Molecular Bismuth Cations: Assessment of Soft Lewis Acidity

Abstract

Three-coordinate cationic bismuth compounds [Bi(diaryl)(EPMe₃ )][SbF₆ ] have been isolated and fully characterized (diaryl=[(C₆ H₄ )₂ C₂ H₂ ]^2- , E=S, Se). They represent rare examples of molecular complexes with Bi⋅⋅⋅EPR₃ interactions (R=monoanionic substituent). The ³¹ P NMR chemical shift of EPMe₃ has been found to be sensitive to the formation of LA⋅⋅⋅EPMe₃ Lewis acid/base interactions (LA=Lewis acid). This corresponds to a modification of the Gutmann-Beckett method and reveals information about the hardness/softness of the Lewis acid under investigation. A series of organobismuth compounds, bismuth halides, and cationic bismuth species have been investigated with this approach and compared to traditional group 13 and cationic group 14 Lewis acids. Especially cationic bismuth species have been shown to be potent soft Lewis acids that may prefer Lewis pair formation with a soft (S/Se-based) rather than a hard (O/N-based) donor. Analytical techniques applied in this work include (heteronuclear) NMR spectroscopy, single-crystal X-ray diffraction analysis, and DFT calculations.

Keywords: HSAB principle; Lewis acids; bismuth; bonding analysis; cationic species.

Scheme 1

Reaction of cationic bismepine 1 with soft donors EPMe₃ to give compounds 2‐EPMe₃ through thf elimination (E=S, Se).

Figure 1

Molecular structures of [Bi(C₆H₄)₂C₂H₂(EPMe₃)][SbF₆] in the solid state: a) E=S: 2‐SPMe₃; b) E=Se: 2‐SePMe₃). Displacement ellipsoids are drawn at the 50 % probability level. Hydrogen atoms and a lattice‐bound CH₂Cl₂ molecule in the structure of 2‐SPMe₃ are omitted for clarity. Selected bond lengths (Å) and bond angles (°): 2‐SPMe₃: Bi1−C1, 2.248(6); Bi1−C14, 2.246(5); Bi1−S1, 2.6105(16); P1−S1, 2.026(2); C1−Bi1−C14, 85.1(2); C1−Bi1−S1, 90.59(15); C14−Bi1−S1, 88.81(14); Bi1−S1−P1, 106.12(8). 2‐SePMe₃: Bi1−C1, 2.237(4); Bi1−C14, 2.240(4); Bi1−Se1, 2.7222(4); P1−Se1, 2.1889(11); C1−Bi1−C14, 87.77(13); C1−Bi1−Se1, 91.24(10); C14−Bi1−Se1, 92.08(10); Bi1−Se1−P1, 100.31(3).

Figure 2

Comparison of acceptor numbers for selected Lewis acids obtained from the Gutmann–Beckett method (top, previous results ^[12] and this work) and modified versions (middle and bottom, this work). “Organobismuth cations” refers to class C compounds (cf. introduction and ref. [12]) without strong directional cation⋅⋅⋅anion interactions.

Figure 3

a) LUMO of [Bi(C₆H₄)₂C₂H₂][SbF₆] with its conformation fixed to that found in the optimized structure of 2‐SePMe₃. b) Selected molecular orbitals (top) and NBOs (bottom) of 2‐SePMe₃ with isovalues of 0.04. c) Resonance structure of the 2‐EPMe₃ according to NRT (E=S, Se; [SbF₆]⁻ omitted for clarity; only one (out of many) resonance structures of the bismepine core is depicted (see text)).