The energetics of proton transfer reactions of hydroxy-substituted ruthenium complexes have been difficult to study in the past due to the similarities in pKa values of the deprotonatable ligands. A method for determining ΔpKa with precision to three decimal places has been developed by Perrin. The method involves an NMR titration where two species in solution, both with one protonatable site, are titrated with small aliquots of acid or base. If distinguishable resonance peaks are able to be seen for each species through the titration, the ΔpKa can be determined. As a proof-of-concept experiment, the method was tested on phenol and 4-methoxyphenol, and the determined ΔpKa was 0.287 +/- 0.003 units with the unsubstituted phenol being more acidic, which follows literature precedence. Computation methods were utilized to see how experimental results compared to theoretical predictions. Using this method on our ruthenium complexes will allow us to observe how small changes in our single-hydroxy-substituted complexes affect ΔpKa. Coupling the ΔpKa values obtained from this method with reduction potentials of the protonated and deprotonated forms of the complexes studied, will give greater insight into the energetics of both ground state and excited state proton coupled electron transfer.

Determining ΔpKa values of phenols and hydroxy-substituted polypyridyl ligands attached to ruthenium

ACS Spring 2022

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