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A tetradentate 8-hydroxyquinoline-based acyl hydrazone ligand (HL1 = 8-hydroxyquinoline-2-carboxaldehyde-(aminourea)hydrochloride) was elaborately used to construct a mononuclear dysprosium complex DyCl3HL1·CH3OH (1) with a nearly ideal pentagonal bipyramid coordination geometry (D5h) surrounding the Dy(iii) ion to achieve the significant performance of single-molecule magnets (SMMs). Meanwhile, the isolated high local symmetry center was successfully kept intact and further bridged to a series of double bipyramid systems by two phenolic oxygen atoms of the acyl hydrazone ligands (HL1 and HL2 = 8-hydroxyquinoline-2-carboxaldehyde-(benzoyl)hydrazine), with the formulae [Dy2Cl4(L1)2(CH3OH)2]·4C5H5N (2) and [Dy2Cl4(L2)2]·2CH3CN (3). In addition, the monodentate co-ligand anion was replaced by a larger sterically hindered ligand and a bidentate monovalent β-diketonate anion to generate [Dy2(tfo)4(L2)2(EtOH)2] (4), [Dy2(tta)4(L2)2(EtOH)2]·2(EtOH) (5) and [Dy2(dbm)4(L2)2(EtOH)2] (6) (tfo = trifluoromethanesulfonic acid, dbm = dibenzoylmethane, tta = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione) with eight-coordinate geometry. Strikingly, the dynamic magnetic measurements revealed that complexes 1-3 did not display the expected significant SMM performance albeit they had high local symmetry. In combination with ab initio calculation, the alignment of the coordination symmetric axis and the magnetic easy axis dominates the molecular magnetic anisotropy, and the magnetic easy axis could be modulated by the distribution of coordination atoms with different electrostatic properties.

A series of dinuclear dysprosium complexes bridged by pyridine-NO ligands with formula [Ln2(BTA)6(pyNO)2] (1Dy, Ln = Dy, 1Y, Ln = Y and 1Gd, Ln = Gd) (BTA = benzoyltrifluoroacetone, pyNO = pyridine-N-oxide) were structurally and magnetically characterized. The X-ray crystallographic analyses of the structures revealed that the NO group serves as the effective bridge to link two Dy(iii) centers and the periphery β-diketonate (BTA) ligands complete the rest of the coordination sphere. The dynamic magnetic measurements revealed that complex 1Dy displayed significant zero-field single-molecule magnetic (SMM) behaviour with 72 K energy barrier and 2.5 K hysteresis temperature. In order to extend this dinuclear system, double N-oxide bridged ligand 4,4'-bpdo(4,4'-bipyridine-N,N'-dioxide) was used, and consequently, a series of one-dimensional chain complexes possessing repeated [Ln2(BTA)6(pyNO)2] units were synthesized with formula [Ln2(BTA)6(4,4'-bpdo)]n·2EtOH (2Dy, Ln = Dy, 2Y, Ln = Y and 2Gd, Ln = Gd). The AC magnetic susceptibility measurements revealed that complex 2Dy exhibited significant zero-field slow magnetic relaxation behavior with a higher effective energy barrier of 87 K and a hysteresis temperature of 3 K than 1Dy albeit the separation between the repeated units is large.

The synthesis, structural characterization and magnetic properties of four heterometallic complexes with formulas Ln 2 M(OQ) 8 [Ln(iii) = Dy, M(ii) = Ca (1), Mg (2); Ln(iii) = Er, M(ii) = Ca (3), Mg (4); HOQ = 8-hydroxyquinoline] are reported. Complexes display a perfectly linear arrangement of three metal ions involving two terminal Ln(iii) ions and one central alkaline earth M(ii) ion, and they are bridged by three phenolato oxygen atoms from three 8-hydroxyquinoline ligands. Direct-current (dc) magnetic susceptibility studies show that there exists a long-range ferromagnetic (FM) interaction between two f-electronic centers in Dy-based complexes 1 and 2 albeit they are separated by diamagnetic alkaline earth ions, and the FM interaction has been successfully reproduced by ab initio calculations. Alternating current (ac) magnetic susceptibility measurements indicate that complexes 1 and 2 exhibit significant single-molecule magnet (SMM) behaviors. This is mostly attributed to the parallel magnetic axes of individual Dy(iii) sites aligning closely to the Dy-M-Dy direction, and the linearly extending magnetic susceptibility tensors of each ion lead to the FM interaction. The dc and ac susceptibility studies of the intramolecularly diluted complexes reveal that the f-f coupling suppresses the QTMs significantly in the absence of the external magnetic field.

A series of HMq-bridged dinuclear dysprosium complexes, namely, [Dy(acac)2(CH3OH)]2(μ-HMq)2 (1), [Dy(DBM)2]2(μ-HMq)2(n-C6H14) (2), [Dy(hmac)2]2(μ-HMq)2 (3) and [Dy(hfac)3]2(μ-HMq)2 (4) (HMq = 2-methyl-8-hydroxyquinoline, acac = acetylacetone, DBM = dibenzoylmethane, hmac = hexamethylacetylacetonate and hfac = hexafluoroacetylacetonate), were structurally and magnetically characterized. X-ray crystallographic analyses of the structures reveal that HMq serves as the effective bridge to link two Dy(III) centers by means of the phenoxyl oxygen and nitrogen atoms and the periphery β-diketonate ligands complete the coordination sphere by bidentate oxygen atoms. The different substituents on the β-diketonate terminal lead to different coordination models mostly due to the steric hindrance of these substituents, and the electron-withdrawing or donating effects likely influence the strength of the ligand fields and the Dy(III) ion anisotropy. Measurements of alternating-current (ac) susceptibility on complexes 1-4 reveal that complexes 3 and 4 display significant zero-field single-molecule magnetic (SMM) behavior with barrier energy Ueff/kB = 14.8 K, τ0 = 1.8 × 10(-5) s and Ueff/kB = 9.2 K, τ0 = 1.7 × 10(-5) s, respectively, whereas 1 and 2 exhibit field-induced SMM behavior, and these differences are attributed to the alteration on the periphery β-diketonate ligands. Their distinct slow magnetic relaxation behaviors were related to their different individual Dy(III) ion magnetic anisotropy and intramolecular coupling, which were confirmed by ab initio calculations.

Three complexes with formula {Dy(q)2(L)}2(μ3-OH)2{Dy(q)(L)}2(solvents) (where q = 8-quinolinolate; L = acetylacetone (acac) with (CH2Cl2)2 = solvent (1), 1,3-diphenyl-1,3-propanedione (DBM) and (CH2Cl2)2 = solvent (2), and hexafluoroacetylacetone (hfac) and CHCl3 = solvent (3)) were structurally and magnetically characterized. They have similar Dy4 structural cores bridged by N and O atoms from 8-quinolinolate, and only differ in the peripheral β-diketonate ligands. The variable-frequency and temperature alternating-current (ac) magnetic susceptibility measurements reveal that complexes 1 and 2 display significant zero-field single-molecule-magnet (SMM) behavior, while complex 3 exhibits the field-induced SMM behavior, albeit they possess the nearly same primary coordination sphere. The ac susceptibility measurement on the diluted samples verified that their relaxation was of purely molecular origin, and their distinct slow magnetic behaviors were related to the replacement of the peripheral β-diketonate ligand, which is responsible for their different individual Dy(iii) ions' magnetic anisotropy and intramolecular coupling, as confirmed by ab initio calculation.

Three dinuclear [Ln2H2OL(1)2(acac)2]·solvent (1, Ln = Gd, solvent = 2CH2Cl2; 2, Ln = Tb, no solvent; 3, Ln = Er, solvent = (C2H5)2O), and two tetranuclear lanthanide clusters [Ln4(μ3-OH)2L(2)2(acac)6]·2(solvent) (4, Ln = Tb, solvent = CH3OH; 5, Ln = Dy, solvent = CH3CN) were characterized in terms of structure, fluorescence and magnetism. The dinuclear lanthanide complexes were constructed by a rigid salen-type ligand H2L(1) = N,N'-bis(salicylidene)-o-phenylenediamine and β-diketonate (acac = acetylacetonate) ligands, while the tetranuclear clusters were formed from the flexible ligand H2L(2) = N,N'-bis(salicylidene)-1,2-ethanediamine. Crystal structure analysis indicates that the rigid ligand favors the double-decker sandwich structure (Ln2L(1)2), in which the two lanthanide ions have different coordination numbers and geometry, while the more flexible ligand (H2L(2)) leads to planar tetranuclear clusters. The relationship between their respective magnetic anisotropy and ligand-field geometries and their fluorescence properties was investigated. The Dy and Tb-containing clusters exhibit typical visible fluorescence properties, and single-molecule magnet behavior is seen in complex 5.

An unusual dinuclear Yb(2) complex isolated using a mixed ligand strategy leads to field-induced SMM behaviour. Low magnetic axiality and a large tunnelling gap lead to significant quantum tunnelling of the magnetisation, which was reduced under an applied static optimum field of 1600 Oe.