Competition between different nucleophilic sites belonging to the same Lewis bases. Part 3. The basicity of arsenic and phosphorus sites relative to borane

Abstract

After establishing some structural characteristics of a few simple AsR₃·BH₃ adducts (R = Me, Et, or Ph) by n.m.r. spectroscopy, the reaction of BH₃ with a ligand (L) containing both phosphorus and arsenic donor sites (Ph₂PCH₂CH₂AsPh₂) has been studied. The superior basicity of phosphorus as compared to arsenic in these adducts is shown by displacement reactions and a calorimetric study. Enthalpic data (in kJ mol^–1) obtained for the gas-liquid reactions (i) and (ii) are ΔH₁=–160.4 and ΔH₂, =–153.4 for M = P, and ΔH₁=–76.1 and ΔH₂=–111.3 2MPh₃(sol)+ B₂H₆(g) [graphic omitted] 2MPh₃·BH₃(sol)(i), MPh₃(sol)+ BH₃(g) [graphic omitted] MPh₃·BH₃(sol)(ii) for M = As. In the case of L, two stages of compiexation may be separated and the corresponding enthalpies (kJ mol^–1) are ΔH₁=–165.3 (2L + B₂H₆) and ΔH₂=–155.6 (L + BH₃) corresponding to complexation of the phosphorus site, and ΔH₁=–60.7 (2L·BH₃+ B₂H₆) and ΔH₂=–103.3 (L·BH₃+ BH₃) corresponding to complexation of the arsenic site.