Chem

We report on a novel manganese(III)–porphyrin complex with the formula [Mn^III(TPP)(3,5-Me₂pyNO)₂]ClO₄⋅CH₃CN (2; 3,5-Me₂pyNO=3,5-dimethylpyridine N-oxide, H₂TPP=5,10,15,20-tetraphenylporphyrin), in which the Mn^III ion is six-coordinate with two monodentate 3,5-Me₂pyNO molecules and a tetradentate TPP ligand to build a tetragonally elongated octahedral geometry. The environment in 2 is responsible for the large and negative axial zero-field splitting (D=−3.8 cm⁻¹), low rhombicity (E/|D|=0.04) of the high-spin Mn^III ion, and, ultimately, for the observation of slow magnetic-relaxation effects (E_a=15.5 cm⁻¹ at H=1000 G) in this rare example of a manganese-based single-ion magnet (SIM). Structural, magnetic, and electronic characterizations were carried out by means of single-crystal diffraction studies, variable-temperature direct- and alternating-current measurements and high-frequency and -field EPR spectroscopic analysis followed by quantum-chemical calculations. Slow magnetic-relaxation effects were also observed in the already known analogous compound [Mn^III(TPP)Cl] (1; E_a=10.5 cm⁻¹ at H=1000 G). The results obtained for 1 and 2 are compared and discussed herein.

Rarely seen: Field-induced slow magnetic-relaxation effects are observed in a manganese(III)–porphyrin complex with bis-axial ligands for the first time (see figure). This complex is a rare example of a manganese-based single-ion magnet and expands the scope of metal-coordination geometries that allow slow magnetic-relaxation effects in mononuclear complexes.

A giant tetrahedral heterometallic polyoxometalate (POM) [Dy₃₀Co₈Ge₁₂W₁₀₈O₄₀₈(OH)₄₂(OH₂)₃₀]⁵⁶⁻, which shows single-molecule magnet (SMM) behavior, is described. This hybrid contains the largest number of 4f ions of any polyoxometalate (POM) reported to date and is the first to incorporate two different 3d–4f and 4f coordination cluster assemblies within same POM framework.

Record SMM POM: A new class of polyoxometalate (POM) containing 3d–4f and 4f aggregates encapsulated by [A-α-GeW₉O₃₄]¹⁰⁻ ligands to give [Dy₃₀Co₈Ge₁₂W₁₀₈O₄₀₈(OH)₄₂(OH₂)₃₀]⁵⁶⁻ is described. This hybrid with single-molecule magnet (SMM) behavior contains the largest number of 4f ions of any POM reported to date and is the first to incorporate two different 3d–4f and 4f coordination cluster assemblies within same POM framework.

Three cationic [Ln₄] squares (Ln=lanthanide) were isolated as single crystals and their structures solved as [Dy₄(μ₄-OH)(HL)(H₂L)₃(H₂O)₄]Cl₂⋅(CH₃OH)₄⋅(H₂O)₈ (1), [Tb₄(μ₄-OH)(HL)(H₂L)₃(MeOH)₄]Cl₂⋅(CH₃OH)₄⋅(H₂O)₄ (2) and [Gd₄(μ₄-OH)(HL)(H₂L)₃(H₂O)₂(MeOH)₂]Br₂⋅(CH₃OH)₄⋅(H₂O)₃ (3). The structures are described as hydroxo-centered squares of lanthanide ions, with each edge of the square bridged by a doubly deprotonated H₂L²⁻ ligand. Alternating current magnetic susceptibility measurements show frequency-dependent out-of-phase signals with two different thermally assisted relaxation processes for 1, whereas no maxima in χ_M“ appears above 2.0 K for complex 2. For 1, the estimated effective energy barrier for these two relaxation processes is 29 and 100 K. Detailed ab initio studies reveal that complex 1 possesses a toroidal magnetic moment. The ab initio calculated anisotropies of the metal ions in complex 1 were employed to simulate the magnetic susceptibility by using the Lines model (POLY_ANISO) and this procedure yields J₁=+0.01 and J₂=−0.01 cm⁻¹ for 1 as the two distinct exchange interactions between the Dy^III ions. Similar parameters are also obtained for complex 1 (and 2) from specific heat measurements. A very weak antiferromagnetic super-exchange interaction (J₁=−0.043 cm⁻¹ and g=1.99) is observed between the metal centers in 3. The magnetocaloric effect (MCE) was estimated by using field-dependent magnetization and temperature-dependent heat-capacity measurements. An excellent agreement is found for the −ΔS_m values extracted from these two measurements for all three complexes. As expected, 3 shows the largest −ΔS_m variation (23 J Kg⁻¹ K⁻¹) among the three complexes. The negligible magnetic anisotropy of Gd indeed ensures near degeneracy in the (2S+1) ground state microstates, and the weak super-exchange interaction facilitates dense population of low-lying excited states, all of which are likely to contribute to the MCE, making complex 3 an attractive candidate for cryogenic refrigeration.

A family of four: A family of rare μ₄-hydroxo centered Ln₄ (Ln=Dy, Tb, Gd) complexes were characterized by single-crystal X-ray diffraction. The two relaxation processes observed in magnetic-susceptibility measurements of Dy₄ were rationalized by detailed ab initio studies through which an exchange interaction between the Dy^III metal centers was computed. The computed results were corroborated by detailed heat-capacity measurements. Of the three complexes, Gd₄ shows the largest −ΔS_m values, which makes Gd₄ an attractive candidate for magnetic refrigeration (see scheme).