Quantifying the electronic effect of substituted phosphine ligands via molecular electrostatic potential

Area of Science:

• Organometallic Chemistry
• Computational Chemistry
• Ligand Design

Background:

• Phosphine ligands (PR(3)) are crucial in organometallic chemistry, influencing metal center reactivity.
• Quantifying the electronic effects of phosphine ligands is essential for predicting and controlling catalytic activity.
• Existing electronic parameters like the Tolman electronic parameter have limitations.

Purpose of the Study:

• To introduce and validate the molecular electrostatic potential minimum (V(min)) as a quantitative measure of phosphine ligand electronic properties.
• To establish correlations between V(min) and established electronic parameters and reactivity metrics.
• To demonstrate the utility of V(min) in phosphine-metal coordination chemistry.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to determine V(min) values for various substituted phosphine ligands.
• V(min) values were compared against the Tolman electronic parameter and pK(a) values of phosphine conjugate acids.
• Reaction energies (DeltaE) for model reactions involving phosphine coordination to metal centers (Ni, Sc) were calculated.
• Electrochemical data (reduction potential, enthalpy change) for iron complexes were analyzed in relation to V(min).

Main Results:

• A strong linear correlation was found between V(min) and the Tolman electronic parameter for phosphine ligands.
• V(min) values showed a proportional relationship with the pK(a) of the conjugate acids of phosphines.
• Reaction energies for Ni(CO)(3) + PR(3) and ScH(3) + PR(3) reactions correlated linearly with V(min), except in cases of strong pi-back-bonding.
• Standard reduction potentials and enthalpy changes for an iron complex also exhibited a linear dependence on V(min).

Conclusions:

• The V(min) value serves as a reliable quantitative measure of the sigma-donating ability of phosphine ligands.
• V(min) offers a more inherent electronic property descriptor compared to parameters like nu-CO and pK(a).
• The V(min) parameter holds significant potential for advancing phosphine-metal coordination chemistry and ligand design.