Heterometallic 3d–4f Complexes as Single‐Molecule Magnets

Heterometallic 3d‐4f complexes are being investigated, for some time, as being useful in molecular magnetism, particularly as single‐molecule magnets (SMMs). This interest is primarily because of the possibility of an increased ligand‐mediated super‐exchange phenomenon between the 3d and 4f metal ions. Such an interaction, apart from bestowing a favorable ground‐state spin to the complex, also assists in reducing quantum tunneling of magnetization that is widely prevalent in SMMs making them to lose magnetization. However, assembling both 3d as well as 4f ions using same ligand system is challenging and involves the design of multi‐site coordination ligands with specific coordination compartments for the 3d and the 4f metal ions while at the same time allowing these disparate metal ions to be linked to each other through a bridging ligating atom. This review presents a summary of the 3d‐4f complexes primarily derived from the author's work while alluding to important examples from the literature. We also provide an outlook for the future design of such complexes.     

Tuning the Magnetization Dynamic Properties of Nd⋅⋅⋅Fe and Nd⋅⋅⋅Co Single‐Molecular Magnets by Introducing 3 d–4 f Magnetic Interactions

By using paramagnetic [Fe(CN)₆]^3? anions in place of diamagnetic [Co(CN)₆]^3? anions, two field‐induced mononuclear single‐molecular magnets, [Nd(18‐crown‐6)(H₂O)₄][Co(CN)₆] ? 2 H₂O ( 1 ) and [Nd(18‐crown‐6)(H₂O)₄][Fe(CN)₆] ? 2 H₂O ( 2 ), have been synthesized and characterized. Single‐crystal X‐ray diffraction analysis revealed that compounds 1 and 2 were ionic complexes. The Nd^III ions were located inside the cavities of the 18‐crown‐6 ligands and were each bound by four water molecules on either side of the crown ether. Magnetic investigations showed that these compounds were both field‐induced single‐molecular magnets. By comparing the slow relaxation behaviors of compounds 1 and 2 , we found significant differences between the direct and Raman processes for these two complexes, with a stronger direct process in compound 2 at low temperatures. Complete active space self‐consistent field (CASSCF) calculations were also performed on two [Nd(18‐crown‐6)(H₂O)₄]³⁺ fragments of compounds 1 and 2 . Ab initio calculations showed that the magnetic anisotropies of the Nd^III centers in complexes 1 and 2 were similar to each other, which indicated that the difference in relaxation behavior was not owing to the magnetic anisotropy of Nd^III. Our analysis showed that the magnetic interaction between the Nd^III ion and the low‐spin Fe^III ion in complex 2 played an important role in enhancing the direct process and suppressing the Raman process of the single‐molecular magnet.     

Tuning the Magnetic Interactions and Relaxation Dynamics of Dy2 Single‐Molecule Magnets

Efficient modulation of single‐molecule magnet (SMM) behavior was realized by deliberate structural modification of the Dy₂ cores of [Dy₂( a ′ povh )₂(OAc)₂(DMF)₂] ( 1 ) and [Zn₂Dy₂( a′povh )₂(OAc)₆] ? 4 H₂O ( 2 ; H₂ a ′ povh =N′‐[amino(pyrimidin‐2‐yl)methylene]‐o‐vanilloyl hydrazine). Compound 1 having fourfold linkage between the two dysprosium ions shows high‐performance SMM behavior with a thermal energy barrier of 322.1 K, whereas only slow relaxation is observed for compound 2 with only twofold connection between the dysprosium ions. This remarkable discrepancy is mainly because of strong axiality in 1 due to one pronounced covalent bond, as revealed by experimental and theoretical investigations. The significant antiferromagnetic interaction derived from bis(μ₂‐O) and two acetate bridging groups was found to be crucial in leading to a nonmagnetic ground state in 1 , by suppressing zero‐field quantum tunneling of magnetization.     

Magnetic Relaxation in Single‐Electron Single‐Ion Cerium(III) Magnets: Insights from Ab Initio Calculations

Detailed ab initio calculations were performed on two structurally different cerium(III) single‐molecule magnets (SMMs) to probe the origin of magnetic anisotropy and to understand the mechanism of magnetic relaxations. The complexes [Ce^III{Zn^II(L)}₂(MeOH)]BPh₄ ( 1 ) and [Li(dme)₃][Ce^III(cot′′)₂] ( 1 ; L=N,N,O,O‐tetradentate Schiff base ligand; 2 ; DME=dimethoxyethane, COT′′=1,4‐bis(trimethylsilyl)cyclooctatetraenyldianion), which are reported to be zero‐field and field‐induced SMMs with effective barrier heights of 21.2 and 30 K respectively, were chosen as examples. CASSCF+RASSI/SINGLE_ANISO calculations unequivocally suggest that m_J|±5/2〉 and |±1/2〉 are the ground states for complexes 1 and 2 , respectively. The origin of these differences is rooted back to the nature of the ligand field and the symmetry around the cerium(III) ions. Ab initio magnetisation blockade barriers constructed for complexes 1 and 2 expose a contrasting energy‐level pattern with significant quantum tunnelling of magnetisation between the ground state Kramers doublet in complex 2 . Calculations performed on several model complexes stress the need for a suitable ligand environment and high symmetry around the cerium(III) ions to obtain a large effective barrier.     

Probing the Origin of Magnetic Anisotropy in a Dinuclear {MnIIICuII} Single‐Molecule Magnet: The Role of Exchange Anisotropy

Using ab initio calculations all the components of the magnetic anisotropy in a dinuclear [Mn^IIICu^IICl(5‐Br‐sap)₂(MeOH)] single‐molecule magnet (SMM) have been computed. These calculations reveal that apart from the single‐ion anisotropy, the exchange anisotropy also plays a crucial role in determining the sign as well as the magnitude of the cluster anisotropy. Developed magneto‐structural correlations suggest that a large ferromagnetic exchange can in fact reduce the ground‐state anisotropy, which is an integral component in the design of SMMs.     

Construction of Nitronyl Nitroxide‐Based 3d–4f Clusters: Structure and Magnetism

Three unprecedented nitronyl nitroxide radical‐bridged 3d–4f clusters, [Ln₂Cu₂(hfac)₁₀(NIT‐3py)₂(H₂O)₂](Ln^III=Y, Gd, Dy), have been obtained from the self‐assembly of Ln(hfac)₃, Cu(hfac)₂, and the radical ligand. The Dy complex shows a slow relaxation of magnetization, representing the first nitronyl nitroxide radical‐based 3d–4f cluster with single‐molecule magnet behavior.     

Large Hexadecametallic {MnIII–LnIII} Wheels: Synthesis,Structural, Magnetic,and Theoretical Characterization

The synthesis, gas sorption studies, magnetic properties, and theoretical studies of new molecular wheels of core type {Mn^III₈Ln^III₈} (Ln=Dy, Ho, Er, Y and Yb), using the ligand mdeaH₂, in the presence of ortho‐toluic or benzoic acid are reported. From the seven wheels studied the {Mn₈Dy₈} and {Mn₈Y₈} analogues exhibit SMM behavior as determined from ac susceptibility experiments in a zero static magnetic field. From DFT calculations a S=16 ground state was determined for the {Mn₈Y₈} complex due to weak ferromagnetic Mn^III–Mn^III interactions. Ab initio CASSCF+RASSI‐SO calculations on the {Mn₈Dy₈} wheel estimated the Mn^III–Dy^III exchange interaction as ?0.1 cm^?1. This weak exchange along with unfavorable single‐ion anisotropy of Dy^III/Mn^III ions, however, led to the observation of SMM behavior with fast magnetic relaxation. The orientation of the g‐anisotropy of the Dy^III ions is found to be perpendicular to the plane of the wheel and this suggests the possibility of toroidal magnetic moments in the cluster. The {Mn₈Ln₈} clusters reported here are the largest heterometallic Mn^IIILn^III wheels and the largest {3d–4f} wheels to exhibit SMM behavior reported to date.     

Magnetic Anisotropy in “Scorpionate” First‐Row Transition‐Metal Complexes: A Theoretical Investigation

In this work we have analyzed in detail the magnetic anisotropy in a series of hydrotris(pyrazolyl)borate (Tp^?) metal complexes, namely [VTpCl]⁺, [CrTpCl]⁺, [MnTpCl]⁺, [FeTpCl], [CoTpCl], and [NiTpCl], and their substituted methyl and tert‐butyl analogues with the goal of observing the effect of the ligand field on the magnetic properties. In the [VTpCl]⁺, [CrTpCl]⁺, [CoTpCl], and [NiTpCl] complexes, the magnetic anisotropy arises as a consequence of out‐of‐state spin–orbit coupling, and covalent changes induced by the substitution of hydrogen atoms on the pyrazolyl rings does not lead to drastic changes in the magnetic anisotropy. On the other hand, much larger magnetic anisotropies were predicted in complexes displaying a degenerate ground state, namely [MnTpCl]⁺ and [FeTpCl], due to in‐state spin–orbit coupling. The anisotropy in these systems was shown to be very sensitive to perturbations, for example, chemical substitution and distortions due to the Jahn–Teller effect. We found that by substituting the hydrogen atoms in [MnTpCl]⁺ and [FeTpCl] by methyl and tert‐butyl groups, certain covalent contributions to the magnetic anisotropy energy (MAE) could be controlled, thereby achieving higher values. Moreover, we showed that the selection of ion has important consequences for the symmetry of the ground spin–orbit term, opening the possibility of achieving zero magnetic tunneling even in non‐Kramers ions. We have also shown that substitution may also contribute to a quenching of the Jahn–Teller effect, which could significantly reduce the magnetic anisotropy of the complexes studied.     

A Linear 3d–4f Tetranuclear CoIII2DyIII2 Single‐Molecule Magnet: Synthesis,Structure, and Magnetic Properties

A linear tetranuclear 3d–4f Co₂Dy₂ cluster assembled from a polydentate Schiff base exhibits single‐molecule magnet (SMM) behavior with an anisotropic barrier of 33.8 K. Due to the presence of diamagnetic cobalt(III) ions, the tetranuclear cluster of 1 behaves magnetically like a dinuclear Dy₂ system. However, the diamagnetic segment might efficiently minimize undesirable intermolecular magnetic interactions, thereby improving the performance of the SMM behavior of 1 . This discrete complex presents us with a unique opportunity to study the magnetic properties and to probe the dynamics of magnetization in a magnetically isolated Dy₂ system.     

Synthesis and characterization of two isostructural 3d–4f coordination compounds based on pyridine‐2,6‐dicarboxylic acid and 4,4′‐bipyridine

The design and synthesis of 3d–4f heterometallic coordination polymers have attracted much interest due to the intriguing diversity of their architectures and topologies. Pyridine‐2,6‐dicarboxylic acid (H₂pydc) has a versatile coordination mode and has been used to construct multinuclear and heterometallic compounds. Two isostructural centrosymmetric 3d–4f coordination compounds constructed from pyridine‐2,6‐dicarboxylic acid and 4,4′‐bipyridine (bpy), namely 4,4′‐bipyridine‐1,1′‐diium diaquabis(μ₂‐pyridine‐2,6‐dicarboxylato)tetrakis(pyridine‐2,6‐dicarboxylato)bis[4‐(pyridin‐4‐yl)pyridinium]cobalt(II)dieuropium(III) octahydrate, (C₁₀H₁₀N₂)[CoEu₂(C₁₀H₉N₂)₂(C₇H₃NO₄)₆(H₂O)₂]·8H₂O, (I), and 4,4′‐bipyridine‐1,1′‐diium diaquabis(μ₂‐pyridine‐2,6‐dicarboxylato)tetrakis(pyridine‐2,6‐dicarboxylato)bis[4‐(pyridin‐4‐yl)pyridinium]cobalt(II)diterbium(III) octahydrate, (C₁₀H₁₀N₂)[CoTb₂(C₁₀H₉N₂)₂(C₇H₃NO₄)₆(H₂O)₂]·8H₂O, (II), were synthesized under hydrothermal conditions and characterized by IR and fluorescence spectroscopy, thermogravimetric analysis and powder X‐ray diffraction. Both compounds crystallize in the triclinic space group P. The Eu^III and Tb^III cations adopt nine‐coordinated distorted tricapped trigonal–prismatic geometries bridged by three pydc^2? ligands. The Co^II cation has a six‐coordination environment formed by two pydc^2? ligands, two bpy ligands and two coordinated water molecules. Adjacent molecules are connected by π–π stacking interactions to form a one‐dimensional chain, which is further extended into a three‐dimensional supramolecular network by multipoint hydrogen bonds.     

Rational Electrostatic Design of Easy‐Axis Magnetic Anisotropy in a ZnII–DyIII–ZnII Single‐Molecule Magnet with a High Energy Barrier

Two novel trinuclear complexes [ZnCl(μ‐L)Ln(μ‐L)ClZn][ZnCl₃(CH₃OH)]?3 CH₃OH (Ln^III=Dy ( 1 ) and Er ( 2 )) have been prepared from the compartmental ligand N,N′‐dimethyl‐N,N′‐bis(2‐hydroxy‐3‐formyl‐5‐bromo‐benzyl)ethylenediamine (H₂L). X‐ray studies reveal that Ln^III ions are coordinated by two [ZnCl(L)]^? units through the phenoxo and aldehyde groups, giving rise to a LnO₈ coordination sphere with square‐antiprism geometry and strong easy‐axis anisotropy of the ground state. Ab initio CASSCF+RASSI calculations carried out on 1 confirm that the ground state is an almost pure M_J=±15/2 Kramers doublet with a marked axial anisotropy, the magnetic moment is roughly collinear with the shortest Dy?O distances. This orientation of the local magnetic moment of the Dy^III ion in 1 is adopted to reduce the electronic repulsion between the oblate electron shape of the M_J=±15/2 Kramers doublet and the phenoxo‐oxygen donor atoms involved in the shortest Dy?O bonds. CASSCF+RASSI calculations also show that the ground and first excited states of the Dy^III ion are separated by 129 cm^?1. As expected for this large energy gap, compound 1 exhibits, in a zero direct‐current field, thermally activated slow relaxation of the magnetization with a large U_eff=140 K. The isostructural Zn–Er–Zn species does not present significant SMM behavior as expected for the prolate electron‐density distribution of the Er^III ion leading to an easy‐plane anisotropy of the ground doublet state.     

Influence of Guest Exchange on the Magnetization Dynamics of Dilanthanide Single‐Molecule‐Magnet Nodes within a Metal–Organic Framework

Multitopic organic linkers can provide a means to organize metal cluster nodes in a regular three‐dimensional array. Herein, we show that isonicotinic acid N‐oxide (HINO) serves as the linker in the formation of a metal–organic framework featuring Dy₂ single‐molecule magnets as nodes. Importantly, guest solvent exchange induces a reversible single‐crystal to single‐crystal transformation between the phases Dy₂(INO)₄(NO₃)₂?2 solvent (solvent=DMF (Dy₂‐DMF), CH₃CN (Dy₂‐CH₃CN)), thereby switching the effective magnetic relaxation barrier (determined by ac magnetic susceptibility measurements) between a negligible value for Dy₂‐DMF and 76 cm^?1 for Dy₂‐CH₃CN. Ab initio calculations indicate that this difference arises not from a significant change in the intrinsic relaxation barrier of the Dy₂ nodes, but rather from a slowing of the relaxation rate of incoherent quantum tunneling of the magnetization by two orders of magnitude.     

Analysis of the Role of Peripheral Ligands Coordinated to ZnII in Enhancing the Energy Barrier in Luminescent Linear Trinuclear Zn‐Dy‐Zn Single‐Molecule Magnets

Three new Dy complexes have been prepared according to a complex‐as‐ligand strategy. Structural determinations indicate that the central Dy ion is surrounded by two LZn units (L^2? is the di‐deprotonated form of the N₂O₂ compartmental N,N′‐2,2‐dimethylpropylenedi(3‐methoxysalicylideneiminato) Schiff base. The Dy ions are nonacoordinate to eight oxygen atoms from the two L ligands and to a water molecule. The Zn ions are pentacoordinate in all cases, linked to the N₂O₂ atoms from L, and the apical position of the Zn coordination sphere is occupied by a water molecule or bromide or chloride ions. These resulting complexes, formulated (LZnX)‐Dy‐(LZnX), are tricationic with X=H₂O and monocationic with X=Br or Cl. They behave as field‐free single‐molecule magnets (SMMs) with effective energy barriers (U_eff) for the reversal of the magnetization of 96.9(6) K with τ₀=2.4×10^?7 s, 146.8(5) K with τ₀=9.2×10^?8 s, and 146.1(10) K with τ₀=9.9×10^?8 s for compounds with Zn?OH₂, Zn?Br, and Zn?Cl motifs, respectively. The Cole–Cole plots exhibit semicircular shapes with α parameters in the range of 0.19 to 0.29, which suggests multiple relaxation processes. Under a dc applied magnetic field of 1000 Oe, the quantum tunneling of magnetization (QTM) is partly or fully suppressed and the energy barriers increase to U_eff=128.6(5) K and τ₀=1.8×10^?8 s for 1 , U_eff=214.7 K and τ₀=9.8×10^?9 s for 2 , and U_eff=202.4 K and τ₀=1.5×10^?8 s for 3 . The two pairs of largely negatively charged phenoxido oxygen atoms with short Dy?O bonds are positioned at opposite sides of the Dy³⁺ ion, which thus creates a strong crystal field that stabilizes the axial M_J=±15/2 doublet as the ground Kramers doublet. Although the compound with the Zn?OH₂ motifs possesses the larger negative charges on the phenolate oxygen atoms, as confirmed by using DFT calculations, it exhibits the larger distortions of the DyO₉ coordination polyhedron from ideal geometries and a smaller U_eff value. Ab initio calculations support the easy‐axis anisotropy of the ground Kramers doublet and predict zero‐field SMM behavior through Orbach and TA‐QTM relaxations via the first excited Kramers doublet, which leads to large energy barriers. In accordance with the experimental results, ab initio calculations have also shown that, compared with water, the peripheral halide ligands coordinated to the Zn²⁺ ions increase the barrier height when the distortions of the DyO₉ have a negative effect. All the complexes exhibit metal‐centered luminescence after excitation into the UV π–π* absorption band of ligand L^2? at λ=335 nm, which results in the appearance of the characteristic Dy^III (⁴F_9/2→⁶H_J/2; J=15/2, 13/2) emission bands in the visible region.     

Heterometallic Fe2II–UV and Ni2II–UV Exchange‐Coupled Single‐Molecule Magnets: Effect of the 3 d Ion on the Magnetic Properties

Uranium‐based compounds have been put forward as ideal candidates for the design of single‐molecule magnets (SMMs) with improved properties, but to date, only two examples of exchange‐coupled 3d–5f SMM containing uranium have been reported and both are based on the Mn^II ion. Here we have synthesized the first examples of exchange‐coupled uranium SMMs based on Fe^II and Ni^II. The SMM behavior of these complexes containing a quasi linear {M?O?U?O?M} core arises from intramolecular Fe?U and Ni?U exchange interactions combined with the high Ising anisotropy of the uranyl(V) moiety. The measured values of the relaxation barrier (53.9±0.9 K in the UFe₂ complex and of 27.4±0.5 K in the UNi₂ complex) show clearly the dependency on the spin value of the transition metal, providing important new information for the future design of improved uranium‐based SMMs.