DFT methods applied to answer the question: how accurate is the ligand acidity constant method for estimating the pKa of transition metal hydride complexes MHXL4 when X is varied?

Abstract

Density functional theory (DFT) is used to calculate the relative free energies of deprotonation of the isomers of iron-group hydride complexes MHXL₄ where M = Fe, Ru, Os, L₄ = (CO)₄ or (PMe₂CH₂CH₂PMe₂)₂ for a wide range of anionic ligands X. The free energies of the most stable isomers are used to calculate relative pK_a values where K_a refers to the acid dissociation constant for the equilibrium MHXL₄ → [MXL₄]⁻ + H⁺. These are used to test the proposal that the pK_a for a given metal complex in THF can be simply calculated by adding the contributions to the total pK_a value from each ligand L; these are called ligand acidity constants (LAC) A_L used in the LAC equation [R. H. Morris, J. Am. Chem. Soc., 2014, 136, 1948–1959]. The A_L are calculated using A_L = 0.2 as a reference for the hydride ligand. The A_L of certain less polarizable X ligands are found to be fairly constant (±1 to ±2 units) and consistent with the proposed LAC method for a range of the complexes considered: 2 for Me⁻, 1 for OH⁻, 1 for NH₂⁻, 0 for B(OCH₂CH₂O⁻)⁻, −1 for OMe⁻, −2 for SH⁻ and −2 for SMe⁻. Other ligands have more variable A_L values (±3 to ±5 units) because of high polarizability or other reasons: 1 for OtBu⁻, −1 for F⁻, −2 for BMe₂⁻, −2 for NMe₂⁻, −3 for Cl⁻, −3 for PMe₂⁻, −3 for Br⁻, −5 for I⁻, −6 for CN⁻ and −12 for SiCl₃⁻. Iodide stabilizes the anion [MIL₄]⁻ more than does fluoride in [MFL₄]⁻ making iodide the more acidifying ligand despite its lower electronegativity. DFT is also used to validate the charge correction in the LAC equation.