In situ hydrolysis of a carbophosphazene ligand leads to one-dimensional lanthanide coordination polymers. Synthesis, structure and dynamic magnetic studies

An in situ hydrolysis of the P-Cl bonds of the carbophosphazene [{NC(NMe2)}2{NPCl2}] (LPCl2) in the presence of hydrated lanthanide(III) nitrates in a dichloromethane and methanol (2 : 1) solvent mixture afforded a series of novel 1D coordination polymers: [{Ln(LHPO2)3(NO3)2(CH3OH)(H2O)} (Cl)]n {where Ln(III) = Gd (1), Tb (2), Dy (3), or Er (4) and LHPO2 is the hydrolyzed carbophosphazene (LPCl2) ligand}. X-ray crystallographic analysis revealed that complexes 1-4 are isostructural and crystallized in the monoclinic crystal system having P21/c space group. The coordination polymers are formed because of the involvement of the geminal P(O)(OH) moieties of the carbophosphazene ligand. Each lanthanide(III) ion is 9-coordinate (9O) in a distorted muffin geometry. Magnetic measurements revealed that both DyIII and ErIII analogues exhibit field-induced single-molecule magnet (SMM) behavior at 0.8 kOe and 2.2 k Oe, respectively. At such dc fields, the dynamic magnetic susceptibility displays complex behavior with a triple magnetic relaxation contribution for 3, while two contributions were identified for 4. The observed static and dynamic magnetic behavior for complexes 1-4 were further rationalized with the aid of BS-DFT and CASSCF/SO-RASSI/SINGLE_ANISO calculations.

Magneto-structural maps and bridged-ligand effect for dichloro-bridged dinuclear copper(ii) complexes: a theoretical perspective

Theoretical understanding of magneto-structural correlations in dichloro-bridged dicopper(ii) complexes can guide the design of magnetic materials having broad-scale applications. However, previous reports suggest these correlations are complicated and unclear. To clarify possible correlations, magnetic coupling constants (J _calc) of variants of a representative {Cu-(μ-Cl)₂-Cu} complex A were calculated through BS-DFT. The variation of the Cu-(μ-Cl)-Cu angle (α), Cu⋯Cu distance (R ₀), and Cu-Cl-Cu-Cl dihedral angle (τ) followed by structural optimization and calculation of the magnetic coupling constant (J _calc) revealed several trends. J _calc increased linearly with R ₀ and τ, and initially increased and then decreased with α. Further, bridging ligand effects on J _calc for dicopper(ii) complexes were evaluated through BS-DFT; the results revealed that J _calc increased with increasing ligand field strength (I^- < Br^- < Cl^- < N₃ ^- < F^-). Furthermore, a linear relationship was found between the spin density of the bridging ligand and J _calc.

Understanding the Exchange Interaction between Paramagnetic Metal Ions and Radical Ligands: DFT and Ab Initio Study on Semiquinonato Cu(II) Complexes

The exchange coupling, represented by the J parameter, is of tremendous importance in understanding the reactivity and magnetic behavior of open-shell molecular systems. In the past, it was the subject of theoretical investigations, but these studies are mostly limited to the interaction between metallic centers. The exchange coupling between paramagnetic metal ions and radical ligands has hitherto received scant attention in theoretical studies, and thus the understanding of the factors governing this interaction is lacking. In this paper, we use DFT, CASSCF, CASSCF/NEVPT2, and DDCI3 methods to provide insight into exchange interaction in semiquinonato copper(II) complexes. Our primary objective is to identify structural features that affect this magnetic interaction. We demonstrate that the magnetic character of Cu(II)-semiquinone complexes are mainly determined by the relative position of the semiquinone ligand to the Cu(II) ion. The results can support the experimental interpretation of magnetic data for similar systems and can be used for the in-silico design of magnetic complexes with radical ligands.

Electronic Structure and Magneto-Structural Correlations Study of Cu2UL Trinuclear Schiff Base Complexes: A 3d-5f-3d Case

The magnetic properties of trinuclear Schiff base complexes M₂AnLⁱ (M^II = Zn, Cu; An^IV = Th, U; Lⁱ = Schiff base; i = 1-4, 6, 7, 9), exhibiting the [M(μ-O)₂]₂U core structure with adjacent M1···U and M2···U and next-adjacent M1···M2 interactions, featuring 3d-5f-3d subsystems, have been investigated theoretically using relativistic ZORA/B3LYP computations combined with the broken symmetry (BS) approach. Bond order and natural population analyses reveal that the covalent contribution to the bonding within the Cu-O-U coordination is important thus favoring superexchange coupling between the transition metal and the uranium magnetic centers. The calculated coupling constants J_CuU between the Cu and U atoms, agree with the observed shift from the antiferromagnetic (AF) character of the L^1,2,3,4 complexes to the ferromagnetic (ferro) of the L^6,7,9 ones. The structural parameters, i.e., the Cu···U distances and the Cu-O-U angles, as well as the electronic factors driving the magnetic couplings are discussed. The analyses are supported by the study of the mixed ZnCuULⁱ and Cu₂ThLⁱ systems, where in the first complex the Cu^II (3d⁹) ion is replaced by the diamagnetic Zn^II (3d¹⁰) one, whereas in the second complex the U^IV (5f²) paramagnetic center is replaced by the diamagnetic Th^IV (5f⁰) one.

Exchange-Driven Slow Relaxation of Magnetization in NiII2LnIII2 (LnIII = Y, Gd, Tb and Dy) Butterfly complexes: Experimental and Theoretical Studies

The tetranuclear NiII2LnIII2 complexes, [{L′2{Ni(MeOH)(μ-OAc)}2(μ3-MeO)2Ln2}; LnIII = YIII (1), GdIII (2), TbIII (3), DyIII (4)] were prepared using a Schiff base ligand, H3L [H3L = 3-((2-hydroxy-3-methoxybenzylidene)amino)-2-(2-hydroxy-3-methoxyphenyl)-2,3-dihydroquinazolin-4(1H)-one, whereas {L′}3- is the...

Synthesis, characterization, magnetism and theoretical analysis of hetero-metallic [Ni2Ln2] partial di-cubane assemblies

A family of four isostructural [Ln₂Ni₂(L)₂(μ₃-OCH₃)₂(μ_1,3-PhCO₂)₂(PhCO₂)₂(CH₃OH)₄]·2CH₃OH [where Ln = Gd (1), Tb (2), Dy (3) and Ho (4)] complexes has been synthesized using Schiff base ligand 2-[{(2-hydroxybenzyl)imino}methyl]-6-methoxyphenol (H₂L). All the complexes possess a partial di-cubane core structure where the growth of the core is contingent upon the ligand anions and solvent generated μ₃-OCH₃ groups. DC magnetic analysis revealed dominating ferromagnetic interactions between the metal ions, however, we find no slow relaxation characteristics in the AC susceptibility. Further insight into the magnetic behavior of the reported complexes was achieved using DFT and CASSCF theoretical calculations, leading to the comprehension of the fast relaxation characteristics observed by magnetometry.

Validation of Ab-Initio-Predicted Magnetic Anisotropies and Magneto-structural Correlations in Linear Hetero-trinuclear DyIII -NiII 2 Compounds

Reported are single crystal SQUID and single crystal high-frequency/high-field EPR data of a trinuclear complex with a rare six-coordinate coordination sphere of a DyIII center coupled to two terminal six-coordinate NiII ions. The analysis of the single crystal spectroscopic parameters allows for an accurate description of the ground state wavefunction. The experimental analysis is supplemented by the analysis of the paramagnetic NMR spectra, allowing for a thorough description of the DyIII center. The experimental data are interpreted on the basis of an ab initio ligand field analysis, and the computed parameters are in good agreement with the experimental observations. This supports the quality of the theoretical approach based on a pseudo-spin Hamiltonian for the electronic ground state. Further support emerges from the ab initio ligand field theory based analysis of a structurally very similar system that, in contrast to the complex reported here, shows single molecule magnetic properties, and this is in agreement with the quantum-chemical prediction and analysis.

A new family of Fe4Ln4 (Ln = DyIII, GdIII, YIII) wheel type complexes with ferromagnetic interaction, magnetocaloric effect and zero-field SMM behavior

Observation of ferromagnetic interactions and single molecule toroic (SMT) behavior in Fe4Ln4 wheel complexes.

Altering the nature of coupling by changing the oxidation state in a {Mn6} cage

Polynuclear transition metal complexes have continuously attracted interest owing to their peculiar electronic and magnetic properties which are influenced by the symmetry and connectivity of the metal centres. Understanding the full electronic picture in such cases often becomes difficult owing to the presence of multiple bridges between metal centres. We have investigated the electronic structure of a {Mn6} cage complex using computational and experimental approaches with the aim to understand the coupling between the manganese centres. The nature of the various coupling pathways has been determined using a novel methodology that involves perturbing the system while retaining the symmetry and analysing the effect on the coupling strength due to the perturbation. Furthermore, we have investigated the magnetic properties of this complex in higher oxidation states which reveals a switch in the nature of coupling from antiferromagnetic to ferromagnetic in addition to stabilisation of intermediate spin states.

Probing the strong magnetic exchange behaviour of transition metal–radical complexes: a DFT case study

Alteration of the structural parameters of metal–radical complexes may pave the way forward for fine tuning the magnetic exchange coupling value as high as >−500 cm⁻¹ – a much sought-after parameter in the area of SMMs.

How important is the coordinating atom in controlling magnetic anisotropy in uranium(iii) single-ion magnets? A theoretical perspective

Theoretical investigation of actinide based nanomagnets is of paramount interest in the field of molecular magnetism as they offer remarkable properties compared to their lanthanide counterparts. Unlike lanthanides, the magnetic properties of actinides can be fine-tuned by modulating the ligand field as they possess a large metal-ligand covalency. In this regard, two complexes reported earlier have gained attention: [U(BcMe)₃] (1) has been found to show Single-ion Magnet (SIM) characteristics whereas isomeric [U(BpMe)₃] (2) does not exhibit any SIM behaviour. To unravel the origin of the differences observed in magnetic anisotropy, a detailed ab initio CASSCF study has been undertaken on the X-ray structure of complexes 1 and 2. Since actinide compounds exhibit strong covalency, the desired active space needs to be benchmarked to address this issue. Here, we have enlarged the active space systematically from CAS(3,7) to CAS(3,12) where all 5f electrons in 5f orbitals are sequentially expanded to include five formally empty 6d orbitals. Our calculations reveal that the incorporation of the 6d_z² orbital is vital in reproducing many experimental observables such as temperature dependent susceptibility, g-factors, ground state m_J level, and ground-state-excited-state gap. Inclusion of this orbital in the reference space is found to describe better the UH-BH agostic interactions leading to significant variations in the computed parameters. Gaining from this understanding, we have carried out extensive bonding analysis within the DFT framework using tools such as Natural Bond Orbital (NBO) and Atoms In Molecule (AIM) to further probe these weak agostic interactions. Also, predictions to enhance the U-ligand covalency using U-sulphur bonds and the role of the U-C distance and C-U-C bite angles in the nature of anisotropy have been studied, and relevant magneto-structural correlations have been developed. Thus our results for the first time provide a comprehensive understanding of uranium based SMMs and offer ways to fine tune the anisotropy for experimental chemists.

Influence of ancillary ligands and solvents on the nuclearity of Ni-Ln complexes

A Schiff base ligand resulting from the reaction of ovanillin and 2,2-dimethyl-1,3-diaminopropane allows the preparation of hetero-dinuclear [Ni-Ln]³⁺ or -trinuclear [Ni-Ln-Ni]³⁺ complexes. Although empirical parameters for rationalizing the strength of the ferromagnetic Ni-Gd interaction have already been discussed in several papers, no systematic study has been devoted to the control of the nuclearity of such complexes. With the help of structural determinations, we demonstrate the role of solvent and of the nature of ancillary ligands, linked to the Ln ions, in nuclearity. For instance, the presence of one chelating nitrato ligand is already sufficient to impede an increase in the nuclearity, while the replacement of nitrato ligands by chloride anions still yields dinuclear Ni-Ln complexes. This experimental result evidences the role of protic solvents. In contrast, the use of lanthanide salts, soluble in non-protic solvents, allows the isolation of dinuclear [Ni-Ln]³⁺ or trinuclear cationic [LNi-Ln-NiL]³⁺ complexes, depending on the Ni/Ln ratio. A further synthetic step can be overtaken by the reaction of a Ni-Ln complex, soluble in a non-protic solvent, with a LM complex (M = Cu, Zn). By doing so, a heterotrinuclear complex made of three different metal ions, two distinct 3d ions and a 4f one, has been isolated and structurally characterized. Note that the Ni coordination number decreases from 6 to 5 on going from the dinuclear complex to the trinuclear one. Also, the replacement of water molecules by chloride ligands in the hexacoordinate Ni complexes induces a net increase of the positive zero-field splitting parameter D to 20 cm^-1, which is supported by ab initio calculations. Although the Ni-Ln (Ln = Gd, Tb, Dy) magnetic interactions are ferromagnetic, the corresponding trinuclear complexes are devoid of SMM properties in the absence of an applied magnetic field.

Solvent-tuned magnetic exchange interactions in Dy2 systems ligated by a μ-phenolato heptadentate Schiff base

A series of binuclear dysprosium compounds, namely, [Dy(api)]₂ (1), [Dy(api)]₂·2CH₂Cl₂ (2), [Dy(Clapi)]₂·2C₄H₈O (3), and [Dy(Clapi)]₂·2C₃H₆O (4) (H₃api = 2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazoline; H₃Clapi = 2-(2′-hydroxy-5′-chlorophenyl)-1,3-bis[3′-aza-4′-(2′′-hydroxy-5′′-chlorophenyl)prop-4′-en-1′-yl]-1,3-imidazolidine), have been isolated by the reactions of salen-type ligands H₃api/H₃Clapi with DyCl₃·6H₂O in different solvent systems. Structural analysis reveals that each salen-type ligand provides a heptadentate coordination pocket (N₄O₃) to encapsulate a Dy^III ion and all of the Dy^III centers in 1–4 adopt a distorted square antiprism geometry with D_4d symmetry. Magnetic studies showed that compound 1 did not exhibit single-molecule magnetic (SMMs) behavior. With the introduction of different lattice solvents, compounds 2–4 showed filed-induced slow magnetic relaxation with barriers U_eff of 18.2 K (2), 28.0 K (3) and 16.4 K (4), respectively. Ab initio calculations were employed to interpret the magnetization behavior of 1–4. The combination of experimental and theoretical data reveal the importance of the weak exchange interaction between the Dy^III ions in the observation of slow magnetic relaxation, and a relaxation mechanism has been developed to rationalize the observed difference in the U_eff values. The different lattice solvents influence Dy–O–Dy bond angles and thus alter the torsion of the square antiprism geometry, consequently resulting in distinct magnetic interactions and the magnetic behavior.

Solvent-tuning changes the magnetic exchange interaction and results in different magnetic relaxation dynamics in Dy₂ systems ligated by a μ-phenolato heptadentate Schiff base.

Contribution of 155Gd Mössbauer data to the study of the magnetic interaction in heterodinuclear 3d–Gd (3d = Cu, Ni) coordination complexes

The observed ¹⁵⁵Gd Mössbauer isomer shifts of 3d–Gd complexes give an experimental proof for the participation of 5d Gd orbitals to the magnetic interaction in these 3d–Gd complexes.

Heterometallic Octanuclear NiII 4LnIII 4 (Ln = Y, Gd, Tb, Dy, Ho, Er) Complexes Containing NiII 2LnIII 2O4 Distorted Cubane Motifs: Synthesis, Structure, and Magnetic Properties

The reaction of 2-methoxy-6-[{2-(2-hydroxyethylamino)ethylimino}methyl] phenol (LH₃) with lanthanide metal salts followed by the addition of nickel acetate allowed isolation of a family of octanuclear complexes, [Ni₄Ln₄(μ₂-OH)₂(μ₃-OH)₄(μ-OOCCH₃)₈(LH₂)₄]·(OH)₂·xH₂O. Single crystal X-ray diffraction studies of these complexes reveal that their central metallic core consists of two tetranuclear [Ni₂Ln₂O₄] cubane subunits fused together by acetate and hydroxide bridges. The magnetic study of these complexes reveals a ferromagnetic interaction between the Ln^III and the Ni^II center. The magnitude of exchange coupling between the Ni^II and Ln^III center, parametrized from the magnetic data of the Gd analogue, gives J = +0.86 cm^-1. The magneto caloric effect, studied for the Ni^II ₄Gd^III ₄ complex, shows a maximum of magnetic entropy change, -ΔS _m = 22.58 J kg^-1 K^-1 at 3 K for an applied external field of 5 T.

Syntheses, crystal structures, magnetic properties and ESI-MS studies of a series of trinuclear CuIIMIICuII compounds (M = Cu, Ni, Co, Fe, Mn, Zn)

Six trinuclear CuIIMIICuII compounds (M = Cu, Ni, Co, Fe, Mn, Zn) derived from the Schiff base ligand, H2L (2 + 1 condensation product of salicylaldehyde and trans-1,2-diaminocyclohexane) are reported in this investigation. The composition of the metal complexes are [{CuIIL(ClO4)}2CuII(H2O)]·2H2O (1), [{CuIIL(ClO4)}{NiII(H2O)2}{CuIIL}]ClO4·CH3COCH3 (2), [{CuIIL(ClO4)}{CoII(CH3COCH3)(H2O)}{CuIIL(CH3COCH3)}]ClO4 (3) and isomorphic [{CuIIL(ClO4)}2MII(CH3OH)2] (4, M = Fe; 5, M = Mn; 6, M = Zn). Two copper(ii) ions in 1-6 occupy N2O2 compartments of two L2- ligands, while the second metal ion occupies the O(phenoxo)4 site provided by the two ligands, i.e., the two metal ions in both CuIIMII pairs are diphenoxo-bridged. Positive ESI-MS of 1-6 reveals some interesting features. Variable-temperature and variable-field magnetic studies reveal moderate or weak antiferromagnetic interactions in 1-6 with the following values of magnetic exchange integrals (H = -2JS 1 S 2 type): J 1 = -136.50 cm-1 and J = 0.00 for the CuIICuIICuII compound 1; J 1 = -22.16 cm-1 and J = -1.97 cm-1 for the CuIINiIICuII compound 2; J 1 = -14.78 cm-1 and J = -1.86 cm-1 for the CuIICoIICuII compound 3; J 1 = -6.35 cm-1 and J = -1.17 cm-1 for the CuIIFeIICuII compound 4; J 1 = -6.02 cm-1 and J = -1.70 cm-1 for the CuIIMnIICuII compound 5; J = -2.25 cm-1 for the CuIIZnIICuII compound 6 (J is between two CuII in the N2O2 compartments; J 1 is between CuII and MII through a diphenoxo bridge).

Modelling spin Hamiltonian parameters of molecular nanomagnets

Molecular nanomagnets encompass a wide range of coordination complexes possessing several potential applications. A formidable challenge in realizing these potential applications lies in controlling the magnetic properties of these clusters. Microscopic spin Hamiltonian (SH) parameters describe the magnetic properties of these clusters, and viable ways to control these SH parameters are highly desirable. Computational tools play a proactive role in this area, where SH parameters such as isotropic exchange interaction (J), anisotropic exchange interaction (Jx, Jy, Jz), double exchange interaction (B), zero-field splitting parameters (D, E) and g-tensors can be computed reliably using X-ray structures. In this feature article, we have attempted to provide a holistic view of the modelling of these SH parameters of molecular magnets. The determination of J includes various class of molecules, from di- and polynuclear Mn complexes to the {3d-Gd}, {Gd-Gd} and {Gd-2p} class of complexes. The estimation of anisotropic exchange coupling includes the exchange between an isotropic metal ion and an orbitally degenerate 3d/4d/5d metal ion. The double-exchange section contains some illustrative examples of mixed valance systems, and the section on the estimation of zfs parameters covers some mononuclear transition metal complexes possessing very large axial zfs parameters. The section on the computation of g-anisotropy exclusively covers studies on mononuclear Dy(III) and Er(III) single-ion magnets. The examples depicted in this article clearly illustrate that computational tools not only aid in interpreting and rationalizing the observed magnetic properties but possess the potential to predict new generation MNMs.

Experimental and theoretical exploration of magnetic exchange interactions and single-molecule magnetic behaviour of bis(η1:η2:μ2-carboxylate)GdIII2/DyIII2 systems

This investigation demonstrates differences in SMM properties and nature of magnetic exchange in closely related DyIII2/GdIII2 compounds.

Synthesis and magneto-structural studies on a new family of carbonato bridged 3d–4f complexes featuring a [CoII3LnIII3(CO3)] (Ln = La, Gd, Tb, Dy and Ho) core: slow magnetic relaxation displayed by the cobalt(ii)–dysprosium(iii) analogue

A new series of carbonato-bridged complexes containing a CoII3LnIII3 core have been synthesized.

Theoretical insights into the origin of magnetic exchange and magnetic anisotropy in {Re(IV)-M(II)} (M = Mn, Fe, Co, Ni and Cu) single chain magnets

Density functional calculations have been performed on a series of {Re(IV)-M(II)} (M = Mn(), Fe(), Co(), Ni(), Cu()) complexes to compute the magnetic exchange interaction between the Re(IV) and M(II) ions, and understand the mechanism of magnetic coupling in this series. DFT calculations yield J values of -5.54 cm(-1), +0.44 cm(-1), +10.5 cm(-1), +4.54 cm(-1) and +19 cm(-1) for complexes respectively, and these estimates are in general agreement with the experimental reports. Using molecular orbital (MO) and overlap integral analysis, we have established a mechanism of coupling for a {3d-5d} pair and the proposed mechanism rationalises both the sign and the magnitude of J values observed in this series. Our proposed mechanism of coupling has five contributing factors: (i) (Re)dyz-dyz(3d) overlap, (ii) (Re)dxz-dxz(3d) overlap, (iii) (Re)dxy-dxy(3d) overlap, (iv) (Re)eg-t2g(3d) overlaps and (v) (Re)eg-eg(3d) overlaps. Here, the first two terms are found to contribute to the antiferromagnetic part of the exchange, while the other three contribute to the ferromagnetic part. The last two terms correspond to the cross-interactions and also contribute to the ferromagnetic part of the exchange. A record high ferromagnetic J value observed for the {Re(IV)-Cu(II)} pair in complex is found to be due to a significant cross interaction between the dz(2) orbital of the Re(IV) ion and the dx(2)-y(2) orbital of the Cu(ii) ion. Magneto-structural correlations are developed for Re-C and M-N bond lengths and Re-C-N and M-N-C bond angles. Among the developed correlations, the M-N-C bond angle is found to be the most sensitive parameter which influences the sign and strength of J values in this series. The J values are found to be more positive (or less negative) as the angle increases, indicating stronger ferromagnetic coupling at linear M-N-C angles. Apart from the magnetic exchange interaction, we have also estimated the magnetic anisotropy of [ReCl4(CN)2](2-) and [(DMF)4(CN)M(II)(CN)] (M(II)-Fe(II), Co(II) and Ni(II)) units using the state-of-the-art ab initio CASSCF/PT2/RASSI-SO/SINGLE_ANISO approach. The calculated D and E values for these building units are found to be in agreement with the available experimental results. Particularly a large positive D computed for the [ReCl4(CN)2](2-) unit was found to arise from dxz/dyz → dxy excitations corresponding to the low-lying doublet states. Similarly, a very large positive D value computed for Fe(II) and Co(II) units are also rationalised based on the corresponding ground state electronic configurations computed. The non-collinearity of the Re(IV) ion and the M(II) ion axial anisotropy (DZZ) axis are found to diminish the anisotropy of the building unit, leading to the observation of moderate relaxation barriers for these molecules.

On The Density Functional Theory Treatment of Lanthanide Coordination Compounds: A Comparative Study in a Series of Cu-Ln (Ln = Gd, Tb, Lu) Binuclear Complexes

The nontrivial aspects of electron structure in lanthanide complexes, considering ligand field (LF) and exchange coupling effects, have been investigated by means of density functional theory (DFT) calculations, taking as a prototypic case study a series of binuclear complexes [LCu(O₂COMe)Ln(thd)₂], where L^2- = N,N'-2,2-dimethyl-propylene-di(3-methoxy-salicylidene-iminato) and Ln = Tb, Lu, and Gd. Particular attention has been devoted to the Cu-Tb complex, which shows a quasi-degenerate nonrelativistic ground state. Challenging the limits of density functional theory (DFT), we devised a practical route to obtain different convergent solutions, permuting the starting guess orbitals in a manner resembling the run of the β electron formally originating from the f⁸ configuration of the Tb(III) over seven molecular orbitals (MOs) with predominant f-type character. Although the obtained states cannot be claimed as the DFT computed split of the ⁷F multiplet, the results are yet interesting numeric experiments, relevant for the ligand field effects. We also performed broken symmetry (BS) DFT estimation of exchange coupling in the Cu-Gd system, using different settings, with Gaussian-type and plane-wave bases, finding a good match with the coupling parameter from experimental data. We also caught BS-type states for each of the mentioned series of different states emulated for the Cu-Tb complex, finding almost equal exchange coupling parameters throughout the seven LF-like configurations, the magnitude of the J parameter being comparable with those of the Cu-Gd system.

Quenching the Quantum Tunneling of Magnetization in Heterometallic Octanuclear {TMIII4 DyIII4 } (TM=Co and Cr) Single-Molecule Magnets by Modification of the Bridging Ligands and Enhancing the Magnetic Exchange Coupling

We report the synthesis, structural characterisation, magnetic properties and provide an ab initio analysis of the magnetic behaviour of two new heterometallic octanuclear coordination complexes containing Co^III and Dy^III ions. Single-crystal X-ray diffraction studies revealed molecular formulae of [Co^III₄ Dy^III₄ (μ-OH)₄ (μ₃ -OMe)₄ {O₂ CC(CH₃ )₃ }₄ (tea)₄ (H₂ O)₄ ]⋅4 H₂ O (1) and [Co^III₄ Dy^III₄ (μ-F)₄ (μ₃ -OH)₄ (o-tol)₈ (mdea)₄ ]⋅ 3 H₂ O⋅EtOH⋅MeOH (2; tea^3- =triply deprotonated triethanolamine; mdea^2- =doubly deprotonated N-methyldiethanolamine; o-tol=o-toluate), and both complexes display an identical metallic core topology. Furthermore, the theoretical, magnetic and SMM properties of the isostructural complex, [Cr^III₄ Dy^III₄ (μ-F₄ )(μ₃ -OMe)_1.25 (μ₃ -OH)_2.75 (O₂ CPh)₈ (mdea)₄ ] (3), are discussed and compared with a structurally similar complex, [Cr^III₄ Dy^III₄ (μ₃ -OH)₄ (μ-N₃ )₄ (mdea)₄ (O₂ CC(CH₃ )₃ )₄ ] (4). DC and AC magnetic susceptibility data revealed single-molecule magnet (SMM) behaviour for 1-4. Each complex displays dynamic behaviour, highlighting the effect of ligand and transition metal ion replacement on SMM properties. Complexes 2, 3 and 4 exhibited slow magnetic relaxation with barrier heights (U_eff ) of 39.0, 55.0 and 10.4 cm^-1 respectively. Complex 1, conversely, did not exhibit slow relaxation of magnetisation above 2 K. To probe the variance in the observed U_eff  values, calculations by using CASSCF, RASSI-SO and POLY_ANISO routine were performed on these complexes to estimate the nature of the magnetic coupling and elucidate the mechanism of magnetic relaxation. Calculations gave values of J_Dy-Dy as -1.6, 1.6 and 2.8 cm^-1 for complexes 1, 2 and 3, respectively, whereas the J_Dy-Cr interaction was estimated to be -1.8 cm^-1 for complex 3. The developed mechanism for magnetic relaxation revealed that replacement of the hydroxide ion by fluoride quenched the quantum tunnelling of magnetisation (QTM) significantly, and led to improved SMM properties for complex 2 compared with 1. However, the tunnelling of magnetisation at low-lying excited states was still operational for 2, which led to low-temperature QTM relaxation. Replacement of the diamagnetic Co^III ions with paramagnetic Cr^III led to Cr^III ⋅⋅⋅Dy^III coupling, which resulted in quenching of QTM at low temperatures for complexes 3 and 4. The best example was found if both Cr^III and fluoride were present, as seen for complex 3, for which both factors additively quenched QTM and led to the observation of highly coercive magnetic hysteresis loops above 2 K. Herein, we propose a synthetic strategy to quench the QTM effects in lanthanide-based SMMs. Our strategy differs from existing methods, in which parameters such as magnetic coupling are difficult to control, and it is likely to have implications beyond the Dy^III SMMs studied herein.

Key role of higher order symmetry and electrostatic ligand field design in the magnetic relaxation of low-coordinate Er(iii) complexes

Our theoretical analysis highlights that both symmetry and a suitable ligand field is required to obtain large barrier heights in SIMs. Key role of Lanthanide–halogen covalency in enhancing U_eff is discussed.

Three series of heterometallic NiII–LnIII Schiff base complexes: synthesis, crystal structures and magnetic characterization

Three series of Ni^II–Ln^III complexes were synthesized using compartmental Schiff base ligands in conjunction with auxiliary ligands. Their magnetic properties have been well studied.