Ligand Field Research Articles

Spontaneous FeIII/FeII redox cycling in single-atom catalysts: Conjugation effect and electron delocalization

The mechanism of spontaneous FeIII/FeII redox cycling in iron-centered single-atom catalysts (I-SACs) is often overlooked. Consequently, pathways for continuous SO4 ·-/HO⋅ generation during peroxymonosulfate (PMS) activation by I-SACs remain unclear. Herein, the evolution of the iron center and ligand in I-SACs was comprehensively investigated. I-SACs could be considered as a coordination complex created by iron and a heteroatom N-doped carbonaceous ligand. The ligand-field theory could well explain the electronic behavior of the complex, whereby electrons delocalized by the conjugation effect of the ligand were confirmed to be responsible for the FeIII/FeII redox cycle. The possible pyridinic ligand in I-SACs was demonstrably weaker than the pyrrolic ligand in FeIII reduction due to its shielding effect on delocalized π orbitals by local lone-pair electrons. The results of this study significantly advance our understanding of the mechanism of spontaneous FeIII/FeII redox cycling and radical generation pathways in the I-SACs/PMS process.

Crystallographic and/or magnetic properties of neutral and cationic uranium(IV) sandwiched phthalocyanine complexes

Neutral and cationic U(IV) sandwiched phthalocyanine (Pc) complexes were prepared. The neutral species, UPc2, was obtained by the reaction of UCl4 and phthalonitrile, and the [UPc2][BF4] crystals were grown by electrolysis. The structures of the complexes were determined crystallographically. A U(IV) ion has two 5f electrons, which carry a magnetic moment localized at the U sites, making the complex magnetically active. The magnetic susceptibility measurement of UPc2 revealed that the system was paramagnetic with local magnetic moments down to 2 K, but these magnetic moments were suppressed, possibly by a ligand field effect. The paramagnetism was also confirmed by the magnetization curve without hysteresis. The cationic complex, [UPc2][BF4] crystalized in a tetragonal structure with the space group P4nc, in which the UPc2 molecules stacked along the tetragonal c-axis. The structure was similar to that of [LnPc2][BF4], but with distinct disorder in the stacking plane. The stabilities of the crystal and molecular structures and the electronic configurations of UPc2 and [UPc2][BF4] were evaluated via the ab initio calculations that included the multiconfigurational nature of the actinide element.

Ferric Pyrophosphate Forms Soluble Iron Coordination Complexes with Zinc Compounds and Solubilizing Agents in Extruded Rice and Predicts Increased Iron Solubility and Bioavailability in Young Women

BackgroundCo-extrusion of ferric pyrophosphate (FePP) with solubilizers, citric acid/trisodium citrate (CA/TSC), or ethylenediaminetetraacetic acid (EDTA) sharply increases iron absorption. Whether this can protect against the inhibition of iron absorption by phytic acid (PA) is unclear. Sodium pyrophosphate (NaPP) may be a new enhancer of iron absorption from FePP. ObjectivesOur objectives were to 1) investigate the ligand coordination of iron, zinc, and solubilizers in extruded rice and test associations with iron solubility and absorption, 2) assess whether co-extrusion of FePP + CA/TSC rice can protect against inhibition of iron absorption by PA; 3) determine the effect of zinc oxide (ZnO) compared with zinc sulfate (ZnSO4), and 4) quantify iron absorption from FePP + NaPP rice. MethodsWe produced labeled 57FePP rice cofortified with ZnSO4 and EDTA, CA/TSC or NaPP, and FePP + EDTA rice with ZnO. We used electron paramagnetic resonance (EPR) to characterize iron-ligand complexes. We measured in vitro iron solubility and fractional iron absorption (FIA) in young women (n = 21, age: 22 ± 2 y, BMI: 21.3 ± 1.5 kg/m2 geometric mean plasma ferritin, 28.5 μg/L) compared with ferrous sulfate (58FeSO4). FIA was compared by linear mixed-effect model analysis. ResultsThe addition of zinc and solubilizers created new iron coordination complexes of Fe(III) species with a weak ligand field at a high-spin state that correlated with solubility (r2 = 0.50, P = 0.02) and absorption (r2 = 0.72, P = 0.02). Phytic acid reduced FIA from FePP + CA/TSC rice by 50% (P < 0.001), to the same extent as FeSO4. FIA from FePP + EDTA + ZnO and FePP + EDTA + ZnSO4 rice did not significantly differ. Mean FIAs from FePP + EDTA + ZnSO4, FePP + CA/TSC + ZnSO4, and FePP + NaPP + ZnSO4 rice were 9% to 11% and did not significantly differ from each other or from FeSO4. ConclusionRice extrusion of FePP with solubilizers resulted in bioavailable iron coordination complexes. In the case of FePP + CA/TSC, PA exerted similar inhibition of FIA as with FeSO4. FePP + NaPP could be a further viable solubilizing agent for rice fortification.This study was registered at clinicaltrials.gov as NCT03703739.

Breaking the Electronic Conductivity Bottleneck of Manganese Oxide Family for High-Power Fluorinated Graphite Composite Cathode by Ligand-Field High-Dimensional Constraining Strategy.

Primary lithium fluorinated graphite (Li/CFx ) batteries with superior energy density are an indispensable energy supply for multiple fields but suffer from sluggish reaction kinetics of the CFx cathode. Designing composite cathodes emerges as a solution to this problem. Despite the optimal composite component for CFx , the manganese oxide family represented by MnO2 is still faced with an intrinsic electronic conductivity bottleneck, which severely limits the power density of the composite cathode. Here, a cation-induced high-dimensional constraining strategy from the perspective of ligand-field stacking structure topological design, which breaks the molecular orbital hybridization of pristine semiconductive oxides to transform them into the high-conductivity metallic state while competitively maintaining structural stability, is proposed. Through first-principles phase diagram calculations, mixed-valent Mn5 O8 ( ) is explored as an ideal high-dimensional constraining material with satisfied conductivity and large-scale production feasibility. Experiments demonstrate that the as-proposed CFx @ Mn5 O8 composite cathode achieves 2.36times the power density (11399 Wkg-1 ) of pristine CFx and a higher CFx conversion ratio (86%). Such a high-dimensional field-constraining strategy is rooted in the established four-quadrant electronic structure tuning framework, which fundamentally changes the orbital symmetry under the ligand field to overcome the common conductivity challenge of wide transition metal oxide materials.

Towards Luminescent Vanadium(II) Complexes with Slow Magnetic Relaxation and Quantum Coherence.

Molecular entities with doublet or triplet ground states find increasing interest as potential molecular quantum bits (qubits). Complexes with higher multiplicity might even function as qudits and serve to encode further quantum bits. Vanadium(II) ions in octahedral ligand fields with quartet ground states and small zero-field splittings qualify as qubits with optical read out thanks to potentially luminescent spin-flip states. We identified two V2+ complexes [V(ddpd)2 ]2+ with the strong field ligand N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine (ddpd) in two isomeric forms (cis-fac and mer) as suitable candidates. The energy gaps between the two lowest Kramers doublets amount to 0.2 and 0.5 cm-1 allowing pulsed EPR experiments at conventional Q-band frequencies (35 GHz). Both isomers possess spin-lattice relaxation times T1 of around 300 μs and a phase memory time TM of around 1 μs at 5 K. Furthermore, the mer isomer displays slow magnetic relaxation in an applied field of 400 mT. While the vanadium(III) complexes [V(ddpd)2 ]3+ are emissive in the near-IR-II region, the [V(ddpd)2 ]2+ complexes are non-luminescent due to metal-to-ligand charge transfer admixture to the spin-flip states.

Influence of Number of Ligands and Point Group on the Electronic Structure of Co2+ Aqua-Complexes

The nucleation process of zeolitic imidazolate frameworks (ZIFs) is, to date, not yet completely understood, making the search for tailored materials very difficult. Recently, it has been shown that, during the formation process, the symmetry of the precursors is reduced by ligand elimination and substitution reactions. The octahedral precursors with simple ligands, such as water, methanol, and/or NO3-, are transformed to five- and finally four-coordinated complexes with imidazole ligands. This reduction of symmetry, caused both by the changing coordination environment and distortions from the perfect symmetry leading to another point group, will have a large influence on the electronic structure and more specifically on the d-orbital splitting. This, in turn, will affect the d-d electronic excitations, which can be followed using UV-vis spectroscopy and which can help to unravel the formation process. In this work, we systematically investigate how the lowering of the number of ligands affects the symmetry and thus the geometry and electronic structure of Co2+ complexes with six, five, and four aqua ligands. Therefore, we first resort to qualitative techniques, such as crystal field theory (CFT) and ligand field theory (LFT), which reveal that the orbital splitting is characteristic for the number of ligands. However, as these techniques are not capable of providing quantitative results without the use of experimental data as input, we perform various computational calculations. Both average of configuration (AOC) and unrestricted density functional theory (UDFT) are thoroughly investigated, and we will determine which technique is the best suited to properly describe the ground state of these systems. To investigate the dependency on the d-orbital occupation, we also investigated V2+, Mn2+, and Ni2+ hexa-aqua-complexes and compared them to the Co systems.

Triplet Spin-State Capped Deltahedral Structures Rather than Singlet Spin-State Oblatocloso Structures as Energetically Favored Dimanganaborane Structures

Density functional studies show that the singlet spin-state flattened oblatocloso deltahedral structures found experimentally in the dimetallaboranes Cp*2Re2Bn-2Hn-2 (Cp* = Me5C5; n = 8-12) of the third row group 7 element rhenium are not favored for analogous dimetallaboranes Cp2Mn2Bn-2Hn-2 (n = 8-14) of its first row congener manganese. Instead, the energetically preferred structures for the dimanganaboranes are higher spin-state triplet and quintet spin-state structures. This appears to be related to the lower ligand field splittings in complexes of the first row transition-metal manganese relative to analogous complexes of the third row transition-metal rhenium. The lowest-energy Cp2Mn2Bn-2Hn-2 (n = 8-13) structures typically have a central MnBn-2 closo deltahedron with one face capped by the second CpMn unit. However, for the 14-vertex Cp2Mn2B12H12 system the lowest-energy structures consist of B12 icosahedra with faces capped by both CpMn units. The thermochemistry of cluster buildup reactions of the type Cp2Mn2Bn-2Hn-2 + BH → Cp2Mn2Bn-1Hn-1 suggests that the 11- and 13-vertex structures are likely to be favored products in reactions of cyclopentadienylmanganese derivatives with borane sources. The paramagnetism of the predicted triplet and quintet spin states for the lowest-energy dimanganaboranes Cp2Mn2Bn-2Hn-2 (n = 8-14) suggests possible applications in novel magnetic materials.

Tuning Fe-spin state of FeN4 structure by axial bonds as efficient catalyst in Li-S batteries

In lithium-sulfur (Li-S) batteries, the root-cause solution is to accelerate the polysulfide conversion to their end products by electrocatalysts. Herein, we put forward a molecular spin engineering to break the symmetry of Fe-N-C and achieve the spin modulation for triplet-to-singlet conversion by the ligand field theory. The Fe-N-C shows an 3d-electronic structure of (dxy)2(dxz)2(dyz)2 and dramatically speeds up the LPSs conversion kinetics. According to the density functional theory (DFT), there is an upshift of energy levels after the adsorption of LPSs on Fe center. As a result, the optimized catalyst delivers a capacity fading rate of 0.026% per cycle at 2 C. In addition, a high capacity of 1042 mA h g−1 is achieved with satisfied capacity retention with the sulfur loading of about 4 mg cm−2. This strategy provides a novel route for the regulation of Fe-spin state and the exploration of catalytic effect in Li-S batteries.

Imidazolin-2-iminato Cerium(IV) Chlorides: Synthesis, Structure and Electrochemical Properties.

The ligand field greatly affects the redox properties of cerium. Herein, cerium(III) and cerium(IV) complexes supported by imidazolin-2-iminato ligands were synthesized and structurally characterized, and their electrochemical properties were investigated. Silylamine elimination of cerium(III) amide Ce{N(SiMe3 )2 }3 with imidazolin-2-imine ImR NH (R=Mes, tBu, iPr) provided imidazolin-2-iminato cerium(III) complexes, [(ImMes N)(ImMes NH)Ce{N(SiMe3 )2 }2 ] (1), [(ImtBu N)2 (ImtBu NH)Ce{N(SiMe3 )2 }] (2) and [(ImiPr N)2 (ImiPr NH)Ce(μ-ImiPr N)]2 (4) in 71-85% yields. These cerium(III) complexes were successfully oxidized by Ph3 CCl or C2 Cl6 to afford imidazolin-2-iminato cerium(IV) chlorides, [(ImMes N)2 Ce{N(SiMe3 )2 }Cl] (5), [(ImtBu N)3 CeCl] (6) and [(ImiPr N)2 Ce(μ-ImiPr N)Cl]2 (7) in 70%-76% yields. All complexes were characterized by the single-crystal X-ray diffraction, which showed that 1, 2, 5 and 6 are monomers while 4 and 7 are dimers. The electrochemical studies indicated that the Ce(III/IV) couples for 5 and 6 are more negative than those of silylamido cerium(IV) complexes, and the Ce(III/IV) couples for 1 and 2 have a similar trend.

Femtosecond Extreme Ultraviolet Spectroscopy of an Iridium Photocatalyst Reveals Oxidation State and Ligand Field Specific Dynamics

Femtosecond X-ray absorption spectroscopy at the Ir O3-edge and N6,7-edges is performed on the photocatalyst iridium(III) tris(2-phenylpyridine), Ir(III)(ppy)3 using a tabletop high-harmonic source. Extreme ultraviolet (XUV) absorption between 44 and 76 eV measures transitions from the Ir 5p3/2 and 4f5/2,7/2 core to 5d valence orbitals, and the position of these spectral features is shown to be sensitive to the oxidation state and ligand field of the metal center. Upon excitation of the singlet metal-to-ligand charge transfer (1MLCT) band at 400 nm, a shift in the spectra due to the formation of the Ir(IV) center is observed, as is the creation of a new spectral feature corresponding to transitions into the t2g hole. Vibrational cooling of the MLCT state on the 3 and 16 ps time scales is measured as changes in the intensity of the transient features. This work establishes XUV spectroscopy as a useful tool for measuring the electronic structure of third row transition metal photosensitizers and catalysts at ultrafast time scales.

μ-Oxo Dimerization Effects on Ground- and Excited-State Properties of a Water-Soluble Iron Porphyrin CO2 Reduction Catalyst

Iron 5,10,15,20-tetra(para-N,N,N-trimethylanilinium)porphyrin (Fe-p-TMA) is a water-soluble catalyst capable of electrochemical and photochemical CO2 reduction. Although its catalytic ability has been thoroughly investigated, the mechanism and associated intermediates are largely unknown. Previous studies proposed that Fe-p-TMA enters catalytic cycles as a monomeric species. However, we demonstrate herein that, in aqueous solutions, Fe-p-TMA undergoes formation of a μ-oxo porphyrin dimer that exists in equilibrium with its monomeric form. The propensity for μ-oxo formation is highly dependent on the solution pH and ionic strength. Indeed, the μ-oxo form is stabilized in the presence of electrolytes that are key components of catalytically relevant conditions. By leveraging the ability to chemically control and spectrally address both species, we characterize their ground-state electronic structures and excited-state photodynamics. Global fitting of ultrafast transient absorption data reveals two distinct excited-state relaxation pathways: a three-component sequential model consistent with monomeric relaxation and a two-component sequential model for the μ-oxo species. Relaxation of the monomeric species is best described as a ligand-to-metal charge transfer (τ1 = ∼500 fs), an ionic strength-dependent metal-to-ligand charge transfer (τ2 = 2-4 ps), and finally relaxation of a ligand field excited state to the ground state (τ3 = 5 ps). Conversely, excited-state relaxation of the μ-oxo species proceeds via cleavage of an FeIII-O bond to generate transient FeIV═O and FeII porphyrin species (τ1 = 2 ps) that recombine to the ground-state μ-oxo species (τ2 = ∼1 ns). This latter lifetime extends to timescales relevant for chemical reactivity. It is therefore emphasized that further consideration of catalyst speciation and chemical microenvironments is necessary for elucidating the mechanisms of catalytic CO2 reduction reactions.

Two-dimensional binary transition metal nitride MN4 ( M = V, Cr, Mn, Fe, Co) with a graphenelike structure and strong magnetic properties

Binary transition metal nitride with a graphene-like structure and strong magnetic properties is rare. Based on the first-principles calculations, we design two kinds of $M{\mathrm{N}}_{4}$ ($M$ = transition metal) monolayers, which are transition metal nitrides with a planar structure, made up of $M{\mathrm{N}}_{4}$ units aligned in the rhombic and square patterns. The two structural lattices have robust stability and good compatibility with different metal atoms, and the underlying mechanism is the combination of $s{p}^{2}$ hybridization, coordinate bond, and $\ensuremath{\pi}$ conjugation. With the metal atom changing from V, Cr, Mn, Fe to Co, the total charge of $M{\mathrm{N}}_{4}$ system increases by one electron in turn, which results in continuous adjustability of the electronic and magnetic properties. The planar ligand field is another feature of the two $M{\mathrm{N}}_{4}$ lattices, which brings about the special splitting of five suborbitals of $3d$ metal atom and gives rise to strong magnetism. Moreover, room-temperature ferromagnetism in square-${\mathrm{CoN}}_{4}$ monolayer with the Curie temperatures of 321 K is determined by solving the Heisenberg model combined with Monte Carlo method.

A cyanide-bridged FeII–MoV-based coordination polymer showing spin crossover

A new cyanide–bridged Fe II –Mo V -based coordination polymer was synthesized. Magnetic and spectroscopic studies indicate that the 2D network displays an incomplete spin crossover (SCO). • A new cyanide-bridged Fe II –Mo V -based coordination polymer was synthesized. It is the first 2D [MoV(CN)]3–-based SCO complex. • X-ray diffraction, magnetic and IR spectra study revealed that 1 exhibited an incomplete spin transition with ca. 62% of the HS Fe II species transformed into the LS Fe II state. • magneto-structural analyses demonstrated that introducing more electron-donative ligands will improve the ligand field strength and therefore the completeness of SCO. Polycyanometallates are widely used in the construction of functional molecular magnetic materials owing to their versatile bonding modes and redox activities. However, only a few octacyanometallate-based complexes showed spin crossover (SCO) properties. In this work, we incorporate the bridging ligand 4,4'-Bis-(1H-imidazol-1-yl-methyl)biphenyl (bimb) into a cyanide-bridged Fe II –Mo V assembly, affording a two-dimensional spin crossover complex of {[Mo V (CN) 8 ][Fe II (bimb) 2 ]·0.5Hbimb} n ∙2H 2 O (1). The complex crystallizes in the P 1 ¯ space group and forms a layered structure. The two-dimensional layer is composed of one-dimensional [Fe II –CN–Mo V ] zigzag chains connected by the bridging ligand of bimb. Magnetic susceptibility measurements revealed that 1 exhibited an incomplete spin transition with ca. 62% of the high-spin (HS) Fe II species transitioned to the low-spin (LS) Fe II state upon cooling from 300 to 100 K.

Reaching the Maximal Unquenched Orbital Angular Momentum L = 3 in Mononuclear Transition-Metal Complexes: Where, When and How?

The conditions for achieving the maximal unquenched orbital angular momentum L = 3 and the highest magnetic anisotropy in mononuclear 3d complexes with axial coordination symmetry are examined in terms of the ligand field theory. It is shown that, apart from the known linear two-coordinate 3d7 complex CoII(C(SiMe2ONaph)3)2 characterized by record magnetic anisotropy and single-molecule magnet (SMM) performance (with the largest known spin-reversal barrier Ueff = 450 cm−1), the maximal orbital angular momentum L = 3 can also be obtained in linear two-coordinate 3d2 complexes (V3+, Cr4+) and in trigonal-prismatic 3d3 (Cr3+, Mn4+) and 3d8 (Co+, Ni2+) complexes. A comparative assessment of the SMM performance of the 3d2, 3d3 and 3d8 complexes indicates that they are unlikely to compete with the record linear complex CoII(C(SiMe2ONaph)3)2, whose magnetic anisotropy is close to the physical limit for a 3d metal.

Molecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes.

Square-planar NiII complexes and their electronically excited states play key roles in cross-coupling catalysis and could offer new opportunities to complement well-known isoelectronic PtII luminophores. Metal-to-ligand charge transfer (MLCT) excited states and their deactivation pathways are particularly relevant in these contexts. We sought to extend the lifetimes of 3MLCT states in square-planar NiII complexes by creating coordination environments that seemed particularly well adapted to the 3d8 valence electron configuration. Using a rigid tridentate chelate ligand, in which a central cyclometalated phenyl unit is flanked by two coordinating N-heterocyclic carbenes, along with a monodentate isocyanide ligand, a very strong ligand field is created. Bulky substituents at the isocyanide backbone furthermore protect the NiII center from nucleophilic attack in the axial directions. UV-Vis transient absorption spectroscopies reveal that upon excitation into 1MLCT absorption bands and ultrafast intersystem crossing to the 3MLCT excited state, the latter relaxes onward into a metal-centered triplet state (3MC). A torsional motion of the tridentate ligand and a NiII-carbon bond elongation facilitate 3MLCT relaxation to the 3MC state. The 3MLCT lifetime gets longer with increasing ligand field strength and improved steric protection, thereby revealing clear design guidelines for square-planar NiII complexes with enhanced photophysical properties. The longest 3MLCT lifetime reached in solution at room temperature is 48 ps, which is longer by a factor of 5-10 compared to previously investigated square-planar NiII complexes. Our study contributes to making first-row transition metal complexes with partially filled d-orbitals more amenable to applications in photophysics and photochemistry.

Chiral Dy(III) Fluorescent Single-Molecule Magnet Based on an Achiral Flexible Ligand

A novel multi-channel barcode module was developed by using chiral co-crystals which contain field-induced SMM behavior and different emission bands. The chiral co-crystals [Zn(H2L)Dy(DBM)2]4(ClO4)4⋅9CH3OH⋅H2O (1a) and [Zn(H2L)Dy(DBM)2]4(ClO4)4⋅8CH3OH⋅0.5H2O (1b) (H4L = 2,2′-[1,2-ethanediylbis[(hydroxyethylimino)methylene]]bis[6-methoxy-4-methyl-phenol], HDBM = dibenzoylmethane) were obtained through one-pot reaction of ZnII and DyIII with the achiral ligands H4L and HDBM. X-ray single crystal diffraction and CD spectroscopy confirmed that they are enantiomers crystallized in P43 (1a) and P41 (1b), both consisting of two ∆-[Zn(H2L)Dy(DBM)2]+ cations, two Λ-[Zn(H2L)Dy(DBM)2]+ cations and four (ClO4)− anions. The presence of DyIII ions endow them with the property of field-induced slow magnetic relaxation. The relatively low energy barrier of 35.0(9) K for complex 1 may be due to the poor axiality of the ligand field caused by the long Dy-Ophenoxy bond lengths and the small Ophenoxy-Dy-Ophenoxy bond angles. Moreover, when the organic ligands H4L (λex = 350 nm) and DyIII (λex = 420 nm) are excited, different emission spectra are observed.

Axial Ligand as a Critical Factor for High-Performance Pentagonal Bipyramidal Dy(III) Single-Ion Magnets.

The choice of axial ligands is of great importance for the construction of high-performance Dy-based single-molecule magnets (SMMs). Here, combining axial ligands Ph3SiO- (anion of triphenylsilanol) and 2,6-dichloro-4-nitro-PhO- (the anion of 2,6-dichloro-4-nitrophenol) with a neutral macrocyclic ligand 2,14-dimethyl-3,6,10,13,19-pentaazabicyclo[13.3.1]nonadeca-1(19),2,13,15,17-pentaene (L2N5) generates two new pentagonal bipyramidal Dy(III) complexes [DyIII(L2N5) (X)2](BPh4) (X = Ph3SiO-, 1; 2,6-dichloro-4-nitro-PhO-, 2) with strong axial ligand fields. Magnetic characterizations show that 1 possesses a large energy barrier above 1000 K and a magnetic hysteresis up to 9 K, whereas 2 only displays field-induced peaks of alternating-current susceptibilities without the hysteresis loop, even though 2 has a similar coordination geometry with 1. Detailed Ab initio calculations indicate an apparent difference in the axial negative charge between both complexes, which is caused by the diverse electron-donating properties of the axial ligands. The present work provides an efficient strategy to enhance the SMMs' properties, which highlights that the electron-donating property of the axial ligands is especially important for constructing the high-performance Dy-based SMMs.

Altering the spin state of Fe-N-C through ligand field modulation of single-atom sites boosts the oxygen reduction reaction

Atomically dispersed Fe-N-C catalysts are the most promising candidates alternatively to Pt-based catalysts for oxygen reduction reaction (ORR). However, the ORR activity of Fe-Nx electrocatalysts in acid are still far from satisfactory; thus far, although some discussions demonstrate the important role of ligand fields of single atom metal-N-C sites on improving catalytic properties, the behind mechanism is still ambiguous. Herein, based on the ligand field theory, the electron spin-state modulation of Fe active centers in SA Fe-N-C achieved from a low-spin state (LS) for FeN5 (Fe-N5-LS) to a high-spin state (HS) for FeN4 (Fe-N4-HS) and FeN3 (Fe-N3-HS) was realized by converting defect-rich pyrrolic N-coordinated FeNx sites, which tune the electron readily penetrating the antibonding π-orbital of oxygen. The Fe-N4-HS exhibits a 3d-electronic structure of t2g3eg2 and significantly accelerate the ORR reaction kinetics. Taking advantage of activity-boosting high spin state (S5/2) of Fe (III), the designed Fe-N4-HS (with two longitudinal parallel coordinated pyrr-N and pyri-N, respectively) catalyst displays excellent ORR activity, which is comparable to commercial Pt/C catalyst. In addition, Fe-N4-HS presents higher proton exchange membrane fuel cell (PEMFC) and Zn-air battery performances than most non-precious-metal electrocatalysts. Our findings provide fundamental and technological insights into the correlation between the electronic spin states/geometric structure and high-efficiency SA Fe-N-C catalysts for ORR process.

High laser performance of an Al3+ and Nd3+-codoping in sodium-borotellurite glass for NIR broadband laser application

The main spectroscopic properties of Nd3+ ions immersed in an oxide glass with composition 59B2O3–10TeO2–30Na2O–1Al2O3:1Nd2O3 (in mol%) has been studied by X-ray diffraction, EDS, SEM, luminescence, transmission, FTIR and Raman spectroscopies. Very high values of the main spectroscopic parameters of Nd3+ ions were determined from the Ωt intensity parameters according to Judd-Ofelt formalism. The 4F3/2 → 4I11/2 NIR transition at 1060 nm of Nd3+ ions is evaluated from the relevant laser parameters reporting a very high quantum efficiency (95%), large effective bandwidth (36 nm), high laser performance quality (9.19 × 10−24 cm2·s) and low saturation intensity (2.04 × 108 W/m2). The excellent non-linear properties of Nd3+ ions are attributed to a microcrystallization inside the glass induced by Al3+ and Nd3+-codoping and a high sodium concentration which intensified the ligand field strengths around Nd3+ ions. Such microcristallization is verified by the presence of several crystalline peaks observed in the FTIR and Raman spectra and identified as Al–O and Te–O linkage vibrations in α-Al2O3 and α-TeO2 phases. As a consequence, a significant reduction of free hydroxyls is appreciated at the midinfrared region in both spectra. Whereas the presence of graphite was detected in the Raman spectrum at 2450 and 2889 cm−1. The great centrosymmetric loss of the Nd3+-sites is indicated by a high value of the Ω2 parameter (36.98 × 10−20 cm2) and confirmed by matched vibrational modes displayed in the deconvoluted Raman and FTIR spectra from 370 to 1750 cm−1 according to the Mutual Exclusion Rule for non-centrosymmetric point groups. The X-ray pattern of the Nd3+-doped oxide glass shows three different amorphous phases generated by delocalized and localized 4f-electrons indicating coexistence of ionic and covalent oxide-metal bonds as characteristic of the invert glasses. However, the compositional parameters confirmed a preferentially covalent domain of oxide-metal bonds as well as presence of crystalline phase, being both structural characteristics consistent with the Judd-Ofelt, transmission, FTIR and Raman results. Finally, the radiative properties are compared with those reported for other Nd3+-activated aluminate glasses suggesting that Al3+ and Nd3+-codoped sodium-borotellurite glass has a great potential for NIR broadband laser application.

Tuning the Coordination Microenvironment of Central Fe Active Site to Boost Water Electrolysis and Oxygen Reduction Activity.

In heterogeneous catalysis, single-atom catalysts are the frontier and important prototypes for many reactions, and revealing the intrinsic structure-activity relationship is presently a critical task, but remains challenging. In this work, water electrolysis and oxygen reduction performances of FeXYi N3 -i (X, Y = B, C, O, P and S; i= 0, 1) moiety in Fe-porphyrin are studied by the first-principles calculations, aiming at unraveling how and why tuning the coordination microenvironment of the active metal atom can improve the activity. It can be concluded that breaking the coordination shell symmetry breaks the well-accepted standard scaling relationship, adjusts *O adsorption behavior and thus optimizes the oxygen evolution reaction (OER) activity, for example, to an extremely low overpotential of 0.17V. In combination with the Fe atom spin configuration and ligand field theory, the dramatically improved OER activity can be well explained. In the present work, the significance of the coordination microenvironment of central metal atom in studies of electrocatalysis is highlighted.