Molecules at the Quantum–Classical Nanoparticle Interface: Giant Mn₇₀ Single-Molecule Magnets of ∼4 nm Diameter

Two Mn₇₀ torus-like molecules have been obtained from the alcoholysis in EtOH and 2-ClC₂H₄OH of [Mn₁₂O₁₂(O₂CMe)₁₆(H₂O)₄]·4H₂O·2MeCO₂H (<b>1</b>) in the presence of NBuⁿ₄MnO₄ and an excess of MeCO₂H. The reaction in EtOH afforded [Mn₇₀O₆₀­(O₂CMe)₇₀­(OEt)₂₀­(EtOH)₁₆­(H₂O)₂₂] (<b>2</b>), whereas the reaction in ClC₂H₄­OH gave [Mn₇₀­O₆₀­(O₂CMe)₇₀­(OC₂H₄Cl)₂₀­(ClC₂H₄OH)₁₈­(H₂O)₂₂] (<b>3</b>). The complexes are nearly isostructural, each possessing a Mn₇₀ torus structure consisting of alternating near-linear [Mn₃(μ₃-O)₄] and cubic [Mn₄(μ₃-O)₂(μ₃-OR)₂] (R = OEt, <b>2</b>; R = OC₂H₄Cl, <b>3</b>) subunits, linked together via <i>syn,syn</i>-μ-bridging MeCO₂^– and μ₃-bridging O^2– groups. <b>2</b> and <b>3</b> have an overall diameter of ∼4 nm and crystallize as highly ordered supramolecular nanotubes. Alternating current (ac) magnetic susceptibility measurements, performed on microcrystalline samples in the 1.8–10 K range and a 3.5 G ac field with oscillation frequencies in the 5–1500 Hz range, revealed frequency-dependent out-of-phase signals below ∼2.4 K for both molecules indicative of the slow magnetization relaxation of single-molecule magnets (SMMs). Single-crystal, magnetization vs field studies on both complexes revealed hysteresis loops below 1.5 K, thus confirming <b>2</b> and <b>3</b> to be new SMMs. The hysteresis loops do not show the steps that are characteristic of quantum tunneling of magnetization (QTM). However, low-temperature studies revealed temperature-independent relaxation rates below ∼0.2 K for both compounds, the signature of ground state QTM. Fitting of relaxation data to the Arrhenius equation gave effective barriers for magnetization reversal (<i>U</i>_eff) of 23 and 18 K for <b>2</b> and <b>3</b>, respectively. Because the Mn₇₀ molecule is close to the classical limit, it was also studied using a method based on the Néel–Brown model of thermally activated magnetization reversal in a classical single-domain magnetic nanoparticle. The field and sweep-rate dependence of the coercive field was investigated and yielded the energy barrier, the spin, the Arrhenius pre-exponential, and the cross-over temperature from the classical to the quantum regime. The validity of this approach emphasizes that large SMMs can be considered as being at or near the quantum–classical nanoparticle interface

Molecules at the Quantum–Classical Nanoparticle Interface: Giant Mn₇₀ Single-Molecule Magnets of ∼4 nm Diameter

Two Mn₇₀ torus-like molecules have been obtained from the alcoholysis in EtOH and 2-ClC₂H₄OH of [Mn₁₂O₁₂(O₂CMe)₁₆(H₂O)₄]·4H₂O·2MeCO₂H (<b>1</b>) in the presence of NBuⁿ₄MnO₄ and an excess of MeCO₂H. The reaction in EtOH afforded [Mn₇₀O₆₀­(O₂CMe)₇₀­(OEt)₂₀­(EtOH)₁₆­(H₂O)₂₂] (<b>2</b>), whereas the reaction in ClC₂H₄­OH gave [Mn₇₀­O₆₀­(O₂CMe)₇₀­(OC₂H₄Cl)₂₀­(ClC₂H₄OH)₁₈­(H₂O)₂₂] (<b>3</b>). The complexes are nearly isostructural, each possessing a Mn₇₀ torus structure consisting of alternating near-linear [Mn₃(μ₃-O)₄] and cubic [Mn₄(μ₃-O)₂(μ₃-OR)₂] (R = OEt, <b>2</b>; R = OC₂H₄Cl, <b>3</b>) subunits, linked together via <i>syn,syn</i>-μ-bridging MeCO₂^– and μ₃-bridging O^2– groups. <b>2</b> and <b>3</b> have an overall diameter of ∼4 nm and crystallize as highly ordered supramolecular nanotubes. Alternating current (ac) magnetic susceptibility measurements, performed on microcrystalline samples in the 1.8–10 K range and a 3.5 G ac field with oscillation frequencies in the 5–1500 Hz range, revealed frequency-dependent out-of-phase signals below ∼2.4 K for both molecules indicative of the slow magnetization relaxation of single-molecule magnets (SMMs). Single-crystal, magnetization vs field studies on both complexes revealed hysteresis loops below 1.5 K, thus confirming <b>2</b> and <b>3</b> to be new SMMs. The hysteresis loops do not show the steps that are characteristic of quantum tunneling of magnetization (QTM). However, low-temperature studies revealed temperature-independent relaxation rates below ∼0.2 K for both compounds, the signature of ground state QTM. Fitting of relaxation data to the Arrhenius equation gave effective barriers for magnetization reversal (<i>U</i>_eff) of 23 and 18 K for <b>2</b> and <b>3</b>, respectively. Because the Mn₇₀ molecule is close to the classical limit, it was also studied using a method based on the Néel–Brown model of thermally activated magnetization reversal in a classical single-domain magnetic nanoparticle. The field and sweep-rate dependence of the coercive field was investigated and yielded the energy barrier, the spin, the Arrhenius pre-exponential, and the cross-over temperature from the classical to the quantum regime. The validity of this approach emphasizes that large SMMs can be considered as being at or near the quantum–classical nanoparticle interface

A Record Anisotropy Barrier for a Single-Molecule Magnet

Structural distortion in a [Mn6] complex switches the magnetic exchange from antiferro- to ferromagnetic, resulting in a single-molecule magnet with a record anisotropy barrier

A Record Anisotropy Barrier for a Single-Molecule Magnet

Structural distortion in a [Mn6] complex switches the magnetic exchange from antiferro- to ferromagnetic, resulting in a single-molecule magnet with a record anisotropy barrier

A Single-Molecule Magnet with a “Twist”

The deliberate structural distortion of a Mn6 compound via the use of a bulky salicylaldoxime derivative switches the intra-triangular magnetic exchange from antiferromagnetic to ferromagnetic resulting in an S = 12 ground state

Microwave-Assisted Synthesis of a Hexanuclear Mn^III Single-Molecule Magnet

The use of microwave heating has improved the reaction rate, enhanced the yield of an inorganic cluster complex, and allowed for the high-temperature/high-pressure synthesis of a MnIII single-molecule magnet

A Single-Molecule Magnet with a “Twist”

The deliberate structural distortion of a Mn6 compound via the use of a bulky salicylaldoxime derivative switches the intra-triangular magnetic exchange from antiferromagnetic to ferromagnetic resulting in an S = 12 ground state

Microwave-Assisted Synthesis of a Hexanuclear Mn^III Single-Molecule Magnet

The use of microwave heating has improved the reaction rate, enhanced the yield of an inorganic cluster complex, and allowed for the high-temperature/high-pressure synthesis of a MnIII single-molecule magnet

Spin Switching via Targeted Structural Distortion

The deliberate “stepwise” structural distortion of the [MnIII6O2(sao)6(O2CR)2L4] (S = 4, Ueff = 28 K) family of SMMs (where sao2- is the dianion of salicylaldoxime or 2-hydroxybenzaldeyhyde oxime and L = MeOH, EtOH) via the use of derivatized oxime ligands and bulky carboxylates leads to a family of single-molecule magnets with larger spin ground states and enhanced blocking temperatures. Replacing sao2- and HCO2- in the molecule [MnIII6O2(sao)6(O2CH)2(MeOH)4] (1), with Et-sao2- (Et-saoH2 = 2-hydroxypropiophenone oxime) and Me3CCO2- (pivalate), produces the complex [MnIII6O2(Et-sao)6(O2CCMe3)2(EtOH)5] (2) that displays an S = 7 ground state with Ueff = 30 K. Replacing Me3CCO2- with PhCO2- produces the complex [MnIII6O2(Et-sao)6(O2CPh)2(EtOH)4(H2O)2] (3) that displays an S = 12 ground state with Ueff = 53 K. The ligand substitution invokes a subtle structural distortion to the core of the molecule evidenced by an increased “twisting” of the oxime moiety (Mn−N−O−Mn) and a change in carboxylate ligation, which, in turn, invokes a dramatic change in the observed magnetic properties by switching weak antiferromagnetic exchange to weak ferromagnetic exchange

Two Frustrated, Bitetrahedral Single-Molecule Magnets

Two unusual mixed-valent {MnIII6MnII} bitetrahedra display frustrated magnetic exchange, leading to S = 13/2 ± 1 and 11/2 ± 1 ground states and slow magnetization relaxation