Local electric fields can interact with dipoles and net charges of reactants, products, and transition states to impart enhanced reactivity in enzymes, at surfaces, and have more recently been investigated at molecular systems. At molecular systems, the incorporation of charged cationic functionalities can generate equivalent internal electric fields that tune both pKa and E1/2. To experimentally demonstrate quantitative examples of the effect of defined electric fields on the bond dissociation free energy of N–H bonds, we report a series of MnVN Schiff base complexes functionalized with a crown ether unit containing a non-redox active cation, Mn+ (n+ = 1+, 2+, or 3+). Bounds for the pKa values for the transient imido complexes were determined, and together with the experimentally measured reduction potentials for the Mn(VI/V) couple, the bond dissociation free energies (BDFEs) were calculated. The results of this study indicate that, despite spanning >600 mV and >9 pKa units across the series, the BDFEs remain relatively constant. We relate this trend to the reactivity of the manganese complexes by exploring net hydrogen-atom transfer (HAT) reactions and breaking thermodynamic scaling relationships. Implications of this study suggest that electric fields may modulate pathways for hydrogen atom transfer reactions or facilitate asynchronous proton-coupled electron transfer pathways.
Thursday, May 6, 2021 - 3:30pm
Dr. Nadia Léonard