Quantitative Assessment of the Stereoelectronic Profile of Phosphine Ligands

Abstract

The stereoelectronic profile of a variety of phosphine ligands (PR3) have been estimated using a combined approach of quantum mechanics (QM) and molecular mechanics (MM). The quantum mechanically derived molecular electrostatic potential minimum (Vmin) of a PR3 ligand at the phosphorus lone pair region provides a direct measure of the total electronic (Eeff) and steric effects (Seff) of the ligand. The difference between the Vmin of unsubstituted PH3 (Vmin(PH3)) and the Vmin of PR3 is considered as Eeff + Seff. It is found that a two-layer QM-MM ONIOM method comprising PH3 in the inner QM layer and the R groups in the outer MM layer is useful to locate the structure of a PR3 ligand in an electronic effect free environment of the substituents. The Vmin of the ONIOM-optimized PR3 at the phosphorus lone-pair region thus provides the quantification of the steric effect as Seff = Vmin(PH3) − Vmin(ONIOM_PR3). Because Vmin(PR3) contains both Eeff and Seff, the Eeff can be easily defined as Eeff = Vmin(ONIOM_PR3) − Vmin(PR3). A modified form of the symmetric deformation coordinate (S4) is calculated for all of the fully optimized and ONIOM-optimized free phosphines to obtain their S4-based steric effect values.24 Good linear correlation between S4 of ONIOM-optimized phosphines and the MESP-based Seff values was obtained. Further, the determination of the stereoelectronic profile of PR3 ligands has been achieved, leading to a general classification of the ligands into four categories, namely, ligands with (i) (+Eeff, +Seff), (ii) (+Eeff, −Seff), (iii) (−Eeff, +Seff), and (iv) (−Eeff, −Seff), where plus and minus signs indicate electron donation and electron withdrawal properties, respectively.