David Collison

Magnetic relaxation pathways in lanthanide single-molecule magnets

Single-molecule magnets are compounds that exhibit magnetic bistability caused by an energy barrier for the reversal of magnetization (relaxation). Lanthanide compounds are proving promising as single-molecule magnets: recent studies show that terbium phthalocyanine complexes possess large energy barriers, and dysprosium and terbium complexes bridged by an N2(3-) radical ligand exhibit magnetic hysteresis up to 13 K. Magnetic relaxation is typically controlled by single-ion factors rather than magnetic exchange (whether one or more 4f ions are present) and proceeds through thermal relaxation of the lowest excited states. Here we report polylanthanide alkoxide cage complexes, and their doped diamagnetic yttrium analogues, in which competing relaxation pathways are observed and relaxation through the first excited state can be quenched. This leads to energy barriers for relaxation of magnetization that exceed 800 K. We investigated the factors at the lanthanide sites that govern this behaviour.

An electrostatic model for the determination of magnetic anisotropy in dysprosium complexes

Understanding the anisotropic electronic structure of lanthanide complexes is important in areas as diverse as magnetic resonance imaging, luminescent cell labelling and quantum computing. Here we present an intuitive strategy based on a simple electrostatic method, capable of predicting the magnetic anisotropy of dysprosium(III) complexes, even in low symmetry. The strategy relies only on knowing the X-ray structure of the complex and the well-established observation that, in the absence of high symmetry, the ground state of dysprosium(III) is a doublet quantized along the anisotropy axis with an angular momentum quantum number mJ=±(15)/2. The magnetic anisotropy axis of 14 low-symmetry monometallic dysprosium(III) complexes computed via high-level ab initio calculations are very well reproduced by our electrostatic model. Furthermore, we show that the magnetic anisotropy is equally well predicted in a selection of low-symmetry polymetallic complexes.

Lanthanide discs chill well and relax slowly

The synthesis, structure and magnetic properties of two isostructural heptametallic lanthanide discs are reported, showing single molecule magnet (SMM) behaviour with a large energy barrier for the dysprosium analogue and a large magnetocaloric effect (MCE) for the gadolinium analogue.

Fungal siderophores: structures, functions and applications

Siderophores are low molecular weight, iron-chelating ligands produced by nearly all microorganisms. Fungi synthesize a wide range of hydroxamate siderophores. This review considers the chemical and biological aspects of these siderophores, their distribution amongst fungal genera and their possible applications. Siderophores function primarily as iron transport compounds. Expression of siderophore biosynthesis and the uptake systems is regulated by internal iron concentrations. Transport of siderophores is an energy-dependent process and is stereoselective, depending on recognition of the metal ion coordination geometry. In addition to transporting iron, siderophores have other functions and effects, including enhancing pathogenicity, acting as intracellular iron storage compounds and suppressing growth of other microorganisms. Siderophores can complex other metals apart from iron, in particular the actinides. Because of their metal-binding ability there are potential applications for siderophores in medicine, reprocessing of nuclear fuel, remediation of metal-contaminated sites and the treatment of industrial waste.

Influence of the N‐Bridging Ligand on Magnetic Relaxation in an Organometallic Dysprosium Single‐Molecule Magnet

Organometallic single-molecule magnet: Studies of two dimeric organometallic dysprosium compounds reveal one to be the first organometallic single-molecule magnet (see picture). A comparison of the magnetic properties and electronic structures of the two compounds shows that Dy⋅⋅⋅Dy interactions have a profound influence on the dynamic magnetic behaviour, while having little effect on the static magnetic measurements.

A dense metal-organic framework for enhanced magnetic refrigeration

X iv :1 21 2. 28 77 v1 [ co nd -m at .m tr lsc i] 1 2 D ec 2 01 2 Magnetic cryocooling with Gd centers in a light and compact framework G. Lorusso, J. W. Sharples, E. Palacios, O. Roubeau, E. K. Brechin, R. Sessoli, A. Rossin, F. Tuna, E. J. L. McInnes, D. Collison, and M. Evangelisti a) Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC − Universidad de Zaragoza, Departamento de F́ısica de la Materia Condensada, 50009 Zaragoza, Spain School of Chemistry and Photon Science Institute, The University of Manchester, M13-9PL Manchester, United Kingdom School of Chemistry, The University of Edinburgh, EH9-3JJ Edinburgh, United Kingdom Department of Chemistry and INSTM, Università degli Studi di Firenze, 50019 Sesto Fiorentino, Italy Istituto di Chimica dei Composti Organometallici (ICCOM), CNR, 50019 Sesto Fiorentino, ItalyThe three-dimensional metal-organic framework Gd(HCOO)3 is characterized by a relatively compact crystal lattice of weakly interacting Gd(3+) spin centers interconnected via lightweight formate ligands, overall providing a remarkably large magnetic:non-magnetic elemental weight ratio. The resulting magnetocaloric effect per unit volume is decidedly superior in Gd(HCOO)3 than in the best known magnetic refrigerant materials for liquid-helium temperatures and low-moderate applied fields.

Direct measurement of dysprosium(III)···dysprosium(III) interactions in a single-molecule magnet.

Lanthanide compounds show much higher energy barriers to magnetic relaxation than 3d-block compounds, and this has led to speculation that they could be used in molecular spintronic devices. Prototype molecular spin valves and molecular transistors have been reported, with remarkable experiments showing the influence of nuclear hyperfine coupling on transport properties. Modelling magnetic data measured on lanthanides is always complicated due to the strong spin-orbit coupling and subtle crystal field effects observed for the 4f-ions; this problem becomes still more challenging when interactions between lanthanide ions are also important. Such interactions have been shown to hinder and enhance magnetic relaxation in different examples, hence understanding their nature is vital. Here we are able to measure directly the interaction between two dysprosium(III) ions through multi-frequency electron paramagnetic resonance spectroscopy and other techniques, and explain how this influences the dynamic magnetic behaviour of the system.

Polyoxometal cations within polyoxometalate anions. Seven-coordinate uranium and zirconium heteroatom groups in [(UO2)12(μ3-O)4(μ2-H2O)12(P2W15O56)4]32− and [Zr4(μ3-O)2(μ2-OH)2(H2O)4 (P2W16O59)2]14−

Abstract Two new composite polyoxotungstate anions with unprecedented structural features, [(UO 2 ) 12 (μ 3 -O) 4 (μ 2 -H 2 O) 12 (P 2 W 15 O 56 ) 4 ] 32− ( 1 ) and [Zr 4 (μ 3 -O) 2 (μ 2 -OH) 2 (H 2 O) 4 (P 2 W 16 O 59 ) 2 ] 14− ( 2 ) contain polyoxo-uranium and -zirconium clusters as bridging units. The anions are synthesized by reaction of Na 12 [P 2 W 15 O 56 ] with solutions of UO 2 (NO 3 ) 2 and ZrCl 4 . The structure of 1 in the sodium salt contains four [P 2 W 15 O 56 ] 12− anions assembled into an overall tetrahedral cluster by means of trigonal bridging groups formed by three equatorial-edge-shared UO 7 pentagonal bipyramids. The structure of anion 2 consists of a centrosymmetric assembly of two [P 2 W 16 O 59 ] 12− anions linked by a {Zr 4 O 2 (OH) 2 (H 2 O) 4 } 10+ cluster. Both complexes in solution yield the expected two-line 31 P-NMR spectra with chemical shifts of −2.95, −13.58 and −6.45, −13.69 ppm, respectively.

Surface coordination chemistry: corrosion inhibition by tetranuclear cluster formation of iron with salicylaldoxime.

A tetranuclear iron cluster is the principal component of the purple coatings produced by treating a mild steel surface with a salicylaldoxime corrosion inhibitor. This was shown by comparison of the spectroscopic data with those of the cluster [{Fe(salH)(HsalH)}4 ], which was obtained from FeCl3 and salicylaldoxime (H2 salH) and has a distorted tetrahedral arrangement of Fe(III) atoms coordinated by terminal (1-) and bridging (2-) salicylaldoximate ligands (the central core of the cluster is depicted).

Probing the local coordination environment and nuclearity of uranyl(VI) complexes in non-aqueous media by emission spectroscopy

We describe the synthesis, solid state and solution properties of two families of uranyl(VI) complexes that are ligated by neutral monodentate and anionic bidentate P=O, P=NH and As=O ligands bearing pendent phenyl chromophores. The uranyl(VI) ions in these complexes possess long-lived photoluminescent LMCT (3)Π(u) excited states, which can be exploited as a sensitive probe of electronic structure, bonding and aggregation behaviour in non-aqueous media. For a family of well defined complexes of given symmetry in trans-[UO(2)Cl(2)(L(2))] (L = Ph(3)PO (1), Ph(3)AsO (2) and Ph(3)PNH (3)), the emission spectral profiles in CH(2)Cl(2) are indicative of the strength of the donor atoms bound in the equatorial plane and the uranyl bond strength; the uranyl LMCT emission maxima are shifted to lower energy as the donor strength of L increases. The luminescence lifetimes in fluid solution mirror these observations (0.87-3.46 μs) and are particularly sensitive to vibrational and bimolecular deactivation. In a family of structurally well defined complexes of the related anion, tetraphenylimidodiphosphinate (TPIP), monometallic complexes, [UO(2)(TPIP)(thf)] (4), [UO(2)(TPIP)(Cy(3)PO)] 5), a bimetallic complex [UO(2)(TPIP)(2)](2) (6) and a previously known trimetallic complex, [UO(2)(TPIP)(2)](3) (7) can be isolated by variation of the synthetic procedure. Complex 7 differs from 6 as the central uranyl ion in 7 is orthogonally connected to the two peripheral ones via uranyl → uranium dative bonds. Each of these oligomers exhibits a characteristic optical fingerprint, where the emission maxima, the spectral shape and temporal decay profiles are unique for each structural form. Notably, excited state intermetallic quenching in the trimetallic complex 7 considerably reduces the luminescence lifetime with respect to the monometallic counterpart 5 (from 2.00 μs to 1.04 μs). This study demonstrates that time resolved and multi-parametric luminescence can be of value in ascertaining solution and structural forms of discrete uranyl(VI) complexes in non-aqueous solution.