Abstract:Two dichloride-bridged dinuclear dysprosium(III) complexes based on salen ligands, namely, [Dy(L 1 )(μ-Cl)(thf)] 2 (1; H 2 L 1 = N,N'-bis(3,5-di-tert-butylsalicylidene)phenylenediamine) and [Dy 2 (L 2 ) 2 (μ-Cl) 2 (thf) 2 ] 2 (2; H 2 L 2 = N,N'-bis(3,5-ditert-butylsalicylidene)ethylenediamine) are reported. These two complexes have two short DyÀ O (PhO) bonds that exhibit angles of ~90°for 1 and ~143°for 2, leading to clear slow relaxation of the magnetization for 2 and not for 1. Compound 2 has a near-identic… Show more
“…Based on the maxima in χ M versus temperature for 1-X , the temperatures at which magnetic exchange influences the magnetic susceptibility are low enough that only the ground Kramers doublet (i.e., ± M J states) of each uranium ion will be thermally populated. , The dipolar interaction, in such a case, will involve through-space interactions between the magnetic moments of the ground Kramers doublets of each uranium ion, with the strength of the interaction decreasing with larger U···U separations, r , by the relation 1/ r 3 . Dipolar interactions dominate magnetic exchange in many multilanthanide complexes (| J dip | is typically on the order of ∼1–5 cm –1 ) and the magnitude of this exchange is strongly dependent on the orientation of the individual Kramers doublets with respect to each other. ,,− Each uranium(III) center in 1-X likely possesses strongly uniaxial magnetic anisotropy (i.e., g z > g x , g y ) as a result of the strong U–Cp i Pr bonding interaction . In the simplest scenario, each uranium ion possesses a completely axial, and maximal, M J = ± 9/2 ground state ( g x , g y , g z = 0, 0, 6.55); assuming full parallel alignment of the magnetic moments, upper limits for | J dip | for 1-Cl , 1-Br , and 1-I are 0.50, 0.44, and 0.39 cm –1 , respectively (see Supporting Information for details).…”
The actinide elements are attractive alternatives to transition metals or lanthanides for the design of exchange-coupled multinuclear single-molecule magnets. However, the synthesis of such compounds is challenging, as is unraveling any contributions from exchange coupling to the overall magnetism. To date, only a few actinide compounds have been shown to exhibit exchange coupling and single-molecule magnetism. Here, we report triangular uranium(III) clusters of the type (Cp iPr5 ) 3 U 3 X (1-X; X = Cl, Br, I; Cp iPr5 = pentaisopropylcyclopentadienyl), which are synthesized via reaction of the aryloxide-bridged precursor (Cp iPr5 ) 2 U 2 (OPh tBu ) 4 with excess Me 3 SiX. Spectroscopic analysis suggests the presence of covalency in the uranium−halide interactions arising from 5f orbital participation in bonding. The dc magnetic susceptibility data reveal the presence of antiferromagnetic exchange coupling between the uranium(III) centers in these compounds, with the strength of the exchange decreasing down the halide series. Ac magnetic susceptibility data further reveal all compounds to exhibit slow magnetic relaxation under zero dc field. In 1-I, which exhibits particularly weak exchange, magnetic relaxation occurs via a Raman mechanism associated with the individual uranium(III) centers. In contrast, for 1-Br and 1-Cl, magnetic relaxation occurs via an Orbach mechanism, likely involving relaxation between ground and excited exchange-coupled states. Significantly, in the case of 1-Cl, magnetic relaxation is sufficiently slow such that open magnetic hysteresis is observed up to 2.75 K, and the compound exhibits a 100-s blocking temperature of 2.4 K. This compound provides the first example of magnetic blocking in a compound containing only actinide-based ions, as well as the first example involving the uranium(III) oxidation state.
“…Based on the maxima in χ M versus temperature for 1-X , the temperatures at which magnetic exchange influences the magnetic susceptibility are low enough that only the ground Kramers doublet (i.e., ± M J states) of each uranium ion will be thermally populated. , The dipolar interaction, in such a case, will involve through-space interactions between the magnetic moments of the ground Kramers doublets of each uranium ion, with the strength of the interaction decreasing with larger U···U separations, r , by the relation 1/ r 3 . Dipolar interactions dominate magnetic exchange in many multilanthanide complexes (| J dip | is typically on the order of ∼1–5 cm –1 ) and the magnitude of this exchange is strongly dependent on the orientation of the individual Kramers doublets with respect to each other. ,,− Each uranium(III) center in 1-X likely possesses strongly uniaxial magnetic anisotropy (i.e., g z > g x , g y ) as a result of the strong U–Cp i Pr bonding interaction . In the simplest scenario, each uranium ion possesses a completely axial, and maximal, M J = ± 9/2 ground state ( g x , g y , g z = 0, 0, 6.55); assuming full parallel alignment of the magnetic moments, upper limits for | J dip | for 1-Cl , 1-Br , and 1-I are 0.50, 0.44, and 0.39 cm –1 , respectively (see Supporting Information for details).…”
The actinide elements are attractive alternatives to transition metals or lanthanides for the design of exchange-coupled multinuclear single-molecule magnets. However, the synthesis of such compounds is challenging, as is unraveling any contributions from exchange coupling to the overall magnetism. To date, only a few actinide compounds have been shown to exhibit exchange coupling and single-molecule magnetism. Here, we report triangular uranium(III) clusters of the type (Cp iPr5 ) 3 U 3 X (1-X; X = Cl, Br, I; Cp iPr5 = pentaisopropylcyclopentadienyl), which are synthesized via reaction of the aryloxide-bridged precursor (Cp iPr5 ) 2 U 2 (OPh tBu ) 4 with excess Me 3 SiX. Spectroscopic analysis suggests the presence of covalency in the uranium−halide interactions arising from 5f orbital participation in bonding. The dc magnetic susceptibility data reveal the presence of antiferromagnetic exchange coupling between the uranium(III) centers in these compounds, with the strength of the exchange decreasing down the halide series. Ac magnetic susceptibility data further reveal all compounds to exhibit slow magnetic relaxation under zero dc field. In 1-I, which exhibits particularly weak exchange, magnetic relaxation occurs via a Raman mechanism associated with the individual uranium(III) centers. In contrast, for 1-Br and 1-Cl, magnetic relaxation occurs via an Orbach mechanism, likely involving relaxation between ground and excited exchange-coupled states. Significantly, in the case of 1-Cl, magnetic relaxation is sufficiently slow such that open magnetic hysteresis is observed up to 2.75 K, and the compound exhibits a 100-s blocking temperature of 2.4 K. This compound provides the first example of magnetic blocking in a compound containing only actinide-based ions, as well as the first example involving the uranium(III) oxidation state.
“…[33][34][35][36][37][38] However, the non-zero angle between the local magnetic anisotropy axes and the associated magnetic exchange produces a QTM-facilitating transverse field and may even quench the SMM behaviour. 39,40,68 It follows that promoting appropriate magnetic coupling in these systems is of great research significance, but not an easy task.…”
Non-radical bridges capable of coupling Ising-type magnetic anisotropy in a collinear manner are rather limited in the construction of dinuclear single-molecule magnets (SMMs). Here we report the first alkoxyborate-bridging SMM...
“…For oblate Dy III –SMMs, a strong axial crystal field (CF) and weak equatorial ligand field are required to balance the electron cloud distribution. , However, for multicore systems, the magnetic coupling is complex because of the different anisotropy of central ions. Therefore, it is of great significance to analyze the relationship between structure and magnetic properties, optimize the anisotropy of single-metal ions, and synthesize ideal structural configurations to construct Dy III SMMs with high performance. − For the development of such SMMs, the selection of suitable ligands is important. Indeed, Schiff base ligands behave as O- and N-multidentate ligands that can readily form many types of coordination pockets to coordinate with Dy III ions.…”
Using the Schiff base ligand H 2 L-pyra (N′-(2hydroxybenzoyl)pyrazine-2-carbohydrazonamide) with multiple dentate sites, the trinuclear Dy III -based complex [Dy 3 (HLpyra) 2 (L-pyra) 2 (CH 3 COO) 3 ]•2H 2 O (1) was synthesized. By analyzing the fragmented assembly process and fine-tuning the b r i d g i n g a n i o n s , c o m p l e x [ D y 4 ( H L -p y r a ) 2 ( L -p y ra) 4 2) with different nuclear numbers was successfully synthesized. Magnetic studies demonstrated that 1 did not exhibit magnetic relaxation behavior under the external field; however, 2 exhibited zero-field single-molecule magnetic relaxation behavior with an effective energy barrier (U eff ) of 197.44 K. This is attributed to the improved anisotropy of the single ion after the normalization of the crystal structure, thus realizing the molecular magnetic switching. Moreover, magnetic dilution analysis of 2 demonstrated that the weak magnetic interaction between metal ions inhibited the occurrence of quantum tunneling of magnetization (QTM), resulting in high-performance single-molecule magnet (SMM) behavior. The reasons for the magnetic difference between these two complexes were analyzed using ab initio calculation and magneto-structural correlations. This study provides a reasonable prediction of the ideal configuration of the approximately parallelogram Dy III -based SMMs, thus offering an effective approach for synthesizing Dy 4 complexes with excellent properties.
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