Density Functional Theory Calculations Support the Additive Nature of Ligand Contributions to the pKaof Iron Hydride Phosphine Carbonyl Complexes.
The acid dissociation constant Kaof 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 pKaof metal hydride complexes. Specifically, the pKavalues 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 pKaover a wide range from approximately -15 for x = 5 to approximately 40 for x = 0. The LAC equation predicts that pKaTHFor pKaDCMincreases 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 pKDCMdata 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 pKavalue as observed.
Davenport Laboratory, Department of Chemistry, University of Toronto , 80 St. George Street, Toronto, Ontario M5S 3H6, Canada.
Molly M H
MM
Sung
Davenport Laboratory, Department of Chemistry, University of Toronto , 80 St. George Street, Toronto, Ontario M5S 3H6, Canada.
Robert H
RH
Morris