Heterometallic Zn₃Ln₃ Ensembles Containing (μ₆‐CO₃) Ligand and Triangular Disposition of Ln³⁺ ions: Analysis of Single‐Molecule Toroic (SMT) and Single‐Molecule Magnet (SMM) Behavior

Abstract

Two new heterometallic Zn Ln (Ln =Dy, Tb) complexes, with a double triangular topology of the metal ions, have been assembled from the polytopic Mannich base ligand 6,6'-{[2-(dimethylamino)ethylazanediyl]bis(methylene)}bis(2-methoxy-4-methylphenol) (H L) with the aid of an in situ generated carbonate ligand from atmospheric CO fixation. Theoretical calculations indicate axial ground states for the Ln ions in these complexes, with their local magnetic moments being almost coplanar and tangential to the Ln atoms that define the equilateral triangle. Therefore, they can be considered as single-molecule toroics (SMTs) with almost zero total magnetic moment. Micro-SQUID measurements on the Dy counterpart show hysteresis loops below 3 K that have an S-shape, with large coercive fields opening upon cooling. This behavior is typical of a single molecule magnet (SMM) with very slow zero-field relaxation. At around ±0.35 T, the loops have a broad step, which is due to a direct relaxation process and corresponds to an acceleration of the relaxation of the magnetization, also observed at this magnetic field from ac susceptibility measurements. Simulations suggest that the broad step corresponds to two level avoidance of crossing points where the spin chiral Kramers doublet meets excited states of the coupled manifold, whose position is defined by exchange and dipole interactions. The Tb counterpart does not exhibit SMM behavior, which is due to the fact that the degeneracy of the ground state of the exchange coupled system is lifted at zero field, thus favoring quantum tunneling of magnetization (QTM).

Magnetic Properties of LnIII–CuII 15‐Metallacrown‐5 Dimers with Terephthalate (LnIII = Pr, Nd, Sm, Eu)

“…Reactions of terephthalate with initial metallacrown blocks {PrCu 5 } 3+ and {NdCu 5 } 3+ resulted in formation of two isostructural discrete complexes {[LnCu 5 (GlyHA) 5 (CO 3 )(H 2 O) 4 ] 2 ( p ‐bdc)} · 15H 2 O (Ln = Pr ( 1 ) and Nd ( 2 )) with carbonate anions coordinated to the Ln III ions in the 15‐metallacrown‐5 units. The presence of carbonate anions is attributed to carbon dioxide uptake from the air, which is quite typical both for 3d and 4f complexes, and was recently reported for some Ln III –Cu II 15‐metallacrown‐5 complexes …”

Heterometallic 3d–4f Complexes as Single‐Molecule Magnets

“…The successful utilization (L 22 H 2 ) in the synthesis of complexes with a [Ni II 3 Ln III 3 ] core prompted us check its efficacy towards the synthesis of Zn II ‐Ln III heterometallic complexes. Hexanuclear [Zn II 3 Ln III 3 ] [Ln III =Tb III ( 173 ), Dy III ( 174 )] complexes possessing double‐triangular topology could be assembled (Figure ) . Interestingly, one CO 3 2− ligand that is bonded to three Ln III centres in a μ 3 ‐fashion is generated in situ through atmospheric CO 2 fixation.…”

“…The M is normalized to its saturation value M s at 1.4 T. Reprinted with permission from Ref. . Copyright 2017 WILEY‐VCH Verlag GmbH & Co.…”

“…[21, 65, 101a, 110] The successful utilization (L 22 H 2 )i nt he synthesis of complexes with a[ Ni II 3 Ln III 3 ]c ore promptedu sc heck its efficacy towards the synthesis of Zn II -Ln III (174)] complexesp ossessing double-triangular topology could be assembled ( Figure 28). [111] Interestingly,o ne CO 3 2À ligand that is bondedt o three Ln III centresi nam 3 -fashion is generated in situ through atmosphericC O 2 fixation. SMM behavior waso bserved for 174 ( Figure 28) In addition to ac susceptibilitye xperiments, hysteresis loops below 3K wereo bserved and were found to be temperatureand sweep-rate dependent.…”

“…Orientationo ft he magnetic anisotropy axes of the Dy III ions in 174 (blue arrows). Reprinted with permission from Ref [111]…”