Exploring the Reactivity of B‐Connected Carboranylphosphines in Frustrated Lewis Pair Chemistry: A New Frame for a Classic System

The primary phosphines MesPH(2) and tBuPH(2) react with 9‐iodo‐m‐carborane yielding B9‐connected secondary carboranylphosphines 1,7‐H(2)C(2)B(10)H(9)‐9‐PHR (R=2,4,6‐Me(3)C(6)H(2) (Mes; 1 a), tBu (1 b)). Addition of tris(pentafluorophenyl)borane (BCF) to 1 a, b resulted in the zwitterionic compounds 1,7‐H(2)C(2)B(10)H(9)‐9‐PHR(p‐C(6)F(4))BF(C(6)F(5))(2) (2 a, b) through nucleophilic para substitution of a C(6)F(5) ring followed by fluoride transfer to boron. Further reaction with Me(2)SiHCl prompted a H−F exchange yielding the zwitterionic compounds 1,7‐H(2)C(2)B(10)H(9)‐9‐PHR(p‐C(6)F(4))BH(C(6)F(5))(2) (3 a, b). The reaction of 2 a, b with one equivalent of R'MgBr (R’=Me, Ph) gave the extremely water‐sensitive frustrated Lewis pairs 1,7‐H(2)C(2)B(10)H(9)‐9‐PR(p‐C(6)F(4))B(C(6)F(5))(2) (4 a, b). Hydrolysis of the B−C(6)F(4) bond in 4 a, b gave the first tertiary B‐carboranyl phosphines with three distinct substituents, 1,7‐H(2)C(2)B(10)H(9)‐9‐PR(p‐C(6)F(4)H) (5 a, b). Deprotonation of the zwitterionic compounds 2 a, b and 3 a, b formed anionic phosphines [1,7‐H(2)C(2)B(10)H(9)‐9‐PR(p‐C(6)F(4))BX(C(6)F(5))(2)](−)[DMSOH](+) (R=Mes, X=F (6 a), R=tBu, X=F (6 b); R=Mes, X=H (7 a), R=tBu, X=H (7 b)). Reaction of 2 a, b with an excess of Grignard reagents resulted in the addition of R’ at the boron atom yielding the anions [1,7‐H(2)C(2)B(10)H(9)‐9‐PR(p‐C(6)F(4))BR’(C(6)F(5))(2)](−) (R=Mes, R’=Me (8 a), R=tBu, R’=Me (8 b); R=Mes, R’=Ph (9 a), R=tBu, R’=Ph (9 b)) with [MgBr(Et(2)O)( n )](+) as counterion. The ability of the zwitterionic compounds 3 a, b to hydrogenate imines as well as the Brønsted acidity of 3 a were investigated.