Unusual behavior of o,o"-dimethylene(tri-p-cresyl) bicyclophosphite as a novel organophosphorus ligand in olefin hydrogenation and hydroformylation

Abstract

A representative of the new class of organophosphorus ligands, viz., o,o"-dimethylene(tri-p-cresyl) bicyclophosphite (BCP), was studied as a promoter of Rh(acac)(CO)₂ in hydrogenation and hydroformylation. BCP enhances the activity and stability of the catalyst much more strongly than analogous organophosphorus ligands used previously (triphenylphosphine, triphenyl phosphite, and etriolphosphite). A reason for this behavior of BCP was studied using NMR spectroscopy, quantum-chemical calculations, and molecular simulation. The high sensitivity of the ¹H NMR signals of the methylene groups of BCP toward complexation appears due to the high density of the highest occupied and lowest unoccupied MO of protons of the CH₂ groups, especially those directed toward the P atom. The ¹H and ³¹P NMR spectra indicate the formation of hydrides of two types (HRh(BCP)₃ and HRh(BCP)₄) directly upon the addition of BCP in amounts exceeding that corresponding to the BCP/Rh = 2 ratio to a solution of Rh((acac)(CO)₂. The most probable source of the hydride ion is the BCP molecule itself, namely, the bridging CH₂ groups. The molecular mechanics simulation showed that in the [Rh(BCP)₃]⁺ complexes the aromatic rings of BCP formed two molecular cavities. These cavities can alternatively open and close, thus providing flexible screening of the catalytic site. This explains the unusual behavior of the Rh complexes with BCP in hydrogenation and hydroformylation.