Molecular Electrostatic Potential Approach to Determining the Steric Effect of Phosphine Ligands in Organometallic Chemistry

Abstract

A two-layer ONIOM(B3LYP/6-31G(d,p):UFF) quantum mechanics (QM)−molecular mechanics (MM) optimization of PR3 ligands, where the QM layer is always constructed as PH3, followed by molecular electrostatic potential (MESP) analysis of the QM layer is suggested as a simple and effective method for evaluating the steric effect of PR3 ligands. The subtle variations in the electron distribution that arise as a result of the steric bulkiness as well as the conformational changes in the substituent groups is well reflected in the value of the MESP minimum (Vmin) located in the QM region. In general, a sterically bulky group has always shown a more negative Vmin than a sterically less bulky group. The difference between the Vmin value of free PH3 and the Vmin value at the QM region of PR3 is used as a measure of the steric effect of the PR3 ligand. This value, designated as MESPsteric, showed a good linear correlation with the cone angle values as well as the average of the intervalence HPH angles found in the QM layer. Further, the difference between the Vmin value at the QM region of PR3 (an indicator of the steric effect) and the Vmin value of the completely optimized PR3 ligand (an indicator of the combined effect of steric and electronic effects) showed a good linear correlation with the Hammett ∑σp constants, which further confirmed the present approach to understanding the steric effect separately from the electronic effect.