Coordination of Pnictogenylboranes Towards Tl(I) Salts and a Tl- Mediated P-P Coupling

Abstract

The coordination chemistry of only Lewis-base (LB)-stabilized pnictogenylboranes EH₂ BH₂ ⋅NMe₃ (E=P, As) towards Tl(I) salts has been studied. The reaction of Tl[BAr^Cl ] (BAr^Cl =[B(3,5-C₆ H₃ Cl₂ )₄ ]^- ) with the corresponding pnictogenylborane results in the formation of [Tl(EH₂ BH₂ ⋅NMe₃ )][BAr^Cl ] (1 a: E=P; 1 b: E=As). Whereas the Tl ion in 1 a/b is monocoordinated, the exchange of the weakly coordinating anion (WCA) in the Tl(I) salt leads to the formation of a trigonal pyramidal coordination mode at the Tl atom by coordination of three equivalents of EH₂ BH₂ ⋅ NMe₃ in [Tl(EH₂ BH₂ ⋅ NMe₃ )₃ ][WCA] (2 a: E=P, WCA=TEF^Cl ; 2 b: E=As, WCA=TEF) (TEF=[Al{OC(CF₃ )₃ }₄ ]^- , TEF^Cl =[Al{(OC(CF₃ )₂ (CCl₃ )}₄ ]^- ). Furthermore, by using two equivalents of PH₂ BH₂ ⋅NMe₃ , a Tl(I)-mediated P-P coupling takes place in CH₂ Cl₂ as solvent resulting in [Me₃ N⋅BH₂ PH₂ PHBH₂ ⋅NMe₃ ][WCA] (WCA=TEF, 3 a; BAr^Cl , 3 b; TEF^Cl , 3 c). In contrast, for the arsenic derivatives 1 b and 2 b, no coupling reaction is observed. The underlying chemical processes are elucidated by quantum chemical computations.

Keywords: boron; main group elements; phosphorus; quantum chemical computation; thallium; weakly coordinating anions.