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The modular construction method provides a feasible way to construct pre-designed cyano-bridged {LnIIIMIV/V} systems with more attractive structures and adjustable interactions between 4d/5d and 4f ions. In this work, the pentadentate Schiff-base ligands H2Ln (H2L1: 2,6-diacetylpyridine bis(nicotinoylhydrazone); H2L2: 2,6-diacetylpyridine bis(isonicotinoylhydrazone)) with multiple coordination sites were designed to coordinate with DyIII ions firstly, and then combined with K4M(CN)8·2H2O (M = W, Mo) by a liquid–liquid diffusion method to successfully synthesize two trinuclear structures {Dy[MIV(CN)8]0.5(HL1)(H2O)3}2·8H2O (1: M = W; 2: M = Mo), and two dinuclear structures {[Dy[MIV(CN)8](H3L2)(H2O)3]}·sol (3: M = W, sol = 11.5H2O; 4: M = Mo, sol = 2CH3OH·9.5H2O). Interestingly, deprotonation occurs in ligand H2L1 during the formation of complexes 1 and 2, while deprotonation/protonation processes both occur in ligand H2L2 during the synthesis of complexes 3 and 4. Magnetic analyses reveal that complexes 1–4 are all field-induced single-ion magnets (SIMs). The energy barriers (Ueff/kB) of complexes 1 and 2 are 92.83 K and 101.08 K, respectively, which are nearly three-fold higher than those of complexes 3 (30.19 K) and 4 (32.77 K). Ab initio calculations were performed on these series of {LnIIIMIV} heteronuclear complexes to provide more insights into the magnetic relaxation properties. This work provides another example of improving the SMM behaviors of cyano-bridged {LnIIIMIV} systems by deprotonation/protonation at different sites in the ligands. [ABSTRACT FROM AUTHOR]

Synthesis of air-stable and high-performance single-molecule magnets (SMMs) is challenging. Here, a heptadentate pentapyridyldiamine (BPA-TPA) ligand and fine-tuned axial phenoxy ligands are used to synthesize two triangular dodecahedral Dy(III) complexes [Dy(BPA-TPA)(4-methoxy-PhO)](BPh4)2·3CH2Cl2 (4) and [Dy(BPA-TPA)(2,4-dimethyl-PhO)](BPh4)2·0.85CH2Cl2 (5). Both complexes have high effective barriers exceeding 400 K and magnetic hysteresis up to 8 K, which is ascribed to one strong and short Dy–O bond combined with seven weak Dy–N bonds. Ab initio calculations reveal the thermally activated quantum tunneling of magnetization through the first excited Kramers doublet, due to the presence of a strong axial Dy–O crystal ligand. Substitution of the phenoxy ligand leads to more constrained vibrations, improving the magnetic hysteresis behavior for 5. [ABSTRACT FROM AUTHOR]

Three mononuclear Co(II) complexes with the compositions of [Co(L1)2] (1), [Co(L2)2(CH3CN)] (2) and [Co(L3)2] (3) (HL1 = 2-((E)-(2-fluorobenzylimino)methyl)-4,6-dibromophenol, HL2 = 2-((E)-(3-fluorobenzylimino)methyl)-4,6-dibromophenol and HL3 = 2-((E)-(4-fluorobenzylimino)methyl)-4,6-dibromophenol) were prepared and structurally determined. The changes in the F-positions in the ligands and solvents led to the formation of these products with various coordination geometries. Both complexes 1 and 3 are four-coordinated and their coordination geometries can be described as tetrahedron and seesaw, whereas complex 2 is five coordinated with a coordination configuration in between trigonal bipyramid and square pyramid. Static magnetic studies reveal that all these complexes exhibit considerable easy-axis magnetic anisotropy. The easy-axis magnetic anisotropy of 1 and 3 mainly derives from the first quartet excited state, whereas that of 2 primarily originates from the first, third and fourth quartet excited states established by theoretical calculations. All the resulting complexes display field-induced slow magnetic relaxation. Complex 3 represents the first Co(II) single ion magnet with a seesaw coordination geometry. Ab initio calculations predict that the magnetic anisotropy will enhance when the seesaw coordination geometry varies from distortion to regulation. [ABSTRACT FROM AUTHOR]

The utilization of two isomorphic ligands with different substituents leads to two centrosymmetric DyIII dimers, namely, [Dy2(bfbpen)2(H2O)2]·2I− (1) and [Dy2(bcbpen)2(H2O)2]·2I−·0.5H2O (2) (H2bfbpen = N,N′-bis-(2-hydroxy-5-fluoro-benzyl)-N,N′-bis-(pyridin-2-ylmethyl)ethylenediamine and H2bcbpen = N,N′-bis-(2-hydroxy-5-chloro-benzyl)-N,N′-bis-(pyridin-2-ylmethyl)ethylenediamine). Although Dy ions in both compounds uniformly exhibit a square-antiprism geometry, the critical difference found in the terminal substituents of the two ligands fine-tunes the local crystal field around Dy centers and the dinuclear molecular structures of 1 and 2. Magnetic investigations unveil that both 1 and 2 display dynamic magnetic relaxation of single-molecule magnet (SMM) behaviour with different energy barriers of 20.9 K for 1 and 72.7 K for 2 under a zero direct-current (dc) field, as well as 26.9 K for 1 under a 1200 Oe dc field. Compared to 1, the stronger uniaxial anisotropy and magnetic exchange in 2 render it a better SMM as evidenced by the higher energy barrier. Ab initio calculations are also performed on both Dy2 compounds to rationalize the observed discrepancy in their magnetic behaviours. The contribution illustrates that the SMM behaviour could be effectively enhanced by means of deliberate local structural manipulation. [ABSTRACT FROM AUTHOR]