Coexistence of Distinct Single‐Ion and Exchange‐Based Mechanisms for Blocking of Magnetization in a Co^II₂Dy^III₂ Single‐Molecule Magnet

Abstract

Research efforts in the quest for new single-molecule magnets (SMMs) have increasingly focused on systems either based on or else incorporating 4f ions. [1] For most pure 3d systems, and especially those containing the Mn III ion such as the original Mn 12 -Ac coordination cluster, spin reorientation is blocked when the ground state spin (S) combines with uniaxial magnetic anisotropy (D) to give an energy barrier to magnetic relaxation with the superexchange interactions between the metal centers leading to a molecular spin ground state and a molecular anisotropy. [2,3] The resultant exchange-based blocking of magnetization can be analyzed using a giant spin model. [4] In systems incorporating highly anisotropic 4f ions [1] it has become clear that magnetic interactions between 4f ions are weak and generally dipolar in nature. Here the single-ion spin and anisotropy become of greater relevance. For example, recent calculations on a Dy 2 SMM showed that the blocking mechanism largely arises from the individual Dy III ions with exchange-based behavior only seen at very low temperatures. [5] In systems combining 3d and 4f ions the aim is to embed highly anisotropic 4f ions into an exchangecoupled molecular 3d system, since 3d-4f interactions can be intermediate in magnitude between 3d-3d and 4f-4f. How-ever, analysis of the origins of the blocking mechanism in such systems is not straightforward and can generally only be achieved through detailed ab initio calculations, such as we recently reported for a Cr 4 Dy 4 SMM. [6] We now present a SMM comprising two Co II and two Dy III ions for which we can demonstrate the novel situation of single-ion blocking of the Dy III ions at higher temperatures with a crossover to molecular exchanged-based blocking at low temperatures.Reaction of Dy(NO 3 ) 3 ·6 H 2 O, Co(NO 3 ) 2 ·6 H 2 O, H 2 L and Et 3 N in the molar ratio 1:1:2:4 in MeOH gives crystalline red powder which was recrystallized from THF giving pink crystals of [Co 2 Dy 2 (L) 4 (NO 3 ) 2 (THF) 2 ]·4 THF (1) in 75% yield. H 2 L is the Schiff-base we previously described [7] resulting from condensation of o-vanillin and 2-aminophenol to give a "pocket ligand" capable of binding two different types of metal ion (see Figure S1 in the Supporting Information).Compound 1 crystallizes in the triclinic space group P " 1 with Z = 1. Within the core of the centrosymmetric complex, the metal ions are linked by four (L) 2À ligands in the butterfly (or defect-dicubane) topology (Figure 1). One of the two crystallographically independent ligands chelates Dy(1) through its imine nitrogen and the two phenolate oxygens O(1) and O(3) (corresponding to pocket I, see Figure S1 in the Supporting Information). Co(1) and Co(1') are linked Figure 1. Molecular structure of the Co 2 Dy 2 complex in 1.

High spin cycles: topping the spin record for a single molecule verging on quantum criticality

“…The opposite is true in terms of a modelling with the FiniteTemperature Lanczos Method (FTLM). 32,33 The method provides very good approximations for thermodynamic functions up to Hilbert space dimensions of 10 10 . So far, the largest system treated using this approach was a Gd III 12 cluster.…”

“…Finding Recent research by us and others into mixed 3d/4f systems [9][10][11] offers various means to increase both spin and total anisotropy. En route we synthesised the Gd-containing isotropic member of a new series of cyclic CCs [Fe 10 Ln 10 (Me-tea) 10 (Me-teaH) 10 (2).…”

“…En route we synthesised the Gd-containing isotropic member of a new series of cyclic CCs [Fe 10 Ln 10 (Me-tea) 10 (Me-teaH) 10 (2). As we will demonstrate below 1 turns out to be a fascinating spin system for two reasons.…”

[LnIII–MnII–LnIII] heterometallic compounds: rare linear SMMs with divalent manganese ions

“…For complex 2, the relaxation time τ follows a thermally-activated mechanism and the plot of ln τ versus 1/T is linear (Fig. 30,31 Conclusions In summary, we have successfully obtained a new family of trinuclear heterometallic Ln III 2 Mn II (Ln = Gd (1), Tb (2), Dy (3), Ho (4) and Er (5)) complexes with a linear type structural topology through the reactions between relevant 3d, 4f metal salts and salicylic aldehyde ligands. Fitting the data to the Arrhenius law produced an effective energy barrier to magnetization reversal of U eff /k B = 18.79(4) K with the pre-exponential factor of τ 0 = 2.56 × 10 −8 s. However, for complex 3, the ln τ versus 1/T plot shows a crossover at around T −1 = 0.1 K −1 , which implies the presence of dual relaxation pathways.…”

A new family of high nuclearity CoII/DyIII coordination clusters possessing robust and unseen topologies

“…16 We recently employed (E)-4-(2-hydroxy-3methoxybenzylideneamino)-2,3-dimethyl-1-phenyl-1,2dihydropyrazol-5-one (HL, Scheme 1) in Co/Ln chemistry, in the absence of co-ligand to give two isoskeletal Co II 3 Dy III 4 CCs formulated as ([Co II 3 Dy III 4 (µ 3 -OH) 6 L 6 (CF 3 SO 3 )] (ClO 4 ) 5 (1) and [Co II 3 Dy III 4 (µ 3 -OH) 6 L 6 (CF 3 SO 3 )] (ClO 4 ) 3 (CF 3 SO 3 ) 2 (2) possessing a disk-like or 3,6M7-1 topology, previously unseen in Co/Ln chemistry and exhibiting interesting magnetic properties. 17 The two different coordination pockets, the limited degree of flexibility of the ligand, the unusual Co/Dy topology and the higher nuclearity afforded in comparison with compounds containing similar ligands, 18 lead us to explore the coordination chemistry of HL in more detail and to undertake a systematic investigation.…”