[(acridine)BCl2]+: A Borenium Cation That Is a Strong Boron- and Carbon-Based Lewis Acid

[(acridine)­BCl2]­[AlCl4] was synthesized by halide abstraction from (acridine)­BCl3 with AlCl3. The hydride ion affinity of the C9 position in [(acridine)­BCl2]+ was calculated to be 14 kcal mol–1 greater than that at boron. [(acridine)­BCl2]­[AlCl4] reacts with 1 equiv of acridine to form the strained boronium cation [(acridine)2BCl2]­[AlCl4] and with P­(mesityl)3 by photoinduced one-electron transfer to form the 9,9′-biacridane moiety by radical coupling. A stable frustrated Lewis pair (FLP) was formed on combining [(acridine)­BCl2]­[AlCl4] and 2,4,6-tri-tert-butylpyridine (TBP), which heterolytically activated H2 at 100 °C. The ultimate location of the hydride from H2 activation was the C9 position of acridine and not boron. Carbon Lewis acid based reactivity also occurred when thieno­[3,2-b]­thiophene was added to the [(acridine)­BCl2]­[AlCl4]/TBP FLP or to [(acridine)2BCl2]­[AlCl4], with arylation of acridine at C9 observed for both.