Characterization of Redox States of Ru(OH₂)(Q)(tpy)²⁺ (Q = 3,5-di-tert-butyl-1,2-benzoquinone, tpy = 2,2′:6′,2″-terpyridine) and Related Species through Experimental and Theoretical Studies

Abstract

The redox states of Ru(OH(2))(Q)(tpy)(2+) (Q = 3,5-di-tert-butyl-1,2-benzoquinone, tpy = 2,2':6',2''-terpyridine) are investigated through experimental and theoretical UV-vis spectra and Pourbaix diagrams. The electrochemical properties are reported for the species resulting from deprotonation and redox processes in aqueous solution. The formal oxidation states of the redox couples in the various intermediate complexes are systematically assigned using electronic structure theory. The controversy over the electronic assignment of ferromagnetic vs. antiferromagnetic coupling is investigated through comparison of ab initio methods and the broken-symmetry density functional theory (DFT) approach. The various pK(a) values and reduction potentials, including the consideration of proton-coupled electron-transfer (PCET) processes, are calculated, and the theoretical version of the Pourbaix diagram is constructed in order to elucidate and assign several previously ambiguous regions in the experimental diagram.

Study of Proton Coupled Electron Transfer in a Biomimetic Dimanganese Water Oxidation Catalyst with Terminal Water Ligands

“…The computations reported in this paper aim to provide a thorough characterization of the 1 → 1 ox redox state transition, complementing earlier studies on the capabilities of DFT methods for predictions of redox potentials of transition metal complexes 17, 35–52. Most of these previous studies investigated functionals based on the generalized gradient approximation (GGA) such as BLYP,53 BP8654 and PBE55 as well as hybrid functionals (e.g., B3LYP56, 57), including studies of oxomanganese complexes 17, 58.…”

Application of Pulse Radiolysis to Mechanistic Investigations of Catalysis Relevant to Artificial Photosynthesis

“…Although pH-dependente lectrochemicale xperiments allowed the determination of the thermodynamic properties of catalytic intermediates (e.g.,e lectrochemicalp otentials and pK a values), PR wasu sed to produce some of the unstable intermediates and comparet heir UV/Vis spectra with those predicted by DFT.F or example, the electronic spectrum of [Ru(OH 2 )(tpy)(SQ)] + (SQ = 3,5-di-tert-butyl-1,2-benzosemiquinone)p roduced by PR is shown in Figure 3 togetherw ith that predicted by theory.G ood agreement between the measured and calculated spectra thus provided additional validation of DFT-predicted intermediates involved in the catalytic cycle. [18] After the first report of am ononuclear ruthenium WOC by Thummeland Zong in 2005, [19] the focus of mechanistic studies has shifted towards single-metals ystems, such as [Ru(OH 2 )(NPM)(4-pic) 2 ] 2 + (NPM = 4-tert-butyl-2,6-di-1',8'-(naphthyrid-2'-yl)pyridine;4pic = 4-picoline), which has been investigated in our laboratory. [20] Based on electrochemical experiments, the first oxidation of [Ru II ÀOH 2 ] 2 + (Ru = Ru(NPM)(4pic) 2 )i sat wo-electron process coupled to the loss of two protons over pH 3-11, which results in the formationo fthe[ Ru IV =O] 2 + species.…”

“…The blueshift of the predicted low-energyband is typical for this level of theory.Redrawn with permission based on Ref. [18].Copyright2 009 American ChemicalS ociety. …”

Oxygen Production from Water: Molecular Catalysts

“…One is the intrinsic complexity of the reaction, where the catalyst is likely to cycle among five different oxidation states, whether metal or ligand based or both. 16 This imposes a requirement for transition metal complexes that need to be sufficiently long lived to be able to perform the reaction and also to be spectroscopically detectable. Another fundamental problem is the unavoidable use of water as solvent, which is a problematic issue because of the limited temperature range in which reactions can be studied, and its high absorptivity prevents the proper use of techniques such as UV-vis-near-IR, EPR, etc., that could provide otherwise valuable information.…”

“…− entity, as shown in equation (16 (16) Furthermore, this interaction between the two active groups is manifested in the fluxional behavior of the molecule, as evidenced by NMR spectroscopy, wherein it undergoes a very fast process that interconverts the C 2 enantiomeric forms associated with the aquo positions above and below the plane of the pyrazolate moiety. This process is so fast at room temperature that the NMR spectrum looks similar to that for a molecule having C 2v symmetry.…”