Ligand Field Strength Research Articles

57Fe Mössbauer and DFT study of the electronic and spatial structures of the iron(II) (pseudo)clathrochelates: the effect of ligand field strength.

Combined experimental 57Fe Mössbauer and theoretical DFT study of a series of iron(II)-centered (pseudo)macrobicyclic analogs and homologs was performed. The field strength of the corresponding (pseudo)encapsulating ligand was found to affect both the spin state of a caged iron(II) ion and the electron density at its nucleus. In a row of the iron(II) tris-dioximates, passing from the non-macrocyclic complex to its monocapped pseudomacrobicyclic analog caused an increase both in the ligand field strength and in the electron density at the Fe2+ ion, and, therefore, a decrease in the isomer shift (IS) value (so-called "semiclathrochelate effect"). Its macrobicyclization, giving the quasiaromatic cage complex, caused a further increase in the two former parameters and a decrease in IS (so-called "macrobicyclic effect"). The trend of their IS values was successfully predicted using the performed quantum-chemical calculations and the corresponding linear correlation with the electron density at their 57Fe nuclei was plotted. A variety of different functionals can be successfully used for such excellent prediction. The slope of this correlation was found to be unaffected by the used functional. In contrast, the predictions of both the sign and the values of quadrupole splitting (QS) for them, based on the theoretical calculations of EFG tensors, were found to be a real great challenge, which could not be solved at the moment even in the case of these C3-pseudosymmetric iron(II) complexes with known XRD structures. The latter experimental data allowed us to deduce a sign of the QSs for them. The straightforwarded molecular design of a (pseudo)encapsulating ligand is proposed to control both the spin state and the redox characteristics of an encapsulated metal ion.

Fine-tuning of the spin-crossover properties of Fe(III) complexes via ligand design.

Exploring the chemical space of a given ligand aiming to modulate its ligand field strength is a versatile strategy for the fine-tuning of physical properties such as the transition temperature (T1/2) of spin-crossover (SCO) complexes. The computational study presented herein aims at systematically exploring the extent to which the ligand substituent effects can modulate T1/2 in two families of Fe(III) SCO systems with a N4O2 coordination environment and at identifying the best descriptors for fast and accurate prediction of changes in T1/2 upon ligand functionalization. B3LYP* calculations show that the attachment of substituents to β-ketoiminato fragments (L1) leads to drastic changes in T1/2, while functionalization of phenolato moieties (L2) allows for a finer degree of control over T1/2. Natural Bond Orbital (NBO) charges of the donor atoms, Hammett parameters for both para and meta-functionalization of L2, and Swain-Lupton parameters for L1 and para-functionalization of L2 have been found to be the suitable descriptors for predicting the changes in T1/2. Further analysis of the ligand-field splitting in such systems rationalizes the observed trends and shows that ligand substituents modify both the σ and π bonds between the Fe(III) center and the ligands. Thus, we provide simple yet reliable guidelines for the rational design of new SCO systems with specific values of T1/2 based on their ligand design.

A colorimetric investigation of copper(II) solutions

Complex ions containing transition metals, elements with incompletely filled d-orbitals, are usually colored, while similar ions from non-transition metals are not. Copper (Cu) is a transition metal with a flexible coordination sphere, meaning it can adopt many coordination geometries, and therefore have various colored complexes. Addition of Chloride (Cl-) to a CuCl2 solution causes color change away from blue toward a yellowish green. We hypothesized that adding acetone to an CuCl2-water solution would cause a similar shift in solution color. We made aqueous CuCl2 solutions with increasing acetone concentrations ranging from 0 to 100 percent and compared the resulting colors to aqueous CuCl2 solutions with increasing added amounts of NaCl to find the closest visual fit in color from the acetone solutions for each added NaCl equivalent. Our experiment showed that increasing the percent acetone in aqueous CuCl2 solutions gave rise to a color change parallel to that of adding Cl-, shifting solution color away from blue and toward yellow. We propose an explanation of this relationship by considering Cu2+ reduction in high acetone concentrations and discussing ligand field strength and the possible effects of tetrahedral versus octahedral geometries on splitting: weaker splitting may similarly affect color through equilibria leaning towards green/yellow products. This investigation could support our understanding of copper coordination and copper complex color behavior with future implications for other copper complex experiments or, in a broader perspective, detection and evaluation of Cu2+ levels in environmental and biological systems.

Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands: a case of dual emission revisited.

Iron N-heterocyclic carbene (FeNHC) complexes with long-lived charge transfer states are emerging as a promising class of photoactive materials. We have synthesized [FeII(ImP)2] (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)) that combines carbene ligands with cyclometalation for additionally improved ligand field strength. The 9 ps lifetime of its 3MLCT (metal-to-ligand charge transfer) state however reveals no benefit from cyclometalation compared to Fe(ii) complexes with NHC/pyridine or pure NHC ligand sets. In acetonitrile solution, the Fe(ii) complex forms a photoproduct that features emission characteristics (450 nm, 5.1 ns) that were previously attributed to a higher (2MLCT) state of its Fe(iii) analogue [FeIII(ImP)2]+, which led to a claim of dual (MLCT and LMCT) emission. Revisiting the photophysics of [FeIII(ImP)2]+, we confirmed however that higher (2MLCT) states of [FeIII(ImP)2]+ are short-lived (<10 ps) and therefore, in contrast to the previous interpretation, cannot give rise to emission on the nanosecond timescale. Accordingly, pristine [FeIII(ImP)2]+ prepared by us only shows red emission from its lower 2LMCT state (740 nm, 240 ps). The long-lived, higher energy emission previously reported for [FeIII(ImP)2]+ is instead attributed to an impurity, most probably a photoproduct of the Fe(ii) precursor. The previously reported emission quenching on the nanosecond time scale hence does not support any excited state reactivity of [FeIII(ImP)2]+ itself.

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.

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.

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.

Efficient water reduction by ruthenium-picolinate dye-sensitized photocatalyst under red light illumination

To utilize a wider region of the solar spectrum for solar-fuel production, we newly synthesized a heteroleptic Ru(II)-picolinate complex, [Ru(H4dpbpy)2(pic)]+ (Rupic; H4dpbpy = H4dpbpy = 2,2′-bipyridine-4,4′-diphosphonic acid, Hpic = picolinic acid) as a red light-absorbing molecular photosensitizer. Rupic exhibited metal-to-ligand charge-transfer (MLCT) absorption and emission (λabs and λem = 491 and 711 nm, respectively) over a longer wavelength region than that observed with the bipyridine derivative [Ru(H4dpbpy)2(bpy)]2+ (Rubpy, bpy = 2,2′-bipyridine). The Ru(III)/Ru(II) redox potential of Rupic (Eox = +1.07 vs NHE) was approximately 0.27 V more negative than that of Rubpy, owing to the weaker ligand field strength of pic than that of bpy. Moreover, as expected, owing to the red-light absorption ability of Rupic, the photocatalytic H2 evolution reaction activity of the Rupic-sensitized Pt-TiO2 nanoparticles (apparent quantum yield, AQY ∼ 3 %) under red light illumination (λ = 625 ± 35 nm) was significantly higher than that of its Rubpy-sensitized analog (AQY ∼ 0.5 %). These results indicate the superior photosensitizing performance of the Ru(II)-picolinate complex for H2 production under sunlight illumination.

Extended π-Conjugated Ligands Tune Excited-State Energies of Iron(II) Polypyridine Dyes

Over the past decade, iron(II) polypyridines have gained a lot of attention as potential chromophores and sensitizers due to the low cost and high abundance of iron. Unfortunately, most iron(II) polypyridines are poor chromophores since their initially excited, photoactive metal-to-ligand charge transfer (MLCT) states quickly decay into non-photoactive metal-centered (MC) states. Many strategies to increase their lifetime have been pursued, built mainly around increasing the ligand field strength of these complexes and thus destabilizing the MC states. In this work, we aim to design a new class of Fe(II) complexes by stabilizing the energies of their MLCT states. To this end, we employ density functional theory (DFT) and time-dependent DFT to investigate a series of Fe(II) complexes, [Fe(L/X)2,4(N^N)]2+/2- where L/X represents either cyanide, isocyanide, or bipyridine ligands and N̂N stands for bidentate-extended π-conjugated ligands derived from the bipyridine. The L/X ligands tune the energetics of the Fe-based t2g molecular orbitals, while the amount of π-conjugation on the N^N ligand impacts the energies of its π and π* orbitals, thus tuning the energetics of the MLCT and the ligand-centered (LC) states. Overall, our results suggest that the use of N^N ligands with the extended π-conjugation is a viable strategy to tune the relative energies of MLCT, LC, and MC states.

Regulating Surface Reaction Kinetics through Ligand Field Effects for Fast and Reversible Aqueous Zinc Batteries

AbstractDesigning water‐deficient solvation sheath of Zn2+ by ligand substitution is a widely used strategy to protect Zn metal anode, yet the intrinsic tradeoff between Zn nucleation/dissolution kinetics and the side hydrogen evolution reaction (HER) remains a huge challenge. Herein, we find boric acid (BA) with moderate ligand field interaction can partially replace H2O molecules in the solvation sheath of Zn2+, forming a stable water‐deficient solvation sheath. It enables fast Zn nucleation/dissolution kinetics and substantially suppressed HER. Crucially, by systematically comparing the ligand field strength and solvation energies between BA and the ever‐reported electrolyte additives, we also find that the solvation energy has a strong correlation with Zn nucleation/dissolution kinetics and HER inhibition ability, displaying a classic volcano behavior. The modulation map could provide valuable insights for solvation sheath design of zinc batteries and beyond.

Regulating Surface Reaction Kinetics through Ligand Field Effects for Fast and Reversible Aqueous Zinc Batteries.

Designing water-deficient solvation sheath of Zn2+ by ligand substitution is a widely used strategy to protect Zn metal anode, yet the intrinsic tradeoff between Zn nucleation/dissolution kinetics and the side hydrogen evolution reaction (HER) remains a huge challenge. Herein, we find boric acid (BA) with moderate ligand field interaction can partially replace H2 O molecules in the solvation sheath of Zn2+ , forming a stable water-deficient solvation sheath. It enables fast Zn nucleation/dissolution kinetics and substantially suppressed HER. Crucially, by systematically comparing the ligand field strength and solvation energies between BA and the ever-reported electrolyte additives, we also find that the solvation energy has a strong correlation with Zn nucleation/dissolution kinetics and HER inhibition ability, displaying a classic volcano behavior. The modulation map could provide valuable insights for solvation sheath design of zinc batteries and beyond.

High‐temperature near‐IR spectral properties and thermal radiation conductivity of (un)colored silicate glass melts

AbstractUsing an emittance technique with a fast CO2 laser heating of glass samples, the high‐temperature absorption spectra in the near‐infrared region of ultrapure and colored (Co‐, Cu‐, Mn‐, and Ni‐doped) glasses are measured. The effects of higher glass temperatures on these absorption spectra are explained in the framework of the ligand field theory. Thus, the temperature‐dependent absorption bands of the previous transition metal ions are assigned to electronic transitions among the ligand field energy levels of these ions. In particular, spectral shifts, spectral broadening, and changes in absorption strength are ascribed to changes in the structural symmetry of the ionic sites in the glass matrix and to changes of the ligand field strength at increasing temperatures.Besides, the temperature‐dependent Rosseland mean absorptions of the sulfate fined soda lime silicate glass melts, colored with the previous transition metal ions, are derived from the absorption spectra. Combining all the data, semiempirical correlations are derived, which predict the Rosseland thermal radiation properties as a function of glass temperature and of glass redox chemistry. The latter property involves the temperature‐dependent concentration of the specific valency of the coloring ions, determined independently, e.g. by a Gibbs minimization redox calculation tool.

Ligand-Field Spectroscopy of Co(III) Complexes and the Development of a Spectrochemical Series for Low-Spin d6 Charge-Transfer Chromophores.

A study of a series of six-coordinate Co(III) complexes has been carried out to quantify spectroscopic parameters for a range of ligands that are commonly employed to realize strong charge-transfer absorptions in low-spin, d6 systems. Identification of any three ligand-field transitions allows for the determination of the splitting parameter (10 Dq) as well as the Racah B and C parameters for a given compound. The data revealed a relatively small spread in the magnitude of 10 Dq, ranging from ca. 23 000 cm-1 in the case of [Co(pyrro-bpy)3]3+ (where pyrro-bpy is 4,4'-dipyrrolidinyl-2,2'-bipyridine) to ca. 26 000 cm-1 for [Co(terpy)2]3+ (where terpy is 2,2':6',2″-terpyridine). Significantly, trends across the series suggest that polypyridyl ligands behave as net π-donors when interacting with Co(III), in contrast to the net π-accepting character they exhibit when bound to second- and third-row metals. The influence of strong σ donation associated with carbene-based ligands was evident from the data acquired for [Co(BMeImPy)2]3+ (where BMeImPy is 3,3'-(pyridine-2,6-diyl)bis(1-methyl-1H-3-imidazolium)), where a 10 Dq value of ca. 30 000 cm-1 was determined. Spectroscopic data were also analyzed for [Fe(bpy)3]2+ using the results on [Co(bpy)3]3+ as a reference point. A value for 10 Dq of 21 000 cm-1 was estimated, indicating a reduction in the ligand-field strength of ca. 3000 cm-1 upon replacing Co(III) with Fe(II). We suggest that this approach of taking advantage of the blueshift of the charge-transfer feature in Co(III) complexes to reveal otherwise obscured ligand-field bands can be a useful tool for the development of new ligand systems to expand the photofunctionality of first-row transition-metal-based chromophores.

The study on BWGd:Nd glass for new laser amplifier: Properties, theoretical and practical investigations

The new laser-amplifier glass, Nd3+ doped tungsten gadolinium borate glass (BWGd:Nd), was investigated in this work. Glasses with different Nd2O3 concentration were prepared by melt-quenching technique. The measured density, refractive index and other calculated physical parameters leaded to the assumption that the degree of Nd–O covalency and the number of NBO increased with addition of Nd2O3 content. The enhancement of both factors possibly increased the strength of ligand field that slightly shifted and split some energy levels of Nd3+. The energy level splitting from 4F5/2 state was an intermediate level which accelerated the Nd3+ population on the 4F3/2 luminescence level by single-phonon relaxation. This electron-phonon coupling and low phonon energy of BWGd host glass was a key for strong Nd3+ luminescence of 4F3/2 → 4I11/2 transition (1074 nm) and high radiative parameters, especially the high quantum efficiency around 96%. Due to high value of Δλeff as well as σem but no long τexp value, this glass is suitable for laser amplifier. In practical laser-amplifier experiment, the gain amplification of developed glass was around 1.5 times under pumping power of 3.55 W. The BWGd:Nd glass performed high ability and may using as laser-amplifier material in the laser fusion and other applied laser system.

A theoretical framework for optical thermometry based on excited-state absorption and lifetimes of Eu3+ compounds

Ratiometric optical thermometers correspond to one of the most extensively studied applications of luminescent lanthanide materials in recent years. Even though several systems were studied involving the Eu3+ ion, few of them explore the absorption from the low-lying 7F1 excited state and its dependence on the ligand field (LF) splitting. This work aims to describe various absorption intensity ratios (AIR) that can be built from the excitation or absorption spectrum of the europium (III) ion by using the absorptions from the 7F0 → 5D0 transition and the different ligand field components of the magnetic-dipole allowed 7F1 → 5D0 transition and the influence of the ligand field strength on the relative thermal sensitivity of such AIR constructions. We have shown that several AIR formulas can be obtained, and they can display opposite behaviors concerning the Ligand Field strength, being suitable for different compounds of non-cubic symmetries. In addition, the ligand field influence on the relative thermal sensitivity obtained by the lifetime thermometry of the Eu3+5D0 level has also been explored.

Mössbauer studies of the ferryl, ferrous and ferric states of dehaloperoxidase from A. ornata

Dehaloperoxidase (DHP) is a multi-functional catalytic globin from the marine worm A. ornata, whose physiological functions include oxygen transport and oxidation of toxic substrates present in its habitat. In the Fe(III) state, DHPA has an isomer shift of 0.42 mm/s, characteristic for high-spin heme proteins. Changes in pH have subtle effects on the electronic structure of DHP in the Fe(III) state detectable in the high-field spectra, which show a pH-dependent mixture of species with different zero-field splittings between 5 and 18 cm−1. The short-lived intermediate obtained by direct reaction of the Fe(III) enzyme with H2O2 has an isomer shift of 0.10 mm/s, indicative of an Fe(IV)-oxo state and of an S = 1 electronic ground state confirmed by variable field studies. The O2-bound state of DHP has an isomer shift of 0.28 mm/s and a high-field spectrum characteristic for diamagnetic heme complexes, similarly to other haemoglobins.Overall, the isomer shift and quadrupole splitting of DHP in the four states studied are expectedly similar to both peroxidases and to myoglobin. The differences in electronic structure between DHP and other heme proteins and enzyme are observed in the high-field Mössbauer spectra of the ferric state, which show pH-dependent zero-field splittings suggesting a heme site in which the ligand field strength at the iron ion is tuned by pH. This tunability is correlated with variable electron-donating properties of the iron, which can perform multiple functions.

Ligand Additivity and Divergent Trends in Two Types of Delocalization Errors from Approximate Density Functional Theory.

The predictive accuracy of density functional theory (DFT) is hampered by delocalization errors, especially for correlated systems such as transition-metal complexes. Two complementary strategies have been developed to reduce delocalization error: eliminating the global curvature with change in charge, and applying a linear response Hubbard U as a measure of local curvature at a metal center at fixed charge in a DFT+U framework. We investigate the relationship between the two delocalization error measures as the ligand field strength is varied with the number of strong-field ligands in a series of heteroleptic complexes or by geometrically constraining the metal-ligand bond length in homoleptic octahedral complexes. We show that across these sets of complexes an inverse relationship generally exists between global and local curvatures. We find that effects of ligand substitution on both measures of delocalization are typically additive, but the quantities seldom coincide.

Regulating electron transfer over asymmetric low-spin Co(II) for highly selective electrocatalysis

Modulating the steric-electronic configuration of metal-organic centers is key for tuning the activity and selectivity of heterogeneous reactions, especially multi-electron transfer reactions. Here, three different asymmetric metal-organic complexes with unique steric-electronic structures are immobilized on nanocarbon for an electron-transfer-controlled oxygen reduction reaction. The strong-field ligand-induced low-spin ( LS ) Co II creates a necessary steric configuration for regulating reaction selectivity through ligand's proton transfer ability, for which acidic diamine ligands facilitate a four-electron transfer (94%), whereas basic ligands drive a highly selective two-electron route (97%). The steric-electronic regulation of the reaction selectivity at catalytic sites is characterized using X-ray absorption spectroscopy, reaction kinetic path analysis, and density functional theory calculation. Our results indicate that an LS state of Co II with asymmetric coordination is necessary to form a unique “flytrap” structure to promote O 2 capture for the subsequent proton-coupled electron transfer, which is regulated by the Brønsted acidity of coordinating ligands. • A series of asymmetric metal-organic complexes are immobilized on nanocarbons • Advanced characterizations are used to explore their structures at the atomic level • Selectivity of ORR is regulated by changing the ligand field strength of the active site • Operando X-ray spectroscopy and computational analysis confirm the reaction mechanism The majority of energy conversion processes such as hydrogen peroxide production, nitrogen fixation, and CO 2 reduction generally involve multi-electron transfer, resulting in multiple by-products and requiring tedious purification process. The development of highly selective electrocatalysts can not only reduce the energy consumption in the production process but also benefit to promote their large-scale application. Here, we report an asymmetric ligand design of molecular “Venus flytrap” at atomic Co II catalysts on a heterogeneous carbon surface for oxygen electrocatalysis with controlled selectivity. Our XAFS and DFT calculations show that the strong-field ligand-induced LS Co II centers offer inimitable steric-electronic advantages in capturing O 2 and the capability to regulate reaction pathways through an H-bonding interaction between the ligands and peroxo intermediate. The proposed concept completes the picture of designer single-atom catalyst by covering the often overlooked exterior coordination sites, and is expected to be generally applicable to other heterogeneous catalyst systems. Steric-electronic configuration regulation plays a key role in achieving highly selective electrocatalysis. As a proof of concept, a series of asymmetric metal-organic complexes were immobilized on a conductive substrate for oxygen reduction reaction (ORR). The strong-field ligand-induced low-spin cobalt sites exhibit an excellent selectivity for four-electron ORR, whereas the medium-field ligand linked Co sites display high efficiency for the two-electron reduction of oxygen into hydrogen peroxide.

Panchromatic Absorption and Oxidation of an Iron(II) Spin Crossover Complex

In order to expand and exploit the useful properties of d6-iron(II) and d5-iron(III) complexes in potential magnetic, photophysical, or magnetooptical applications, crucial ligand-controlled parameters are the ligand field strength in a given coordination mode and the availability of suitable metal and ligand frontier orbitals for charge-transfer processes. The push-pull ligand 2,6-diguanidylpyridine (dgpy) features low-energy π* orbitals at the pyridine site and strongly electron-donating guanidinyl donors combined with the ability to form six-membered chelate rings for optimal metal-ligand orbital overlap. The electronic ground states of the pseudo-octahedral d6- and d5-complexes mer-[Fe(dgpy)2]2+, cis-fac-[Fe(dgpy)2]2+, and mer-[Fe(dgpy)2]3+ as well as their charge-transfer (CT) and metal-centered (MC) excited states are probed by variable temperature UV/vis absorption, NMR, EPR, and Mössbauer spectroscopy, magnetic susceptibility measurements at variable temperature as well as quantum chemical calculations.

Guest-Induced Multistep-to-One-Step Reversible Spin Transition with Enhanced Hysteresis in a 2D Hofmann Framework.

The interplay of host-guest interactions and controlled modulation of spin-crossover (SCO) behavior is one of the most exploited topics regarding data storage, molecular sensing, and optical technologies. In this work, we demonstrate the experimental approach of tuning the SCO behavior via controlled modulation of the spin-state cooperativity in a 2D Hofmann coordination polymer, [FeIIPd(CN)4(L)2]·1.3MeOH (1·1.3MeOH; L = methyl isonicotinate). Removal of the solvent changes the four-step transition to a complete one-step spin transition with an enhanced hysteresis width (∼20 K). Structural analysis of solvated (1·1.3MeOH) and partially desolvated (1·0.3MeOH) compounds reveals that the crystal system changes from a monoclinic C2/c space group to an orthorhombic Imma space group, where the FeII sites are present in a more symmetrically equivalent environment. Consequently, the axial ligand-field (LF) strength and face-to-face interactions of the ligands were increased by removing the guest, which is reflected in the highly cooperative SCO in 1 (desolvated compound). Also, the shift of the CN bond stretching frequencies and decrease of their relative intensities from the variable-temperature Raman spectroscopic measurements further corroborate the SCO behavior. Besides, theoretical calculations reveal that the singlet (1Γ) LF terms decrease by removing guest molecules, enhancing the molecular population in the low-spin state at low temperature, as experimentally observed for 1. Notably, fine tuning of the SCO behavior via host-guests interactions provides a great opportunity to design potential chemosensors.