Ligand Field Research Articles

Ligand Field and Charge Transfer in Transition-Metal Compounds

Ligand Field and Charge Transfer in Transition-Metal Compounds

Synthetic Studies on Bioactive Substances: From Research on Water-soluble Compounds to the Field of Lipophilic Ligands

This is a personal review of my chemistry research on retirement from Teikyo University. Under the guidance of Prof. Masaji Ohno of the Faculty of Pharmaceutical Sciences, The University of Tokyo, my research career started with the synthesis of water-soluble basic natural compounds, including the first artificial bleomycin showing potent molecular-oxygen activation effects and DNA binding abilities. While studying abroad at Eidgenössische Technische Hochschule (ETH) under the guidance of Prof. Albert Eschenmoser, I studied the formation of ribose under prebiotic conditions. The condensation reaction between formaldehyde and glycolaldehyde phosphate produced ribose in far greater yield than the formose reaction. In the School of Pharmacy, Showa University, I conducted research in nucleic acid chemistry to synthesize, for example, anomeric spiro-nucleosides using radical chemistry and oligonucleotides that interacted with the κB motif. After moving to Teikyo University in 1999, I engaged in studies on the synthesis of vitamin D derivatives, included in fat-soluble vitamins, with selective biological activities without calcemic side-effects, and discovered, for example, 2α-(3-hydroxypropyl)-19-nor-1α,25-dihydroxyvitamin D3 (MART-10), which exhibits potent anticancer activity in vivo, 2α-[2-(tetrazol-2-yl)ethyl]-1α,25-dihydroxyvitamin D3 (AH-1), which shows greater bone-forming effects than natural active vitamin D3 in vivo, and the A-ring-converted vitamin D derivative KK-052, which is an in vivo selective inhibitor of sterol regulatory-element binding protein (SREBP), a master transcription factor of lipogenesis, independent of the vitamin D canonical activity through a vitamin D receptor.

Modification of the Spin Transition Properties in Hetero‐Anionic Iron(II)‐Triazole Complexes

AbstractThe phenomenon which is called spin crossover is known to occur in some coordination compounds with an octahedral ligand field and electron configurations from 3d4 to 3d7. Thereby, a reversible transition between spin states (high spin and low spin state) is possible, through several external stimuli. Iron(II) triazole complexes exhibit this phenomenon at a wide range of temperatures depending on the ligands and anions used. For this reason, they are often considered for several possible practical applications. It is also possible to combine ligands or anions to modify the transition temperature. The latter of which was rarely discussed in the past. In this study we synthesized a series of iron(II)‐4‐Aminotriazole complexes, with different ratios of chloride‐ and tetrafluoroborate‐anions, of the formula [Fe(Atrz)3]Cl2−X(BF4)X. We show that the combination of these anions leads to transition temperatures between those of their corresponding pure anion complexes. We furthermore present that a simple modification of the synthesis leads to a possible easy way of fine‐tuning transitions temperatures.

High Spin‐State Modulation of Catalytic Centers by Weak Ligand Field for Promoting Sulfur Redox Reaction in Lithium‐Sulfur Batteries

High Spin‐State Modulation of Catalytic Centers by Weak Ligand Field for Promoting Sulfur Redox Reaction in Lithium‐Sulfur Batteries

High Spin-State Modulation of Catalytic Centers by Weak Ligand Field for Promoting Sulfur Redox Reaction in Lithium-Sulfur Batteries.

The spin state of transition-metal compounds in lithium-sulfur batteries (LSBs) significantly impacts the electronic properties and the kinetics of sulfur redox reactions (SRR). However, accurately designing the spin state remains challenging, which is crucial for understanding the structure-performance relationship and developing high-performance electrocatalysts. Herein, the CoF2, specifically the Co2+ with 3d7 electrons in a high-spin state distribution (t2g 5eg 2), were tailored predictably for the first time through the weak coordination field effect of the F element. Both DFT calculations and experimental results confirm that the spin state of Co2+ transitions from low- to high-spin configurations and strongly interacts with sulfur species through Co-S and Li-F bonds during the SRR process. This interaction weakens the S-S bond, promoting its facile cleavage from both ends while also facilitating the rapid and uniform nucleation of Li2S2/Li2S, thus resulting in LSBs with a capacity of 447.7 mAh g-1 at 10 C rates and stable cycling for 1000 cycles, with an acceptable practical capacity of 585 mAh g-1 at a high sulfur loading mass of 10 mg cm-2. This work achieves rational control of the active Co2+ d electron state through the field effect and enriches the application of spin control to accelerate SRR in LSBs.

An Efficient Analytical Method for Determining Transition Temperatures in Core‐Shell Spin Crossover Nanoparticles Described by an Extended Ising‐Like Model

AbstractThis contribution is dedicated to the analysis of the size and interaction parameters effects on the thermal properties of square‐shaped cross‐spin transition core‐shell nanoparticles. The present study is carried out by varying the shell size at fixed core size and vice versa. In this problem, the core and shell are active from the spin transition point of view, while having different properties in terms of ligand field (transition temperatures) and interactions. The resulting nanoparticle is described by an Ising‐type model including the three interaction parameters JCC, JSS and JCS coupling core‐core, shell‐shell and core‐shell sites, respectively. The thermodynamic investigations have been carried out in 3 different ways: (i) by Monte Carlo Entropic Sampling (MCES) for small size nanoparticles ( ) (ii) by usual Monte Carlo Metropolis for higher dimensions (iii) by an analytical method deducing the equilibrium temperature of the system from the various local environments. We demonstrate that, as long as a clear plateau region occurs in the thermal‐dependence of the high‐spin fraction, an excellent agreement is obtained, allowing to predict analytically the evolution of the transition temperatures. The limitations of the method are discussed in the case of strong interference between the thermal dependences of the core and shell.

Slow Magnetic Relaxation in a Californium Complex.

We report the synthesis and characterization of the macrocyclic californium derivative Na[Cf(H2O)(DOTA)] (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate), 1-Cf, which was studied in comparison to its dysprosium counterpart, Na[Dy(H2O)(DOTA)], 1-Dy. Divergent spectroscopic and magnetic behaviors were observed between 1-Cf and 1-Dy. Based upon spectroscopic measurements, we propose that accessible 5f → 6d transitions (potentially operating in tandem with charge-transfer transitions) are the major contributors to the observed broadband photoluminescence in 1-Cf. Dc magnetic susceptibility data for 1-Cf revealed lower magnetic moments than those previously observed for 1-Dy and expected for an f9 free ion, which calculations suggest is the result of greater ligand field effects. Notably, 1-Cf displays slow magnetic relaxation on the time scale of ac susceptibility measurements, making it the first example of a californium-based single-molecule magnet. A side-by-side comparison of the ac susceptibility data reveals magnetic relaxation properties that widely differ between 1-Cf and 1-Dy. This divergent relaxation behavior is attributed mainly to the inherent difference in spin-orbit coupling between Dy3+ and Cf3+.

Enhancement of Effective Energy Barrier and Magnetic Blocking Temperature in Tetraoxolene Radical Coupled Dinuclear Dysprosium Complex.

A well-judged combination of a high axial ligand field and a bridging radical ligand in a dinuclear lanthanide complex provides a single-molecule magnet with a higher effective energy barrier for magnetic relaxation and blocking temperature compared to its non-radical analog due to significant magnetic exchange coupling between radical and Ln(III) ions. In this work, we report two chloranilate (CA) bridged dinuclear dysprosium complexes, [{(bbpen)Dy(μ2-CA)Dy(bbpen)}] (1Dy) and [{(bbpen)Dy(μ2-CA⋅)Dy(bbpen)}-{CoCp2}+] (2Dy), where 2Dy is the radical bridged Dy-complex obtained via the chemical reduction of bridging CA moiety (H2bbpen=N,N'-bis(2-hydroxybenzyl)-N,N'-bis(2-methylpyridyl)ethylenediamine). The presence of high electronegative phenoxide moiety enhances the axial anisotropy of pseudo-square antiprismatic Dy(III) ions. The diffused spin of radical is efficiently coupled with anisotropic Dy(III) centres and decreases the quantum tunnelling of magnetization (QTM) in the magnetic relaxation process. The magnetic relaxation of 1Dy follows Orbach, Raman, and QTM processes whereas for 2Dy it follows Orbach and Raman Processes. Due to less involvement of the QTM relaxation process, 2Dy shows a higher thermal energy barrier (Ueff=700 K) and a high blocking temperature (6.7 K), compared to its non-radical analog. Remarkably, the radical coupled 2Dy complex shows the highest energy barrier among the radical bridged Dy(III)-based SMMs to date.

Hydrothermally synthesized cobalt oxide/polydimethylsiloxane based photothermal absorber for superior thermal energy conversion and water evaporation application

Solar-driven interfacial evaporation has garnered worldwide interest in recent years due to its unique vapor generating capacity using sustainable solar energy. Many photoabsorber have been studied for conversion of photothermal energy and heat absorption. Unfortunately, the majority of the absorber materials in supply are pricey, and the installation procedures tend to be intricate. This research focuses on the ongoing difficulty of creating cost-efficient photothermal materials that have excellent light absorption and simple manufacturing processes. We created a novel composites coated with Cobalt oxide and polydimethylsiloxane (Co3O4/PDMS) which successfully produce energy and purify water utilizing an extensive spectrum of solar energy. Hydrothermally synthesized Co3O4 particles exhibit distinct optical properties in the UV–Vis region due to ligand field transitions and charge transfer between Co2⁺ and Co³⁺ ions. Additionally, these particles exhibits a strong absorption in the NIR region due to the intervalence charge transfer and d-d transitions, enhancing their photothermal activity. This culminates in outstanding light-to-heat transformation in the Co3O4/PDMS composite, which maintains a surface temperature of 42.7 °C compared to 33.7 °C for pristine PDMS under standard 1 sun intensity for 5 min. The flexible Co3O4/PDMS composite transfers solar energy to electric energy, producing ∼99 mV with 1 sun irradiation, while bare PDMS only achieved a voltage of 61 mV under 1 sun circumstances. An efficient double layer Co3O4/PDMS@MF sponge achieved an evaporation rate of 1.33 kg m−2 h−1 with the photothermal conversion efficiency of 68.8 %. These results motivate thorough investigation in photothermal potential of Co3O4, revealing the promising possibilities for harnessing solar-thermal energy and presents a novel method for using solar power to purify water and generate electricity.

Layer-By-Layer Magnetic Ordering via Idle Spins and the Optical Signature of Jahn-Teller Cr2+ Ions in Sr2Cr(PO4)2.

Blue/turquoise crystals of Sr2CrII(PO4)2 with prismatic shape and edge-length of up to 1 mm were obtained by a vapor-phase moderated solid-state reaction at 1273 K in sealed silica tubes. Its crystal structure was solved and refined from a triply twinned ("trilling") crystal [Pbca (no. 61), Z = 12, a = 10.7064(6) Å, b = 9.2730(5) Å, c = 21.2720(7) Å, R1 = 0.038]. Sr2Cr(PO4)2 belongs to the small family of inorganic solids containing divalent chromium, where the rare Cr2+ ions are stabilized by the inductive effect of the phosphate groups. As expected from its d4 (S = 2) electronic configuration, the Jahn-Teller effect (JT) is prominent, leading for the two independent Cr2+ ions to square-pyramidal Cr(1)O4+1 and square-planar Cr(2)O4 coordination within a 3D chromium phosphate network [CrII2(PO4)4]8. Topologically, the Cr(1) and Cr(2) cations are arranged in separate alternating layers stacked along the c axis. In their respective layers, Cr(1) shows a gapped 2D topology and only weak interaction with the adjacent Cr(2) layers. However, below TN1 ∼11.3 K, Cr(1) orders antiferromagnetically into a noncollinear structure, leaving nearly paramagnetic Cr(2) idle spins, strongly frustrated by the Cr(1) moments of the next layers. On further cooling, below TN2 ∼3.6 K, the ordering of Cr(2) occurs via an additional magnetic irreducible representation, which splits the Cr(1) into Cr(1)a and Cr(1)b orbits, thus lifting the frustration on Cr(2). The corresponding P21ca.29.99 magnetic space group forces a crystal symmetry lowering, plausibly signed by a change of the magnetostrictive coefficient from positive to negative below TN2. The optical transitions observed for the JT d4 ions are in good agreement with our crystal picture from the DFT calculations. A detailed analysis within the angular overlap model explains the surprisingly different d orbital splitting by the ligand field for the chromophores Cr(1)O4+1 and Cr(2)O4.

Unveiling the Local Structure and the Ligand Field of Organic Cation Preintercalated Vanadate Cathode for Aqueous Zinc-Ion Batteries

Unveiling the Local Structure and the Ligand Field of Organic Cation Preintercalated Vanadate Cathode for Aqueous Zinc-Ion Batteries

Stereoelectronic Tuning of Bioinspired Nonheme Iron(IV)-Oxo Species by Amide Groups in Primary and Secondary Coordination Spheres for Selective Oxygenation Reactions.

Two mononuclear iron(II) complexes, [(6-amide2-BPMEN)FeII](OTf)2 (1) and [(6-amide-Me-BPMEN)FeII(OTf)](OTf) (2), supported by two BPMEN-derived (BPMEN = N1,N2-dimethyl-N1,N2-bis(pyridine-2-yl-methyl)ethane-1,2-diamine) ligands bearing one or two amide functionalities have been isolated to study their reactivity in the oxygenation of C-H and C═C bonds using isopropyl 2-iodoxybenzoate (iPr-IBX ester) as the oxidant. Both 1 and 2 contain six-coordinate high-spin iron(II) centers in the solid state and in solution. The 6-amide2-BPMEN ligand stabilizes an S = 1 iron(IV)-oxo intermediate, [(6-amide2-BPMEN)FeIV(O)]2+ (1A). The oxidant (1A) oxygenates the C-H and C═C bonds with a high selectivity. Oxidant 1A, upon treatment with 2,6-lutidine, is transformed into another oxidant [{(6-amide2-BPMEN)-(H)}FeIV(O)]+ (1B) through deprotonation of an amide group, resulting in a stronger equatorial ligand field and subsequent stabilization of the triplet ground state. In contrast, no iron-oxo species could be observed from complex 2 and [(6-Me2-BPMEN)FeII(OTf)2] (3) under similar experimental conditions. The iron(IV)-oxo oxidant 1A shows the highest A/K selectivity in cyclohexane oxidation and 3°/2° selectivity in adamantane oxidation reported for any synthetic nonheme iron(IV)-oxo complexes. Theoretical investigation reveals that the hydrogen bonding interaction between the -NH group of the noncoordinating amide group and Fe═O core smears out the equatorial charge density, reducing the triplet-quintet splitting, and thus helping complex 1A to achieve better reactivity.

Active lattice oxygen trigger for propane total oxidation: [MnO6] tunnel-restrained Cu2+ chain in α-MnO2

The elimination of propane is particularly important in volatile organic compounds (VOCs) emissions control. We designed a special Cu-type α-MnO2 with tunnel-restrained Cu2+ chain for propane total oxidation. It can effectively elevate the reaction rate and reduce the apparent activation energy. Cu2+ mainly stabilizes as tunnel [CuO4] configurations, changing the charge distribution and ligand field inside the tunnel, compared to the original ionic [KO8] and [KO12] in α-MnO2. The covalency between Cu−O as well as exacerbated Jahn-Teller distortion of [MnO6] occur. The resulting charge transfer of Cu−O−Mn units via bridging O atoms further affects the outside-tunnel properties, specifically, facilitating oxygen vacancy formation and lattice oxygen activation. Moreover, abundant oxygen vacancies can promote the dehydrogenation of propane to generate alcohols at low temperatures. The above intermediates undergo successively oxidation with activated lattice oxygen to produce C3∼C1 carboxylates as temperature elevates.

Metal-Ligand Covalency in the Valence Excited States of Metal Dithiolenes Revealed by S 1s3p Resonant Inelastic X-ray Scattering.

Metallo dithiolene complexes with biological and catalytic relevance are well-known for having strong metal-ligand covalency, which dictates their valence electronic structures. We present the resonant sulfur Kβ (1s3p) X-ray emission spectroscopy (XES) for a series of Ni and Cu bis(dithiolene) complexes to reveal the ligand sulfur contributions to both the occupied and unoccupied valence orbitals. While S K-edge X-ray absorption spectroscopy played a critical role in identifying the covalency of the unoccupied orbitals of metal dithiolenes, the present focus on XES explores the occupied density of states. For a series of [Cu(mnt)2]n- and [Ni(mnt)2]n- anions and dianions, a comparison of the nonresonant and resonant S Kβ XES spectra highlights the dramatic improvement in spectral resolution and corresponding ability to differentiate subtle changes in occupied electronic structure across the series. Furthermore, the use of resonant inelastic X-ray scattering (RIXS) probes the valence excited states and the core-valence couplings of the complexes. By employing a theoretical approach based on time-dependent density functional theory to interpret the RIXS spectra, we reveal how metal-ligand covalency influences the excited state energies and covalencies. We identify the low energy excited states as having the same symmetry as the nominal "ligand field" or "d-d" states that typically dominate the photophysics of 3d metal complexes but with significant metal-ligand charge transfer character dictated by their covalency. These results suggest that strong metal-ligand covalency can be used to influence the charge-transfer photochemistry of first row transition metal complexes.

The interplay between structural features and the efficiency of the energy transfer process in terbium complexes with triarylcyclopentadienyl ligands

Terbium and gadolinium complexes with triarylcyclopentadienyl ligands having electron-donating methoxy substituents in the ortho- (CpPh2Ar−o) and para- (CpPh2Ar−p) positions of one of the phenyl rings have been synthesized. Different structural types, including tetranuclear [{CpPh2Ar−oTb (THF)}2 (μ2-Cl)2(μ3-Cl)3K (THF)]2 (Tb1), [{CpPh2Ar−oTb}2 (μ2-Cl)2 (μ3-Cl)3K]2 (Tb3), [{(CpPh2Ar−oTb)2 (μ2-Cl)3}2 (μ2-Cl)2] (Tb5), [{CpPh2Ar−pTb (THF)}2 (μ2-Cl)2 (μ3-Cl)3K (THF)]2 (Tb6), binuclear [{CpPh2Ar−oTb (THF)Cl}2 (μ2-Cl)2] (Tb4), [{CpPh2Ar−pTb (THF)}2 (μ2-Cl)2 (μ3-Cl)3K (THF)]2 (Tb7), [CpPh2Ar−p2Tb (μ2-Cl) (μ3-Cl)K]2 (Tb8) and mononuclear [CpPh2Ar−o2TbCl] (Tb2) complexes have been obtained and confirmed by single crystal X-ray diffraction analysis. All designed terbium complexes exhibit high quantum yields of the photoluminescence (up to 65 %). The introduction of a methoxy group into one of the phenyl rings of the triarylcyclopentadienyl ligand leads to a more than twofold increase in the quantum yield of the terbium ion luminescence. The estimated values of radiative rate krad of the terbium ion vary in the range of 0.23–0.69 ms. The presence of the methoxy group as well as the mutual disposition of the phenyl rings promotes the appearance of an intraligand charge transfer states involved in the energy transfer process. Due to a lowering of the energy of the interconfigurational 4f −5d transitions caused by the relatively strong crystal field of the Cp ligand in bis(cyclopentadienyl) terbium complexes, the quantum yield of the photoluminescence is high at the short lifetimes of the 5D4 level of the terbium ion.

Influence of η1‐Acetylide Ligands on the Spin State and Catalytic Activity of Iron(II) PNP Pincer Complexes

Here, we report the preparation, structures and reactivity of iron(II) PNP pincer complexes bearing one, two or three η1‐acetylide ligands bound to the metal center. The described complexes are accessible through the reaction of either one, two or ≥ 3 equivalent of LiCCR (R = SiMe3, Ph) with the dibromide complex [Fe(PNP)Br2] (1). Through increasing the number of η1‐acetylides the overall ligand field strength can be modulated resulting in S = 2, S = 1 and S = 0 spin states for mono‐, bis‐ and tris‐acetylated complexes, respectively. These findings are supported by DFT calculations on the spin state spitting in these complexes and appear to affect their binding affinity towards additional co‐ligands. The Fe(II) mono‐, bis‐ and tris‐acetylides were tested as catalyst in the catalytic dimerization of terminal alkynes. These test reactions revealed that only the bis‐acetylated complexes are catalytically competent while mono‐acetylated species are inactive. The tris‐acetylated complexes were found to be off‐cycle intermediates in the presence of lithium acetylides.

Retraction Note: Towards superior structural, optical and magnetic properties, and ligand field effects of B2O3–ZnO–CoO glass doped with alkaline metal earth cations

Retraction Note: Towards superior structural, optical and magnetic properties, and ligand field effects of B2O3–ZnO–CoO glass doped with alkaline metal earth cations

Unraveling the Stability and Magnetic Properties of Bis-Hydrated Mn(II) Complexes via Tailored Ligand Design.

Exploring the electronic structure and dynamic behavior of Mn(II) complexes reveals fascinating magnetic properties and prospective biomedical applications. In this study, we investigate the solvent phase dynamics of heptacoordinated Mn(II) complexes through ab initio molecular dynamics simulations and density functional theory (DFT) calculations with effectively varying temperatures. We observed that the complex with high stability ([Mn(pmpa)(H2O)2]) remains relatively rigid as the temperature increases to 90 °C, with only a minor change in its radial distribution functions (RDFs), compared to the RDF peaks at 25 °C. To elucidate the impact of halogens on the magnetic anisotropy of seven-coordinated Mn(II) complexes, we performed both DFT and multireference calculations. This shows that the zero-field splitting (ZFS) parameter D follows the order D(I)> D(Br)> D(Cl). We observed a significant increase in the D-value following the substitution of soft Se-donors in the equatorial position and heavier halogens in the axial position. The D-value of halogen derivatives of Se-analogues varies in the order of D(Cl) < D(I) < D(Br), deviating from the regular spectrochemical series with the discrepancy between the covalency of the Mn(II)-Se bond and the ligand field strength. We anticipate that this study will enhance our understanding of the solvent phase dynamics and structural aspects of ZFS in various Mn(II) complexes with different electronic environments.

Nanocrystallized copper phosphate endowed with photothermal conversion and photoacoustic imaging capabilities

Copper-based nanomaterials have garnered significant attention for their potential applications in photothermal (PT) conversion and photoacoustic (PA) imaging due to their unique optical properties. However, current strategies often involve complex synthesis processes and the incorporation of additional materials, which limit their practical applicability. Here, we employed a straightforward nanocrystallization method to synthesize copper phosphate nanoparticles (Cu3(PO4)2 NPs), converting free copper ions into solid-phase nano-sized particles. This approach not only simplifies the synthesis process but also enhances the optical and thermal properties of the resulting Cu3(PO4)2 NPs. The Cu3(PO4)2 NPs achieved a temperature increase of over 20∘C and a PT conversion efficiency of 10.5% under 808 nm laser irradiation. Furthermore, Cu3(PO4)2 NPs exhibited strong, concentration-dependent PA signals, which enabled effective deep tissue imaging in vivo. The enhanced properties of Cu3(PO4)2 NPs can be attributed to the nanocrystallization of the particles, which alters the local environment of copper ions and enhances d−d transitions through specific ligand field effects. These effects increase light absorption, particularly in the NIR region, by facilitating better overlap of electronic states and enabling more efficient non-radiative relaxation processes. This highlights the significance of nano-structuring in developing advanced nanomaterials with tailored functionalities for biomedical applications.

Ligand field exciton annihilation in bulk CrCl3.

The layered van der Waals material CrCl3 exhibits very strongly bound ligand field excitons that control optoelectronic applications and are connected with magnetic ordering by virtue of their d-orbital origin. Time-resolved photoluminescence of these exciton populations at room temperature shows that their relaxation is dominated by exciton-exciton annihilation and that the spontaneous decay lifetime is very long. These observations allow the rough quantification of the exciton annihilation rate constant and contextualization in light of a recent theory of universal scaling behavior of the annihilation process.