Ligand acidity constants as calculated by density functional theory for PF3 and N-Heterocyclic carbene ligands in hydride complexes of Iron(II)

The deprotonation of metal-hydride intermediates is a key step in some catalytic processes, making the pKa of these intermediates a notable characteristic in the design of catalysts. The acid dissociation constant (Ka) in THF can be approximated using the Ligand Acidity Constant (LAC) method which involves adding acidity constants AL for each of the ligands in the conjugate base complex and correcting for its charge and the stability of the hydride complexes. Here density Functional Theory (DFT) is used to calculate the free energies, in THF solvent, of deprotonation of six series of hydride complexes with the formula [FeH(CO)(5-x)(L)x]+ where L is the neutral ligand PF3 or N-heterocyclic carbene (NHC). This involved finding among several isomers the isomer of lowest energy. From these energies, pKa values are derived. Plots of pKa of [FeH(CO)(5-x)(L)x]+ vs x are linear and AL values are obtained from the slopes of the lines. Plots of the HOMO energies of the conjugate bases Fe(CO)(5-x)(L)x are also linear showing that the pKa of the Fe(II) hydride cations and the energies of the non-bonding d electrons in the Fe(0) complexes are directly related. The strongly acidifying PF3 ligand has AL = −4.1 while the strongly donating, weakly acidifying NHC ligands have AL ranging from 5.6 to 8.6 depending on the structure. The AL of PF3 is the same as that of CO. The saturated NHC-2 with two methyl groups on the nitrogens (SIMe) is more donating (less acidifying) than the analagous unsaturated NHC-1 (IMe). Replacing the backbone hydrogens of NHC-1 with phenyl groups (NHC-4) has no effect on AL. There is a trend to less donating and more acidifying NHC ligands as the size of the wingtip groups is changed from (H,Me) in NHC-5 to (Me,Me) in NHC-1 to (iPr,iPr) in NHC-3. Thus the protic carbene NHC-5 is the most donating in this series.