Bing-Wu Wang

An organometallic single-ion magnet.

An organometallic single-ion magnet is synthesized with only 19 non-hydrogen atoms featuring an erbium ion sandwiched by two different aromatic ligands. This molecule displays a butterfly-shaped hysteresis loop at 1.8 K up to even 5 K. Alternating-current (ac) susceptibility measurement reveals the existence of two thermally activated magnetic relaxation processes with the energy barriers as high as 197 and 323 K, respectively.

A Mononuclear Dysprosium Complex Featuring Single-Molecule-Magnet Behavior†

Single-molecule magnets (SMMs) have received much attention owing to their quantum tunneling and slow relaxation. These behaviors can be observed when the molecule has large ground spin state with a uniaxial magnetic anisotropy, namely negative zero-field splitting (ZFS) parameter D. Besides a large ground state, to increase the SMMs energy barrier and blocking temperature, it is of fundamental importance to control the magnetic anisotropy. It is also of great complexity to understand the conditions that determine the anisotropy or zero-field splitting properties for a cluster. As a result, SMMs containing only one spin carrier are of great interest because of the simplification in the analysis of local anisotropy and ZFS. Recently, some molecules with isolated 3d or 4f metal ions were observed to show a direct-current (dc) field-induced slow magnetic relaxation. Studies show that this kind of dc-field-dependent relaxation phenomenon is thermally activated, and the quench of fast quantum tunneling by the dc field could give rise to the slow relaxation. However, as commented by Benelli and Gatteschi, it is an unexpected and puzzling behavior, and the underlying mechanism is still unclear. Ishikawa et al. reported that the phthalocyanine (Pc) double-decker anion complexes [Pc2Ln] with a single Tb or Dy ion show slow relaxation without a dc field. A single Er ion in the polyoxometallate system [ErW10O36] 9 showed similar relaxation behavior in zero static magnetic field. The first single-actinide complex [U(Ph2BPz2)3] (containing the U III ion; Ph2BPz2 = diphenylbis(pyrazolylborate)) reported by Rinehart and Long shows a similar slow relaxation. Owing to the single-ion features, these complexes could be called “single-ion magnets”. In these complexes, ligand fields with a high-order single axis Cn (n> 2; n = 4 for Ref. [8, 9] and n = 3 for Ref. [10]) split the (2J + 1) degenerate ground states into new sublevels, which produces a uniaxial anisotropy, thus giving rise to a higher energy barrier for relaxation. The Dy ion, which possesses a Kramers ground state of H15/2, is an appealing paramagnetic source for the construction of SMMs in a suitable ligand-field symmetry and strength. Moreover, among reported SMMs, some of them are clusters containing Dy : Dy2, [11] Dy3, [12] Dy4, [13] Dy5, [14] Dy-containing chain, and tens of Dycontaining 3d–4f clusters. All three reported types of single-ion magnets are found with a high-order single axis defining the local symmetry. Pursuing this clue, we synthesized a series of mononuclear Ln compounds with a local symmetry close to D4d. Crystal analysis shows that the isomorphous complexes consist of a neutral mononuclear [Ln(acac)3(H2O)2] complex (Ln = Dy, Ho, Er, acac = acetylacetonate) together with an uncoordinated water molecule and an uncoordinated ethanol molecule (Figure 1a). In the Dy complex, Dy is coordinated by eight oxygen atoms with Dy O bonds of 2.311–2.434 , six of which come from the acetylacetonate ligand and two from coordinated water molecules. The eight oxygen atoms form an approximately square-antiprismatic coordination polyhedron, and the local symmetry of Dy is nearly D4d (Figure 1b). Actually, a similar lanthanide acac complex was firstly reported in 1968, but the uncoordinated water and

Series of Lanthanide Organometallic Single-Ion Magnets

The synthesis, structures, and magnetic properties of a series of lanthanide organometallic mixed sandwich molecules, (Cp*)Ln(COT), are investigated, where Cp* is the pentamethylcyclopentadiene anion and COT is the cyclooctatetraene dianion and Ln represents Tb(III), Dy(III), Ho(III), Er(III), and Tm(III). Among the five complexes, Dy and Ho complexes are determined to be single-ion magnets in addition to the previously reported Er complex. Both Dy and Ho complexes show obvious quantum tunneling magnetization relaxation in the absence of a static field. The diluted Ho complex behaves two sets of thermally activated relaxation as we reported previously in Er due to the COT ring static disorder. A stair-shaped hysteresis for the Er compound can be observed at 1.6 K with Hc = 1 kOe at a sweeping rate over 700 Oe/s. The quantum tunneling decoherence relaxation rate increases from Er to Ho to Dy, which may be caused by the relative increase of transverse anisotropy coming from the larger tilting of the two aromatic rings within the molecule. The fine electronic structure is analyzed with ligand-field theory employing the effective Hamiltonian method. The zero-field splitting is determined to be Ising type, and the energy gap between the ground state and the first excited one is consistent with the barrier obtained by Arrhenius analysis.

Zero-field slow magnetic relaxation from single Co(II) ion: a transition metal single-molecule magnet with high anisotropy barrier

An air-stable star-shaped CoIICoIII3 complex with only one paramagnetic Co(II) ion in the D3 coordination environment has been synthesized from a chiral Schiff base ligand. Magnetic studies revealed that this complex exhibits slow magnetic relaxation in the absence of an applied dc field, which is one of the main characteristics of single-molecule magnets (SMMs). The relaxation barrier is as high as 109 K, which is quite large among transition-metal ion-based SMMs. The complex represents the first example of zero-field SMM behavior in a mononuclear six oxygen-coordinate Co(II) complex.

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.

Two-Coordinate Co(II) Imido Complexes as Outstanding Single-Molecule Magnets

The pursuit of single-molecule magnets (SMMs) with better performance urges new molecular design that can endow SMMs larger magnetic anisotropy. Here we report that two-coordinate cobalt imido complexes featuring highly covalent Co═N cores exhibit slow relaxation of magnetization under zero direct-current field with a high effective relaxation barrier up to 413 cm-1, a new record for transition metal based SMMs. Two theoretical models were carried out to investigate the anisotropy of these complexes: single-ion model and Co-N coupling model. The former indicates that the pseudo linear ligand field helps to preserve the first-order orbital momentum, while the latter suggests that the strong ferromagnetic interaction between Co and N makes the [CoN]+ fragment a pseudo single paramagnetic ion, and that the excellent performance of these cobalt imido SMMs is attributed to the inherent large magnetic anisotropy of the [CoN]+ core with |MJ = ± 7/2⟩ ground Kramers doublet.

Constructing single-chain magnets by supramolecular π-π stacking and spin canting: a case study on manganese(III) corroles.

A single-chain magnet (SCM) was constructed from manganese(III) 5,10,15-tris(pentafluorophenyl)corrole complex [Mn(III) (tpfc)] through supramolecular π-π stacking without bridging ligands. In the crystal structures, [Mn(tpfc)] molecules crystallized from different solvents, such as methanol, ethyl acetate, and ethanol, exhibit different molecular orientations and intermolecular π-π interaction or weak Mn⋅⋅⋅O interaction to form a supramolecular one-dimensional motif or dimer. These three complexes show very different magnetic behaviors at low temperature. Methanol solvate 1 shows obvious frequency dependence of out-of-phase alternating-current magnetic susceptibility below 2 K and a magnetization hysteresis loop with a coercive field of 400 Oe at 0.5 K. It is the first example of spin-canted supramolecular single-chain magnet due to weak π-π stacking interaction. By fitting the susceptibility data χ(M) T (20-300 K) of 1 with the spin Hamiltonian expression H = -2J Σ(i=1)(n-1) S(Ai) S(Ai+1) + D Σ(i) S((iZ)(2)), the intrachain magnetic coupling parameter transmitted by π-π interaction of -0.31 cm(-1) and zero field splitting parameter D of -2.59 cm(-1) are obtained. Ethyl acetate solvate 2 behaves as an antiferromagnetic chain without ordering or slow magnetic relaxation down to 0.5 K. The magnetic susceptibility data χ(M) T (20-300 K) of 2 was fitted by assuming the spin Hamiltonian H = -2JΣ(i=1)(n-1) S(Ai) S(Ai+1), and the intrachain antiferromagnetic coupling constant of -0.07 cm(-1) is much weaker than that of 1. Ethanol solvate 3 with a dimer motif shows field-induced single-molecule magnet like behavior below 2.5 K. The exchange coupling constant J within the dimer propagated by π-π interaction is -0.14 cm(-1) by fitting the susceptibility data χ(M) T (20-300 K) with the spin Hamiltonian H = -2J S(A) S(B) + β(S((A)g(A)) + S((B)g(B)))H. The present studies open a new way to construct SCMs from anisotropic magnetic single-ion units through weak intermolecular interactions in the absence of bridging ligands.

An A-Site Mixed-Ammonium Solid Solution Perovskite Series of [(NH2NH3)x(CH3NH3)1−x][Mn(HCOO)3] (x=1.00–0.67)

The A-site mixed-ammonium solid solutions of metal-organic perovskites [(NH2 NH3 )x (CH3 NH3 )1-x ][Mn(HCOO)3 ] (x=1.00-0.67) exhibit para- to ferroelectric diffuse phase transitions with lowered transition temperatures from x=1.00 to 0.67. These properties are due to the decreased framework distortion and polarization in their low temperature ferroelectric phases caused by the increased CH3 NH3 (+) concentration.

Solvent Responsive Magnetic Dynamics of a Dinuclear Dysprosium Single‐Molecule Magnet

A new dysprosium(III) phosphonate dimer {Dy(notpH4)(NO3)(H2O)}2·8H2O (1) [notpH6=1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid)] that contains two equivalent Dy(III) ions with a three-capped trigonal prism environment is reported. Complex 1 can be transformed into {Dy(notpH4)(NO3)(H2O)}2 (2) in a reversible manner by desorption and absorption of solvent water at ambient temperature. This process is accompanied by a large dielectric response. Magnetic studies reveal that both 1 and 2 show thermally activated magnetization relaxation as expected for single-molecule magnets. Moreover, the magnetic dynamics of the two compounds can be manipulated by controlling the number of solvent molecules at room temperature.

First Fe(II)-based cyano-bridged single molecule magnet [CrIIIFeII2] with a large anisotropy

The first Fe(ii)-based cyanometalate single molecule magnet [Cr(III)Fe(II)(2)] exhibits fully visible, frequency-dependent ac susceptibilities above 2 K.