Field-induced Single-molecule Magnet Behavior Research Articles

Manipulating a series of Ln-based helix chains or dinuclear complexes by designing two types of Schiff-based ligands: Structure and magnetic properties

As an important means of regulating ligands, steric hindrance effect is always the first choice to construct structures. It is rare to assemble complexes with different topological and structural types by increasing or decreasing the steric hindrance of ligands and to regulate their properties. Here, we successfully obtained two types of lanthanide structures [Ln(H2L1)(NO3)3]n (1: Dy; 2: Gd, H2L1 ​= ​6,6'-((1E,1′E)-((2,2-dimethylpropane-1,3-diyl)bis(azanylylidene))bis(methanylylidene))bis(2-methoxyphenol)) and [Ln2(H2L2)3(NO3)6]·(CH3CN)x·(CH3OH)y (3: Dy, x ​= ​3, y ​= ​0; 4: Gd, x ​= ​y ​= ​2, H2L2 ​= ​6,6'-((1E,1′E)-((2,2-dimethylpropane-1,3-diyl)bis(azanylylidene))bis(methanylylidene))bis(2-ethoxyphenol)), which are controlled by ligands with the steric hindrance effect. For the all structures, the ligands are distorted when they coordinate with lanthanide ions, which make the ligands form helical structures around Ln(III) ions. For chains 1 and 2, there is a single twisted ligand bridging to form a one-dimensional spiral chain between the Ln(III) ion and the Ln(III) ion. But for complexes 3 and 4, there is a helical dinuclear structure formed by the cross-bridging of a double twisted ligand between the Ln(III) ion and the Ln(III) ion. AC magnetic susceptibilities show that chain 1 is a single-molecule magnet (SMM), which exhibits obviously frequency dependent behavior with the energy barriers of 39.6 (3) K and 44.6 (5) K, but complex 3 shows field-induced single-molecule magnetic behavior. The magnetocaloric effects (MCEs) show that chain 2 has bigger value of S than that of complex 4.

Field-Induced Single-Molecule Magnets of Dysprosium Involving Quinone Derivatives

The coordination reaction of the [Dy(hfac)3(H2O)2] units (hfac− = 1,1,1,5,5,5-hexafluoroacetylacetonate) with the two quinone-based derivatives 4,7-di-tert-butyl-2-(3,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-ylidene)benzo[d][1,3]dithiole-5,6-dione (L1) and 7,8-dithiabicyclo[4.2.0]octa-1,5-diene-3,4-dione,2,5bis(1,1-dimethylethyl) (L2) led respectively to the complexes [Dy(hfac)3(H2O)(L1)] (1) and [Dy(hfac)3(H2O) (L2)]⋅(C6H14)(CH2Cl2) (2)⋅(C6H14)(CH2Cl2). X-ray structures on single crystal of 1 and 2⋅(C6H14)(CH2Cl2) revealed the coordination of the DyIII on the bischelating oxygenated quinone site and the formation of dimeric species through hydrogen bonds. Ac magnetic measurements highlighted field-induced single-molecule magnet behavior with magnetic relaxation through a Raman process.

Multi-Nuclear Rare-Earth-Implanted Tartaric Acid-Functionalized Selenotungstates and Their Fluorescent and Magnetic Properties.

A family of multinuclear rare-earth (RE)-implanted H2tart2--functionalized selenotungstates (STs) [H2N(CH3)2]13H{[W2O5(OH)2(H2tart)2](H2tart){[W3O6RE2(H2O)6][SeW9O33]2}2}·31H2O [RE = Eu3+ (1), Tb3+ (2), Dy3+ (3), Ho3+ (4), Y3+ (5); H4tart = d-tartaric acid] have been afforded by a simple one-pot aqueous reaction and were structurally characterized. Intriguingly, their isomorphous organic-inorganic hybrid anion {[W2O5(OH)2(H2tart)2](H2tart){[W3O6RE2(H2O)6][SeW9O33]2}2}14- includes two sandwich-type {[W3O6[RE2(H2O)6][SeW9O33]2}4- dimeric units with a W-O-RE heterometal core, which are further joined by two H2tart2--decorated dinuclear tungsten-oxo {W2O5(OH)2(H2tart)2} clusters and a bridging H2tart2- ligand, contributing to a surprising Mobius band-like configuration. It is worth emphasizing that three H2tart2- ligands coordinate with tungsten centers rather than RE cations. For all we know, 1-5 delegate the infrequent RE-implanted STs functionalized by triplicate H2tart2- bridges. Furthermore, fluorescent performances of 1-4 as well as magnetic properties of 2-4 have been surveyed. The solid-state fluorescence emission spectra prove that each of them undoubtedly shows the characteristic emission peaks of RE cores, while alternating-current susceptibility measurements suggest field-induced single-molecule magnetic behavior in 3.

Magnetic Anisotropy: Structural Correlation of a Series of Chromium(II)-Amidinate Complexes.

Systematic substituent variations on amidinate ligands bring delicate changes of CrN4 coordination in a family of chromium(II) complexes with the common formula of Cr(RNC(CH3)NR)2, where R = iPr (1), Cy (2), Dipp (Dipp = 2, 6-diisopropylphenyl) (3), and tBu (4). With the largest substituent group, 4 shows the largest distortion of the N4 coordination geometry from square-planar to seesaw shape, which leads to its field-induced single-molecule magnet (SMM) behavior. This is an indication that 4 has the strongest axial magnetic anisotropy and/or optimized magnetic relaxation process. Combined with high-frequency/field electron paramagnetic resonance (HF-EPR) experiments and ab initio calculations, we deduce that the smallest energy gap between ground 4Ψ0 and the first excited 4Ψ1 orbitals in 4 contributes the most to its strongest magnetic anisotropy. Moreover, the lower E value of 4 ensures its being a field-induced SMM. Specifically, the D and E values were found to be correlated to the dihedral angle between the ΔN1CrN2 and ΔN3CrN4 triangles, indicating that distortion from ideal square-planar geometry to the seesaw help increase axial magnetic anisotropy and suppress the transversal part. Thus, the study on this system not only expands the family of Cr(II)-based SMMs but also contributes to a deeper understanding of magneto-structural correlation in four-coordinate Cr(II) SMMs.

Single-Ion Magnet and Photoluminescence Properties of Lanthanide(III) Coordination Polymers Based on Pyrimidine-4,6-Dicarboxylate

Herein, we report the magnetic and photoluminescence characterization of coordination polymers (CP) built from the combination of lanthanide(III) ions, pyrimidine-4,6-dicarboxylate (pmdc) ligand and a co-ligand with formula {[Dy(μ-pmdc)(μ-ox)0.5(H2O)3]·2H2O}n (1-Dy), {[Dy(μ3-pmdc)(μ-ox)0.5(H2O)2] ~2.33H2O}n (2-Dy), {[Dy2(μ3-pmdc)(μ4-pmdc)(μ-ox)(H2O)3]·5H2O}n (3-Dy), {[Ln(μ3-pmdc)(μ-ox)0.5(H2O)2]·H2O}n (where Ln(III) = Nd (4-Nd), Sm (4-Sm), Eu (4-Eu) and Dy (4-Dy)) and {[Dy(μ4-pmdc)(NO3)(H2O)]·H2O}n (5-Dy). It must be noted the presence of oxalate anion acting as ditopic co-ligand in compounds 1-Dy, 2-Dy, 3-Dy and 4-Ln, whereas in 5-Dy the nitrate anion plays the role of terminal co-ligand. Direct current measurements carried out for the dysprosium-based CPs reveal almost negligible interactions between Dy3+ ions within the crystal structure, which is confirmed by computed values of the exchange parameters J. In addition, alternating current measurements show field-induced single-molecule magnet (SMM) behavior in compounds 1-Dy, 2-Dy, 4-Dy and 5-Dy, whereas slight-frequency dependence is also observed in 3-Dy. Solid state emission spectra performed at room temperature for those compounds emitting in visible region confirm the occurrence of significant ligand-to-lanthanide charge transfer in view of the strong characteristic emissions for all lanthanide ions. Emission decay curves were also recorded to estimate the emission lifetimes for the reported compounds, in addition to the absolute quantum yields. Among them, the high quantum yield of 25.0% measured for 4-Eu is to be highlighted as a representative example of the good emissive properties of the materials.

Hydrogen bond-induced abrupt spin crossover behaviour in 1-D cobalt(II) complexes - the key role of solvate water molecules.

The magnetic properties and structural aspects of the 1-D cobalt(ii) complexes, [Co(pyterpy)Cl2]·2H2O (1·2H2O; pyterpy = 4'-(4'''-pyridyl)-2,2':6',2''-terpyridine) and [Co(pyethyterpy)Cl2]·2H2O (2·2H2O; pyethyterpy = 4'-((4'''-pyridyl)ethynyl)-2,2':6',2''-terpyridine) are reported. In each complex the central cobalt(ii) ion displays an octahedral coordination environment composed of three nitrogen donors from the terpyridine moiety, a nitrogen donor from a pyridyl group and two chloride ligands which occupy the axial sites. 1·2H2O exhibits abrupt spin-crossover (SCO) behaviour (T1/2↓ = 218 K; T1/2↑ = 227 K) along with a thermal hysteresis loop, while 2·2H2O and the dehydrated species 1 and 2 exhibit high-spin (HS) states at 2-300 K as well as field-induced single-molecule magnet (SMM) behaviour attributed to the presence of magnetic anisotropic HS cobalt(ii) species (S = 3/2). 1·2H2O exhibited reversible desorption/resorption of its two water molecules, revealing reversible switching between SCO and SMM behaviour triggered by the dehydration/rehydration processes. Single crystal X-ray structural analyses revealed that 1·2H2O crystalizes in the orthorhombic space group Pcca while 2 and 2·2H2O crystallize in the monoclinic space group P2/n. Each of the 1-D chains formed by 1·2H2O in the solid state are bridged by hydrogen bonds between water molecules and chloride groups to form a 2-D layered structure. The water molecules bridging 1-D chains in 1·2H2O interact with the chloride ligands occupying the axial positions, complementing the effect of Jahn-Teller distortion and contributing to the abrupt SCO behaviour and associated stabilization of the LS state.

Single-molecule magnets within polyoxometalate-based frameworks.

As an extension of our interest in polyoxometalates (POMs) and lanthanoids, we report the design and synthesis of two polyoxometalate-based frameworks under hydrothermal conditions; [Ho4(PDA)4(H2O)11][(SiO4)@W12O36]·8H2O (1) and [Tb4(PDA)4(H2O)12][(SiO4)@W12O36]·4H2O (2) (H2PDA = 1,10-phenanthroline-2,9-dicarboxylic acid). Both hybrids have been characterized by elemental analysis, Fourier transform infrared spectroscopy, thermogravimetric analysis, and powder/single-crystal X-ray diffraction. According to the structural analysis, 1 and 2 consist of 2D-cationic coordination polymers based on the respective lanthanoids and PDA2- as well as Keggin anions that reside in the interspaces between two adjacent layers as discrete counterions connected by extensive hydrogen bonding. Although the overall structures of 1 and 2 are composed of cationic and anionic layers, there are many differences in the cationic layers such as various coordination modes of PDA2-, different void shapes, and the existence of dinuclear Tb(III) units only in 2. Frameworks 1 and 2 were further characterized by dc and ac magnetic measurements and both exhibit slow relaxation of magnetization at low temperatures under an applied dc field. Their single-molecule magnet (SMM) properties are investigated, where weak field-induced SMM behaviour is observed at low temperatures in dynamic magnetic studies.

Ten-Coordinate Lanthanide [Ln(HL)(L)] Complexes (Ln = Dy, Ho, Er, Tb) with Pentadentate N3O2-Type Schiff-Base Ligands: Synthesis, Structure and Magnetism

A series of five neutral mononuclear lanthanide complexes [Ln(HL)(L)] (Ln = Dy3+, Ho3+ Er3+ and Tb3+) with rigid pentadentate N3O2-type Schiff base ligands, H2LH (1-Dy, 3-Ho, 4-Er and 6-Tb complexes) or H2LOCH3, (2-Dy complex) has been synthesized by reaction of two equivalents of 1,1′-(pyridine-2,6-diyl)bis(ethan-1-yl-1-ylidene))dibenzohydrazine (H2LH, [H2DAPBH]) or 1,1′-(pyridine-2,6-diyl)bis(ethan-1-yl-1-ylidene))di-4-methoxybenzohydrazine (H2LOCH3, [H2DAPMBH]) with common lanthanide salts. The terbium complex [Tb(LH)(NO3)(H2O)2](DME)2 (5-Tb) with one ligand H2LH was also obtained and characterized. Single crystal X-ray analysis shows that complexes 1–4 have the composition {[Ln3+(HL)−(L)2−] solv} and similar molecular structures. In all the compounds, the central Ln3+ ion is chelated by two interlocked pentadentate ligands resulting in the coordination number of ten. Each lanthanide ion is coordinated by six nitrogen atoms and four oxygen atoms of the two N3O2 chelating groups forming together a distorted bicapped square antiprismatic polyhedron N6O4 with two capping pyridyl N atoms in the apical positions. The ac magnetic measurements reveal field-induced single-molecule magnet (SMM) behavior of the two dysprosium complexes (with barriers of Ueff = 29 K at 800 Oe in 1-Dy and Ueff = 70 K at 300 Oe in 2-Dy) and erbium complex (Ueff = 87 K at 1500 Oe in 4-Er); complex 3-Ho with a non-Kramers Ho3+ ion is SMM-silent. Although 2-Dy differs from 1-Dy only by a distant methoxy-group in the phenyl ring of the ligand, their dynamic magnetic properties are markedly different. This feature can be due to the difference in long-range contributions (beyond the first coordination sphere) to the crystal-field (CF) potential of 4f electrons of Dy3+ ion that affects magnetic characteristics of the ground and excited CF states. Magnetic behavior and the electronic structure of Ln3+ ions of 1–4 complexes are analyzed in terms of CF calculations.

Water Molecule-Induced Reversible Magnetic Switching in a Bis-Terpyridine Cobalt(II) Complex Exhibiting Coexistence of Spin Crossover and Orbital Transition Behaviors.

The development of molecule-based switchable materials remains an important challenge in the field of molecular science. Achievement of a structural phase transition induced by adsorption/desorption of guest molecules in spin crossover (SCO) Co(II) compounds is of significant interest because of the possibility that the spin state of the magnetic anisotropic high-spin (HS, S = 3/2) and low-spin (LS, S = 1/2) states can be switched via the induced changes in associated intermolecular interactions. In this study, we demonstrated a reversible magnetic switching associated with spin state conversion, along with a single-crystal to single-crystal (SCSC) phase transition induced by dehydration/rehydration. [Co(terpy)2](BF4)2·H2O (1·H2O; terpy = 2,2':6',2''-terpyridine) assembles in the solid state via π-π and CH-π interactions involving adjacent terpyridine cores along the ab direction to form two-dimensional (2D) layered domains. 1·H2O exhibits gradual and incomplete SCO, from fully HS to ca. 0.5 HS, and the field-induced single-molecule magnet (SMM) behavior attributed to the presence of the anisotropic partial high-spin Co(II) species. 1·H2O undergoes a SCSC transformation accompanied by a change from the tetragonal space group I41/a to P42/n via a dehydration process. Dehydrated 1 exhibits a reverse thermal hysteresis behavior (T1/2↑ = 287 K; T1/2↓ = 270 K) in the gradual SCO region from fully HS to ca. 0.5 HS, followed by an ordinary thermal hysteresis (T'1/2↑ = 195 K; T'1/2↓ = 155 K) to fully LS Co(II). A temperature-dependent single-crystal X-ray structural analysis revealed that the reverse hysteresis can be attributed to an order/disorder structural phase transition of the BF4- anions involving a symmetry breaking to yield the monoclinic space group P21/n and orbital (angular momentum) transition (LT). Both the SCSC phase transition and magnetic behavior are switchable by dehydration/rehydration processes; thus 1 again adsorbs water at room temperature to give both the original structure and its magnetic behavior.

Redox Modulation of Field-Induced Tetrathiafulvalene-Based Single-Molecule Magnets of Dysprosium

The complexes [Dy2(tta)6(H2SQ)] (Dy-H2SQ) and [Dy2(tta)6(Q)]·2CH2Cl2 (Dy-Q) (tta− = 2-thenoyltrifluoroacetonate) were obtained from the coordination reaction of the Dy(tta)3·2H2O units with the 2,2′-benzene-1,4-diylbis(6-hydroxy-4,7-di-tert-butyl-1,3-benzodithiol-2-ylium-5-olate ligand (H2SQ) and its oxidized form 2,2′-cyclohexa-2,5-diene-1,4-diylidenebis(4,7-di-tert-butyl-1,3-benzodithiole-5,6-dione (Q). The chemical oxidation of H2SQ in Q induced an increase in the coordination number from 7 to 8 around the DyIII ions and by consequence a modulation of the field-induced Single-Molecule Magnet behavior. Computational results rationalized the magnetic properties of each of the dinuclear complexes.

Alignment of Axial Anisotropy in a 1D Coordination Polymer shows Improved Field Induced Single Molecule Magnet Behavior over a Mononuclear Seven Coordinated FeII Complex.

Herein, we report a CN-bridged alternating FeII -NiII 1D chain to ensure the alignment of axial anisotropy and improve the single molecule magnet (SMM) behavior in seven coordinated FeII compound. The chain was constructed from hepta coordinated Fe(II) complex as an anisotropic building unit and diamagnetic nickel tetra cyanate as a bridging ligand. The magnetic measurements show the easy-axis anisotropy of the seven coordinated Fe(II) complex and field induced SMM behavior with spin reversal energy barrier Ueff =61(2) K (42 cm-1 ) and pre-exponential relaxation time τ0 =1.9×10-8 s. The detailed analysis of the relaxation dynamics discloses that the Orbach process plays an important role in slow relaxation of magnetization for this compound. Notably, this example represents a remarkable energy barrier observed in hepta coordinated Fe(II) SMMs. The ab initio calculations estimate the magnitude of axial anisotropy and show the parallel orientation of the anisotropic axis throughout the 1D polymeric chain. In addition, it is also reported that the presence of weak π accepter ligands in the distorted axial position enhance the easy-axis anisotropy.

Remarkable Energy Barrier for Magnetization Reversal in 3D and 2D Dysprosium‐Chloranilate‐Based Coordination Polymers

Herein, we report two coordination polymers (CPs)[{Dy(Cl2An)1.5(CH3OH)}.4.5H2O]n(CP1) and [Dy(Cl2An)1.5(DMF)2]n(CP2) (where Cl2An2-= chloroanilate (2,5-dihydroxy-1,4-benzoquinone dianion))exhibiting field inducedsingle molecule magnet behavior with moderate barrier of magnetization reversible.Detailed structural and topological analysis disclose that CP1possesses three dimensional network whereas CP2has a two dimensional layered type structure. In both CPs, the magnetic measurements show weak antiferromagnetic exchange interaction between the dysprosium centers and field induced slow magnetic relaxation with the barrier of 175(9) K (CP1) and 145(7) K (CP2) for CPs1and2, respectively.Notably, thevaluesfor CP1and CP2represent remarkableenergy barrier of magnetization reversalin anymetal-chloranilate based 3D (CP1) and 2D (CP2)coordinationpolymers [Anchor] .The temperature and field dependence of relaxation time indicate the presence of multiple relaxation like direct, QTM, Raman, and Orbach processes in both the CPs. The theoreticalab initiocalculations reinforce the experimentally observed higher energy barrier in CP1as compared to CP2due to the presence of large transverse anisotropy on the ground state in the later compound. The average value of the transition magnetic moment between the computed low-lying spin-orbit states also rationalized the possible relaxation process as Orbach and Raman process through the first excited state. The BS-DFT calculations have been carried out to provide more insight into the exchange interaction.

Field-induced single-molecule magnet behavior in a Dy-based coordination polymer

A new mononuclear dysprosium-based coordination polymer {Dy-CP}is investigated for the magnetic properties. DC susceptibility of the Dy-CP does not exhibit any long-range ordering down to 1.8 K and the negative value of the Curie constant () indicate the dominance of antiferromagnetic interactions between the Dy (III) spins. AC susceptibility of the Dy-CP exhibits absence of single molecular magnet (SMM) behavior at zero field and shows signal of quantum tunneling magnetization (QTM) below 8 K. However, on the superimposition of a DC field (3 kOe), a frequency-dependent relaxation peak emerged at and the QTM signal is suppressed at higher fields. The intermediate value of reflects the formation of superparamagnetic state. The magnetic susceptibility analysis (Arrhenius fit) yields characteristics pre-relaxation factor and energy barrier , indicating the slow spin relaxation. The Cole-Cole fit shows further evidence for the single ion spin relaxation. Thus, the magnetic measurements support the SMM behavior in the Dy-CP under the application of DC magnetic field.

Ligand effect of two Dy(III) complexes on single-molecule magnetism

The organic ligands (E)-3-(pyridin-2-ylmethyleneamino)propane-1,2-diol (H2L1) and (E)-2-((2,3-dihydroxypropylimino)methyl)quinolin-8-ol (H3L2) were reacted with Dy(NO3)3·6H2O at 80 °C to obtain the binuclear complex [Dy2(HL1)2(NO3)4] (1) and the trinuclear complex [Dy3(HL2)3(OH)(H2O)(CH3OH)2](NO3)2·CH3CN (2), where the H2L1 and H3L2 ligands were obtained from picolinaldehyde or 8-hydroxyquinoline-2-carbaldehyde and 3-aminopropane-1,2-diol by an in-situ reaction. Magnetic investigations showed that complex 1 exhibited field-induced single-molecule magnetic behavior with an energy barrier of 4.65(6) K under a 1200 Oe field. By contrast, complex 2 was a clear single-molecule magnet, with energy barriers of 7.7(2) and 25.2 (3) K under different dc-fields.

Family of Chiral ZnII-LnIII (Ln = Dy and Tb) Heterometallic Complexes Derived from the Amine-Phenol Ligand Showing Multifunctional Properties.

A family of chiral 3d-4f heterometallic complexes, namely, [Zn2Ln(R,R-L)2(H2O)4](ClO4)3) [Ln = Dy (1), Tb (3)], [Zn2Ln(S,S-L)2(H2O)4](ClO4)3 [Ln = Dy (2), Tb (4)], [Zn2Ln2(R,R-L)2(CO3)2(NO3)2]·2CH3OH [Ln = Dy (5), Tb (7)], and [Zn2Ln2(S,S-L)2(CO3)2(NO3)2]·2CH3OH [Ln = Dy (6), Tb (8)] {H2L = cyclohexane-1,2-diylbis(azanediyl)bis(methylene)bis(2-methoxyphenol)}, has been synthesized and characterized. Crystal structure analysis reveals that complexes 1-4 are isostructural trinuclear clusters crystallized in chiral space group C2221, and 5-8 are isostructural tetranuclear clusters crystallized in chiral space group P1. Interestingly, the adjacent [ZnLn] units within the tetranuclear cluster in 5-8 are bridged by two carbonate anions via in situ incorporation of CO2 from air. Magnetic measurements indicate that complexes 1 and 3 exhibit field-induced single-molecule magnet behavior with energy barriers (Ueff) of 22.46 and 38.70 K (or 41.87 K), respectively. Complex 5 displays typical SMM behavior with Ueff = 19.61 K under zero dc field, while for complex 7, no obvious out-of-phase signals are observed even under 2 kOe dc field, the absence of SMM behavior. The solid-state luminescence studies reveal that all complexes display the characteristic fluorescence emission of lanthanide ions. Furthermore, the Kurtz-Perry measurements reveal these complexes are potential nonlinear optical materials.

Investigation of magneto-structural relation based on a series of mononuclear dysprosium single-ion magnets with high Oh symmetry

Two mononuclear Dy(III) complexes [Dy(HL)2(CH3OH)Cl3] (1) and [Dy(L)2(CH3CH2OH)Cl3] (2) were obtained by organic ligand 5,7-dibromo-2-methyl-8-quinolinol (HL) with DyCl3·6H2O. Structure analysis suggested that the Dy(III) center in those two complexes had six octahedral coordinative configurations, but the symmetry of complex 2 (1.251) was obviously higher than that of complex 1 (1.419). The coordinated solvent molecule contributes significantly on the geometry of Dy(III) ions in the two diverse complexes. And magnetic investigations showed that complex 1 exhibited a weak field-induced single-molecule magnetic (SIM) behavior, but complex 2 showed obvious SIM behavior. For complex 2, the energy barriers were 36.8 (1) K and 50.8 (9) K, respectively. They are typical SMM examples with rare six coordination of Dy(III) ions.

Field-induced single molecule magnet behavior of a dinuclear cobalt(ii) complex: a combined experimental and theoretical study

Two dinuclear cobalt(ii) complexes, [(dmso)CoIIL1(μ-(m-NO2)C6H4COO)CoII(NCS)] (1) and [(dmso)CoIIL2(μ-(m-NO2)C6H4COO)CoII(NCS)] (2) [dmso = dimethylsulfoxide, H2L1 = (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol) and H2L2 = (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol)] have been synthesized and structurally characterized by single-crystal X-ray diffraction, magnetic-susceptibility measurements and various spectroscopic techniques. Each complex contains a cobalt(ii) center with a slightly distorted octahedral geometry and a second cobalt(ii) center with a distorted trigonal prismatic one. To obtain insight into the physical nature of weak non-covalent interactions, we have extensively used the Bader's quantum theory of atoms-in-molecules (QTAIM). In addition, the non-covalent interaction reduced density gradient (NCI-RDG) methods established the presence of such non-covalent intermolecular interactions. Variable temperature magnetic susceptibility measurements show that both cobalt centers in each complex are in the high spin state (S = 3/2) and both complexes show weak ferromagnetic couplings through the double phenoxido bridges (J = 3.36(3) cm-1 in 1 and 4.56(2) cm-1 in 2). The magnetic properties of both complexes can be fitted to a Co(ii) dimer model including similar orbital reduction factors (α = -0.94(1) for 1 and -0.85(1) for 2) although different zero field splitting parameters D(1) = 11.0(4) cm-1 and D(2) = 19.5(4) cm-1 in 1 and D(1) = 8.2(4) cm-1 and D(2) = -1.3(4) cm-1 in 2. AC magnetic measurements reveal that the CoII2 unit in complex 2 exhibits field-induced slow relaxation of the magnetization at low temperatures and high frequencies.

Two azido-bridged [2×2] cobalt(ii) grids featuring single-molecule magnet behaviour.

The self-assembly of Co(ii) salts, pyridazine derivatives and azides afforded two azido-bridged [2×2] grid-type complexes {[(L)4CoII4(N3)4][BPh4]4}·sol (1, L = 3,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)pyridazine (pzdz) and sol = 4CH3CN·3CHCl3·2CH3OH and 2, L = 3,6-di(pyridin-2-yl)pyridazine (pydz) and sol = 4CH3CN). Upon comparison with other related grid-like complexes, the incorporation of end-on azido-bridges resulted in overall intramolecular ferromagnetic couplings, and thus endowed complexes 1 and 2 single molecule magnet behaviour with field-induced slow magnetic relaxation.

3d–4f Metallacrown complexes with a new sandwich core: synthesis, structures and single molecule magnet behavior

Two heterometallic metallacrown complexes, [Er{Cu4(butyrat)4}2]·Cl3·MeOH·26H2O (1) and [Yb{Cu4(butyrat)4}2]·Cl3·MeOH·26H2O (2) (H2butyrat = 3-aminobutyric hydroxamic acid), have been reported.

Dinuclear Lanthanide(III) Complexes Showing Single-Molecule Magnet Behaviour and Optical Properties

Three dinuclear lanthanide(iii) complexes with an acetate bridge, which are synthesised by a Salen-type Schiff base and β-diketonate ligands, i.e. [Ln2L2(TTA)4(OAc)2]·CH2Cl2 (Ln=Eu (1), Gd (2), Dy (3); TTA=thenoyltrifluoroacetonate, H2L=N,N′-ethylenebis(salicylideneimine), are investigated. We obtained the structures of complexes 1–3 by X-ray crystallography. Notably, in terms of the structure of these complexes, what is intriguing is that the acetate groups link two Lniii ions, whereas the cadmium ions do not coordinate. Lanthanide-based luminescence is exhibited by complex 1, which exists in both the solid state and a methanol solution. Through magnetic analysis, it is found that a field-induced single-molecule magnet behaviour is exhibited by complex 3, and the energy barrier is shown to be Ueff=45.97K.