrocene Cation [(FeCp)2{μ‐C10H6(BPh)2}]+

“…[26][27][28] Boron-bridged oligo-and polyferrocenes are particularly intriguing owing to the opportunity to switch between tri-and tetracoordinate boron through the binding of nucleophiles, which in turn influences the degree of electronic communication between the ferrocene moieties. [28][29][30] An early example of a MV boron-bridged oligoferrocene is the zwitterionic borate Fc II 3 Fc III B (Fc= ferrocenyl), which features a tetra-coordinate boron bridge. Although an intervalence charge-transfer (IVCT) band was detected in the NIR region, a localized structure was identified in the solid state by X-ray diffraction.…”

Synthesis, Structures and Comparison of Neutral Complexes Formed by2‐(2′‐Pyridyl)indole and d6 Transition Metals

“…[6][7][8][9] Recent work has concerned not only pyind complexes as catalysts in hydroarylation [10] and the Heck reaction, [11] but also its use in optical devices. [12][13][14][15][16] It has also been used in an organometallic mimic for the protein kinase staurosporin. [17][18][19][20][21] The authors of the latter complex pointed out the "solely structural role" of an inert ruthenium complex in the biological activity of this pyind derivative, and they achieved enhanced activity and selectivity of the biologically active centre by modifying the distant ruthenium's ligand sphere.…”