An Inorganic Analogue of Borylcyclopentadienide

Cyclopentadienide (Cp^-) is a ubiquitous species in organic and organometallic chemistry. Its derivatization has been investigated enthusiastically to impart an additional function to the molecules. For instance, replacement of the skeletal carbon atoms by boron and nitrogen atoms yields 1,2-diaza-3,5-diborolanide (CB₂N₂^-), which has recently been focused on as an inorganic analogue of Cp^-. In addition, the introduction of heteroatoms onto the skeletal carbon atom can be another strategy to vary the properties of Cp^-. Among them, boryl-substituted Cp^- (boryl-Cp^-) has interesting properties originated by the π-accepting nature of the boryl group, such as the contribution of the borataalkene structure. In this communication, we prepared the first potassium salt of boryl-substituted CB₂N₂^- (boryl-CB₂N₂^-) <b>3</b>. The solution- and solid-state structures of <b>3</b> were determined by NMR and single crystal X-ray diffraction analyses, respectively. In the crystal structure, the C-B bond lengths are lying in the range of C-B bond lengths for the reported borataalkene species. Computational studies suggested that the C-B(O₂C₆H₄) bond in <b>3</b> is likely a polarized single bond rather than a double bond. Compound <b>3</b> can serve as a C-centered anion, which was demonstrated by protonation. In addition, competitive reaction between compounds <b>3</b> and <b>1</b>_<b>H</b> revealed that boryl-CB₂N₂^- <b>3</b> is less basic than hydrogen-substituted CB₂N₂^- <b>1</b>_<b>H</b>.