Single-molecule magnets (SMMs) are a class of coordination compounds that attract attention because of their magnetic bistability.[1] These finite-size molecules show a slow relaxation of the magnetization at low temperatures owing to an energy barrier for spin reversal resulting in a hysteresis of the magnetization which is of purely molecular origin. [2, 3] SMMs promise access to dynamic random access memory (RAM) devices for quantum computing and to ultimate high-density memory storage devices in which each bit of digital information might be stored on a single molecule.[4] The energy barrier (D S t ·S t 2 ) for spin reversal arises from the combination of a high spin ground state S t and a magnetic anisotropy of the easy-axis type (negative zero-field splitting parameter D S t ). [2,5] Since the discovery of the fascinating SMM behavior of Mn 12 , a lot of synthetic efforts have been devoted to the preparation of new molecules with an increased anisotropy barrier and impressive new structural motives have been reported. [6] The necessary requirements for SMMs are a high spin ground state S t and a strong magnetic anisotropy D S t . To rationally design polynuclear complexes with high spin ground states S t , a control of the exchange couplings is highly desirable. The main component of the magnetic anisotropy of the ground state (D S t ) usually comes from the projection of the single-site anisotropies (D i ) onto the spin ground state S t , while dipolar and anisotropic interactions yield only minor contributions. [7,8] Since zero-field splittings are tensor quantities, the projection of the single-site zerofield splittings onto the spin ground state may vanish when the metal-ion arrangement approaches a cubic symmetry. Thus, a rational design of SMMs requires a control of the molecular topology which can not be achieved by simply increasing the nuclearity of complexes and using small bridging ligands. Another prerequisite for a SMM to function as a data storage is the minimization of the quantum-mechanical magnetization tunneling which provides an alternative pathway for spinreversal and thus the loss of information. This tunneling mechanism is directly related to the rhombic component of the magnetic anisotropy expressed by E S t which is exactly zero for complexes with at least a threefold axis. Thus, there are several requirements for a targeted synthesis of SMMs which must be considered when designing a polynucleating ligand of low flexibility.We have designed the C 3 -symmetric triplesalen ligand C (Scheme 1) which combines the phloroglucinol bridging unit (A) with the coordination environment of a salen ligand (B). [9] In trinuclear Cu II [10] and Mo V [11] complexes the phloroglucinol bridging unit A acts as a ferromagnetic coupler by the spin-polarization mechanism. To introduce magnetic anisotropy we have been choosing a salen-like coordination environment B which causes a pronounced magnetic anisotropy through its strong ligand field in the basal plane. [8,12] A well documented example is t...

Star-shaped complex [Fe(III)[Fe(III)(L1)2]3] (3) was synthesized starting from N-methyldiethanolamine H2L1 (1) and ferric chloride in the presence of sodium hydride. For 3, two different high-spin iron(III) ion sites were confirmed by Mössbauer spectroscopy at 77 K. Single-crystal X-ray structure determination revealed that 3 crystallizes with four molecules of chloroform, but, with only three molecules of dichloromethane. The unit cell of 3.4CHCl3 contains the enantiomers (delta)-[(S,S)(R,R)(R,R)] and (lambda)-[(R,R)(S,S)(S,S)], whereas in case of 3.3CH2Cl2 four independent molecules, forming pairs of the enantiomers [lambda-(R,R)(R,R)(R,R)]-3 and [lambda-(S,S)(S,S)(S,S)]-3, were observed in the unit cell. According to SQUID measurements, the antiferromagnetic intramolecular coupling of the iron(III) ions in 3 results in a S = 10/2 ground state multiplet. The anisotropy is of the easy-axis type. EPR measurements enabled an accurate determination of the ligand-field splitting parameters. The ferric star 3 is a single-molecule magnet (SMM) and shows hysteretic magnetization characteristics below a blocking temperature of about 1.2 K. However, weak intermolecular couplings, mediated in a chainlike fashion via solvent molecules, have a strong influence on the magnetic properties. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) were used to determine the structural and electronic properties of star-type tetranuclear iron(III) complex 3. The molecules were deposited onto highly ordered pyrolytic graphite (HOPG). Small, regular molecule clusters, two-dimensional monolayers as well as separated single molecules were observed. In our STS measurements we found a rather large contrast at the expected locations of the metal centers of the molecules. This direct addressing of the metal centers was confirmed by DFT calculations.

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Reaction of lithium tetrachloromanganate(II) with N-n-butyldiethanolamine H2L3 (3) in the presence of LiH leads to the formation of wheel-shaped, mixed-valent heptanuclear, neutral complex {MnII subset[MnII2MnIII4Cl6(L3)6]} (4). The manganese wheel crystallizes in the triclinic space group P as 4.2CHCl3 or 4.3THF when either diethyl ether or n-pentane was allowed to diffuse into solutions of 4 in chloroform or tetrahydrofuran. The oxidation states of each manganese ion in 4.2CHCl3 or 4.3THF were assigned on the basis of detailed symmetry, bond length, and charge considerations, as well as by the Jahn-Teller axial elongation observed for the manganese(III) ions, and were further supported by cyclic voltammetry. The analysis of the SQUID magnetic susceptibility data for complex 4.2CHCl3 showed that the intramolecular magnetic coupling of the manganese(II,III) ions is dominated by ferromagnetic exchange interactions. This results in an S = 27/2 ground-state multiplet at low magnetic field. At fields higher than 0.68 T, the energetically lowest state is given by the mS = 31/2 component of the S = 31/2 multiplet due to the Zeeman effect. The ligand-field-splitting parameters were determined by anisotropy SQUID measurements on single crystalline samples along the crystallographic x, y, and z axes (D = -0.055 K, E = 6.6 mK) and by high-frequency electron spin resonance measurements on a polycrystalline powder of 4.2CHCl3 (D = -0.068 K, E = 9.7 mK). The resulting barrier height for magnetization reversal amounts to U approximately 10 K. Finally, 2DEG Hall magnetization measurements revealed that 4.2CHCl3 shows single-molecule magnet behavior up to the blocking temperature of about 0.6 K with closely spaced steps in the hysteresis because of the quantum tunneling of the magnetization.

Einzelmolekülmagnete sind eine Klasse von Koordinationsverbindungen, die wegen ihrer magnetischen Bistabilität auf großes Interesse gestoßen sind.[1] Diese endlichen Moleküle zeigen bei tiefen Temperaturen wegen einer Energiebarriere für die Spinumkehr eine langsame Relaxation der Magnetisierung, aus der eine Hysterese in der Magnetisierung resultiert, die rein molekularen Ursprungs ist. [2, 3] Wir haben den C 3 -symmetrischen Tripelsalen-Liganden C (Schema 1) entworfen, der eine verbrückende PhloroglucinEinheit (A) mit der Koordinationsumgebung eines SalenLiganden (B) vereint.[9] Die Einheit A führt in dreikernigen Cu II - [10] und Mo V