The Role of Chelating Phosphine Rhodium Complexes in Dehydrocoupling Reactions of Amine-Boranes: A Theoretical Investigation Attempting To Rationalize the Observed Behaviors

A quantum-chemical investigation of the dehydrocoupling reaction of the secondary amine-borane Me2HNBH3 assisted by phosphine chelating [Rh(Ph2P(CH2)n-PPh2)(C6H5F)]+ (n = 3–5) complexes to ultimately afford the cyclic dimer [Me2NBH2]2 is reported. The hypothesis, proposed on the basis of experimental evidence, that the catalytic efficiency of such systems is due to formation of Rh(III) dihydride complexes, which rapidly lose H2 and reform Rh(I) species, has been explored, together with the influence that the structure of the ligand (namely, the chelating phosphine P–Rh–P bite angle) has on the rate of the reaction. Along the pathway that our computational analysis has indicated as the most likely, the first step of the dehydrogenation reaction is the concerted B–H hydride and N–H proton transfer from an additional amine-borane molecule to the rhodium center of the formed [Rh(Ph2P(CH2)n-PPh2)(η2-Me2HNBH3)]+ complexes. The reaction proceeds by formation of dihydrogen complexes, which eliminate molecular hydrogen and restore the σ-amine-borane complexes. The impact of the bite angle on the kinetics has been rationalized in terms of both the distortions to the geometry of stationary points around the metal center and the strength of the Rh–B interaction with the amine-borane ligand. The final cyclic dimer is formed by off-metal coupling of the released aminoboranes. A plausible explanation of the observed induction period is also given.