Sheila M. J. Aubin

Manganese carboxylate clusters: From structural aesthetics to single-molecule magnets

Abstract An overview is provided of some recent developments in manganese carboxylatecluster chemistry. A variety of synthetic methodologies are described together with the structuresof the resultant products, which span metal nuclearities of 4 to 18 and oxidation states of II to IV,including mixed-valency. The spins of the ground states of these products are often large andsometimes abnormally large, and in certain cases when this is coupled to a sufficiently largemagnetoanisotropy, the clusters function as single-molecule magnets i.e., they can be magnetizedby an external magnetic field below a critical temperature. These complexes display hysteresis inmagnetization vs . magnetic field studies, and clear evidence for quantum tunnelling ofmagnetization. Such results establish single-molecule magnetism as a new magnetic phenomenon,and one that holds great promise for next-century technological applications.

Magnetization tunneling in single-molecule magnets

The quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecules magnets (SMM’s). SMM’s are molecules that function as nanomagnets. Magnetization tunneling is described for two crystallographically different forms of [Mn12O12(O2CC6H4-p-Me)16(H2O)4] solvate. The two Mn12 complexes are isomers that both differ in the positioning of the H2O and carboxylate ligands and also in the orientations of the Jahn–Teller elongation at the Mn III ions. The magnetization versus magnetic field hysteresis loop is quite different for the two isomeric Mn12 complexes. One Mn12 complex exhibits a magnetization hysteresis loop that is characteristic of considerably faster magnetization tunneling than in the other Mn12 isomer. The lower symmetry and greater rhombic zero-field splitting are the origin of the faster magnetization tunneling. Frequency-dependent ac magnetic susceptibility and dc magnetization decay data are presented to characterize the magnetization relaxation rate versus temperature responses of three mixed-valence Mn4 complexes. In all three cases, the Arrhenius plot of the logarithm of the magnetization relaxation rate versus the inverse absolute temperature shows a temperature-dependent region as well as a temperature-independent region. The temperature-independent magnetization rate is definitive evidence of magnetization tunneling in the lowest-energy zero-field component of the ground state.

Single-Molecule Magnets: Characterization of Complexes Exhibiting Out-of-Phase AC Susceptibility Signals

Abstract The structures and characterization are described of complexes that exhibit an out-of-phase AC magnetic susceptibility x m” The dodecanuclear complexes [Mn12O12(O2CR)16(H2O)4]z contain 8MnIII, 4MnIV or MnII, 7MnIII, 4MnIV and possess ground state spins of S = 9 or 10 (z = 0) or S = 19/2 (z = −1). The distorted-cubane complexes [Mn4O3X(O2CR)3(dbm)3] contain 3MnIII, MnIV and have been prepared with a variety of X groups; in every case, the ground state is S = 9/2. The anion [Mn11O10X6(O2CPh)11(H2O)4]2− (9MnIII, 2MnIV; X = NCO−, N3 −) contains two [Mn4O3X] distorted-cubanes linked by a [Mn3O4]+ unit; the ground state is S = 5. The cation [V4O2(O2CEt)7(bpy)2]+ (4VIII) has a [V4O2]8+ butterfly core and a S = 3 ground state. All these complexes exhibit out-of-phase AC magnetic susceptibility signals at low temperatures (<8 K). These signals are indicative of slow relaxation of the magnetization, unable to keep in phase with the oscillating AC magnetic field, and are taken as evidence for single-molecul...

Internal magnetic structure of Mn12 acetate by polarized neutron diffraction

The internal magnetic structure of [Mn12 O12 (CD3 COO)16 (H2 O)4 ]2CD3 COOH 4H2 O as determined by polarized-beam single-crystal neutron diffraction is reported. The standard picture, in which the inner tetrahedron of (S = 3/2) Mn4+ ions is polarized antiparallel to an outer ring of eight (S = 2) Mn3+ ions, is confirmed directly. While the total magnetization for the molecule is in good agreement with bulk measurements, the individual moment components on each of the three symmetry-independent Mn sites are less than predicted by the standard picture. There is no evidence for net moments on the oxygen atoms, but overlap of positive and negative magnetization on the oxygen sites cannot be ruled out.

[Mn3O(O2CPh)6(py)2]2(4,4′-bpy) and [Mn9O7(O2CC6H4-p-OMe)13 (4,4′-bpy)]2: new multinuclear manganese complexes

Abstract The synthesis, X-ray crystal structures and magnetic properties of two new types of dimers of multinuclear manganese aggregates are described. [Mn 3 O(O 2 CPh) 6 (py) 2 ] 2 (4,4′-bpy) is prepared from the reaction of the trinuclear starting material [Mn 3 O(O 2 CPh) 6 (py) 2 (H 2 O)] with 4,4′-bipyridine. This compound can be described as a dimer of trinuclear units and has been characterized crystallographically as part of a unit cell which also contains a monomeric, trinuclear species. Dimers of enneanuclear manganese complexes linked by two bifunctional units (4,4′-bipyridine or trans -1,2-bis(4-pyridylethene)) may be prepared from the tetranuclear starting material, (su0)Bu 4 N)[Mn 4 O 2 -(O 2 CR) 9 (H 2 O)], where R = Ph or C 6 H 4 - p -OMe. The new octadecanuclear complex [Mn 9 O 7 (O 2 CC 6 H 4 - p -OMe) 13 (4,4′-bpy)] 2 has been structurally characterized. This complex shows magnetic behavior which can be fit by considering only magnetic interactions within the individual Mn 9 O 7 units.

A new manganese cluster topology capable of yielding high-spinspecies: mixed-valence[Mn7(OH)3Cl3(hmp)9]2+ with S ≥ 10

The reaction between MnCl 2 ·4H 2 O, Hhmp (Hhmp = 2-hydroxymethylpyridine) and NEt 4 MnO 4 in MeCN gives NEt 4 [Mn 7 (OH) 3 Cl 3 (hmp) 9 ]Cl[MnCl 4 ] 1; the 4 Mn II , 3 Mn III cation comprises a Mn 6 hexagon containing a central Mn ion, and it possesses a S ≥ 10 ground state.

Single-molecule magnets: isomeric [Mn12O12(O2CC6H4Me-4)16(H2O)4] complexes exhibiting different rates of resonant magnetization tunnelling

Two different isomeric forms of [Mn12O12(O2CC6H4Me-4)16(H2O)4], differing in the positioning of H2O ligands, are structurally characterized and shown to have considerably different magnetization hysteresis loops.

Single-molecule magnets : Magnetization relaxation and quantum tunneling in dodecanuclear manganese complexes

Abstract The syntheses and characterization of three new dodecanuclear manganese complexes with the composition [Mn12O12(O2CR)16(H2O)4] are Complexes 1-3 can be prepared with different solvate molecules in the crystal. The X-ray structure of 1–8(CH2Cl2) (1a) has been determined. The nature of the solvation influences the out-of-phase AC magnetic susceptibility, χm ″ Some samples show one χm ″ peak in the 5–6 K range, others show a χm ″ peak in the 2–3 K range, and even other compounds show both peaks. These χm ″ peaks are indicative of a slow relaxation of a single-molecule magnet, where the magnetization of the molecule cannot keep in phase with the oscillating AC magnetic field. Possible explanations for the two χm ″ peaks are given. Hysteresis loops are seen below −2.5 K. Step-like features are seen on the hysteresis loops measured for a single crystal of the R = Et (6) complex. Quantum mechanical tunneling could be the origin of these step-like features.

Single molecule magnets: High frequency electron paramagnetic resonance study of two isomeric forms of an Mn12 molecule

Different crystallographic forms of the single molecule magnet [Mn12O12(O2CR)16(H2O)4] (complex 1) with a given R substituent have been isolated. The two different isomeric forms of the p-methylbenzoate complex crystallize as [Mn12O12(O2CC6H4-p-Me)16(H2O)4]⋅(HO2CC6H4-p-Me) (complex 2) and [Mn12O12(O2CC6H4-p-Me)16(H2O)4]⋅3H2O (complex 3). In complex 2, one MnIII ion has an abnormal Jahn–Teller distortion axis oriented at an oxide ion, and thus 2 and 3 are Jahn-Teller isomers. This reduces the symmetry of the core of complex 2 compared with that of complex 3. Complex 2 likely has a larger tunneling matrix element and this explains why this complex shows an out-of-phase ac peak (χM″) in the signal in the 2–3 K region, whereas complex 3 has its χM″ peak in the 4–7 K range, i.e., the rate of tunneling of magnetization is greater in complex 2 than complex 3. High frequency electron paramagnetic resonance (HFEPR) experiments were performed on both isomers. Computed simulations of the experimental HFEPR data yiel...

Resonant magnetization tunneling in single-molecule magnets

Abstract Data are presented for three different types of single-molecule magnets (SMMs): [Mn12O12(O2CR)16(H2O)4],[Cation][Mn12O12(O2CR)16(H2O)4], and the distorted cubane [MnIV MnIII 3O3X(O2CR)3L3] complexes. All three types of complexes exhibit slow magnetization relaxation at temperatures below 5 K. Each molecule can change the direction of its magnetization only slowly at these low temperatures. Manisfetations of this are seen in magnetization hysteresis loops and in the presence of frequency-dependent out-of-phase ac magnetic susceptibility signals (X′M). It is shown that the neutral Mn12 SMMs exist in isomeric forms that differ in the positoning of the 4H2O and 16 carboxylate ligands. The orgin of the two X′M signals for the neutral Mn12 complexes is discussed. All Mn12 complexes exhibit magnetization hysteresis loops with steps seen at regular increments of magnetic field. The step heights and coercive fields for the loops vary from one Mn12 SMM to another. Step-structured hysteresis loops are als...