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

Abstract

The primary phosphines MesPH₂ and tBuPH₂ react with 9-iodo-m-carborane yielding B9-connected secondary carboranylphosphines 1,7-H₂ C₂ B₁₀ H₉ -9-PHR (R=2,4,6-Me₃ C₆ H₂ (Mes; 1 a), tBu (1 b)). Addition of tris(pentafluorophenyl)borane (BCF) to 1 a, b resulted in the zwitterionic compounds 1,7-H₂ C₂ B₁₀ H₉ -9-PHR(p-C₆ F₄ )BF(C₆ F₅ )₂ (2 a, b) through nucleophilic para substitution of a C₆ F₅ ring followed by fluoride transfer to boron. Further reaction with Me₂ SiHCl prompted a H-F exchange yielding the zwitterionic compounds 1,7-H₂ C₂ B₁₀ H₉ -9-PHR(p-C₆ F₄ )BH(C₆ F₅ )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-H₂ C₂ B₁₀ H₉ -9-PR(p-C₆ F₄ )B(C₆ F₅ )₂ (4 a, b). Hydrolysis of the B-C₆ F₄ bond in 4 a, b gave the first tertiary B-carboranyl phosphines with three distinct substituents, 1,7-H₂ C₂ B₁₀ H₉ -9-PR(p-C₆ F₄ H) (5 a, b). Deprotonation of the zwitterionic compounds 2 a, b and 3 a, b formed anionic phosphines [1,7-H₂ C₂ B₁₀ H₉ -9-PR(p-C₆ F₄ )BX(C₆ F₅ )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-H₂ C₂ B₁₀ H₉ -9-PR(p-C₆ F₄ )BR'(C₆ F₅ )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(Et₂ 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.

Keywords: anions; carboranes; dihydrogen activation; frustrated Lewis pairs; phosphorus; zwitterions.

Figure 1

Dichotomy regarding the electronic effect of a carboranyl moiety on phosphine substituents.

Figure 2

Known examples of carborane derivatives studied in the context of FLPs and/or featuring an intramolecular FLP.

Figure 3

Selected examples of C₆F₄‐bridged zwitterionic compounds in FLP chemistry.

Scheme 1

Syntheses of compounds 1  a, b, 1  a‐BH₃, 1  b‐BH₃ and 2  a, b. The ³¹P{¹H} NMR signal of 2  b is displayed as the sum of a 1 : 1 : 1 : 1 quartet (blue; ³¹P−¹¹B coupling) and a 1 : 1 : 1 : 1 : 1 : 1 : 1 septet (red; ³¹P−¹⁰B coupling). Compounds 1  a and 1  b, as well as all compounds derived from them, are P‐chiral and were obtained as racemic mixtures. For clarity, in this and all following figures and schemes, only one enantiomer is shown.

Figure 4

Molecular structures of 1  a (left), 1  a‐BH₃ (middle) and 1  b‐BH₃ (right) in the solid state with thermal ellipsoids drawn at the 50 % probability level. Only the non‐oxidized molecule in the asymmetric unit is depicted for 1  a, and in the case of 1  b‐BH₃, only the molecule with the highest site occupancy factor (0.92) is depicted. Hydrogen atoms (except for PH and BH₃) are omitted for clarity.

Figure 5

Molecular structures of 2  a (left) and 3  a (right) in the solid state with thermal ellipsoids drawn at the 50 % probability level. The carboranyl, mesityl and C₆F₅ substituents are represented as wire frames. The hydrogen atoms (except for PH and BH groups other than the carboranyl moiety) and disorders are omitted for clarity. The dashed lines show the intermolecular PH⋅⋅⋅FB or PH⋅⋅⋅FC interactions.

Scheme 2

Syntheses of compounds 3  a, b–9  a, b.

Figure 6

Molecular structure of 3  b in the solid state with thermal ellipsoids drawn at the 50 % probability level. The carboranyl and C₆F₅ substituents are represented as wire frames, and the hydrogen atoms (except for PH and BH other than the carboranyl moiety) are omitted for clarity.

Scheme 3

Proposed reaction mechanism (RIJK‐DSD PBEP86‐D3BJ/def2 TZVP energies relative to the reactant are shown in kJ mol⁻¹).

Scheme 4

Acid–base equilibrium of 3  a and 7  a with various amines (B). B=triethylamine, 4‐Br‐aniline, N‐Me‐aniline, aniline, pyridine and 2‐MeO‐pyridine.

Scheme 5

Hydrogenation of imine tBuN=C(H)Ph with 3  a, b and subsequent formation of the phosphine borates 10  a, b.