Electronic communication in oligonuclear ferrocene complexes with anionic four-coordinate boron bridges

Abstract

The di- and trinuclear ferrocene species Li[Fc-BPh(2)-Fc] (Li[]) and Li(2)[Fc-BPh(2)-fc-BPh(2)-Fc] (Li(2)[]) have been investigated with regard to their electrochemical properties and the degree of intervalence charge-transfer after partial oxidation. Li[] shows two distinct one-electron redox waves for its chemically equivalent ferrocenyl substituents in the cyclic voltammogram (E(1/2) = -0.38 V, -0.64 V; vs. FcH/FcH(+)). The corresponding values of Li(2)[] are E(1/2) = -0.45 V (two-electron process) and -1.18 V. All these redox events are reversible at r. t. on the time scale of cyclic voltammetry. X-ray crystallography on the mixed-valent Fe(II)(2)Fe(III) complex Li(12-c-4)(2)[] reveals the centroid-centroid distance between the cyclopentadienyl rings of each of the terminal ferrocenyl substituents (3.329 A) to be significantly smaller than in the central 1,1'-ferrocenediyl fragment (3.420 A). This points towards a charge-localized structure (on the time scale of X-ray crystallography) with the central iron atom being in the Fe(III) state. Mössbauer spectroscopic measurements on Li(12-c-4)(2)[] lend further support to this interpretation. Spectroelectrochemical measurements on Li[] and Li(2)[] in the wavelength range between 300-2800 nm do not show bands interpretable as intervalence charge-transfer absorptions for the mixed-valent states. All data accumulated so far lead to the conclusion that electronic interaction between the individual Fe atoms in Li[] and Li(2)[] occurs via a through-space pathway and/or is electrostatic in nature.

A Quest for Ligand‐Unsupported Li+–π Interactions in Mono‐, Di‐, and Tritopic Lithium Arylborates

“…We therefore conclude that the attachment of anionic borate substituents may help to stabilize a broader variety of Li + -π interactions. This conclusion is supported by the solid-state structures of IV, [30] V, [31] and VI, [32] which all feature Li + -benzene π coordination ( Figure 1). In IV-VI, the desired structural motif is still enforced by a chelating side arm.…”

“Tail–Tail Dimerization” of Ferrocene Amino Acid Derivatives

“…Oligoferrocenes have attracted enormous interest in the past years because of their applicability in molecular electronics and multi‐redox catalysis 1–4. Linear processablepoly(ferrocenylenes) were pioneered by Manners, who obtained these compounds by the ring‐opening polymerization of ER 2 ‐bridged [1]ferrocenophanes (E = C, Si, Ge, Sn), and Wagner, who used BR n linkers 5–9…”

Unexpected Generation of Diastereomers by Double Diboration of a Dialkyne

“…This is possibly due to the perpendicular orientation of the p orbital at boron towards the conjugated spacer-system, thus insulating it from the ferrocene moieties. [36] It should be added that the B=N double bonds (vide infra) could also reduce the degree of electrochemical communication by lowering the Lewis acidity of the boron centers. [37] Subsequent experiments have shown that the diastereomers 4 and 5 can easily be interconverted.…”