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At least 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

What Is the Right Level of Activation of a High-Spin {FeNO} 7 Complex to Enable Direct N–N Coupling? Mechanistic Insight into Flavodiiron NO Reductases

Flavodiiron nitric oxide reductases (FNORs), found in pathogenic bacteria, are capable of reducing nitric oxide (NO) to nitrous oxide (N 2 O) to detoxify NO released by the human immune system. Previously, we reported the first FNOR model system that mediates direct NO reduction (Dong, H. T.; et al. J. Am. Chem. Soc. 2018, 140, 13429-13440), but no intermediate of the reaction could be characterized. Here, we present a new set of model complexes that, depending on the ligand substitution, can either mediate direct NO reduction or stabilize a highly activated high-spin (hs) {FeNO} 7 complex, the first intermediate of the reaction. The precursors, [{Fe II (MPA-(RPhO) 2 )} 2 ] (1, R = H and 2, R = t Bu, Me), were prepared first and fully characterized. Complex 1 (without steric protection) directly reduces NO to N 2 O almost quantitatively, which constitutes only the second example of this reaction in model systems. Contrarily, the reaction of sterically protected 2 with NO forms the stable mononitrosyl complex 3, which shows one of the lowest N-O stretching frequencies (1689 cm -1 ) observed so far for a mononuclear hs-{FeNO} 7 complex. Here this study confirms that an N-O stretch & LE;1700 cm -1 represents the appropriate level of activation of the FeNO unit to enable direct NO reduction. The higher activation level of these hs-{FeNO} 7 complexes required for NO reduction compared to those formed in FNORs emphasizes the importance of hydrogen bonding residues in the active sites of FNORs to activate the bound NO ligands for direct N-N coupling and N2O formation. The implications of these results for FNORs are further discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Two-Coordinate Coinage Metal Complexes as Solar Photosensitizers

Generating a sustainable fuel from sunlight plays an important role in meeting the energy demands of the modern age. Herein we report two-coordinate carbene-metal-amide (cMa, M = Cu(I) and Au(I)) complexes can be used as sensitizers to promote the light driven reduction of water to hydrogen. The cMa complexes studied here absorb visible photons (ε vis > 10 3 M -1 cm -1 ), maintain long excited state lifetimes (τ ~ 0.2-1 μs) and perform stable photo-induced charge transfer to a target substrate with high photoreducing potential (E +/* up to 2.33 V vs. Fc +/o based on a Rehm-Weller analysis). We pair these coinage metal complexes with a cobalt-glyoxime electrocatalyst to photocatalytically generate hydrogen and compare the performance of the copper- and gold-based cMa complexes. We also find that these two-coordinate complexes presented can perform photo-driven hydrogen production from water without the addition of the cobalt-glyoxime electrocatalyst. In this “catalyst free” system the cMa sensitizer partially decomposes to give metal nanoparticles that catalyze water reduction. Furthermore, this work identifies two-coordinate coinage metal complexes as promising abundant metal, solar fuels photosensitizers that offer exceptional tunability and photoredox properties.

14 SOLAR ENERGY↗
DOE OSTI2026

Pursuing Heteroleptic Ligand Design Principles for Photoactive Fe Complexes with Ultrafast X-ray Emission and Variable-Temperature Optical Spectroscopies

Understanding the key parameters that govern the photophysical and photochemical properties of transition metal complexes is essential for the development of efficient photosensitizers for photocatalytic applications. Achieving this objective necessitates clear and detailed investigations of their electronic excited states, for which time-resolved metal Kβ X-ray emission spectroscopy (XES) has proven highly effective. Here, we present a time-resolved Fe Kβ XES study of a heteroleptic Fe(II) polypyridyl carbene complex, [Fe(phen) 2 (C 4 H 10 N 4 )] 2+ (1; phen = 1,10-phenanthroline), utilizing both the valence-to-core and Kβ mainline spectral regions, complemented by variable-temperature transient optical absorption (VT-TA) spectroscopy. Detailed analysis of the time-resolved Kβ XES data, supported by density functional theory (DFT) calculations and an Eyring analysis of the VT-TA data, reveals parallel excited state relaxation dynamics that support an assignment of the long-lived excited state to a triplet metal-centered state. Placing these results in the context of prior studies of heteroleptic Fe(II) polypyridyl cyanide complexes motivated a series of DFT calculations to investigate the effects of ligand structural flexibility and arrangement. These calculations reinforce the experimentally derived conclusion that constraining structural flexibility with multidentate ligands significantly impacts the excited state relaxation dynamics. Furthermore, our study emphasizes that the arrangement of strong field ligands in heteroleptic complexes substantially affects the energy of Jahn–Teller active triplet metal-centered states in low-spin d 6 metal complexes. Together, these findings provide synthetic design principles for extending metal-to-ligand charge transfer excited state lifetimes of heteroleptic Fe complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Detection of an Iridium-Dihydrogen Complex: A Proposed Intermediate in Ionic Hydrogenation

Addition of high pressures of H2 to five-coordinate [(tBu)4(POCOP)Ir(CO)(H)]OTf [(tBu)4(POCOP) = ?3-C6H3-2,6- (OP(tBu)2)2] complexes results in observation of two new iridium-dihydrogen complexes. If the aryl moiety of the POCOP ligand is substituted with an electron withdrawing protonated dimethylamino group at the para position, hydrogen coordination is enhanced. Five-coordinate Ir-H complexes generated by addition of triflic acid to (tBu)4(POCOP)Ir(CO) species show an Ir-H 1H NMR chemical shift dependence on the number of equivalents of acid present. It is proposed that excess triflic acid in solution facilitates triflate dissociation from iridium, resulting in unsaturated five-coordinate Ir-H complexes. The five-coordinate iridium-hydride complexes were found to catalyze H/D exchange between H2 and CD3OD. The existence of the dihydrogen complexes as well as isotope exchange reactions provide evidence for proposed ionic hydrogenation mechanistic intermediates for glycerol deoxygenation.

Goldberg, Jonathan M.↗
DOE OSTI2017

A Six-Coordinate Peroxynitrite Low-Spin Iron(III) Porphyrinate Complex—The Product of the Reaction of Nitrogen Monoxide (·NO (g) ) with a Ferric-Superoxide Species

Peroxynitrite ( – OON=O, PN) is a reactive nitrogen species (RNS) which can effect deleterious nitrative or oxidative (bio)chemistry. It may derive from reaction of superoxide anion (O 2 •– ) with nitric oxide (·NO) and has been suggested to form an as-yet unobserved bound heme-iron-PN intermediate in the catalytic cycle of nitric oxide dioxygenase (NOD) enzymes, which facilitate a ·NO homeostatic process, i.e., its oxidation to the nitrate anion. Here, a discrete six-coordinate low-spin porphyrinate-Fe III complex [(P Im )Fe III ( – OON=O)] (P Im ; a porphyrin moiety with a covalently tethered imidazole axial “base” donor ligand) has been identified and characterized by various spectroscopies (UV–vis, NMR, EPR, XAS, resonance Raman) and DFT calculations, following its formation at –80 °C by addition of ·NO (g) to the heme-superoxo species, [(P Im )Fe III (O 2 •– )]. DFT calculations confirm that is a six-coordinate low-spin species with the PN ligand coordinated to iron via its terminal peroxidic anionic O atom with the overall geometry being in a cis-configuration. Complex thermally transforms to its isomeric low-spin nitrato form [(P Im )Fe III (NO 3 – )]. While previous (bio)chemical studies show that phenolic substrates undergo nitration in the presence of PN or PN-metal complexes, in the present system, addition of 2,4-di- tert -butylphenol ( 2,4 DTBP) to complex does not lead to nitrated phenol; the nitrate complex still forms. Furthermore, DFT calculations reveal that the phenolic H atom approaches the terminal PN O atom (farthest from the metal center and ring core), effecting O–O cleavage, giving nitrogen dioxide (·NO 2 ) plus a ferryl compound [(P Im )Fe IV =O] (7); this rebounds to give [(P Im )Fe III (NO 3 – )].The generation and characterization of the long sought after ferriheme peroxynitrite complex has been accomplished.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

New Rh 2 (II,II) Complexes for Solar Energy Applications: Panchromatic Absorption and Excited-State Reactivity

In this work, the new heteroleptic paddlewheel complexes cis-[Rh 2 (μ-form) 2 (μ-np) 2 ][BF 4 ] 2 , where form = p-ditolylformamidinate (DTolF) or p-difluorobenzylformamidinate (F-form) and np = 1,8-napthyridyine, and cis-Rh 2 (μ-form) 2 (μ-npCOO) 2 (npCOO – = 1,8-naphthyridine-2-carboxylate), were synthesized and characterized. The complexes absorb strongly throughout the ultraviolet (λ max = 300 nm, ε = 20 300 M –1 cm –1 ) and visible regions (λ max = 640 nm ε = 3500 M –1 cm –1 ), making them potentially useful new dyes with panchromatic light absorption for solar energy conversion applications. Ultrafast and nanosecond transient absorption and time-resolved infrared spectroscopies were used to characterize the identity and dynamics of the excited states, where singlet and triplet Rh 2 /form-to-naphthyridine, metal/ligand-to-ligand charge-transfer (ML-LCT) excited states were observed in all four complexes. The npCOO – complexes exhibit red-shifted absorption profiles extending into the near-IR and undergo photoinitiated electron transfer to generate reduced methyl viologen, a species that persists in the presence of a sacrificial donor. The energy of the triplet excited state of each complex was estimated from energy-transfer quenching experiments using a series of organic triplet donors (E( 3 ππ*) from 1.83 to 0.78 eV). The singlet reduction (+0.6 V vs Ag/AgCl) potentials, and singlet and triplet oxidation potentials (-1.1 and -0.5 V vs Ag/AgCl, respectively) were determined. Finally, based on the excited-state lifetimes and redox properties, these complexes represent a new class of light absorbers with potential application as dyes for charge injection into semiconductor solar cells and in sensitizer-catalyst assemblies for photocatalysis that operate with irradiation from the ultraviolet to ~800 nm.

14 SOLAR ENERGY↗
DOE OSTI2018

Proton-Coupled Electron Transfer to a Molybdenum Ethylene Complex Yields a β-Agostic Ethyl: Structure, Dynamics and Mechanism

In this paper, the interconversion of molybdenum ethylene and ethyl complexes by proton-coupled electron transfer (PCET) is described, an unusual transformation in organometallic chemistry. The cationic molybdenum ethylene complex [( Ph Tpy)(PPh 2 Me) 2 Mo(C 2 H 4 )][BArF 24 ] ([1-C 2 H 4 ] + ; Ph Tpy = 4'-Ph-2,2',6',2"-terpyridine, ArF 24 = [C 6 H 3 -3,5-(CF 3 ) 2 ] 4 ) was synthesized, structurally characterized, and its electronic structure established by a combination of spectroscopic and computational methods. The overall electronic structure is best described as a molybdenum(III) complex with a metallacyclopropane and a redox neutral terpyridine ligand. Addition of the nonclassical ammine complex [( Ph Tpy)(PPh 2 Me) 2 Mo(NH 3 )][BArF 24 ] ([1-NH 3 ] + ) to [1-C 2 H 4 ] + resulted in a net C–H bond-forming PCET reaction to yield the molybdenum ethyl [( Ph Tpy)(PPh 2 Me) 2 Mo(CH 2 CH 3 )][BArF 24 ] ([1-CH 2 CH 3 ] + ) and amido [( Ph Tpy)(PPh 2 Me) 2 Mo(NH 2 )][BArF 24 ] ([1-NH 2 ] + ) compounds. The reaction was reversed by addition of 2,4,6-tritert-butylphenoxyl radical to [1-CH 2 CH 3 ] + . The solid-state structure of [1-CH 2 CH 3 ] + established a β-agostic ethyl ligand that is maintained in solution as judged by variable temperature 1 H and 13 C NMR experiments. A combination of variable-temperature NMR experiments and isotopic labeling studies were used to probe the dynamics of [1-CH 2 CH 3 ] + and established restricted β-agostic -CH 3 rotation at low temperature (ΔG ‡ = 9.8 kcal mol –1 at -86 °C) as well as ethyl isomerization by β-hydride elimination-olefin rotation-reinsertion (ΔH ‡ = 19.3 ± 0.6 kcal mol –1 ; ΔS ‡ = 3.4 ± 1.7 cal mol –1 K –1 ). The β-(C–H) bond-dissociation free energy (BDFE) in [1-CH 2 CH 3 ] + was determined experimentally as 57 kcal mol –1 (THF) supported by a DFT-computed value of 52 kcal/mol –1 (gas phase). Comparison of pK a and electrochemical data for the complexes [1-C 2 H 4 ] + and [1-NH 3 ] + in combination with a deuterium kinetic isotope effect (k H /k D ) of 3.5(2) at 23 °C support a PCET process involving initial electron transfer followed by protonation leading to the formation of [1-CH 2 CH 3 ] + and [1-NH 2 ] + or a concerted pathway. Lastly, the data presented herein provides a structural, thermochemical and mechanistic foundation for understanding the PCET reactivity of organometallic complexes with alkene and alkyl ligands.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Experimental and Theoretical Studies on the Fragmentation of Gas-Phase Uranyl-, Neptunyl- and Plutonyl-Diglycolamide Complexes

Fragmentation of actinyl(VI) complexes UVIO2(L)22+, NpVIO2(L)22+ and PuVIO2(L)22+ (L = tetramethyl-3-oxa-glutaramide, TMOGA) produced by electrospray ionization was examined in the gas phase by collision induced dissociation (CID) in a quadrupole ion trap mass spectrometer. Cleavage of the C-Oether bond was observed for all three complexes, with dominant products being UVIO2(L)(L-86)+ with charge reduction, and NpVIO2(L)(L-101)2+ and PuVIO2(L)(L-101)2+ with charge conservation. The neptunyl and plutonyl complexes also exhibited substantial L+ loss to give pentavalent complexes NpVO2(L)+ and PuVO2(L)+, whereas the uranyl complex did not, consistent with the comparative An 5f-orbital energies and the AnVIO22+/AnVO2+ (An = U, Np, Pu) reduction potentials. CID of NpVO2(L)2+ and PuVO2(L)2+ was dominated by neutral ligand loss to form NpVO2(L)+ and PuVO2(L)+, which hydrated by addition of residual water in the ion trap; UVO2(L)2+ was not observed. Theoretical calculations of the structures and bonding of the AnVIO2(L)22+ complexes using density functional theory reveal that the metal centers are coordinated by six oxygen atoms from the two TMOGA ligands. The results are compared with radiolytic decomposition of TMOGA in solution.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2014

Observation of a Rare Earth Ion–Extractant Complex Arrested at the Oil–Water Interface During Solvent Extraction

Selective extraction of metal ions from a complex aqueous mixture into an organic phase is used to separate toxic or radioactive metals from polluted environments and nuclear waste, as well as to produce industrially relevant metals, such as rare earth ions. Selectivity arises from the choice of an extractant amphiphile, dissolved in the organic phase, which interacts preferentially with the target metal ion. The extractant-mediated process of ion transport from an aqueous to an organic phase takes place at the aqueous–organic interface; nevertheless, little is known about the molecular mechanism of this process despite its importance. Although state-of-the-art X-ray scattering is uniquely capable of probing molecular ordering at a liquid–liquid interface with subnanometer spatial resolution, utilizing this capability to investigate interfacial dynamical processes of short temporal duration remains a challenge. We show that a temperature-driven adsorption transition can be used to turn the extraction on and off by controlling adsorption and desorption of extractants at the oil–water interface. Lowering the temperature through this transition immobilizes a supramolecular ion–extractant complex at the interface during the extraction of rare earth erbium ions. Under the conditions of these experiments, the ion–extractant complexes condense into a two-dimensional inverted bilayer, which is characterized on the molecular scale with synchrotron X-ray reflectivity and fluorescence measurements. Raising the temperature above the transition leads to Er ion extraction as a result of desorption of ion–extractant complexes from the interface into the bulk organic phase. XAFS measurements of the ion–extractant complexes in the bulk organic phase demonstrate that they are similar to the interfacial complexes.

Bu, Wei↗
DOE OSTI2015

Visible Light Sensitized CO 2 Activation by the Tetraaza [Co II N 4 H(MeCN)] 2+ Complex Investigated by FT-IR Spectroscopy and DFT Calculations

In situ FT-IR measurements and electronic structure calculations are reported for the reduction of CO 2 catalyzed by the macrocyclic complex [Co II N 4 H] 2+ (N 4 H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene). Beginning from the [Co II N 4 H] 2+ resting state of the complex in wet acetonitrile solution, two different visible light sensitizers with substantially different reducing power are employed to access reduced states. Accessing reduced states of the complex with a [Ru(bpy) 3 ] 2+ sensitizer yields an infrared band at 1670 cm -1 attributed to carboxylate, which is also observed for an authentic sample of the one-electron reduced complex [CoN 4 H(MeCN)] + in CO 2 saturated acetonitrile solution. The results are interpreted based on calculations using the pure BP86 functional that correctly reproduces experimental geometries. Continuum solvation effects are also included. The calculations show that Co is reduced to CoIin the first reduction, which is consistent with experimental d-d spectra of square Co(I) macrocycle complexes. The energy of the CO 2 adduct of the one-electron reduced catalyst complex is essentially the same as for [CoN 4 H(MeCN)] + , which implies that only a fraction of the latter forms an adduct with CO 2 . By contrast, the calculations indicate a crucial role for redox noninnocence of the macrocyclic ligand in the doubly reduced state, [Co I (N 4 H) -• ], and show that [Co I (N 4 H) -• ] binds partially reduced CO 2 fairly strongly. Experimentally accessing [Co I (N 4 H)-•] with an Ir(bpy) 3 sensitizer with greater reducing power closes the catalytic cycle as FT-IR spectroscopy shows CO production. Use of isotopically substituted C 18 O 2 also shows clear evidence for 18 O-substituted byproducts from CO 2 reduction to CO.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Strengthening of Calcite Assemblages Through Chemical Complexation Reactions

Abstract Utilization of subsurface reservoirs, including fluid extraction/injection, can induce stress changes and modify in‐situ chemical equilibrium causing subcritical fracturing and deformation. Here, we show that chemical complexation reactions at the newly created fracture surfaces within the crack tip limit fracturing during consolidation of granular calcite assemblages. Previously, we showed that analogous chemical complexation reactions can increase in‐situ fracture toughness in pre‐fractured single crystal calcite. To test the chemical complexation effect under subsurface conditions, we consolidated samples under increasing hydrostatic pressure to induce widespread intergranular fracturing in the presence of fluid containing common anions. Measured consolidation, acoustic emissions, and microfracturing demonstrate that deformation correlates with complexation affinity for calcium in calcite and aqueous anions. We observed that deformation is lowest in the presence of strongly complexing anions (e.g., sulphate), provided sufficient anion diffusion rates. This understanding is important for predicting reservoir effective strength in the presence of complex aqueous fluids.

Choens, R. C.↗
DOE OSTI2024

Selecting Appropriate Model Complexity: An Example of Tracer Inversion for Thermal Prediction in Enhanced Geothermal Systems

Abstract A major challenge in the inversion of subsurface parameters is the ill‐posedness issue caused by the inherent subsurface complexities and the generally spatially sparse data. Appropriate simplifications of inversion models are thus necessary to make the inversion process tractable and meanwhile preserve the predictive ability of the inversion results. In this study, we investigate the effect of model complexity on fracture aperture inversion and thermal performance prediction in a field‐scale EGS model. Principal component analysis was used to map the aperture field to a low‐dimensional latent space. The complexity of the inversion model was quantitatively represented by the percentage of total variance in the original aperture fields preserved by the latent space. Tracer, pressure and flow rate data were used to invert for fracture aperture through an ensemble‐based inversion method, and the inferred aperture field was used to predict thermal performance. With an over‐simplified aperture model, ensemble collapse occurred. The inverted aperture models failed to resolve necessary flow and transport features, leading to a biased thermal performance prediction. A complex aperture model involved excessive features and was prone to overinterpreting the inversion data. Both the tracer/pressure/flow rate data reproduction and thermal prediction showed significant uncertainties, making it difficult to properly estimate long‐term thermal performance. Fortunately, our results indicate that there exists an appropriate model complexity which can simultaneously match inversion data and predict thermal performance with an acceptable uncertainty. The quality of the fit of tracer data appears to be a useful indicator of such an appropriate model complexity.

15 GEOTHERMAL ENERGY↗
DOE OSTI2015

Chirality-selected phase behaviour in ionic polypeptide complexes

In this study, polyelectrolyte complexes present new opportunities for self-assembled soft matter. Factors determining whether the phase of the complex is solid or liquid remain unclear. Ionic polypeptides enable examination of the effects of stereochemistry on complex formation. Here we demonstrate that chirality determines the state of polyelectrolyte complexes, formed from mixing dilute solutions of oppositely charged polypeptides, via a combination of electrostatic and hydrogen-bonding interactions. Fluid complexes occur when at least one of the polypeptides in the mixture is racemic, which disrupts backbone hydrogen-bonding networks. Pairs of purely chiral polypeptides, of any sense, form compact, fibrillar solids with a β-sheet structure. Analogous behaviour occurs in micelles formed from polypeptide block copolymers with polyethylene oxide, where assembly into aggregates with either solid or fluid cores, and eventually into ordered phases at high concentrations, is possible. Chirality is an exploitable tool for manipulating material properties in polyelectrolyte complexation.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Atomic structure of the Y complex of the nuclear pore

The nuclear pore complex (NPC) is the principal gateway for transport into and out of the nucleus. Selectivity is achieved through the hydrogel-like core of the NPC. The structural integrity of the NPC depends on ~15 architectural proteins, which are organized in distinct subcomplexes to form the >40-MDa ring-like structure. In this paper, we present the 4.1-Å crystal structure of a heterotetrameric core element ('hub') of the Y complex, the essential NPC building block, from Myceliophthora thermophila. Using the hub structure together with known Y-complex fragments, we built the entire ~0.5-MDa Y complex. Our data reveal that the conserved core of the Y complex has six rather than seven members. Finally, evolutionarily distant Y-complex assemblies share a conserved core that is very similar in shape and dimension, thus suggesting that there are closely related architectural codes for constructing the NPC in all eukaryotes.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

Munc18 and Munc13 serve as a functional template to orchestrate neuronal SNARE complex assembly

The transition of the Munc18-1/syntaxin-1 complex to the SNARE complex, a key step involved in exocytosis, is regulated by Munc13-1, SNAP-25 and synaptobrevin-2, but the underlying mechanism remains elusive. Here, we identify an interaction between Munc13-1 and the membrane-proximal linker region of synaptobrevin-2, and reveal its essential role in transition and exocytosis. Upon this interaction, Munc13-1 not only recruits synaptobrevin-2-embedded vesicles to the target membrane but also renders the synaptobrevin-2 SNARE motif more accessible to the Munc18-1/syntaxin-1 complex. Afterward, the entry of SNAP-25 leads to a half-zippered SNARE assembly, which eventually dissociates the Munc18-1/syntaxin-1 complex to complete SNARE complex formation. Furthermore, our data suggest that Munc18-1 and Munc13-1 together serve as a functional template to orchestrate SNARE complex assembly.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Atomically precise control of rotational dynamics in charged rare-earth complexes on a metal surface

Abstract Complexes containing rare-earth ions attract great attention for their technological applications ranging from spintronic devices to quantum information science. While charged rare-earth coordination complexes are ubiquitous in solution, they are challenging to form on materials surfaces that would allow investigations for potential solid-state applications. Here we report formation and atomically precise manipulation of rare-earth complexes on a gold surface. Although they are composed of multiple units held together by electrostatic interactions, the entire complex rotates as a single unit when electrical energy is supplied from a scanning tunneling microscope tip. Despite the hexagonal symmetry of the gold surface, a counterion at the side of the complex guides precise three-fold rotations and 100% control of their rotational directions is achieved using a negative electric field from the scanning probe tip. This work demonstrates that counterions can be used to control dynamics of rare-earth complexes on materials surfaces for quantum and nanomechanical applications.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Cryo-EM structure of the Saccharomyces cerevisiae Rpd3L histone deacetylase complex

The Rpd3L histone deacetylase (HDAC) complex is an ancient 12-subunit complex conserved in a broad range of eukaryotes that performs localized deacetylation at or near sites of recruitment by DNA-bound factors. Here we describe the cryo-EM structure of this prototypical HDAC complex that is characterized by as many as seven subunits performing scaffolding roles for the tight integration of the only catalytic subunit, Rpd3. The principal scaffolding protein, Sin3, along with Rpd3 and the histone chaperone, Ume1, are present in two copies, with each copy organized into separate lobes of an asymmetric dimeric molecular assembly. The active site of one Rpd3 is completely occluded by a leucine side chain of Rxt2, while the tips of the two lobes and the more peripherally associated subunits exhibit varying levels of flexibility and positional disorder. The structure reveals unexpected structural homology/analogy between unrelated subunits in the fungal and mammalian complexes and provides a foundation for deeper interrogations of structure, biology, and mechanism of these complexes, as well as for the discovery of HDAC complex-specific inhibitors.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

The synthesis and characterization of an iron(VII) nitrido complex

Abstract Complexes of iron in high oxidation states are captivating research subjects due to their pivotal role as active intermediates in numerous catalytic processes. Structural and spectroscopic studies of well-defined model complexes often provide evidence of these intermediates. In addition to the fundamental molecular and electronic structure insights gained by these complexes, their reactivity also affects our understanding of catalytic reaction mechanisms for small molecule and bond-activation chemistry. Here, we report the synthesis, structural and spectroscopic characterization of a stable, octahedral Fe(VI) nitrido complex and an authenticated, unique Fe(VII) species, prepared by one-electron oxidation. The super-oxidized Fe(VII) nitride rearranges to an Fe(V) imide through an intramolecular amination mechanism and ligand exchange, which is characterized spectroscopically and computationally. This enables combined reactivity and stability studies on a single molecular system of a rare high-valent complex redox pair. Quantum chemical calculations complement the spectroscopic parameters and provide evidence for a diamagnetic ( S = 0) d 2 Fe(VI) and a genuine S = 1/2, d 1 Fe(VII) configuration of these super-oxidized nitrido complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗