“Covalent Hydration” Reactions in Model Monomeric Ru 2,2′-Bipyridine Complexes: Thermodynamic Favorability as a Function of Metal Oxidation and Overall Spin States

Abstract

Density functional theory (DFT) has been used to investigate the plausibility of water addition to the simple mononuclear ruthenium complexes, [(NH(3))(3)(bpy)Ru═O](2+/3+) and [(NH(3))(3)(bpy)RuOH](3+), in which the OH fragment adds to the 2,2'-bipyridine (bpy) ligand. Activation of bpy toward water addition has frequently been postulated within the literature, although there exists little definitive experimental evidence for this type of "covalent hydration". In this study, we examine the energetic dependence of the reaction upon metal oxidation state, overall spin state of the complex, as well as selectivity for various positions on the bipyridine ring. The thermodynamic favorability is found to be highly dependent upon all three parameters, with free energies of reaction that span favorable and unfavorable regimes. Aqueous addition to [(NH(3))(3)(bpy)Ru═O](3+) was found to be highly favorable for the S = 1/2 state, while reduction of the formal oxidation state on the metal center makes the reaction highly unfavorable. Examination of both facial and meridional isomers reveals that when bipyridine occupies the position trans to the ruthenyl oxo atom, reactivity toward OH addition decreases and the site preferences are altered. The electronic structure and spectroscopic signatures (EPR parameters and simulated spectra) have been determined to aid in recognition of "covalent hydration" in experimental systems. EPR parameters are found to uniquely characterize the position of the OH addition to the bpy as well as the overall spin state of the system.

Design and Optimization of Catalysts Based on Mechanistic Insights Derived from Quantum Chemical Reaction Modeling

“…This prediction gave a solid conceptual foundation to a widely accepted notion that mononuclear metallic systems should, in theory, be tunable toward effective water oxidation. This new mechanistic understanding outlined above led to the development of highly effective multinuclear complexes − as well as mononuclear catalysts. − …”

Electron Paramagnetic Resonance Analysis of a Transient Species Formed During Water Oxidation Catalyzed by the Complex Ion [(bpy)2Ru(OH2)]2O4+

“…Indeed, evidence for formation of this type of intermediate during catalytic cycling of the "blue dimer" has been obtained in the form of NIR optical spectra in photocatalyzed reactions, 39 pulse radiolysis experiments that characterize "blue dimer"−OH adducts, 36 and theoretical studies that establish the plausibility of this reaction in bipyridine complexes containing highly oxidized Ru centers. 80 However, it is unclear how the unpaired spins on the metal center and ligand would interact in such a species (e.g., {4,5}-bpyOH • ) and, more pointedly, we do not observe any 1 H or 14 N hyperfine contributions that would be indicative of a bipyridyl radical, refer to Figure 7 for the assigned coupling constants. Further, that the {5,5} containing solutions for functionalized bipyridine ligands gives essentially the same EPR spectrum would seem to preclude bipyridine hydroxylation.…”

Reaction Mechanism of Water Oxidation Catalyzed by Iron Tetraamido Macrocyclic Ligand Complexes – A DFT Study

“…An alternative target for the attack would be the ligand radical cation, which has been proposed for the blue dimer. [42,43] Therefore, one water molecule was added to 6 to form 17. We found that the oxo group acts as a general base and removes a proton from the water molecule, concomitant with a nucleophilic attack on one of the benzene carbon atoms by the emerging hydroxide ion.…”