George Christou

Exchange-biased quantum tunnelling in a supramolecular dimer of single-molecule magnets

Various present and future specialized applications of magnets require monodisperse, small magnetic particles, and the discovery of molecules that can function as nanoscale magnets was an important development in this regard. These molecules act as single-domain magnetic particles that, below their blocking temperature, exhibit magnetization hysteresis, a classical property of macroscopic magnets. Such ‘single-molecule magnets’ (SMMs) straddle the interface between classical and quantum mechanical behaviour because they also display quantum tunnelling of magnetization and quantum phase interference. Quantum tunnelling of magnetization can be advantageous for some potential applications of SMMs, for example, in providing the quantum superposition of states required for quantum computing. However, it is a disadvantage in other applications, such as information storage, where it would lead to information loss. Thus it is important to both understand and control the quantum properties of SMMs. Here we report a supramolecular SMM dimer in which antiferromagnetic coupling between the two components results in quantum behaviour different from that of the individual SMMs. Our experimental observations and theoretical analysis suggest a means of tuning the quantum tunnelling of magnetization in SMMs. This system may also prove useful for studying quantum tunnelling of relevance to mesoscopic antiferromagnets.

Enhancing the Quantum Properties of Manganese–Lanthanide Single‐Molecule Magnets: Observation of Quantum Tunneling Steps in the Hysteresis Loops of a {Mn12Gd} Cluster

Single-molecule magnets (SMMs) are individual molecules that function as single-domain nanoscale magnetic particles. A SMM derives its properties from a combination of a high-spin ground state (S) and an easy axis type of magnetoanisotropy (negative zero-field splitting parameter,D), which results in a significant energy barrier to the reversal of the magnetization vector. Such species display both classical magnetization hysteresis, quantum tunneling of magnetization (QTM), and quantum phase interference. Thus, SMMs represent a molecular (“bottom-up”) route to nanoscale magnetism, with potential technological applications in information storage and spintronics at the molecular level, and use as quantum bits (qubits) in quantum computation by exploiting the QTM through the anisotropy barrier. The upper limit to the barrier (U) is given by S jD j or (S-1/4) jD j for integer and half-integer S, respectively. In practice, QTM through upper regions of the barrier makes the true or the effective barrier (Ueff) lower than that of U. Ideally, the QTM can be observed and studied in magnetization vs. DC (direct current) field hysteresis loops, appearing as distinct step-like features at periodic field values, at which levels on either side of the anisotropy barrier to relaxation are in resonance. The steps are thus field positions at which the magnetization relaxation rate increases owing to the onset of QTM. Such steps are a diagnostic signature of resonant QTM, and have been clearly seen only for a few classes of compounds, such as manganese, iron, and nickel SMMs. 7, 8] The most fruitful source of SMMs is the manganese carboxylate chemistry. The prototype was the [Mn12O12(O2CR)16(H2O)4] family, [2,4, 9] and a number of others have since been discovered; almost all have been transition metal clusters, and the vast majority of them have been manganese clusters containing at least some manganese(III) ions. As the search for new SMMs expanded, several groups explored mixed transition metal/lanthanide (Ln) compounds, and particularly Mn–Ln ones, as an attractive area; these efforts were greatly stimulated by the Cu2Tb2 SMM reported by Matsumoto and co-workers. The strategy is obviously to take advantage of the lanthanide ion s significant spin, and/or its large anisotropy, as reflected in a largeD value, to generate SMMs distinctly different from the homometallic ones. Indeed, there are now several Mn–Ln SMMs, including Mn11Ln4, [11] Mn11Gd2, [12] Mn5Ln4, [13a] and Mn6Dy6 [13b] . Many of them have exhibited magnetization hysteresis loops, but unfortunately none of them have displayed resolved QTM steps in these loops. Thus, the incorporation of lanthanide ions has led to a degradation of the quantum properties, as reflected in the QTM steps. The likeliest reason for the degradation of the quantum properties is the step broadening owing to the low-lying excited states resulting from very weak exchange interactions involving the 4f metal ion(s). Herein we report a new structural type in mixed Mn–Ln SMMs having a {Mn12Gd} 38+ core, in which clear QTM steps have been observed in the hysteresis loops of a mixed 3d–4f SMM for the first time. As a result, the D value of a 3d–4f SMM can be measured directly for the first time from the hysteresis data, that is, from magnetic field separation between the steps. The reaction of Mn(O2CPh)2, nBu4NMnO4, Gd(NO3)3, and PhCO2H in a 4:1:4:32 molar ratio in nitromethane produced a dark brown solution, which upon filtration and slow evaporation of the solvent resulted in crystals of [Mn12GdO9(O2CPh)18(O2CH)(NO3)(HO2CPh)] (1) in 40% yield. The structure of 1 consists of a {MnMn11} 35+ cluster with a central Gd ion (Figure 1). The {Mn12Gd} 38+ core is held together by seven m4-O 2 and two m3-O 2 ions. Peripheral ligation is provided by a m4-, three m3-, fourteen m-benzoate groups, a m3-formate group, a chelating NO3 on Mn12, and a terminal benzoic acid on Mn5. The formate probably comes from oxidation of nitromethane by the highly oxidizing MnO4 reagent. The metal oxidation states and the protonation levels of O ions were established by bond valence sum (BVS) calculations and the observation of manganese(III) Jahn–Teller (JT) elongation axes (Figure S1). All manganese atoms are six-coordinate, whereas the gadolinium [*] Dr. T. C. Stamatatos, Prof. Dr. G. Christou Department of Chemistry, University of Florida Gainesville, FL 32611-7200 (USA) Fax: (+1)352-392-8757 E-mail: christou@chem.ufl.edu

Cobalt single-molecule magnet

A cobalt molecule that functions as a single-molecule magnet, [Co4(hmp)4(MeOH)4Cl4], where hmp− is the anion of hydroxymethylpyridine, is reported. The core of the molecule consists of four Co(II) cations and four hmp− oxygen atom ions at the corners of a cube. Variable-field and variable-temperature magnetization data have been analyzed to establish that the molecule has a S=6 ground state with considerable negative magnetoanisotropy. Single-ion zero-field interactions (DSz2) at each cobalt ion are the origin of the negative magnetoanisotropy. A single crystal of the compound was studied by means of a micro-superconducting quantum interference device magnetometer in the range of 0.040–1.0 K. Hysteresis was found in the magnetization versus magnetic field response of this single crystal.

Synthetic model of the asymmetric [Mn3CaO4] cubane core of the oxygen-evolving complex of photosystem II

The laboratory synthesis of the oxygen-evolving complex (OEC) of photosystem II has been the objective of synthetic chemists since the early 1970s. However, the absence of structural information on the OEC has hampered these efforts. Crystallographic reports on photosystem II that have been appearing at ever-improving resolution over the past ten years have finally provided invaluable structural information on the OEC and show that it comprises a [Mn3CaO4] distorted cubane, to which is attached a fourth, external Mn atom, and the whole unit attached to polypeptides primarily by aspartate and glutamate carboxylate groups. Such a heterometallic Mn/Ca cubane with an additional metal attached to it has been unknown in the literature. This paper reports the laboratory synthesis of such an asymmetric cubane-containing compound with a bound external metal atom, [(1)] . All peripheral ligands are carboxylate or carboxylic acid groups. Variable-temperature magnetic susceptibility data have established 1 to possess an S = 9/2 ground state. EPR spectroscopy confirms this, and the Davies electron nuclear double resonance data reveal similar hyperfine couplings to those of other MnIV species, including the OEC S2 state. Comparison of the X-ray absorption data with those for the OEC reveal 1 to possess structural parameters that make it a close structural model of the asymmetric-cubane OEC unit. This geometric and electronic structural correspondence opens up a new front in the multidisciplinary study of the properties and function of this important biological unit.

Manganese carboxylate clusters: From structural aesthetics to single-molecule magnets

Abstract An overview is provided of some recent developments in manganese carboxylatecluster chemistry. A variety of synthetic methodologies are described together with the structuresof the resultant products, which span metal nuclearities of 4 to 18 and oxidation states of II to IV,including mixed-valency. The spins of the ground states of these products are often large andsometimes abnormally large, and in certain cases when this is coupled to a sufficiently largemagnetoanisotropy, the clusters function as single-molecule magnets i.e., they can be magnetizedby an external magnetic field below a critical temperature. These complexes display hysteresis inmagnetization vs . magnetic field studies, and clear evidence for quantum tunnelling ofmagnetization. Such results establish single-molecule magnetism as a new magnetic phenomenon,and one that holds great promise for next-century technological applications.

Exchange bias in Ni4 single-molecule magnets

Abstract The syntheses and physical properties are reported for three single-molecule magnets (SMMs) with the composition [Ni(hmp)(ROH)Cl]4, where R is CH3 (complex 1), CH2CH3 (complex 2) or CH2CH2C(CH3)3 (complex 3) and hmp− is the monoanion of 2-hydroxymethylpyridine. The core of each complex is a distorted cube formed by four NiII ions and four alkoxide hmp− oxygen atoms at alternating corners. Ferromagnetic exchange interactions give a S=4 ground state. Single crystal high-frequency EPR spectra clearly indicate that each of the complexes has a S=4 ground state and that there is negative magnetoanisotropy, where D is negative for the axial zero-field splitting DŜz2. Magnetization versus magnetic field measurements made on single crystals with a micro-SQUID magnetometer indicate these Ni4 complexes are SMMs. Exchange bias is seen in the magnetization hysteresis loops for complexes 1 and 2.

Mn21Dy Cluster with a Record Magnetization Reversal Barrier for a Mixed 3d/4f Single-Molecule Magnet

A high-oxidation-state Mn(III,IV)21Dy(III) cluster with an unusual structure is reported. It also possesses a record barrier to magnetization reversal for a 3d/4f single-molecule magnet (SMM) and provides insight into how the full benefit of lanthanides to the mixed 3d/4f SMM field might be realized.

Azide Groups in Higher Oxidation State Manganese Cluster Chemistry: From Structural Aesthetics to Single-Molecule Magnets

This Forum Article overviews the recent amalgamation of two long-established areas, manganese/oxo coordination cluster chemistry involving the higher Mn(II)/Mn(IV) oxidation states and transition-metal azide (N(3)(-)) chemistry. The combination of azide and alkoxide- or carboxylate-containing ligands in Mn chemistry has led to a variety of new polynuclear clusters, high-spin molecules, and single-molecule magnets, with metal nuclearities ranging from Mn(4) to Mn(32) and with ground-state spin values as large as S = 83/2. The organic bridging/chelating ligands are discussed separately as follows: (i) pyridyl alkoxides [the anions of 2-(hydroxymethyl)pyridine (hmpH), 2,6-pyridinedimethanol (pdmH(2)), and the gem-diol form of di-2-pyridyl ketone (dpkdH(2))]; (ii) non-pyridyl alkoxides [the anions of 1,1,1-tris(hydroxymethyl)ethane (thmeH(3)), triethanolamine (teaH(3)), and N-methyldiethanolamine (mdaH(2))]; (iii) other alcohols [the anions of 2,6-dihydroxymethyl-4-methylphenol (LH(3)) and Schiff bases]; (iv) pyridyl monoximes/dioximes [the anions of methyl-2-pyridyl ketone oxime (mpkoH), phenyl-2-pyridyl ketone oxime (ppkoH), and 2,6-diacetylpyridine dioxime (dapdoH(2))]; (v) non-pyridyl oximes [the anions of salicylaldoxime (saoH(2)) and its derivatives R-saoH(2)]. The large structural diversity of the resulting complexes stems from the combined ability of the azide and organic ligands to adopt a variety of ligation and bridging modes. The combined work demonstrates the synthetic novelty that arises when azide is used in conjunction with alcohol-based chelates, the aesthetic beauty of the resulting molecules, and the often fascinating magnetic properties that these compounds possess. This continues to emphasize the extensive and remarkable ability of Mn chemistry to satisfy a variety of different tastes.

A Mn17 octahedron with a giant ground-state spin: occurrence in discrete form and as multidimensional coordination polymers.

A [Mn(III)(11)Mn(II)(6)(mu(4)-O)(8)(mu(3)-L)(4)](25+) (L = N(3)(-) or OCN(-)) octahedral unit is reported, occurring within 1D (1)(infinity) and 2D (2)(infinity) coordination polymers, as well as the corresponding 0D discrete cluster 3. It possesses a giant ground-state spin value, determined in the case of 3 to be S = 37, the second largest to be reported to date. In addition, compound 3 displays single-molecule magnet (SMM) behavior, and is thus the largest-spin SMM.

Covalently Linked Dimers of Clusters: Loop‐ and Dumbbell‐Shaped Mn24 and Mn26 Single‐Molecule Magnets

Molecular clusters of paramagnetic 3d transition metals continue to be a major research area because of their fascinating physical properties and their complex structures. In particular, they often have high-spin ground states and easy-axis-type magnetic anisotropy, giving a significant energy barrier to reversal of the magnetization vector. Thus, at sufficiently low temperatures they function as nanoscale magnetic particles. Such single-molecule magnets (SMMs) also straddle the classical/quantum interface by displaying not just classical magnetization hysteresis but also quantum tunneling of magnetization (QTM) and quantum phase interference. SMMs represent a molecular, or “bottom-up”, route to nanoscale magnetic materials, with potential applications in information storage and spintronics at the molecular level and use as quantum bits (qubits) in quantum computation. The upper limit to the barrier (U) is given by S 2 jD j or (S 2 1/4) jD j for integer and halfinteger spins (S), respectively; in practice, QTM through upper regions of the barrier makes the true or effective barrier (Ueff) less than U. Manganese carboxylate chemistry has been the main source of new SMMs, and we are therefore developing new synthetic methods to Mn clusters of various types. The N3 ion bridging in the 1,1-fashion (end-on) is a strong ferromagnetic mediator for a wide range of M-N-M angles, and thus it opens an attractive route to new high-spin Mn clusters and SMMs. In past work, we have shown that azide and the bidentate N,O chelate hmp (the anion of 2-(hydroxymethyl)pyridine) or the tridentate N,O,O chelates pdmH /pdm (the anions of 2,6-pyridinedimethanol) yield [Mn10O4(N3)4(hmp)12] 2+ with S= 22 and [Mn25O18(OH)2(N3)12(pdm)6(pdmH)6] 2+ with S= 51/2, respectively. Both clusters are high-spin molecules, but with smallUeff values, and include coordinated azide groups as ancillary ligands. In the present work, we have explored reactions of Mn precursors with azide and the potentially tetradentate N,N,O,O gem-diolate of di-2-pyridylketone, (py)2C(O)2 2 (dpkd ), formed in situ from dpk, which has previously been a useful route to non-azido metal clusters. We considered dpkd particularly attractive because it can be