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.     

Ligand π‐Radical Interaction with f‐Shell Unpaired Electrons in Phthalocyaninato–Lanthanoid Single‐Molecule Magnets: A Solution NMR Spectroscopic and DFT Study

The phthalocyaninato double‐decker complexes [M(obPc)₂]⁰ (M= Y^III, Tb^III, Dy^III; obPc=2,3,9,10,16,17,23,24‐octabutoxyphthalocyaninato), along with their reduced ([M(obPc)₂]^?[P(Ph)₄]⁺; M=Tb^III, Dy^III) and oxidized ([M(obPc)₂]⁺[SbCl₆]^? (M=Y^III, Tb^III) counterparts were studied with ¹H, ¹³C and 2D NMR. From the NMR data of the neutral (i.e., with one unpaired electron in the ligands) and anionic Tb^III complexes, along with the use of dispersion corrected DFT methods, it was possible to separate the metal‐centered and ligand‐centered contributions to the hyperfine NMR shift. These contributions to the ¹H and ¹³C hyperfine NMR shifts were further analyzed in terms of pseudocontact and Fermi contact shifts. Furthermore, from a combination of NMR data and DFT calculations, we have determined the spin multiplicity of the neutral complexes [M(obPc)₂]⁰ (M=Tb^III and Dy^III) at room temperature. From the NMR data of the cationic Tb^III complex, for which actually no experimental structure determination is available, we have analyzed the structural changes induced by oxidation from its neutral/anionic species and shown that the interligand distance decreases upon oxidation. The fast electron exchange process between the neutral and anionic Tb^III double‐decker complexes was also studied.     

A Uranium‐Based UO2+–Mn2+ Single‐Chain Magnet Assembled trough Cation–Cation Interactions

Single‐chain magnets (SCMs) are materials composed of magnetically isolated one‐dimensional (1D) units exhibiting slow relaxation of magnetization. The occurrence of SCM behavior requires the fulfillment of stringent conditions for exchange and anisotropy interactions. Herein, we report the synthesis, the structure, and the magnetic characterization of the first actinide‐containing SCM. The 5f–3d heterometallic 1D chains [{[UO₂(salen)(py)][M(py)₄](NO₃)}]_n, (M=Cd ( 1 ) and M=Mn ( 2 ); py=pyridine) are assembled trough cation–cation interaction from the reaction of the uranyl(V) complex [UO₂(salen)py][Cp*₂Co] (Cp*=pentamethylcyclopentadienyl) with Cd(NO₃)₂ or Mn(NO₃)₂ in pyridine. The infinite UMn chain displays a high relaxation barrier of 134±0.8 K (93±0.5 cm^?1), probably as a result of strong intra‐chain magnetic interactions combined with the high Ising anisotropy of the uranyl(V) dioxo group. It also exhibits an open magnetic hysteresis loop at T<6 K, with an impressive coercive field of 3.4 T at 2 K.     

Iron Polyoxometalate Single‐Molecule Magnets

Iron sandwich on a tungstate bun : Two new polyoxotungstates with paramagnetic iron(III) heteroatoms (see structure, W blue, Fe yellow, O red) possess S=15/2 and S=5 ground states. Both compounds are single‐molecule magnets, and the hexairon species shows large hysteresis (see picture) and quantum tunneling effects at low temperature. Electrochemical studies indicate that these species are stable in solution for a wide range of pH values.

     

Density functional study of the conformers of Co4‐based single‐molecule magnet

We present a detailed density functional theory‐based investigation on the geometry and electronic structure of the [Co₄(hmp)₄(MeOH)₄Cl₄] molecule. It is experimentally found to behave as a molecular magnet. The all‐electron electronic structure calculations and geometry optimization of the 88‐atom molecule were carried out within the generalized gradient approximation to the exchange correlation energy. We also study the electronic structures and geometries of a few low‐lying conformers of this molecule. It is found that the magnetic anisotropy energy is highly sensitive to the geometric structure of the molecule. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 324–331, 2003     

Rational Design of a Lanthanide‐Based Complex Featuring Different Single‐Molecule Magnets

The rational synthesis of the 2‐{1‐methylpyridine‐N‐oxide‐4,5‐[4,5‐bis(propylthio)tetrathiafulvalenyl]‐1H‐benzimidazol‐2‐yl}pyridine ligand ( L ) is described. It led to the tetranuclear complex [Dy₄(tta)₁₂( L )₂] ( Dy‐Dy₂‐Dy ) after coordination reaction with the precursor Dy(tta)₃?2 H₂O (tta^?=2‐thenoyltrifluoroacetonate). The X‐ray structure of Dy‐Dy₂‐Dy can be described as two terminal mononuclear units bridged by a central antiferromagnetically coupled dinuclear complex. The terminal N₂O₆ and central O₈ environments are described as distorted square antiprisms. The ac magnetism measurements revealed a strong out‐of‐phase signal of the magnetic susceptibility with two distinct sets of data. The high‐ and low‐frequency components were attributed to the two terminal mononuclear single‐molecule magnets (SMMs) and the central dinuclear SMM, respectively. A magnetic hysteresis loop was detected at very low temperature. From both structural and magnetic points of view, the tetranuclear SMM Dy‐Dy₂‐Dy is a self‐assembly of two known mononuclear SMMs bridged by a known dinuclear SMM.     

Butterfly‐Shaped Pentanuclear Dysprosium Single‐Molecule Magnets

Two new “butterfly‐shaped” pentanuclear dysprosium(III) clusters, [Dy₅(μ₃‐OH)₃(opch)₆(H₂O)₃] ? 3 MeOH ? 9 H₂O ( 1 ) and [Dy₅(μ₃‐OH)₃(Hopch)₂(opch)₄(MeOH)(H₂O)₂] ? (ClO₄)₂ ? 6 MeOH ? 4 H₂O ( 2 ), which were based on the heterodonor‐chelating ligand o‐vanillin pyrazine acylhydrazone (H₂opch), have been successfully synthesized by applying different reaction conditions. Single‐crystal X‐ray diffraction analysis revealed that the butterfly‐shaped cores in both compounds were comparable. However, their magnetic properties were drastically different. Indeed, compound 1 showed dual slow‐relaxation processes with a transition between them that corresponded to energy gaps (Δ) of 8.1 and 37.9 K and pre‐exponential factors (τ₀) of 1.7×10^?5 and 9.7×10^?8 s for the low‐ and high‐temperature domains, respectively, whilst only a single relaxation process was noted for compound 2 (Δ=197 K, τ₀=3.2×10^?9 s). These significant disparities are most likely due to the versatile coordination of the H₂opch ligands with particular keto–enol tautomerism, which alters the strength of the local crystal field and, hence, the nature or direction of the easy axes of anisotropic dysprosium ions.     

Enantioselective Self‐Assembly of Triangular Dy3 Clusters with Single‐Molecule Magnet Behavior

Three pairs of enantiopure chiral triangular Ln₃ clusters, [Ln₃L_{RRRRRR/SSSSSS}(μ₃‐OH)₂(H₂O)₂(SCN)₄]?xCH₃OH?yH₂O ( R ‐Dy₃ , Ln=Dy, x=6, y=0; S ‐Dy₃ , Ln=Dy, x=6, y=1; R ‐Ho₃ , Ln=Ho, x=6, y=1; S ‐Ho₃ , Ln=Ho, x=6, y=1; R ‐Er₃ , Ln=Er, x=6, y=0; S ‐Er₃ , Ln=Er, x=6, y=1), have been successfully synthesized by a rational enantioselective synthetic strategy. The core of triangular Ln₃ is bound in the central N₆O₃ of the macrocyclic ligand, and the coordination spheres of Ln ions are completed by four SCN^? anions and two H₂O molecules in axial positions of the macrocycle. The circular dichroism (CD) and vibrational circular dichroism (VCD) spectra of the enantiomers demonstrate that the chirality is successfully transferred from the ligands to the resulting Ln₃ clusters. Ac susceptibility measurements reveal that single‐molecule magnet behavior occurs for both enantiopure clusters of R ‐Dy₃ and S ‐Dy₃ . This work is one of the few examples of the successful design of a pair of triangular Dy₃ clusters showing simultaneously slow magnetic relaxation and optical activity, and this might open up new opportunities to develop novel multifunctional materials.     

Magnetic Bistability of Individual Single‐Molecule Magnets Grafted on Single‐Wall Carbon Nanotubes

A POM to remember : Hexanuclear Fe^III polyoxometalate (POM) single‐molecule magnets (see structure) can be noncovalently assembled on the surface of single‐wall carbon nanotubes. Complementary characterization techniques (see TEM image and magnetic hysteresis loops) demonstrate the integrity and bistability of the individual molecules, which could be used to construct single‐molecule memory devices.

     

Angular‐Resolved Magnetometry Beyond Triclinic Crystals Part II: Torque Magnetometry of Cp*ErCOT Single‐Molecule Magnets

The experimental investigation of the molecular magnetic anisotropy in crystals in which the magnetic centers are symmetry related, but do not have a parallel orientation has been approached by using torque magnetometry. A single crystal of the orthorhombic organometallic Cp*ErCOT [Cp*=pentamethylcyclopentadiene anion (C₅Me₅^?); COT=cyclooctatetraenedianion (C₈H₈^2?)] single‐molecule magnet, characterized by the presence of two nonparallel families of molecules in the crystal, has been investigated above its blocking temperature. The results confirm an Ising‐type anisotropy with the easy direction pointing along the pseudosymmetry axis of the complex, as previously suggested by out‐of‐equilibrium angular‐resolved magnetometry. The use of torque magnetometry, not requiring the presence of magnetic hysteresis, proves to be even more powerful for these purposes than standard single‐crystal magnetometry. Furthermore, exploiting the sensitivity and versatility of this technique, magnetic anisotropy has been investigated up to 150 K, providing additional information on the crystal‐field splitting of the ground J multiplet of the Er^III ion.     

[UIII{N(SiMe2tBu)2}3]: A Structurally Authenticated Trigonal Planar Actinide Complex

We report the synthesis and characterization of the uranium(III) triamide complex [U^III(N**)₃] [ 1 , N**=N(SiMe₂tBu)₂^?]. Surprisingly, complex 1 exhibits a trigonal planar geometry in the solid state, which is unprecedented for three‐coordinate actinide complexes that have exclusively adopted trigonal pyramidal geometries to date. The characterization data for [U^III(N**)₃] were compared with the prototypical trigonal pyramidal uranium(III) triamide complex [U^III(N“)₃] (N”=N(SiMe₃)₂^?), and taken together with theoretical calculations it was concluded that pyramidalization results in net stabilization for [U^III(N“)₃], but this can be overcome with very sterically demanding ligands, such as N**. The planarity of 1 leads to favorable magnetic dynamics, which may be considered in the future design of U^III single‐molecule magnets.     

Heterodinuclear Lanthanoid‐Containing Polyoxometalates: Stepwise Synthesis and Single‐Molecule Magnet Behavior

Polyoxometalates (POMs) with heterodinuclear lanthanoid cores, TBA₈H₄[{Ln(μ₂‐OH)₂Ln′}(γ‐SiW₁₀O₃₆)₂] ( LnLn′ ; Ln=Gd, Dy; Ln′=Eu, Yb, Lu; TBA=tetra‐n‐butylammonium), were successfully synthesized through the stepwise incorporation of two types of lanthanoid cations into the vacant sites of lacunary [γ‐SiW₁₀O₃₆]^8? units without the use of templating cations. The incorporation of a Ln³⁺ ion into the vacant site between two [γ‐SiW₁₀O₃₆]^8? units afforded mononuclear Ln³⁺‐containing sandwich‐type POMs with vacant sites ( Ln1 ; TBA₈H₅[{Ln(H₂O)₄}(γ‐SiW₁₀O₃₆)₂]; Ln=Dy, Gd, La). The vacant sites in Ln1 were surrounded by coordinating W? O and Ln? O oxygen atoms. On the addition of one equivalent of [Ln′(acac)₃] to solutions of Dy1 or Gd1 in 1,2‐dichloroethane (DCE), heterodinuclear lanthanoid cores with bis(μ₂‐OH) bridging ligands, [Dy(μ₂‐OH)₂Ln′]⁴⁺, were selectively synthesized ( LnLn′ ; Ln=Dy, Gd; Ln′=Eu, Yb, Lu). On the other hand, La1 , which contained the largest lanthanoid cation, could not accommodate a second Ln′³⁺ ion. DyLn′ showed single‐molecule magnet behavior and their energy barriers for magnetization reversal (ΔE/k_B) could be manipulated by adjusting the coordination geometry and anisotropy of the Dy³⁺ ion by tuning the adjacent Ln′³⁺ ion in the heterodinuclear [Dy(μ₂‐OH)₂Ln′]⁴⁺ cores. The energy barriers increased in the order: DyLu (ΔE/k_B=48 K)< DyYb (53 K)< DyDy (66 K)< DyEu (73 K), with an increase in the ionic radii of Ln′³⁺; DyEu showed the highest energy barrier.     

Self‐Assembly of a Giant Tetrahedral 3 d–4 f Single‐Molecule Magnet within a Polyoxometalate System

A giant tetrahedral heterometallic polyoxometalate (POM) [Dy₃₀Co₈Ge₁₂W₁₀₈O₄₀₈(OH)₄₂(OH₂)₃₀]^56?, which shows single‐molecule magnet (SMM) behavior, is described. This hybrid contains the largest number of 4f ions of any polyoxometalate (POM) reported to date and is the first to incorporate two different 3d–4f and 4f coordination cluster assemblies within same POM framework.     

A Trigonal‐Pyramidal Erbium(III) Single‐Molecule Magnet

Given the recent advent of mononuclear single‐molecule magnets (SMMs), a rational approach based on lanthanides with axially elongated f‐electron charge cloud (prolate) has only recently received attention. We report herein a new SMM, [Li(THF)₄[Er{N(SiMe₃)₂}₃Cl]?2 THF, which exhibits slow relaxation of the magnetization under zero dc field with an effective barrier to the reversal of magnetization (ΔE_eff/k_B=63.3 K) and magnetic hysteresis up to 3 K at a magnetic field sweep rate of 34.6 Oe s^?1. This work questions the theory that oblate or prolate lanthanides must be stabilized with the appropriate ligand framework in order for SMM behavior to be favored.     

Adding Remnant Magnetization and Anisotropic Exchange to Propeller‐like Single‐Molecule Magnets through Chemical Design

The selective replacement of the central iron(III) ion with vanadium(III) in a tetrairon(III) propeller‐shaped single‐molecule magnet has allowed us to increase the ground spin state from S=5 to S=13/2. As a consequence of the pronounced anisotropy of vanadium(III), the blocking temperature for the magnetization has doubled. Moreover, a significant remnant magnetization, practically absent in the parent homometallic molecule, has been achieved owing to the suppression of zero‐field tunneling of the magnetization for the half‐integer molecular spin. Interestingly, the contribution of vanadium(III) to the magnetic anisotropy barrier occurs through the anisotropic exchange interaction with iron(III) spins and not through single ion anisotropy as in most single‐molecule magnets.