We report the synthesis, crystal structures, and magnetic properties of three Co(II) and two Ln(III) (Dy, Tb) complexes, all containing the tridentate ligand 2,6-bis(3-methoxypropyl-1H-benzimidazol-2-yl)pyridine (L). The pentacoordinate Co(II) complexes [Co(L)Cl2] (1) and [Co(L)Br2] (2) adopt distorted square-pyramidal geometries, while the heptacoordinate complexes [Co(L)(κ2-NO3)2] (3) contain two independent complex molecules with capped trigonal prismatic and pentagonal-bipyramidal shape of polyhedra. Lanthanide complexes [Dy(L)Cl3(H2O)] (4) and [Tb(L)Cl3(H2O)] (5) also adopt pentagonal-bipyramidal coordination environments and display characteristic 4f-4f emission bands in the solid state at 3 K. Ab initio calculations reproduce the observed anisotropy trends and rationalize the magnetic behaviour. Pentacoordinate Co(II) centres require an explicit Griffith-Figgis Hamiltonian approach, while heptacoordinate Co(II) complex 3 exhibits axial anisotropy rationalised by spin Hamiltonian formalism. Analysis of static magnetic properties of complexes 4 and 5 also revealed the notable magnetic anisotropy of the Ln(III) ions. AC susceptibility measurements reveal field-supported slow relaxation of magnetisation (SRM) in 1-4, with complexes 1-3 exhibiting single-channel relaxation and 4 displaying two relaxation pathways. In contrast, compound 5 shows no evidence of SRM. Analysis of the temperature dependence of the relaxation times yielded effective energy barrier (Ueff) values of 15.9(7) K for 1, 25.0(5) K for 2, 27(1) K for 3, and 33(3) K for 4. Comparative analysis across 3d and 4f systems highlights the crucial role of subtle ligand-field distortions and donor charge distribution in governing SIM performance. These findings expand the chemistry of pyridyl-benzimidazole-derived ligands and provide new design principles for both transition-metal and lanthanide single-ion magnets.

Correction for 'Unveiling field-induced single-ion magnetism in pentacoordinate and heptacoordinate cobalt(II), dysprosium(III) and terbium(III) complexes with a tridentate bis(benzimidazole)pyridine ligand' by Nikoleta Malinová et al., Dalton Trans., 2026, 55, 3975-3989, https://doi.org/10.1039/D5DT02849C.

We report the synthesis, structural characterization, and multifunctional properties of three new iron(II) spin-crossover (SCO) complexes based on the azo-functionalized ligand 2-{4-[trans-phenyldiazenyl]-pyridine-2-yl}-1H-benzimidazole (L): [Fe(L)3](ClO4)2·C3H6O (1), ([Fe(L)3](CF3SO3)2) (2) and [Fe(L)3](BF4)2·C6H14O (3). Single-crystal X-ray diffraction reveals isostructural [Fe(L)3]2+ cations with octahedral {FeN6} coordination environments, adopting a low-spin state at 100 K. Temperature-dependent magnetic measurements and Mössbauer spectroscopy demonstrate gradual thermal SCO behaviour, with transition temperatures tuneable by solvation and counter-anion effects. All three complexes exhibit light-induced excited spin-state trapping (LIESST) at low temperatures, with photo-conversion yields reaching up to 63%. The photoisomerization of the azo unit was investigated in solution and in solid thin films, revealing efficient trans → cis switching in solution and a remarkably stable cis form in the solid state under ambient conditions. Multireference computational studies supported by TDDFT calculations provide insight into the photoswitching mechanism and indicate possible triplet sensitisation of the azo bond located in the vicinity of the high-spin Fe(II) centre. Furthermore, the successful fabrication of Langmuir-Blodgett monolayer and multilayer films was demonstrated, with AFM and XPS confirming molecular-level film organization and preservation of metal-ligand coordination at the surface. Together, these results establish azo-functionalized pyridyl-benzimidazole ligands as a versatile platform for integrating thermal SCO, light responsiveness, and surface assembly, offering promising prospects for multifunctional molecular switches and surface-integrated photomagnetic devices.

4-cyclohexyl-1,2,4-triazole (4-C6trz) was synthesized and used as a ligand to prepare Fe(II) complex of the formula [FeII(4-C6trz)3](BF4)24H2O. The complex was characterized spectroscopically, magnetically, and thermally. The characterization of the complex proved the polymeric nature of the complex and the triazole ligand acted as a bridging ligand. The prepared structure has the formula confirmed to be [FeII(4-C6trz)3](BF4)24H2O. The chi T product of [FeII(4-C6trz)3](BF4)24H2O was almost constant over 200 - 400 K, and the measured value (3.6 cm3mol-1K) is expected to be the paramagnetic high-spin Fe(II) complexes (S = 2). Density Functional Theory (DFT) calculations were conducted on [FeII(4-C6trz)3](BF4)24H2O to rationalize the experimentally observed magnetic behavior measured using the magnetometer. The results predict that the 4-cyclohexyl-1,2,4-triazole (4-C6trz) ligand provides a pi-acceptor environment in the first coordination, which helps to show spin crossover transition with a transition temperature of 215 K.

A heptadentate 15-membered pyridine-based macrocyclic ligand containing two pyridine-N-oxide pendant arms (L4 = 3,12-bis((pyridine-1-oxide-2-yl)methyl)-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca-1(18),14,16-triene) was synthesized together with its first-row transition metal complexes with the general formula [M(L4)](ClO4)2·1DMF (MII = Mn (1), Fe (2), Co (3), and Ni (4); DMF = N,N'-dimethylformamide), which were thoroughly investigated. According to the obtained X-ray crystal structures, all complexes possess axially compressed pentagonal bipyramidal geometry with a coordination number of 7 for 1-3 or 5 + 2 for Ni(II) complex 4 with a large Jahn-Teller distortion. Fe(II), Co(II), and Ni(II) complexes 2, 3, and 4 show pronounced magnetic anisotropy (D = 4.47, 30.10, -7.58 cm-1, respectively). The magnetic properties of the studied complexes were supported by theoretical calculations, which corresponded very well to the experimental data for magnetic anisotropy. Furthermore, complex 3 showed a field-induced single-molecule magnet behavior described best by the combination of direct (DHm = 145 K-1s-1) and Raman (C = 0.58 K-ns-1 for n = 5.76) relaxation processes. Magneto-structural correlation for Fe(II)/Co(II)/Ni(II) complexes with L4 and previously studied structurally similar ligands revealed a significant impact of the coordination ability of the functional group in pendant arms on the final magnetic anisotropy (π-acceptors appear to be more suitable).

The synthesis and characterization of four new iron(III) coordination compounds with saltrien-like hexadentate Schiff base ligands Ln, prepared by condensation between triethylenetetramine and 2-hydroxy-3,5-dimethylbenzophenone (L1, C1-C3) or 2-hydroxy-5-methoxybenzophenone (L2, C4), are reported. The complexes [Fe(Ln)]X·mCH3CN (X = SeCN- for C1 and C4, SCN- for C2 and BPh4- for C3, m = 0 for C3, m = 1 for C1 and C4, m = 2 for C2) were structurally characterized, and their spin-crossover (SCO) was monitored by magnetic measurements, X-ray powder diffraction analysis, and EPR spectroscopy. Intermolecular interactions relevant to SCO were analyzed through Hirshfeld surface maps and QT-AIM calculations. All compounds exhibit SCO above room temperature in their solvated forms, and ab initio calculations were employed to probe their electronic structures. While the computed 2T2g-6A1g gaps and 10Dq energies are consistent across the whole series, the experimental T1/2 values do not directly reflect these energy differences. Instead, SCO is predominantly controlled by crystal packing effects, including intermolecular connectivity, internal pressure, lattice rigidity, and solvation. Upon heating, solvent removal in C1 and C2 shifts their SCO to below room-temperature. The desolvated compounds C1d and C2d exhibit sharp SCO with wide hysteresis, while C2d additionally features a second gradual step (C1d: T1/2 = 82 K/166 K; C2d: T(1)1/2 = 170 K/153 K, T(2)1/2 = 110 K). Furthermore, both compounds are LIESST active upon blue light irradiation (T(LIESST)=57 K for C1d and 36 K for C2d). These results underscore the crucial role of ligand flexibility, solvation, and intermolecular interactions on SCO and highlight the potential of these iron(III) complexes in molecular switching applications.

Three variants of 2-(pyridine-2-yl)-1H-benzimidazole bidentate ligands, with distinct pentyl substituents (n-pentyl (L1), (2S)-2-methylbutyl (L2S), and (2R)-2-methylbutyl (L2R)), were synthesized and employed to prepare magnetically bistable cobalt(ii) and iron(ii) complexes. The cobalt(ii) complexes [Co(L1)(kappa 2-NO3)2(CH3CN)] (1), [Co(L2S)(kappa 2-NO3)2(H2O)] (2S), [Co(L2R)(kappa 2-NO3)2(H2O)] (2R) and [Co(L2)(kappa 2-NO3)2(H2O)] (2rac) exhibit field supported slow relaxation of magnetisation, while the iron(ii) complexes [Fe(L2S)3](CF3SO3)2H2OCH3NO2C5H12O (3S) and [Fe(L2S)3]1.5H2O (3R) exhibit thermal spin crossover allocated above room temperature. Structural analysis revealed that the heptacoordinated cobalt(ii) complexes adopt a distorted pentagonal bipyramidal geometry, whereas the iron(ii) compounds contain hexacoordinated complex cations. Coordination compounds incorporating the chiral ligands L2S and L2R are optically active, with their enantiomeric relationship confirmed by circular dichroism spectroscopy. Computational studies provided insights into electron density distributions and bonding energetics (DFT and QT-AIM), predicted zero-field splitting parameters (CASSCF/NEVPT2), and quantified energies of the d-orbital, ligand field terms and their multiplets (via AILFT). Magnetic investigations of the cobalt(ii) complexes yielded experimental ZFS parameters and revealed distinct magnetization relaxation mechanisms: 1 exhibits relaxation governed by a phonon bottle-neck process, whereas 2S and 2R involve a combination of Raman and direct relaxation processes. Notably, compounds 2S and 2R, despite being enantiomers and isostructural, exhibited distinct relaxation dynamics, attributable to differential phonon coupling pathways modulated by the chiral substituents. This is the first report demonstrating enantiomer-dependent slow relaxation of magnetisation behaviour in cobalt(ii)-based SMMs. These findings underscore the critical role of stereochemistry in modulating spin dynamics and magnetic bistability, providing design principles for future chiral magnetic materials.

Spin-state switching in iron(II) complexes composed of ligands featuring moderate ligand-field strength-for example, 2,6-bi(1H-pyrazol-1-yl)pyridine (BPP)-is dependent on many factors. Herein, we show that spin-state switching in isomeric iron(II) complexes composed of BPP-based ligands-ethyl 2,6-bis(1H-pyrazol-1-yl)isonicotinate (BPP-COOEt, L1) and (2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)methylacetate (BPP-CH2OCOMe, L2)-is dependent on the nature of the substituent at the BPP skeleton. Bi-stable spin-state switching-with a thermal hysteresis width (ΔT1/2) of 44 K and switching temperature (T1/2) = 298 K in the first cycle-is observed for complex 1·CH3CN composed of L1 and BF4- counter anions. Conversely, the solvent-free isomeric counterpart of 1·CH3CN-complex 2a, composed of L2 and BF4- counter anions-was trapped in the high-spin (HS) state. For one of the polymorphs of complex 2b·CH3CN-2b·CH3CN-Y, Y denotes yellow colour of the crystals-composed of L2 and ClO4- counter anions, a gradual and non-hysteretic SCO is observed with T1/2 = 234 K. Complexes 1·CH3CN and 2b·CH3CN-Y also underwent light-induced spin-state switching at 5 K due to the light-induced excited spin-state trapping (LIESST) effect. Structures of the low-spin (LS) and HS forms of complex 1·CH3CN revealed that spin-state switching goes hand-in-hand with pronounced distortion of the trans-N{pyridyl}-Fe-N{pyridyl} angle (ϕ), whereas such distortion is not observed for 2b·CH3CN-Y. This observation points that distortion is one of the factors making the spin-state switching of 1·CH3CN hysteretic in the solid state. The observation of bi-stable spin-state switching with T1/2 centred at room temperature for 1·CH3CN indicates that technologically relevant spin-state switching profiles based on mononuclear iron(II) complexes can be obtained.

Four novel Co(II) coordination compounds 1-4 of the general formula [Co(Ln)2][Co(NCY)4]·mCH3CN (where Ln are tridentate ligands L1 = 2,6-bis(1-hexyl-1H-benzimidazol-2-yl)pyridine for 1 and 2; L2 = 2,6-bis(1-octyl-1H-benzimidazol-2-yl)pyridine for 3; L3 = 2,6-bis(1-dodecyl-1H-benzimidazol-2-yl)pyridine for 4, Y = O for 1, 3, and 4 and Y = S for 2; m = 0 for 1 and 3, m = 0.5 for 2 and m = 2 for 4) were prepared and characterised. The molecular structures of all four compounds consist of the hexacoordinate complex cation [Co(Ln)2]2+ and tetracoordinate complex anion [Co(NCY)4]2-, with distorted octahedral and tetrahedral symmetry of coordination polyhedra, respectively. The electronic structures of all compounds feature an orbitally non-degenerate ground state well-separated from the lowest excited state, which allows the analysis of the magnetic anisotropy by the spin Hamiltonian model. ZFS parameters, derived from both CASSCF-NEVPT2 calculations and magnetic data analysis, indicate that tetrahedral anions [Co(NCY)4]2- exhibit small axial parameters |D| spanning the range of 2.2 to 7.7 cm-1, while octahedral cations [Co(Ln)2]2+ display significantly larger |D| parameters in the range of 37 to 95 cm-1. For 1-3, the Fourier-transform infrared magnetic spectroscopy (FIRMS) revealed a reasonable transmission with a magnetic absorption around the expected value for the ZFS accompanied by features allowing to identify phonon frequencies and simulate spin-phonon couplings. Dynamic magnetic investigations unveiled the field-induced slow relaxation of magnetisation, with maximal relaxation times (τ) of 92(2) μs for 2 at 2 K and BDC = 0.3 T. The temperature evolution of τ was analysed using a combination of Orbach, direct and Raman relaxations (Ueff = 8(1) K (5.6 cm-1)) or Orbach, direct and spin-phonon induced relaxations (Ueff = 10.3(9) K (7.2 cm-1)). The rest of the complexes, namely 1, 3, and 4 show field-induced slow relaxation of magnetisation with τ smaller than 16 μs.

Two bidentate ligands (L1 = 1-pentyl-2-(pyridin-2-yl)-1H-benzimidazole and L2 = 1-heptyl-2-(pyridin-2-yl)-1H-benzimidazole) were employed for the synthesis of five mononuclear Fe(ii) coordination compounds 1-5 containing perchlorate, tetrafluoroborate and triflate counterions. Single-crystal X-ray diffraction analysis confirmed the expected molecular structures of all the reported compounds, revealing a moderately distorted octahedral geometry of {FeN6} coordination chromophores. All five compounds exhibit thermal spin crossover with T1/2 temperatures allocated above 400 K. The theoretical calculations supported the experimental magnetic investigation and helped to explain the electronic structures of the reported complexes with respect to the occurrence of thermal spin state switching. In addition, compound 4 was employed for the preparation of Langmuir-Blodgett films and fabrication of molecular films using the method of spontaneous evaporation of the subphase. While the formation of Langmuir-Blodgett films was unsuccessful due to the instability of the compound at the water/air interface, the latter technique allowed the formation of molecular films of 4 with well-defined thickness and homogeneity.

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.

The crystal structure of [Cu3(C7H5O3)4(C20H20NO5)2(H2O)2]·2(H2O) (1) and analysis of temperature and field dependence of magnetic susceptibility is reported in this work. The structure of 1 is composed of trinuclear complex units and water molecules. The middle copper atom occupies the center of symmetry. N, O-bonded (6,7-dimethoxy-isoquinolin-1-yl)-(3,4-di­methoxy-phenyl)-methanolato ligands, 2-hydroxybenzoates with bridging carboxylic groups, and oxo-bridged water molecules connect the middle Cu(II) atom with the terminal copper atoms. Two 2-hydroxybenzoates coordinate the terminal copper atoms via one carboxylic oxygen and an O atom of the hydroxyl group. The analysis of copper coordination by bond-valence sum approach and relevant structural correlation is consistent with hexacoordinated Cu(II) centers. Cu···Cu separation is 3.0269(3) Å. The magnetism of 1 shows a strong ferromagnetic interaction between the neighboring metallic centers accompanied by very weak antiferromagnetic intermolecular interactions. The complex units are mutually held by π···π stack interactions of 2-hydroxybenzoates and hydrogen bonds. A new N,O bonded ligands, (R, S)-[(6,7-dimethoxy-isoquinolin-1-yl)-(3,4-dimethoxy-phenyl)-methanolate] coordinate the terminal atoms of the trinuclear copper(II) complex.

The review presents several families of spin crossover (SCO) active Fe(II) coordination compounds with photoactive N-donor heterocyclic ligands, in which the photoinduced structural changes can activate reversible change of spin state and thus control magnetic properties under isothermal conditions. Detailed description of structural, spectral, and magnetic behavior for selected examples of photoisomerizable coordination compounds are provided. From the application point of view, light is an excellent tool to control SCO properties. The first and best known approach called Light Induced Excited Spin State Trapping (LIESST) has a significant technological limitation due to low temperatures (< 120 K) required for the trapping and existence of photoexcited metastable states. The second and more elegant approach known as Ligand-Driven Light-Induced Spin Crossover (LD-LISC) seems to be a very suitable strategy utilizing light-induced structural changes to control the spin. Isomerization of photoswitchable groups, such as azobenzenes or stilbenes, can cause reversible transformation between two isomeric forms after exposition to selective wavelengths at ambient temperature. A very recent third approach, the Guest-Driven Light-Induced Spin Crossover (GD-LISC) effect employing the photoisomerizable guest molecules to control the spin state has also been introduced.

Two hexacoordinated Co( ii ) complexes were prepared, characterised, and deposited on silicon surface in the logical structures. The magnetic anisotropy and slow relaxation of magnetisation was studied by experimental and theoretical approaches.

The novel tridentate ligand L (2,6-bis(1-(n-decyl)-1H-benzimidazol-2-yl)pyridine) was used for the synthesis of mononuclear Co(II) complex 1 with the general formula [Co(L)Br2]. A single-crystal X-ray structural investigation confirmed the expected molecular structure, and noncovalent contacts were inspected by a Hirschfeld surface analysis. The electronic structure of square-pyramidal complex 1 contains an orbitally degenerate ground state which predetermines the use of the Griffith-Figgis Hamiltonian for the analysis of magnetic properties. CASSCF-NEVPT2 calculations and far-infrared mag-netic spectroscopy show excellent agreement with the Griffith- Figgis Hamiltonian parameters obtained from the magnetic investigation. The high and negative value of the axial crystal field parameter Delta ax and the calculated g-tensor components suggest the axial magnetic anisotropy of 1. The low-temperature X-band EPR spectra were analyzed within a simplified effective spin-1/2 Hamiltonian to determine effective g-tensor components of the ground Kramers doublet, which agree with the electronic structure predicted within the CASSCF-NEVPT2 theory. An AC magnetic investigation revealed field-supported single-channel slow relaxation of magnetization with maximum relaxation time tau approximate to 28 ms at low temperatures. The comprehensive analysis of the field and temperature evolution of tau indicates that direct, Raman, and Orbach processes are all involved in slow relaxation of magnetization in 1.

We present a theoretical and experimental study of two tetracoordinate Co(ii)-based complexes with semi-coordination interactions, i.e., non-covalent interactions involving the central atom. We argue that such interactions enhance the thermal and structural stability of the compounds, making them appropriate for deposition on substrates, as demonstrated by their successful deposition on graphene. DC magnetometry and high-frequency electron spin resonance (HF-ESR) experiments revealed an axial magnetic anisotropy and weak intermolecular antiferromagnetic coupling in both compounds, supported by theoretical predictions from complete active space self-consistent field calculations complemented by N-electron valence state second-order perturbation theory (CASSCF-NEVPT2), and broken-symmetry density functional theory (BS-DFT). AC magnetometry demonstrated that the compounds are field-induced single-ion magnets (SIMs) at applied static magnetic fields, with slow relaxation of magnetization governed by a combination of quantum tunneling, Orbach, and direct relaxation mechanisms. The structural stability under ambient conditions and after deposition was confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Theoretical modeling by DFT of different configurations of these systems on graphene revealed n-type doping of graphene originating from electron transfer from the deposited molecules, confirmed by electrical transport measurements and Raman spectroscopy.

An anticancer azo bond-containing half-sandwich Ir(iii) complex oxidizes ascorbate to dehydroascorbate, and ascorbate recovers in the presence of reduced glutathione.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Series of phenylazo substituted pyridyl-benzimidazole ligands and their ferrous complexes have been prepared. Photoirradiation experiments allowed to investigate E–Z isomerisation of ligands, which was further rationalized by computational study.