R. David Britt

Electrochemical Water Oxidation with Cobalt-Based Electrocatalysts from pH 0–14: The Thermodynamic Basis for Catalyst Structure, Stability, and Activity

Building upon recent study of cobalt-oxide electrocatalysts in fluoride-buffered electrolyte at pH 3.4, we have undertaken a mechanistic investigation of cobalt-catalyzed water oxidation in aqueous buffering electrolytes from pH 0-14. This work includes electrokinetic studies, cyclic voltammetric analysis, and electron paramagnetic resonance (EPR) spectroscopic studies. The results illuminate a set of interrelated mechanisms for electrochemical water oxidation in alkaline, neutral, and acidic media with electrodeposited Co-oxide catalyst films (CoO(x)(cf)s) as well as for a homogeneous Co-catalyzed electrochemical water oxidation reaction. Analysis of the pH dependence of quasi-reversible features in cyclic voltammograms of the CoO(x)(cf)s provides the basis for a Pourbaix diagram that closely resembles a Pourbaix diagram derived from thermodynamic free energies of formation for a family of Co-based layered materials. Below pH 3, a shift from heterogeneous catalysis producing O(2) to homogeneous catalysis yielding H(2)O(2) is observed. Collectively, the results reported here provide a foundation for understanding the structure, stability, and catalytic activity of aqueous cobalt electrocatalysts for water oxidation.

EPR Evidence for Co(IV) Species Produced During Water Oxidation at Neutral pH

Thin-film water oxidation catalysts (Co-Pi) prepared by electrodeposition from phosphate electrolyte and Co(NO(3))(2) have been characterized by electron paramagnetic resonance (EPR) spectroscopy. Co-Pi catalyst films exhibit EPR signals corresponding to populations of both Co(II) and Co(IV). As the deposition voltage is increased into the region where water oxidation prevails, the population of Co(IV) rises and the population of Co(II) decreases. The changes in the redox speciation of the film can also be induced, in part, by prolonged water oxidation catalysis in the absence of additional catalyst deposition. These results provide spectroscopic evidence for the formation of Co(IV) species during water oxidation catalysis at neutral pH.

EPR/ENDOR characterization of the physical and electronic structure of the OEC Mn cluster

Electron paramagnetic resonance (EPR) spectroscopy has often played a crucial role in characterizing the various cofactors and processes of photosynthesis, and photosystem II and its oxygen evolving chemistry is no exception. Until recently, the application of EPR spectroscopy to the characterization of the oxygen evolving complex (OEC) has been limited to the S2-state of the Kok cycle. However, in the past few years, continuous wave-EPR signals have been obtained for both the S0- and S1-state as well as for the S2 (radical)(Z)-state of a number of inhibited systems. Furthermore, the pulsed EPR technique of electron spin echo electron nuclear double resonance spectroscopy has been used to directly probe the 55Mn nuclei of the manganese cluster. In this review, we discuss how the EPR data obtained from each of these states of the OEC Kok cycle are being used to provide insight into the physical and electronic structure of the manganese cluster and its interaction with the key tyrosine, Y(Z).

Electronic Structure Description of a [Co(III)3Co(IV)O4] Cluster: A Model for the Paramagnetic Intermediate in Cobalt-Catalyzed Water Oxidation

Multifrequency electron paramagnetic resonace (EPR) spectroscopy and electronic structure calculations were performed on [Co(4)O(4)(C(5)H(5)N)(4)(CH(3)CO(2))(4)](+) (1(+)), a cobalt tetramer with total electron spin S = 1/2 and formal cobalt oxidation states III, III, III, and IV. The cuboidal arrangement of its cobalt and oxygen atoms is similar to that of proposed structures for the molecular cobaltate clusters of the cobalt-phosphate (Co-Pi) water-oxidizing catalyst. The Davies electron-nuclear double resonance (ENDOR) spectrum is well-modeled using a single class of hyperfine-coupled (59)Co nuclei with a modestly strong interaction (principal elements of the hyperfine tensor are equal to [-20(±2), 77(±1), -5(±15)] MHz). Mims (1)H ENDOR spectra of 1(+) with selectively deuterated pyridine ligands confirm that the amount of unpaired spin on the cobalt-bonding partner is significantly reduced from unity. Multifrequency (14)N ESEEM spectra (acquired at 9.5 and 34.0 GHz) indicate that four nearly equivalent nitrogen nuclei are coupled to the electron spin. Cumulatively, our EPR spectroscopic findings indicate that the unpaired spin is delocalized almost equally across the eight core atoms, a finding corroborated by results from DFT calculations. Each octahedrally coordinated cobalt ion is forced into a low-spin electron configuration by the anionic oxo and carboxylato ligands, and a fractional electron hole is localized on each metal center in a Co 3d(xz,yz)-based molecular orbital for this essentially [Co(+3.125)(4)O(4)] system. Comparing the EPR spectrum of 1(+) with that of the catalyst film allows us to draw conclusions about the electronic structure of this water-oxidation catalyst.

The state of manganese in the photosynthetic apparatus. 3. Light-induced changes in X-ray absorption (K-edge) energies of manganese in photosynthetic membranes

Abstract Photosynthetic water oxidation by higher plants proceeds as though five intermediates, S0-S4, operate in a cyclic fashion. In this study of the manganese involvement in the process, a low temperature EPR signal is used as an indicator of S-state composition for manganese X-ray absorption K-edge measurements of a spinach Photosystem II preparation. A dramatic change is observed in the edge properties between samples prepared in states S1 and either S2 or S3, establishing a direct relation between the local environment of Mn and the S-state composition. Samples in S2 or S3 exhibit a broadening of the principal absorption peak and a shift to higher energy by as much as 2.5 eV relative to S1 samples. The magnitude of these changes is directly related to the EPR signal intensity induced by illumination. Models are discussed in which these data may be interpreted in terms of a conformation-induced change in Mn ligation and/or oxidation during the S1 to S2 transition.

The state of manganese in the photosynthetic apparatus: 4. Structure of the manganese complex in Photosystem II studied using EXAFS spectroscopy. The S1 state of the O2-evolving Photosystem II complex from spinach

Abstract The structure of the Mn complex in the oxygen-evolving system and its mechanistic relation to photosynthetic oxygen evolution are poorly understood, though many studies have established that membrane-bound Mn plays an active role. Recently established procedures for isolating oxygen-evolving subchloroplast Photosystem II (PS II) preparations and the discovery of a light-induced multiline EPR signal attributable to the S2 state of the O2-evolving complex have facilitated the preparation of samples well characterized in the S1 and S2 states. We have used extended X-ray absorption fine structure (EXAFS) spectroscopy to probe the ligand environment of Mn in PS II particles from spinach, and in this report we present our results. The essential feature of the EXAFS results are that at least two Mn atoms per PS II reaction center occur as a binuclear species with a metal-metal distance of approx. 2.7 A, with low Z atoms, N or O, at a distance of approx. 1.75 A and at approx. 1.98 A, which are characteristic of bridging and terminal ligands. These results agree well with those derived from whole chloroplasts that provided the first evidence for a binuclear manganese complex (Kirby, J.A., Robertson, A.S., Smith, J.P., Thompson, A.C., Cooper, S.R. and Klein, M.P. (1981) J. Am. Chem. Soc. 103, 5529–5537).

A High-Spin Iron(IV)–Oxo Complex Supported by a Trigonal Nonheme Pyrrolide Platform

We report the generation and characterization of a new high-spin iron(IV)-oxo complex supported by a trigonal nonheme pyrrolide platform. Oxygen-atom transfer to [(tpa(Mes))Fe(II)](-) (tpa(Ar) = tris(5-arylpyrrol-2-ylmethyl)amine) in acetonitrile solution affords the Fe(III)-alkoxide product [(tpa(Mes2MesO))Fe(III)](-) resulting from intramolecular C-H oxidation with no observable ferryl intermediates. In contrast, treatment of the phenyl derivative [(tpa(Ph))Fe(II)](-) with trimethylamine N-oxide in acetonitrile solution produces the iron(IV)-oxo complex [(tpa(Ph))Fe(IV)(O)](-) that has been characterized by a suite of techniques, including mass spectrometry as well as UV-vis, FTIR, Mössbauer, XAS, and parallel-mode EPR spectroscopies. Mass spectral, FTIR, and optical absorption studies provide signatures for the iron-oxo chromophore, and Mössbauer and XAS measurements establish the presence of an Fe(IV) center. Moreover, the Fe(IV)-oxo species gives parallel-mode EPR features indicative of a high-spin, S = 2 system. Preliminary reactivity studies show that the high-spin ferryl tpa(Ph) complex is capable of mediating intermolecular C-H oxidation as well as oxygen-atom transfer chemistry.

General and efficient simulation of pulse EPR spectra

We present a rather general and efficient method of simulating electron-spin echo spectra for spin systems where the microwave frequency does not simultaneously excite EPR transitions that share a common level. The approach can handle arbitrary pulse sequences with microwave pulses of arbitrary length and strength. The signal is computed as a sum over signals from the electron coherence transfer pathways contributing to the detected echo. For each pathway, amplitudes and frequencies of the signal components are computed and used to construct a spectral histogram from which the time-domain signal is obtained. For multinuclear spin systems, the nuclear subspace is factorized to accelerate the computation. The method is also applicable to high electron spin systems with significant zero-field splitting and to pulse electron-nuclear double resonance experiments. The method is implemented in the software package EasySpin, and several illustrative calculations are shown.

The HydG enzyme generates an Fe(CO)2(CN) synthon in assembly of the FeFe hydrogenase H-cluster.

Three iron-sulfur proteins–HydE, HydF, and HydG–play a key role in the synthesis of the [2Fe]H component of the catalytic H-cluster of FeFe hydrogenase. The radical S-adenosyl-l-methionine enzyme HydG lyses free tyrosine to produce p-cresol and the CO and CN− ligands of the [2Fe]H cluster. Here, we applied stopped-flow Fourier transform infrared and electron-nuclear double resonance spectroscopies to probe the formation of HydG-bound Fe-containing species bearing CO and CN− ligands with spectroscopic signatures that evolve on the 1- to 1000-second time scale. Through study of the 13C, 15N, and 57Fe isotopologs of these intermediates and products, we identify the final HydG-bound species as an organometallic Fe(CO)2(CN) synthon that is ultimately transferred to apohydrogenase to form the [2Fe]H component of the H-cluster. Vibrational spectroscopy traces the origin of carbon monoxide and cyanide ligands in the active site of di-iron hydrogenase enzymes. [Also see Perspective by Pickett] Sourcing CO and Cyanide Hydrogenase enzymes derive their activity in part from the coordination of CO and cyanide ligands to metals in their active sites. Recent work elucidated the jettisoning of a tyrosine side chain at the outset of the biosynthetic pathway toward these ligands in the di-iron class of hydrogenase. Kuchenreuther et al. (p. 424; see the Perspective by Pickett) now apply stopped-flow infrared spectroscopy to uncover the next portion of the pathway, during which the residual tyrosine fragment is further broken down into CO and CN− ligands at a single iron center in an iron sulfur cluster associated with the HydG enzyme.

A Redox Series of Aluminum Complexes: Characterization of Four Oxidation States Including a Ligand Biradical State Stabilized via Exchange Coupling

Electrophilic activation and subsequent reduction of substrates is in general not possible because highly Lewis acidic metals lack access to multiple redox states. Herein, we demonstrate that transition metal-like redox processes and electronic structure and magnetic properties can be imparted to aluminum(III). Bis(iminopyridine) complexes containing neutral, monoanionic, and dianionic iminopyridine ligands (IP) have been characterized structurally and electronically; yellow (IP)AlCl(3) (1), deep green (IP(-))(2)AlCl (2) and (IP(-))(2)Al(CF(3)SO(3)) (3), and deep purple [(IP(2-))Al](-) (5) are presented. The mixed-valent, monoradical complex (IP(-))(IP(2-))Al is unstable toward C-C coupling, and [(IP(2-))Al](2-)(μ-IP-IP)(2-) (4) has been isolated. Variable-temperature magnetic susceptibility and EPR spectroscopy measurements indicate that the biradical character of the ligand-based triplet in 2 is stabilized by strong antiferromagnetic exchange coupling mediated by aluminum(III): J = -230 cm(-1) for Ĥ = -2J(Ŝ(L(1))·Ŝ(L(2))). Coordination geometry-dependent (IP(-))-(IP(-)) communication through aluminum(III) is observed electrochemically. The cyclic voltammogram of trigonal bipyramidal 2 displays successive ligand-based oxidation events for the two IP(1-/0) processes, at -0.86 and -1.20 V vs SCE. The 0.34 V spacing between redox couples corresponds to a conproportionation constant of K(c) = 10(5.8) for the process (IP(-))(2)AlCl + (IP)(2)AlCl → 2(IP(-))(IP)AlCl consistent with Robin and Day Class II mixed-valent behavior. Tetrahedral 5 displays localized, Class I behavior as indicated by closely spaced redox couples. Furthermore, CVs of 2 and 5 indicate that changes in the coordination environment of the aluminum center shift the potentials for the IP(1-/0) and IP(2-/1-) redox couples by up to 0.9 V.