Co-reporter:Christopher M. Lemon, David C. Powers, Penelope J. Brothers, and Daniel G. Nocera
Inorganic Chemistry September 18, 2017 Volume 56(Issue 18) pp:10991-10991
Publication Date(Web):September 5, 2017
DOI:10.1021/acs.inorgchem.7b01302
The triplet state of gold(III) corroles is exploited for optical oxygen sensing. We report intense phosphorescence for gold(III) corroles in the near-IR, an optical window that is ideal for tissue transparency. Moreover, the triplet excited-state emission exhibits significant changes in intensity and lifetime over the 0–160 Torr O2 pressure range. This renders these compounds sensitive at biologically relevant pressures and overcomes the spectral limitations of palladium and platinum porphyrins for oxygen sensing in biology.
Co-reporter:Xiaowen Feng, Seung Jun Hwang, Jun-Liang Liu, Yan-Cong Chen, Ming-Liang Tong, and Daniel G. Nocera
Journal of the American Chemical Society November 22, 2017 Volume 139(Issue 46) pp:16474-16474
Publication Date(Web):October 25, 2017
DOI:10.1021/jacs.7b09699
Magneto-structural correlation studies of mononuclear intermediate S = 3/2 Fe(III) complexes, (PMe3)2FeCl3 (1) and (PMe2Ph)2FeCl3 (2), demonstrate the influence of local symmetry on magnetic anisotropy. Symmetric compound 1 is characterized by a zero-field splitting (ZFS) parameter of D = −50(2) cm–1, leading to the observation of slow magnetic relaxation with an energy barrier of 81 cm–1 along with magnetic hysteresis up to 4 K, whereas symmetrically perturbed compound 2 displays a much reduced ZFS parameter of D = −17(1) cm–1 and energy barrier of Ueff = 46 cm–1.
Co-reporter:Julia M. Stauber, Glen E. Alliger, Daniel G. Nocera, and Christopher C. Cummins
Inorganic Chemistry July 17, 2017 Volume 56(Issue 14) pp:7615-7615
Publication Date(Web):June 30, 2017
DOI:10.1021/acs.inorgchem.7b01335
The preparation of a selective turn-on colorimetric fluoride sensor was achieved through single cobalt(II) ion insertion into a macrobicyclic cryptand. Monometallic [Co(mBDCA-5t-H3)]− (1) and [Zn(mBDCA-5t-H3)]− (2) complexes were prepared in 74 and 84% yields, respectively. Structural characterization of 1 confirmed the presence of a proximal hydrogen-bonding network consisting of carboxamide N–H donors. The reaction of 1 with F– was accompanied by a distinct colorimetric turn-on response in mixed aqueous/organic media, and 1 was capable of selective fluoride sensing in the presence of large quantities of potentially competitive anions. Complex 1 represents a unique example of a fluoride sensor wherein selective F– binding takes place directly at a transition-metal center and induces a color change based upon metal-centered transitions. The metal(II) fluoride complexes [F⊂Co(mBDCA-5t-H3)]2– (3) and [F⊂Zn(mBDCA-5t-H3)]2– (4) were both fully characterized, including single crystal X-ray analyses. Fluoride binding is synergistic involving hydrogen-bond donors from the second-coordination sphere together with metal(II) ion complexation.
Co-reporter:Tuncay Ozel, Benjamin A. Zhang, Ruixuan Gao, Robert W. Day, Charles M. Lieber, and Daniel G. Nocera
Nano Letters July 12, 2017 Volume 17(Issue 7) pp:4502-4502
Publication Date(Web):June 16, 2017
DOI:10.1021/acs.nanolett.7b01950
Development of new synthetic methods for the modification of nanostructures has accelerated materials design advances to furnish complex architectures. Structures based on one-dimensional (1D) silicon (Si) structures synthesized using top-down and bottom-up methods are especially prominent for diverse applications in chemistry, physics, and medicine. Yet further elaboration of these structures with distinct metal-based and polymeric materials, which could open up new opportunities, has been difficult. We present a general electrochemical method for the deposition of conformal layers of various materials onto high aspect ratio Si micro- and nanowire arrays. The electrochemical deposition of a library of coaxial layers comprising metals, metal oxides, and organic/inorganic semiconductors demonstrate the materials generality of the synthesis technique. Depositions may be performed on wire arrays with varying diameter (70 nm to 4 μm), pitch (5 μ to 15 μ), aspect ratio (4:1 to 75:1), shape (cylindrical, conical, hourglass), resistivity (0.001–0.01 to 1–10 ohm/cm2), and substrate orientation. Anisotropic physical etching of wires with one or more coaxial shells yields 1D structures with exposed tips that can be further site-specifically modified by an electrochemical deposition approach. The electrochemical deposition methodology described herein features a wafer-scale synthesis platform for the preparation of multifunctional nanoscale devices based on a 1D Si substrate.Keywords: Core−shell structures; electrochemistry; hybrid layers; one-dimensional structures; silicon nanowire arrays; wafer-scale deposition;
Co-reporter:David Gygi, Seung Jun Hwang, and Daniel G. Nocera
The Journal of Organic Chemistry December 1, 2017 Volume 82(Issue 23) pp:12933-12933
Publication Date(Web):November 9, 2017
DOI:10.1021/acs.joc.7b02571
A concise benchtop and scalable synthesis of pyridine–diimine (PDI) ligand frameworks is presented using inexpensive commercial starting materials as opposed to previous syntheses of these ligands, which have confronted long and tedious routes that employ toxic and often difficult to scale pyrophoric reagents. The streamlined synthesis is derived from the facile delivery of 4-functionalized diacetylpyridines from a Minisci reaction using pyruvic acid, silver nitrate, and persulfate. As the PDI ligand scaffold has been adopted for a range of catalytic applications, the ability to modulate the electronic properties of the ligand with facility may be useful for optimizing a variety of catalytic transformations.
Co-reporter:Casey N. Brodsky;Ryan G. Hadt;Dugan Hayes;Benjamin J. Reinhart;Nancy Li;Lin X. Chen
PNAS 2017 114 (15 ) pp:3855-3860
Publication Date(Web):2017-04-11
DOI:10.1073/pnas.1701816114
The Co4O4 cubane is a representative structural model of oxidic cobalt oxygen-evolving catalysts (Co-OECs). The Co-OECs are active
when residing at two oxidation levels above an all-Co(III) resting state. This doubly oxidized Co(IV)2 state may be captured in a Co(III)2(IV)2 cubane. We demonstrate that the Co(III)2(IV)2 cubane may be electrochemically generated and the electronic properties of this unique high-valent state may be probed by
in situ spectroscopy. Intervalence charge-transfer (IVCT) bands in the near-IR are observed for the Co(III)2(IV)2 cubane, and spectroscopic analysis together with electrochemical kinetics measurements reveal a larger reorganization energy
and a smaller electron transfer rate constant for the doubly versus singly oxidized cubane. Spectroelectrochemical X-ray absorption
data further reveal systematic spectral changes with successive oxidations from the cubane resting state. Electronic structure
calculations correlated to experimental data suggest that this state is best represented as a localized, antiferromagnetically
coupled Co(IV)2 dimer. The exchange coupling in the cofacial Co(IV)2 site allows for parallels to be drawn between the electronic structure of the Co4O4 cubane model system and the high-valent active site of the Co-OEC, with specific emphasis on the manifestation of a doubly
oxidized Co(IV)2 center on O–O bond formation.
Co-reporter:Michael Huynh;Tuncay Ozel;Chong Liu;Eric C. Lau
Chemical Science (2010-Present) 2017 vol. 8(Issue 7) pp:4779-4794
Publication Date(Web):2017/06/26
DOI:10.1039/C7SC01239J
Oxygen evolution reaction (OER) catalysts that are earth-abundant and are active and stable in acid are unknown. Active catalysts derived from Co and Ni oxides dissolve at low pH, whereas acid stable systems such as Mn oxides (MnOx) display poor OER activity. We now demonstrate a rational approach for the design of earth-abundant catalysts that are stable and active in acid by treating activity and stability as decoupled elements of mixed metal oxides. Manganese serves as a stabilizing structural element for catalytically active Co centers in CoMnOx films. In acidic solutions (pH 2.5), CoMnOx exhibits the OER activity of electrodeposited Co oxide (CoOx) with a Tafel slope of 70–80 mV per decade while also retaining the long-term acid stability of MnOx films for OER at 0.1 mA cm−2. Driving OER at greater current densities in this system is not viable because at high anodic potentials, Mn oxides convert to and dissolve as permanganate. However, by exploiting the decoupled design of the catalyst, the stabilizing structural element may be optimized independently of the Co active sites. By screening potential–pH diagrams, we replaced Mn with Pb to prepare CoFePbOx films that maintained the high OER activity of CoOx at pH 2.5 while exhibiting long-term acid stability at higher current densities (at 1 mA cm−2 for over 50 h at pH 2.0). Under these acidic conditions, CoFePbOx exhibits OER activity that approaches noble metal oxides, thus establishing the viability of decoupling functionality in mixed metal catalysts for designing active, acid-stable, and earth-abundant OER catalysts.
Co-reporter:Chong Liu;Shannon N. Nangle;Brendan C. Colón;Pamela A. Silver
Faraday Discussions 2017 (Volume 198) pp:529-537
Publication Date(Web):2017/06/02
DOI:10.1039/C6FD00231E
Interfacing the CO2-fixing microorganism, Ralstonia eutropha, to the energy derived from hydrogen produced by water splitting is a viable approach to achieving renewable CO2 reduction at high efficiencies. We employ 13C-labeling to report on the nature of CO2 reduction in the inorganic water splitting|R. eutropha hybrid system. Accumulated biomass in a reactor under a 13C-enriched CO2 atmosphere may be sampled at different time points during CO2 reduction. Converting the sampled biomass into gaseous CO2 allows the 13C/12C ratio to be determined by gas chromatography-mass spectrometry. After 2 hours of inoculation and the initiation of water splitting, the microbes adapted and began to convert CO2 into biomass. The observed time evolution of the 13C/12C ratio in accumulated biomass is consistent with a Monod model for carbon fixation. Carbon dioxide produced by catabolism was found to be minimal. This rapid response of the bacteria to a hydrogen input and to subsequent CO2 reduction at high efficiency are beneficial to achieving artificial photosynthesis for the storage of renewable energy.
Co-reporter:Shannon N Nangle, Kelsey K Sakimoto, Pamela A Silver, Daniel G Nocera
Current Opinion in Chemical Biology 2017 Volume 41(Volume 41) pp:
Publication Date(Web):1 December 2017
DOI:10.1016/j.cbpa.2017.10.023
•Hybrid biological-inorganic systems use renewable electrical energy to synthesize chemicals from CO2.•Interfacing inorganic electrodes with CO2-fixing microorganisms has moved from proof-of-concept to emerging technology.•Efforts to improve the biocompatibility of electrodes have pushed energy efficiencies beyond that of natural photosynthesis.•Work to improve the chemocompatibility of microorganisms expands the product scope and robustness towards industrialization.An expanding renewable energy market to supplant petrochemicals has motivated synthesis technologies that use renewable feedstocks, such as CO2. Hybrid biological-inorganic systems provide a sustainable, efficient, versatile, and inexpensive chemical synthesis platform. These systems comprise biocompatible electrodes that transduce electrical energy either directly or indirectly into bioavailable energy, such as H2 and NAD(P)H. In combination, specific bacteria use these energetic reducing equivalents to fix CO2 into multi-carbon organic compounds. As hybrid biological-inorganic technologies have developed, the focus has shifted from phenomenological and proof-of-concept discovery towards enhanced energy efficiency, production rate, product scope, and industrial robustness. In this review, we highlight the progress and the state-of-the-art of this field and describe the advantages and challenges involved in designing bio- and chemo- compatible systems.
Co-reporter:Cyrille Costentin
PNAS 2017 114 (51 ) pp:13380-13384
Publication Date(Web):2017-12-19
DOI:10.1073/pnas.1711836114
Principles for designing self-healing water-splitting catalysts are presented together with a formal kinetics model to account
for the key chemical steps needed for self-healing. Self-healing may be realized if the catalysts are able to self-assemble
at applied potentials less than that needed for catalyst turnover. Solution pH provides a convenient handle for controlling
the potential of these two processes, as demonstrated for the cobalt phosphate (CoPi) water-splitting catalyst. For Co2+ ion that appears in solution due to leaching from the catalyst during turnover, a quantitative description for the kinetics
of the redeposition of the ion during the self-healing process has been derived. The model reveals that OER activity of CoPi occurs with negligible film dissolution in neutral pH for typical cell geometries and buffer concentrations.
Co-reporter:Cyrille Costentin
PNAS 2017 114 (51 ) pp:13380-13384
Publication Date(Web):2017-12-19
DOI:10.1073/pnas.1711836114
Principles for designing self-healing water-splitting catalysts are presented together with a formal kinetics model to account
for the key chemical steps needed for self-healing. Self-healing may be realized if the catalysts are able to self-assemble
at applied potentials less than that needed for catalyst turnover. Solution pH provides a convenient handle for controlling
the potential of these two processes, as demonstrated for the cobalt phosphate (CoPi) water-splitting catalyst. For Co2+ ion that appears in solution due to leaching from the catalyst during turnover, a quantitative description for the kinetics
of the redeposition of the ion during the self-healing process has been derived. The model reveals that OER activity of CoPi occurs with negligible film dissolution in neutral pH for typical cell geometries and buffer concentrations.
Co-reporter:Cyrille Costentin
PNAS 2017 114 (51 ) pp:13380-13384
Publication Date(Web):2017-12-19
DOI:10.1073/pnas.1711836114
Principles for designing self-healing water-splitting catalysts are presented together with a formal kinetics model to account
for the key chemical steps needed for self-healing. Self-healing may be realized if the catalysts are able to self-assemble
at applied potentials less than that needed for catalyst turnover. Solution pH provides a convenient handle for controlling
the potential of these two processes, as demonstrated for the cobalt phosphate (CoPi) water-splitting catalyst. For Co2+ ion that appears in solution due to leaching from the catalyst during turnover, a quantitative description for the kinetics
of the redeposition of the ion during the self-healing process has been derived. The model reveals that OER activity of CoPi occurs with negligible film dissolution in neutral pH for typical cell geometries and buffer concentrations.
Co-reporter:Cyrille Costentin
PNAS 2017 114 (51 ) pp:13380-13384
Publication Date(Web):2017-12-19
DOI:10.1073/pnas.1711836114
Principles for designing self-healing water-splitting catalysts are presented together with a formal kinetics model to account
for the key chemical steps needed for self-healing. Self-healing may be realized if the catalysts are able to self-assemble
at applied potentials less than that needed for catalyst turnover. Solution pH provides a convenient handle for controlling
the potential of these two processes, as demonstrated for the cobalt phosphate (CoPi) water-splitting catalyst. For Co2+ ion that appears in solution due to leaching from the catalyst during turnover, a quantitative description for the kinetics
of the redeposition of the ion during the self-healing process has been derived. The model reveals that OER activity of CoPi occurs with negligible film dissolution in neutral pH for typical cell geometries and buffer concentrations.
Co-reporter:Kanchana RavichandranEllen C. Minnihan, Qinghui Lin, Kenichi Yokoyama, Alexander T. Taguchi, Jimin Shao, Daniel G. Nocera, JoAnne Stubbe
Biochemistry 2017 Volume 56(Issue 6) pp:
Publication Date(Web):January 19, 2017
DOI:10.1021/acs.biochem.6b01145
Escherichia coli class Ia ribonucleotide reductase (RNR) is composed of two subunits that form an active α2β2 complex. The nucleoside diphosphate substrates (NDP) are reduced in α2, 35 Å from the essential diferric-tyrosyl radical (Y122•) cofactor in β2. The Y122•-mediated oxidation of C439 in α2 occurs by a pathway (Y122 ⇆ [W48] ⇆ Y356 in β2 to Y731 ⇆ Y730 ⇆ C439 in α2) across the α/β interface. The absence of an α2β2 structure precludes insight into the location of Y356 and Y731 at the subunit interface. The proximity in the primary sequence of the conserved E350 to Y356 in β2 suggested its importance in catalysis and/or conformational gating. To study its function, pH–rate profiles of wild-type β2/α2 and mutants in which 3,5-difluorotyrosine (F2Y) replaces residue 356, 731, or both are reported in the presence of E350 or E350X (X = A, D, or Q) mutants. With E350, activity is maintained at the pH extremes, suggesting that protonated and deprotonated states of F2Y356 and F2Y731 are active and that radical transport (RT) can occur across the interface by proton-coupled electron transfer at low pH or electron transfer at high pH. With E350X mutants, all RNRs were inactive, suggesting that E350 could be a proton acceptor during oxidation of the interface Ys. To determine if E350 plays a role in conformational gating, the strong oxidants, NO2Y122•-β2 and 2,3,5-F3Y122•-β2, were reacted with α2, CDP, and ATP in E350 and E350X backgrounds and the reactions were monitored for pathway radicals by rapid freeze-quench electron paramagnetic resonance spectroscopy. Pathway radicals are generated only when E350 is present, supporting its essential role in gating the conformational change(s) that initiates RT and masking its role as a proton acceptor.
Co-reporter:Nancy Li;D. Kwabena Bediako;Ryan G. Hadt;David C. Bell;Thomas J. Kempa;Lin X. Chen;Dugan Hayes;Felix von Cube
PNAS 2017 Volume 114 (Issue 7 ) pp:1486-1491
Publication Date(Web):2017-02-14
DOI:10.1073/pnas.1620787114
Iron doping of nickel oxide films results in enhanced activity for promoting the oxygen evolution reaction (OER). Whereas
this enhanced activity has been ascribed to a unique iron site within the nickel oxide matrix, we show here that Fe doping
influences the Ni valency. The percent of Fe3+ doping promotes the formation of formal Ni4+, which in turn directly correlates with an enhanced activity of the catalyst in promoting OER. The role of Fe3+ is consistent with its behavior as a superior Lewis acid.
Co-reporter:Kelsey K. Sakimoto;Brendan C. Colón;Pamela A. Silver;Chong Liu
PNAS 2017 Volume 114 (Issue 25 ) pp:6450-6455
Publication Date(Web):2017-06-20
DOI:10.1073/pnas.1706371114
We demonstrate the synthesis of NH3 from N2 and H2O at ambient conditions in a single reactor by coupling hydrogen generation from catalytic water splitting to a H2-oxidizing bacterium Xanthobacter autotrophicus, which performs N2 and CO2 reduction to solid biomass. Living cells of X. autotrophicus may be directly applied as a biofertilizer to improve growth of radishes, a model crop plant, by up to ∼1,440% in terms of
storage root mass. The NH3 generated from nitrogenase (N2ase) in X. autotrophicus can be diverted from biomass formation to an extracellular ammonia production with the addition of a glutamate synthetase
inhibitor. The N2 reduction reaction proceeds at a low driving force with a turnover number of 9 × 109 cell–1 and turnover frequency of 1.9 × 104 s–1⋅cell–1 without the use of sacrificial chemical reagents or carbon feedstocks other than CO2. This approach can be powered by renewable electricity, enabling the sustainable and selective production of ammonia and
biofertilizers in a distributed manner.
Co-reporter:Kanchana R. Ravichandran, Alexander T. Taguchi, Yifeng Wei, Cecilia Tommos, Daniel G. Nocera, and JoAnne Stubbe
Journal of the American Chemical Society 2016 Volume 138(Issue 41) pp:13706-13716
Publication Date(Web):September 20, 2016
DOI:10.1021/jacs.6b08200
Escherichia coli class Ia ribonucleotide reductase (RNR) converts ribonucleotides to deoxynucleotides. A diferric-tyrosyl radical (Y122•) in one subunit (β2) generates a transient thiyl radical in another subunit (α2) via long-range radical transport (RT) through aromatic amino acid residues (Y122 ⇆ [W48] ⇆ Y356 in β2 to Y731 ⇆ Y730 ⇆ C439 in α2). Equilibration of Y356•, Y731•, and Y730• was recently observed using site specifically incorporated unnatural tyrosine analogs; however, equilibration between Y122• and Y356• has not been detected. Our recent report of Y356• formation in a kinetically and chemically competent fashion in the reaction of β2 containing 2,3,5-trifluorotyrosine at Y122 (F3Y122•-β2) with α2, CDP (substrate), and ATP (effector) has now afforded the opportunity to investigate equilibration of F3Y122• and Y356•. Incubation of F3Y122•-β2, Y731F-α2 (or Y730F-α2), CDP, and ATP at different temperatures (2–37 °C) provides ΔE°′(F3Y122•–Y356•) of 20 ± 10 mV at 25 °C. The pH dependence of the F3Y122• ⇆ Y356• interconversion (pH 6.8–8.0) reveals that the proton from Y356 is in rapid exchange with solvent, in contrast to the proton from Y122. Insertion of 3,5-difluorotyrosine (F2Y) at Y356 and rapid freeze-quench EPR analysis of its reaction with Y731F-α2, CDP, and ATP at pH 8.2 and 25 °C shows F2Y356• generation by the native Y122•. FnY-RNRs (n = 2 and 3) together provide a model for the thermodynamic landscape of the RT pathway in which the reaction between Y122 and C439 is ∼200 meV uphill.
Co-reporter:Guillaume Passard; Andrew M. Ullman; Casey N. Brodsky
Journal of the American Chemical Society 2016 Volume 138(Issue 9) pp:2925-2928
Publication Date(Web):February 13, 2016
DOI:10.1021/jacs.5b12828
The selective four electron, four proton, electrochemical reduction of O2 to H2O in the presence of a strong acid (TFA) is catalyzed at a dicobalt center. The faradaic efficiency of the oxygen reduction reaction (ORR) is furnished from a systematic electrochemical study by using rotating ring disk electrode (RRDE) methods over a wide potential range. We derive a thermodynamic cycle that gives access to the standard potential of O2 reduction to H2O in organic solvents, taking into account the presence of an exogenous proton donor. The difference in ORR selectivity for H2O vs H2O2 depends on the thermodynamic standard potential as dictated by the pKa of the proton donor. The model is general and rationalizes the faradaic efficiencies reported for many ORR catalytic systems.
Co-reporter:Ryan G. Hadt, Dugan Hayes, Casey N. Brodsky, Andrew M. Ullman, Diego M. Casa, Mary H. Upton, Daniel G. Nocera, and Lin X. Chen
Journal of the American Chemical Society 2016 Volume 138(Issue 34) pp:11017-11030
Publication Date(Web):August 12, 2016
DOI:10.1021/jacs.6b04663
The formation of high-valent states is a key factor in making highly active transition-metal-based catalysts of the oxygen evolution reaction (OER). These high oxidation states will be strongly influenced by the local geometric and electronic structures of the metal ion, which are difficult to study due to spectroscopically active and complex backgrounds, short lifetimes, and limited concentrations. Here, we use a wide range of complementary X-ray spectroscopies coupled to DFT calculations to study Co(III)4O4 cubanes and their first oxidized derivatives, which provide insight into the high-valent Co(IV) centers responsible for the activity of molecular and heterogeneous OER catalysts. The combination of X-ray absorption and 1s3p resonant inelastic X-ray scattering (Kβ RIXS) allows Co(IV) to be isolated and studied against a spectroscopically active Co(III) background. Co K- and L-edge X-ray absorption data allow for a detailed characterization of the 3d-manifold of effectively localized Co(IV) centers and provide a direct handle on the t2g-based redox-active molecular orbital. Kβ RIXS is also shown to provide a powerful probe of Co(IV), and specific spectral features are sensitive to the degree of oxo-mediated metal–metal coupling across Co4O4. Guided by the data, calculations show that electron–hole delocalization can actually oppose Co(IV) formation. Computational extension of Co4O4 to CoM3O4 structures (M = redox-inactive metal) defines electronic structure contributions to Co(IV) formation. Redox activity is shown to be linearly related to covalency, and M(III) oxo inductive effects on Co(IV) oxo bonding can tune the covalency of high-valent sites over a large range and thereby tune E0 over hundreds of millivolts. Additionally, redox-inactive metal substitution can also switch the ground state and modify metal–metal and antibonding interactions across the cluster.
Co-reporter:Andrew M. Ullman; Casey N. Brodsky; Nancy Li; Shao-Liang Zheng
Journal of the American Chemical Society 2016 Volume 138(Issue 12) pp:4229-4236
Publication Date(Web):February 24, 2016
DOI:10.1021/jacs.6b00762
Differential electrochemical mass spectrometry (DEMS) analysis of the oxygen isotopologues produced by 18O-labeled Co-OEC in H216O reveals that water splitting catalysis proceeds by a mechanism that involves direct coupling between oxygens bound to dicobalt edge sites of Co-OEC. The edge site chemistry of Co-OEC has been probed by using a dinuclear cobalt complex. 17O NMR spectroscopy shows that ligand exchange of OH/OH2 at Co(III) edge sites is slow, which is also confirmed by DEMS experiments of Co-OEC. In borate (Bi) and phosphate (Pi) buffers, anions must be displaced to allow water to access the edge sites for an O–O bond coupling to occur. Anion exchange in Pi is slow, taking days to equilibrate at room temperature. Conversely, anion exchange in Bi is rapid (kassoc = 13.1 ± 0.4 M–1 s–1 at 25 °C), enabled by facile changes in boron coordination. These results are consistent with the OER activity of Co-OEC in Bi and Pi. The Pi binding kinetics are too slow to establish a pre-equilibrium sufficiently fast to influence the oxygen evolution reaction (OER), consistent with the zero-order dependence of Pi on the OER current density; in contrast, Bi exchange is sufficiently facile such that Bi has an inhibitory effect on OER. These complementary studies on Co-OEC and the dicobalt edge site mimic allow for a direct connection, at a molecular level, to be made between the mechanisms of heterogeneous and homogeneous OER.
Co-reporter:David C. Powers, Seung Jun Hwang, Bryce L. Anderson, Haifeng Yang, Shao-Liang Zheng, Yu-Sheng Chen, Timothy R. Cook, François P. Gabbaï, and Daniel G. Nocera
Inorganic Chemistry 2016 Volume 55(Issue 22) pp:11815-11820
Publication Date(Web):October 31, 2016
DOI:10.1021/acs.inorgchem.6b01887
Halogen photoelimination is the critical energy-storing step of metal-catalyzed HX-splitting photocycles. Homo- and heterobimetallic Pt(III) complexes display among the highest quantum efficiencies for halogen elimination reactions. Herein, we examine in detail the mechanism and energetics of halogen elimination from a family of binuclear Pt(III) complexes featuring meridionally coordinated Pt(III) trichlorides. Transient absorption spectroscopy, steady-state photocrystallography, and far-infrared vibrational spectroscopy suggest a halogen elimination mechanism that proceeds via two sequential halogen-atom-extrusion steps. Solution-phase calorimetry experiments of the meridional complexes have defined the thermodynamics of halogen elimination, which show a decrease in the photoelimination quantum efficiency with an increase in the thermochemically defined Pt–X bond strength. Conversely, when compared to an isomeric facial Pt(III) trichloride, a much more efficient photoelimination is observed for the fac isomer than would be predicted based on thermochemistry. This difference in the fac vs mer isomer photochemistry highlights the importance of stereochemistry on halogen elimination efficiency and points to a mechanism-based strategy for achieving halogen elimination reactions that are both efficient and energy storing.
Co-reporter:Lisa Olshansky, Brandon L. Greene, Chelsea Finkbeiner, JoAnne Stubbe, and Daniel G. Nocera
Biochemistry 2016 Volume 55(Issue 23) pp:3234-3240
Publication Date(Web):May 9, 2016
DOI:10.1021/acs.biochem.6b00292
The Escherichia coli class Ia ribonucleotide reductase (RNR) achieves forward and reverse proton-coupled electron transfer (PCET) over a pathway of redox active amino acids (β-Y122 ⇌ β-Y356 ⇌ α-Y731 ⇌ α-Y730 ⇌ α-C439) spanning ∼35 Å and two subunits every time it turns over. We have developed photoRNRs that allow radical transport to be phototriggered at tyrosine (Y) or fluorotyrosine (FnY) residues along the PCET pathway. We now report a new photoRNR in which photooxidation of a tryptophan (W) residue replacing Y356 within the α/β subunit interface proceeds by a stepwise ET/PT (electron transfer then proton transfer) mechanism and provides an orthogonal spectroscopic handle with respect to radical pathway residues Y731 and Y730 in α. This construct displays an ∼3-fold enhancement in photochemical yield of W• relative to F3Y• and a ∼7-fold enhancement relative to Y•. Photogeneration of the W• radical occurs with a rate constant of (4.4 ± 0.2) × 105 s–1, which obeys a Marcus correlation for radical generation at the RNR subunit interface. Despite the fact that the Y → W variant displays no enzymatic activity in the absence of light, photogeneration of W• within the subunit interface results in 20% activity for turnover relative to wild-type RNR under the same conditions.
Co-reporter:Evan C. Jones, Qihan Liu, Zhigang Suo, and Daniel G. Nocera
ACS Nano 2016 Volume 10(Issue 5) pp:5321
Publication Date(Web):April 13, 2016
DOI:10.1021/acsnano.6b01335
Reactive interface patterning promoted by lithographic electrochemistry serves as a method for generating submicrometer scale structures. We use a binary-potential step on a metallic overlayer on silicon to fabricate radial patterns of cobalt oxide on the nanoscale. The mechanism for pattern formation has heretofore been ill-defined. The binary potential step allows the electrochemical boundary conditions to be controlled such that initial conditions for a scaling analysis are afforded. With the use of the scaling analysis, a mechanism for producing the observed pattern geometry is correlated to the sequence of electrochemical steps involved in the formation of the submicrometer structures. The patterning method is facile and adds to electrochemical micromachining techniques employing a silicon substrate.Keywords: electrochemical micromachining; finite element simulation; nanopattern; oxide catalysts; silicon
Co-reporter:Zongyou Yin;Casandra Cox;Michel Bosman;Xiaofeng Qian;Zhengqing Liu;Na Li;Hongyang Zhao;Yaping Du;Ju Li
Science Advances 2016 Volume 2(Issue 9) pp:
Publication Date(Web):
DOI:10.1126/sciadv.1501425
A novel facile strategy was developed to synthesize MgO nanocrystals for producing H2 through photodecomposing methanol.
Co-reporter:Chong Liu;Brendan C. Colón;Marika Ziesack;Pamela A. Silver
Science 2016 Vol 352(6290) pp:1210-1213
Publication Date(Web):03 Jun 2016
DOI:10.1126/science.aaf5039
Artificial photosynthesis steps up
Photosynthesis fixes CO2 from the air by using sunlight. Industrial mimics of photosynthesis seek to convert CO2 directly into biomass, fuels, or other useful products. Improving on a previous artificial photosynthesis design, Liu et al. combined the hydrogen-oxidizing bacterium Raistonia eutropha with a cobalt-phosphorus water-splitting catalyst. This biocompatible self-healing electrode circumvented the toxicity challenges of previous designs and allowed it to operate aerobically. When combined with solar photovoltaic cells, solar-to-chemical conversion rates should become nearly an order of magnitude more efficient than natural photosynthesis.
Science, this issue p. 1210
Co-reporter:Ama K. Turek;Dr. David J. Hardee;Andrew M. Ullman; Daniel G. Nocera; Eric N. Jacobsen
Angewandte Chemie 2016 Volume 128( Issue 2) pp:549-554
Publication Date(Web):
DOI:10.1002/ange.201508060
Abstract
Quinones are important organic oxidants in a variety of synthetic and biological contexts, and they are susceptible to activation towards electron transfer through hydrogen bonding. Whereas this effect of hydrogen bond donors (HBDs) has been observed for Lewis basic, weakly oxidizing quinones, comparable activation is not readily achieved when more reactive and synthetically useful electron-deficient quinones are used. We have successfully employed HBD-coupled electron transfer as a strategy to activate electron-deficient quinones. A systematic investigation of HBDs has led to the discovery that certain dicationic HBDs have an exceptionally large effect on the rate and thermodynamics of electron transfer. We further demonstrate that these HBDs can be used as catalysts in a quinone-mediated model synthetic transformation.
Co-reporter:Christopher M. Lemon;Michael Huynh;Andrew G. Maher;Bryce L. Anderson;Dr. Eric D. Bloch;Dr. David C. Powers;Dr. Daniel G. Nocera
Angewandte Chemie International Edition 2016 Volume 55( Issue 6) pp:2176-2180
Publication Date(Web):
DOI:10.1002/anie.201509099
Abstract
The ground state electronic structure of copper corroles has been a topic of debate and revision since the advent of corrole chemistry. Computational studies formulate neutral Cu corroles with an antiferromagnetically coupled CuII corrole radical cation ground state. X-ray photoelectron spectroscopy, EPR, and magnetometry support this assignment. For comparison, CuII isocorrole and [TBA][Cu(CF3)4] were studied as authentic CuII and CuIII samples, respectively. In addition, the one-electron reduction and one-electron oxidation processes are both ligand-based, demonstrating that the CuII centre is retained in these derivatives. These observations underscore ligand non-innocence in copper corrole complexes.
Co-reporter:Christopher M. Lemon;Michael Huynh;Andrew G. Maher;Bryce L. Anderson;Dr. Eric D. Bloch;Dr. David C. Powers;Dr. Daniel G. Nocera
Angewandte Chemie 2016 Volume 128( Issue 6) pp:2216-2220
Publication Date(Web):
DOI:10.1002/ange.201509099
Abstract
The ground state electronic structure of copper corroles has been a topic of debate and revision since the advent of corrole chemistry. Computational studies formulate neutral Cu corroles with an antiferromagnetically coupled CuII corrole radical cation ground state. X-ray photoelectron spectroscopy, EPR, and magnetometry support this assignment. For comparison, CuII isocorrole and [TBA][Cu(CF3)4] were studied as authentic CuII and CuIII samples, respectively. In addition, the one-electron reduction and one-electron oxidation processes are both ligand-based, demonstrating that the CuII centre is retained in these derivatives. These observations underscore ligand non-innocence in copper corrole complexes.
Co-reporter:Ama K. Turek;Dr. David J. Hardee;Andrew M. Ullman; Daniel G. Nocera; Eric N. Jacobsen
Angewandte Chemie International Edition 2016 Volume 55( Issue 2) pp:539-544
Publication Date(Web):
DOI:10.1002/anie.201508060
Abstract
Quinones are important organic oxidants in a variety of synthetic and biological contexts, and they are susceptible to activation towards electron transfer through hydrogen bonding. Whereas this effect of hydrogen bond donors (HBDs) has been observed for Lewis basic, weakly oxidizing quinones, comparable activation is not readily achieved when more reactive and synthetically useful electron-deficient quinones are used. We have successfully employed HBD-coupled electron transfer as a strategy to activate electron-deficient quinones. A systematic investigation of HBDs has led to the discovery that certain dicationic HBDs have an exceptionally large effect on the rate and thermodynamics of electron transfer. We further demonstrate that these HBDs can be used as catalysts in a quinone-mediated model synthetic transformation.
Co-reporter:Michael Huynh; Chenyang Shi; Simon J. L. Billinge
Journal of the American Chemical Society 2015 Volume 137(Issue 47) pp:14887-14904
Publication Date(Web):November 2, 2015
DOI:10.1021/jacs.5b06382
Electrodeposited manganese oxide films (MnOx) are promising stable oxygen evolution catalysts. They are able to catalyze the oxygen evolution reaction in acidic solutions but with only modest activity when prepared by constant anodic potential deposition. We now show that the performance of these catalysts is improved when they are “activated” by potential cycling protocols, as measured by Tafel analysis (where lower slope is better): upon activation the Tafel slope decreases from ∼120 to ∼70 mV/decade in neutral conditions and from ∼650 to ∼90 mV/decade in acidic solutions. Electrochemical, spectroscopic, and structural methods were employed to study the activation process and support a mechanism where the original birnessite-like MnOx (δ-MnO2) undergoes a phase change, induced by comproportionation with cathodically generated Mn(OH)2, to a hausmannite-like intermediate (α-Mn3O4). Subsequent anodic conditioning from voltage cycling or water oxidation produces a disordered birnessite-like phase, which is highly active for oxygen evolution. At pH 2.5, the current density of activated MnOx (at an overpotential of 600 mV) is 2 orders of magnitude higher than that of the original MnOx and begins to approach that of Ru and Ir oxides in acid.
Co-reporter:Julia M. Stauber; Eric D. Bloch; Konstantinos D. Vogiatzis; Shao-Liang Zheng; Ryan G. Hadt; Dugan Hayes; Lin X. Chen; Laura Gagliardi; Daniel G. Nocera;Christopher C. Cummins
Journal of the American Chemical Society 2015 Volume 137(Issue 49) pp:15354-15357
Publication Date(Web):November 11, 2015
DOI:10.1021/jacs.5b09827
A dicobalt(II) complex, [Co2(mBDCA-5t)]2– (1), demonstrates a cofacial arrangement of trigonal monopyramidal Co(II) ions with an inter-metal separation of 6.2710(6) Å. Reaction of 1 with potassium superoxide generates an encapsulated Co–O–Co core in the dianionic complex, [Co2O(mBDCA-5t)]2– (2); to form the linear Co–O–Co core, the inter-metal distance has diminished to 3.994(3) Å. Co K-edge X-ray absorption spectroscopy data are consistent with a +2 oxidation state assignment for Co in both 1 and 2. Multireference complete active space calculations followed by second-order perturbation theory support this assignment, with hole equivalents residing on the bridging O-atom and on the cryptand ligand for the case of 2. Complex 2 acts as a 2-e– oxidant toward substrates including CO and H2, in both cases efficiently regenerating 1 in what represent net oxygen-atom-transfer reactions. This dicobalt system also functions as a catalase upon treatment with H2O2.
Co-reporter:Thomas J. Kempa; D. Kwabena Bediako; Evan C. Jones; Charles M. Lieber
Journal of the American Chemical Society 2015 Volume 137(Issue 11) pp:3739-3742
Publication Date(Web):March 5, 2015
DOI:10.1021/ja5118717
The development of high-throughput and scalable techniques for patterning inorganic structures is useful for the improved function and efficiency of photonic and energy conversion devices. Here we demonstrate a facile and rapid electrochemical method for patterning periodic metallic and nonmetallic submicron structures over large areas. Si substrates have been patterned with arrays of periodically spaced lines, rings, squares, and terraces of main-group and transition-metal oxides. In addition to planar substrates, three-dimensional surfaces and their vertical sidewalls have been patterned. The features are 20(±1) nm high and 360(±15) nm wide, and their period is finely tunable in situ from 500 nm to 7 μm. These features exhibit <3% variation in period and are rapidly patterned in <2 min. We demonstrate the versatility of the technique by rapidly patterning an efficient water splitting catalyst, Co phosphate oxide (CoPi), and show that the integrated materials system performs water splitting with complete Faradaic efficiency. More generally, the ability to pattern submicron structures over large areas in a facile, reliable, and timely manner may be useful for the fabrication of devices for energy, meta-material, and sensing applications.
Co-reporter:Bon Jun Koo; Michael Huynh; Robert L. Halbach; JoAnne Stubbe
Journal of the American Chemical Society 2015 Volume 137(Issue 37) pp:11860-11863
Publication Date(Web):August 25, 2015
DOI:10.1021/jacs.5b05955
The presentation of two phenols on a xanthene backbone is akin to the tyrosine dyad (Y730 and Y731) of ribonucleotide reductase. X-ray crystallography reveals that the two phenol moieties are cofacially disposed at 4.35 Å. Cyclic voltammetry reveals that phenol oxidation is modulated within the dyad, which exhibits a splitting of one-electron waves with the second oxidation of the phenol dyad occurring at larger positive potential than that of a typical phenol. In contrast, a single phenol appended to a xanthene exhibits a two-electron process, consistent with reported oxidation pathways of phenols in acetonitrile. The perturbation of the phenol potential by stacking is reminiscent of a similar effect for guanines stacked within DNA base pairs.
Co-reporter:Matthew Nava; Nazario Lopez; Peter Müller; Gang Wu; Daniel G. Nocera;Christopher C. Cummins
Journal of the American Chemical Society 2015 Volume 137(Issue 46) pp:14562-14565
Publication Date(Web):October 14, 2015
DOI:10.1021/jacs.5b08495
The reactivity of peroxide dianion O22– has been scarcely explored in organic media due to the lack of soluble sources of this reduced oxygen species. We now report the finding that the encapsulated peroxide cryptate, [O2⊂mBDCA-5t-H6]2– (1), reacts with carbon monoxide in organic solvents at 40 °C to cleanly form an encapsulated carbonate. Characterization of the resulting hexacarboxamide carbonate cryptate by single crystal X-ray diffraction reveals that carbonate dianion forms nine complementary hydrogen bonds with the hexacarboxamide cryptand, [CO3⊂mBDCA-5t-H6]2– (2), a conclusion that is supported by spectroscopic data. Labeling studies and 17O solid-state NMR data confirm that two-thirds of the oxygen atoms in the encapsulated carbonate derive from peroxide dianion, while the carbon is derived from CO. Further evidence for the formation of a carbonate cryptate was obtained by three methods of independent synthesis: treatment of (i) free cryptand with K2CO3; (ii) monodeprotonated cryptand with PPN[HCO3]; and (iii) free cryptand with TBA[OH] and atmospheric CO2. This work demonstrates CO oxidation mediated by a hydrogen-bonding anion receptor, constituting an alternative to transition-metal catalysis.
Co-reporter:Seung Jun Hwang; David C. Powers; Andrew G. Maher; Bryce L. Anderson; Ryan G. Hadt; Shao-Liang Zheng; Yu-Sheng Chen
Journal of the American Chemical Society 2015 Volume 137(Issue 20) pp:6472-6475
Publication Date(Web):May 7, 2015
DOI:10.1021/jacs.5b03192
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.
Co-reporter:Seung Jun Hwang, David C. Powers, Andrew G. Maher and Daniel G. Nocera
Chemical Science 2015 vol. 6(Issue 2) pp:917-922
Publication Date(Web):08 Oct 2014
DOI:10.1039/C4SC02357A
Photoactivation of M–X bonds is a challenge for photochemical HX splitting, particularly with first-row transition metal complexes because of short intrinsic excited state lifetimes. Herein, we report a tandem H2 photocycle based on combination of a non-basic photoredox phosphine mediator and nickel metal catalyst. Synthetic studies and time-resolved photochemical studies have revealed that phosphines serve as photochemical H-atom donors to Ni(II) trihalide complexes to deliver a Ni(I) centre. The H2 evolution catalytic cycle is closed by sequential disproportionation of Ni(I) to afford Ni(0) and Ni(II) and protolytic H2 evolution from the Ni(0) intermediate. The results of these investigations suggest that H2 photogeneration proceeds by two sequential catalytic cycles: a photoredox cycle catalyzed by phosphines and an H2-evolution cycle catalyzed by Ni complexes to circumvent challenges of photochemistry with first-row transition metal complexes.
Co-reporter:David Y. Song, Arturo A. Pizano, Patrick G. Holder, JoAnne Stubbe and Daniel G. Nocera
Chemical Science 2015 vol. 6(Issue 8) pp:4519-4524
Publication Date(Web):08 Jun 2015
DOI:10.1039/C5SC01125F
Proton-coupled electron transfer (PCET) is a fundamental mechanism important in a wide range of biological processes including the universal reaction catalysed by ribonucleotide reductases (RNRs) in making de novo, the building blocks required for DNA replication and repair. These enzymes catalyse the conversion of nucleoside diphosphates (NDPs) to deoxynucleoside diphosphates (dNDPs). In the class Ia RNRs, NDP reduction involves a tyrosyl radical mediated oxidation occurring over 35 Å across the interface of the two required subunits (β2 and α2) involving multiple PCET steps and the conserved tyrosine triad [Y356(β2)–Y731(α2)–Y730(α2)]. We report the synthesis of an active photochemical RNR (photoRNR) complex in which a Re(I)-tricarbonyl phenanthroline ([Re]) photooxidant is attached site-specifically to the Cys in the Y356C-(β2) subunit and an ionizable, 2,3,5-trifluorotyrosine (2,3,5-F3Y) is incorporated in place of Y731 in α2. This intersubunit PCET pathway is investigated by ns laser spectroscopy on [Re356]-β2:2,3,5-F3Y731-α2 in the presence of substrate, CDP, and effector, ATP. This experiment has allowed analysis of the photoinjection of a radical into α2 from β2 in the absence of the interfacial Y356 residue. The system is competent for light-dependent substrate turnover. Time-resolved emission experiments reveal an intimate dependence of the rate of radical injection on the protonation state at position Y731(α2), which in turn highlights the importance of a well-coordinated proton exit channel involving the key residues, Y356 and Y731, at the subunit interface.
Co-reporter:Christopher M. Lemon; Robert L. Halbach; Michael Huynh
Inorganic Chemistry 2015 Volume 54(Issue 6) pp:2713-2725
Publication Date(Web):February 25, 2015
DOI:10.1021/ic502860g
Corroles are an emergent class of fluorophores that are finding an application and reaction chemistry to rival their porphyrin analogues. Despite a growing interest in the synthesis, reactivity, and functionalization of these macrocycles, their excited-state chemistry remains undeveloped. A systematic study of the photophysical properties of β-substituted corroles was performed on a series of free-base β-brominated derivatives as well as a β-linked corrole dimer. The singlet and triplet electronic states of these compounds were examined with steady-state and time-resolved spectroscopic methods, which are complemented with density functional theory (DFT) and time-dependent DFT calculations to gain insight into the nature of the electronic structure. Selective bromination of a single molecular edge manifests in a splitting of the Soret band into x and y polarizations, which is a consequence of asymmetry of the molecular axes. A pronounced heavy atom effect is the primary determinant of the photophysical properties of these free-base corroles; bromination decreases the fluorescence quantum yield (from 15% to 0.47%) and lifetime (from 4 ns to 80 ps) by promoting enhanced intersystem crossing, as evidenced by a dramatic increase in knr with bromine substitution. The nonbrominated dimer exhibits absorption and emission features comparable to those of the tetrabrominated derivative, suggesting that oligomerization provides a means of red-shifting the spectral properties akin to bromination but without decreasing the fluorescence quantum yield.
Co-reporter:Robert L. Halbach; Thomas S. Teets
Inorganic Chemistry 2015 Volume 54(Issue 15) pp:7335-7344
Publication Date(Web):July 13, 2015
DOI:10.1021/acs.inorgchem.5b00856
The reduction of O2 to H2O mediated by a series of electronically varied rhodium hydride complexes of the form cis,trans-RhIIICl2H(CNAd)(P(4-X-C6H4)3)2 (2) (CNAd = 1-adamantylisocyanide; X = F (2a), Cl (2b), Me (2c), OMe (2d)) was examined through synthetic and kinetic studies. Rhodium(III) hydride 2 reacts with O2 to afford H2O with concomitant generation of trans-RhIIICl3(CNAd)(P(4-X-C6H4)3)2 (3). Kinetic studies of the reaction of the hydride complex 2 with O2 in the presence of HCl revealed a two-term rate law consistent with an HX reductive elimination (HXRE) mechanism, where O2 binds to a rhodium(I) metal center and generates an η2-peroxo complex intermediate, trans-RhIIICl(CNAd)(η2-O2)(P(4-X-C6H4)3)2 (4), and a hydrogen-atom abstraction (HAA) mechanism, which entails the direct reaction of O2 with the hydride. Experimental data reveal that the rate of reduction of O2 to H2O is enhanced by electron-withdrawing phosphine ligands. Complex 4 was independently prepared by the addition of O2 to trans-RhICl(CNAd)(P(4-X-C6H4)3)2 (1). The reactivity of 4 toward HCl reveals that such peroxo complexes are plausible intermediates in the reduction of O2 to H2O. These results show that the given series of electronically varied rhodium(III) hydride complexes facilitate the reduction of O2 to H2O according to a two-term rate law comprising HXRE and HAA pathways and that the relative rates of these two pathways, which can occur simultaneously and competitively, can be systematically modulated by variation of the electronic properties of the ancillary ligand set.
Co-reporter:Seung Jun Hwang, Bryce L. Anderson, David C. Powers, Andrew G. Maher, Ryan G. Hadt, and Daniel G. Nocera
Organometallics 2015 Volume 34(Issue 19) pp:4766-4774
Publication Date(Web):September 4, 2015
DOI:10.1021/acs.organomet.5b00568
Endothermic halogen elimination reactions, in which molecular halogen photoproducts are generated in the absence of chemical traps, are rare. Inspired by the proclivity of mononuclear Ni(III) complexes to participate in challenging bond-forming reactions in organometallic chemistry, we targeted Ni(III) trihalide complexes as platforms to explore halogen photoelimination. A suite of Ni(III) trihalide complexes supported by bidentate phosphine ligands has been synthesized and characterized. Multinuclear NMR, EPR, and electronic absorption spectroscopies, as well as single-crystal X-ray diffraction, have been utilized to characterize this suite of complexes as distorted square pyramidal, S = 1/2 mononuclear Ni(III) complexes. All complexes participate in clean halogen photoelimination in solution and in the solid state. Evolved halogen has been characterized by mass spectrometry and quantified chemically. Energy storage via halogen elimination was established by solution-phase calorimetry measurements; in all cases, halogen elimination is substantially endothermic. Time-resolved photochemical experiments have revealed a relatively long-lived photointermediate, which we assign to be a Ni(II) complex in which the photoextruded chlorine radical interacts with a ligand-based aryl group. Computational studies suggest that the observed intermediate arises from a dissociative LMCT excited state. The participation of secondary coordination sphere interactions to suppress back-reactions is an attractive design element in the development of energy-storing halogen photoelimination involving first-row transition metal complexes.
Co-reporter:Thomas J. Kempa;D. Kwabena Bediako;Sun-Kyung Kim;Hong-Gyu Park
PNAS 2015 Volume 112 (Issue 17 ) pp:5309-5313
Publication Date(Web):2015-04-28
DOI:10.1073/pnas.1504280112
A patterning method termed “RIPPLE” (reactive interface patterning promoted by lithographic electrochemistry) is applied to
the fabrication of arrays of dielectric and metallic optical elements. This method uses cyclic voltammetry to impart patterns
onto the working electrode of a standard three-electrode electrochemical setup. Using this technique and a template stripping
process, periodic arrays of Ag circular Bragg gratings are patterned in a high-throughput fashion over large substrate areas.
By varying the scan rate of the cyclically applied voltage ramps, the periodicity of the gratings can be tuned in situ over
micrometer and submicrometer length scales. Characterization of the periodic arrays of periodic gratings identified point-like
and annular scattering modes at different planes above the structured surface. Facile, reliable, and rapid patterning techniques
like RIPPLE may enable the high-throughput and low-cost fabrication of photonic elements and metasurfaces for energy conversion
and sensing applications.
Co-reporter:Janice S. Chen;Jeffery C. Way;D. Kwabena Bediako;Pamela A. Silver;Joseph P. Torella;Christopher J. Gagliardi;Brendan Colón
PNAS 2015 Volume 112 (Issue 8 ) pp:2337-2342
Publication Date(Web):2015-02-24
DOI:10.1073/pnas.1424872112
Photovoltaic cells have considerable potential to satisfy future renewable-energy needs, but efficient and scalable methods
of storing the intermittent electricity they produce are required for the large-scale implementation of solar energy. Current
solar-to-fuels storage cycles based on water splitting produce hydrogen and oxygen, which are attractive fuels in principle
but confront practical limitations from the current energy infrastructure that is based on liquid fuels. In this work, we
report the development of a scalable, integrated bioelectrochemical system in which the bacterium Ralstonia eutropha is used to efficiently convert CO2, along with H2 and O2 produced from water splitting, into biomass and fusel alcohols. Water-splitting catalysis was performed using catalysts that
are made of earth-abundant metals and enable low overpotential water splitting. In this integrated setup, equivalent solar-to-biomass
yields of up to 3.2% of the thermodynamic maximum exceed that of most terrestrial plants. Moreover, engineering of R. eutropha enabled production of the fusel alcohol isopropanol at up to 216 mg/L, the highest bioelectrochemical fuel yield yet reported
by >300%. This work demonstrates that catalysts of biotic and abiotic origin can be interfaced to achieve challenging chemical
energy-to-fuels transformations.
Co-reporter:Bryce L. Anderson, Andrew G. Maher, Matthew Nava, Nazario Lopez, Christopher C. Cummins, and Daniel G. Nocera
The Journal of Physical Chemistry B 2015 Volume 119(Issue 24) pp:7422-7429
Publication Date(Web):January 30, 2015
DOI:10.1021/jp5110505
The encapsulation of peroxide dianion by hexacarboxamide cryptand provides a platform for the study of electron transfer of isolated peroxide anion. Photoinitiated electron transfer (ET) between freely diffusing Ru(bpy)32+ and the peroxide dianion occurs with a rate constant of 2.0 × 1010 M–1 s–1. A competing electron transfer quenching pathway is observed within an ion pair. Picosecond transient spectroscopy furnishes a rate constant of 1.1 × 1010 s–1 for this first-order process. A driving force dependence for the ET rate within the ion pair using a series of Ru(bpy)32+ derivatives allows for the electronic coupling and reorganization energies to be assessed. The ET reaction is nonadiabatic and dominated by a large inner-sphere reorganization energy, in accordance with that expected for the change in bond distance accompanying the conversion of peroxide dianion to superoxide anion.
Co-reporter:Lisa Olshansky, Arturo A. Pizano, Yifeng Wei, JoAnne Stubbe, and Daniel G. Nocera
Journal of the American Chemical Society 2014 Volume 136(Issue 46) pp:16210-16216
Publication Date(Web):October 29, 2014
DOI:10.1021/ja507313w
Ribonucleotide reductases (RNRs) catalyze the conversion of nucleotides to deoxynucleotides in all organisms. Active E. coli class Ia RNR is an α2β2 complex that undergoes reversible, long-range proton-coupled electron transfer (PCET) over a pathway of redox active amino acids (β-Y122 → [β-W48] → β-Y356 → α-Y731 → α-Y730 → α-C439) that spans ∼35 Å. To unmask PCET kinetics from rate-limiting conformational changes, we prepared a photochemical RNR containing a [ReI] photooxidant site-specifically incorporated at position 355 ([Re]-β2), adjacent to PCET pathway residue Y356 in β. [Re]-β2 was further modified by replacing Y356 with 2,3,5-trifluorotyrosine to enable photochemical generation and spectroscopic observation of chemically competent tyrosyl radical(s). Using transient absorption spectroscopy, we compare the kinetics of Y· decay in the presence of substrate and wt-α2, Y731F-α2 ,or C439S-α2, as well as with 3′-[2H]-substrate and wt-α2. We find that only in the presence of wt-α2 and the unlabeled substrate do we observe an enhanced rate of radical decay indicative of forward radical propagation. This observation reveals that cleavage of the 3′-C–H bond of substrate by the transiently formed C439· thiyl radical is rate-limiting in forward PCET through α and has allowed calculation of a lower bound for the rate constant associated with this step of (1.4 ± 0.4) × 104 s–1. Prompting radical propagation with light has enabled observation of PCET events heretofore inaccessible, revealing active site chemistry at the heart of RNR catalysis.
Co-reporter:Michael Huynh ; D. Kwabena Bediako
Journal of the American Chemical Society 2014 Volume 136(Issue 16) pp:6002-6010
Publication Date(Web):March 26, 2014
DOI:10.1021/ja413147e
First-row metals have been a target for the development of oxygen evolution reaction (OER) catalysts because they comprise noncritical elements. We now report a comprehensive electrochemical characterization of manganese oxide (MnOx) over a wide pH range, and establish MnOx as a functionally stable OER catalyst owing to self-healing, is derived from MnOx redeposition that offsets catalyst dissolution during turnover. To study this process in detail, the oxygen evolution mechanism of MnOx was investigated electrokinetically over a pH range spanning acidic, neutral, and alkaline conditions. In the alkaline pH regime, a ∼60 mV/decade Tafel slope and inverse first-order dependence on proton concentration were observed, whereas the OER acidic pH regime exhibited a quasi-infinite Tafel slope and zeroth-order dependence on proton concentration. The results reflect two competing mechanisms: a one-electron one-proton PCET pathway that is dominant under alkaline conditions and a Mn3+ disproportionation process, which predominates under acidic conditions. Reconciling the rate laws of these two OER pathways with that of MnOx electrodeposition elucidates the self-healing characteristics of these catalyst films. The intersection of the kinetic profile of deposition and that of water oxidation as a function of pH defines the region of kinetic stability for MnOx and importantly establishes that a non-noble metal oxide OER catalyst may be operated in acid by exploiting a self-healing process.
Co-reporter:Andrew M. Ullman ; Yi Liu ; Michael Huynh ; D. Kwabena Bediako ; Hongsen Wang ; Bryce L. Anderson ; David C. Powers ; John J. Breen ; Héctor D. Abruña
Journal of the American Chemical Society 2014 Volume 136(Issue 50) pp:17681-17688
Publication Date(Web):November 18, 2014
DOI:10.1021/ja5110393
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.
Co-reporter:David C. Powers ; Bryce L. Anderson ; Seung Jun Hwang ; Tamara M. Powers ; Lisa M. Pérez ; Michael B. Hall ; Shao-Liang Zheng ; Yu-Sheng Chen
Journal of the American Chemical Society 2014 Volume 136(Issue 43) pp:15346-15355
Publication Date(Web):September 29, 2014
DOI:10.1021/ja508218v
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.
Co-reporter:David C. Powers, Seung Jun Hwang, Shao-Liang Zheng, and Daniel G. Nocera
Inorganic Chemistry 2014 Volume 53(Issue 17) pp:9122-9128
Publication Date(Web):August 19, 2014
DOI:10.1021/ic501136m
Two-electron mixed-valence compounds promote the rearrangement of the two-electron bond photochemically. Such complexes are especially effective at managing the activation of hydrohalic acids (HX). Closed HX-splitting cycles require proton reduction to H2 and halide oxidation to X2 to be both accomplished, the latter of which is thermodynamically and kinetically demanding. Phosphazane-bridged Rh2 catalysts have been especially effective at activating HX via photogenerated ligand-bridged intermediates; such intermediates are analogues of the classical ligand-bridged intermediates proposed in binuclear elimination reactions. Herein, a new family of phosphazane-bridged Rh2 photocatalysts has been developed where the halide-bridged geometry is designed into the ground state. The targeted geometries were accessed by replacing previously used alkyl isocyanides with aryl isocyanide ligands, which provided access to families of Rh2L1 complexes. H2 evolution with Rh2 catalysts typically proceeds via two-electron photoreduction, protonation to afford Rh hydrides, and photochemical H2 evolution. Herein, we have directly observed each of these steps in stoichiometric reactions. Reactivity differences between Rh2 chloride and bromide complexes have been delineated. H2 evolution from both HCl and HBr proceeds with a halide-bridged Rh2 hydride photoresting state. The H2-evolution efficiency of the new family of halide-bridged catalysts is compared to a related catalyst in which ligand-bridged geometries are not stabilized in the molecular ground state, and the new complexes are found to more efficiently facilitate H2 evolution.
Co-reporter:Andrew M. Ullman, Xianru Sun, Daniel J. Graham, Nazario Lopez, Matthew Nava, Rebecca De Las Cuevas, Peter Müller, Elena V. Rybak-Akimova, Christopher C. Cummins, and Daniel G. Nocera
Inorganic Chemistry 2014 Volume 53(Issue 10) pp:5384-5391
Publication Date(Web):April 28, 2014
DOI:10.1021/ic500759g
A peroxide dianion (O22–) can be isolated within the cavity of hexacarboxamide cryptand, [(O2)⊂mBDCA-5t-H6]2–, stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O2)⊂mBDCA-5t-H6]2– with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler–Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O22– occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O22– within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal–air batteries and the use of [(O2)⊂mBDCA-5t-H6]2– as a chemical reagent.
Co-reporter:Christopher M. Lemon, Peter N. Curtin, Rebecca C. Somers, Andrew B. Greytak, Ryan M. Lanning, Rakesh K. Jain, Moungi G. Bawendi, and Daniel G. Nocera
Inorganic Chemistry 2014 Volume 53(Issue 4) pp:1900-1915
Publication Date(Web):October 21, 2013
DOI:10.1021/ic401587r
Acidity, hypoxia, and glucose levels characterize the tumor microenvironment rendering pH, pO2, and pGlucose, respectively, important indicators of tumor health. To this end, understanding how these parameters change can be a powerful tool for the development of novel and effective therapeutics. We have designed optical chemosensors that feature a quantum dot and an analyte-responsive dye. These noninvasive chemosensors permit pH, oxygen, and glucose to be monitored dynamically within the tumor microenvironment by using multiphoton imaging.
Co-reporter:Casandra R. Cox;Tonio Buonassisi;Jungwoo Z. Lee
PNAS 2014 Volume 111 (Issue 39 ) pp:14057-14061
Publication Date(Web):2014-09-30
DOI:10.1073/pnas.1414290111
Direct solar-to-fuels conversion can be achieved by coupling a photovoltaic device with water-splitting catalysts. We demonstrate
that a solar-to-fuels efficiency (SFE) > 10% can be achieved with nonprecious, low-cost, and commercially ready materials.
We present a systems design of a modular photovoltaic (PV)–electrochemical device comprising a crystalline silicon PV minimodule
and low-cost hydrogen-evolution reaction and oxygen-evolution reaction catalysts, without power electronics. This approach
allows for facile optimization en route to addressing lower-cost devices relying on crystalline silicon at high SFEs for direct
solar-to-fuels conversion.
Co-reporter:Yi Liu
The Journal of Physical Chemistry C 2014 Volume 118(Issue 30) pp:17060-17066
Publication Date(Web):March 28, 2014
DOI:10.1021/jp5008347
Nanoparticle (NP) cobalt–phosphate (Co-Pi) water oxidation catalysts are prepared as thin films by anodic electrodeposition from solutions of Co2+ dissolved in proton-accepting electrolytes. Compositional and structural insight into the nature of the catalyst film is provided from advanced spectroscopy. Infrared spectra demonstrate that counteranions incorporate into the Co-Pi thin films and that the phosphate ion, among various anion electrolytes, exhibits the highest binding affinity to the cobalt centers. Atomic force microscopy images show a highly porous morphology of the thin film that is composed of Co-Pi NPs. Whereas conventional X-ray powder diffraction technique shows catalyst films to be amorphous, synchrotron-based X-ray grazing incidence diffraction reveals well-defined diffraction patterns that are indicative of long-range ordering within the film. Azimuthal scans imply that as-prepared films possess a highly preferred orientation and texture on the electrode surface.
Co-reporter:Daniel J. Graham and Daniel G. Nocera
Organometallics 2014 Volume 33(Issue 18) pp:4994-5001
Publication Date(Web):August 4, 2014
DOI:10.1021/om500300e
The ability to control proton translocation is essential for optimizing electrocatalytic reductions in acidic solutions. We have synthesized a series of new hangman iron porphyrins with hanging groups of differing proton-donating abilities and evaluated their electrocatalytic hydrogen-evolving ability using foot-of-the-wave analysis. In the presence of excess triphenylphosphine, iron porphyrins initiate proton reduction electrocatalysis upon reduction to FeI. By changing the proton-donating ability of the hanging group, we can affect the rate of catalysis by nearly 3 orders of magnitude. The presence of an acid/base moiety in the second coordination sphere results in a marked increase in turnover frequency when extrapolated to zero overpotential.
Co-reporter:Michael Huynh ; D. Kwabena Bediako ; Yi Liu
The Journal of Physical Chemistry C 2014 Volume 118(Issue 30) pp:17142-17152
Publication Date(Web):March 31, 2014
DOI:10.1021/jp501768n
We investigate the mechanisms of nucleation and steady-state growth of a manganese oxide catalyst (MnOx) electrodeposited from Mn2+ solutions in a weakly basic electrolyte. Early catalyst growth was probed through chronoamperometry transients, which were fit to reveal a progressive nucleation mechanism for initial catalyst formation. Time-dependent atomic force microscopy snapshots of the electrode surface reveal a rapid increase in nucleus size together with a sluggish rise in coverage, which is also characteristic of progressive nucleation. Electrochemical kinetic studies of the catalyst growth yield a Tafel slope of approximately 2.3 × RT/2F and a rate law consisting of a second-order and inverse fourth-order dependence on [Mn2+] and proton activity, respectively. These results are consistent with a deposition mechanism involving rate-limiting disproportionation of aqueous Mn3+, resolving a longstanding ambiguity surrounding the deposition of manganese oxides under nonacidic conditions.
Co-reporter:Daniel J. Graham;Dr. Shao-Liang Zheng ;Dr. Daniel G. Nocera
ChemSusChem 2014 Volume 7( Issue 9) pp:2449-2452
Publication Date(Web):
DOI:10.1002/cssc.201402242
Abstract
We report a multi-gram scale synthesis of methyl 6-formyl-4-dibenzofurancarboxylate and its subsequent use in the gram scale synthesis of a dibenzofuran-functionalized hangman porphyrin containing a pendant carboxylic acid (HPD-CO2H). HPD-CO2H can be isolated as a free carboxylic acid in high purity with minimal purification. Post-synthetic modification of HPD-CO2H allows for the introduction of any desired pendant group in good yields, resulting in a practical amount of hangman porphyrin ligand with an easily customizable second coordination sphere. The cobalt complexes of these hangman porphyrins are shown to be active proton reduction electrocatalysts.
Co-reporter:Brian H. Solis;D. Kwabena Bediako;Manolis M. Roubelakis;Andrew G. Maher;Matthew B. Chambers;Sharon Hammes-Schiffer;Chang Hoon Lee;Dilek K. Dogutan
PNAS 2014 Volume 111 (Issue 42 ) pp:15001-15006
Publication Date(Web):2014-10-21
DOI:10.1073/pnas.1414908111
The hangman motif provides mechanistic insights into the role of pendant proton relays in governing proton-coupled electron
transfer (PCET) involved in the hydrogen evolution reaction (HER). We now show improved HER activity of Ni compared with Co
hangman porphyrins. Cyclic voltammogram data and simulations, together with computational studies using density functional
theory, implicate a shift in electrokinetic zone between Co and Ni hangman porphyrins due to a change in the PCET mechanism.
Unlike the Co hangman porphyrin, the Ni hangman porphyrin does not require reduction to the formally metal(0) species before
protonation by weak acids in acetonitrile. We conclude that protonation likely occurs at the Ni(I) state followed by reduction,
in a stepwise proton transfer–electron transfer pathway. Spectroelectrochemical and computational studies reveal that upon
reduction of the Ni(II) compound, the first electron is transferred to a metal-based orbital, whereas the second electron
is transferred to a molecular orbital on the porphyrin ring.
Co-reporter:D. Kwabena Bediako ; Cyrille Costentin ; Evan C. Jones ; Daniel G. Nocera ;Jean-Michel Savéant
Journal of the American Chemical Society 2013 Volume 135(Issue 28) pp:10492-10502
Publication Date(Web):June 12, 2013
DOI:10.1021/ja403656w
Solar-driven electrochemical transformations of small molecules, such as water splitting and CO2 reduction, pertinent to modern energy challenges, require the assistance of catalysts preferably deposited on conducting or semiconducting surfaces. Understanding mechanisms and identifying the factors that control the functioning of such systems are required for rational catalyst optimization and improved performance. A methodology is proposed, in the framework of rotating disk electrode voltammetry, to analyze the current responses expected in the case of a semigeneral reaction scheme involving a proton-coupled catalytic reaction associated with proton-coupled electron hopping through the film as rate controlling factors in the case where there is no limitation by substrate diffusion. The predictions concern the current density vs overpotential (Tafel) plots and their dependence on buffer concentration (including absence of buffer), film thickness and rotation rate. The Tafel plots may have a variety of slopes (e.g., F/RT ln 10, F/2RT ln 10, 0) that may even coexist within the overpotential range of a single plot. We show that an optimal film thickness exists beyond which the activity of the film plateaus. Application to water oxidation by films of a cobalt-based oxidic catalyst provides a successful test of the applicability of the proposed methodology, which also provides further insight into the mechanism by which these cobalt-based films catalyze the oxidation of water. The exact nature of the kinetic and thermodynamic characteristics that have been derived from the analysis is discussed as well as their use in catalyst benchmarking.
Co-reporter:Arturo A. Pizano ; Lisa Olshansky ; Patrick G. Holder ; JoAnne Stubbe
Journal of the American Chemical Society 2013 Volume 135(Issue 36) pp:13250-13253
Publication Date(Web):August 8, 2013
DOI:10.1021/ja405498e
Substrate turnover in class Ia ribonucleotide reductase (RNR) requires reversible radical transport across two subunits over 35 Å, which occurs by a multistep proton-coupled electron-transfer mechanism. Using a photooxidant-labeled β2 subunit of Escherichia coli class Ia RNR, we demonstrate photoinitiated oxidation of a tyrosine in an α2:β2 complex, which results in substrate turnover. Using site-directed mutations of the redox-active tyrosines at the subunit interface, Y356F(β) and Y731F(α), this oxidation is identified to be localized on Y356. The rate of Y356 oxidation depends on the presence of Y731 across the interface. This observation supports the proposal that unidirectional PCET across the Y356(β)–Y731(α)–Y730(α) triad is crucial to radical transport in RNR.
Co-reporter:Andrew M. Ullman
Journal of the American Chemical Society 2013 Volume 135(Issue 40) pp:15053-15061
Publication Date(Web):August 29, 2013
DOI:10.1021/ja404469y
A heptanuclear cobalt cluster was synthesized in two different oxidation states, Co(II)7 and a mixed valence Co(III)Co(II)6, as a soluble model of a cobalt–phosphate/borate (Co-OEC) water splitting catalyst. Crystallographic characterization indicates similar cluster cores, distinguished primarily at the central Co atom. An anion associates to the cluster cores via hydrogen bonding. Using an isotope exchange method, an anomalously slow self-exchange electron transfer rate constant (kobs = 1.53 × 10–3 M–1 s–1 at 40 °C and 38 mM [OTf] in MeCN), as compared to that predicted from semiclassical Marcus theory, supports a charge transfer process that is accelerated by dissociation of the anion from the oxidized cluster. This mechanism sheds light on the inverse dependence of anions in the self-repair mechanism of Co-OECs. Moreover, because H2O cannot directly bridge cobalt centers, owing to the encapsulation of the central Co within the cluster core, the observed results address a long-standing controversy surrounding the Co2+/3+ self-exchange electron transfer reaction of the hexaaqua complex.
Co-reporter:Thomas J. Kempa ; Sun-Kyung Kim ; Robert W. Day ; Hong-Gyu Park ; Daniel G. Nocera ;Charles M. Lieber
Journal of the American Chemical Society 2013 Volume 135(Issue 49) pp:18354-18357
Publication Date(Web):November 26, 2013
DOI:10.1021/ja411050r
Enhanced synthetic control of the morphology, crystal structure, and composition of nanostructures can drive advances in nanoscale devices. Axial and radial semiconductor nanowires are examples of nanostructures with one and two structural degrees of freedom, respectively, and their synthetically tuned and modulated properties have led to advances in nanotransistor, nanophotonic, and thermoelectric devices. Similarly, developing methods that allow for synthetic control of greater than two degrees of freedom could enable new opportunities for functional nanostructures. Here we demonstrate the first regioselective nanowire shell synthesis in studies of Ge and Si growth on faceted Si nanowire surfaces. The selectively deposited Ge is crystalline, and its facet position can be synthetically controlled in situ. We use this synthesis to prepare electrically addressable nanocavities into which solution soluble species such as Au nanoparticles can be incorporated. The method furnishes multicomponent nanostructures with unique photonic properties and presents a more sophisticated nanodevice platform for future applications in catalysis and photodetection.
Co-reporter:Christopher L. Farrow ; D. Kwabena Bediako ; Yogesh Surendranath ; Daniel G. Nocera ;Simon J. L. Billinge
Journal of the American Chemical Society 2013 Volume 135(Issue 17) pp:6403-6406
Publication Date(Web):April 2, 2013
DOI:10.1021/ja401276f
Continual improvements in solar-to-fuels catalysis require a genuine understanding of catalyst structure–function relationships, not only with respect to local order, but also intermediate-range structure. We report the X-ray pair distribution function analysis of the nanoscale order of an oxidic cobalt-based water-splitting catalyst and uncover an electrolyte dependence in the intermediate-range structure of catalyst films. Whereas catalyst films formed in borate electrolyte (CoBi) exhibit coherent domains consisting of 3–4 nm cobaltate clusters with up to three layers, films deposited in phosphate electrolyte (CoPi) comprise significantly smaller clusters that are not coherently stacked. These structural insights are correlated with marked differences in activity between CoPi and CoBi films.
Co-reporter:David C. Powers ; Bryce L. Anderson
Journal of the American Chemical Society 2013 Volume 135(Issue 50) pp:18876-18883
Publication Date(Web):November 18, 2013
DOI:10.1021/ja408787k
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.
Co-reporter:David C. Powers, Matthew B. Chambers, Thomas S. Teets, Noémie Elgrishi, Bryce L. Anderson and Daniel G. Nocera
Chemical Science 2013 vol. 4(Issue 7) pp:2880-2885
Publication Date(Web):17 Apr 2013
DOI:10.1039/C3SC50462J
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.
Co-reporter:Christopher M. Lemon, Elizabeth Karnas, Moungi G. Bawendi, and Daniel G. Nocera
Inorganic Chemistry 2013 Volume 52(Issue 18) pp:10394-10406
Publication Date(Web):August 26, 2013
DOI:10.1021/ic4011168
Supramolecular assemblies of a quantum dot (QD) associated to palladium(II) porphyrins have been developed to detect oxygen (pO2) in organic solvents. Palladium porphyrins are sensitive in the 0–160 Torr range, making them ideal phosphors for in vivo biological oxygen quantification. Porphyrins with meso pyridyl substituents bind to the surface of the QD to produce self-assembled nanosensors. Appreciable overlap between QD emission and porphyrin absorption features results in efficient Förster resonance energy transfer (FRET) for signal transduction in these sensors. The QD serves as a photon antenna, enhancing porphyrin emission under both one- and two-photon excitation, demonstrating that QD-palladium porphyrin conjugates may be used for oxygen sensing over physiological oxygen ranges.
Co-reporter:Matthew B. Chambers, Stanislav Groysman, Dino Villagrán, and Daniel G. Nocera
Inorganic Chemistry 2013 Volume 52(Issue 6) pp:3159-3169
Publication Date(Web):February 22, 2013
DOI:10.1021/ic302634q
Mononuclear Fe(II) and Fe(III) complexes residing in a trigonal tris(ditox) (ditox = tBu2(Me)CO–) ligand environment have been synthesized and characterized. The Fe(III) ditox complex does not react with oxidants such as PhIO, whereas NMe3O substitutes a coordinated tetrahydrofuran (THF) in the apical position without undergoing oxo transfer. In contrast, the Fe(II) ditox complex reacts rapidly with PhIO or Me3NO in THF or cyclohexadiene to furnish a highly reactive intermediate, which cleaves C–H bonds to afford the Fe(III)–hydroxide complex. When generated in 1,2-difluorobenze, this intermediate can be intercepted to oxidize phosphines to phosphine oxide. The fast rates at which these reactions occur is attributed to a particularly weak ligand field imparted by the tris(alkoxide) ancillary ligand environment.
Co-reporter:Michael P. Marshak and Daniel G. Nocera
Inorganic Chemistry 2013 Volume 52(Issue 3) pp:1173-1175
Publication Date(Web):January 22, 2013
DOI:10.1021/ic3023612
The reaction of Na(OSitBu2Me) with CrCl3 yields solid [Cr(OSitBu2Me)3]n (1), which can be crystallized in the presence of excess Na(OSitBu2Me) to yield [Na(THF)][Cr(OSitBu2Me)4] (2). This complex is oxidized to yield Cr(OSitBu2Me)4 (3), a crystalline chromium(IV) siloxide complex that is air- and moisture-stable. Electronic spectroscopic analysis of the absorption spectrum of 3 indicates a particularly weak ligand field (ΔT = 7940 cm–1) and covalent Cr–O bonding. 3 provides the first structural and spectroscopic characterization of a homoleptic chromium(IV) siloxide complex and provides a benchmark for tetrahedral chromium(IV) ions residing in solid oxide lattices.
Co-reporter:Dr. Chang Hoon Lee;Dr. Dino Villágran;Dr. Timothy R. Cook; Jonas C. Peters; Daniel G. Nocera
ChemSusChem 2013 Volume 6( Issue 8) pp:1541-1544
Publication Date(Web):
DOI:10.1002/cssc.201300068
Abstract
Metal complexes of derivatized 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentane (bapa) ligands were prepared from 4-substituted diacetylpyridine derivatives by templated condensation with 3,3′-diaminodipropylamine in the presence of a metal halide or nitrate. The diacetylpyridine derivatives with Pacman and Hangman scaffolds are delivered from borylation of the 4-postion of diacetylpyridine and subsequent Suzuki coupling with the appropriate Hangman or Pacman backbone. Electrochemical examination of the parent [Co(bapa)]2+ scaffold reveals it to be a catalyst for the hydrogen evolution reaction (HER) in acetonitrile. Similar studies of the Hangman complex appear to be obscured by trace amounts of residual palladium remaining from the Suzuki coupling reaction to provide a cautionary note for the use of such cross-coupling methodologies in the preparation of HER catalysts.
Co-reporter:Lisa Olshansky; JoAnne Stubbe
Journal of the American Chemical Society () pp:
Publication Date(Web):December 29, 2015
DOI:10.1021/jacs.5b09259
Ribonucleotide reductase (RNR) catalyzes the conversion of ribonucleotides to deoxyribonucleotides to provide the monomeric building blocks for DNA replication and repair. Nucleotide reduction occurs by way of multistep proton-coupled electron transfer (PCET) over a pathway of redox active amino acids spanning ∼35 Å and two subunits (α2 and β2). Despite the fact that PCET in RNR is rapid, slow conformational changes mask examination of the kinetics of these steps. As such, we have pioneered methodology in which site-specific incorporation of a [ReI] photooxidant on the surface of the β2 subunit (photoβ2) allows photochemical oxidation of the adjacent PCET pathway residue β-Y356 and time-resolved spectroscopic observation of the ensuing reactivity. A series of photoβ2s capable of performing photoinitiated substrate turnover have been prepared in which four different fluorotyrosines (FnYs) are incorporated in place of β-Y356. The FnYs are deprotonated under biological conditions, undergo oxidation by electron transfer (ET), and provide a means by which to vary the ET driving force (ΔG°) with minimal additional perturbations across the series. We have used these features to map the correlation between ΔG° and kET both with and without the fully assembled photoRNR complex. The photooxidation of FnY356 within the α/β subunit interface occurs within the Marcus inverted region with a reorganization energy of λ ≈ 1 eV. We also observe enhanced electronic coupling between donor and acceptor (HDA) in the presence of an intact PCET pathway. Additionally, we have investigated the dynamics of proton transfer (PT) by a variety of methods including dependencies on solvent isotopic composition, buffer concentration, and pH. We present evidence for the role of α2 in facilitating PT during β-Y356 photooxidation; PT occurs by way of readily exchangeable positions and within a relatively “tight” subunit interface. These findings show that RNR controls ET by lowering λ, raising HDA, and directing PT both within and between individual polypeptide subunits.
Co-reporter:Michael Huynh, Tuncay Ozel, Chong Liu, Eric C. Lau and Daniel G. Nocera
Chemical Science (2010-Present) 2017 - vol. 8(Issue 7) pp:NaN4794-4794
Publication Date(Web):2017/05/05
DOI:10.1039/C7SC01239J
Oxygen evolution reaction (OER) catalysts that are earth-abundant and are active and stable in acid are unknown. Active catalysts derived from Co and Ni oxides dissolve at low pH, whereas acid stable systems such as Mn oxides (MnOx) display poor OER activity. We now demonstrate a rational approach for the design of earth-abundant catalysts that are stable and active in acid by treating activity and stability as decoupled elements of mixed metal oxides. Manganese serves as a stabilizing structural element for catalytically active Co centers in CoMnOx films. In acidic solutions (pH 2.5), CoMnOx exhibits the OER activity of electrodeposited Co oxide (CoOx) with a Tafel slope of 70–80 mV per decade while also retaining the long-term acid stability of MnOx films for OER at 0.1 mA cm−2. Driving OER at greater current densities in this system is not viable because at high anodic potentials, Mn oxides convert to and dissolve as permanganate. However, by exploiting the decoupled design of the catalyst, the stabilizing structural element may be optimized independently of the Co active sites. By screening potential–pH diagrams, we replaced Mn with Pb to prepare CoFePbOx films that maintained the high OER activity of CoOx at pH 2.5 while exhibiting long-term acid stability at higher current densities (at 1 mA cm−2 for over 50 h at pH 2.0). Under these acidic conditions, CoFePbOx exhibits OER activity that approaches noble metal oxides, thus establishing the viability of decoupling functionality in mixed metal catalysts for designing active, acid-stable, and earth-abundant OER catalysts.
Co-reporter:David Y. Song, Arturo A. Pizano, Patrick G. Holder, JoAnne Stubbe and Daniel G. Nocera
Chemical Science (2010-Present) 2015 - vol. 6(Issue 8) pp:NaN4524-4524
Publication Date(Web):2015/06/08
DOI:10.1039/C5SC01125F
Proton-coupled electron transfer (PCET) is a fundamental mechanism important in a wide range of biological processes including the universal reaction catalysed by ribonucleotide reductases (RNRs) in making de novo, the building blocks required for DNA replication and repair. These enzymes catalyse the conversion of nucleoside diphosphates (NDPs) to deoxynucleoside diphosphates (dNDPs). In the class Ia RNRs, NDP reduction involves a tyrosyl radical mediated oxidation occurring over 35 Å across the interface of the two required subunits (β2 and α2) involving multiple PCET steps and the conserved tyrosine triad [Y356(β2)–Y731(α2)–Y730(α2)]. We report the synthesis of an active photochemical RNR (photoRNR) complex in which a Re(I)-tricarbonyl phenanthroline ([Re]) photooxidant is attached site-specifically to the Cys in the Y356C-(β2) subunit and an ionizable, 2,3,5-trifluorotyrosine (2,3,5-F3Y) is incorporated in place of Y731 in α2. This intersubunit PCET pathway is investigated by ns laser spectroscopy on [Re356]-β2:2,3,5-F3Y731-α2 in the presence of substrate, CDP, and effector, ATP. This experiment has allowed analysis of the photoinjection of a radical into α2 from β2 in the absence of the interfacial Y356 residue. The system is competent for light-dependent substrate turnover. Time-resolved emission experiments reveal an intimate dependence of the rate of radical injection on the protonation state at position Y731(α2), which in turn highlights the importance of a well-coordinated proton exit channel involving the key residues, Y356 and Y731, at the subunit interface.
Co-reporter:Seung Jun Hwang, David C. Powers, Andrew G. Maher and Daniel G. Nocera
Chemical Science (2010-Present) 2015 - vol. 6(Issue 2) pp:NaN922-922
Publication Date(Web):2014/10/08
DOI:10.1039/C4SC02357A
Photoactivation of M–X bonds is a challenge for photochemical HX splitting, particularly with first-row transition metal complexes because of short intrinsic excited state lifetimes. Herein, we report a tandem H2 photocycle based on combination of a non-basic photoredox phosphine mediator and nickel metal catalyst. Synthetic studies and time-resolved photochemical studies have revealed that phosphines serve as photochemical H-atom donors to Ni(II) trihalide complexes to deliver a Ni(I) centre. The H2 evolution catalytic cycle is closed by sequential disproportionation of Ni(I) to afford Ni(0) and Ni(II) and protolytic H2 evolution from the Ni(0) intermediate. The results of these investigations suggest that H2 photogeneration proceeds by two sequential catalytic cycles: a photoredox cycle catalyzed by phosphines and an H2-evolution cycle catalyzed by Ni complexes to circumvent challenges of photochemistry with first-row transition metal complexes.
Co-reporter:David C. Powers, Matthew B. Chambers, Thomas S. Teets, Noémie Elgrishi, Bryce L. Anderson and Daniel G. Nocera
Chemical Science (2010-Present) 2013 - vol. 4(Issue 7) pp:NaN2885-2885
Publication Date(Web):2013/04/17
DOI:10.1039/C3SC50462J
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.
Co-reporter:Julia M. Stauber, Peter Müller, Yizhe Dai, Gang Wu, Daniel G. Nocera and Christopher C. Cummins
Chemical Science (2010-Present) 2016 - vol. 7(Issue 12) pp:NaN6933-6933
Publication Date(Web):2016/07/06
DOI:10.1039/C6SC01754A
Cofacial bimetallic tin(II) ([Sn2(mBDCA-5t)]2−, 1) and lead(II) ([Pb2(mBDCA-5t)]2−, 2) complexes have been prepared by hexadeprotonation of hexacarboxamide cryptand mBDCA-5t-H6 together with double Sn(II) or Pb(II) insertion. Reaction of 1 with elemental sulfur or selenium generates di-tin polychalcogenide complexes containing μ-E and bridging μ-E5 ligands where E = S or Se, and the Sn(II) centers have both been oxidized to Sn(IV). Solution and solid-state UV-Vis spectra of [(μ-S5)Sn2(μ-S)(mBDCA-5t)]2− (4) indicate that the complex acts reversibly as a source of S3˙− in DMF solution with a Keq = 0.012 ± 0.002. Reductive removal of all six chalcogen atoms is achieved through treatment of [(μ-E5)Sn2(μ-E)(mBDCA-5t)]2− with PR3 (R = tBu, Ph, OiPr) to produce six equiv. of the corresponding EPR3 compound with regeneration of di-tin(II) cryptand complex 1.
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![2-[(4-methoxyphenyl)methyl]benzoic Acid](http://img.cochemist.com/ccimg/5400/5398-17-4_b.png)
![2-Anthracenecarboxylic acid,9,10-bis[[[(2-boronophenyl)methyl]methylamino]methyl]-](/data/chemimg/1326300/790257-31-7.png)
![2-Anthracenecarboxylic acid,9,10-bis[[[(2-boronophenyl)methyl]methylamino]methyl]-](/data/chemimg/1326300/790257-31-7_b.png)
![Nickel,dibromo[1,2-ethanediylbis[diphenylphosphine-kP]]-, (SP-4-2)- (9CI)](http://img.cochemist.com/ccimg/14700/14647-21-3.png)
![Nickel,dibromo[1,2-ethanediylbis[diphenylphosphine-kP]]-, (SP-4-2)- (9CI)](http://img.cochemist.com/ccimg/14700/14647-21-3_b.png)
![Phenol, 2-methoxy-6-[(methylimino)methyl]-](http://img.cochemist.com/ccimg/13700/13664-27-2.png)
![Phenol, 2-methoxy-6-[(methylimino)methyl]-](http://img.cochemist.com/ccimg/13700/13664-27-2_b.png)
![2,2'-[(2,3,4,5,6-pentafluorophenyl)methylene]bis-1H-Pyrrole](http://img.cochemist.com/ccimg/167500/167482-91-9.png)
![2,2'-[(2,3,4,5,6-pentafluorophenyl)methylene]bis-1H-Pyrrole](http://img.cochemist.com/ccimg/167500/167482-91-9_b.png)
