Base-enhanced catalytic water oxidation by a carboxylate–bipyridine Ru(II) complex

Development of rapid, robust water oxidation catalysts remains an essential element in solar water splitting by artificial photosynthesis. We report here dramatic rate enhancements with added buffer bases for a robust Ru(II) polypyridyl catalyst with a calculated half-time for water oxidation of ∼7 μs in 1.0 M phosphate. The results of detailed kinetic studies provide insight into the water oxidation mechanism and an important role for added buffer bases in accelerating water oxidation by concerted atom–proton transfer

Isolation and Characterization of a Dihydroxo-Bridged Iron(III,III)(μ-OH)₂ Diamond Core Derived from Dioxygen

Dioxygen addition to coordinatively unsaturated [Fe­(II)­(O^Me2N₄(6-Me-DPEN))]­(PF₆) (<b>1</b>) is shown to afford a complex containing a dihydroxo-bridged Fe­(III)₂(μ-OH)₂ diamond core, [Fe^III(O^Me2N₄(6-Me-DPEN))]₂(μ-OH)₂(PF₆)₂·(CH₃CH₂CN)₂ (<b>2</b>). The diamond core of <b>2</b> resembles the oxidized methane monooxygenase (MMOox) resting state, as well as the active site product formed following H-atom abstraction from Tyr-OH by ribonucleotide reductase (RNR). The Fe-OH bond lengths of <b>2</b> are comparable with those of the MMOHox suggesting that MMOHox contains a Fe­(III)₂(μ-OH)₂ as opposed to Fe­(III)₂(μ-OH)­(μ-OH₂) diamond core as had been suggested. Isotopic labeling experiments with ¹⁸O₂ and CD₃CN indicate that the oxygen and proton of the μ-OH bridges of <b>2</b> are derived from dioxygen and acetonitrile. Deuterium incorporation (from CD₃CN) suggests that an unobserved intermediate capable of abstracting a H-atom from CH₃CN forms en route to <b>2</b>. Given the high C–H bond dissociation energy (BDE = 97 kcal/mol) of acetonitrile, this indicates that this intermediate is a potent oxidant, possibly a high-valent iron oxo. Consistent with this, iodosylbenzene (PhIO) also reacts with <b>1</b> in CD₃CN to afford the deuterated Fe­(III)₂(μ-OD)₂ derivative of <b>2</b>. Intermediates are not spectroscopically observed in either reaction (O₂ and PhIO) even at low-temperatures (−80 °C), indicating that this intermediate has a very short lifetime, likely due to its highly reactive nature. Hydroxo-bridged <b>2</b> was found to stoichiometrically abstract hydrogen atoms from 9,10-dihydroanthracene (C–H BDE = 76 kcal/mol) at ambient temperatures

Isolation and Characterization of a Dihydroxo-Bridged Iron(III,III)(μ-OH)₂ Diamond Core Derived from Dioxygen

Dioxygen addition to coordinatively unsaturated [Fe­(II)­(O^Me2N₄(6-Me-DPEN))]­(PF₆) (<b>1</b>) is shown to afford a complex containing a dihydroxo-bridged Fe­(III)₂(μ-OH)₂ diamond core, [Fe^III(O^Me2N₄(6-Me-DPEN))]₂(μ-OH)₂(PF₆)₂·(CH₃CH₂CN)₂ (<b>2</b>). The diamond core of <b>2</b> resembles the oxidized methane monooxygenase (MMOox) resting state, as well as the active site product formed following H-atom abstraction from Tyr-OH by ribonucleotide reductase (RNR). The Fe-OH bond lengths of <b>2</b> are comparable with those of the MMOHox suggesting that MMOHox contains a Fe­(III)₂(μ-OH)₂ as opposed to Fe­(III)₂(μ-OH)­(μ-OH₂) diamond core as had been suggested. Isotopic labeling experiments with ¹⁸O₂ and CD₃CN indicate that the oxygen and proton of the μ-OH bridges of <b>2</b> are derived from dioxygen and acetonitrile. Deuterium incorporation (from CD₃CN) suggests that an unobserved intermediate capable of abstracting a H-atom from CH₃CN forms en route to <b>2</b>. Given the high C–H bond dissociation energy (BDE = 97 kcal/mol) of acetonitrile, this indicates that this intermediate is a potent oxidant, possibly a high-valent iron oxo. Consistent with this, iodosylbenzene (PhIO) also reacts with <b>1</b> in CD₃CN to afford the deuterated Fe­(III)₂(μ-OD)₂ derivative of <b>2</b>. Intermediates are not spectroscopically observed in either reaction (O₂ and PhIO) even at low-temperatures (−80 °C), indicating that this intermediate has a very short lifetime, likely due to its highly reactive nature. Hydroxo-bridged <b>2</b> was found to stoichiometrically abstract hydrogen atoms from 9,10-dihydroanthracene (C–H BDE = 76 kcal/mol) at ambient temperatures

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