Exchange Interactions Switch Tunneling: A Comparative Experimental and Theoretical Study on Relaxation Dynamics by Targeted Metal Ion Replacement

Effect of coordinated anions on ferromagnetically coupled Dy2 zero-field single-molecule magnets

A new hydrazone Schiff base ligand was condensed from 2-hydroxy-3-methoxybenzaldehyde and pyrimidine-4-carbohydrazide {H2L = (E)-N'-(2-hydroxy-3-methoxybenzylidene)pyrimidine-4-carbohydrazide}, which was used to assemble two new Dy2 complexes Dy2L2(DMF)2(NO3)2 (1) and Dy2L2(DMF)2(AcO)2 (2). Notably, the coordinated anions have a subtle effect on the coordination configurations of the Dy3+ ions and the magnetic properties of the two Dy2 complexes. The Dy3+ ions in 1 and 2 have the same N2O5 coordination environment but show the triangular dodecahedron and the biaugmented trigonal prism coordination configurations, respectively. Magnetic measurements revealed that both 1 and 2 have intramolecular ferromagnetic interactions between the Dy3+ ions and show single-molecule magnet behaviors at 0 Oe, with Ueff/k values of 58.2 K for 1 and 59.9 K for 2. These magnetic properties may be explained by theoretical calculations.

An Effectively Uncoupled Gd8 Cluster Formed through Fixation of Atmospheric CO2 Showing Excellent Magnetocaloric Properties

The [Gd8(opch)8(CO3)4(H2O)8]·4H2O·10MeCN coordination cluster (1) crystallises in P1¯. The Gd8 core is held together by four bridging carbonates derived from atmospheric CO2 as well as the carboxyhydrazonyl oxygens of the 2-hydroxy-3-methoxybenzylidenepyrazine-2-carbohydrazide (H2opch) Schiff base ligands. The magnetic measurements show that the GdIII ions are effectively uncoupled as seen from the low Weiss constant of 0.05 K needed to fit the inverse susceptibility to the Curie-Weiss law. Furthermore, the magnetisation data are consistent with the Brillouin function for eight independent GdIII ions. These features lead to a magnetocaloric effect with a high efficiency which is 89% of the theoretical maximum value.

Galván I, Alavi A, Aleotti F, Aquilante F, Autschbach J, Avagliano D, Baiardi A, Bao JJ, Battaglia S, Birnoschi L, Blanco-González A, Bokarev SI, Broer R, Cacciari R, Calio PB, Carlson RK, Carvalho Couto R, Cerdán L, Chibotaru LF, Chilton NF, Church JR, Conti I, Coriani S, Cuéllar-Zuquin J, Daoud RE, Dattani N, Decleva P, de Graaf C, Delcey M, De Vico L, Dobrautz W, Dong SS, Feng R, Ferré N, Filatov(Gulak) M, Gagliardi L, Garavelli M, González L, Guan Y, Guo M, Hennefarth MR, Hermes MR, Hoyer CE, Huix-Rotllant M, Jaiswal VK, Kaiser A, Kaliakin DS, Khamesian M, King DS, Kochetov V, Krośnicki M, Kumaar AA, Larsson ED, Lehtola S, Lepetit MB, Lischka H, López Ríos P, Lundberg M, Ma D, Mai S, Marquetand P, Merritt ICD, Montorsi F, Mörchen M, Nenov A, Nguyen VHA, Nishimoto Y, Oakley MS, Olivucci M, Oppel M, Padula D, Pandharkar R, Phung QM, Plasser F, Raggi G, Rebolini E, Reiher M, Rivalta I, Roca-Sanjuán D, Romig T, Safari AA, Sánchez-Mansilla A, Sand AM, Schapiro I, Scott TR, Segarra-Martí J, Segatta F, Sergentu DC, Sharma P, Shepard R, Shu Y, Staab JK, Straatsma TP, Sørensen LK, Tenorio BNC, Truhlar DG, Ungur L, Vacher M, Veryazov V, Voß TA, Weser O, Wu D, Yang X, Yarkony D, Zhou C, Zobel JP, Lindh R. The OpenMolcas Web: A Community-Driven Approach to Advancing Computational Chemistry

The developments of the open-source OpenMolcas chemistry software environment since spring 2020 are described, with a focus on novel functionalities accessible in the stable branch of the package or via interfaces with other packages. These developments span a wide range of topics in computational chemistry and are presented in thematic sections: electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report offers an overview of the chemical phenomena and processes OpenMolcas can address, while showing that OpenMolcas is an attractive platform for state-of-the-art atomistic computer simulations.

Magneto-thermal properties and slow magnetic relaxation in Mn(II)Ln(III) complexes: influence of magnetic coupling on the magneto-caloric effect

A family of Mn(II)Ln(III) dinuclear and tetranuclear complexes (Ln = Gd and Dy) has been prepared from the compartmental ligands N,N'-dimethyl-N,N'-bis(2-hydroxy-3-formyl-5-bromobenzyl)ethylenediamine (H₂L¹) and N,N',N''-trimethyl-N,N''-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H₂L²). The Mn(II)Gd(III) complexes exhibit antiferromagnetic interactions between Mn(II) and Gd(III) ions in most cases, which are supported by Density Functional Theory (DFT) calculations. Experimental magneto-structural correlations carried out for the reported complexes and other related complexes found in bibliography show that the highest ferromagnetic coupling constants are observed in di-μ-phenoxido bridged complexes, which is due to the planarity of the Mn-(μ-O)₂-Gd bridging fragment and to the high Mn-O-Gd angles. The effect of these angles has been studied by DFT calculations performed on a di-μ-phenoxido doubly bridged model. The magneto-thermal properties of the Mn(II)Gd(III) based complexes have also been measured, concluding that the magnitude of the Magneto-Caloric Effect (MCE) is due to the strength rather than to the nature of the magnetic coupling. Moreover, when two Mn(II)Gd(III) dinuclear units are connected by two carbonato-bridging ligands the MCE is enhanced, obtaining a maximum magnetic entropy change of 36.4 Jkg^-1 K^-1 at ΔB = 7 T and T = 2.2 K. On the other hand, one of the dinuclear Mn(II)Dy(III) complexes displays Single-Molecule Magnet (SMM) behaviour with an energy barrier of 14.8 K under an applied external field of 1000 Oe.

Self-assembly of non-macrocyclic triangular Ni3Ln clusters

The synthesis and structural characterisation of four new heterometallic tetranuclear complexes is reported. Three L₃Ni₃Ln type complexes, where Ln = La (C1), Eu (C2), and Gd (C3), have been fully characterised including DC and AC magnetic measurements. A fourth complex featuring a diamagnetic Ba^II ion at its centre is also reported with structural characterisation. Structural elucidation showed that all four complexes successfully self-assembled from a stoichiometric mixture of the acyclic ligand, 1,4-diformylnaphthalene-2,3-diol, with nickel(II) nitrate and the appropriate heavy metal salt to produce the same near planar Ni₃MO₁₂ core. Ferromagnetic interactions were found to dominate the ground state of C3, exhibiting a maximal spin ground state of 13/2. The exchange coupling is quantitatively discussed along with the nickel(II) zero-field splitting effect. AC magnetic susceptibility experiments were carried out, but no frequency dependent signals were observed and thus no observable slow relaxation of magnetisation.

A nonsymmetric Dy2 single-molecule magnet with two relaxation processes triggered by an external magnetic field: a theoretical and integrated EPR study of the role of magnetic-site dilution

The 1 : 2 reaction between Dy(O₂CMe)₃·4H₂O and 1-acetyl-2-napthol (LH) in MeOH has provided access to the complex [Dy₂L₆(MeOH)]·MeOH (1·MeOH) in a good yield. The structures of the isomorphous complex 1·MeOH and its doped diamagnetic yttrium analogue [Dy_0.14Y_1.86L₆(MeOH)]·MeOH (Dy@Y2) have been determined by single-crystal X-ray crystallography and characterized based on elemental analyses, IR spectra, and powder X-ray patterns. Combined dc and ac magnetic susceptibility and the magnetization data for 1 suggest that this complex shows slow magnetic relaxation. Under a 0 Oe dc field, a single relaxation mechanism is seen while two magnetization relaxation processes are evident under a 1500 G external magnetic field. The fit to the Arrhenius law has been performed using the 1.8-10 K ac data, affording an effective barrier for the magnetization reversal of 13 K and 7 K under the external dc field. Theoretical studies have been performed using ab initio and density functional methodologies to understand the electronic structure and the magnetic relaxation dynamics resulting from the single Dy^III ion as well as from the dinuclear exchange-coupled states. Rich powder EPR spectra at the X-band and Q-band were obtained from Dy@Y2, as well as from the 1·MeOH dimer, while simulation studies revealed the ferromagnetic nature of the interaction between the Dy^III ions in accordance with theoretical studies.

Are lanthanide-transition metal direct bonds a route to achieving new generation {3d-4f} SMMs?

Lanthanide based single-molecule magnets are gaining wide attention due to their potential applications in emerging technologies. One of the main challenges in this area is quenching quantum tunnelling of magnetisation (QTM), which often undercuts the magnetisation reversal barrier. Among the several strategies employed, enhancing exchange coupling has been studied in detail, with large exchanges resulting in stronger quenching of QTM effects. Lanthanides, however, suffer from weak exchanges offered by the deeply buried 4f orbitals and the numerous attempts to enhance the exchange coupling in the {3d-4f} pairs have not exceeded values larger than 30 cm^-1. In this work, using a combination of DFT and the ab initio CASSCF/RASSI-SO method, we have explored lanthanide-transition metal direct bonds as a tool to quench QTM effects. In this direction, we have modelled [PyCp₂LnMCp(CO)₂] (Ln = Gd(III), Dy(III), and Er(III) and M = V(0), Mn(0), Co(0) and Fe(I) and here PyCp₂ = [2,6-(CH₂C₅H₃)₂C₅H₃N]₂^- using [PyCp₂DyFeCp(CO)₂] as an example as reported by Nippe et al. (C. P. Burns, X. Yang, J. D. Wofford, N. S. Bhuvanesh, M. B. Hall and M. Nippe, Angew. Chem., Int. Ed. 2018, 57, 8144). Bonding analysis reveals a dative Ln-TM bond with a donation of π(V/Mnd_xy-π*CO) to 5d_z² (Gd) in the case of Gd-V and Gd-Mn and 4s(Co) to 5d_xy/5d_yz (Gd) for Gd-Co with the transition metal ion being found in the low-spin S = ½ configurations in all the cases. B3LYP/TZV (Gd;CSDZ) calculations on [PyCp₂GdMCp(CO)₂] yield J_Gd-V = -46.1 cm^-1, J_Gd-Mn = -57.1 cm^-1, J_Gd-Co = +55.3 cm^-1, J_Gd-Fe⁺ = +13.9 cm^-1, J_Gd-Vhs = -162.1 cm^-1 and J_Gd-Mnhs = -343.9 cm^-1 and unveiling record-high J values for {3d-4f} complexes. The mechanism of magnetic coupling is developed, which discloses the dominating presence of strong 3d-4f orbital overlaps in most of the cases studied, leading to antiferromagnetic exchange. When these overlaps are weaker and 3d to Gd(5d_z²), charge transfer dominates, yielding a ferromagnetic coupling for the Gd-Co/Gd-Fe⁺ complexes. Calculations performed on the anisotropic Dy(III) and Er(III) complexes reveal that the ground state g_zz axis lies along the Cp-Ln-Cp axis and the Ln-TM bonds, respectively. Thus the Ln-TM bond hinders the single-ion anisotropy of Dy(III) by offering equatorial ligation and lowering the m_J = ±½ state energy, and at the same time, helping in enhancing the axiality of Er(III). When strong {3d-4f} exchange couplings are introduced, record-high barrier heights as high as 229 cm^-1 were accomplished. Furthermore, the exchange coupling annihilates the QTM effects and suggests the lanthanide-transition metal direct bond as a viable alternative to enhance exchange coupling to bring {3d-4f} complexes back in the race for high-blocking SMMs.

Hetero-tri-spin systems: an alternative stairway to the single molecule magnet heaven?

The search for molecule-based magnetic materials has stimulated over the years the development of extremely rich coordination chemistry. Various combinations of spin carriers have been investigated and illustrated by a plethora of hetero-spin complexes: 3d-nd, 3d-4f, 2p-3d, and 2p-4f. More recently, two other classes of hetero-spin complexes have grown rapidly: compounds containing three different paramagnetic metal ions, or one radical and two different paramagnetic metal ions (all within the same molecular entity). Such new classes of systems represent a challenge both from a synthetic and theoretical point of view. Indeed, the synthetic control and the understanding of the spin topology effect on the overall magnetic behavior from first-principles is a difficult problem to be solved. The presence of different spin carriers in a single molecule makes such compounds particularly interesting because they offer the possibility of developing new magnetic properties, different from those of hetero-bi-spin or homo-spin systems. A critical overview taking the case of 2p-3d-4f complexes is the focus of this perspective paper. An original organic picture of the state-of-art in this field and new hints about the main directions that should be pursued to achieve hetero-tri-spin systems with large anisotropy barriers, low quantum tunneling of magnetization and, possibly, large blocking temperatures are provided in this article through an analysis based on numerically revisiting already published data and a critical survey of the literature reported so far. The reasons for the limited success obtained for the largely used 3d-2p-4f topology are given along with the ones explaining the failure for the 2p-4f-3d case. The still never synthesized linear 2p-3d-4f spin topology seemed to be the most promising one based on the results obtained for the unique closed hetero-tri-spin closed triangular system synthesized so far.

Recent advances in lanthanide coordination polymers and clusters with magnetocaloric effect or single-molecule magnet behavior

Molecular magnetorefrigerant materials for low-temperature magnetic refrigeration and single-molecule magnets for high-density information storage and quantum computing have received extensive attention from chemists and magnetic experts. Lanthanide ions with unique magnetic properties have always been considered as ideal candidates for the construction of such materials. This frontier article focuses on Gd^III-based molecular magnetorefrigerants and lanthanide-based single-molecule magnets and highlights the most significant advances.

Attaining record-high magnetic exchange, magnetic anisotropy and blocking barriers in dilanthanofullerenes

While the blocking barrier (U eff) and blocking temperature (T B) for "Dysprocenium" SIMs have been increased beyond liquid N2 temperature, device fabrication of these molecules remains a challenge as low-coordinate Ln3+ complexes are very unstable. Encapsulating the lanthanide ion inside a cage such as a fullerene (called endohedral metallofullerene or EMF) opens up a new avenue leading to several Ln@EMF SMMs. The ab initio CASSCF calculations play a pivotal role in identifying target metal ions and suitable cages in this area. Encouraged by our earlier prediction on Ln2@C79N, which was verified by experiments, here we have undertaken a search to enhance the exchange coupling in this class of molecules beyond the highest reported value. Using DFT and ab initio calculations, we have studied a series of Gd2@C2n (30 ≤ 2n ≤ 80), where an antiferromagnetic J Gd⋯Gd of -43 cm-1 was found for a stable Gd2@C38-D 3h cage. This extremely large and exceptionally rare 4f⋯4f interaction results from a direct overlap of 4f orbitals due to the confinement effect. In larger cages such as Gd2@C60 and Gd2@C80, the formation of two centre-one-electron (2c-1e-) Gd-Gd bonds is perceived. This results in a radical formation in the fullerene cage leading to its instability. To avoid this, we have studied heterofullerenes where one of the carbon atoms is replaced by a nitrogen atom. Specifically, we have studied Ln2@C59N and Ln2@C79N, where strong delocalisation of the electron yields a mixed valence-like behaviour. This suggests a double-exchange (B) is operational, and CASSCF calculations yield a B value of 434.8 cm-1 and resultant J Gd-rad of 869.5 cm-1 for the Gd2@C59N complex. These parameters are found to be two times larger than the world-record J reported for Gd2@C79N. Further ab initio calculations reveal an unprecedented U cal of 1183 and 1501 cm-1 for Dy2@C59N and Tb2@C59N, respectively. Thus, this study offers strong exchange coupling as criteria for new generation SMMs as the existing idea of enhancing the blocking barrier via crystal field modulation has reached its saturation point.

Ring-forming transformation associated with hydrazone changes of hexadecanuclear dysprosium phosphonates

{Dy16(μ6-C10H7PO3)2(μ5-C10H7PO3)8(spch)8(μ3-OH)2(μ2-OH)2(μ2-AcO)6(μ3-COO)2(DMF)2(H2O)6}·0.5CH3OH·4.5H2O (1) and {Dy16(μ5-C10H7PO3)4(μ3-C10H7PO3)12(μ2-C10H7PO3H)8(opch)4(DMF)8(MeOH)4}·2.5CH3OH·3H2O (2), where H2spch is ((E)-N'-(2-hydroxybenzylidene)pyrazine-2-carbohydrazide, C10H7PO3H2 is 1-naphthylphosphonic acid and H2opch is (E)-N'-(2-hydroxy-3-methoxybenzylidene)pyrazine-2-carbohydrazide, were successfully synthesized by varying the hydrazone ligands in the Dy-phosphonate system. It is important that the ellipsoidal core experiences a ring forming structural transformation to the supramolecular square motif upon the incorporation of an ortho-methoxy substituent into the hydrazone. Alternating-current (ac) magnetic susceptibility studies of 1 and 2 suggest that similar single molecule magnet behaviors occur for these two complexes. The result represents an effective molecular assembly tactic to develop highly complicated lanthanide coordination clusters through the multicomponent self-assembly of the coalescence of phosphonate- and hydrazone-based ligands and metal salts.

Slow magnetic relaxation in a homo dinuclear Dy(iii) complex in a pentagonal bipyramidal geometry

We hereby report a dinuclear Dy(iii) complex, [Dy(LH₃)Cl₂]₂·2Et₂O (1) (LH₄ = 2,3-dihydroxybenzylidene)-2-(hydroxyimino)propanehydrazide where both the metal centres are in a pentagonal bipyramidal (PBP) geometry with the axial positions being occupied by negatively charged Cl^- ions. The complex as well as it's 10% diluted analogue (1₁₀) do not show zero-field SMM behaviour. However, in the presence of small optimum dc fields the slow relaxation of magnetization was displayed. The effective energy barrier for 1₁₀ at 800 Oe of applied field was extracted as 83(17) K with τ₀ = 2(4) × 10^-12 s. Through a combined experimental and ab initio electronic structure calculations studies we have thoroughly analysed the role of the ligand field around the Dy(iii), present in pentagonal bipyramidal geometry, in contributing to the slow relaxation of magnetization.

Exchange-Biasing in a Dinuclear Dysprosium(III) Single-Molecule Magnet with a Large Energy Barrier for Magnetisation Reversal

A dichlorido-bridged dinuclear dysprosium(III) single-molecule magnet [Dy₂ L₂ (μ-Cl)₂ (thf)₂ ] has been made by using a diamine-bis(phenolate) ligand, H₂ L. Magnetic studies show an energy barrier for magnetisation reversal (U_eff ) around 1000 K. An exchange-biasing effect is clearly seen in magnetic hysteresis with steps up to 3 K. Ab initio calculations exclude the possibility of a pure dipolar origin of this effect leading to the conclusion that super-exchange through the chloride bridging ligands is important.

Monometallic lanthanide salicylhydrazone complexes exhibiting strong near-infrared luminescence

We report the synthesis of monometallic lanthanide complexes supported by one salicylhydrazone ligand. Complexes of neodymium, erbium, and ytterbium have been synthesized, and exhibit near infrared luminescence with modest to high quantum yields. The luminescence of ytterbium is increased by a factor of five when comparing our complex to other salicylhydrazone-supported complexes.

A supramolecular chain of dimeric Dy single molecule magnets decorated with azobenzene ligands

Dy^III dimers decorated with photo-isomerizable azobenzene ligands behave as single-molecule magnets and self-organize into a supramolecular chain. Ab initio calculations, magnetic and optical properties are reported.