P-substituted phosphine–boranes: Gas phase acidities, basicities and dihydrogen release. A comparison to amine–boranes

Abstract

We present the gas phase stabilities, acidities and basicities for several previously uninvestigated phosphine–boranes, and compare them to respective amine–boranes. Although both species seem to be very similar, it was found that only acidities correlate in a reasonable manner (R2=0.874). According to MP2/6-311+G(d,p) calculations the P-substitutions increase the stability of the H3P⋯BH3 complex up to 14kcal/mol. Phosphorine–borane was the only complex less stable than the unsubstituted H3P⋯BH3. Replacing the phosphine with respective amine decreases the acidity but raises the basicity of the complexes. A complex that can be described as R1R2R3P⋯BH2+⋯H2 forms upon protonation and the dihydrogen removal from such species is an exergonic process. Typical ΔG for the dissociation reaction ranges from 0.1 to −2.6kcal/mol but there are notable exceptions. For example, the protonated benzyl phosphine–borane contains a B⋯H–H⋯π interaction which is replaced with stronger B–Cπ interaction upon hydrogen removal thus leading to more favorable H2 removal. The bonds and interactions present in protonated species have been studied using Bader’s Atoms in Molecules approach.