Heavier Diferrocenylpnictogenium Ions

Abstract

The synthesis, characterization and reactivity of the diferrocenylphosphenium ion [Fc₂P]⁺ was extended to the heavier diferrocenylpnictogenium ions, [Fc₂E]⁺ (E=As, Sb, Bi). The lighter diferrocenylnitrenium ion, [Fc₂N]⁺, was detected by mass spectrometry, but could not be isolated. The molecular structures of [Fc₂E]⁺ (E=P, As, Sb, Bi) reveal intramolecular coordination of the iron atoms, which counterbalance the electron deficiency of the pnictogens without affecting the strong Lewis acidities, which were determined according to the method of Gutmann and Beckett. The (electro-)chemical oxidation and reduction afforded the dications [Fc₂E]²⁺ (E=P (unstable), As) and the neutral dipnictogens Fc₂EEFc₂ (E=P, As).

Keywords: Carbene analogues; Cations; Ferrocene; Lewis acids; Pnictogenium ions.

Scheme 1

Ferrocenyl stabilized main group Lewis acids.

Scheme 2

Synthesis of the diferrocenylpnictogenium ions 1E[B(C₆F₅)₄] (E=P, As, Sb, Bi).

Figure 1

Molecular structures of [Fc₂E]⁺ (1E, E=P, As, Sb, Bi) showing 50 % probability ellipsoids and the essential atomic numbering scheme. Hydrogen atoms and [B(C₆F₅)₄]⁻ counterions are omitted for clarity.

Figure 2

⁵⁷Fe Mößbauer spectra of 1E[B(C₆F₅)₄] (E=As, Sb, Bi) and 5As[B(C₆F₅)₄]₂, collected in solid state at 80 K. Red and blue lines represent the individual subspectra, the black line is the overall fitting curve.

Figure 3

Normalized UV‐vis absorption spectra and absorption maxima of 1E[B(C₆F₅)₄] (E=P (pink), λ_max=504.4, 398.5, 341.3 nm; E=As (blue) λ_max=515.9, 327.1 nm; E=Sb (yellow) λ_max=503.8 nm; E=Bi (dark red) λ_max=548.1 nm).

Figure 4

Electrospray mass spectrum (CH₂Cl₂, positive mode) of reaction mixture of Fc₂NH with [Ph₃C][B(C₆F₅)₄].

Scheme 3

Reduction of the pnictogenium ions [Fc₂E][B(C₆F₅)₄] (1E, E=P, As).

Figure 5

Molecular structures of Fc₂EEFc₂ (4E, E=P, As) showing 50 % probability ellipsoids and the essential atomic numbering scheme. Hydrogen atoms are omitted for clarity.

Scheme 4

Oxidation of Fc₂EEFc₂ (4E, E=P, As).

Scheme 5

Oxidation of [Fc₂E][B(C₆F₅)₄] (1E, E=P, As).

Figure 6

Normalized UV‐vis absorption spectra of 1As (blue) and 5As (green).

Scheme 6

Possible resonance structures of [Fc₂E]²⁺ (5E, E=P, As).

Figure 7

Spin density of 5As, representation at +/−0.02 a.u. (blue/green).

Figure 8

Cyclic voltammograms (0.5 mM, CH₂Cl₂, [n‐Bu₄N][B(C₆F₅)₄]) of A) 1P and 4P, B) 1As and 4As, C) 1Sb and 1Bi, potential sweep rate 50 mV s⁻¹. Plotting convention: IUPAC (direction of the scan: toward positive potentials (to the right); initial potential: −0.1 V).