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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

A nanobody suite for yeast scaffold nucleoporins provides details of the nuclear pore complex structure

Nuclear pore complexes (NPCs) are the main conduits for molecular exchange across the nuclear envelope. The NPC is a modular assembly of ~500 individual proteins, called nucleoporins or nups. Most scaffolding nups are organized in two multimeric subcomplexes, the Nup84 or Y complex and the Nic96 or inner ring complex. Working in S. cerevisiae, and to study the assembly of these two essential subcomplexes, we here develop a set of twelve nanobodies that recognize seven constituent nucleoporins of the Y and Nic96 complexes. These nanobodies all bind specifically and with high affinity. We present structures of several nup-nanobody complexes, revealing their binding sites. Additionally, constitutive expression of the nanobody suite in S. cerevisiae detect accessible and obstructed surfaces of the Y complex and Nic96 within the NPC. Overall, this suite of nanobodies provides a unique and versatile toolkit for the study of the NPC.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2011

Redox Chemistry in Thin Layers of Organometallic Complexes Prepared Using Ion Soft Landing

Soft landing (SL) of mass-selected ions is used to introduce catalytically-active metal complexes complete with organic ligands into the gas phase and then to transfer them as ions onto an inert surface. This is part of an effort to prepare materials with defined active sites and thus achieve molecular design of surfaces in a highly controlled way. Solution-phase electrochemical studies have shown that VIVO(salen) reacts in the presence of acid to form VVO(salen)+ and the deoxygenated VIII(salen)+ complex -- a key intermediate in the four electron reduction of O2 by vanadium-salen. In this work, the VVO(salen)+ and [NiII(salen)+H]+ complexes were generated by electrospray ionization and mass-selected before being deposited into an inert fluorinated self-assembled monolayer (FSAM) surface on gold. A time dependence study after ion deposition showed loss of O from VVO(salen)+ forming VIII(salen)+ over a four-day period, indicating a slow interfacial reduction process. Similar results were obtained when other protonated molecules were co-deposited with VVO(salen)+ on the FSAM surface. In all these experiments oxidation of the VIII(salen)+ product occurred upon exposure to oxygen or to air. The cyclic regeneration of VVO(salen)+ upon exposure to molecular oxygen and its subsequent reduction to VIII(salen)+ in vacuum completes the catalytic cycle of O2 reduction by the immobilized vanadium-salen species. Moreover, our results represent the first evidence of formation of reactive organometallic complexes on substrates in the absence of solvent. Remarkably, deoxygenation of the oxo-vanadium complex, previously observed only in highly acidic non-aqueous solvents, occurs on the surface in the UHV environment using an acid which is deposited into the inert monolayer. This acid can be a protonated metal complex, e.g. [NiII(salen)+H]+ or an organic acid such as protonated diaminododecane.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Spectroscopic characterization of a new Re(i) tricarbonyl complex with a thiosemicarbazone derivative: towards sensing and electrocatalytic applications

This work describes the preparation of a new thiosemicarbazone derivative, (Z)- N -ethyl-2-(6-oxo-1,10-phenanthrolin-5(6 H )-ylidene)hydrazinecarbothioamide (phet) and its respective Re( I ) tricarbonyl chloro complex, fac -[ReCl(CO) 3 (phet)]. The spectroscopic, photophysical and electrochemical properties of the new complex were fully investigated through steady state and time-resolved techniques along with computational calculations. In fac -[ReCl(CO) 3 (phet)], the new ligand is coordinated to the metal center through the pyridyl rings of the phenanthroline moiety. The unbound electron pairs in the S atom of the bending thiosemicarbazone group induce new low energy lying electronic transitions. Consequently, enhanced visible light absorption up to 550 nm is observed in acetonitrile due to the overlap between MLCT Re→phet and IL phet(n→π*) transitions. The absorption bands and emission quantum yields of fac -[ReCl(CO) 3 (phet)] are sensitive to proton concentration due to an acid-basic equilibrium in the N atoms of the thiosemicarbazone. Proton dissociation constants of 10.0 ± 0.1 and 11.4 ± 0.2 were determined respectively for the ground and excited states of the new complex. Spectral changes could also be observed in the presence of Zn 2+ cations which can be further explored for sensing applications. The electrochemical behavior of the new complex was studied in detail, revealing up to four one electron reduction processes in the range from 0 to -2.4 V vs. Fc + /Fc. With support of DFT calculations, the first three processes are ascribed to the reduction of the coordinated phet ligand followed by the Re I/0 reduction and consequent Cl - release. Furthermore, the new complex was able to act as an electrocatalyst for CO 2 reduction into CO ( E onset = -1.92 V vs. Fc + /Fc), with a turnover frequency of 2.81 s -1 and turnover number of 24 ± 1 in anhydrous acetonitrile, being the first Re( I ) tricarbonyl complex with a thiosemicarbazone derivative described for this goal. The detailed characterization carried out here can drive the development of new Re( I )-thiosemicarbazone derivatives for different applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Ion-specific clustering of metal-amphiphile complexes in rare earth separations

The nanoscale structure of a complex fluid can play a major role in the selective adsorption of ions at the nanometric interfaces, which is crucial in industrial and technological applications. Here we study the effect of anions and lanthanide ions on the nanoscale structure of a complex fluid formed by metal-amphiphile complexes, using small angle X-ray scattering. The nano- and mesoscale structures we observed can be directly connected to the preferential transfer of light (La and Nd) or heavy (Er and Lu) lanthanides into the complex fluid from an aqueous solution. While toluene-based complex fluids containing trioctylmethylammonium-nitrate (TOMA-nitrate) always show the same mesoscale hierarchical structure regardless of lanthanide loading and prefer light lanthanides, those containing TOMA-thiocyanate show an evolution of the mesoscale structure as a function of the lanthanide loading and prefer heavy lanthanides. The hierarchical structure indicates the presence of attractive interactions between ion-amphiphile aggregates, causing them to form clusters. A clustering model that accounts for the hard sphere repulsions and short-range attractions between the aggregates has been adapted to model the X-ray scattering results. The new model successfully describes the nanoscale structure and helps in understanding the mechanisms responsible for amphiphile assisted ion transport between immiscible liquids. Accordingly, our results imply different mechanisms of lanthanide transport depending on the anion present in the complex fluid and correspond with anion-dependent trends in rare earth separations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Communication: Photoactivation of nucleobase bound platinum{sup II} metal complexes: Probing the influence of the nucleobase

We present UV laser action spectra (220-300 nm) of isolated nucleobase-bound Pt{sup II}(CN){sub 4}{sup 2−} complexes, i.e., Pt(CN){sub 4}{sup 2−}⋅M, where M = uracil, thymine, cytosine, and adenine. These metal complex-nucleobase clusters represent model systems for identifying the fundamental photophysical and photochemical processes occurring in photodynamic platinum (II) drug therapies that target DNA. This is the first study to explore the specific role of the nucleobase in the photophysics of the aggregate complex. Each of the complexes studied displays a broadly similar absorption spectra, with a strong λ{sub max} ∼ 4.7 eV absorption band (nucleobase localized chromophore) and a subsequent increase in the absorption intensity towards higher spectral-energy (Pt(CN){sub 4}{sup 2−} localized chromophore). However, strikingly different band widths are observed across the series of complexes, decreasing in the order Pt(CN){sub 4}{sup 2−}⋅Thymine > Pt(CN){sub 4}{sup 2−}⋅Uracil > Pt(CN){sub 4}{sup 2−}⋅Adenine > Pt(CN){sub 4}{sup 2−}⋅Cytosine. Changes in the bandwidth of the ∼4.7 eV band are accompanied by distinctive changes in the photofragment product ions observed following photoexcitation, with the narrower-bandwidth complexes showing a greater propensity to decay via electron detachment decay. We discuss these observations in the context of the distinctive nucleobase-dependent excited state lifetimes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Structural insights into the assembly of the histone deacetylase-associated Sin3L/Rpd3L corepressor complex

Acetylation is correlated with chromatin decondensation and transcriptional activation, but its regulation by histone deacetylase (HDAC)-bearing corepressor complexes is poorly understood. Here, we describe the mechanism of assembly of the mammalian Sin3L/Rpd3L complex facilitated by Sds3, a conserved subunit deemed critical for proper assembly. Sds3 engages a globular, helical region of the HDAC interaction domain (HID) of the scaffolding protein Sin3A through a bipartite motif comprising a helix and an adjacent extended segment. Sds3 dimerizes through not only one of the predicted coiled-coil motifs but also, the segment preceding it, forming an ~150-Å-long antiparallel dimer. Contrary to previous findings in yeast, Sin3A rather than Sds3 functions in recruiting HDAC1 into the complex by engaging the latter through a highly conserved segment adjacent to the helical HID subdomain. In the resulting model for the ternary complex, the two copies of the HDACs are situated distally and dynamically because of a natively unstructured linker connecting the dimerization domain and the Sin3A interaction domain of Sds3; these features contrast with the static organization described previously for the NuRD (nucleosome remodeling and deacetylase) complex. Finally, the Sds3 linker features several conserved basic residues that could potentially maintain the complex on chromatin by nonspecific interactions with DNA after initial recruitment by sequence-specific DNA-binding repressors.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2013

Purification and SAXS Analysis of the Integrin Linked Kinase, PINCH, Parvin (IPP) Heterotrimeric Complex

The heterotrimeric protein complex containing the integrin linked kinase (ILK), parvin, and PINCH proteins, termed the IPP complex, is an essential component of focal adhesions, where it interacts with many proteins to mediate signaling from integrin adhesion receptors. Here we conduct a biochemical and structural analysis of the minimal IPP complex, comprising full-length human ILK, the LIM1 domain of PINCH1, and the CH2 domain of α-parvin. We provide a detailed purification protocol for IPP and show that the purified IPP complex is stable and monodisperse in solution. Using small-angle X-ray scattering (SAXS), we also conduct the first structural characterization of IPP, which reveals an elongated shape with dimensions 120×60×40 Å. Flexibility analysis using the ensemble optimization method (EOM) is consistent with an IPP complex structure with limited flexibility, raising the possibility that inter-domain interactions exist. However, our studies suggest that the inter-domain linker in ILK is accessible and we detect no inter-domain contacts by gel filtration analysis. This study provides a structural foundation to understand the conformational restraints that govern the IPP complex.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2023

Eu(III) and Cm(III) Complexation by the Aminocarboxylates NTA, EDTA, and EGTA Studied with NMR, TRLFS, and ITC—An Improved Approach to More Robust Thermodynamics

The complex formation of Eu(III) and Cm(III) was studied via tetradentate, hexadentate, and octadentate coordinating ligands of the aminopolycarboxylate family, viz., nitrilotriacetate (NTA 3- ), ethylenediaminetetraacetate (EDTA 4- ), and ethylene glycol-bis(2-aminoethyl ether)-N,N,N',N'-tetraacetate (EGTA 4- ), respectively. Based on the complexones’ pK a values obtained from 1 H nuclear magnetic resonance (NMR) spectroscopic pH titration, complex formation constants were determined by means of the parallel-factor-analysis-assisted evaluation of Eu(III) and Cm(III) time-resolved laser-induced fluorescence spectroscopy (TRLFS). This was complemented by isothermal titration calorimetry (ITC), providing the enthalpy and entropy of the complex formation. This allowed us to obtain genuine species along with their molecular structures and corresponding reliable thermodynamic data. The three investigated complexones formed 1:1 complexes with both Eu(III) and Cm(III). Besides the established Eu(III)–NTA 1:1 and 1:2 complexes, we observed, for the first time, the existence of a Eu(III)–NTA 2:2 complex of millimolar metal and ligand concentrations. Demonstrated for thermodynamic studies on Eu(III) and Cm(III) interaction with complexones, the utilized approach is commonly applicable to many other metal–ligand systems, even to high-affinity ligands.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Polyelectrolyte Complexation of Oligonucleotides by Charged Hydrophobic—Neutral Hydrophilic Block Copolymers

Polyelectrolyte complex micelles (PCMs, core-shell nanoparticles formed by complexation of a polyelectrolyte with a polyelectrolyte-hydrophilic neutral block copolymer) offer a solution to the critical problem of delivering therapeutic nucleic acids, Despite this, few systematic studies have been conducted on how parameters such as polycation charge density, hydrophobicity, and choice of charged group influence PCM properties, despite evidence that these strongly influence the complexation behavior of polyelectrolyte homopolymers. In this article, we report a comparison of oligonucleotide PCMs and polyelectrolyte complexes formed by poly(lysine) and poly((vinylbenzyl) trimethylammonium) (PVBTMA), a styrenic polycation with comparatively higher charge density, increased hydrophobicity, and a permanent positive charge. All of these differences have been individually suggested to provide increased complex stability, but we find that PVBTMA in fact complexes oligonucleotides more weakly than does poly(lysine), as measured by stability versus added salt. Using small angle X-ray scattering and electron microscopy, we find that PCMs formed from both cationic blocks exhibit very similar structure-property relationships, with PCM radius determined by the cationic block size and shape controlled by the hybridization state of the oligonucleotides. These observations narrow the design space for optimizing therapeutic PCMs and provide new insights into the rich polymer physics of polyelectrolyte self-assembly.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

The Straightening of a River Meander Leads to Extensive Losses in Flow Complexity and Ecosystem Services

To assist river restoration efforts we need to slow the rate of river degradation. This study provides a detailed explanation of the hydraulic complexity loss when a meandering river is straightened in order to motivate the protection of river channel curvature. We used computational fluid dynamics (CFD) modeling to document the difference in flow dynamics in nine simulations with channel curvature (C) degrading from a well-established tight meander bend (C = 0.77) to a straight channel without curvature (C = 0). To control for covariates and slow the rate of loss to hydraulic complexity, each of the nine-channel realizations had equivalent bedform topography. The analyzed hydraulic variables included the flow surface elevation, streamwise and transverse unit discharge, flow velocity at streamwise, transverse, and vertical directions, bed shear stress, stream function, and the vertical hyporheic flux rates at the channel bed. The loss of hydraulic complexity occurred gradually when initially straightening the channel from C = 0.77 to C = 0.33 (i.e., the radius of the channel is three-times the channel width), and additional straightening incurred rapid losses to hydraulic complexity. Other studies have shown hydraulic complexity provides important riverine habitat and is positively correlated with biodiversity. This study demonstrates how hydraulic complexity can be gradually and then rapidly lost when unwinding a river, and hopefully will serve as a cautionary tale.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Molecular mechanisms of assembly and TRIP13-mediated remodeling of the human Shieldin complex

The Shieldin complex, composed of REV7, SHLD1, SHLD2, and SHLD3, protects DNA double-strand breaks (DSBs) to promote nonhomologous end joining. The AAA + ATPase TRIP13 remodels Shieldin to regulate DNA repair pathway choice. Here we report crystal structures of human SHLD3–REV7 binary and fused SHLD2–SHLD3–REV7 ternary complexes, revealing that assembly of Shieldin requires fused SHLD2–SHLD3 induced conformational heterodimerization of open (O-REV7) and closed (C-REV7) forms of REV7. We also report the cryogenic electron microscopy (cryo-EM) structures of the ATPγS-bound fused SHLD2–SHLD3–REV7–TRIP13 complexes, uncovering the principles underlying the TRIP13-mediated disassembly mechanism of the Shieldin complex. We demonstrate that the N terminus of REV7 inserts into the central channel of TRIP13, setting the stage for pulling the unfolded N-terminal peptide of C-REV7 through the central TRIP13 hexameric channel. The primary interface involves contacts between the safety-belt segment of C-REV7 and a conserved and negatively charged loop of TRIP13. This process is mediated by ATP hydrolysis-triggered rotatory motions of the TRIP13 ATPase, thereby resulting in the disassembly of the Shieldin complex.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2018

Crystal Structure of a Nickel(II) Complex with the Macrocyclic Azadibenzo-14-crown-4 Ether Ligand Containing the Embedded Di(α-pyridyl)bispidone Moiety

The crystal structure of the nickel(II) complex with substituted azadibenzo-14-crown-4 ether containing the embedded di(α-pyridyl)bispidone moiety having the formula [L(H{sub 2}O)Ni(NO{sub 3}) Ni(H{sub 2}O)L](ClO{sub 4}){sub 3} · 0.7EtOH was studied by X-ray diffraction (T = 150(2) K, R = 0.0628). In the complex cation two five-coordinate nickel complexes are bridged by the nitrate anion to form a dimer. The distorted octahedral environment of Ni atoms is formed by four nitrogen atoms of the molecule L and the oxygen atoms of the water molecule and the NO{sub 3}{sup -} anion. In the nickel complex, like in the cobalt and copper complexes studied previously, the bispidine moiety of the molecule L adopts a chair-chair conformation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Tuning the Spin Transition and Carrier Type in Rare-Earth Cobaltates via Compositional Complexity

There is growing interest in material candidates with properties that can be engineered beyond traditional design limits. Compositionally complex oxides (CCO), often called high entropy oxides, are excellent candidates, wherein a lattice site shares more than four cations, forming single-phase solid solutions with unique properties. However, the nature of compositional complexity in dictating properties remains unclear, with characteristics that are difficult to calculate from first principles. Here, in this study, compositional complexity is demonstrated as a tunable parameter in a spin-transition oxide semiconductor La 1- x (Nd, Sm, Gd, Y) x/4 CoO 3 , by varying the population x of rare earth cations over 0.00≤ x≤ 0.80. Across the series, increasing complexity is revealed to systematically improve crystallinity, increase the amount of electron versus hole carriers, and tune the spin transition temperature and on-off ratio. At high a population (x = 0.8), Seebeck measurements indicate a crossover from hole-majority to electron-majority conduction without the introduction of conventional electron donors, and tunable complexity is proposed as new method to dope semiconductors. First principles calculations combined with angle resolved photoemission reveal an unconventional doping mechanism of lattice distortions leading to asymmetric hole localization over electrons. Thus, tunable complexity is demonstrated as a facile knob to improve crystallinity, tune electronic transitions, and to dope semiconductors beyond traditional means.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Self-assembly of {alpha}-helical polypeptides driven by complex coacervation.

Reported is the ability of alpha-helical polypeptides to self-assemble with oppositely-charged polypeptides to form liquid complexes while maintaining their alpha-helical secondary structure. Coupling the alpha-helical polypeptide to a neutral, hydrophilic polymer and subsequent complexation enables the formation of nanoscale coacervate-core micelles. While previous reports on polypeptide complexation demonstrated a critical dependence of the nature of the complex (liquid versus solid) on chirality, the alpha-helical structure of the positively charged polypeptide prevents the formation of beta-sheets, which would otherwise drive the assembly into a solid state, thereby, enabling coacervate formation between two chiral components. The higher charge density of the assembly, a result of the folding of the alpha-helical polypeptide, provides enhanced resistance to salts known to inhibit polypeptide complexation. The unique combination of properties of these materials can enhance the known potential of fluid polypeptide complexes for delivery of biologically relevant molecules.

chirality↗
DOE OSTI2023

Intramolecular 1,2 C-H Addition of o -Methyl Groups to Form Unique Ruthenium Pincer Tuck-in Complexes

New RPN H P ligands containing 2,4-xylyl (4mXPN H P) and mesityl (MesPN H P) groups on the phosphorus atoms were synthesized. 4mXPN H P reacts with [(cymene)RuCl 2 ] 2 followed by PMe 3 to produce κ 3 -4mXPN H PRu(PMe 3 )Cl 2 . MesPN H P reacts with [(cymene)RuCl 2 ] 2 to produce monomeric κ 3 -MesPN H PRuCl 2 that reacts with CO forming κ 3 -MesPN H PRu(CO)Cl 2 . Surprisingly, dehydrohalogenation of these complexes results in the activation of ortho methyl groups of the pincer ligands, rather than formation of κ 3 -RPNPRuLCl complexes. This results from transient κ 3 -RPNPRuLCl formation followed by 1,2-addition of an ortho C-H bond across the Ru-amide bond. The transient amide complex of κ 4 -4mXPN H PRu(PMe 3 )Cl was trapped with CO forming κ 3 -4mXPNPRu(PMe 3 )(CO)Cl. In contrast, κ 4 -MesPNHPRu(CO)Cl does not react with ligands to trap the expected amide complex of reverse C-H addition. Instead, CO and PMe 3 displace the chloride ligand forming cationic complexes. In both cases, hemi-lability of the pincer ligand was observed spectroscopically. In conclusion, the new complexes serve as precursors to moderately active catalysts for the acceptorless dehydrogenative coupling of n-butanol.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Molecular Tethering or Aggregation: Is the Existence of Charge-Transfer Bands Indicative of the Formation of Blue-Box/Tetrathiafulvalene Inclusion Complexes?

The interaction between tetrathiafulvalene and tetracation cyclobis(paraquat-p-phenylene) fragments—the key elements of many rotaxane systems—was investigated theoretically by using ab-initio second-order perturbation methods. In addition to the inclusion complex observed in the solid state, a thermodynamically stable “exterior” complex was identified. Calculation of the UV/Vis spectra for the inclusion and the exterior complexes indicated that the charge-transfer band that is often used to predict the formation of the inclusion complexes in solution is, in reality, due to the exterior mode of complexation. These results suggest that UV/Vis spectroscopy is not a reliable method for assigning the complexation modes in TTF:BB4+ rotaxanes and related systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Dramatic Influence of an Anionic Donor on the Oxygen-Atom Transfer Reactivity of a Mn V -Oxo Complex

Addition of an anionic donor to an Mn V (O) porphyrinoid complex causes a dramatic increase in 2-electron oxygen-atom-transfer (OAT) chemistry. The 6-coordinate [Mn V (O)(TBP 8 Cz)(CN)] – was generated from addition of Bu 4 N + CN – to the 5-coordinate Mn V (O) precursor. The cyanide-ligated complex was characterized for the first time by Mn K-edge X-ray absorption spectroscopy (XAS) and gives Mn—O=1.53 Å, Mn—CN=2.21 Å. In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN– complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e – -reduced Mn III (CN) – complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24 000-fold versus the same reaction for the parent 5-coordinate complex. An Eyring analysis gives ΔH ≠ = 14 kcal mol –1 , ΔS ≠ = –10 cal mol –1 K –1 . Computational studies fully support the structures, spin states, and relative reactivity of the 5- and 6-coordinate Mn V (O) complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Spectroscopic and Computational Investigation of Low-Spin MnIII Bis(scorpionate) Complexes

Six-coordinate Mn III complexes are typically high-spin (S = 2), however, the scorpionate ligand, both in its traditional, hydridotris(pyrazolyl)borate form, Tp - and Tp* - (the latter with 3,5dimethylpyrazole substituents) and in an aryltris(carbene)borate (i.e. , N -heterocyclic carbene, NHC) form, [Ph(MeIm) 3 B] - , (MeIm = 3-methylimidazole) lead to formation of bis(scorpionate) complexes of Mn III with spin triplet ground states; three of which were investigated herein: [Tp 2 Mn]SbF 6 (1SBF 6 ), [Tp* 2 Mn]SbF 6 (2SBF 6 ), and [{Ph(MeIm) 3 B} 2 Mn]CF 3 SO 3 (3CF 3 SO 3 ). These trigonally symmetric complexes were studied experimentally by magnetic circular dichroism (MCD) spectroscopy (the propensity of 3 to oxidize to Mn IV precluded collection of useful MCD data) including variable temperatures and fields (VTVH-MCD) and computationally by ab initio CASSCF/NEVPT2 methods. These combined experimental and theoretical techniques establish the 3 A 2g electronic ground state for the three complexes, and provide information on the energy of the “conventional” high-spin excited state ( 5 E g ) and other, triplet excited states. These results show the electronic effect of pyrazole ring substituents in comparing 1 and 2. The tunability of the scorpionate ligand, even by perhaps the simplest change (from pyrazole in 1 to 3,5-dimethylpyrazole in 2) is quantitatively manifested through perturbations in ligand-field excited-state energies that impact ground-state zero-field splittings. The comparison with the NHC donor is much more dramatic. In 3, the stronger σ-donor properties of the NHC lead to a quantitatively different electronic structure, so that the lowest lying spin triplet excited state, 3 E g , is much closer in energy to the ground state than in 1 or 2. The zero-field splitting (zfs) parameters of the three complexes were calculated and in the case of 1 and 2 compare closely to experiment (lower by < 10%, < 2 cm -1 in absolute terms); for 3 the large magnitude zfs is reproduced, although there is ambiguity about its sign. The comprehensive picture obtained for these bis(scorpionate) Mn III complexes thus provides quantitative insight into the role played by the scorpionate ligand in stabilizing unusual electronic structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗