Readily exchangeable water molecules are commonly found in the active sites of oxidoreductases, yet the overwhelming majority of studies on small-molecule mimics of these enzymes entirely ignores the contribution of water to the reactivity. Studies of how these enzymes can continue to function in spite of the presence of highly oxidizing species are likewise limited. The mononuclear Mn complex with the potentially hexadentate ligand N-(2-hydroxy-5-methylbenzyl)-N,N',N'-tris(2-pyridinylmethyl)-1,2-ethanediamine (L) was previously found to act as both a HO-responsive MRI contrast agent and a mimic of superoxide dismutase (SOD). Here, we studied this complex in aqueous solutions at different pH values in order to determine its (i) acid-base equilibria, (ii) coordination equilibria, (iii) substitution lability and operative mechanisms for water exchange, (iv) redox behavior and ability to participate in proton-coupled electron transfer (PCET) reactions, (v) SOD activity and reductive activity toward both oxygen and superoxide, and (vi) mechanism for its transformation into the binuclear Mn complex with L-L and its hydroxylated derivatives. The conclusions drawn from potentiometric titrations, low-temperature mass spectrometry, temperature- and pressure-dependent O NMR spectroscopy, electrochemistry, stopped-flow kinetic analyses, and EPR measurements were supported by the structural characterization and quantum chemical analysis of proposed intermediate species. These comprehensive studies enabled us to determine how transiently bound water molecules impact the rate and mechanism of SOD catalysis. Metal-bound water molecules facilitate the PCET necessary for outer-sphere SOD activity. The absence of the water ligand, conversely, enables the inner-sphere reduction of both superoxide and dioxygen. The L complex maintains its SOD activity in the presence of OH and Mn-oxo species by channeling these oxidants toward the synthesis of a functionally equivalent binuclear Mn species.

The protonation of thiourea was reinvestigated in aqueous and non-aqueous media using potentiometric, UV-vis, NMR, MS spectroscopy, as well as DFT calculations. In aqueous medium, no evidence for protonation could be found in the pH range 0 to 7, in agreement with typical pK a values (acid dissociation constants of protonated thiourea) quoted in the literature of around Ϫ1.2. Most of the protonation constants quoted in the lite-

The perchlorate complexes of a series of half‐sandwich monoaqua cations Cp*Ir(A−B)(H2O)2+/+ with A−B = prol (D/L‐proline anion), picac (picolinic acid anion), R,R‐dach [(−)‐(1R,2R)‐1,2‐diaminocyclohexane], R,R‐dpen [(+)‐(1R,2R)‐1,2‐diphenylethylenediamine], phen (o‐phenanthroline), and bpy (2,2′‐bipyridine) (Cp* = η5‐pentamethylcyclopentadienyl anion) have been prepared and characterized. An X‐ray structure analysis of Cp*Ir(R,R‐dach)(H2O)(ClO4)2·H2O has revealed that the cation Cp*Ir(R,R‐dach)(H2O)2+ has a distorted pseudo‐octahedral coordination geometry. In the case of A−B = prol, crystallization from water led to the trinuclear complex [Cp*Ir(D‐prol)]3(ClO4)3, which has also been characterized by X‐ray structure analysis. The experimental data suggest that in aqueous solution the trinuclear proline complex dissociates to form the cation Cp*Ir(D‐prol)(H2O)+. The proton dissociation constants of the coordinated water in Cp*Ir(A−B)(H2O)2+/+ have been determined as pKa = 7.5 (A−B = bpy) and pKa = 7.1 (A−B = R,R‐dach and picac). Substitution of the water in Cp*Ir(A−B)(H2O)2+/+ by the monodentate ligands L = py (pyridine), DMS (dimethyl sulfide), TU (thiourea), and monodentate anions according to the Equation Cp*Ir(A−B)(H2O)2+/+ + L → Cp*Ir(A−B)L2+/+ + H2O has been studied by multi‐wavelength stopped‐flow spectrophotometry in aqueous solution at I = 0.2 M. This kinetic investigation, carried out at different concentrations, temperatures, and pressures, showed that the process obeys second‐order kinetics, where rate = kL[Cp*Ir(A−B)H2O2+/+][L]. The magnitude of the second‐order rate constant kL depends on the nature of both A−B and L. The data for kL have been found to range from 6.4 × 104 M−1s−1 (A−B = D‐prol; L = TU) to 10.5 M−1s−1 (A−B = bpy; L = py) at 298 K. The activation parameters for water substitution at Cp*Ir(A−B)(H2O)2+/+ (A−B = bpy, R,R‐dach, and picac) by L = TU have been evaluated. The activation volumes of ΔV≠ = +2.3, +7.4, and +7.3 cm3 mol−1, respectively, are supportive of an Id mechanism. The results regarding the kinetic lability of the coordinated water in the monoaqua cations Cp*Ir(A−B)(H2O)2+/+ are compared to those obtained for the triaqua cation Cp*Ir(H2O)32+.

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