Marshall Luban

Classical and Quantum Magnetism in Giant Keplerate Magnetic Molecules

Complementary theoretical modeling methods are presented for the classical and quantum Heisenberg model to explain the magnetic properties of nanometer-sized magnetic molecules. Excellent quantitative agreement is achieved between our experimental data down to 0.1 K and for fields up to 60 Tesla and our theoretical results for the giant Keplerate species {Mo72Fe30}, by far the largest paramagnetic molecule synthesized to date.

Molecular Growth of a Core–Shell Polyoxometalate

The self-assembly of large metal oxide clusters usually proceeds via condensation steps thermodynamically driven by charge and nucleophilicity of the growing transient cluster fragments. Polyoxometalate (POM) chemistry has accumulated many strategies to interfere with these basic formation principles, aiming at both directed molecular growth and targeted functionalization by selective introduction of metal centers or organic moieties. POM structures integrating 3d or 4d transition-metal ions in particular attest to this approach, and they have led to a rich class of molecular materials ranging from molecular magnets to oxidation catalysts. In this context, polyoxotungstate clusters provide rigid and redox-stable scaffolds based on building-block-type fragments that are frequently derived from archetypal structures such as the Keggin, Dawson, or Lindqvist species. Additional heterometal cations coordinating to or interconnecting these nucleophilic structures are key to the reactivity and the electronic and magnetic characteristics of the resulting adducts. This situation is exemplified by the spherical {M72L30} Keplerate clusters that have been realized both as polymolybdate (M = Mo) and polytungstate (M = W) structures containing a variety of heterometal linkers (L = V, Cr, Fe, etc.). These clusters, comprising unique spin polytopes that represent molecular analogues of Kagom lattices, constitute structural platforms for subsequent reactions, ranging from redox reactions and partial heterometal exchange to condensations to oneand two-dimensional coordination networks—all of which alter the clusters magnetic characteristics while retaining the basic cluster structure. Moreover, recent development of POM-based single-molecule magnets raises the hope that magnetic POMs may find their way into areas such as molecular spintronics or quantum computing. However, the controlled growth of large metal oxide clusters remains elusive: precise prediction of the outcome is very difficult given the involvement of many, often labile, metal–oxygen bonds. The self-assembly mechanisms that underlie the formation of larger POMs in aqueous reaction solutions are barely established and are not compatible with the synthetic controls available in classical coordination chemistry, as exemplified by the use of molecular tectons, characterized by their specific connectivity constraints, in the rational production of supramolecular aggregates or porous metal–organic frameworks. We postulate that this roadblock in controlling the molecular growth steps in polyoxotungstate chemistry can be partially circumvented by kinetic control of competing reactions, as illustrated by the template-induced formation of a 4.3 nm manganese(III) polyoxotungstate cluster anion [Mn40P32W VI 224O888] 144 (1). The preparation of 1 starts from metastable [a-H2P2W12O48] 12 ({P2W12}), a hexavacant phosphotungstate derived from the plenary [a-P2W18O62] 6

Rotational modes in molecular magnets with antiferromagnetic Heisenberg exchange

In an effort to understand the low-temperature behavior of recently synthesized molecular magnets, we present numerical evidence for the existence of a rotational band in systems of quantum spins interacting with nearest-neighbor antiferromagnetic Heisenberg exchange. While this result has previously been noted for ring arrays with an even number of spin sites, we find that it also applies for rings with an odd number of sites as well as for all of the polytope configurations we have investigated (tetrahedron, cube, octahedron, icosahedron, triangular prism, and axially truncated icosahedron). It is demonstrated how the rotational band levels can, in many cases, be accurately predicted using the underlying sublattice structure of the spin array. We illustrate how the characteristics of the rotational band can provide valuable estimates for the low-temperature magnetic susceptibility.

{Mo24Fe12} Macrocycles: Anion Templation with Large Polyoxometalate Guests

POM and circumstance: Nanometer-sized polyoxometalates (POMs) bring a new direction to anion-templated supramolecular chemistry. The Keggin (left) and Dawson-type (right) polyoxoanions direct the assembly of giant metallomacrocycles through an array of weak hydrogen-bonding interactions. The concerted action of multiple hydrogen bonds keeps the templating guests embedded within the hosts, even in the solution state.

Metamagnetic Phase Transition of the Antiferromagnetic Heisenberg Icosahedron

The observation of hysteresis effects in single molecule magnets like Mn12-acetate has initiated ideas of future applications in storage technology. The appearance of a hysteresis loop in such compounds is an outcome of their magnetic anisotropy. In this Letter we report that magnetic hysteresis occurs in a spin system without any anisotropy, specifically where spins mounted on the vertices of an icosahedron are coupled by antiferromagnetic isotropic nearest-neighbor Heisenberg interaction giving rise to geometric frustration. At T = 0 this system undergoes a first-order metamagnetic phase transition at a critical field Bc between two distinct families of ground state configurations. The metastable phase of the system is characterized by a temperature and field dependent survival probability distribution.

Paramagnetic Keplerate “Necklaces” Synthesized by a Novel Room‐Temperature Solid‐State Reaction: Controlled Linking of Metal‐Oxide‐Based Nanoparticles

Crystal engineering with nanoobjects? Spherical structurally well-defined molybdenum-oxide-based giant clusters can be appropriately functionalized to a crystalline material with the remarkable property of having discrete cluster units which get covalently linked to form chains (see picture) through Fe-O-Mo bonds after having approached each other as a result of the release of crystal water upon drying.

Probing spin frustration in high-symmetry magnetic nanomolecules by inelastic neutron scattering

We report cold-neutron inelastic neutron scattering measurements on deuterated samples of the giant polyoxomolybdate magnetic molecule {Mo72Fe30}. The 30 s = 5/2 Fe+3 ions occupy the vertices of an icosidodecahedron, and interact via antiferromagnetic nearest neighbor coupling. The measurements reveal a band of magnetic excitations near E ~ 0.6 meV. The spectrum broadens and shifts to lower energy as the temperature is increased, and also is strongly affected by magnetic fields. The results can be interpreted within the context of an effective three-sublattice spin Hamiltonian.

Multiple nearest-neighbor exchange model for the frustrated magnetic molecules {Mo72Fe30} and {Mo72Cr30}

Our measurements of the differential susceptibility

Hysteresis loops and adiabatic Landau-Zener-Stückelberg transitions in the magnetic molecule {V6}.

\ensuremath{\partial}M/\ensuremath{\partial}H

Spin dynamics of quantum and classical Heisenberg dimers

of the frustrated magnetic molecules