Synthesis and Structural Characterization of a Series of Mn^IIIOR Complexes, Including a Water-Soluble Mn^IIIOH That Promotes Aerobic Hydrogen-Atom Transfer

Hydrogen-atom-transfer (HAT) reactions are a class of proton-coupled electron-transfer (PCET) reactions used in biology to promote substrate oxidation. The driving force for such reactions depends on both the oxidation potential of the catalyst and the p<i>K</i>_a value of the proton-acceptor site. Both high-valent transition-metal oxo M^IVO (M = Fe, Mn) and lower-valent transition-metal hydroxo compounds M^IIIOH (M = Fe, Mn) have been shown to promote these reactions. Herein we describe the synthesis, structure, and reactivity properties of a series of Mn^IIIOR compounds [R = ^<i>p</i>NO₂Ph (<b>5</b>), Ph (<b>6</b>), Me (<b>7</b>), H (<b>8</b>)], some of which abstract H atoms. The Mn^IIIOH complex <b>8</b> is water-soluble and represents a rare example of a stable mononuclear Mn^IIIOH. In water, the redox potential of <b>8</b> was found to be pH-dependent and the Pourbaix (<i>E</i>_p,c vs pH) diagram has a slope (52 mV pH^–1) that is indicative of the transfer a single proton with each electron (i.e., PCET). The two compounds with the lowest oxidation potential, hydroxide- and methoxide-bound <b>7</b> and <b>8</b>, are found to oxidize 2,2′,6,6′-tetramethylpiperidin-1-ol (TEMPOH), whereas the compounds with the highest oxidation potential, phenol-ligated <b>5</b> and <b>6</b>, are shown to be unreactive. Hydroxide-bound <b>8</b> reacts with TEMPOH an order of magnitude faster than methoxide-bound <b>7</b>. Kinetic data [<i>k</i>_H/<i>k</i>_D = 3.1 (<b>8</b>); <i>k</i>_H/<i>k</i>_D = 2.1 (<b>7</b>)] are consistent with concerted H-atom abstraction. The reactive species <b>8</b> can be aerobically regenerated in H₂O, and at least 10 turnovers can be achieved without significant degradation of the “catalyst”. The linear correlation between the redox potential and pH, obtained from the Pourbaix diagram, was used to calculate the bond dissociation free energy (BDFE) = 74.0 ± 0.5 kcal mol^–1 for Mn^IIOH₂ in water, and in MeCN, its BDFE was estimated to be 70.1 kcal mol^–1. The reduced protonated derivative of <b>8</b>, [Mn^II(S^Me2N₄(tren))­(H₂O)]⁺ (<b>9</b>), was estimated to have a p<i>K</i>_a of 21.2 in MeCN. The ability (<b>7</b>) and inability (<b>5</b> and <b>6</b>) of the other members of the series to abstract a H atom from TEMPOH was used to estimate either an upper or lower limit to the Mn^IIO­(H)­R p<i>K</i>_a based on their experimentally determined redox potentials. The trend in p<i>K</i>_a [21.2 (R = H) > 16.2 (R = Me) > 13.5 (R = Ph) > 12.2 (R = ^<i>p</i>NO₂Ph)] is shown to oppose that of the oxidation potential <i>E</i>_p,c [−220 (R = ^<i>p</i>NO₂Ph) > −300 (R = Ph) > −410 (R = Me) > −600 (R = H) mV vs Fc^+/0] for this particular series

Synthesis and Structural Characterization of a Series of Mn^IIIOR Complexes, Including a Water-Soluble Mn^IIIOH That Promotes Aerobic Hydrogen-Atom Transfer

Hydrogen-atom-transfer (HAT) reactions are a class of proton-coupled electron-transfer (PCET) reactions used in biology to promote substrate oxidation. The driving force for such reactions depends on both the oxidation potential of the catalyst and the p<i>K</i>_a value of the proton-acceptor site. Both high-valent transition-metal oxo M^IVO (M = Fe, Mn) and lower-valent transition-metal hydroxo compounds M^IIIOH (M = Fe, Mn) have been shown to promote these reactions. Herein we describe the synthesis, structure, and reactivity properties of a series of Mn^IIIOR compounds [R = ^<i>p</i>NO₂Ph (<b>5</b>), Ph (<b>6</b>), Me (<b>7</b>), H (<b>8</b>)], some of which abstract H atoms. The Mn^IIIOH complex <b>8</b> is water-soluble and represents a rare example of a stable mononuclear Mn^IIIOH. In water, the redox potential of <b>8</b> was found to be pH-dependent and the Pourbaix (<i>E</i>_p,c vs pH) diagram has a slope (52 mV pH^–1) that is indicative of the transfer a single proton with each electron (i.e., PCET). The two compounds with the lowest oxidation potential, hydroxide- and methoxide-bound <b>7</b> and <b>8</b>, are found to oxidize 2,2′,6,6′-tetramethylpiperidin-1-ol (TEMPOH), whereas the compounds with the highest oxidation potential, phenol-ligated <b>5</b> and <b>6</b>, are shown to be unreactive. Hydroxide-bound <b>8</b> reacts with TEMPOH an order of magnitude faster than methoxide-bound <b>7</b>. Kinetic data [<i>k</i>_H/<i>k</i>_D = 3.1 (<b>8</b>); <i>k</i>_H/<i>k</i>_D = 2.1 (<b>7</b>)] are consistent with concerted H-atom abstraction. The reactive species <b>8</b> can be aerobically regenerated in H₂O, and at least 10 turnovers can be achieved without significant degradation of the “catalyst”. The linear correlation between the redox potential and pH, obtained from the Pourbaix diagram, was used to calculate the bond dissociation free energy (BDFE) = 74.0 ± 0.5 kcal mol^–1 for Mn^IIOH₂ in water, and in MeCN, its BDFE was estimated to be 70.1 kcal mol^–1. The reduced protonated derivative of <b>8</b>, [Mn^II(S^Me2N₄(tren))­(H₂O)]⁺ (<b>9</b>), was estimated to have a p<i>K</i>_a of 21.2 in MeCN. The ability (<b>7</b>) and inability (<b>5</b> and <b>6</b>) of the other members of the series to abstract a H atom from TEMPOH was used to estimate either an upper or lower limit to the Mn^IIO­(H)­R p<i>K</i>_a based on their experimentally determined redox potentials. The trend in p<i>K</i>_a [21.2 (R = H) > 16.2 (R = Me) > 13.5 (R = Ph) > 12.2 (R = ^<i>p</i>NO₂Ph)] is shown to oppose that of the oxidation potential <i>E</i>_p,c [−220 (R = ^<i>p</i>NO₂Ph) > −300 (R = Ph) > −410 (R = Me) > −600 (R = H) mV vs Fc^+/0] for this particular series

Synthesis and Structural Characterization of a Series of Mn^IIIOR Complexes, Including a Water-Soluble Mn^IIIOH That Promotes Aerobic Hydrogen-Atom Transfer

Hydrogen-atom-transfer (HAT) reactions are a class of proton-coupled electron-transfer (PCET) reactions used in biology to promote substrate oxidation. The driving force for such reactions depends on both the oxidation potential of the catalyst and the p<i>K</i>_a value of the proton-acceptor site. Both high-valent transition-metal oxo M^IVO (M = Fe, Mn) and lower-valent transition-metal hydroxo compounds M^IIIOH (M = Fe, Mn) have been shown to promote these reactions. Herein we describe the synthesis, structure, and reactivity properties of a series of Mn^IIIOR compounds [R = ^<i>p</i>NO₂Ph (<b>5</b>), Ph (<b>6</b>), Me (<b>7</b>), H (<b>8</b>)], some of which abstract H atoms. The Mn^IIIOH complex <b>8</b> is water-soluble and represents a rare example of a stable mononuclear Mn^IIIOH. In water, the redox potential of <b>8</b> was found to be pH-dependent and the Pourbaix (<i>E</i>_p,c vs pH) diagram has a slope (52 mV pH^–1) that is indicative of the transfer a single proton with each electron (i.e., PCET). The two compounds with the lowest oxidation potential, hydroxide- and methoxide-bound <b>7</b> and <b>8</b>, are found to oxidize 2,2′,6,6′-tetramethylpiperidin-1-ol (TEMPOH), whereas the compounds with the highest oxidation potential, phenol-ligated <b>5</b> and <b>6</b>, are shown to be unreactive. Hydroxide-bound <b>8</b> reacts with TEMPOH an order of magnitude faster than methoxide-bound <b>7</b>. Kinetic data [<i>k</i>_H/<i>k</i>_D = 3.1 (<b>8</b>); <i>k</i>_H/<i>k</i>_D = 2.1 (<b>7</b>)] are consistent with concerted H-atom abstraction. The reactive species <b>8</b> can be aerobically regenerated in H₂O, and at least 10 turnovers can be achieved without significant degradation of the “catalyst”. The linear correlation between the redox potential and pH, obtained from the Pourbaix diagram, was used to calculate the bond dissociation free energy (BDFE) = 74.0 ± 0.5 kcal mol^–1 for Mn^IIOH₂ in water, and in MeCN, its BDFE was estimated to be 70.1 kcal mol^–1. The reduced protonated derivative of <b>8</b>, [Mn^II(S^Me2N₄(tren))­(H₂O)]⁺ (<b>9</b>), was estimated to have a p<i>K</i>_a of 21.2 in MeCN. The ability (<b>7</b>) and inability (<b>5</b> and <b>6</b>) of the other members of the series to abstract a H atom from TEMPOH was used to estimate either an upper or lower limit to the Mn^IIO­(H)­R p<i>K</i>_a based on their experimentally determined redox potentials. The trend in p<i>K</i>_a [21.2 (R = H) > 16.2 (R = Me) > 13.5 (R = Ph) > 12.2 (R = ^<i>p</i>NO₂Ph)] is shown to oppose that of the oxidation potential <i>E</i>_p,c [−220 (R = ^<i>p</i>NO₂Ph) > −300 (R = Ph) > −410 (R = Me) > −600 (R = H) mV vs Fc^+/0] for this particular series

Synthesis and Structural Characterization of a Series of Mn^IIIOR Complexes, Including a Water-Soluble Mn^IIIOH That Promotes Aerobic Hydrogen-Atom Transfer

Hydrogen-atom-transfer (HAT) reactions are a class of proton-coupled electron-transfer (PCET) reactions used in biology to promote substrate oxidation. The driving force for such reactions depends on both the oxidation potential of the catalyst and the p<i>K</i>_a value of the proton-acceptor site. Both high-valent transition-metal oxo M^IVO (M = Fe, Mn) and lower-valent transition-metal hydroxo compounds M^IIIOH (M = Fe, Mn) have been shown to promote these reactions. Herein we describe the synthesis, structure, and reactivity properties of a series of Mn^IIIOR compounds [R = ^<i>p</i>NO₂Ph (<b>5</b>), Ph (<b>6</b>), Me (<b>7</b>), H (<b>8</b>)], some of which abstract H atoms. The Mn^IIIOH complex <b>8</b> is water-soluble and represents a rare example of a stable mononuclear Mn^IIIOH. In water, the redox potential of <b>8</b> was found to be pH-dependent and the Pourbaix (<i>E</i>_p,c vs pH) diagram has a slope (52 mV pH^–1) that is indicative of the transfer a single proton with each electron (i.e., PCET). The two compounds with the lowest oxidation potential, hydroxide- and methoxide-bound <b>7</b> and <b>8</b>, are found to oxidize 2,2′,6,6′-tetramethylpiperidin-1-ol (TEMPOH), whereas the compounds with the highest oxidation potential, phenol-ligated <b>5</b> and <b>6</b>, are shown to be unreactive. Hydroxide-bound <b>8</b> reacts with TEMPOH an order of magnitude faster than methoxide-bound <b>7</b>. Kinetic data [<i>k</i>_H/<i>k</i>_D = 3.1 (<b>8</b>); <i>k</i>_H/<i>k</i>_D = 2.1 (<b>7</b>)] are consistent with concerted H-atom abstraction. The reactive species <b>8</b> can be aerobically regenerated in H₂O, and at least 10 turnovers can be achieved without significant degradation of the “catalyst”. The linear correlation between the redox potential and pH, obtained from the Pourbaix diagram, was used to calculate the bond dissociation free energy (BDFE) = 74.0 ± 0.5 kcal mol^–1 for Mn^IIOH₂ in water, and in MeCN, its BDFE was estimated to be 70.1 kcal mol^–1. The reduced protonated derivative of <b>8</b>, [Mn^II(S^Me2N₄(tren))­(H₂O)]⁺ (<b>9</b>), was estimated to have a p<i>K</i>_a of 21.2 in MeCN. The ability (<b>7</b>) and inability (<b>5</b> and <b>6</b>) of the other members of the series to abstract a H atom from TEMPOH was used to estimate either an upper or lower limit to the Mn^IIO­(H)­R p<i>K</i>_a based on their experimentally determined redox potentials. The trend in p<i>K</i>_a [21.2 (R = H) > 16.2 (R = Me) > 13.5 (R = Ph) > 12.2 (R = ^<i>p</i>NO₂Ph)] is shown to oppose that of the oxidation potential <i>E</i>_p,c [−220 (R = ^<i>p</i>NO₂Ph) > −300 (R = Ph) > −410 (R = Me) > −600 (R = H) mV vs Fc^+/0] for this particular series

Synthesis and Structural Characterization of a Series of Mn^IIIOR Complexes, Including a Water-Soluble Mn^IIIOH That Promotes Aerobic Hydrogen-Atom Transfer

Hydrogen-atom-transfer (HAT) reactions are a class of proton-coupled electron-transfer (PCET) reactions used in biology to promote substrate oxidation. The driving force for such reactions depends on both the oxidation potential of the catalyst and the p<i>K</i>_a value of the proton-acceptor site. Both high-valent transition-metal oxo M^IVO (M = Fe, Mn) and lower-valent transition-metal hydroxo compounds M^IIIOH (M = Fe, Mn) have been shown to promote these reactions. Herein we describe the synthesis, structure, and reactivity properties of a series of Mn^IIIOR compounds [R = ^<i>p</i>NO₂Ph (<b>5</b>), Ph (<b>6</b>), Me (<b>7</b>), H (<b>8</b>)], some of which abstract H atoms. The Mn^IIIOH complex <b>8</b> is water-soluble and represents a rare example of a stable mononuclear Mn^IIIOH. In water, the redox potential of <b>8</b> was found to be pH-dependent and the Pourbaix (<i>E</i>_p,c vs pH) diagram has a slope (52 mV pH^–1) that is indicative of the transfer a single proton with each electron (i.e., PCET). The two compounds with the lowest oxidation potential, hydroxide- and methoxide-bound <b>7</b> and <b>8</b>, are found to oxidize 2,2′,6,6′-tetramethylpiperidin-1-ol (TEMPOH), whereas the compounds with the highest oxidation potential, phenol-ligated <b>5</b> and <b>6</b>, are shown to be unreactive. Hydroxide-bound <b>8</b> reacts with TEMPOH an order of magnitude faster than methoxide-bound <b>7</b>. Kinetic data [<i>k</i>_H/<i>k</i>_D = 3.1 (<b>8</b>); <i>k</i>_H/<i>k</i>_D = 2.1 (<b>7</b>)] are consistent with concerted H-atom abstraction. The reactive species <b>8</b> can be aerobically regenerated in H₂O, and at least 10 turnovers can be achieved without significant degradation of the “catalyst”. The linear correlation between the redox potential and pH, obtained from the Pourbaix diagram, was used to calculate the bond dissociation free energy (BDFE) = 74.0 ± 0.5 kcal mol^–1 for Mn^IIOH₂ in water, and in MeCN, its BDFE was estimated to be 70.1 kcal mol^–1. The reduced protonated derivative of <b>8</b>, [Mn^II(S^Me2N₄(tren))­(H₂O)]⁺ (<b>9</b>), was estimated to have a p<i>K</i>_a of 21.2 in MeCN. The ability (<b>7</b>) and inability (<b>5</b> and <b>6</b>) of the other members of the series to abstract a H atom from TEMPOH was used to estimate either an upper or lower limit to the Mn^IIO­(H)­R p<i>K</i>_a based on their experimentally determined redox potentials. The trend in p<i>K</i>_a [21.2 (R = H) > 16.2 (R = Me) > 13.5 (R = Ph) > 12.2 (R = ^<i>p</i>NO₂Ph)] is shown to oppose that of the oxidation potential <i>E</i>_p,c [−220 (R = ^<i>p</i>NO₂Ph) > −300 (R = Ph) > −410 (R = Me) > −600 (R = H) mV vs Fc^+/0] for this particular series

Synthesis, Protonation, and Reduction of Ruthenium–Peroxo Complexes with Pendent Nitrogen Bases

Cyclopentadienyl and pentamethylcyclopentadienyl ruthenium­(II) complexes have been synthesized with cyclic (RPCH₂NR′CH₂)₂ ligands, with the goal of using these [Cp^R′′Ru­(P^R₂N^R′₂)]⁺ complexes for catalytic O₂ reduction to H₂O (R = <i>t</i>-butyl, phenyl; R′ = benzyl, phenyl; R″ = methyl, H). In each compound, the Ru is coordinated to the two phosphines, positioning the amines of the ligand in the second coordination sphere where they may act as proton relays to a bound dioxygen ligand. The phosphine, amine, and cyclopentadienyl substituents have been systematically varied in order to understand the effects of each of these parameters on the properties of the complexes. These Cp^R″Ru­(P^R₂N^R′₂)⁺ complexes react with O₂ to form η²-peroxo complexes, which have been characterized by NMR, IR, and X-ray crystallography. The peak reduction potentials of the O₂ ligated complexes have been shown by cyclic voltammetry to vary as much as 0.1 V upon varying the phosphine and amine. In the presence of acid, protonation of these complexes occurs at the pendent amine, forming a hydrogen bond between the protonated amine and the bound O₂. The ruthenium–peroxo complexes decompose upon reduction, precluding catalytic O₂ reduction. The irreversible reduction potentials of the protonated O₂ complexes depend on the basicity of the pendent amine, giving insight into the role of the proton relay in facilitating reduction

A C–C Bonded Phenoxyl Radical Dimer with a Zero Bond Dissociation Free Energy

The 2,6-di-<i>tert</i>-butyl-4-methoxyphenoxyl radical is shown to dimerize in solution and in the solid state. The X-ray crystal structure of the dimer, the first for a para-coupled phenoxyl radical, revealed a bond length of 1.6055(23) Å for the C4–C4a bond. This is significantly longer than typical C–C bonds. Solution equilibrium studies using both optical and IR spectroscopies showed that the <i>K</i>_eq for dissociation is 1.3 ± 0.2 M at 20 °C, indicating a C–C bond dissociation free energy of −0.15 ± 0.1 kcal mol^–1. Van’t Hoff analysis gave an exceptionally small bond dissociation enthalpy (BDE) of 6.1 ± 0.5 kcal mol^–1. To our knowledge, this is the smallest BDE measured for a C–C bond. This very weak bond shows a large deviation from the correlation of C–C bond lengths and strengths, but the computed force constant follows Badger’s rule

Electron Transfer Mediator Effects in the Oxidative Activation of a Ruthenium Dicarboxylate Water Oxidation Catalyst

The mechanism of electrocatalytic water oxidation by the water oxidation catalyst, ruthenium 2,2′-bipyridine-6,6′-dicarboxylate (bda) bis-isoquinoline (isoq), [Ru­(bda)­(isoq)₂], <b>1</b>, at metal oxide electrodes has been investigated. At indium-doped tin oxide (ITO), diminished catalytic currents and increased overpotentials are observed compared to glassy carbon (GC). At pH 7.2 in 0.5 M NaClO₄, catalytic activity is enhanced by the addition of [Ru­(bpy)₃]²⁺ (bpy = bipyridine) as a redox mediator. Enhanced catalytic rates are also observed at ITO electrodes derivatized with the surface-bound phosphonic acid derivative [Ru­(4,4′-(PO₃H₂)₂bpy)­(bpy)₂]²⁺, <b>RuP</b>²⁺. Controlled potential electrolysis with measurement of O₂ at ITO with and without surface-bound RuP²⁺ confirm that water oxidation catalysis occurs. Remarkable rate enhancements are observed with added acetate and phosphate, consistent with an important mechanistic role for atom-proton transfer (APT) in the rate-limiting step as described previously at GC electrodes

Synthesis and Electrocatalytic Water Oxidation by Electrode-Bound Helical Peptide Chromophore–Catalyst Assemblies

Artificial photosynthesis based on dye-sensitized photoelectrosynthesis cells requires the assembly of a chromophore and catalyst in close proximity on the surface of a transparent, high band gap oxide semiconductor for integrated light absorption and catalysis. While there are a number of approaches to assemble mixtures of chromophores and catalysts on a surface for use in artificial photosynthesis based on dye-sensitized photoelectrosynthesis cells, the synthesis of discrete surface-bound chromophore–catalyst conjugates is a challenging task with few examples to date. Herein, a versatile synthetic approach and electrochemical characterization of a series of oligoproline-based light-harvesting chromophore–water-oxidation catalyst assemblies is described. This approach combines solid-phase peptide synthesis for systematic variation of the backbone, copper­(I)-catalyzed azide–alkyne cycloaddition (CuAAC) as an orthogonal approach to install the chromophore, and assembly of the water-oxidation catalyst in the final step. Importantly, the catalyst was found to be incompatible with the conditions both for amide bond formation and for the CuAAC reaction. The modular nature of the synthesis with late-stage assembly of the catalyst allows for systematic variation in the spatial arrangement of light-harvesting chromophore and water-oxidation catalyst and the role of intrastrand distance on chromophore–catalyst assembly properties. Controlled potential electrolysis experiments verified that the surface-bound assemblies function as water-oxidation electrocatalysts, and electrochemical kinetics data demonstrate that the assemblies exhibit greater than 10-fold rate enhancements compared to the homogeneous catalyst alone

Electron Transfer Mediator Effects in Water Oxidation Catalysis by Solution and Surface-Bound Ruthenium Bpy-Dicarboxylate Complexes

Electrocatalytic water oxidation by the catalyst, ruthenium 2,2′-bipyridine-6,6′-dicarboxylate (bda) bis-isoquinoline (isoq), [Ru­(bda)­(isoq)₂], <b>1</b>, was investigated at metal oxide electrodes surface-derivatized with electron transfer (ET) mediators. At indium-doped tin oxide (ITO) in pH 7.2 in H₂PO₄^–/HPO₄^2– buffers in 0.5 M NaClO₄ with added acetonitrile (MeCN), the catalytic activity of <b>1</b> is enhanced by the surface-bound redox mediators [Ru (4,4′-PO₃H₂-bpy)­(4,4′-R-bpy)₂]²⁺ (<b>RuPbpyR</b>_<b>2</b><b>²⁺</b>, R = Br, H, Me, or OMe, bpy = 2,2′-bipyridine). Rate-limiting ET between the Ru³⁺ form of the mediator and the Ru^IV(O) form in the [Ru^V/IV(O)]^+/0 couple of <b>1</b> is observed at relatively high concentrations of HPO₄^2– buffer base under conditions where O···O bond formation is facilitated by atom-proton transfer (APT). For the solution [Ru­(bpy)₃]^3+/2+ mediator couple and <b>1</b> as the catalyst, catalytic currents vary systematically with the concentration of mediator and the HPO₄^2– buffer base concentration. Electron transfer mediation of water oxidation catalysis was also investigated on nanoparticle TiO₂ electrodes co-loaded with catalyst [Ru­(bda)­(py-4-O­(CH₂)₃-PO₃H₂)₂], <b>2</b>, (py = pyridine) and <b>RuPbpyR</b>_<b>2</b><b>²⁺</b> (R = H, Me, or OMe) with an interplay between rate-limiting catalyst oxidation and rate-limiting O···O bond formation by APT. Lastly, the co-loaded assembly <b>RuPbpyR</b>_<b>2</b><b>²⁺</b> + <b>2</b> has been investigated in a dye-sensitized photoelectrosynthesis cell for water splitting