Density Functional Theory Calculations Support the Additive Nature of Ligand Contributions to the pKa of Iron Hydride Phosphine Carbonyl Complexes

Molly M H Sung; Robert H Morris

doi:10.1021/acs.inorgchem.6b01274

Density Functional Theory Calculations Support the Additive Nature of Ligand Contributions to the pK_a of Iron Hydride Phosphine Carbonyl Complexes

Inorg Chem. 2016 Oct 3;55(19):9596-9601. doi: 10.1021/acs.inorgchem.6b01274. Epub 2016 Sep 12.

Authors

Molly M H Sung¹, Robert H Morris¹

Affiliation

¹ Davenport Laboratory, Department of Chemistry, University of Toronto , 80 St. George Street, Toronto, Ontario M5S 3H6, Canada.

PMID: 27616349
DOI: 10.1021/acs.inorgchem.6b01274

Abstract

The acid dissociation constant K_a of a transition-metal hydride complex is a key thermodynamic quantity for evaluating reactivity and stability of the complexes and their conjugate bases in stoichiometric and catalytic reactions. It can be estimated using a simple ligand acidity constant (LAC) empirical equation for a wide range of complexes. Here, we provide the first density functional theory (DFT) study that supports the additive nature of ligand contributions to the pK_a of metal hydride complexes. Specifically, the pK_a values of iron hydride complexes [FeH(CO)_x(PR₃)_(5-x)]⁺ in either tetrahydrofuran or dichloromethane solutions are estimated using the LAC method and DFT calculations. There is a linear correlation between these two methods, and both predict a surprisingly linear increase in pK_a over a wide range from approximately -15 for x = 5 to approximately 40 for x = 0. The LAC equation predicts that pK_a^THF or pK_a^DCM increases by 9 units with the replacement of each CO ligand with a trialkylphosphine ligand in a stepwise fashion, whereas the DFT calculations predict the step size will be approximately 11. The two methods agree with pK^DCM data available for x = 3 and qualitative data for x = 1 and 0, but further quantitative measurements over a wider range are needed to firmly establish the trend. The free energy of protonation and the energy of the highest occupied molecular orbital of Fe(CO)_x(PR₃)_(5-x) (mainly nonbonding d electrons) increase linearly with phosphine substitution, and this increases the pK_a value as observed.