Breaking Symmetry Relaxes Structural and Magnetic Restraints, Suppressing QTM in Enantiopure Butterfly Fe2 Dy2 SMMs*

The {Fe2 Dy2 } butterfly systems can show single molecule magnet (SMM) behaviour, the nature of which depends on details of the electronic structure, as previously demonstrated for the [Fe2 Dy2 (μ3 -OH)2 (Me-teaH)2 (O2 CPh)6 ] compound, where the [N,N-bis-(2-hydroxyethyl)-amino]-2-propanol (Me-teaH3 ) ligand is usually used in its racemic form. Here, we describe the consequences for the SMM properties by using enantiopure versions of this ligand and present the first homochiral 3d/4 f SMM, which could only be obtained for the S enantiomer of the ligand for [Fe2 Dy2 (μ3 -OH)2 (Me-teaH)2 (O2 CPh)6 ] since the R enantiomer underwent significant racemisation. To investigate this further, we prepared the [Fe2 Dy2 (μ3 -OH)2 (Me-teaH)2 (O2 CPh)4 (NO3 )2 ] version, which could be obtained as the RS-, R- and S-compounds. Remarkably, the enantiopure versions show enhanced slow relaxation of magnetisation. The use of the enantiomerically pure ligand suppresses QTM, leading to the conclusion that use of enantiopure ligands is a "gamechanger" by breaking the cluster symmetry and altering the intimate details of the coordination cluster's molecular structure.

Inorganic Approach to Stabilizing Nanoscale Toroidicity in a Tetraicosanuclear Fe18Dy6 Single Molecule Magnet

Cyclic coordination clusters (CCCs) are proving to provide an extra dimension in terms of exotic magnetic behavior as a result of their finite but cyclized chain structures. The Fe₁₈Dy₆ CCC is a Single Molecule Magnet with the highest nuclearity among Ln containing clusters. The three isostructural compounds [Fe₁₈Ln₆(μ-OH)₆(ampd)₁₂(Hampd)₁₂(PhCO₂)₂₄](NO₃)₆·38MeCN for Ln = Dy^III (1), Lu^III (2), or Y^III (3), where H₂ampd = 2-amino-2-methyl-1,3-propanediol, are reported. These can be described in terms of the cyclization of six {Fe₃Ln(μOH)(ampd)₂(Hampd)₂(PhCO₂)₄}⁺ units with six nitrate counterions to give the neutral cluster. The overall structure consists of two giant Dy₃ triangles sandwiching a strongly antiferromagnetically coupled Fe₁₈ ring, leading to a toroidal arrangement of the anisotropy axis of the Dy ions, making this the biggest toroidal arrangement on a molecular level known so far.

Direct surface visualization of biofilms with high spin coordination clusters using Magnetic Resonance Imaging

Magnetic Resonance Imaging is a powerful tool for the investigation of a biofilms' physical structure determining mass transport behavior which is of major importance in biofilm research. The entire biofilm is imaged in situ non-invasively and non-destructively on a meso-scale. In this study, different contrast agents were applied to study the biofilm's properties with the focus on mass transport, which is achieved by varying the contrast agents with respect to their NMR and interaction properties. The spatio-temporal tracking of these cluster, molecular and particulate contrast agents in biofilms was achieved by T1-, T2-weighted and proton density images during short (20h) and long (14 d) term exposures. The best biofilm surface visualization was observed when applying a new high spin coordination cluster (Fe10Gd10) showing a high affinity to the biofilm's surface and a fast immobilization within minutes. Contrarily, the small molecular contrast agents show no immobilization and fully penetrated into the biofilm. A concentration equilibrium was observed which was confirmed in back diffusion experiments. Interactions between larger nanoparticulate contrast agents and the biofilm required hours to achieve immobilization. Thus, the penetration depth into the biofilm is predominantly size-dependent. Here, it is shown that biofilm surface interactions can be observed in situ and spatio-temporarily resolved. The reported methodology demonstrates a new means to explore mass transfer of various substances in biofilms.

STATEMENT OF SIGNIFICANCE

In biofilm research, the investigation of the biofilms' physical structure is of high relevance for the understanding of mass transport processes. However, commonly used imaging techniques for biofilm imaging such as CLSM or electron microscopy rarely visualize the real biofilm due to their invasiveness and destructiveness. Magnetic Resonance Imaging (MRI) represents the ideal tool to image the biofilm in situ, non-invasively and non-destructively with a spatial resolution of several 10μm. To gain specific structural and functional information, a variety of MRI contrast agents (molecular and particulate) was applied with different properties for the first time. Results elucidate the interactions between the biofilms' surface and the contrast agents and open a new field for biotechnological applications by functional contrast enhancement.

Ligand field variations: tuning the toroidal moment of Dy6 rings

We show how the toroidal moment in Dy₆ cyclic coordination clusters can be tuned and optimised.

Squashed {Fe2IIIM4III} octahedra (M = Y, Gd, Dy) from the first use of the cyanoacetate ligand in 3d/4f coordination chemistry

The first use of the cyanoacetate ligand in 3d/4f-metal chemistry leads to {Fe₂^IIIM₄^III} (M = Dy, Gd, Y) coordination clusters with a squashed octahedral core structure; the Dy^III compound exhibits slow relaxation of the magnetisation.

Characterisation and application of ultra-high spin clusters as magnetic resonance relaxation agents

Highly paramagnetic coordination clusters with a {Fe₁₀Ln₁₀} core are used as NMR relaxation agents with a first application in a biofilm study.

Nuclear magnetic resonance relaxivities: investigations of ultrahigh-spin lanthanide clusters from 10 MHz to 1.4 GHz

Paramagnetic relaxation enhancement is often explored in magnetic resonance imaging in terms of contrast agents and in biomolecular nuclear magnetic resonance (NMR) spectroscopy for structure determination. New ultrahigh-spin clusters are investigated with respect to their NMR relaxation properties. As their molecular size and therefore motional correlation times as well as their electronic properties differ significantly from those of conventional contrast agents, questions about a comprehensive characterization arise. The relaxivity was studied by field-dependent longitudinal and transverse NMR relaxometry of aqueous solutions containing Fe(III)(10)Dy(III)(10) ultrahigh-spin clusters (spin ground state 100/2). The high-field limit was extended to 32.9 T by using a 24 MW resistive magnet and an ultrahigh-frequency NMR setup. Interesting relaxation dispersions were observed; the relaxivities increase up to the highest available fields, which indicates a complex interplay of electronic and molecular correlation times.

Magnetic anisotropy and exchange coupling in a family of isostructural Fe(III)2Ln(III)2 complexes

The reaction of [Fe3O(O2CPh)6(H2O)3](O2CPh) with lanthanide/rare earth nitrate salts in the presence of triethanolamine (H3tea) in acetonitrile/methanol solution yields a series of compounds with isostructural tetranuclear core motifs [Fe(III)2Ln(III)2(μ3-OH)2(teaH)2(O2CCPh)6]·3MeCN (Ln = Ce (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7), Dy (8), Ho (9), Er (10), Tm (11), Yb (12), Y (13)). In all cases the core topology is a defect-dicubane planar or "butterfly" Fe2Ln2 motif. Compounds 1-13 were investigated using a combination of experimental techniques and theoretical studies. Magnetic susceptibility measurements were carried out on all compounds. The magnetic coupling between the two Fe(III) centres is antiferromagnetic, with J(FeFe) ca.-6.71(4) cm(-1), while the Fe-Ln couplings are much weaker, e.g. J(FeGd) = 0.18(1) cm(-1). Compounds 6, 7, 8 and 13 were selected for Mössbauer studies in order to investigate the influence of isotropic (Gd(III)), highly anisotropic non-Kramers and Kramers (Tb(III) and Dy(III)) and diamagnetic (Y(III)) rare earth ions on the local environment of the Fe(III) centres. Compounds 3, 6, 8 and 13 were also studied using X-Band EPR spectroscopy. For 13, with the diamagnetic Y(III) ion, this made it possible to obtain the D, E, J(FeFe) and g parameters for the iron centres. It is shown that the low-temperature spectra of compounds 3, 6 and 8 are determined by magnetic properties of rare-earth ions and the dipole-dipole interactions between the Ln(III) ions. The Fe-Ln interactions were confirmed as very weak and dipolar in nature by the temperature dependence of EPR spectra at T > 20 K.