Magnetic Bistability of Isolated Giant‐Spin Centers in a Diamagnetic Crystalline Matrix

Polynuclear single‐molecule magnets (SMMs) were diluted in a diamagnetic crystal lattice to afford arrays of independent and iso‐oriented magnetic units. Crystalline solid solutions of an Fe₄ SMM and its Ga₄ analogue were prepared with no metal scrambling for Fe₄ molar fractions x down to 0.01. According to high‐frequency EPR and magnetic measurements, the guest SMM species have the same total spin (S=5), anisotropy, and high‐temperature spin dynamics found in the pure Fe₄ phase. However, suppression of intermolecular magnetic interactions affects magnetic relaxation at low temperature (40 mK), where quantum tunneling (QT) of the magnetization dominates. When a magnetic field is applied along the easy magnetic axis, both pure and diluted (x=0.01) phases display pronounced steps at evenly spaced field values in their hysteresis loops due to resonant QT. The pure Fe₄ phase exhibits additional steps which are firmly ascribed to two‐molecule QT transitions. Studies on the field‐dependent relaxation rate showed that the zero‐field resonance sharpens by a factor of five and shifts from about 8 mT to exactly zero field on dilution, in agreement with the calculated variation of dipolar interactions. The tunneling efficiency also changes significantly as a function of Fe₄ concentration: the zero‐field resonance is significantly enhanced on dilution, while tunneling at ±0.45 T becomes less efficient. These changes were rationalized on the basis of a dipolar shuffling mechanism and transverse dipolar fields, whose effect was analyzed by using a multispin model. Our findings directly prove the impact of intermolecular magnetic couplings on SMM behavior and disclose the magnetic response of truly isolated giant spins in a diamagnetic crystalline environment.     

Quantum tunneling of magnetization and related phenomena in molecular materials

Molecules comprising a large number of coupled paramagnetic centers are attracting much interest because they may show properties which are intermediate between those of simple paramagnets and classical bulk magnets and provide unambiguous evidence of quantum size effects in magnets. To date, two cluster families, usually referred to as Mn12 and Fe8, have been used to test theories. However, it is reasonable to predict that other classes of molecules will be discovered which have similar or superior properties. To do this it is necessary that synthetic chemists have a good understanding of the correlation between the structure and properties of the molecules, for this it is necessary that concepts such as quantum tunneling, quantum coherence, quantum oscillations are understood. The goal of this article is to review the fundamental concepts needed to understand quantum size effects in molecular magnets and to critically report what has been done in the field to date.     

Modelling the Magnetic Behaviour of Square‐Pyramidal CoII5 Aggregates: Tuning SMM Behaviour through Variations in the Ligand Shell

Three new μ₄‐bridged Co^II₅ clusters with similar core motifs have been synthesised with the use of N‐tert‐butyldiethanolamine (tbdeaH₂) and pivalic acid (piv): [Co^II₅(μ₄‐N₃)(tbdea)₂(μ‐piv)₄(piv)(CH₃CN)₂]?CH₃CN ( 1 ), [Co^II₅(μ₄‐Cl)(Cl)(tbdea)₂(μ‐piv)₄(pivH)₂] ( 2 ) and [Co^II₅(μ₄‐N₃)(Cl)(tbdea)₂(μ‐piv)₄(pivH)₂] ( 3 ). Magnetic measurements were performed for all three compounds. It was found that while the chloride‐bridged cluster 2 does not show an out‐of‐phase signal, which excludes single‐molecule magnet (SMM) behaviour, the azide‐bridged compounds 1 and 3 show out‐of‐phase signals as well as frequency dependence of the ac susceptibility, as expected for SMMs. We confirmed that 1 is a SMM with zero‐field quantum tunnelling of the magnetisation at 1.8 K. Compound 3 is likely a SMM with a blocking temperature well below 1.8 K. We established a physical model to fit the χT versus T and M versus B curves of the three compounds to reproduce the observed SMM trend. The analysis showed that small changes in the ligand shell modify not only the magnitude of exchange constants, but also affect the J and g matrices in a non‐trivial way.     

Effects of 3d-4f magnetic exchange interactions on the dynamics of the magnetization of Dy(III)-M(II)-Dy(III) trinuclear clusters

[{Dy(hfac)(3)}(2){Fe(bpca)(2)}] x CHCl(3) ([Dy(2)Fe]) and [{Dy(hfac)(3)}(2){Ni(bpca)(2)}]CHCl(3) ([Dy(2)Ni]) (in which hfac(-)=1,1,1,5,5,5-hexafluoroacetylacetonate and bpca(-)=bis(2-pyridylcarbonyl)amine anion) were synthesized and characterized. Single-crystal X-ray diffraction shows that [Dy(2)Fe] and [Dy(2)Ni] are linear trinuclear complexes. Static magnetic susceptibility measurements reveal a weak ferromagnetic exchange interaction between Ni(II) and Dy(III) ions in [Dy(2)Ni], whereas the use of the diamagnetic Fe(II) ion leads to the absence of magnetic exchange interaction in [Dy(2)Fe]. Dynamic susceptibility measurements show a thermally activated behavior with the energy barrier of 9.7 and 4.9 K for the [Dy(2)Fe] and [Dy(2)Ni] complexes, respectively. A surprising negative effect of the ferromagnetic exchange interaction has been found and has been attributed to the structural conformation of these trinuclear complexes.     

A Linear 3d–4f Tetranuclear CoIII2DyIII2 Single‐Molecule Magnet: Synthesis,Structure, and Magnetic Properties

A linear tetranuclear 3d–4f Co₂Dy₂ cluster assembled from a polydentate Schiff base exhibits single‐molecule magnet (SMM) behavior with an anisotropic barrier of 33.8 K. Due to the presence of diamagnetic cobalt(III) ions, the tetranuclear cluster of 1 behaves magnetically like a dinuclear Dy₂ system. However, the diamagnetic segment might efficiently minimize undesirable intermolecular magnetic interactions, thereby improving the performance of the SMM behavior of 1 . This discrete complex presents us with a unique opportunity to study the magnetic properties and to probe the dynamics of magnetization in a magnetically isolated Dy₂ system.     

Heterometallic [Mn5-Ln4] single-molecule magnets with high anisotropy barriers

The reaction of [Mn6O2(Piv)(10)(4-Me-py)(2.5)(PivH)(1.5)] (1) (py: pyridine, Piv: pivilate) with N-methyldiethanolamine (mdeaH2) and Ln(NO3)3 x 6 H2O in MeCN leads to a series of nonanuclear compounds [Mn5Ln4(O)6(mdea)2(mdeaH)2(Piv)6(NO3)4(H2O)2]2 MeCN (Ln=Tb(III) (2), Dy(III) (3), Ho(III) (4), Y(III) (5)). Single-crystal X-ray diffraction shows that compounds 2-5 are isostructural, with the central core composed of two distorted {Mn(IV)Mn(III)Ln2O4} cubanes sharing a Mn(IV) vertex, representing a new heterometallic 3d-4f motif for this class of ligand. The four new compounds display single-molecule magnet (SMM) behaviour, which is modulated by the lanthanide ion used. Moreover, the values found for Delta(eff) and tau(o) for 3 of 38.6 K and 3.0 x 10(-9) s respectively reveal that the complex 3 exhibits the highest energy barrier recorded so far for 3d-4f SMMs. The slow relaxation of the magnetisation for 3 was confirmed by mu-SQUID measurements on an oriented single crystal and the observation of M versus H hysteresis loops below 1.9 K.     

Magnetic “Molecular Oligomers” Based on Decametallic Supertetrahedra: A Giant Mn49 Cuboctahedron and its Mn25Na4 Fragment

Two nanosized Mn₄₉ and Mn₂₅Na₄ clusters based on analogues of the high‐spin (S=22) [Mn^III₆Mn^II₄(μ₄‐O)₄]¹⁸⁺ supertetrahedral core are reported. Mn₄₉ and Mn₂₅Na₄ complexes consist of eight and four decametallic supertetrahedral subunits, respectively, display high virtual symmetry (O_h), and are unique examples of clusters based on a large number of tightly linked high nuclearity magnetic units. The complexes also have large spin ground‐state values (Mn₄₉: S=61/2; Mn₂₅Na₄: S=51/2) with the Mn₄₉ cluster displaying single‐molecule magnet (SMM) behavior and being the second largest reported homometallic SMM.     

Butterfly‐Shaped Pentanuclear Dysprosium Single‐Molecule Magnets

Two new “butterfly‐shaped” pentanuclear dysprosium(III) clusters, [Dy₅(μ₃‐OH)₃(opch)₆(H₂O)₃] ? 3 MeOH ? 9 H₂O ( 1 ) and [Dy₅(μ₃‐OH)₃(Hopch)₂(opch)₄(MeOH)(H₂O)₂] ? (ClO₄)₂ ? 6 MeOH ? 4 H₂O ( 2 ), which were based on the heterodonor‐chelating ligand o‐vanillin pyrazine acylhydrazone (H₂opch), have been successfully synthesized by applying different reaction conditions. Single‐crystal X‐ray diffraction analysis revealed that the butterfly‐shaped cores in both compounds were comparable. However, their magnetic properties were drastically different. Indeed, compound 1 showed dual slow‐relaxation processes with a transition between them that corresponded to energy gaps (Δ) of 8.1 and 37.9 K and pre‐exponential factors (τ₀) of 1.7×10^?5 and 9.7×10^?8 s for the low‐ and high‐temperature domains, respectively, whilst only a single relaxation process was noted for compound 2 (Δ=197 K, τ₀=3.2×10^?9 s). These significant disparities are most likely due to the versatile coordination of the H₂opch ligands with particular keto–enol tautomerism, which alters the strength of the local crystal field and, hence, the nature or direction of the easy axes of anisotropic dysprosium ions.     

Pure trinuclear 4f single-molecule magnets: synthesis, structures, magnetism and ab initio investigation

A family of linear Dy₃ and Tb₃ clusters have been facilely synthesized from the reactions of DyCl₃, the polydentate 3‐methyloxysalicylaldoxime (MeOsaloxH₂) ligand with auxiliary monoanionic ligands, such as trichloroacetate, NO₃^?, OH^?, and Cl^?. Complexes 1 – 5 contain a nearly linear Ln₃ core, with similar Ln???Ln distances (3.6901(4)–3.7304(3) Å for the Dy₃ species, and 3.7273(3)–3.7485(5) Å for the Tb₃ species) and Ln???Ln???Ln angles of 157.036(8)–159.026(15)° for the Dy₃ species and 157.156(8)–160.926(15)° for the Tb₃ species. All three Ln centers are bridged by the two doubly‐deprotonated [MeOsalox]^2? ligands and two of the four [MeOsaloxH]^? ligands through the N,O‐η²‐oximato groups and the phenoxo oxygen atoms (Dy‐O‐Dy angles=102.28(16)–106.85(13)°; Tb‐O‐Tb angles=102.00(11)–106.62(11)°). The remaining two [MeOsaloxH]^? ligands each chelate an outer Ln^III center through their phenoxo oxygen and oxime nitrogen atoms. Magnetic studies reveal that both Dy₃ and Tb₃ clusters exhibit significant ferromagnetic interactions and that the Dy₃ species behave as single‐molecule magnets, expanding upon the recent reports of the pure 4f type SMMs.     

Ferromagnetic NiII Discs

A family of planar disc‐like hexa‐, octa‐ and decametallic Ni^II complexes exhibit dominant ferromagnetic exchange. The deca‐ and octametallic clusters [Ni^II₁₀(tmp)₂(N₃)₈(acac)₆(MeOH)₆] ( 1 , H₃tmp=1,1,1‐tris(hydroxymethyl)propane; acac=acetylacetonate) and [Ni^II₈(thme)₂(O₂CPh)₄(Cl)₆(MeCN)₆(H₂O)₂] ( 2 , H₃thme=1,1,1‐tris(hydroxymethyl)ethane) represent rare examples of Ni^II‐based single‐molecule magnets, and [Ni^II₁₀] ( 1 ) possesses the largest barrier to magnetisation reversal of any Ni^II single‐molecule magnet to date.     

Cyanide-bridged [Fe8M6] clusters displaying single-molecule magnetism (M=Ni) and electron-transfer-coupled spin transitions (M=Co)

Cyanide‐bridged metal complexes of [Fe₈M₆(μ‐CN)₁₄(CN)₁₀ (tp)₈(HL)₁₀(CH₃CN)₂][PF₆]₄?n CH₃CN?m H₂O (HL=3‐(2‐pyridyl)‐5‐[4‐(diphenylamino)phenyl]‐1H‐pyrazole), tp^?=hydrotris(pyrazolylborate), 1 : M=Ni with n=11 and m=7, and 2 : M=Co with n=14 and m=5) were prepared. Complexes 1 and 2 are isomorphous, and crystallized in the monoclinic space group P2₁/n. They have tetradecanuclear cores composed of eight low‐spin (LS) Fe^III and six high‐spin (HS) M^II ions (M=Ni and Co), all of which are bridged by cyanide ions, to form a crown‐like core structure. Magnetic susceptibility measurements revealed that intramolecular ferro‐ and antiferromagnetic interactions are operative in 1 and in a fresh sample of 2 , respectively. Ac magnetic susceptibility measurements of 1 showed frequency‐dependent in‐ and out‐of‐phase signals, characteristic of single‐molecule magnetism (SMM), while desolvated samples of 2 showed thermal‐ and photoinduced intramolecular electron‐transfer‐coupled spin transition (ETCST) between the [(LS‐Fe^II)₃(LS‐Fe^III)₅(HS‐Co^II)₃(LS‐Co^III)₃] and the [(LS‐Fe^III)₈(HS‐Co^II)₆] states.