Slow magnetic relaxation at zero field in the tetrahedral complex [Co(SPh)4]2-

Construction of a general library for the rational design of nanomagnets and spin qubits based on mononuclear f-block complexes. The polyoxometalate case

This paper belongs to a series of contributions aiming at establishing a general library that helps in the description of the crystal field (CF) effect of any ligand on the splitting of the J ground states of mononuclear f-element complexes. Here, the effective parameters associated with the oxo ligands (effective charges and metal-ligand distances) are extracted from the study of the magnetic properties of the first two families of single-ion magnets based on lanthanoid polyoxometalates (POMs), formulated as [Ln(W5O18)2](9-) and [Ln(β2-SiW11O39)2](13-) (Ln = Tb, Dy, Ho, Er, Tm, Yb). This effective CF approach provides a good description of the lowest-lying magnetic levels and the associated wave functions of the studied systems, which is fully consistent with the observed magnetic behavior. In order to demonstrate the predictive character of this model, we have extended our model in a first step to calculate the properties of the POM complexes of the early 4f-block metals. In doing so, [Nd(W5O18)2](9-) has been identified as a suitable candidate to exhibit SMM behavior. Magnetic experiments have confirmed such a prediction, demonstrating the usefulness of this strategy for the directed synthesis of new nanomagnets. Thus, with an effective barrier of 51.4 cm(-1) under an applied dc field of 1000 Oe, this is the second example of a Nd(3+)-based single-ion magnet.

Identification of the chromophore in the apatite pigment [Sr10(PO4)6(Cu(x)OH(1-x-y))2]: linear OCuO- featuring a resonance Raman effect, an extreme magnetic anisotropy, and slow spin relaxation

A new chromophore has been identified in copper-doped apatite pigments having the general composition [Sr(10)(PO(4))(6)(Cu(x)OH(1-x-y))(2)], in which x=0.1, 0.3 and y=0.01-0.42. By using X-ray absorption spectroscopy, low-temperature magnetization measurements, and synchrotron X-ray powder structure refinement, it has been shown that the oxygenated compounds contain simultaneously diamagnetic Cu(1+) and paramagnetic Cu(3+) with S=1. Cu(3+) is located at the same crystallographic position as Cu(1+), being linearly coordinated by two oxygen atoms and forming the OCuO(-) anion. The Raman spectroscopy study of [A(10)(PO(4))(6)(Cu(x)OH(1-x-y))(2)], in which A=Ca, Sr, Ba, reveals resonance bands at 651-656 cm(-1) assigned to the symmetric stretching vibration (ν(1)) of OCuO(-). The strontium apatite pigment exhibits a strong paramagnetic anisotropy with an unprecedentedly large negative zero-field splitting parameter (D) of ≈-400 cm(-1). The extreme magnetic anisotropy causes slow magnetization relaxation with relaxation times (τ) up to 0.3 s at T=2 K, which relates the compounds to single-ion magnets. At low temperature, τ is limited by a spin quantum-tunneling, whereas at high temperature a thermally activated relaxation prevails with U(eff)≈48 cm(-1). Strong dependence of τ on the paramagnetic center concentration at low temperature suggests that the spin-spin relaxation dominates in the spin quantum-tunneling process. The compound is the first example of a d-metal-based single-ion magnet with S=1, the smallest spin at which an energy barrier arises for the spin flipping.

Slow magnetic relaxation in a mononuclear eight-coordinate cobalt(II) complex

The quest for the single-molecular magnets (SMMs) based on mononuclear transition-metal complexes is focused on the low-coordinate species. No transition-metal complex with a coordination number of eight has been shown to exhibit SMM properties. Here the magnetic studies have been carried out for a mononuclear, eight-coordinate cobalt(II)-12-crown-4 (12C4) complex [Co(II)(12C4)2](I3)2(12C4) (1) with a large axial zero-field splitting. Magnetic measurements show field-induced, slow magnetic relaxation under an applied field of 500 Oe at low temperature. The magnetic relaxation time τ was fitted by the Arrhenius model to afford an energy barrier of Ueff = 17.0 cm(-1) and a preexponential factor of τ0 = 1.5 × 10(-6) s. The work here presents the first example of the eight-coordinate, mononuclear, 3d metal complex exhibiting the slow magnetic relaxation.

Single-molecule magnetism in a pentacoordinate cobalt(II) complex supported by an antenna ligand

Pentacoordinate complex [CoL(3)Cl2] with a tridentate antenna-like ligand L(3) forms a dimer held by short π-π stacking with head-to-head contacts at 3.4 Å. The direct-current (dc) magnetic susceptibility and magnetization data confirm weak ferromagnetic interaction and a large-magnetic anisotropy, D/hc = 150 cm(-1) and E/hc = 11.6 cm(-1). The system shows superparamagnetic behavior at low temperature that depends upon the applied magnetic field. At Bdc = 0.2 T, a low-frequency peak at the out-of-phase susceptibility is seen (ν ∼ 0.3 Hz), whereas the onset of the second peak appears at ν > 1500 Hz, indicating the existence of two slow relaxation processes.

Magnetic anisotropy of mononuclear Ni(II) complexes: on the importance of structural diversity and the structural distortions

Mononuclear Ni(II) complexes are particularly attractive in the area of single-molecule magnets as the axial zero-field splitting (D) for the Ni(II) complexes is in the range of -200 to +200 cm(-1) . Despite this advantage, very little is known on the origin of anisotropy across various coordination ligands, coordination numbers, and particularly what factors influence the D parameter in these complexes. To answer some of these questions, herein we have undertaken a detailed study of a series of mononuclear Ni(II) complexes with ab initio calculations. Our results demonstrate that three prominent spin-conserved low-lying d-d transitions contribute significantly to the D value. Variation in the sign and the magnitude of D values are found to correlate to the specific structural distortions. Apart from the metal-ligand bond lengths, two different parameters, namely, Δα and Δβ, which are correlated to the cis angles present in the coordination environment, are found to significantly influence the axial D values. Developed magneto-structural D correlations suggest that the D values can be enhanced significantly by fine tuning the structural distortion in the coordination environment. Calculations performed on a series of Ni(II) models with coordination numbers two to six unfold an interesting observation-the D parameter increases significantly upon a reduction in coordination number compared with a reference octahedral coordination. Besides, if high symmetry is maintained, even larger coordination numbers yield large D values.

A SIM-MOF: three-dimensional organisation of single-ion magnets with anion-exchange capabilities

The formation of a metal-organic framework (MOF) with nodes that have single-molecule magnet (SMM) behaviour has been achieved by using mononuclear lanthanoid analogues, also known as single-ion magnets (SIMs), which enormously simplifies the challenging issue of making SMM-MOFs. Here we present a rational design of a family of MOFs, [Ln(bipyNO)4](TfO)3⋅x solvent (Ln=Tb (1); Dy (2); Ho (3); Er (4); TfO=triflate), in which the lanthanoid centres have an square-antiprismatic coordination environment suitable for SIM behaviour. Magnetic measurements confirm the existence of slow magnetic relaxation typical of SMMs, which has been rationalised by means of a radial effective charge model. In addition, we have explored the incorporation of bulky polyoxometalates (POMs) into the cavities of the SIM-MOF by anion exchange, finding that they do not interfere with the slow magnetic relaxation. This demonstrates the robustness of the frameworks and opens the possibility of incorporating non-innocent anions.

Single-molecule magnet engineering: building-block approaches

Tailoring the specific magnetic properties of any material relies on the topological control of the constituent metal ion building blocks. Although this general approach does not seem to be easily applied to traditional inorganic bulk magnets, coordination chemistry offers a unique tool to delicately tune, for instance, the properties of molecules that behave as "magnets", the so-called single-molecule magnets (SMMs). Although many interesting SMMs have been prepared by a more or less serendipitous approach, the assembly of predesigned, isolatable molecular entities into higher nuclearity complexes constitutes an elegant and fascinating strategy. This Feature article focuses on the use of building blocks or modules (both terms being used indiscriminately) to direct the structure, and therefore also the magnetic properties, of metal ion complexes exhibiting SMM behaviour.

Giant magnetic anisotropy and tunnelling of the magnetization in Li₂(Li(1-x)Fe(x))N

Large magnetic anisotropy and coercivity are key properties of functional magnetic materials and are generally associated with rare earth elements. Here we show an extreme, uniaxial magnetic anisotropy and the emergence of magnetic hysteresis in Li₂(Li(1-x)Fe(x))N. An extrapolated, magnetic anisotropy field of 220 T and a coercivity field of over 11 T at 2 K outperform all known hard ferromagnets and single-molecular magnets. Steps in the hysteresis loops and relaxation phenomena in striking similarity to single-molecular magnets are particularly pronounced for x≪1 and indicate the presence of nanoscale magnetic centres. Quantum tunnelling, in the form of temperature-independent relaxation and coercivity, deviation from Arrhenius behaviour and blocking of the relaxation, dominates the magnetic properties up to 10 K. The simple crystal structure, the availability of large single crystals and the ability to vary the Fe concentration make Li₂(Li(1-x)Fe(x))N an ideal model system to study macroscopic quantum effects at elevated temperatures and also a basis for novel functional magnetic materials.

Simple mononuclear cobalt(II) complex: a single-molecule magnet showing two slow relaxation processes

Complex [Co(PPh3)2Br2] possesses intermediate magnetic anisotropy, D/hc = -13 cm(-1). It displays superparamagnetic behavior either in the absence of the magnetic field or in fields of Bdc = 0.05, 0.1, 0.15, and 0.2 T. At Bdc = 0.1 T, the barrier to spin reversal U/kB = 37 K and the extrapolated relaxation time τ0 = 9.4 × 10(-11) s confirm its classification as a single-molecule magnet. At Bdc = 0.2 T, two relaxation processes are evidenced.

Switching off the single-molecule magnet properties of the [CoII(Me6tren)(OH2)]2+ module by complexation with trans-[RuIII(salen)(CN)2]−

A new mononuclear pentacoordinate Co^II SMM was synthesized and associated with trans-[Ru^III(salen)(CN)₂]⁻ to form a trinuclear complex without SMM properties.

Assembly of heterobimetallic NiII–LnIII (LnIII = DyIII, TbIII, GdIII, HoIII, ErIII, YIII) complexes using a ferrocene ligand: slow relaxation of the magnetization in DyIII, TbIII and HoIII analogues

Structure and characterization of a new family of dinuclear 3d–4f heterobimetallic complexes [LNi(H₂O)(μ-OAc)Ln(NO₃)₂]·CH₃CN; {Ln = Dy^III (1), Tb^III (2), Ho^III (3), Gd^III (4), Er^III (5), Y^III (6)} using a ferrocene-based compartmental ligand H₂L.

Unprecedented magnetic relaxation via the fourth excited state in low-coordinate lanthanide single-ion magnets: a theoretical perspective

How to enhance the barrier heights in lanthanide based SIMs? Using ab initio calculations, we have explored the role of symmetry and the coordination number on the magnetization dynamics of four three-coordinate Ln(iii) SIMs.

Heptanuclear lanthanide [Ln7] clusters: from blue-emitting solution-stable complexes to hybrid clusters

A family of heptanuclear lanthanide [Ln₇] complexes was isolated and fully characterized by magnetic measurements, NMR and absorption–emission spectroscopy.

Huge magnetic anisotropy in a trigonal-pyramidal nickel(II) complex

The work presented herein shows the experimental and theoretical studies of a mononuclear nickel(II) complex with the largest magnetic anisotropy ever reported. The zero-field-splitting D parameter, extracted from the fits of the magnetization and susceptibility measurements, shows a large value of -200 cm(-1), in agreement with the theoretical value of -244 cm(-1) obtained with the CASPT2-RASSI method.

Inspiration from old molecules: field-induced slow magnetic relaxation in three air-stable tetrahedral cobalt(II) compounds

We have investigated the dynamics of the magnetization of three four-coordinate mononuclear cobalt(II) compounds, which are synthesized conveniently and are air stable. Slow magnetic relaxation effects were observed for the compounds in the presence of a dc magnetic field.

Evolvement of molecular nanomagnets in China

Molecular nanomagnets have been undergoing development for 20 years since the first single-molecule magnet (SMM), Mn₁₂Ac, was characterized as the molecule-behaved magnet. The multi-disciplinary scientists promoted the magnetic characteristics to be more suitable for use in information science and spintronics. The concept of molecular nanomagnets has also evolved to include single-chain magnets (SCMs), single-ion magnets (SIMs) and even magnetic molecules that showed only slow magnetic relaxation, in addition to the initial cluster-type SMMs. In this review, several aspects, including SMMs, SCMs and SIMs, are introduced briefly through some representative examples. In particular, the contribution of Chinese chemists is highlighted in the design, synthesis and understanding of various types of molecular nanomagnets.