Mechanism of Catalytic Water Oxidation by the Ruthenium Blue Dimer Catalyst: Comparative Study in D₂O versus H₂O

Dooshaye Moonshiram; Vatsal Purohit; Javier J Concepcion; Thomas J Meyer; Yulia Pushkar

doi:10.3390/ma6020392

Mechanism of Catalytic Water Oxidation by the Ruthenium Blue Dimer Catalyst: Comparative Study in D₂O versus H₂O

Materials (Basel). 2013 Jan 30;6(2):392-409. doi: 10.3390/ma6020392.

Authors

Dooshaye Moonshiram¹, Vatsal Purohit², Javier J Concepcion³, Thomas J Meyer⁴, Yulia Pushkar⁵

Affiliations

¹ Department of Physics, Purdue University, 525 Northwestern West Lafayette, IN 47907, USA. dmoonshi@purdue.edu.
² Department of Physics, Purdue University, 525 Northwestern West Lafayette, IN 47907, USA. vatsal.purohit15@gmail.com.
³ Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA. jconcepc@email.unc.edu.
⁴ Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA. tjmeyer@unc.edu.
⁵ Department of Physics, Purdue University, 525 Northwestern West Lafayette, IN 47907, USA. ypushkar@purdue.edu.

Abstract

Water oxidation is critically important for the development of energy solutions based on the concept of artificial photosynthesis. In order to gain deeper insight into the mechanism of water oxidation, the catalytic cycle for the first designed water oxidation catalyst, cis,cis-[(bpy)₂(H₂O)Ru^IIIORu^III(OH₂)(bpy)₂]⁴⁺ (bpy is 2,2-bipyridine) known as the blue dimer (BD), is monitored in D₂O by combined application of stopped flow UV-Vis, electron paramagnetic resonance (EPR) and resonance Raman spectroscopy on freeze quenched samples. The results of these studies show that the rate of formation of BD[4,5] by Ce(IV) oxidation of BD[3,4] (numbers in square bracket denote oxidation states of the ruthenium (Ru) centers) in 0.1 M HNO₃, as well as further oxidation of BD[4,5] are slower in D₂O by 2.1-2.5. Ce(IV) oxidation of BD[4,5] and reaction with H₂O result in formation of an intermediate, BD[3,4]', which builds up in reaction mixtures on the minute time scale. Combined results under the conditions of these experiments at pH 1 indicate that oxidation of BD[3,4]' is a rate limiting step in water oxidation with the BD catalyst.

Keywords: Ru complexes; UV-Vis stopped flow kinetics; catalysis of water oxidation; deuterium isotope effect; time-resolved X-band EPR spectroscopy; time-resolved resonance Raman.