Multistep Redox Processes and Intramolecular Charge Transfer in Ferrocene-Based 2,2‘-Bipyridylboronium Salts

Abstract

A one-step high-yield synthesis of donor-acceptor complexes [1A]X to [1D]X, [2A]X 2 , and [4A]X 4 with one, two, and four cationic [B(R)bipy] + acceptors (R ) Me, Br, OEt, NC 4 H 8 ; X) Br, PF 6 ; bipy ) 2,2′-bipyridine) covalently attached to a ferrocene donor is described. Apart from 2,2′-bipyridine, 1,10-phenanthroline and 2,2′-bipyrimidine have also been used as chelating amines. With the exception of R ) Br, water-stable compounds are obtained in all cases. In DMF solution and under an inert atmosphere, [1A]PF 6 , [2A](PF 6 ) 2 , and [4A]-(PF 6 ) 4 behave as reversible three-step redox systems, capable of storing three, five, and nine electrons, respectively. In their cationic state, the complexes possess an intense purple color, which can be attributed to charge-transfer interactions between the ferrocene unit and the electron-poor B(R)bipy substituent(s). This is confirmed by the ESR spectrum of the monoreduced species [1A] 0 , which features a line shape indicating considerable admixture of the ligand and metal orbitals.

Electronic Communication in a Novel Five-Step Redox System

“…However, on the basis of relative peak heights in the cyclic voltammogram, it is reasonable to assume that the second cathodic couple also involves two subsequent one-electron additions (E°′ ) -1.55 and -1.67 V). In agreement with previous findings, 12 we assign the two couples of reductions as centered on the two bipyridylboronium groups. In contrast to [5] 2+ , the reduction of the unbridged analogue [2a] 2+ occurs through two single stepped two-electron processes (E°′ ) -0.98 and -1.71 V), which excludes the presence of electrochemically detectable through-space interactions.…”

“…Following this strategy, we have synthesized cations [1] + -[3] 4+ , in which up to four 2,2′-bipyridylboronium substituents, [B(Me)bipy] + , are attached to a ferrocene core ( Figure 1). 12 Similar to the well-known Diquat, 2,2′-bipyridylboronium ions [4] + were found to be perfectly reversible two-step redox systems. 13 Consequently, [1] + -[3] 4+ behave as highly redox-active species capable of storing three, five, and nine electrons, respectively.…”

“…[1][2][3][4][5][6][7][8] Our group has recently suggested a novel concept for the generation of charge transfer polymers 9-11 and highly efficient electron sponges 12 exploiting the spontaneous formation of boron-nitrogen and boronoxygen bonds. Following this strategy, we have synthesized cations [1] + - [3] 4+ , in which up to four 2,2′-bipyridylboronium substituents, [B(Me)bipy] + , are attached to a ferrocene core ( Figure 1).…”

Unsymmetrically Substituted 9,10‐Dihydro‐9,10‐diboraanthracenes as Versatile Building Blocks for Boron‐Doped π‐Conjugated Systems

“…Most of the 1 H NMR signals of the coordinating 2,2'-bipyridyl ligand appear at lower field than those of the free base. The same is true for most of the 13 C NMR signals, a characteristic exception [25] being the resonance of bipyC-2,2', which experiences an upfield shift of 9.3 ppm in 11. The X-ray crystal structure analysis of 11 is in full accord with the structure established by NMR spectroscopy (see the Supporting Information).…”

C5H4BR2 Bending in Ferrocenylboranes: A Delocalized Through‐Space Interaction Between Iron and Boron

“…Compound 4 (orthorhombic, Pbca ; Figure 2) was chosen for a comparison of the structural characteristics of a ferrocenylborane possessing a three‐coordinate boron center and its respective Lewis base adduct with a four‐coordinate boryl group ( 4 a ; monoclinic, P 2 1 / c ; Figure 3). Bromoboranes R 2 BBr, such as 2 and 3 , are not suitable for this kind of investigation, because they tend to form boronium salts [R 2 B(LB) 2 ] + Br − upon reaction with strong Lewis bases (LB) 27. The free Lewis acid 4 exhibits a dip angle α * of 13.0°, which is reduced to a value of α * = 1.1° in the BN adduct 4 a .…”