Theocharis C. Stamatatos

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

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.

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

A high-nuclearity 3d/4f metal oxime cluster: an unusual Ni(8)Dy(8) "core-shell" complex from the use of 2-pyridinealdoxime.

The initial employment of 2-pyridinealdoxime in 3d/4f chemistry has led to a Ni(II)(8)Dy(III)(8) cluster with an unprecedented metal topology; the compound has an unusual structure, is the highest-nuclearity metal oxime cluster to date, and exhibits slow magnetization relaxation.

Unusual structural types in nickel cluster chemistry from the use of pyridyl oximes: Ni5, Ni12Na2, and Ni14 clusters.

Syntheses, crystal structures, and magnetochemical characterization are reported for the three new nickel(II) clusters [Ni(14)(OH)(4)(N(3))(8)(pao)(14)(paoH)(2)(H(2)O)(2)](ClO(4))(2) (1), [Ni(12)Na(2)(OH)(4)(N(3))(8)(pao)(12)(H(2)O)(10)](OH)(2) (2), and [Ni(5)(ppko)(5)(H(2)O)(7)](NO(3))(5) (3) (paoH = pyridine-2-carbaldehyde oxime, ppkoH = di-2-pyridyl ketone oxime). The reaction of Ni(ClO(4))(2).6H(2)O with paoH and NBu(n)(4)N(3) in H(2)O/MeCN in the presence of NEt(3), gave 1 in 65% yield. Complex 2 was obtained in 60% yield from the reaction of NiCl(2).6H(2)O with paoH and NaN(3) in H(2)O/MeCN in the presence of NaOH. The reaction of Ni(NO(3))(2).6H(2)O with ppkoH in EtOH in the presence of LiOH afforded complex 3 in 75% yield. The complexes all contain novel core topologies. The core of 1 comprises a central Ni(4) rhombus between two Ni(5). Complex 1 is the largest metal/oxime cluster discovered to date, as well as the first Ni(II)(14) coordination complex and the largest Ni(II)/N(3)(-) cluster. Complex 2 has a Ni(12)Na(2) topology that is very similar to that of 1, but with two central Ni(II) atoms of 1 replaced with Na(I) atoms. The core of 3 consists of four Ni(II) atoms forming a highly distorted tetrahedron, with the fifth Ni(II) atom lying almost on one of the edges. Variable-temperature, solid-state dc susceptibility and magnetization studies were carried out on complexes 1-3, and these were complemented with ac susceptibility data for 1 and 2. Fitting of the obtained M/(Nmu(B)) vs H/T data by matrix diagonalization and including axial zero-field splitting (D) gave ground-state spin (S) and D values of S = 6, D = -0.12(3) cm(-1) for 1 and S = 3, D = -0.20(5) cm(-1) for each of the two essentially noninteracting S = 3 Ni(6) subunits of 2. The data for 3 indicate antiferromagnetic exchange interactions and an S = 1 ground state. A simple 2-J model was found to be adequate to describe the variable-temperature dc susceptibility data. The combined work demonstrates the ligating flexibility of pao(-) and ppko(-), and their usefulness in the synthesis of polynuclear Ni(x) clusters with or without the presence of ancillary ligands.

A new family of nonanuclear lanthanide clusters displaying magnetic and optical properties.

The initial employment of 2-(hydroxymethyl)pyridine in 4f metal chemistry has afforded a new family of Ln(III)(9) clusters with a sandglass-like topology and dual physical properties; the Dy(III) member shows single-molecule magnetism behavior, while the Eu(III) analogue exhibits intense red photoluminescence.

{Mn6}n single-chain magnet bearing azides and di-2-pyridylketone-derived ligands.

The synthesis, structure, and magnetochemical characterization of a new manganese single-chain magnet are reported. The compound is a chain of repeating Mn(6) units bridged by end-on azide groups and exhibits magnetization hysteresis loops.

New Fe4, Fe6, and Fe8 Clusters of Iron(III) from the Use of 2-Pyridyl Alcohols: Structural, Magnetic, and Computational Characterization

The syntheses, crystal structures, magnetochemical characterization, and theoretical calculations are reported for three new iron clusters [Fe 6O 2(NO3) 4(hmp) 8(H 2O) 2](NO3)2 (1), [Fe4(N3)6(hmp)6] (2), and [Fe8O3(OMe)(pdm)4(pdmH) 4(MeOH)2](ClO4)5 (3) (hmpH=2-(hydroxymethyl)pyridine; pdmH2=2,6-pyridinedimethanol). The reaction of hmpH with iron(III) sources such as Fe(NO3) 3.9H2O in the presence of NEt 3 gave 1, whereas 2 was obtained from a similar reaction by adding an excess of NaN3. Complex 3 was obtained in good yield from the reaction of pdmH 2 with Fe(ClO4)3.6H2O in MeOH in the presence of an organic base. The complexes all possess extremely rare or novel core topologies. The core of 1 comprises two oxide-centered [Fe3(mu3-O)](7+) triangular units linked together at two of their apexes by two sets of alkoxide arms of hmp(-) ligands. Complex 2 contains a zigzag array of four Fe (III) atoms within an [Fe4(mu-OR) 6](6+) core, with the azide groups all bound terminally. Finally, complex 3 contains a central [Fe 4(mu4-O)](10+) tetrahedron linked to two oxide-centered [Fe3(mu3-O)](7+) triangular units. Variable-temperature, solid-state dc and ac magnetization studies were carried out on complexes 1-3 in the 5.0-300 K range. Fitting of the obtained magnetization versus field (H) and temperature (T) data by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) established that 1 possesses an S=3 ground-state spin, with g=2.08, and D=-0.44 cm(-1). The magnetic susceptibility data for 2 up to 300 K were fit by matrix diagonalization and gave J1=-9.2 cm(-1), J2=-12.5 cm(-1), and g=2.079, where J 1 and J 2 are the outer and middle nearest-neighbor exchange interactions, respectively. Thus, the interactions between the Fe(III) centers are all antiferromagnetic, giving an S=0 ground state for 2. Similarly, complex 3 was found to have an S=0 ground state. Theoretically computed values of the exchange constants in 2 were obtained with DFT calculations and the ZILSH method and were in good agreement with the values obtained from the experimental data. Exchange constants obtained with ZILSH for 3 successfully rationalized the experimental S = 0 ground state. The combined work demonstrates the ligating flexibility of pyridyl-alcohol chelates and their usefulness in the synthesis of new polynuclear Fex clusters without requiring the copresence of carboxylate ligands.