Palladium Acetate Trimer: Understanding Its Ligand-Induced Dissociation Thermochemistry Using Isothermal Titration Calorimetry, X-ray Absorption Fine Structure, and P-31 Nuclear Magnetic Resonance

摘要

Palladium acetate trimer, Pd-3(OAc)(6), is a common precursor for Pd nanoparticle synthesis and an important precatalyst for homogeneous catalysis reactions. Added ligands (L, e.g., phosphines) are well-known to dissociate the Pd-3(OAc)(6) trimer into Pd(OAc)(2) monomers and form Pd(OAc)(2)(L)(2) complexes. Despite the importance of the trimer dissociation and ligand-monomer binding thermodynamics on the properties of the resulting Pd complex, little is known about either reaction. Using a combination of isothermal titration calorimetry (ITC), X-ray absorption fine structure (XAFS), and P-31 nuclear magnetic resonance (P-31 NMR) on trimer samples containing monomer impurities, we developed a methodology and a trimer-monomer ITC model to obtain the Gibbs free energy, enthalpy, and entropy for both reactions in toluene and quantify the monomer content. The results provide the following, previously inaccessible, quantitative insights on the reactions. Trioctylphosphine (TOP)-monomer binding reaction is enthalpically driven (Delta H = -187 +/- 8 kJ/mol), but the entropy loss due to binding (-T Delta S = 84 +/- 8 kJ/mol) was significant. However, while the trimer dissociation is enthalpically uphill (Delta H = 323 +/- 35 kJ/mol), the increase in entropy due to the formation of monomers was very large (-T Delta S = -266 +/- 35 kJ/mol). The entropy gains and losses for both reactions have major contributions to each reaction Gibbs free energy and almost compensate for each other, making the overall reaction (i.e., Pd-3(OAc)(6) + 6 TOP <-). 3 Pd(OAc)(2)(TOP)(2)) enthalpically driven. The results show the importance of determining the relative enthalpy and entropy contributions to the free energies for the complete thermochemical cycle of Pd acetate trimer dissociation and reaction with TOP. The methodology can be generalized to other ligands and metal or nonmetal homopolymers to obtain detailed thermochemistry of thermodynamically or kinetically unfavorable reactions.