Characterization of sites of different thermodynamic affinities on the same metal center via isothermal titration calorimetry

We investigate the binding thermodynamics of a series of phosphorus ligands to a model compound, PdCl2(solv)2, where solv refers to a molecule of solvent, using isothermal titration calorimetry (ITC). ITC allows for the quantification of the equilibrium binding constant, the binding enthalpy, and the binding stoichiometry all in a single experiment. For systems in which two equivalents of ligand were able to bind to the Pd center, the binding sites on each Pd center in solution showed a different thermodynamic affinity for the same ligand. Changes in binding modes between different phosphorus ligands were due to steric bulk and poor electron-donating ability of such ligands. Our results demonstrate ligand binding was strongly enthalpy-driven due to solvent reorganization, which is the rearrangement of solvent molecules in the bulk solvent and the solvent molecules surrounding the solvated species.

Graphical abstractBinding of phosphorus ligands to PdCl2(solv)2 where solv represents a molecule of solvent in solution was examined using isothermal titration calorimetry (ITC) to understand the effects of the ligand and solvent properties on the thermodynamics of binding. Electronic and steric characteristics of the ligands as well as the abilities of the solvents to interact with the solvated Pd centers contribute to the observed thermodynamic parameters. Ligand binding is enthalpy-driven due to solvent reorganization, though entropy can contribute greatly depending on the structure of the ligand.Figure optionsDownload full-size imageDownload high-quality image (45 K)Download as PowerPoint slideHighlights► We use calorimetry to study monodentate P ligand binding to a solvated Pd(II) salt. ► Ligands exhibit different thermodynamic affinities for sites on the Pd centers. ► Electron-donating solvents reduce the affinity of the metal center for P ligands. ► Ligand binding is enthalpy-driven due to solvent reorganization.