Methylbismuth: an organometallic bismuthinidene biradical

Abstract

We report the generation, spectroscopic characterization, and computational analysis of the first free (non-stabilized) organometallic bismuthinidene, BiMe. The title compound was generated in situ from BiMe₃ by controlled homolytic Bi-C bond cleavage in the gas phase. Its electronic structure was characterized by a combination of photoion mass-selected threshold photoelectron spectroscopy and DFT as well as multi-reference computations. A triplet ground state was identified and an ionization energy (IE) of 7.88 eV was experimentally determined. Methyl abstraction from BiMe₃ to give [BiMe₂]^• is a key step in the generation of BiMe. We reaveal a bond dissociation energy of 210 ± 7 kJ mol^-1, which is substantially higher than the previously accepted value. Nevertheless, the homolytic cleavage of Me-BiMe₂ bonds could be achieved at moderate temperatures (60-120 °C) in the condensed phase, suggesting that [BiMe₂]^• and BiMe are accessible as reactive intermediates under these conditions.

Scheme 1. Low-valent group 15 compounds: (a) isolable nitrene and phosphinidene (singlet ground state). (b) stabilized organometallic bismuthinidenes (singlet ground state; occupied bismuth 6s-orbital omitted for clarity); (c) free (non-stabilized) singlet and triplet species that may be envisaged for BiMe (only two bismuth p-orbitals shown; p-orbital used for Bi–Me bonding and occupied s-orbital are omitted for clarity). R = 2,6-Me₂-C₆H₃; 2,6-iPr₂-C₆H₃; R′ = 2,6-[(4-tBu-C₆H₄)₂CH]₂-4-Me-C₆H₃.

Fig. 1. Photoionization mass spectra recorded at 8.5 eV without pyrolysis (top trace), low pyrolysis power (T ≈ 470 K, center trace) and medium pyrolysis power (T ≈ 600 K, bottom trace). The signal at m/z = 91 is due to a background signal from previous experiments.

Scheme 2. Controlled, stepwise abstraction of CH₃ radicals from 1 in the gas phase by flash pyrolysis.

Fig. 2. Mass-selected threshold photoelectron spectrum of m/z = 224. The simulation based on 3 as the carrier (blue line) fits the experiment well. Transitions into the X^{+ 2}A′′ ground state of the ion are given as grey bars, transitions into the A^{+ 2}A′ state as red bars.

Fig. 3. Ms-TPE spectrum of Bi(CH₃)₂, m/z = 239. The vibrational progression is due to the symmetric bismuth–carbon stretch and its combination with torsional motion. The blue line represents the simulated spectrum based on 2.

Fig. 4. Abstraction and spin trapping of methyl radicals from BiMe₃.

Fig. 5. (a) Synthesis of BiMe(SPh)₂ (8) from BiMe₃ and (SPh)₂ with methane as a detected by-product. (b and c) Molecular structure of 8 in the solid state with one formula unit shown in (b) and a cutout of the coordination polymer shown in (c). Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms and one set of split positions of disordered atoms are omitted for clarity. For detailed discussion of structural parameters see ESI. Selected bond lengths (Å) and angles (°): Bi1–C1, 2.208(10); Bi1–S1, 2.736(2); Bi1–S2, 2.699(2); C1–Bi1–S1, 89.6(3); C1–Bi1–S2, 89.4(3); S1–Bi1–S2, 93.18(7).