Computational Studies of "Covalent Hydration" Reactions Involving Non-innocent 2-2'-bipyridine Ligands Within Ru Water Oxidation Catalysts
EMSL Project ID
44726
Abstract
The notion that bipyridine ligands in catalysts such as [Ru(bpy)3]3+ and the "blue dimer" [(bpy)2Ru(OH2)]2O4+ might participate in water oxidation through formation of OH-substituted reaction transients has existed in the literature for some time. Our recent combined computational and EPR study examined the plausibility of water addition to Ru "blue dimer" as well as to simple mononuclear ruthenium complexes ([(NH3)3(bpy)Ru=O]2+/3+, and (NH3)3(bpy)Ru-OH]3+), in which the OH fragment adds to 2,2'-bipyridine (bpy) ligand, the energetic dependence of this type of reaction upon metal oxidation state, overall spin state of the complex, as well as the selectivity for various positions on the bipyridine ring. Aqueous addition to [(NH3)3(bpy)Ru=O]3+ was found to be highly favorable for the lowest spin states, while reduction of the formal oxidation state on the metal center makes the reaction highly unfavorable. These low-spin reactions produce stable adducts wherein internal electron transfer has occurred, effectively generating a one-electron oxidized ligand pi-cation coordinated to a reduced (d5 or d6) Ru atom. The internal electronic rearrangements are reminiscent of bimolecular reactions between free and Ru-complexed bpyOH radical adducts and weak oxidants which have been proposed from studies involving radiolytically-generated OH radicals. In these latter studies, the oxidized bpyOH+ pi-cation appears to undergo rapid addition of hydroxide ion to produce a ring-substituted diol. The analogous reaction within the model monomeric catalysts would be addition of a second water molecule to generate more stable ring-substituted diols. Within the proposed work, we will continue the study of monomeric Ru complexes wherein we focus upon determining the activation barriers for the formation of covalent hydrates and potential channels for their secondary decay to further species. Collectively, our independent experimental work and the theoretical studies performed at EMSL will constitute the most stringent test yet applied to the concept of bipyridine "noninnocence" and should allow confirmation or denial of the plausibility of this type of pathway in water oxidation. As such, these studies are also relevant to a broader set of immine-based water oxidation catalysts, including the recently described mononuclear 6-coordinate ions that contain no aqua bound ligands.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2011-10-01
End Date
2014-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Clark AE, and JK Hurst. 2011. "Mechanisms of Water Oxidation Catalyzed by Ruthenium Coordination Complexes." In Progress in Inorganic Chemistry, vol. 57, ed. K. D. Karlin. John Wiley & Sons, Inc., Hoboken, NJ.
Ozkanlar A, and AE Clark. 2012. "Sensitivity of the Properties of Ruthenium “Blue Dimer” to Method, Basis Set, and Continuum Model." Journal of Chemical Physics 136(20):204104. doi:10.1063/1.4719937
Ozkanlar A, JL Cape, JK Hurst, and AE Clark. 2011. "“Covalent Hydration” Reactions in Model Monomeric Ru 2,20-Bipyridine Complexes: Thermodynamic Favorability as a Function of Metal Oxidation and Overall Spin States." Inorganic Chemistry 50(17):8177-8087. doi:10.1021/ic200646h