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 (V-min) of a PR3 ligand at the phosphorus lone pair region provides a direct measure of the total electronic (E-eff) and steric effects (S-eff) of the ligand. The difference between the V-min of unsubstituted PH3 (V-min(PH3)) and the V-min of PR3 is considered as E-eff +S-eff. 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 V-min of the ONIOM-optimized PR3 at the phosphorus lone-pair region thus provides the quantification of the steric effect as S-eff = V-min(PH3) - V-min(ONIOM_PR3). Because V-min(PR3) contains both E-eff and S-eff, the E-eff can be easily defined as E-eff = V-min(ONIOM-PR3) -V-min(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 Set 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) (+E-eff, +S-eff), (ii) (+E-eff, -S-eff), (iii) (-E-eff, +S-eff), and (iv) (-E-eff, -S-eff), where plus and minus signs indicate electron donation and electron withdrawal properties, respectively.