Molecular Bismuth Cations: Assessment of Soft Lewis Acidity

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.