Harvard University

Graduate Student

Graduate student studying high-valent Pd chemistry for both C-H bond oxidation as well as PET imaging applications in Tobias Ritter's research group.

Harvard University

Post-Doctoral Fellow

Post-doctoral fellow studying molecular photocatalysts for solar energy conversion in Dan Nocera's research group.

Texas A&M University

Assistant Professor

David worked at Texas A&M University as a Assistant Professor

Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Chemical Science

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Chemical Science

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

ACS Catalysis

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H2 by protonation of a formally Co(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the meso carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co(II) species is reduced to a monoanionic singlet Co(I) species. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms.

Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Chemical Science

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

ACS Catalysis

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H2 by protonation of a formally Co(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the meso carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co(II) species is reduced to a monoanionic singlet Co(I) species. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms.

Halide-Bridged Binuclear HX-Splitting Catalysts

Inorganic Chemistry

Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Chemical Science

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

ACS Catalysis

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H2 by protonation of a formally Co(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the meso carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co(II) species is reduced to a monoanionic singlet Co(I) species. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms.

Halide-Bridged Binuclear HX-Splitting Catalysts

Inorganic Chemistry

A Transition State Analogue for the Oxidation of Binuclear Palladium(II) to Binuclear Pd(III) Complexes

Organometallics

Cooperative metal–metal (M–M) redox chemistry has the potential to lower activation barriers for redox transformations relevant to catalysis. Pd2(III,III) complexes, generated by oxidation of Pd2(II,II) complexes, have recently been implicated as intermediates in a variety of Pd-catalyzed C–H oxidation reactions. M–M redox synergy, mediated by Pd–Pd bond formation and cleavage, has been proposed to facilitate both oxidation and reductive elimination steps during various Pd-catalyzed directed C–H oxidation reactions. Herein, we report a transition state mimic for the oxidation of Pd2(II,II) complexes which suggests that M–M redox synergy is involved in the oxidation of Pd2(II,II) complexes to Pd2(III,III) complexes.

Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Chemical Science

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

ACS Catalysis

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H2 by protonation of a formally Co(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the meso carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co(II) species is reduced to a monoanionic singlet Co(I) species. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms.

Halide-Bridged Binuclear HX-Splitting Catalysts

Inorganic Chemistry

A Transition State Analogue for the Oxidation of Binuclear Palladium(II) to Binuclear Pd(III) Complexes

Organometallics

Cooperative metal–metal (M–M) redox chemistry has the potential to lower activation barriers for redox transformations relevant to catalysis. Pd2(III,III) complexes, generated by oxidation of Pd2(II,II) complexes, have recently been implicated as intermediates in a variety of Pd-catalyzed C–H oxidation reactions. M–M redox synergy, mediated by Pd–Pd bond formation and cleavage, has been proposed to facilitate both oxidation and reductive elimination steps during various Pd-catalyzed directed C–H oxidation reactions. Herein, we report a transition state mimic for the oxidation of Pd2(II,II) complexes which suggests that M–M redox synergy is involved in the oxidation of Pd2(II,II) complexes to Pd2(III,III) complexes.

Metal-Metal Bond Containing Complexes as Catalysts for C-H Functionalization

Progress in Inorganic Chemistry

This chapter explores examples in which binuclear complexes are involved in C[BOND]H functionalization reactions and explores the effect of M[BOND]M bonds in catalysis. Functionalization of C[BOND]H bonds is a field of current interest, and many methodologies for achieving selective C[BOND]H functionalization continue to appear in the literature. The chapter discusses mixed-valent Ru2(II,III) dimers, having an Ru-Ru bond order of 2.5. Although the topics of Rh2 and Pd2 C[BOND]H functionalization have been reviewed extensively, this chapter focuses on the relationship between electronic structure and reaction mechanisms. It compares the chemistries of Rh2 and Pd2 complexes and discusses the ways in which the intriguing geometric and electronic structure of these species may facilitate C[BOND]H functionalization reactions.

Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Chemical Science

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

ACS Catalysis

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H2 by protonation of a formally Co(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the meso carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co(II) species is reduced to a monoanionic singlet Co(I) species. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms.

Halide-Bridged Binuclear HX-Splitting Catalysts

Inorganic Chemistry

A Transition State Analogue for the Oxidation of Binuclear Palladium(II) to Binuclear Pd(III) Complexes

Organometallics

Cooperative metal–metal (M–M) redox chemistry has the potential to lower activation barriers for redox transformations relevant to catalysis. Pd2(III,III) complexes, generated by oxidation of Pd2(II,II) complexes, have recently been implicated as intermediates in a variety of Pd-catalyzed C–H oxidation reactions. M–M redox synergy, mediated by Pd–Pd bond formation and cleavage, has been proposed to facilitate both oxidation and reductive elimination steps during various Pd-catalyzed directed C–H oxidation reactions. Herein, we report a transition state mimic for the oxidation of Pd2(II,II) complexes which suggests that M–M redox synergy is involved in the oxidation of Pd2(II,II) complexes to Pd2(III,III) complexes.

Metal-Metal Bond Containing Complexes as Catalysts for C-H Functionalization

Progress in Inorganic Chemistry

This chapter explores examples in which binuclear complexes are involved in C[BOND]H functionalization reactions and explores the effect of M[BOND]M bonds in catalysis. Functionalization of C[BOND]H bonds is a field of current interest, and many methodologies for achieving selective C[BOND]H functionalization continue to appear in the literature. The chapter discusses mixed-valent Ru2(II,III) dimers, having an Ru-Ru bond order of 2.5. Although the topics of Rh2 and Pd2 C[BOND]H functionalization have been reviewed extensively, this chapter focuses on the relationship between electronic structure and reaction mechanisms. It compares the chemistries of Rh2 and Pd2 complexes and discusses the ways in which the intriguing geometric and electronic structure of these species may facilitate C[BOND]H functionalization reactions.

Water Oxidation Catalysis by Co(II) Impurities in Co(III)4O4 Cubanes

Journal of the American Chemical Society

The observed water oxidation activity of the compound class Co4O4(OAc)4(Py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis.

Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Chemical Science

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

ACS Catalysis

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H2 by protonation of a formally Co(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the meso carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co(II) species is reduced to a monoanionic singlet Co(I) species. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms.

Halide-Bridged Binuclear HX-Splitting Catalysts

Inorganic Chemistry

A Transition State Analogue for the Oxidation of Binuclear Palladium(II) to Binuclear Pd(III) Complexes

Organometallics

Cooperative metal–metal (M–M) redox chemistry has the potential to lower activation barriers for redox transformations relevant to catalysis. Pd2(III,III) complexes, generated by oxidation of Pd2(II,II) complexes, have recently been implicated as intermediates in a variety of Pd-catalyzed C–H oxidation reactions. M–M redox synergy, mediated by Pd–Pd bond formation and cleavage, has been proposed to facilitate both oxidation and reductive elimination steps during various Pd-catalyzed directed C–H oxidation reactions. Herein, we report a transition state mimic for the oxidation of Pd2(II,II) complexes which suggests that M–M redox synergy is involved in the oxidation of Pd2(II,II) complexes to Pd2(III,III) complexes.

Metal-Metal Bond Containing Complexes as Catalysts for C-H Functionalization

Progress in Inorganic Chemistry

This chapter explores examples in which binuclear complexes are involved in C[BOND]H functionalization reactions and explores the effect of M[BOND]M bonds in catalysis. Functionalization of C[BOND]H bonds is a field of current interest, and many methodologies for achieving selective C[BOND]H functionalization continue to appear in the literature. The chapter discusses mixed-valent Ru2(II,III) dimers, having an Ru-Ru bond order of 2.5. Although the topics of Rh2 and Pd2 C[BOND]H functionalization have been reviewed extensively, this chapter focuses on the relationship between electronic structure and reaction mechanisms. It compares the chemistries of Rh2 and Pd2 complexes and discusses the ways in which the intriguing geometric and electronic structure of these species may facilitate C[BOND]H functionalization reactions.

Water Oxidation Catalysis by Co(II) Impurities in Co(III)4O4 Cubanes

Journal of the American Chemical Society

The observed water oxidation activity of the compound class Co4O4(OAc)4(Py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis.

Photocrystallographic Observation of Halide-Bridged Intermediates in Halogen Photoeliminations

Journal of the American Chemical Society

Polynuclear transition metal complexes, which frequently constitute the active sites of both biological and chemical catalysts, provide access to unique chemical transformations that are derived from metal–metal cooperation. Reductive elimination via ligand-bridged binuclear intermediates from bimetallic cores is one mechanism by which metals may cooperate during catalysis. We have established families of Rh2 complexes that participate in HX-splitting photocatalysis in which metal–metal cooperation is credited with the ability to achieve multielectron photochemical reactions in preference to single-electron transformations. Nanosecond-resolved transient absorption spectroscopy, steady-state photocrystallography, and computational modeling have allowed direct observation and characterization of Cl-bridged intermediates (intramolecular analogues of classical ligand-bridged intermediates in binuclear eliminations) in halogen elimination reactions. On the basis of these observations, a new class of Rh2 complexes, supported by CO ligands, has been prepared, allowing for the isolation and independent characterization of the proposed halide-bridged intermediates. Direct observation of halide-bridged structures establishes binuclear reductive elimination as a viable mechanism for photogenerating energetic bonds.

Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Chemical Science

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

ACS Catalysis

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H2 by protonation of a formally Co(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the meso carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co(II) species is reduced to a monoanionic singlet Co(I) species. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms.

Halide-Bridged Binuclear HX-Splitting Catalysts

Inorganic Chemistry

A Transition State Analogue for the Oxidation of Binuclear Palladium(II) to Binuclear Pd(III) Complexes

Organometallics

Cooperative metal–metal (M–M) redox chemistry has the potential to lower activation barriers for redox transformations relevant to catalysis. Pd2(III,III) complexes, generated by oxidation of Pd2(II,II) complexes, have recently been implicated as intermediates in a variety of Pd-catalyzed C–H oxidation reactions. M–M redox synergy, mediated by Pd–Pd bond formation and cleavage, has been proposed to facilitate both oxidation and reductive elimination steps during various Pd-catalyzed directed C–H oxidation reactions. Herein, we report a transition state mimic for the oxidation of Pd2(II,II) complexes which suggests that M–M redox synergy is involved in the oxidation of Pd2(II,II) complexes to Pd2(III,III) complexes.

Metal-Metal Bond Containing Complexes as Catalysts for C-H Functionalization

Progress in Inorganic Chemistry

This chapter explores examples in which binuclear complexes are involved in C[BOND]H functionalization reactions and explores the effect of M[BOND]M bonds in catalysis. Functionalization of C[BOND]H bonds is a field of current interest, and many methodologies for achieving selective C[BOND]H functionalization continue to appear in the literature. The chapter discusses mixed-valent Ru2(II,III) dimers, having an Ru-Ru bond order of 2.5. Although the topics of Rh2 and Pd2 C[BOND]H functionalization have been reviewed extensively, this chapter focuses on the relationship between electronic structure and reaction mechanisms. It compares the chemistries of Rh2 and Pd2 complexes and discusses the ways in which the intriguing geometric and electronic structure of these species may facilitate C[BOND]H functionalization reactions.

Water Oxidation Catalysis by Co(II) Impurities in Co(III)4O4 Cubanes

Journal of the American Chemical Society

The observed water oxidation activity of the compound class Co4O4(OAc)4(Py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis.

Photocrystallographic Observation of Halide-Bridged Intermediates in Halogen Photoeliminations

Journal of the American Chemical Society

Polynuclear transition metal complexes, which frequently constitute the active sites of both biological and chemical catalysts, provide access to unique chemical transformations that are derived from metal–metal cooperation. Reductive elimination via ligand-bridged binuclear intermediates from bimetallic cores is one mechanism by which metals may cooperate during catalysis. We have established families of Rh2 complexes that participate in HX-splitting photocatalysis in which metal–metal cooperation is credited with the ability to achieve multielectron photochemical reactions in preference to single-electron transformations. Nanosecond-resolved transient absorption spectroscopy, steady-state photocrystallography, and computational modeling have allowed direct observation and characterization of Cl-bridged intermediates (intramolecular analogues of classical ligand-bridged intermediates in binuclear eliminations) in halogen elimination reactions. On the basis of these observations, a new class of Rh2 complexes, supported by CO ligands, has been prepared, allowing for the isolation and independent characterization of the proposed halide-bridged intermediates. Direct observation of halide-bridged structures establishes binuclear reductive elimination as a viable mechanism for photogenerating energetic bonds.

Two-Electron Photoreduction of a Ni(II) Halide Enables H2 Evolution from HCl

Journal of the American Chemical Society

Photochemical HX splitting requires the management of two protons and the execution of multielectron photoreactions. Herein, we report a photoinduced two-electron reduction of a polypyridyl Ni(II) chloride complex that provides a route to H2 evolution from HCl. The excited states of Ni complexes are too short to participate directly in HX activation, and hence, the excited state of a photoredox mediator is exploited for the activation of HX at the Ni(II) center. Nanosecond transient absorption (TA) spectroscopy has revealed that the excited state of the polypyridine results in a photoreduced radical that is capable of mediating HX activation by producing a Ni(I) center by halogen-atom abstraction. Disproportionation of the photogenerated Ni(I) intermediate affords Ni(II) and Ni(0) complexes. The Ni(0) center is capable of reacting with HX to produce H2 and the polypyridyl Ni(II) dichloride, closing the photocycle for H2 generation from HCl.

Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Chemical Science

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

ACS Catalysis

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H2 by protonation of a formally Co(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the meso carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co(II) species is reduced to a monoanionic singlet Co(I) species. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms.

Halide-Bridged Binuclear HX-Splitting Catalysts

Inorganic Chemistry

A Transition State Analogue for the Oxidation of Binuclear Palladium(II) to Binuclear Pd(III) Complexes

Organometallics

Cooperative metal–metal (M–M) redox chemistry has the potential to lower activation barriers for redox transformations relevant to catalysis. Pd2(III,III) complexes, generated by oxidation of Pd2(II,II) complexes, have recently been implicated as intermediates in a variety of Pd-catalyzed C–H oxidation reactions. M–M redox synergy, mediated by Pd–Pd bond formation and cleavage, has been proposed to facilitate both oxidation and reductive elimination steps during various Pd-catalyzed directed C–H oxidation reactions. Herein, we report a transition state mimic for the oxidation of Pd2(II,II) complexes which suggests that M–M redox synergy is involved in the oxidation of Pd2(II,II) complexes to Pd2(III,III) complexes.

Metal-Metal Bond Containing Complexes as Catalysts for C-H Functionalization

Progress in Inorganic Chemistry

This chapter explores examples in which binuclear complexes are involved in C[BOND]H functionalization reactions and explores the effect of M[BOND]M bonds in catalysis. Functionalization of C[BOND]H bonds is a field of current interest, and many methodologies for achieving selective C[BOND]H functionalization continue to appear in the literature. The chapter discusses mixed-valent Ru2(II,III) dimers, having an Ru-Ru bond order of 2.5. Although the topics of Rh2 and Pd2 C[BOND]H functionalization have been reviewed extensively, this chapter focuses on the relationship between electronic structure and reaction mechanisms. It compares the chemistries of Rh2 and Pd2 complexes and discusses the ways in which the intriguing geometric and electronic structure of these species may facilitate C[BOND]H functionalization reactions.

Water Oxidation Catalysis by Co(II) Impurities in Co(III)4O4 Cubanes

Journal of the American Chemical Society

The observed water oxidation activity of the compound class Co4O4(OAc)4(Py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis.

Photocrystallographic Observation of Halide-Bridged Intermediates in Halogen Photoeliminations

Journal of the American Chemical Society

Polynuclear transition metal complexes, which frequently constitute the active sites of both biological and chemical catalysts, provide access to unique chemical transformations that are derived from metal–metal cooperation. Reductive elimination via ligand-bridged binuclear intermediates from bimetallic cores is one mechanism by which metals may cooperate during catalysis. We have established families of Rh2 complexes that participate in HX-splitting photocatalysis in which metal–metal cooperation is credited with the ability to achieve multielectron photochemical reactions in preference to single-electron transformations. Nanosecond-resolved transient absorption spectroscopy, steady-state photocrystallography, and computational modeling have allowed direct observation and characterization of Cl-bridged intermediates (intramolecular analogues of classical ligand-bridged intermediates in binuclear eliminations) in halogen elimination reactions. On the basis of these observations, a new class of Rh2 complexes, supported by CO ligands, has been prepared, allowing for the isolation and independent characterization of the proposed halide-bridged intermediates. Direct observation of halide-bridged structures establishes binuclear reductive elimination as a viable mechanism for photogenerating energetic bonds.

Two-Electron Photoreduction of a Ni(II) Halide Enables H2 Evolution from HCl

Journal of the American Chemical Society

Photochemical HX splitting requires the management of two protons and the execution of multielectron photoreactions. Herein, we report a photoinduced two-electron reduction of a polypyridyl Ni(II) chloride complex that provides a route to H2 evolution from HCl. The excited states of Ni complexes are too short to participate directly in HX activation, and hence, the excited state of a photoredox mediator is exploited for the activation of HX at the Ni(II) center. Nanosecond transient absorption (TA) spectroscopy has revealed that the excited state of the polypyridine results in a photoreduced radical that is capable of mediating HX activation by producing a Ni(I) center by halogen-atom abstraction. Disproportionation of the photogenerated Ni(I) intermediate affords Ni(II) and Ni(0) complexes. The Ni(0) center is capable of reacting with HX to produce H2 and the polypyridyl Ni(II) dichloride, closing the photocycle for H2 generation from HCl.

Trap-Free Halogen Photoelimination from Mononuclear Ni(III) Complexes

Journal of the American Chemical Society

Halogen photoelimination reactions constitute the oxidative half-reaction of closed HX-splitting energy storage cycles. Here, we report high-yielding, endothermic Cl2 photoelimination chemistry from mononuclear Ni(III) complexes. On the basis of time-resolved spectroscopy and steady-state photocrystallography experiments, a mechanism involving ligand-assisted halogen elimination is proposed. Employing ancillary ligands to promote elimination offers a strategy to circumvent the inherently short-lived excited states of 3d metal complexes for the activation of thermodynamically challenging bonds.

Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Chemical Science

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

ACS Catalysis

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H2 by protonation of a formally Co(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the meso carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co(II) species is reduced to a monoanionic singlet Co(I) species. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms.

Halide-Bridged Binuclear HX-Splitting Catalysts

Inorganic Chemistry

A Transition State Analogue for the Oxidation of Binuclear Palladium(II) to Binuclear Pd(III) Complexes

Organometallics

Cooperative metal–metal (M–M) redox chemistry has the potential to lower activation barriers for redox transformations relevant to catalysis. Pd2(III,III) complexes, generated by oxidation of Pd2(II,II) complexes, have recently been implicated as intermediates in a variety of Pd-catalyzed C–H oxidation reactions. M–M redox synergy, mediated by Pd–Pd bond formation and cleavage, has been proposed to facilitate both oxidation and reductive elimination steps during various Pd-catalyzed directed C–H oxidation reactions. Herein, we report a transition state mimic for the oxidation of Pd2(II,II) complexes which suggests that M–M redox synergy is involved in the oxidation of Pd2(II,II) complexes to Pd2(III,III) complexes.

Metal-Metal Bond Containing Complexes as Catalysts for C-H Functionalization

Progress in Inorganic Chemistry

This chapter explores examples in which binuclear complexes are involved in C[BOND]H functionalization reactions and explores the effect of M[BOND]M bonds in catalysis. Functionalization of C[BOND]H bonds is a field of current interest, and many methodologies for achieving selective C[BOND]H functionalization continue to appear in the literature. The chapter discusses mixed-valent Ru2(II,III) dimers, having an Ru-Ru bond order of 2.5. Although the topics of Rh2 and Pd2 C[BOND]H functionalization have been reviewed extensively, this chapter focuses on the relationship between electronic structure and reaction mechanisms. It compares the chemistries of Rh2 and Pd2 complexes and discusses the ways in which the intriguing geometric and electronic structure of these species may facilitate C[BOND]H functionalization reactions.

Water Oxidation Catalysis by Co(II) Impurities in Co(III)4O4 Cubanes

Journal of the American Chemical Society

The observed water oxidation activity of the compound class Co4O4(OAc)4(Py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis.

Photocrystallographic Observation of Halide-Bridged Intermediates in Halogen Photoeliminations

Journal of the American Chemical Society

Polynuclear transition metal complexes, which frequently constitute the active sites of both biological and chemical catalysts, provide access to unique chemical transformations that are derived from metal–metal cooperation. Reductive elimination via ligand-bridged binuclear intermediates from bimetallic cores is one mechanism by which metals may cooperate during catalysis. We have established families of Rh2 complexes that participate in HX-splitting photocatalysis in which metal–metal cooperation is credited with the ability to achieve multielectron photochemical reactions in preference to single-electron transformations. Nanosecond-resolved transient absorption spectroscopy, steady-state photocrystallography, and computational modeling have allowed direct observation and characterization of Cl-bridged intermediates (intramolecular analogues of classical ligand-bridged intermediates in binuclear eliminations) in halogen elimination reactions. On the basis of these observations, a new class of Rh2 complexes, supported by CO ligands, has been prepared, allowing for the isolation and independent characterization of the proposed halide-bridged intermediates. Direct observation of halide-bridged structures establishes binuclear reductive elimination as a viable mechanism for photogenerating energetic bonds.

Two-Electron Photoreduction of a Ni(II) Halide Enables H2 Evolution from HCl

Journal of the American Chemical Society

Photochemical HX splitting requires the management of two protons and the execution of multielectron photoreactions. Herein, we report a photoinduced two-electron reduction of a polypyridyl Ni(II) chloride complex that provides a route to H2 evolution from HCl. The excited states of Ni complexes are too short to participate directly in HX activation, and hence, the excited state of a photoredox mediator is exploited for the activation of HX at the Ni(II) center. Nanosecond transient absorption (TA) spectroscopy has revealed that the excited state of the polypyridine results in a photoreduced radical that is capable of mediating HX activation by producing a Ni(I) center by halogen-atom abstraction. Disproportionation of the photogenerated Ni(I) intermediate affords Ni(II) and Ni(0) complexes. The Ni(0) center is capable of reacting with HX to produce H2 and the polypyridyl Ni(II) dichloride, closing the photocycle for H2 generation from HCl.

Trap-Free Halogen Photoelimination from Mononuclear Ni(III) Complexes

Journal of the American Chemical Society

Halogen photoelimination reactions constitute the oxidative half-reaction of closed HX-splitting energy storage cycles. Here, we report high-yielding, endothermic Cl2 photoelimination chemistry from mononuclear Ni(III) complexes. On the basis of time-resolved spectroscopy and steady-state photocrystallography experiments, a mechanism involving ligand-assisted halogen elimination is proposed. Employing ancillary ligands to promote elimination offers a strategy to circumvent the inherently short-lived excited states of 3d metal complexes for the activation of thermodynamically challenging bonds.

Oxidation of Carbon-Metal Bonds

Comprehensive Organic Synthesis