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.