Bernd Pilawa

Ferromagnetic interactions mediated by syn–anti carboxylate bridging in tetranuclear copper(II) compounds

Abstract The tridentate ligand N-(2-hydroxypropyl)aminopropionic acid (H2hap) and the tripod ligand nitrilodi(propionic acid) acetic acid (H3ndpa) have been synthesised and found to interact with Cu(II) to produce tetranuclear compounds. In the cyclic complex [Cu(Hhap)(NO3)]4 (1), the four Cu(II) centres form a slightly buckled square linked via syn–anti carboxylate bridges. In [Cu4(ndpa)2(H2O)8(NO3)2]·2H2O (2), and [Cu4(ndpa)2(H2O)10]SO4·6H2O (3), two Cu(II) centres are each chelated by a ndpa3− ligand which in turn are linked by coordination of the outer carboxylate oxygens on the longer propionate arms of the ligands to two further {Cu(H2O)3}2+ units resulting in a rhomboid array of Cu(II) centres. In the closely related complexes [Cu4(ndpa)2(H2O)6Cl2]·4H2O (4), and [Cu4(ndpa)2(H2O)6Br2]·4H2O (5), replacement of one aqua ligand by an outer carboxylate oxygen from the shorter acetate arm of an adjacent complex results in the formation of infinite sheets of linked tetranuclear units. Magnetic studies performed between 1.8 and 300 K on these complexes indicate that ferromagnetic or antiferromagnetic coupling between Cu(II) ions can be observed depending on the geometry of the carboxylate bridging mode. Compound 1 shows a paramagnetic behaviour with a ferromagnetic interaction (J/kB=+3.9 K, S=2 ground state) between adjacent Cu(II) ions through equatorial–equatorial syn–anti carboxylate bridges. In compound 2, within the rhombus motif ferromagnetic (J1/kB=+12.1 K) and antiferromagnetic (J2/kB=−0.4 K) interactions through syn–anti bridges but in equatorial–equatorial and equatorial–axial modes, respectively, are present. Finally, in the 2-D 4 and 5 the linkage of the rhombus motif, which is isolated in 2, by equatorial–axial anti–anti carboxylate bridges, leads to dominating ferromagnetic interactions (J′=+0.2 K) between the ferromagnetically interacting Cu(II) dimers (J1=+8.8 K).

Superconductivity of a Crystalline Ga84-Cluster Compound

We report on magnetization and resistivity measurements of a cluster compound in which negatively charged Ga84 entities are strongly bonded to organic ligands and confined in an ionic crystal structure. The macroscopic single crystals have a resistivity in the range of 100 Ωcm at room temperature with a semiconducting-like temperature dependence. They reveal a superconducting transition at Tc≈7.2 K and an upper critical field of Bc2=13.8 T. The presence of the superconducting state in the regularly arranged Ga84 clusters implies an electronic coupling between the individual clusters.

Magnetic properties of new Fe6(triethanolaminate(3-))6 spin-clusters

Abstract We report the magnetic properties of three cyclic hexanuclear iron(III) spin-clusters ([Fe 6 (tea) 6 ] · 6MeOH ( 1 ), [NaFe 6 (tea) 6 ]ClO 4 · 2MeOH ( 2 ) and [LiFe 6 (tea) 6 ]ClO 4 · 2MeOH ( 3 ), (tea: triethanolaminate(3-)). The intra-molecular exchange coupling between the Fe III -spins is determined by measurements of the magnetic susceptibility in the range 2–300 K. The zero-field splitting of the lowest cluster spin states ( E/k B 200K) is studied by ESR (electron-spin resonance) at frequencies between 9.5 and 350 GHz. The level splitting is analyzed by the irreducible-tensor operator technique.

New concepts for molecular magnets

Miller and Epstein (1994) define molecular magnets as magnetic materials which are prepared by the low-temperature methods of the preparative chemistry. This definition includes molecular crystals of neutral radicals, radical salts and charge transfer complexes as well as metal complexes and polymers with unpaired spins (Dormann 1995). The challenge of molecular magnets consists in tailoring magnetic properties by specific modifications of the molecular units. The combination of magnetism with mechanical or electrical properties of molecular compounds promise materials of high technical interest (Gatteschi 1994a and 1994b, Mohwald 1996) and both the chemical synthesis of new molecular materials with magnetic properties as well as the physical investigation and explanation of these properties is important, in order to achieve any progress. This work deals with the physical characterization of the magnetic properties of molecular materials. It is organized as follows. In the first part molecular crystals of neutral radicals are studied. After briefly discussing the general magnetic properties of these materials and after an overview over the physical principles of exchange interaction between organic radicals I focus on the interplay between the crystallographic structure and the magnetic properties of various derivatives of the verdazyl and nitronyl nitroxide radicals. The magnetic properties of metal complexes are the subject of the second part. After an overview over the experimental and theoretical tools which are used for the investigation of the magnetic properties I shortly discuss the exchange coupling of transition metal ions and the magnetic properties of complexes of two and three metal ions. Special emphasis is given to spin cluster compounds. Spin cluster denote complexes of many magnetic ions. They are attractive as building blocks of molecular magnets as well as magnetic model compounds for the study of spin frustration, molecular super-paramagnetism and quasi one-dimensional magnets.

Absorption spectra and Zeeman effect of the trivalent holmium ion in compounds with tetragonal zircon structure. I. Ho3+ in YVO4

The optical absorption spectrum of Ho3+ in tetragonal (Ho0.1,Y0.9)VO4 was studied in the range from 11000 to 27600 cm-1 at temperatures between 1.4 and 85 K in magnetic fields up to 4 T. Fields along the crystal c axis were used to determine the splitting factors of the doublets. Fields perpendicular to c were applied to induce transitions from the lowest singlet of the ground term, which are forbidden without the field. The energies and the symmetry types of almost all crystal-field components could be established with great certainty.

Homoleptische Mangan(III,III)‐ und Mangan(III,IV)‐Zweikernkomplexe mit deprotonierter D‐Mannose als Ligand aus wäßriger Lösung

Ausgerechnet mit einem „reduzierenden” Zucker – mit D-Mannose – gelingt die Herstellung eines gemischtvalenten Komplexes von Mangan in den Oxidationsstufen +III und +IV. Ba2[MnIIIMnIV(β-D-ManfH−5)2]Cl⋅14 H2O (Struktur des Anions siehe rechts; Manf=Mannofuranose) bildet sich in wasrig-alkalischer Losung quantitativ durch Oxidation eines analogen Mn2III-Komplexes mit Sauerstoff. Aus wasrig-neutraler Losung der Mn2III-Vorstufe entsteht der MnIIIMnIV-Zweikernkomplex durch Disproportionierung.

Anisotropy of the electron spin-resonance linewidth of

This paper presents measurements of the angular variation of the position and width of ESR lines in three mutually perpendicular planes of a single crystal of in the temperature range 12 - 200 K at two widely differing microwave frequencies. The analysis is carried out in terms of the anisotropic exchange and anisotropic Zeeman interaction. The variations of the anisotropy of the linewidth can be attributed to a temperature dependence of the orientation and the principal values of the exchange tensor.

NMR analysis and the magnetic properties of GdMn6Ge6

Abstract The magnetic phase diagram of GdMn6Ge6 was deduced from magnetisation measurements of powder samples. In agreement with the results of molecular field calculations it contains a paramagnetic phase, a ferrimagnetic phase where the magnetisation of ferromagnetically coupled Mn and Gd sublattices points in opposite directions, a spin flop phase and a phase where ferromagnetically ordered Gd and Mn planes are present, but the interaction between these planes results in a macroscopic magnetisation smaller than that in the ferri-phase. For the first time the zero-field magnetic resonance of 73Ge-nuclei has been observed at 77 K and 4.2 K. A separation of the Zeeman and quadrupolar contributions yields the hyperfine fields and the electric field gradients at the three inequivalent Ge sites. The enhancement observed indicates the presence of some net magnetisation in the low-temperature zero-field phase. Applying an external magnetic field, the sign of the hyperfine field at the Ge nuclei was determined.

Magnetic resonance of the new neutral Al cluster radical [Al7R6]

Static magnetometry and continuous-wave and pulsed-electron spin resonance (ESR) is used for the characterization of the neutral radical cluster [Al(Al3R3)2]• (R=N(SiMe2Ph)2). For temperatures below 25 K, we prove its monoradical character, a g-factor of 1.997–1.991, and a three time larger hyperfine interaction with the central than with the six outer 27Al nuclei of the [Al7R6]• cluster radical. Temperature dependence of the low-temperature spin-lattice relaxation is analysed. At temperatures above 30 K, the effective moment per cluster increases, and the hyperfine interaction with the seven 27Al nuclei is equalized.

Crystal-field spectra of trivalent holmium in HoVO4, HoAsO4, HoPO4 and Y(OH)3 in the infrared region

The polarized absorption spectra of Ho3+ in HoVO4, HoAsO4, HoPO4 and (Ho0.1,Y0.9) (OH)3 were studied in the range from 5000 to 12000 cm-1. Nearly all crystal-field components were found and their crystal quantum numbers mu , nu and S could be assigned.