Shape and Spin State in Four-Coordinate Transition-Metal Complexes: The Case of the d6 Configuration

Levamisole Based Co(II) Single-Ion Magnet

A new Co(II) complex, [Co(NCS)2(L)2] (1) has been synthesized based on levamisole (L) as a new ligand. Single-crystal X-ray diffraction analyses confirm that the Co(II) ion is having a distorted tetrahedral coordination geometry in the complex. Notably strong intramolecular S⋅⋅⋅S and S⋅⋅⋅N interactions has been confirmed by employing Quantum Theory of Atoms in Molecules (QTAIM). These intramolecular interactions occur among the sulfur and nitrogen atoms of the levamisole ligands and also the nitrogen atoms of the thiocyanate. Direct current (dc) magnetic analyses reveal presence of zero field splitting (ZFS) and large magnetic anisotropy on Co(II). Detailed ab initio ligand field theory calculations quantitatively predicted the magnitude of ZFS. Prominent field-induced single-ion magnet (SIM) behavior was observed for 1 from dynamic magnetization measurements. Slow magnetic relaxation follows an Orbach mechanism with the effective energy barrier Ueff=29.6 (7) K and relaxation time τo=1.4 (4)×10-9 s.

Selective fluoride sensing by a novel series of lanthanide-based one-dimensional coordination polymers through intramolecular proton transfer

A novel series of one-dimensional coordination polymers (CPs) is achieved via a facile one-pot synthesis strategy employing the nitrate salts of trivalent lanthanides, a pentadentate chelating ligand, and triphenylphosphine oxide at a controlled stoichiometry under ambient conditions. All the CPs are characterized comprehensively using spectroscopic, X-ray crystallographic and magnetometric studies. The CPs are found to be thermally stable up to a significantly high temperature and resistant to water for an indefinite time. They are photoactive and exhibit selective fluoride ion (F-) sensing with excellent efficiency both colorimetrically and fluorimetrically in the solid-state as well as in solution. The presence of F- concomitantly sensitizes the photoluminescence enhancement and visual decolourization of the CPs in solution owing to the ground-state intra-molecular proton transfer. The photophysical response of the CPs to F- in solution was found to be instantaneous (<30 s). The sensitivity of detection is observed to be significantly high over a wide range of F- concentrations, covering the beneficial and detrimental domains of F- concentrations in drinking water. The limit of detection (LoD) under ambient conditions was found to be in the micromolar (μM) range-the best being 0.22 μM found using UV-vis spectrometry and 7.5 μM using fluorimetry. In comparison, the USEPA standard cut-off for the upper limit of F- concentration in drinking water is 211 μM, and the LoD of measuring F- concentration using the USEPA standard method using a fluoride-selective electrode is 26.3 μM. The CPs display markedly high selectivity toward F- with negligible-to-no interference from the commonly abundant ions (Cl-, Br-, I-, CH3CO2-, CO32-, SO42-, HPO42-, NH4+, Na+, K+, Mg2+, and Ca2+) in terms of UV-vis spectral change. Moreover, they also exhibit solid-state IR-spectrometric sensitivity towards F- under ambient conditions.

The Divalent Lanthanoid Triflates Ln(CF3SO3)2(CH3CN) (Ln=Sm, Eu): Structure, Luminescence, and Magnetism

The reaction of the trivalent lanthanoide triflates Ln(OTf)3 (Ln=Sm, Eu; OTf=CF3SO3 -) with the respective metals in acetonitrile leads to the Ln(II)-triflates Eu(OTf)2(CH3CN) (monoclinic, P21/n, Z=4, a=1053.54(1), b=610.28(5), c=1946.92(2) pm, β =98.611(4)) and Sm(OTf)2(CH3CN) (monoclinic, P21/n, Z=4, a=1054.41(4), b=612.16(2), c=1952.65(7) pm, β =98.524(2)). The isotypic strontium compound Sr(OTf)2(CH3CN) (monoclinic, P21/n, Z=4, a=1056.39(5), b=610.05(3), c=1950.1(1) pm, β =98.900(2)°) has been obtained from SrCO3 and triflic acid. The compounds have been investigated by X-ray diffraction, vibrational spectroscopy, luminescence spectroscopy, cyclic voltammetry, thermal analysis, and magnetic measurements.

Unusually short unsupported Au(III)⋯Au(III) aurophilic contacts in emissive lanthanide tetracyanoaurate(III) complexes

A series of [Au(CN)4]- salts with lanthanide 2,2'-bipyridine dioxide cations features Au(III) aurophilic interactions between [Au(CN)4]- groups, with Au⋯Au distances of 3.3603(4) Å and 3.4354(4) Å that are shorter than any previously reported. Computations predict the interactions to be weakly attractive; packing effects appear to also contribute to the close contacts. The materials are emissive: there is no Au(III)-based luminescence, but for Ln = Eu the PLQY of 29% is surprisingly high compared to related analogues.

A very rare fluorescent chemosensor of zinc(II) exhibiting AIEE, ESIPT and TICT: Spectroscopic, crystallographic and theoretical exploration

The basic systems in this study are HL (1; 1:2 condensation product of 2,6-diformyl-4-ethylphenol and o-anisidine) and its ZnII and CdII complexes of composition [ZnII(LH)Cl2]·CH3OH (2) and [CdII(LH)Cl2] (3), all of which are synthesized and characterized by CHN elemental analyses, single crystal X-ray crystallography, powder X-ray diffraction (PXRD) and fourier transform infrared (FT-IR) spectrum. It has been established from the following experimental and theoretical studies that 1 is a fluorescent turn on sensor of ZnII ion and it exhibits all of excited state intramolecular proton transfer (ESIPT), photoinduced electron transfer (PET), twisted intramolecular charge transfer (TICT) and aggregation induced enhanced emission (AIEE): (i) Detailed absorption and emission (steady state / time resolved) studies in various single solvents, in solvent mixtures, with pH variation, with various single metal ions, with mixtures of metal ions, on varying temperature and on varying viscosity; (ii) dynamic light scattering (DLS) and scanning electron microscopy (SEM) in solvent mixtures; (iii) density functional theory (DFT) and time dependent density functional theory (TD-DFT) calculations in ground and excites states of 1-3. It is shown that 1 can be efficaciously applied in inkless writing with the "write - erase - write" facility. The mechanisms/reasons of the observed properties have been addressed. The difference in fluorescence of ZnII and CdII complexes, unusual case of crystal structures of probe and complexes with ZnII and CdII, unusual features in the structures of 2 and 3 as well as a structure-property correlation have been discussed.

Controlling Electron Delocalization in Vanadium-Based Hybrid Bronzes through Molecular Templation

We show that the conductivity of hybrid vanadium bronzes-mixed-valence organic-inorganic vanadium oxides-can be tuned over six orders of magnitude through judicious choice of molecular component. By systematically varying the steric profile, charge density, and propensity to hydrogen bond across a series of eight diammonium-based molecules, we engender multiple distinct motifs of V-O connectivity within the two-dimensional vanadium oxide layers of a family of bulk crystalline hybrid materials. A combination of single-crystal and powder X-ray diffraction analysis, variable-temperature electrical transport measurements, and a range of spectroscopic methods, including UV/Visible diffuse reflectance, X-ray photoelectron, and electron paramagnetic resonance are employed to probe how vanadium oxide layer topology correlates with electron localization. Specifically, alkylammonium molecules yield hybrids featuring more corrugated layers that contain V-O tetrahedra as well as a higher ratio of corner-sharing to edge-sharing polyhedra and that exhibit highly localized electronic behavior, while alkyl bipyridinium molecules yield more regular layers with polyhedral edge-sharing that show substantially delocalized electronic behavior. This work allows for the development of design principles based on structure-property relationships and brings the charge transport capabilities of hybrid vanadium bronzes to more technologically relevant levels.

Geometry and UV-Vis Spectra of Au3+ Complexes with Hydrazones Derived from Pyridoxal 5'-Phosphate: A DFT Study

Gold(III) complexes with different ligands can provide researchers with a measure against pathogenic microorganisms with antibiotic resistance. We reported in our previous paper that the UV-Vis spectra of different protonated species of complexes formed by gold(III) and five hydrazones derived from pyridoxal 5'-phosphate are similar to each other and to the spectra of free protonated hydrazones. The present paper focuses on the reasons of the noted similarity in electron absorption spectra. The geometry of different protonated species of complexes of gold(III) and hydrazones (15 structures in total) was optimized using the density functional theory (DFT). The coordination polyhedron of gold(III) bond critical points were further studied to identify the symmetry of the gold coordination sphere and the type of interactions that hold the complex together. The UV-Vis spectra were calculated using TD DFT methods. The molecular orbitals were analyzed to interpret the calculated spectra.

Tuning symmetry and magnetic blocking of an exchange-coupled lanthanide ion in isomeric, tetrametallic complexes: [LnCl6(TiCp2)3]

The synthesis and magnetic properties of two pairs of isomeric, exchange-coupled complexes, [LnCl₆(TiCp₂)₃] (Ln = Gd, Tb), are reported. In each isomeric pair, the central lanthanide ion adopts either a pseudo-octahedral (O-Ln) or trigonal prismatic geometry (TP-Ln) yielding complexes with C ₁ or C _3h molecular symmetry, respectively. Ferromagnetic exchange coupling is observed in TP-Ln as indicated by the increases in χ _m T below 30 K. For TP-Gd, a fit to the susceptibility reveals ferromagnetic coupling between the Gd³⁺ ion and the Ti³⁺ ions (J = 2.90(1) cm^-1). In contrast to O-Tb, which shows no single-molecule magnetic behavior, the TP-Tb complex presents slow magnetic relaxation with a 100s-blocking temperature of 2.3 K and remanent magnetization at zero field up to 3 K. The calculated electronic structures of both compounds imply that trigonal prismatic geometry of TP-Tb is critical to the observed magnetic behavior.

Analysis of the First Ion Coordination Sphere: A Toolkit to Analyze the Coordination Sphere of Ions

Rapid and accurate approaches to characterizing the coordination structure of an ion are important for designing ligands and quantifying structure-property trends. Here, we introduce AFICS (Analysis of the First Ion Coordination Sphere), a tool written in Python 3 for analyzing the structural and geometric features of the first coordination sphere of an ion over the course of molecular dynamics simulations. The principal feature of AFICS is its ability to quantify the distortion a coordination geometry undergoes compared to uniform polyhedra. This work applies the toolkit to analyze molecular dynamics simulations of the well-defined coordination structure of aqueous Cr³⁺ along with the more ambiguous structure of aqueous Eu³⁺ chelated to ethylenediaminetetraacetic acid. The tool is targeted for analyzing ions with fluxional or irregular coordination structures (e.g., solution structures of f-block elements) but is generalized such that it may be applied to other systems.

Magnetic Anisotropy and Relaxation of Pseudotetrahedral [N2 O2 ] Bis-Chelate Cobalt(II) Single-Ion Magnets Controlled by Dihedral Twist Through Solvomorphism

The methanol solvomorph 1 ⋅ 2MeOH of the cobalt(II) complex [Co(L^Sal,2-Ph )₂ ] (1) with the sterically demanding Schiff-base ligand 2-(([1,1'-biphenyl]-2-ylimino)methyl)phenol (HL^Sal,2-Ph ) shows the thus far largest dihedral twist distortion between the two chelate planes compared to an ideal pseudotetrahedral arrangement. The cobalt(II) ion in 1 ⋅ 2MeOH exhibits an easy-axis anisotropy leading to a spin-reversal barrier of 55.3 cm^-1 , which corresponds to an increase of about 17 % induced by the larger dihedral twist compared to the solvent-free complex 1. The magnetic relaxation for 1 ⋅ 2MeOH is significantly slower compared to 1. An in-depth frequency-domain Fourier-transform (FD-FT) THz-EPR study not only allowed the direct measurement of the magnetic transition between the two lowest Kramers doublets for the cobalt(II) complexes, but also revealed the presence of spin-phonon coupling. Interestingly, a similar dihedral twist correlation is also observed for a second pair of cobalt(II)-based solvomorphs, which could be benchmarked by FD-FT THz-EPR.

Porous and Stable Zn-Series Metal-Organic Frameworks as Efficient Catalysts for Grafting Wood Nanofibers with Polycaprolactone via a Copolymerization Approach

A hydrothermal method was used to synthesize two highly stable Zn(II) metal-organic frameworks (MOFs), namely, [Zn₂(L)₂(HIPA)]_n (1) and [Zn₉(L)₆(BTEC)₃(H₂O)₄·6H₂O]_n (2) (HL = 3-amino-1H-1,2,4-triazole, H₂HIPA = 5-hydroxyisophthalic acid, H₄BTEC = benzene-1,2,4,5-tetracarboxylic acid). The physicochemical properties of 1 and 2 were characterized using a range of analytical techniques. The scanning electron microscopy images confirmed the stability of the MOFs under heating at 120 °C for 12 h. Following their preparation, the two MOFs were used as catalysts in the grafting of poly(ε-caprolactone) on wood nanofibers (WNFs) by means of a homogeneous ring-opening polymerization protocol in an ionic liquid. The grafting ratio achieved using catalyst 1 was higher than that achieved for catalyst 2, wherein a maximum of 92.43% was obtained using the former. Under comparable reaction conditions, the grafting ratio of 1 was found to be significantly higher than those achieved using 4-dimethylamino pyridine, Sn(Oct)₂, and UiO-67 catalysts. In addition, fluorescence emission was detected from the residual catalysts present in the products. The calculated electrostatic potentials and average local ionization energies indicated that the grafting of ε-caprolactone on the WNFs follows a "coordination-insertion" mechanism. Overall, these two new and efficient MOF catalysts have the potential to replace highly toxic traditional catalysts in polymerization reactions. The grafted cellulose material with fluorescence emission may also be suitable for use in biomedical applications.

[ReH3 (PPh3 )4 ] - A Key Compound in the Rhenium Hydride Chemistry

The chemistry of the rhenium trihydrido complex [ReH₃ (PPh₃ )₄ ] (1) has been reinvestigated. An improved synthesis and the solid-state structure of the compound as well as several reactions are reported. The solid-state structure of 1 is similar to that of [TcH₃ (PPh₃ )₄ ] having a capped-octahedral coordination sphere. The PPh₃ ligands surround the Re atom in a trigonal-pyramidal mode with a short apical Re-P bond (2.300(2) Å) and three longer basal bonds (2.429(2)-2.449(2) Å). Reactions of 1 with monodentate phosphines such as PMe₃ or PBu₃ give the mono-substituted complexes [ReH₃ (PPh₃ )₃ (PMe₃ )] (2) and [ReH₃ (PPh₃ )₃ (PBu₃ )] (3) under retention of the apical PPh₃ ligand and substitution of one of the basal PPh₃ ligands. The stability of the phosphine trihydride complexes decreases in the order PPh₃ >PMe₃ >PBu₃ . Treatment of [ReH₃ (PPh₃ )₄ ] with trityl hexafluorophosphate in CH₃ CN does not result in a hydride abstraction, but gives the tetrahydrido cation [ReH₄ (NCCH₃ )(PPh₃ )₃ ]⁺ (4), while reactions with nitriles give unstable azavinylidene complexes of the composition [ReH₂ (PPh₃ )₃ (NC(H)R)] (5). They are formed by an insertion of the nitrile into a Re-H bond. The solid-state structure of the methyl derivative [ReH₂ (PPh₃ )₃ (NC(H)CH₃ )] (5 a) was determined showing a linear Re-N-C unit with rhenium-nitrogen and nitrogen-carbon double bonds, while the N=CH-C bond is clearly bent with an angle of 124°. Two previously unknown polymorphs of [ReH₅ (PPh₃ )₃ ] were isolated from reactions of 1 with HOC₆ H₃ (CH₃ )₂ and thiourea after prolonged heating in toluene and characterized by IR spectroscopy and X-ray diffraction.

Experimental and theoretical investigations on three DyIII4 single molecule magnets: structural and magneto-structural correlations

The work in this report describes the syntheses, crystal structures, dc/ac magnetic behaviour, and theoretical calculations (both ab initio CASSCF and DFT) of three defect dicubane/planar butterfly type tetradysprosium(III) compounds of compositions [Dy^III₄L₄(μ₃-OH)₂(carboxylate)₂(dmf)₂] (carboxylate = formate (1), acetate (2), propionate (3)), where H₂L = 2-(2-hydroxy-3-ethoxybenzylideneamino)phenol. In the butterfly type structures, two Dy^III centres (Dy_b) occupy the body positions while two other (Dy_w) units occupy the wing positions. SHAPE analyses reveal that the coordination geometries of the Dy_b and Dy_w centres, both octacoordinated, are triangular dodecahedron (TDD) and square antiprism (SAPR), respectively. Variable-temperature magnetic susceptibility measurements give an indication of weak antiferromagnetic interactions and variable-field magnetization measurements reveal strong anisotropy in all the three compounds. The variable-temperature/frequency in-phase/out-of-phase AC susceptibility data reveal that all these three compounds are SMMs with two relaxation channels under zero dc field; slow relaxation (SR) and fast relaxation (FR) processes could be assigned to the SAPR (Dy_w) and TDD (Dy_b) metal centres, respectively. The simulated U_eff and τ₀ values are: 49.0 cm^-1 and 1.76 × 10^-7 s for 1, 30.3 cm^-1 and 1.51 × 10^-8 s for 2 and 23.4 cm^-1 and 9.64 × 10^-7 s for 3. Furthermore, ab initio CASSCF/RASSI-SO/SINGLE_ANISO calculations reveal that the ground state of Dy^III centres are axial in nature with a dominating contribution from m_J = |±15/2>. The magnetization relaxation occurs via the first excited KD resulting in the large computed blocking barrier of Dy_w (SAPR) centres compared to that of the Dy_b (TDD) centres which corroborates the experimental measurements. The exchange parameters obtained from DFT calculations are generally in line with those obtained from the fitting of χ_MT vs. T in POLY_ANISO calculations. Interesting structural and magneto-structural correlations have been found, which are the major outcomes of this investigation.

Rigid N3O2-Pentadentate Ligand-Assisted Octacoordinate Mononuclear Ln(III) Complexes: Syntheses, Characterization, and Slow Magnetization Relaxation

A series of air-stable mononuclear octacoordinate Ln(III) complexes, [(L)Ln(TPPO)₃]OTf (Ln = Y (1·Y); Gd (1·Gd); Tb (1·Tb); Dy (1·Dy); Ho (1·Ho); and Er (1·Er)) and [(L)Ln(TPPO)(NO₃)] (Ln = Y (2·Y) and Dy (2·Dy)), are synthesized employing a rigid N₃O₂-pentadentate chelating ligand as the basis ligand and meridional ancillary ligands (where H₂L = 2,6-diacetylpyridine bis-benzoylhydrazone, TPPO = triphenylphosphine oxide, and OTf^- = trifluoromethanesulfonate). All the complexes are synthesized under aerobic conditions and characterized comprehensively by spectroscopic and X-ray crystallographic techniques. Magnetic property investigation on the polycrystalline solid samples of 1·Ln (Ln = Gd, Tb, Dy, Ho, and Er) and 2·Dy are reported. A field-induced single-molecule magnet behavior was observed for the Dy derivatives. 1·Dy exhibits the highest effective energy barrier of magnetization reversal, U _eff/k _B = 47 K under H _dc = 1 kOe among the complexes presented herein.

Versatile Macrocyclic Platform for the Complexation of [natY/90Y]Yttrium and Lanthanide Ions

We report a macrocyclic ligand (H3L6) based on a 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane platform containing three acetate pendant arms and a benzyl group attached to the fourth nitrogen atom of the macrocycle. The X-ray structures of the YL6 and TbL6 complexes reveal nine coordination of the ligand to the metal ions through the six nitrogen atoms of the macrocycle and three oxygen atoms of the carboxylate pendants. A combination of NMR spectroscopic studies (1H, 13C, and 89Y) and DFT calculations indicated that the structure of the YL6 complex in the solid state is maintained in an aqueous solution. The detailed study of the emission spectra of the EuL6 and TbL6 complexes revealed Ln3+-centered emission with quantum yields of 7.0 and 60%, respectively. Emission lifetime measurements indicate that the ligand offers good protection of the metal ions from surrounding water molecules, preventing the coordination of water molecules. The YL6 complex is remarkably inert with respect to complex dissociation, with a lifetime of 1.7 h in 1 M HCl. On the other hand, complex formation is fast (∼1 min at pH 5.4, 2 × 10-5 M). Studies using the 90Y-nuclide confirmed fast radiolabeling since [90Y]YL6 is nearly quantitatively formed (radiochemical yield (RCY) > 95) in a short time over a broad range of pH values from ca. 2.4 to 9.0. Challenging experiments in the presence of excess ethylenediaminetetraacetic acid (EDTA) and in human serum revealed good stability of the [90Y]YL6 complex. All of these experiments combined suggest the potential application of H3L6 derivatives as Y-based radiopharmaceuticals.

Schiff Base Tetranuclear Zn2Ln2 Single-Molecule Magnets bridged by Hydroxamic acid in association with Near-Infrared Luminescence

A series of Zn-Ln heteronuclear SMMs formed by hexadentate compartment Schiff base Zn-precursor and lanthanoid ions were structurally and magnetically characterized, in which the two [Zn-Ln] moieties are bridged by...

Beyond Thermodynamics: Assessing the Dynamical Softness of Hydrated Ions from First Principles

Ion (de)hydration is a key rate-determining step in interfacial processes from corrosion to electrochemical energy storage. However, predicting the kinetics of ion (de)hydration remains challenging, prompting the use of static proxies such as hydration energy and valence. While useful for assessing thermodynamic preferences, such descriptors cannot fully capture the dynamical softness of the hydration shell that dictates kinetics. Accordingly, we use first-principles molecular dynamics to analyze hydration shell softness for a diverse set of metal cations. Three dynamic metrics are introduced to intuitively describe the bond rigidity, shape deformability, and exchange fluidity of the solvation shell. Together, these metrics capture the relevant physics in the static descriptors, while offering a far more complete and efficient representation for the overall propensity for (de)hydration. Application to the hydrated ion set demonstrates a weak connection between dynamical softness and hydration energy, confirming that dynamical descriptors of hydration are key for correctly describing ion transfer processes.

Cis-Divacant Octahedral Fe(II) in a Dimensionally Reduced Family of 2-(Pyridin-2-yl)pyrrolide Complexes

Four-coordinate transition-metal complexes can adopt a diverse array of coordination geometries, with square planar and tetrahedral coordination being the most prevalent. Previously, we reported the synthesis of a trinuclear Fe(II) complex, Fe3TPM2, supported by a 3-fold-symmetric 2-pyridylpyrrolide ligand [i.e., tris(5-(pyridin-2-yl)-1H-pyrrol-2-yl)methane] that featured a rare cis-divacant octahedral (CDO) geometry at each Fe(II) center. Here, a series of truncated 2-pyridylpyrrolide ligands are described that support mono- and binuclear Fe(II) complexes that also exhibit CDO geometries. Metalation of the tetradentate ligand bis[5-(pyridin-2-yl)-1H-pyrrol-2-yl]methane (H2BPM) in tetrahydrofuran (THF) results in the binuclear complex Fe2(BPM)2(THF)2 in which both Fe(II) ions are octahedrally coordinated. The coordinated THF solvent ligands are labile: THF dissociation leads to Fe2(BPM)2, which features five-coordinate Fe(II) ions. The Fe-Fe distance in these binuclear complexes can be elongated by ligand methylation. Metalation of bis[5-(6-methylpyridin-2-yl)-1H-pyrrol-2-yl]methane (H2BPMMe) in THF leads to the formation of four-coordinate, CDO Fe(II) centers in Fe(BPMMe)2. Further ligand truncation affords bidentate ligands 2-(1H-pyrrol-2-yl)pyridine (PyrPyrrH) and 2-methyl-6-(1H-pyrrol-2-yl)pyridine (PyrMePyrrH). Metalation of these ligands in THF affords six-coordinate complexes Fe(PyrPyrr)2(THF)2 and Fe(PyrMePyrr)2(THF)2. Dissociation of labile solvent ligands provides access to four-coordinate Fe(II) complexes. Ligand disproportionation at Fe(PyrPyrr)2 results in the formation of Fe(PyrPyrr)3 and Fe(0). Ligand methylation suppresses this disproportionation and enables isolation of Fe(PyrMePyrr)2, which is rigorously CDO. Complete ligand truncation, by separating the 2-pyridylpyrrolide ligands into the constituent monodentate pyridyl and pyrrolide donors, affords Fe(Pyr)2(Pyrr)2 in which Fe(II) is tetrahedrally coordinated. Computational analysis indicates that the potential energy surface that dictates the coordination geometry in this family of four-coordinate complexes is fairly flat in the vicinity of CDO coordination. These synthetic studies provide the structural basis to explore the implications of CDO geometry on Fe-catalyzed reactions.

Improving the single-molecule magnet properties of two pentagonal bipyramidal Dy3+ compounds by the introduction of both electron-withdrawing and -donating groups

Two mononuclear Dy³⁺ compounds [Dy(bmbpen-F)X] (X = Cl, 1; Br, 2) with a pentagonal bipyramidal (PBP) geometry were obtained from N,N'-bis-(5-methyl-2-hydroxybenzyl)-N,N'-bis(5-fluoro-2-methylpyridyl)ethylenediamine (H₂bmbpen-F) and dysprosium halides. The magnetic anisotropy and single-molecule magnet (SMM) behavior of these PBP compounds were regulated by introducing both electron-withdrawing F atoms into the equatorial pyridine rings and electron-donating -CH₃ groups into the axial phenolic hydroxyl rings. The results of magnetic characterization show that 1 and 2 exhibit single molecule magnet behavior with magnetization reversal barriers of 990(13) and 1189(16) K under a zero dc external field and magnetic hysteresis loops up to 26 K and 36 K, respectively. The results of ab initio calculations are consistent with the experimental observations, confirming that the simultaneous introduction of electron-withdrawing groups into the equatorial positions and electron-donating groups into the axial positions can lead to PBP Dy-SMMs with improved properties.

Theoretical investigation of tetrahedral distortion of four-coordinate iron(II) centres in FePd(CN)4

The tetrahedral distortion of iron(ii) centres in the cyanide-bridged framework FePd(CN)4 was recently demonstrated experimentally. Here, we theoretically confirmed the electronically driven tetrahedral distortion of iron(ii) by comparing the density of states and total energies of FePd(CN)4 (d6) and ZnPd(CN)4 (d10). The calculation results suggested that a Jahn-Teller-like effect is caused on the tetrahedral geometry by the electronic effect of unequally occupied non-bonding 3d orbitals in the corresponding structure.

Responsive Four-Coordinate Iron(II) Nodes in FePd(CN)4

Metal node design is crucial for obtaining structurally diverse coordination polymers (CPs) and metal-organic frameworks with desirable properties; however, Fe^II ions are exclusively six-coordinated. Herein, we present a cyanide-bridged three-dimensional (3D) CP, FePd(CN)₄ , bearing four-coordinate Fe^II ions, which is synthesized by thermal treatment of a two-dimensional (2D) six-coordinate Fe^II CP, Fe(H₂ O)₂ Pd(CN)₄ ⋅4 H₂ O, to remove water molecules. Atomic-resolution transmission electron microscopy and powder X-ray and neutron diffraction measurements revealed that the FePd(CN)₄ structure is composed of a two-fold interpenetrated PtS topology network, where the Fe^II center demonstrates an intermediate geometry between tetrahedral and square-planar coordination. This four-coordinate Fe^II center with the distorted geometry can act as a thermo-responsive flexible node in the PtS network.

The Tyranny of Arm-Wrestling Methyls on Iron(II) Spin State in Pseudo-Octahedral [Fe(didentate)3] Complexes

The connection of a sterically constrained 3-methyl-pyrazine ring to a N-methyl-benzimidazole unit to give the unsymmetrical α,α’-diimine ligand L5 has been programmed for the design of pseudo-octahedral spin-crossover [Fe(L5)3]2+ units, the transition temperature (T1/2) of which occurs in between those reported for related facial tris-didentate iron chromophores fitted with 3-methyl-pyridine-benzimidazole in a LaFe helicate (T1/2 ~ 50 K) and with 5-methyl-pyrazine-benzimidazole L2 ligands (T1/2 ~350 K). A thorough crystallographic analysis of [Fe(L5)3](ClO4)2 (I), [Ni(L5)3](ClO4)2 (II), [Ni(L5)3](BF4)2∙H2O (III), [Zn(L5)3](ClO4)2 (IV), [Ni(L5)3](BF4)2∙1.75CH3CN (V), and [Zn(L5)3](BF4)2∙1.5CH3CN (VI) shows the selective formation of pure facial [M(L5)3]2+ cations in the solvated crystals of the tetrafluoroborate salts and alternative meridional isomers in the perchlorate salts. Except for a slightly larger intra-strand interannular twist between the aromatic heterocycles in L5, the metric parameters measured in [Zn(L5)3]2+ are comparable to those reported for [Zn(L2)3]2+, where L2 is the related unconstrained ligand. This similitude is reinforced by comparable ligand-field strengths (∆oct) and nephelauxetic effects (as measured by the Racah parameters B and C) extracted from the electronic absorption spectra recorded for [Ni(L5)3]2+ and [Ni(L2)3]2+. In this context, the strictly high-spin behavior observed for [Fe(L5)3]2+ within the 5–300 K range contrasts with the close to room-temperature spin-crossover behavior of [Fe(L2)3]2+ (T1/2 = 349(5) K in acetonitrile). This can be unambiguously assigned to an intraligand arm wrestling match operating in bound L5, which prevents the contraction of the coordination sphere required for accommodating low-spin FeII. Since the analogous 3-methyl-pyridine ring in [Fe(L3)3]2+ derivatives are sometimes compatible with spin-crossover properties, the consequences of repulsive intra-strand methyl–methyl interactions are found to be amplified in [Fe(L5)3]2+ because of the much lower basicity of the 3-methyl-pyrazine ring and the resulting weaker thermodynamic compensation. The decrease of the stability constants by five orders of magnitude observed in going from [M(L2)3]2+ to [M(L5)3]2+ (M = NiII and ZnII) is diagnostic for the operation of this effect, which had been not foreseen by the authors.

Monitoring Fe(II) Spin-State Equilibria via Eu(III) Luminescence in Molecular Complexes: Dream or Reality?

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and noninvasive monitoring of magnetic information storage. In order to approach this goal at the molecular level, three segmental ligand strands, L4-L6, were reacted with stoichiometric mixtures of divalent d-block cations (M(II) = Fe(II) or Zn(II)) and trivalent lanthanides (Ln(III) = La(III) or Eu(III)) in acetonitrile to give C₃-symmetrical dinuclear triple-stranded helical [LnM(Lk)₃]⁵⁺ cations, which can be crystallized with noncoordinating counter-anions. The divalent metal M(II) is six-coordinate in the pseudo-octahedral sites produced by the facial wrapping of the three didentate binding units, the ligand field of which induces variable Fe(II) spin-state properties in [LnFe(L4)₃]⁵⁺ (strictly high-spin), [LnFe(L5)₃]⁵⁺ (spin-crossover (SCO) around room temperature), and [LnFe(L6)₃]⁵⁺ (SCO at very low temperature). The introduction of the photophysically active Eu(III) probe in [EuFe(Lk)₃]⁵⁺ results in europium-centered luminescence modulated by variable intramolecular Eu(III) → Fe(II) energy-transfer processes. The kinetic analysis implies Eu(III) → Fe(II) quenching efficiencies close to 100% for the low-spin configuration and greater than 95% for the high-spin state. Consequently, the sensitivity of indirect luminescence detection of Fe(II) spin crossover is limited by the resulting weak Eu(III)-centered emission intensities, but the dependence of the luminescence on the temperature unambiguously demonstrates the potential of indirect lanthanide-based spin-state monitoring at the molecular scale.

Heterometallic 3d–4f {Co2Gd4} phosphonates: new members of the potential magnetic cooler family

Three new 3d–4f heterometallic phosphonate complexes with the formula [Na₂CoIII2Gd₄(L)₆(bpy)₂(OR)₂]·nSolv (L = SAA³⁻(1), BSAA³⁻(2), and NAA³⁻(3); bpy = 2,2′-bipyridine; OR = OH and OMe; Solv = H₂O and MeOH) were isolated and studied in detail.

One-dimensional lanthanide coordination polymers supported by pentadentate Schiff-base and diphenyl phosphate ligands: single molecule magnet behavior and photoluminescence

Four one-dimensional lanthanide coordination polymers constructed from Ln₂ dimers showing single molecule magnet behavior and photoluminescence are reported.

Field-induced slow magnetic relaxation in two-dimensional and three-dimensional Co(ii) coordination polymers

Two coordination polymers formulated as [Co(1,4-bimb)_0.5(5-aip)(H₂O)]_n (1) and [Co(1,4-bib)_1.5(5-hip)(H₂O)]_n (2) (1,4-bimb = 1,4-bis(imidazol-1-ylmethyl)benzene, 5-aip = 5-aminoisophthalic acid, 1,4-bib = 1,4-bis(1-imidazolyl)benzene and 5-hip = 5-hydroxyisophthalic acid) have been prepared and structurally characterized. Complex 1 is a two-dimensional (2D) network where Co(ii) is six coordinate in a CoO₄N₂ coordination environment, while the structure of 2 consists of a three-dimensional (3D) framework built from mononuclear Co(ii) units with distorted octahedral geometry as nodes. Static magnetic studies show that first-order orbital angular momentum may play an important role in the magnetic properties of 1, whereas strong easy-axis anisotropy (D = -102 cm^-1) was observed in 2. Alternating current (ac) susceptibility measurements demonstrate that both the complexes display field-induced single ion magnet (SIM) behavior.

New examples of triangular terbium(iii) and holmium(iii) and hexagonal dysprosium(iii) single molecule toroics

The structural, magnetic and theoretical aspects are described for three triangular lanthanide complexes, [Tb(OH)(teaH₂)₃(paa)₃]Cl₂ (1), [Dy(OH)(teaH₂)₃(paa)₃]Cl₂ (2) and [Ho(OH)(teaH₂)₃(paa)₃]Cl₂ (3), and a hexanuclear wheel of formula [Dy(pdeaH)₆(NO₃)₆] (4) [teaH₃ = triethanolamine, paaH = N-(2-pyridyl)-acetoacetamide and pdeaH₃ = 3-[bis(2-hydroxyethyl)amino]propan-1-ol]. Each complex displays single molecule toroidal behaviour as rationalised using high-level ab initio calculations. Complexes 2 and 3 are the first examples of mixed moment single molecule toroidal complexes featuring non-Kramers ions.

A rare chloride-bridged dysprosium chain with slow magnetic relaxation: a thermally activated mechanism via a second-excited state promoted by magnetic interactions

Magnetic interactions induced by chloride bridges promote the slow magnetic relaxation in a dysprosium chain to pass through the second-excited state.

Structural and thermodynamic aspects of water–carbonate exchange equilibrium for MIII/IV–EDTA–carbonate systems

The carbonate anion readily displaces water molecules in the [Er(EDTA)(H₂O)₂]⁻ complex and though it is mainly an entropy driven process, the formation of the [Er(EDTA)(CO₃)]^3– complex is additionally enthalpy stabilized.

Exploration of SMM behavior of Ln2 complexes derived from thianaphthene-2-carboxylic acid

Observations were made of field-induced SMM property of a dysprosium-based dinuclear complex derived from thianaphthene-2-carboxylic acid ligand.

Mononuclear and trinuclear DyIII SMMs with Schiff-base ligands modified by nitro-groups: first triangular complex with a N–N pathway

Three Dy^III SMMs were obtained with Schiff-base ligands containing nitro-groups. The triangular complex supported by the cis–trans form of the hnc³⁻ ligand displayed the first air-stable triangular complex with a N–N pathway.