Figure 3.

(a) ¹H NMR spectrum of a CD₃CN solution of BBR^4(+•) that was prepared in situ by the addition of 4 equiv of Cp₂Co to a solution of BBR·8PF₆. The spectrum is abridged in the region of the residual CD₂HCN and HOD signals, and the height of the ^tBu resonance is truncated. See Figure S4 for the full spectrum. (b) Partial ¹H NMR spectra, collected over a temperature range of −20 to −57 °C, of a solution of BBR^4(+•) in CD₃COCD₃ that was prepared in situ by stirring a solution of BBR·8PF₆ over Zn dust. Although the resonances arising from the BBR^4(+•) cation are best resolved in the spectrum collected at −57 °C, signal H is obscured by the H₂O/HOD signal at this temperature. In the interest of preserving space, the ^tBu resonance of BBR^4(+•) is not displayed. See Figure S14 for the full ¹H NMR spectrum recorded at −57 °C.