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.