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

Abstract

[(acridine)BCl 2 ][AlCl 4 ] was synthesized by halide abstraction from (acridine)BCl 3 with AlCl 3 . The hydride ion affinity of the C9 position in [(acridine)BCl 2 ] + was calculated to be 14 kcal mol –1 greater than that at boron. [(acridine)BCl 2 ][AlCl 4 ] reacts with 1 equiv of acridine to form the strained boronium cation [(acridine) 2 BCl 2 ][AlCl 4 ] 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)BCl 2 ][AlCl 4 ] and 2,4,6-tri- tert -butylpyridine (TBP), which heterolytically activated H 2 at 100 °C. The ultimate location of the hydride from H 2 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)BCl 2 ][AlCl 4 ]/TBP FLP or to [(acridine) 2 BCl 2 ][AlCl 4 ], with arylation of acridine at C9 observed for both.