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All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Iron Carbonyl Complexes of [2.2.2]Hericene as a Rigid Tris(1,3-diene) Ligand

Hericene is an unusual hexaolefin consisting of three 1,3-diene units located on a rigid bicyclo [2.2.2]octane framework that restricts the geometrical relationships of metal atoms bonded to these olefinic units. In order to explore possible effects of this rigidity limiting metal–metal interaction in polynuclear derivatives possibly stabilizing coordinatively unsaturated species, the structures and energetics of the hericene iron carbonyl complexes (hericene)Fem(CO)n (m = 1, n = 3; m = 2, n = 6, 5; m = 3, n = 9, 8) have been investigated by density functional theory. The lowest-energy (hericene)Fem(CO)3m (m = 1, 2, 3) structures have the cavities of the hericene ligand filled with a single Fe(CO)3 moiety bonded to a 1,3-diolefin unit. Such species have been synthesized by the reaction of Fe2(CO)9 with hericene. For the (hericene)Fe2(CO)5 system, the lowest energy structures are singlet structures with an Fe(CO)3 unit bonded to a 1,3-diene unit in one hericene cavity and an Fe(CO)2 unit in another hericene cavity bonded to three C=C double bonds from two 1,3-diene units. Higher energy (hericene)Fe2(CO)5 structures include a structure in which a single hericene cavity contains a Fe2(CO)4(µ-CO) moiety with each iron atom bonded to a 1,3-diene unit. In addition, both singlet and triplet (hericene)Fe2(CO)5 structures are found in which an Fe(CO)3 moiety and an Fe(CO)2 moiety located in separate hericene cavities are each bonded to a 1,3-diene unit. The lowest-energy (hericene)Fe3(CO)8 structures have two hericene cavities containing Fe(CO)3 moieties fully bonded to 1,3-diene units and a third hericene cavity containing an Fe(CO)2 moiety fully bonded to a 1,3-diene unit.

Luo, Jinfeng↗
DOE OSTI2021

Using molten salts to probe outer-coordination sphere effects on lanthanide( III )/( II ) electron-transfer reactions

Controlling structure and reactivity by manipulating the outer-coordination sphere around a given reagent represents a longstanding challenge in chemistry. Despite advances toward solving this problem, it remains difficult to experimentally interrogate and characterize outer-coordination sphere impact. Here, this work describes an alternative approach that quantifies outer-coordination sphere effects. It shows how molten salt metal chlorides (MCl n ; M = K, Na, n = 1; M = Ca, n = 2) provided excellent platforms for experimentally characterizing the influence of the outer-coordination sphere cations (M n+ ) on redox reactions accessible to lanthanide ions; Ln 3+ + e 1– → Ln 2+ (Ln = Eu, Yb, Sm; e 1– = electron). As a representative example, X-ray absorption spectroscopy and cyclic voltammetry results showed that Eu 2+ instantaneously formed when Eu 3+ dissolved in molten chloride salts that had strongly polarizing cations (like Ca 2+ from CaCl 2 ) via the Eu 3+ + Cl 1– → Eu 2+ + ½Cl 2 reaction. Conversely, molten salts with less polarizing outer-sphere M 1+ cations (e.g., K 1+ in KCl) stabilized Ln 3+ . For instance, the Eu 3+ /Eu 2+ reduction potential was >0.5 V more positive in CaCl 2 than in KCl. In accordance with first-principle molecular dynamics (FPMD) simulations, we postulated that hard M n+ cations (high polarization power) inductively removed electron density from Ln n+ across Ln–Cl···M n+ networks and stabilized electron-rich and low oxidation state Ln 2+ ions. Conversely, less polarizing M n+ cations (like K 1+ ) left electron density on Lnn+ and stabilized electron-deficient and high-oxidation state Ln 3+ ions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Search for a right-handed W boson and a heavy neutrino in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A search is presented for a right-handed W boson (W R ) and a heavy neutrino (N), in a final state consisting of two same-flavor leptons (ee or μμ) and two quarks. The search is performed with the CMS experiment at the CERN LHC using a data sample of proton-proton collisions at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 138 fb -1 . The search covers two regions of phase space, one where the decay products of the heavy neutrino are merged into a single large-area jet, and one where the decay products are well separated. The expected signal is characterized by an excess in the invariant mass distribution of the final-state objects. No significant excess over the standard model background expectations is observed. The observations are interpreted as upper limits on the product of W R production cross sections and branching fractions assuming that couplings are identical to those of the standard model W boson. For N masses m N equal to half the W R mass m W R (m N = 0.2 TeV), m W R is excluded at 95% confidence level up to 4.7 (4.8) and 5.0 (5.4) TeV for the electron and muon channels, respectively. This analysis provides the most stringent limits on the W R mass to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

2D Homologous Series SrFM n BiS n +2 (M = Pb, Ag 0.5 Bi 0.5 ; n = 0, 1) and Commensurately Modulated Sr 2 F 2 Bi 2/3 S 2

In this work, we report three new mixed-anion two-dimensional (2D) compounds: SrFPbBiS 3 , SrFAg 0.5 Bi 1.5 S 3 , and Sr 2 F 2 Bi 2/3 S 2 . Their structures as well as the parent compound SrFBiS 2 were refined using single-crystal X-ray diffraction data, with the sequence of SrFBiS 2 , SrFPbBiS 3 , and SrFAg 0.5 Bi 1.5 S 3 defining the new homologous series SrFM n BiS n+2 (M = Pb, Ag 0.5 Bi 0.5 ; n= 0, 1). Sr 2 F 2 Bi 2/3 S 2 has a different structure, which is modulated with a q vector of 1/3b* and was refined in superspace group X2/m(0 β 0)00 as well as in the 1x3x1 superstructure with space group C2/m (with similar results). Sr 2 F 2 Bi 2/3 S 2 features hexagonal layers of alternating [Sr 2 F 2 ] 2+ and [Bi 2/3 S 2 ] 2- , and the modulated structure arises from the unique ordering pattern of Sr 2+ cations. SrFPbBiS 3 , SrFAg 0.5 Bi 1.5 S 3 , and Sr 2 F 2 Bi 2/3 S 2 are semiconductors with band gaps of 1.31, 1.21, and 1.85 eV, respectively. The latter compound exhibits room temperature red photoluminescence at ~ 700 nm.

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

Measurement-based quantum simulation of Abelian lattice gauge theories

Numerical simulation of lattice gauge theories is an indispensable tool in high energy physics, and their quantum simulation is expected to become a major application of quantum computers in the future. In this work, for an Abelian lattice gauge theory in d d spacetime dimensions, we define an entangled resource state (generalized cluster state) that reflects the spacetime structure of the gauge theory. We show that sequential single-qubit measurements with the bases adapted according to the former measurement outcomes induce a deterministic Hamiltonian quantum simulation of the gauge theory on the boundary. Our construction includes the (2+1) ( 2 + 1 ) -dimensional Abelian lattice gauge theory simulated on three-dimensional cluster state as an example, and generalizes to the simulation of Wegner’s lattice models M_{(d,n)} M ( d , n ) that involve higher-form Abelian gauge fields. We demonstrate that the generalized cluster state has a symmetry-protected topological order with respect to generalized global symmetries that are related to the symmetries of the simulated gauge theories on the boundary.Our procedure can be generalized to the simulation of Kitaev’s Majorana chain on a fermionic resource state. We also study the imaginary-time quantum simulation with two-qubit measurements and post-selections, and a classical-quantum correspondence, where the statistical partition function of the model M_{(d,n)} M ( d , n ) is written as the overlap between the product of two-qubit measurement bases and the wave function of the generalized cluster state.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Bootstrapping Mixed MN Correlators in 3D

The recent emergence of the modern conformal bootstrap method for the study of conformal field theories (CFTs) has enabled the revisiting of old problems in classical critical phenomena described by three-dimensional CFTs. The study of such CFTs with O(m)^n \rtimes S_n O ( m ) n ⋊ S n global symmetry, also known as models, is pursued in this work. Systems of mixed correlators involving scalar operators in two different representations of the global symmetry group are considered. Isolated allowed regions are found in parameter space for various values of m m and n n . These ``islands’’ can be separated into two qualitative groups: those close to the unitarity bound and those further away. As a by-product of our analysis generic tensor structures required to bootstrap any G^n \rtimes S_n G n ⋊ S n theory with G G arbitrary are worked out.

Kousvos, Stefanos Robert↗
DOE OSTI2024

Constraining nucleon effective masses with flow and stopping observables from the S π RIT experiment

Properties of the nuclear equation of state (EoS) can be probed by measuring the dynamical properties of nucleus-nucleus collisions. In this study, we present the directed flow (υ 1 ), elliptic flow (υ 2 ) and stopping (VarXZ) measured in fixed target Sn + Sn collisions at 270 AMeV with the SπRIT Time Projection Chamber. We perform Bayesian analyses in which EoS parameters are varied simultaneously within the Improved Quantum Molecular Dynamics-Skyrme (ImQMD-Sky) transport code to obtain a multivariate correlated constraint. The varied parameters include symmetry energy, S 0 , and slope of the symmetry energy, L, at saturation density, isoscalar effective mass, $m^{⁎}_{S}$/$m_{N}$ , isovector effective mass, $m^{⁎}_{υ}$/$m_{N}$ and the in-medium cross-section enhancement factor η. We find that the flow and VarXZ observables are sensitive to the splitting of proton and neutron effective masses and the in-medium cross-section. Comparisons of ImQMD-Sky predictions to the SπRIT data suggest a narrow range of preferred values for $m^{⁎}_{S}$/$m_{N}$, $m^{⁎}_{υ}$/$m_{N}$ and η.

Astronomy & Astrophysics↗
DOE OSTI2020

Chemical Vapor Deposition for Atomically Dispersed and Nitrogen Coordinated Single Metal Site Catalysts

Abstract Atomically dispersed and nitrogen coordinated single metal sites (M‐N‐C, M=Fe, Co, Ni, Mn) are the popular platinum group‐metal (PGM)‐free catalysts for many electrochemical reactions. Traditional wet‐chemistry catalyst synthesis often requires complex procedures with unsatisfied reproducibility and scalability. Here, we report a facile chemical vapor deposition (CVD) strategy to synthesize the promising M‐N‐C catalysts. The deposition of gaseous 2‐methylimidazole onto M‐doped ZnO substrates, followed by an in situ thermal activation, effectively generated single metal sites well dispersed into porous carbon. In particular, an optimal CVD‐derived Fe‐N‐C catalyst exclusively contains atomically dispersed FeN 4 sites with increased Fe loading relative to other catalysts from wet‐chemistry synthesis. The catalyst exhibited outstanding oxygen‐reduction activity in acidic electrolytes, which was further studied in proton‐exchange membrane fuel cells with encouraging performance.

Liu, Shengwen↗
DOE OSTI2023

Enumeration of Moiré patterns of a hexagonal twisted bilayer: Applications to intercalated transition metals

A real-space method using generating integers is used to determine possible commensurate lattice Moiré patterns for a bilayer of two equal hexagonal lattices, which can in principle be extended to lattice mismatched bilayers. These Moiré patterns can be classified by a pair of relatively prime integers (n,m), wherein a rotation θ(n,m) of the top hexagonal lattice maps its lattice vector (n,m) to (m,n) of the bottom lattice. Within this formulation, the area of the commensurate supercell is proportional to (n2+m2+nm) and the number of coincident lattice sites per supercell is given by (n−m)2. Taking bilayer boron nitride (BN) as an example, we present how to systematically generate Moiré patterns and explore the differences in local chemistry in the interstitial region by impurity intercalation. Systematic calculations of the properties of intercalated 3d transition metals were performed in an h-BN (4,3) bilayer, corresponding to a rotation of 9.43 degrees. These calculations reveal that local symmetry in the intercalated region significantly affect the energetics and magnetization of the intercalated species. These results highlight that Moiré pattern physics is not limited to optoelectronic/electronic phenomena, such as interfacial exciton formation or magic angle superconductivity, but it also produces chemical and magnetic atomic site selectivity, which may play important roles in adsorption, catalysis, or quantum information.

Physics↗
DOE OSTI2025

Ferroelectric epitaxial Al(Sc/B)N/Mo/SiC heterostructures for high operating temperature devices

Advances in wurtzite nitride ferroelectrics of Al 1-x M x N (⁠ M = Sc or B) have led to novel capabilities, which must be integrated into existing fabrication processes. In the case of electronics operating > 200°C, a movement toward SiC-based platforms enables better performance over conventional Si, primarily due to the higher bandgap and lower intrinsic carrier concentration of SiC. Hence, the challenge is to develop a deposition process to integrate Al 1-x M x M x N ferroelectrics with elevated temperature-compatible materials for high temperature electronics such as the non-volatile memory component. We demonstrate epitaxial Al M N/Mo/SiC heterostructures, which provides both the crystalline and surface features that promote high-quality ferroelectric nitride film growth. Omega scans of the Mo (110) reflection exhibit a full width at half max of < 0.02° (40 arc sec) and the (0002) peak of the subsequently grown nitride film had a value of 1.1° for 160 nm thick Al 0.7 Sc 0.3 N and 1.3° for 400 nm thick Al 0.94 B 0.06 N. The crystallographic relationships found between the Al M N, Mo, and SiC layers indicate an advancement in sputter deposition of epitaxial films. Ferroelectric switching is also shown at 400 °C in both samples via polarization-electric field hysteresis and pulsed measurements, which exhibited P r values >100 μC cm -2 and E c between 3 and 4 M V cm -1 ⁠, despite a large presence of oxygen in both Al M N films ranging between 4 and 5 at.%, revealed by compositional analysis. This study demonstrates the process for synthesizing high-crystal quality ferroelectric nitride films, which can be used in extremely high temperature applications.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Modeling dark- and light-induced crystal structures and single-crystal optical absorption spectra of ruthenium-based complexes that undergo SO 2 -linkage photoisomerization

A family of coordination complexes of the type [Ru(SO 2 )(NH 3 ) 4 X] m+ Y n – (m, n = 1 or 2) exhibit optical switching capabilities in their single-crystal states. This striking effect is caused by the light-induced formation of SO 2 -linkage photoisomers, which are metastable if kept at suitably cool temperatures. We modeled the dark- and light-induced states of these large crystalline complexes via plane-wave (PW)- and molecular-orbital (MO)-based density functional theory (DFT) and time-dependent DFT in order to calculate their structural and optical properties; the calculated results are compared with experimental data. We show that the PW-DFT-based periodic models replicate the structural properties of these complexes more effectively than the MO-DFT-based molecular-fragment models, observing only small deviations in key bond lengths relative to the experimentally derived crystal structures. The periodic models were also found to more effectively simulate trends seen in experimental optical absorption spectra, with optical absorbance and coverage of the visible region increasing with the formation of the photoinduced geometries. The contribution of the metastable photoisomeric species more heavily focuses on the lower-energy end of the spectra. Spectra generated from the molecular-fragment models are limited by the geometry of the fragment used and the number of excited-state roots considered in those calculations. In general, periodic models outperform the molecular-fragment models owing to their ability to better appreciate the periodic phenomena that are present in these crystalline materials as opposed to MO approaches, which are finite methods. We thus demonstrate that PW-DFT-based periodic models should be considered as a more than viable method for simulating the optical and electronic properties of these single-crystal optical switches.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

PGM-Free Oxygen-Reduction Catalyst Development for Proton-Exchange Membrane Fuel Cells: Challenges, Solutions, and Promises

Proton-exchange membrane fuel cells (PEMFCs) are efficient and clean hydrogen energy technologies for transportation and stationary applications. Highly active and durable low-cost cathode catalysts for the oxygen-reduction reaction (ORR) under challenging acidic environments are desperately needed to address the cost and durability issues of PEMFCs. The most promising platinum group metal (PGM)-free catalysts for the ORR in acidic media are atomically dispersed and nitrogen-coordinated metal site catalysts denoted as M–N–C, M = Fe, Co, or Mn. Due to significant efforts in the past few decades, these catalysts have demonstrated much-improved ORR activity and promising initial fuel cell performance approaching traditional Pt/C catalysts. However, the insufficient long-term stability (up to 5000 h) under PEMFC operation represents a primary technical barrier to making current PGM-free catalysts less viable yet in PEMFCs. In this Account, we highlight recent advances in synthesizing efficient PGM-free catalysts for the ORR in PEMFCs, emphasizing effective strategies to improve mass and intrinsic activity and the possible degradation mechanisms. In particular, a chemical doping method based on the zeolitic imidazolate framework (ZIF)-8 represents the key to developing efficient M–N–C catalysts containing atomically dispersed and nitrogen-coordinated single metal active sites (i.e., MN 4 ). The newly acquired understanding of the formation mechanism of MN 4 active sites during the thermal activation and its correlation to catalytic properties guide the rational catalyst design rather than relying on current trial-and-error approaches. Considerable efforts have further been invested in increasing the active site density and enhancing intrinsic activity by regulating carbon-phase structures and the local coordination environment. Furthermore, these highly active catalysts usually suffer from significant activity loss during the ORR. Therefore, breaking the activity–stability trade-off is the key to simultaneously achieving activity and stability in one catalyst, which is discussed on the basis of our recent successes in regulating local carbon structures surrounding active single metal sites. Significant research efforts toward understanding the degradation mechanisms and improving the lifetime of PGM-free catalysts are still crucial for viable applications in the future. Novel electrode designing strategies are needed to translate the PGM-free catalysts’ ORR activity to solid-state electrolyte-based membrane electrode assemblies (MEAs) with robust three-phase (i.e., gas–liquid–solid) interfaces for efficient charge and mass transports for performance improvement. On the basis of our effort at the University at Buffalo supported by ElectroCat Consortium associated with U.S. DOE’s Hydrogen and Fuel Cell Technologies Office, we provide a perspective on PGM-free cathode catalysts concerning remaining bottlenecks and future opportunities, aiming to inspire the community in both mechanistic understanding and technological development.

08 HYDROGEN↗
DOE OSTI2020

Base pairing, structural and functional insights into N 4 -methylcytidine (m 4 C) and N 4 ,N 4 -dimethylcytidine (m 4 2 C) modified RNA

The N 4 -methylation of cytidine (m 4 C and m 4 2 C) in RNA plays important roles in both bacterial and eukaryotic cells. In this work, we synthesized a series of m 4 C and m 4 2 C modified RNA oligonucleotides, conducted their base pairing and bioactivity studies, and solved three new crystal structures of the RNA duplexes containing these two modifications. Our thermostability and X-ray crystallography studies, together with the molecular dynamic simulation studies, demonstrated that m 4 C retains a regular C:G base pairing pattern in RNA duplex and has a relatively small effect on its base pairing stability and specificity. By contrast, the m 4 2 C modification disrupts the C:G pair and significantly decreases the duplex stability through a conformational shift of native Watson-Crick pair to a wobble-like pattern with the formation of two hydrogen bonds. This double-methylated m 4 2 C also results in the loss of base pairing discrimination between C:G and other mismatched pairs like C:A, C:T and C:C. The biochemical investigation of these two modified residues in the reverse transcription model shows that both mono- or di-methylated cytosine bases could specify the C:T pair and induce the G to T mutation using HIV-1 RT. In the presence of other reverse transcriptases with higher fidelity like AMV-RT, the methylation could either retain the normal nucleotide incorporation or completely inhibit the DNA synthesis. These results indicate the methylation at N 4 -position of cytidine is a molecular mechanism to fine tune base pairing specificity and affect the coding efficiency and fidelity during gene replication.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Density functional studies of defects and defect-related luminescence in M g 3 N 2

M g 3 N 2 is a promising material for light-emitting-diode applications. However, the microscopic origin of the broad yellow photoluminescence in M g 3 N 2 remains unknown. In this study, defect properties and defect-related optical transitions are investigated based on the hybrid functional calculation. Our results show that the nitrogen vacancy introduces multiple localized defect states within the band gap, which play a dominant role in luminescent properties of M g 3 N 2 . Common impurities like hydrogen, oxygen, and carbon and their complexes with native defects are also studied. Compared with isolated V Mg , V Mg -H and V Mg – O N complexes have shallower transition levels. Our calculated optical excitation and emission peaks associated with isolated V N are in good agreement with those observed in experiments. In addition, the impurity C N is also a potential source for the observed yellow emission in M g 3 N 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2023

Search for Z' bosons decaying to pairs of heavy Majorana neutrinos in proton-proton collisions at $ \sqrt{s} $ = 13 TeV

A search for the production of pairs of heavy Majorana neutrinos (N ℓ ) from the decays of Z' bosons is performed using the CMS detector at the LHC. The data were collected in proton-proton collisions at a center-of-mass energy of $\sqrt s$ = 13 TeV, with an integrated luminosity of 138 fb –1 . The signature for the search is an excess in the invariant mass distribution of the final-state objects, two same-flavor leptons (e or μ) and at least two jets. No significant excess of events beyond the expected background is observed. Upper limits at 95% confidence level are set on the product of the Z' production cross section and its branching fraction to a pair of N ℓ , as functions of N ℓ and Z' boson masses (m N ℓ and m Z' , respectively) for m Z' from 0.4 to 4.6 TeV and m N ℓ from 0.1 TeV to m Z' /2. In the theoretical framework of a left-right symmetric model, exclusion bounds in the m N ℓ -m Z' plane are presented in both the electron and muon channels. The observed upper limit on m Z' reaches up to 4.42 TeV. These are the most restrictive limits to date on the mass of N ℓ as a function of the Z' boson mass.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Vibrational signatures of HNO 3 acidity when complexed with microhydrated alkali metal ions, M + ∙(HNO 3 )(H 2 O) n =5 (M = Li, K, Na, Rb, Cs), at 20 K

The speciation of strong acids like HNO 3 under conditions of restricted hydration is an important factor in the rates of chemical reactions at the air-water interface. Here we explore the trade-offs in play when HNO 3 is attached to alkali ions (Li + -Cs + ) with four water molecules in their primary hydration shells. This is achieved by analyzing the vibrational spectra of the M + ∙(HNO 3 )(H 2 O) 5 clusters cooled to about 20 K in a cryogenic photofragmentation mass spectrometer. The local acidity of the acidic OH group is estimated by the extent of the red shift in its stretching frequency when attached to a single water molecule. Here, the persistence of this local structural motif (HNO 3 -H 2 O) in all of these alkali metal clusters enables us to determine the competition between the effect of the direct complexation of the acid with the cation, which acts to enhance acidity, and the role of the water network in the first hydration shell around the ions, which acts to counter (screen) the intrinsic effect of the ion. Analysis of the vibrational features associated with the acid molecule as well as those of the water network reveal how cooperative interactions in the micro hydration regime conspire to effectively offset the intrinsic enhancement of HNO 3 acidity afforded by attachment to the smaller cations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Effect of Redox-Inactive Metal Ion–Nickel(III) Interactions on the Redox Properties and Proton-Coupled Electron Transfer Reactivity

We report Mononuclear nickel(II) and nickel(III) complexes of a bisamidate-bisalkoxide ligand, (NMe 4 ) 2 [Ni- II (HMPAB)] (1) and (NMe 4 )[Ni III (HMPAB)] (2), respectively, have been synthesized and characterized by various spectroscopic techniques including X-ray crystallography. The reaction of redox-inactive metal ions (M n+ = Ca 2+ , Mg 2+ , Zn 2+ , Y 3+ , and Sc 3+ ) with 2 resulted in 2-M n+ adducts, which was assessed by an array of spectroscopic techniques including X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and reactivity studies. The X-ray structure of Ca 2+ coordinated to Ni(III) complexes, 2Ca 2+ T, was determined and exhibited an average Ni-Ca distance of 3.1253 angstrom, close to the metal ions' covalent radius. XAS analysis of 2-Ca 2+ and 2-Y 3+ in solution further revealed an additional coordination to Ca and Y in the 2-M n+ adducts with shortened Ni-M distances of 2.15 and 2.11 Å, respectively, implying direct bonding interactions between Ni and Lewis acids (LAs). Such a short interatomic distance between Ni(III) and M is unprecedented and was not observed before. EPR analysis of 2 and 2-M n+ species, moreover, displayed rhombic signals with g av > 2.12 for all complexes, supporting the +III oxidation state of Ni. The Ni III /Ni II redox potential of 2 and 2-M n+ species was determined, and a plot of E 1/ 2 of 2-M n+ versus pK a of [M(H 2 O) n ] m+ exhibited a linear relationship, implying that the Ni III /Ni II potential of 2 can be tuned with different redox-inactive metal ions. Reactivity studies of 2 and 2-M n+ with different 4-X-2,6-ditert-butylphenol (4-XDTBP) and other phenol derivatives were performed, and based on kinetic studies, we propose the involvement of a proton-coupled electron transfer (PCET) pathway. Analysis of the reaction products after the reaction of 2 with 4-OMe-DTBP showed the formation of a Ni(II) complex (1a) where one of the alkoxide arms of the ligand is protonated. A pK a value of 24.2 was estimated for 1a. The reaction of 2-M n+ species was examined with 4-OMe-DTBP, and it was observed that the k 2 values of 2-M n+ species increase by increasing the Lewis acidity of redox-inactive metal ions. However, the obtained k 2 values for 2-M n+ species are much lower compared to the k 2 value for 2. Such a variation of PCET reactivity between 2 and 2-M n+ species may be attributed to the interactions between Ni(III) and LAs. Our findings show the significance of the secondary coordination sphere effect on the PCET reactivity of Ni(III) complexes and furnish important insights into the reaction mechanism involving high-valent nickel species, which are frequently invoked as key intermediates in Ni-mediated enzymatic reactions, solar-fuel catalysis, and biomimetic/synthetic transformation reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗