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

Abstract

A series of mixed-valent (MV) complexes [(FeCp)2(mu-C10H6(BPh)2)]+X ([1+]X; X=I 5, PF6, SbF6, B(C6F5)4) were prepared by oxidation of diboradiferrocene [(FeCp)2(mu-C10H6(BPh)2)] (1) with I 2, AgPF6, and AgSbF6, respectively, and through anion exchange of the I 5(-) salt with [Li(Et2O)x][B(C6F5)4] in the case of X=B(C6F5)4. The MV state of the cation was investigated in solution by multinuclear NMR spectroscopy, CV, and UV/Vis-NIR absorption spectroscopy, and in the solid state by IR spectroscopy, single-crystal X-ray crystallography, and Mössbauer spectroscopy. The cyclic voltammogram of 1 shows two distinct redox waves with a large redox splitting of Delta E=510 mV in CH2Cl2 and the NIR spectrum for the mono-oxidized species displays an intervalence charge-transfer band at around 1500 to 1700 nm depending on the specific counterion present. The X-ray crystal structures of [1+]X show inversion-symmetric cations with X=I 5 and B(C6F5)4 and unsymmetric valence-trapped structures composed of one ferrocene and one ferrocenium moiety with X=PF6 and SbF6. Mössbauer data for X=PF6 are consistent with valence trapping at all temperatures between 90 and 343 K. In comparison, fast electron transfer is evident on the Mössbauer timescale for X=I 5 and temperature-dependent behavior is observed for X=B(C6F5)4. The anion dependence of the X-ray structural and Mössbauer data is discussed in the context of crystal symmetry and the possibility of static and dynamic disorder effects is considered.

Metal atom dynamics in four triferrocenylmethane derivatives and the crystal structure of Fc3COH

“…(b) 57 Fe M össbauer effect (ME) spectroscopy: The air stable powdered compounds were mixed with a small quantity of BN (to ensure random crystallite orientation) and examined in transmission geometry at various temperatures as detailed earlier. 7,8 Temperature was monitored using the Daswin 9 software and the transmission rate was monitored before and after each data point acquisition. Spectrometer calibration was effected using a 20 mg cm -2 a-Fe absorber at room temperature, and all isomer shifts (IS) are reported with respect to the centroid of such spectra.…”

Synthesis and supramolecular assembly of the bifunctional borinic acid [1,2-fcB(OH)]2

“…1 Among the most common boron heterocycles are borole (A) 2 and diborabenzene (B), 3 as well as their benzannulated congeners, the borafluorene (C) 4 and diboraanthracene (D) 5 derivatives. We have investigated in detail the corresponding redox-active ferrocene-fused diborabenzene systems E. [6][7][8][9][10] We have developed several synthetic routes to compounds of this type, using 1,1′-distannylated and 1,2-dimercurated ferrocene precursors. 8,10 An especially noteworthy property of compounds E is the strong electronic coupling between the ferrocene moieties across the electron-deficient diboron bridge.…”

“…8,10 An especially noteworthy property of compounds E is the strong electronic coupling between the ferrocene moieties across the electron-deficient diboron bridge. 7,8 Moreover, the Lewis acidity of the borane moieties is significantly enhanced by oxidation of the appended ferrocenes. 7,9 We recently became interested in incorporating these redox-active Lewis acids into supramolecular polymers, taking advantage of hydrogen bonding and reversible covalent B-O bond formation.…”

Intervalence charge transfer across noncovalent interactions on vinyl silyl bridged biferrocenyl compounds

“…Among all metallocenes, ferrocene (Fc) is one of the most commonly used as building block, as its redox behavior is very well described in literature. It is because of this well-behaved redox chemistry of ferrocene that it constitutes an ideal test molecule to study the possibilities of new molecular bridges that link several metallocene units [4][5][6][7][8][9][10].…”