OH Complex Research Articles

A Ruthenium(III)-Oxyl Complex Bearing Strong Radical Character.

Proton-coupled electron-transfer oxidation of a RuII -OH2 complex, having an N-heterocyclic carbene ligand, gives a RuIII -O. species, which has an electronically equivalent structure of the RuIV =O species, in an acidic aqueous solution. The RuIII -O. complex was characterized by spectroscopic methods and DFT calculations. The oxidation state of the Ru center was shown to be close to +3; the Ru-O bond showed a lower-energy Raman scattering at 732 cm-1 and the Ru-O bond length was estimated to be 1.77(1) Å. The RuIII -O. complex exhibits high reactivity in substrate oxidation under catalytic conditions; particularly, benzaldehyde and the derivatives are oxidized to the corresponding benzoic acid through C-H abstraction from the formyl group by the RuIII -O. complex bearing a strong radical character as the active species.

A Ruthenium(III)–Oxyl Complex Bearing Strong Radical Character

AbstractProton‐coupled electron‐transfer oxidation of a RuII−OH2 complex, having an N‐heterocyclic carbene ligand, gives a RuIII−O. species, which has an electronically equivalent structure of the RuIV=O species, in an acidic aqueous solution. The RuIII−O. complex was characterized by spectroscopic methods and DFT calculations. The oxidation state of the Ru center was shown to be close to +3; the Ru−O bond showed a lower‐energy Raman scattering at 732 cm−1 and the Ru−O bond length was estimated to be 1.77(1) Å. The RuIII−O. complex exhibits high reactivity in substrate oxidation under catalytic conditions; particularly, benzaldehyde and the derivatives are oxidized to the corresponding benzoic acid through C−H abstraction from the formyl group by the RuIII−O. complex bearing a strong radical character as the active species.

Lanthanide Tetrazolate Complexes Combining Single-Molecule Magnet and Luminescence Properties: The Effect of the Replacement of Tetrazolate N3 by β-Diketonate Ligands on the Anisotropy Energy Barrier.

Three new sets of mononuclear Ln(III) complexes of general formulas [LnL3 ]⋅CH3 OH [Ln(III) =Yb (1), Er (2), Dy (3), Gd (4), and Eu (5)], [LnL2 (tmh)(CH3 OH)]⋅n H2 O⋅m CH3 OH [Ln(III) =Yb (1 b), Er (2 b), Dy (3 b), Gd (4 b)], and [LnL2 (tta)(CH3 OH)]⋅CH3 OH [Ln(III) =Yb (1 c), Er (2 c), Dy (3 c), Gd (4 c)] were prepared by the reaction of Ln(CF3 SO3 )⋅n H2 O salts with the tridentate ligand 2-(tetrazol-5-yl)-1,10-phenanthroline (HL) and, for the last two sets, additionally with the β-diketonate ligands 2,2,6,6-tetramethylheptanoate (tmh) and 2-thenoyltrifluoroacetonate (tta), respectively. In the [LnL3 ]⋅CH3 OH complexes the Ln(III) ions are coordinated to three phenanthroline tetrazolate ligands with an LnN9 coordination sphere. Dynamic ac magnetic measurements on 1-3 reveal that these complexes only exhibit single-molecule magnet (SMM) behavior when an external dc magnetic field is applied, with Ueff values of 11.7 K (1), 16.0 K (2), and 20.2 K (3). When the tridentate phenanthroline tetrazolate ligand is replaced by one molecule of methanol and the β-diketonate ligand tmh (1 b-3 b) or tta (1 c-3 c), a significant increase in Ueff occurs and, in the case of the Dy(III) complexes 3 b and 3 c, out-of-phase χ'' signals below 15 and 10 K, respectively, are observed in zero dc magnetic field. CASSCF+RASSI ab initio calculations performed on the Dy(III) complexes support the experimental results. Thus, for 3 the ground Kramers' doublet is far from being axial and the first excited state is found to be very close in energy to the ground state, so the relaxation barrier in this case is almost negligible. Conversely, for 3 b and 3 c, the ground Kramers' doublet is axial with a small quantum tunneling of the magnetization, and the energy difference between the ground and first Kramers' doublets is much higher, which allows these compounds to behave as SMMs at zero field. Moreover, these calculations support the larger Ueff observed for 3 b compared to 3 c. Additionally, the solid-state photophysical properties of 1, 2, 4, and 5 show that the phenanthroline tetrazolate ligand can act as an effective antenna to sensitize the characteristic Yb(III) , Er(III) , and Eu(III) emissions through an energy-transfer process.

Effects of hydrogen sulfide on the heme coordination structure and catalytic activity of the globin-coupled oxygen sensor AfGcHK.

AfGcHK is a globin-coupled histidine kinase that is one component of a two-component signal transduction system. The catalytic activity of this heme-based oxygen sensor is due to its C-terminal kinase domain and is strongly stimulated by the binding of O2 or CO to the heme Fe(II) complex in the N-terminal oxygen sensing domain. Hydrogen sulfide (H2S) is an important gaseous signaling molecule and can serve as a heme axial ligand, but its interactions with heme-based oxygen sensors have not been studied as extensively as those of O2, CO, and NO. To address this knowledge gap, we investigated the effects of H2S binding on the heme coordination structure and catalytic activity of wild-type AfGcHK and mutants in which residues at the putative O2-binding site (Tyr45) or the heme distal side (Leu68) were substituted. Adding Na2S to the initial OH-bound 6-coordinate Fe(III) low-spin complexes transformed them into SH-bound 6-coordinate Fe(III) low-spin complexes. The Leu68 mutants also formed a small proportion of verdoheme under these conditions. Conversely, when the heme-based oxygen sensor EcDOS was treated with Na2S, the initially formed Fe(III)-SH heme complex was quickly converted into Fe(II) and Fe(II)-O2 complexes. Interestingly, the autophosphorylation activity of the heme Fe(III)-SH complex was not significantly different from the maximal enzyme activity of AfGcHK (containing the heme Fe(III)-OH complex), whereas in the case of EcDOS the changes in coordination caused by Na2S treatment led to remarkable increases in catalytic activity.

Theoretical Study on Gas Phase Reactions of OH Hydrogen-Abstraction from Formyl Fluoride with Different Catalysts

The mechanisms and kinetics of the gas phase reactions that the hydrogen atom in formyl fluoride (FCHO) abstracted by OH in the presence of water, formic acid (FA), or sulfuric acid (SA) are theoretically investigated at the CCSD(T)/6-311++G(3df, 3pd)//M06-2X/6-311++G(3df, 3pd) level of theory. The calculated results show that the barriers of the transition states involving catalysts are lowered to −2.89, −6.25, and −7.76 kcal/mol from 3.64 kcal/mol with respect to the separate reactants, respectively, which reflects that those catalysts play an important role in reducing the barrier of the hydrogen abstraction reaction of FCHO with OH. Additionally, using conventional transition state theory with Eckart tunneling correction, the kinetic data demonstrate that the entrance channel X⋯FCHO+OH (X=H2O, FA, or SA) is significantly more favorable than the pathway X⋯OH+FCHO. Moreover, the rate constants of the reactions of FCHO with OH radical with H2O, FA, or SA introduced are computed to be smaller than that of the naked OH+FCHO reaction because the concentration of the formed X⋯FCHO or X⋯OH complex is quite low in the atmosphere.

Facet-dependent trapping and dynamics of excess electrons at anatase TiO2 surfaces and aqueous interfaces.

Excess electrons from intrinsic defects, dopants and photoexcitation play a key role in many of the properties of TiO2. Understanding their behaviour is important for improving the performance of TiO2 in energy-related applications. We focus on anatase, the TiO2 polymorph most relevant in photocatalysis and solar energy conversion. Using first-principles simulations, we investigate the states and dynamics of excess electrons from different donors near the most common anatase (101) and (001) surfaces and aqueous interfaces. We find that the behaviour of excess electrons depends strongly on the exposed anatase surface, the environment and the character of the electron donor. Whereas no electron trapping is observed on the (101) surface in vacuo, an excess electron at the aqueous (101) interface can trigger water dissociation and become trapped into a stable surface Ti(3+)-bridging OH complex. By contrast, electrons avoid the (001) surface, indicating that oxidation reactions are favoured on this surface. Our results provide a bridge between surface science experiments and observations of crystal-face-dependent photocatalysis on anatase, and support the idea that optimization of the ratio between {101} and {001} facets could provide a way to enhance the photocatalytic activity of this material.

Theory Revealing Unusual Non-Rebound Mechanisms Responsible for the Distinct Reactivities of O═MnIV═O and [HO–MnIV–OH]2+ in C–H Bond Activation

This article uses theory to address the origins of the reactivity differences between MnIV═O and MnIV–OH complexes, having identical ligand spheres and metal oxidation states, toward 9,10-dihydroanthracene (DHA) under different pH conditions. Theory discovers different non-rebound mechanisms leading to unique products for the two complexes. One of these is a novel mechanism that operates under basic conditions and that rationalizes the formation of anthraquinone through an anthracene radical anion intermediate. In addition, the calculations reveal a rich mechanistic scheme having blended hydrogen atom transfer and proton-coupled electron transfer (HAT/PCET) with both proton transfer/electron transfer (PT/ET) and electron transfer/proton transfer (ET/PT) characters. The distinct nature of the transition states, such as PT/ET and ET/PT, for the second H-abstraction reactions from the substrate radical by the MnIV═O and MnIV–OH complexes accounts for the observed product distributions for these two species. ...

DFT-PBE, DFT-D, and MP2 Studies on the H2O•••HNH and HOH•••NH2 Hydrogen Bonds in Water-Aniline Complexes

DFT-GGA method of Perdew-Burke-Ernzerhof (PBE) is used with aug-cc-PVTZ, 6-311++G**, and Def2-TZVP large basis sets to study the hydrogen bond interactions between oxygen lone pair as a donor electron with hydrogen atom connected to the aniline’s nitrogen as an electron acceptor (H2O···HNH-Ph), and nitrogen lone pair with hydrogen of water molecule (Ph-H2N···HOH), both in the gas phase. In some cases, MP2/Def2-TZVP is also carried out to test the results of DFT. To analyze donor-acceptor interactions of two above hydrogen bonds, natural bond orbital (NBO), natural energy decomposition analysis (NEDA), natural population analysis (NPA), and quantum theory of atoms in molecules (QTAIM) methods have been used in a detailed manner. Results show that the hydrogen bond in PhH2N···HOH is partially covalent, while hydrogen bond in PhHNH···OH2 is totally electrostatic. For the PhHNH···OH2 complex, there is a large gap between MP2 and PBE results which can be filled by incorporating dispersion terms in the DFT calculations. In all calculations, nitrogen atom of aniline is an stronger electron donor than the oxygen atom of water in the gas phase. PhH2N···HOH has higher electron density than PhHNH···OH2. NBO data shows that the stabilization energy due to the charge transfer for HOH···NH2Ph complex is more than that in H2O···HNHPh complex. The inversion barrier energy was also calculated at the level of PBE/Def2-TZVP without and with dispersion term, and results show that the barrier energy for PhH2N···HOH and PhHNH···OH2 complexes, are about 104 and 103 kcal mol-1, without, and 8.14 and 7.03 kcal mol-1, with dispersion, respectively.

Determining the Oxidation State of Small, Hydroxylated Metal-Oxide Nanoparticles with Infrared Absorption Spectroscopy

Metal-oxide nanoparticles (MONPs) are of broad interest as catalysts and catalyst supports in automotive, industrial, and biomedical applications. In water-containing environments, MONPs adsorb hydroxyl groups at nanoparticle corners, edges, and faces. While adsorbed surface groups will influence MONP properties, including oxidation state and the availability and accessibility of catalytically active sites, determination of the density, distribution, and effects of such groups is experimentally challenging. Here, focusing on CeO2 nanoparticles (CNPs) as a representative MONP system, we report computational and experimental results showing that, for small, hydroxylated CNPs, the presence or absence of strongly infrared (IR) absorbing O–H stretching modes associated with O–H–OH complexes present on fully oxidized CNPs can be used to probe the configuration of surface-adsorbed hydroxyl groups and the CNP net oxidation state. As the vibrational modes of O–H–OH complexes are distinct from those of molecular water (3240 vs 3404 cm–1), these results demonstrate that IR absorption spectroscopy can be used to determine the oxidation state of small, hydroxylated CNPs, as well as other MONPs, more generally, in water-containing environments.

H-treatment impact on conductive-filament formation and stability in Ta2O5-based resistive-switching memory cells

In this article, we evidence the lower formation energy and improved stability of the conductive filament (CF) formed in TiN\Ta2O5\Ta resistive-switching memory cells treated in NH3 atmosphere at 400 °C. This annealing treatment results in (i) lower forming voltage, (ii) lower CF resistance, and (iii) longer retention lifetime of the oxygen-vacancy (Vo) chain constituting the CF. Atomistic insights into these processes are provided by ab initio calculations performed for hydrogen (H) species incorporated in non-stoichiometric Ta2O5 supercells: (i) Vo formation energy is reduced by the presence of H, (ii) Vo-chain CF conductivity is increased by Vo + OH complex formation, and (iii) Vo-chain retention is strengthened by the stable Vo + OH complex. As a result, efficient CF formation and excellent state stability are obtained after 15 days at 250 °C.

Facile formation of homoleptic Au(III) trications via simultaneous oxidation and ligand delivery from [PhI(pyridine)2](2+).

We report the first examples of Au(III) tricationic complexes bound only by neutral monodentate ligands, which are a new class of gold reagents. Oxidative addition to the bis-pyridine Au(I) cation, [Au(4-DMAP)2](+), using a series of dicationic I(III) oxidants of the general form [PhI(L)2](2+) (L = pyridine, 4-DMAP, 4-cyanopyridine) allows ready access to homoleptic and pseudo-homoleptic Au(III) complexes [Au(4-DMAP)2(L)2](3+). The facile oxidative addition of Au(I) species additionally demonstrates the efficacy of PhI(L)2](2+) reagents as halide-free oxidants for Au(I). Comparisons are made via attempts to oxidize NHC-Au(I)Cl, where introduction of the chloride anion results in complex mixtures via ligand and chloride exchange, demonstrating the advantage of using the pyridine-based homoleptic compounds. The new Au(III) trications show intriguing reactivity with water, yielding dinuclear oxo-bridged and rare terminal Au(III)-OH complexes.

Infrared studies of a hybrid CO2-reduction photocatalyst consisting of a molecular Re(I) complex grafted on Kaolin

We previously investigated hybrid photocatalysts prepared by grafting Re(bpy)(CO)3Cl, where bpy=2,2′-bipyridine, onto different solid-state surfaces. The hybrid photocatalysts lost reactivity in solar CO2 reduction although the diimine-tricarbonyl coordination of Re(I) was maintained after photocatalysis. In this study, we synthesized a hybrid photocatalyst by grafting the Re(I) complex on Kaolin and further explored the interactions between CO2 and the hybrid photocatalyst using in situ infrared spectroscopy. Our infrared studies at the gas–surface interface revealed the formation of two surface Re(I) complexes in the presence of CO2 and triethanolamine, a basic electron donor. One of the surface Re(I) complexes is considered as the real catalyst for CO2 reduction, while the other complex might be associated with the deactivation of our hybrid photocatalysts in CO2 reduction. The layered structure of Kaolin likely contributed to the formation of a surface Re(I)-OH complex.

Fe-catalyzed photoreduction of Cr(VI) with dicarboxylic acid (C2–C5): divergent reaction pathways

Reduction of carcinogenic hexavalent chromium (Cr(VI)) by ferrous iron (Fe(II)) to essentially nontoxic trivalent chromium (Cr(III)) is a simple and effective method for Cr(VI) decontamination. In this study, the photoreduction efficacy of Cr(VI) in acidic Fe(III)/Fe(II) solutions was examined in the presence of dicarboxylic acids (DCA) (i.e. oxalic acid (Oxal), malonic acid (Mal), succinic acid (Suc), and glutaric acid (Glu)). Rates of Cr(VI) reduction under UV irradiation depend on the photochemistry of Fe(III)/Fe(II) complexes with DCA, following the order of Oxal > Suc ≈ Glu > Mal. Fe(III)-oxalato complexes are favorable for transforming Cr(VI) due to the formation of relative abundance of Fe(II) and reductive radicals such as CO2·-, O2·- after photoreaction, whereas Fe(III)-Mal complexes upon UV irradiation generate the lowest amount of Fe(II) among all the Fe-DCA complexes, and thus have poor capacity for Cr(VI) reduction. The effects of Suc and Glu on the Cr(VI) reduction are insignificant since the Fe(III)-OH complexes are still the main Fe(III) species due to their weak chelating abilities with Fe(III). This work has important implications for selecting the favorable ligands of Fe(III) for light-induced Cr(VI) transformation and designing the methods for treating wastewater with low-molecular-weight acids and Cr(VI).

Saturation kinetics in phenolic O-H bond oxidation by a mononuclear Mn(III)-OH complex derived from dioxygen.

The mononuclear hydroxomanganese(III) complex, [Mn(III)(OH)(dpaq)](+), which is supported by the amide-containing N5 ligand dpaq (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) was generated by treatment of the manganese(II) species, [Mn(II)(dpaq)](OTf), with dioxygen in acetonitrile solution at 25 °C. This oxygenation reaction proceeds with essentially quantitative yield (greater than 98% isolated yield) and represents a rare example of an O2-mediated oxidation of a manganese(II) complex to generate a single product. The X-ray diffraction structure of [Mn(III)(OH)(dpaq)](+) reveals a short Mn-OH distance of 1.806(13) Å, with the hydroxo moiety trans to the amide function of the dpaq ligand. No shielding of the hydroxo group is observed in the solid-state structure. Nonetheless, [Mn(III)(OH)(dpaq)](+) is remarkably stable, decreasing in concentration by only 10% when stored in MeCN at 25 °C for 1 week. The [Mn(III)(OH)(dpaq)](+) complex participates in proton-coupled electron transfer reactions with substrates with relatively weak O-H and C-H bonds. For example, [Mn(III)(OH)(dpaq)](+) oxidizes TEMPOH (TEMPOH = 2,2'-6,6'-tetramethylpiperidine-1-ol), which has a bond dissociation free energy (BDFE) of 66.5 kcal/mol, in MeCN at 25 °C. The hydrogen/deuterium kinetic isotope effect of 1.8 observed for this reaction implies a concerted proton-electron transfer pathway. The [Mn(III)(OH)(dpaq)](+) complex also oxidizes xanthene (C-H BDFE of 73.3 kcal/mol in dimethylsulfoxide) and phenols, such as 2,4,6-tri-t-butylphenol, with BDFEs of less than 79 kcal/mol. Saturation kinetics were observed for phenol oxidation, implying an initial equilibrium prior to the rate-determining step. On the basis of a collective body of evidence, the equilibrium step is attributed to the formation of a hydrogen-bonding complex between [Mn(III)(OH)(dpaq)](+) and the phenol substrates.

Synthesis and photocatalysis performances of bismuth oxynitrate photocatalysts with layered structures.

A new type of layered oxy-acid salt of bismuth oxynitrate was synthesized by a simple hydrothermal method. The obtained bismuth oxy-nitrates consist of a Bi2O2(2+) layered module inserted into the interlamellar anion modules of NO3(-) and OH(-). Varying amounts of NO3(-) and OH(-) complexes on the surface of the bismuth oxynitrate were also obtained by adjusting the precursor pH before hydrothermal treatment. It was found that the sample prepared with the precursor pH = 5.00 presented the highest photocatalytic activity, with a rate constant of 0.05 min(-1), which is 2 and 6.7 times higher than those presented by the samples with the precursor pH = 7.00 and 1.22, respectively. The largest cathodic to anodic photocurrent switching was also presented by the sample with the precursor pH = 5.00, which can be reasonably attributed to NO3(-) complexes on the surface of the bismuth oxynitrate. The NO3(-) complexes could efficiently migrate the photo-induced holes to the surface of the semiconductor.

Mn K-edge X-ray absorption studies of oxo- and hydroxo-manganese(IV) complexes: experimental and theoretical insights into pre-edge properties.

Mn K-edge X-ray absorption spectroscopy (XAS) was used to gain insights into the geometric and electronic structures of [MnII(Cl)2(Me2EBC)], [MnIV(OH)2(Me2EBC)]2+, and [MnIV(O)(OH)(Me2EBC)]+, which are all supported by the tetradentate, macrocyclic Me2EBC ligand (Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Analysis of extended X-ray absorption fine structure (EXAFS) data for [MnIV(O)(OH)(Me2EBC)]+ revealed Mn–O scatterers at 1.71 and 1.84 Å and Mn–N scatterers at 2.11 Å, providing the first unambiguous support for the formulation of this species as an oxohydroxomanganese(IV) adduct. EXAFS-determined structural parameters for [MnII(Cl)2(Me2EBC)] and [MnIV(OH)2(Me2EBC)]2+ are consistent with previously reported crystal structures. The Mn pre-edge energies and intensities of these complexes were examined within the context of data for other oxo- and hydroxomanganese(IV) adducts, and time-dependent density functional theory (TD-DFT) computations were used to predict pre-edge properties for all compounds considered. This combined experimental and computational analysis revealed a correlation between the Mn–O(H) distances and pre-edge peak areas of MnIV=O and MnIV–OH complexes, but this trend was strongly modulated by the MnIV coordination geometry. Mn 3d-4p mixing, which primarily accounts for the pre-edge intensities, is not solely a function of the Mn–O(H) bond length; the coordination geometry also has a large effect on the distribution of pre-edge intensity. For tetragonal MnIV=O centers, more than 90% of the pre-edge intensity comes from excitations to the Mn=O σ* MO. Trigonal bipyramidal oxomanganese(IV) centers likewise feature excitations to the Mn=O σ* molecular orbital (MO) but also show intense transitions to 3dx2–y2 and 3dxy MOs because of enhanced 3d-4px,y mixing. This gives rise to a broader pre-edge feature for trigonal MnIV=O adducts. These results underscore the importance of reporting experimental pre-edge areas rather than peak heights. Finally, the TD-DFT method was applied to understand the pre-edge properties of a recently reported S = 1 MnV=O adduct; these findings are discussed within the context of previous examinations of oxomanganese(V) complexes.

Imaging Dynamics on the F + H 2 O → HF + OH Potential Energy Surfaces from Wells to Barriers

The study of gas-phase reaction dynamics has advanced to a point where four-atom reactions are the proving ground for detailed comparisons between experiment and theory. Here, a combined experimental and theoretical study of the dissociation dynamics of the tetra-atomic FH2O system is presented, providing snapshots of the F + H2O → HF + OH reaction. Photoelectron-photofragment coincidence measurements of the dissociative photodetachment (DPD) of the F(-)(H2O) anion revealed various dissociation pathways along different electronic states. A distinct photoelectron spectrum of stable FH-OH complexes was also measured and attributed to long-lived Feshbach resonances. Comparison to full-dimensional quantum calculations confirms the sensitivity of the DPD measurements to the subtle dynamics on the low-lying FH2O potential energy surfaces over a wide range of nuclear configurations and energies.

Reactivity of a Ru(III)-hydroxo complex in substrate oxidation in water.

A mononuclear Ru(III)-OH complex oxidizes substrates such as hydroquinones in water through a pre-equilibrium process based on adduct formation by hydrogen bonding between the Ru(III)-OH complex and the substrates. The reaction mechanism switches from hydrogen atom transfer to electron transfer depending on the oxidation potential of substrates.

Dioxygen activation by an organometallic Pd(ii) precursor: formation of a Pd(iv)–OH complex and its C–O bond formation reactivity

The complex (Me3tacn)Pd(II)(CH2CMe2C6H4) is readily oxidized by O2 or H2O2 to yield the Pd(IV)-OH complex [(Me3tacn)Pd(IV)(OH)(CH2CMe2C6H4)](+). Thermolysis of this product leads to the selective C(sp(2))-O reductive elimination of 2-tert-butylphenol, no C(sp(3))-O elimination product being detected. This system represents a rare example of selective C(sp(2))-O bond formation that is relevant to Pd-catalyzed aerobic C-H hydroxylation reactions.

Preparation and Properties of an MnIV-Hydroxide Complex: Proton and Electron Transfer at a Mononuclear Manganese Site and its Relationship to the Oxygen Evolving Complex within Photosystem II.

Photosynthetic water oxidation is catalyzed by a Mn4O5Ca cluster with an unprecedented arrangement of metal ions in which a single manganese center is bonded to a distorted Mn3O4Ca cubane-like structure. Several mechanistic proposals describe the unique manganese center as a site for water binding and subsequent formation of a high valent Mn-oxo center that reacts with a M-OH unit (M = Mn or CaII) to form the O-O bond. The conversion of low valent Mn-OHn (n = 1,2) to a Mn-oxo species requires that a single manganese site be able to accommodate several oxidation states as the water ligand is deprotonated. To study these processes, the preparation and characterization of a new monomeric MnIV-OH complex is described. The MnIV-OH complex completes a series of well characterized Mn-OH and Mn-oxo complexes containing the same primary and secondary coordination spheres; this work thus demonstrates that a single ligand can support mononuclear Mn complexes spanning four different oxidation states (II through V) with oxo and hydroxo ligands that are derived from water. Moreover, we have completed a thermodynamic analysis based on this series of manganese complexes to predict the formation of high valent Mn-oxo species; we demonstrated that the conversion of a MnIV-OH species to a MnV-oxo complex would likely occur via a stepwise proton transfer-electron transfer mechanism. The large dissociation energy for the MnIVO-H bond (~95 kcal/mol) diminished the likelihood that other pathways are operative within a biological context. Furthermore, these studies showed that reactions between Mn-OH and Mn-oxo complexes lead to non-productive, one-electron processes suggesting that initial O-O bond formation with the OEC does not involve an Mn-OH unit.