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.

The discovery that individual molecules can function as magnets provided a new, "bottom-up" approach to nanoscale magnetic materials, [1][2][3] and such molecules have since been called single-molecule magnets (SMMs). [4] Each molecule is a single-domain magnetic particle that, below its blocking temperature, exhibits the classical macroscale property of a magnet, namely magnetization hysteresis. In addition, SMMs straddle the classical/quantum interface in also displaying quantum tunneling of magnetization (QTM) [5,6] and quantum phase interference, [7] which are the properties of the microscale. SMMs have various potential applications, including very high-density information storage with each bit stored as the magnetization orientation of an individual molecule, and as quantum bits for quantum computing [8] by taking advantage of the quantum superposition of states provided by the QTM. For a number of reasons, including facilitating development of techniques for addressing individual SMMs, we have sought to synthesize SMMs of very-large dimensions (by molecular standards). In effect, can the molecular (or bottomup) approach reach the size regime of the classical (or topdown) approach to nanoscale magnetic materials? Indeed, herein we report a giant Mn 84 SMM. It has a 4 nm diameter torus structure, exhibits both magnetization hysteresis and QTM, and crystallizes as supramolecular nanotubes.The (2) with (N n Bu 4 )(MnO 4 ) in MeOH that contained a little acetic acid, followed by filtration and layering of the filtrate with chloroform. After several weeks, the well-formed reddish-brown crystals were isolated in a 20 % overall yield, based on the total available Mn equivalents. The compound crystallizes in hexagonal space group P6 with the asymmetric unit containing 1/6 of the molecule and approximately 25 water and 2 chloroform solvent mole-

We have structurally and magnetically characterized a total of 12 complexes based on the Single-Molecule Magnet (SMM) [MnIII6O2(sao)6(O2CH)2(MeOH) 4] (1) (where sao2- is the dianion of salicylaldoxime or 2-hydroxybenzaldeyhyde oxime) that display analogous structural cores but remarkably different magnetic behaviors. Via the use of derivatized oxime ligands and bulky carboxylates we show that it is possible to deliberately increase the value of the spin ground state of the complexes [Mn6O2(Me-sao)6(O2CCPh3)2(EtOH)4] (2), [Mn6O2(Et-sao)6(O2CCMe3)2(EtOH)5] (3), [Mn6O2(Et-sao)6(O2CPh2OPh)2(EtOH)4] (4), [Mn6O2(Et-sao)6(O2CPh4OPh)2(EtOH)4(H2O)2] (5), [Mn6O2(Me-sao)6(O2CPhBr)2(EtOH)6] (6), [Mn6O2(Et-sao)6(O2CPh)2(EtOH)4(H2O)2] (7), [Mn6O2(Et-sao)6{O2CPh(Me)2}2(EtOH)6] (8), [Mn6O2(Et-sao)6(O2C11H15)2(EtOH)6] (9), [Mn6O2(Me-sao)6(O2C-th)2(EtOH)4(H2O)2] (10), [Mn6O2(Et-sao)6(O2CPhMe)2(EtOH)4(H2O)2] (11), and [Mn6O2(Et-sao)6(O2C12H17)2(EtOH)4(H2O)2] (12) (Et-saoH2 = 2-hydroxypropiophenone oxime, Me-saoH2 = 2-hydroxyethanone oxime, HO2CCPh3 = triphenylacetic acid, HO2CCMe3 = pivalic acid, HO2CPh2OPh = 2-phenoxybenzoic acid, HO2CPh4OPh = 4-phenoxybenzoic acid, HO2CPhBr = 4-bromobenzoic acid, HO2CPh(Me)2 = 3,5-dimethylbenzoic acid, HO2C11H15 = adamantane carboxylic acid, HO2C-th = 3-thiophene carboxylic acid, HO2CPhMe = 4-methylbenzoic acid, and HO2C12H17 = adamantane acetic acid) in a stepwise fashion from S = 4 to S = 12 and, in-so-doing, enhance the energy barrier for magnetization reorientation to record levels. The change from antiferromagnetic to ferromagnetic exchange stems from the "twisting" or "puckering" of the (-Mn-N-O-)3 ring, as evidenced by the changes in the Mn-N-O-Mn torsion angles.

No abstract

The discovery that individual molecules can function as magnets provided a new, "bottom-up" approach to nanoscale magnetic materials, [1][2][3] and such molecules have since been called single-molecule magnets (SMMs).[4] Each molecule is a single-domain magnetic particle that, below its blocking temperature, exhibits the classical macroscale property of a magnet, namely magnetization hysteresis. In addition, SMMs straddle the classical/quantum interface in also displaying quantum tunneling of magnetization (QTM) [5,6] and quantum phase interference, [7] which are the properties of the microscale. SMMs have various potential applications, including very high-density information storage with each bit stored as the magnetization orientation of an individual molecule, and as quantum bits for quantum computing [8] by taking advantage of the quantum superposition of states provided by the QTM. For a number of reasons, including facilitating development of techniques for addressing individual SMMs, we have sought to synthesize SMMs of very-large dimensions (by molecular standards). In effect, can the molecular (or bottomup) approach reach the size regime of the classical (or topdown) approach to nanoscale magnetic materials? Indeed, herein we report a giant Mn 84 SMM. It has a 4 nm diameter torus structure, exhibits both magnetization hysteresis and QTM, and crystallizes as supramolecular nanotubes.The (2) with (N n Bu 4 )(MnO 4 ) in MeOH that contained a little acetic acid, followed by filtration and layering of the filtrate with chloroform. After several weeks, the well-formed reddish-brown crystals were isolated in a 20 % overall yield, based on the total available Mn equivalents. The compound crystallizes in hexagonal space group P6 with the asymmetric unit containing 1/6 of the molecule and approximately 25 water and 2 chloroform solvent mole-

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 Mn70 torus-like molecules have been obtained from the alcoholysis in EtOH and 2-ClC2H4OH of [Mn12O12(O2CMe)16(H2O)4]·4H2O·2MeCO2H (1) in the presence of NBu(n)4MnO4 and an excess of MeCO2H. The reaction in EtOH afforded [Mn70O60(O2CMe)70(OEt)20(EtOH)16(H2O)22] (2), whereas the reaction in ClC2H4OH gave [Mn70O60(O2CMe)70(OC2H4Cl)20(ClC2H4OH)18(H2O)22] (3). 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, 2; R = OC2H4Cl, 3) subunits, linked together via syn,syn-μ-bridging MeCO2(-) and μ3-bridging O(2-) groups. 2 and 3 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 2 and 3 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 (Ueff) of 23 and 18 K for 2 and 3, 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 Mn4 single-molecule magnet (SMM), with a well isolated spin ground state of S = 9/2, is used as a model system to study Landau-Zener (LZ) tunneling in the presence of weak intermolecular dipolar and exchange interactions. The anisotropy constants D and B are measured with minor hysteresis loops. A transverse field is used to tune the tunnel splitting over a large range. Using the LZ and inverse LZ method, it is shown that these interactions play an important role in the tunnel rates. Three regions are identified: (i) at small transverse fields, tunneling is dominated by single tunnel transitions; (ii) at intermediate transverse fields, the measured tunnel rates are governed by reshuffling of internal fields, (iii) at larger transverse fields, the magnetization reversal starts to be influenced by the direct relaxation process, and many-body tunnel events may occur. The hole digging method is used to study the next-nearest neighbor interactions. At small external fields, it is shown that magnetic ordering occurs which does not quench tunneling. An applied transverse field can increase the ordering rate. Spin-spin cross-relaxations, mediated by dipolar and weak exchange interactions, are proposed to explain additional quantum steps.