Stergios Piligkos

Molecular engineering of antiferromagnetic rings for quantum computation.

The substitution of one metal ion in a Cr-based molecular ring with dominant antiferromagnetic couplings allows the engineering of its level structure and ground-state degeneracy. Here we characterize a Cr7Ni molecular ring by means of low-temperature specific-heat and torque-magnetometry measurements, thus determining the microscopic parameters of the corresponding spin Hamiltonian. The energy spectrum and the suppression of the leakage-inducing S mixing render the Cr7Ni molecule a suitable candidate for the qubit implementation, as further substantiated by our quantum-gate simulations.

The Importance of Being Exchanged: [GdIII4MII8(OH)8(L)8(O2CR)8]4+ Clusters for Magnetic Refrigeration

One of the most promising applications for molecules built from paramagnetic metal ions is low temperature magnetic refrigeration.[1] Indeed recent studies have suggested that molecular coolers can outperform any conventionally-employed solid-state refrigerant material by orders of magnitude.[2] In order to do so, molecules must possess a combination of a large spin ground state (S), with negligible anisotropy (Dcluster = 0), weak magnetic exchange between the constituent metal ions and a relatively large metal:non-metal mass ratio (i.e. a large magnetic density).[1b] These molecular pre-requisites suggest the use of lanthanide ions and, in particular, the f7 ion Gd3+ in the construction of homoand heterometallic (Gd-3d) clusters, and a sensible starting point is the synthesis of GdIII-CuII clusters since previous studies have shown this combination favours ferromagnetic exchange.[3] Here we introduce a rather remarkable new family of compounds of general formula [Ln4M8(OH)8(L)8(O2CR)8](X)4 in which almost all the constituent parts – the lanthanide ions (Ln3+), the transition metal ions (M2+), the bridging ligand L, the carboxylates and the counter anions (X) can be exchanged. In each case the structure remains essentially the same and this allows for a thorough understanding of the individual contributions to the magneto-caloric effect (MCE). In this communication we describe the three family members [Gd4M8(OH)8(L)8(O2CR)8](ClO4)4 (M = Zn, R = CHMe2, 1; M = Cu, R = CHMe2, 2; M = Ni, R = CH2Me, 3; LH = 2(hydroxymethyl)pyridine) and show how the identity of the transition metal and the sign of the magnetic exchange are vital components to consider when designing molecular coolers. For the sake of brevity we provide a generic structure description, highlighting any differences. The core (Figure 1 shows complex 2) of the molecule consists of a square (or wheel) of four cornersharing {Gd2M2O4} cubanes. The shared corners are the Gd ions which thus themselves form an inner {Gd4} square, each edge of which is occupied by two μ3-OH ions which further bridge to a MII ion. The μ3-L ions chelate the M2+ ions and use their O-arm to further bridge to the second M2+ ion in the same cubane and to one Gd ion. There are two carboxylates per cubane, each μ-bridging across a M2+...Gd square face, alternately above and below the plane of the {Gd4} square.

Calix[4]arene-based single-molecule magnets

Calix[4]arene Based Single-Molecule Magnets** Georgios Karotsis, Simon J. Teat, Wolfgang Wernsdorfer, Stergios Piligkos, Scott J. Dalgarno* and Euan K. Brechin* Mr. G. Karotsis, Dr. E. K. Brechin, School of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, UK. Fax: (+44)-131-650-6453 E-mail: ebrechin@staffmail.ed.ac.uk Dr. S. J. Dalgarno, School of Engineering and Physical Sciences, Heriot-Watt University, Riccarton, Edinburgh, EH14 4AS. Fax: (+44)-131-451-3180 E-mail: S.J.Dalgarno@hw.ac.uk Prof. Dr. W. Wernsdorfer, Institut Neel, CNRS, Grenoble Cedex 9, France. Dr. S. Piligkos, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100, Denmark. Dr. S. J. Teat, Advanced Light Source, Berkeley Laboratory, 1 Cyclotron Road, MS6R2100, Berkeley, CA 94720, USA. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231. Single-molecule magnets (SMMs) [1] have been the subject of much interest in recent years because their molecular nature and inherent physical properties allow the crossover between classical and quantum physics to be observed. [2] The macroscopic observation of quantum phenomena - tunneling between different spin states, [3] quantum interference between tunnel paths [4] - not only allows scientists to study quantum mechanical laws in great detail, but also provides model systems with which to investigate the possible implementation of spin- based solid state qubits [5] and molecular spintronics. [6] The isolation of small, simple SMMs is therefore an exciting prospect. To date almost all SMMs have been made via the self-assembly of 3d metal ions in the presence of bridging/chelating organic ligands. [7] However, very recently an exciting new class of SMMs, based on 3d metal clusters (or single lanthanide ions) housed within polyoxometalates, [8] has appeared. These types of molecule, in which the SMM is completely encapsulated within (or shrouded by) a “protective” organic or inorganic sheath have much potential for design and manipulation: for example, for the removal of unwanted dipolar interactions, the introduction of redox activity, or to simply aid functionalisation for surface grafting. [9] Calix[4]arenes are cyclic (typically bowl-shaped) polyphenols that have been used extensively in the formation of versatile self-assembled supramolecular structures. [10] Although many have been reported, p- t But-calix[4]arene and calix[4]arene (TBC4 and C4 respectively, Figure 1A) are frequently encountered due to a) synthetic accessibility, and b) vast potential for alteration at either the upper or lower rim of the macrocyclic framework. [11] Within the field of supramolecular chemistry, TBC4 is well known for interesting polymorphic behavior and phase transformations within anti-parallel bi-layer arrays, while C4 often forms self-included trimers. [12] The polyphenolic nature of calix[n]arenes (where n = 4 – 8) also suggests they should be excellent candidates as ligands for the isolation of molecular magnets, but to date their use in the isolation of paramagnetic cluster compounds is rather limited. [13] Herein we present the first Mn cluster and the first SMM to be isolated using any methylene bridged calix[n]arene - a ferromagnetically coupled mixed-valence [Mn III2 Mn II2 ] complex housed between either two TBC4s or two C4s. Reaction of MnBr 2 with TBC4 and NEt 3 in a solvent mixture of MeOH/DMF results in the formation of the complex [Mn III2 Mn II2 (OH) 2 (TBC4) 2 (DMF) 6 ] (1) which crystallises as purple blocks that are in the monoclinic space group P2 1 /c. The cluster (Figure 1B) comprises a planar diamond or butterfly-like [Mn III2 Mn II2 (OH) 2 ] core in which the wing tip Mn ions (Mn1) are in the 3+ oxidation state and the body Mn ions (Mn2) in the 2+ oxidation state. This is a common structural type in Mn SMM chemistry, [14] but the oxidation state distribution here is highly unusual, being “reversed” from the norm in which the body Mn ions are almost always 3+. Indeed the “reversed” core has been seen only once before, in the cluster [Mn III2 Mn II2 (teaH) 2 (acac) 4 (MeOH) 2 ] 2+ (2) (teaH 3 = triethanolamine) and its analogues. [15] The Mn 3+ ions are in distorted octahedral geometries with the Jahn-Teller axes defined by O5(DMF)-Mn1-O6(OH). The four equatorial sites are occupied by the oxygen atoms (O1-O4) of the TBC4, two of which bridge in a µ 2 -fashion to the central Mn 2+ ions (Mn1-O4-Mn2, 103.5°; Mn1-O1-Mn2, 105.4°). These are connected to each other (Mn2-O6-Mn2’, 94.7°) and to the Mn 3+ ions (Mn1-O6-Mn2, 100.4°; Mn1- O6-Mn2’, 98.8°) via two µ 3 -bridging OH - ions, with the two remaining equatorial sites (completing the distorted octahedral geometry on Mn2) filled by terminal DMF molecules. There are no inter-molecular H-bonds between symmetry equivalents of 1, with the closest

Design of Single-Molecule Magnets: Insufficiency of the Anisotropy Barrier as the Sole Criterion

Determination of the electronic energy spectrum of a trigonal-symmetry mononuclear Yb(3+) single-molecule magnet (SMM) by high-resolution absorption and luminescence spectroscopies reveals that the first excited electronic doublet is placed nearly 500 cm(-1) above the ground one. Fitting of the paramagnetic relaxation times of this SMM to a thermally activated (Orbach) model {τ = τ0 × exp[ΔOrbach/(kBT)]} affords an activation barrier, ΔOrbach, of only 38 cm(-1). This result is incompatible with the spectroscopic observations. Thus, we unambiguously demonstrate, solely on the basis of experimental data, that Orbach relaxation cannot a priori be considered as the main mechanism determining the spin dynamics of SMMs. This study highlights the fact that the general synthetic approach of optimizing SMM behavior by maximization of the anisotropy barrier, intimately linked to the ligand field, as the sole parameter to be tuned, is insufficient because of the complete neglect of the interaction of the magnetic moment of the molecule with its environment. The Orbach mechanism is expected dominant only in the cases in which the energy of the excited ligand field state is below the Debye temperature, which is typically low for molecular crystals and, thus, prevents the use of the anisotropy barrier as a design criterion for the realization of high-temperature SMMs. Therefore, consideration of additional design criteria that address the presence of alternative relaxation processes beyond the traditional double-well picture is required.

[ReF6]2−: A Robust Module for the Design of Molecule‐Based Magnetic Materials

A facile synthesis of the [ReF6 ](2-) ion and its use as a building block to synthesize magnetic systems are reported. Using dc and ac magnetic susceptibility measurements, INS and EPR spectroscopies, the magnetic properties of the isolated [ReF6 ](2-) unit in (PPh4 )2 [ReF6 ]⋅2 H2 O (1) have been fully studied including the slow relaxation of the magnetization observed below ca. 4 K. This slow dynamic is preserved for the one-dimensional coordination polymer [Zn(viz)4 (ReF6 )]∞ (2, viz=1-vinylimidazole), demonstrating the irrelevance of low symmetry for such magnetization dynamics in systems with easy-plane-type anisotropy. The ability of fluoride to mediate significant exchange interactions is exemplified by the isostructural [Ni(viz)4 (ReF6 )]∞ (3) analogue in which the ferromagnetic Ni(II) -Re(IV) interaction (+10.8 cm(-1) ) dwarfs the coupling present in related cyanide-bridged systems. These results reveal [ReF6 ](2-) to be an unique new module for the design of molecule-based magnetic materials.

A Family of Calix{[}4]arene-Supported {[}(Mn2Mn2II)-Mn-III] Clusters

In the cone conformation calix[4]arenes possess lower-rim polyphenolic pockets that are ideal for the complexation of various transition-metal centres. Reaction of these molecules with manganese salts in the presence of an appropriate base (and in some cases co-ligand) results in the formation of a family of calixarene-supported [Mn(III)(2)Mn(II)(2)] clusters that behave as single-molecule magnets (SMMs). Variation in the alkyl groups present at the upper-rim of the cone allows for the expression of a degree of control over the self-assembly of these SMM building blocks, whilst retaining the general magnetic properties. The presence of various different ligands around the periphery of the magnetic core has some effect over the extended self-assembly of these SMMs.

Direct observation of a ferri-to-ferromagnetic transition in a fluoride-bridged 3d–4f molecular cluster

We report on the synthesis, crystal structure and magnetic characterisation of the trinuclear, fluoride-bridged, molecular nanomagnet [Dy(hfac)3(H2O)–CrF2(py)4–Dy(hfac)3(NO3)] (1) (hfacH = 1,1,1,5,5,5-hexafluoroacetylacetone, py = pyridine) and a closely related dinuclear species [Dy(hfac)4–CrF2(py)4]·½CHCl3 (2). Element-specific magnetisation curves obtained on 1 by X-ray magnetic circular dichroism (XMCD) allow us to directly observe the field-induced transition from a ferrimagnetic to a ferromagnetic arrangement of the Dy and Cr magnetic moments. By fitting a spin-Hamiltonian model to the XMCD data we extract a weak antiferromagnetic exchange coupling of j = −0.18 cm−1 between the DyIII and CrIII ions. The value found from XMCD is consistent with SQUID magnetometry and inelastic neutron scattering measurements. Furthermore, alternating current susceptibility and muon-spin relaxation measurements reveal that 1 shows thermally activated relaxation of magnetisation with a small effective barrier for magnetisation reversal of Δeff = 3 cm−1. Density-functional theory calculations show that the Dy–Cr couplings originate from superexchange via the fluoride bridges.

A classification of spin frustration in molecular magnets from a physical study of large odd-numbered-metal, odd electron rings

The term “frustration” in the context of magnetism was originally used by P. W. Anderson and quickly adopted for application to the description of spin glasses and later to very special lattice types, such as the kagomé. The original use of the term was to describe systems with competing antiferromagnetic interactions and is important in current condensed matter physics in areas such as the description of emergent magnetic monopoles in spin ice. Within molecular magnetism, at least two very different definitions of frustration are used. Here we report the synthesis and characterization of unusual nine-metal rings, using magnetic measurements and inelastic neutron scattering, supported by density functional theory calculations. These compounds show different electronic/magnetic structures caused by frustration, and the findings lead us to propose a classification for frustration within molecular magnets that encompasses and clarifies all previous definitions.

Toward Molecular 4f Single-Ion Magnet Qubits

Quantum coherence is detected in the 4f single-ion magnet (SIM) Yb(trensal), by isotope selective pulsed EPR spectroscopy on an oriented single crystal. At X-band, the spin-lattice relaxation (T1) and phase memory (Tm) times are found to be independent of the nuclei bearing, or not, a nuclear spin. The observation of Rabi oscillations of the spin echo demonstrates the possibility to coherently manipulate the system for more than 70 rotations. This renders Yb(trensal), a sublimable and chemically modifiable SIM, an excellent candidate for quantum information processing.

Fluoride-Bridged {GdIII3MIII2} (M=Cr, Fe, Ga) Molecular Magnetic Refrigerants†

The reaction of fac-[M(III)F3(Me3tacn)]⋅x H2O with Gd(NO3)3⋅5H2O affords a series of fluoride-bridged, trigonal bipyramidal {Gd(III)3M(III)2} (M = Cr (1), Fe (2), Ga (3)) complexes without signs of concomitant GdF3 formation, thereby demonstrating the applicability even of labile fluoride-complexes as precursors for 3d-4f systems. Molecular geometry enforces weak exchange interactions, which is rationalized computationally. This, in conjunction with a lightweight ligand sphere, gives rise to large magnetic entropy changes of 38.3 J kg(-1)  K(-1) (1) and 33.1 J kg(-1)  K(-1) (2) for the field change 7 T→0 T. Interestingly, the entropy change, and the magnetocaloric effect, are smaller in 2 than in 1 despite the larger spin ground state of the former secured by intramolecular Fe-Gd ferromagnetic interactions. This observation underlines the necessity of controlling not only the ground state but also close-lying excited states for successful design of molecular refrigerants.