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

摘要

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, L-4 = (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 AL 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 AL 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.