Ziming Sun

Single-Molecule Magnets: Different Rates of Resonant Magnetization Tunneling in Mn12 Complexes**

Two new molecular magnets, the dodecanuclear manganese complexes of the type [Mn12 O12 (O2 CR)16 (H2 O)4 ] (known for R = CH3 , new for R = o-ClC6 H4 , o-BrC6 H4 ) have contributed to a better quantum mechanical understanding of single-molecule magnets. For instance, appreciable differences in the steps seen in the magnetization hysteresis loops of these high-spin clusters are attributed to changes in the rates of magnetization tunneling from one complex to another.

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: control by a single solvent molecule of Jahn-Teller isomerism in [Mn12O12(O2CCH2But)16(H2O)4]

Faster- and slower-relaxing versions of the title Mn12 compound have been obtained in pure forms that crystallize in the same space group and differ only in the identity of one lattice solvent molecule; solvent loss causes isomerization from the faster- to the slower-relaxing form.

ZIGZAG CHAIN OF HEPTA-COORDINATE MANGANESE COMPLEXES

Abstract The preparation of the polymeric chain complex, }[Mn(bic)(H2O)]2 · 2Br · 2H2O){n (2) is reported. Characterization is made by single X-ray structure, IR and magnetochemistry data. Each seven-coordinate manganese ion has a slightly distorted pentagonal bipyramidal coordination geometry. Bicine is a hexadentate ligand using one nitrogen, three oxygen and one μ2-oxygen atom from the carboxylate group to coordinate to two MnII ions. The two hydroxyl groups are not deprotonated, and the μ2-oxygen atom of the carboxylate moiety of the bic− ligand is a bridge leading to a polymeric chain. One water molecule coordinates to the MnII ion, and also forms a strong hydrogen bond with other non-coordinated water molecules. Both non-coordinated water and counter Br− ion are located between the zigzag chains. Weak pairwise antiferromagnetic exchange interactions (J = −1.42 cm−1) are found within the chain complex due to antiparallel geometry orientations of neighboring MnII centers.

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...

ONE-DIMENSIONAL POLYMERIC BICINATE MANGANESE COMPLEX

Abstract The synthesis, X-ray structure and magnetochemical properties of the one-dimensional polymeric bicinate (bic−) manganese complex {[Mn(bic)Cl]2·2(H2O)}n are reported. Each manganese ion has distorted octahedral coordination geometry. Bicine is a pentadentate ligand using one nitrogen, and four oxygen atoms to coordinate to the MnII ion. The two hydroxyl groups are not deprotonated and the carboxylate moiety of the bic− ligand ion is a bridge leading to a one-dimensional chain. The water molecules are not coordinated to the MnII ions, but are involved in hydrogen-bonding interactions with the chain. Only weak pairwise antiferromagnetic exchange interactions (J = −0.30 cm−1) are found within the chain complex due to the carboxylate bridges.

New example of Jahn-Teller isomerism in [Mn12O12(O2CR)16(H2O)4] complexes

Abstract The isolation and characterization of a new pair of Jahn-Teller isomers of [Mn 12 O 12 (O 2 CCH 2 Bu t ) 16 (H 2 O) 4 ] ( 3 ) are reported: complex 3 ·CH 2 Cl 2 ·MeNO 2 ( 3a ) and 3 ·CH 2 Cl 2 ·MeCN ( 3b ). These Jahn-Teller isomers have been crystallized in the same triclinic space group, and differ only in one abnormally oriented Jahn-Teller axis in 3a and in the identity of one solvent molecule of crystallization. Magnetic data show much faster magnetization relaxation for 3a (the low-temperature, LT, form) compared with 3b (the high-temperature, HT, form), exhibiting a peak in the out-of-phase magnetic susceptibility in the 2–4 and 5–7 K ranges, respectively. Magnetization vs. DC field scans on aligned crystals of 3a and 3b display hysteresis with coercivities that vary with temperature, as expected for single-molecule magnets. The hysteresis loops also exhibit the step features that are the signature of quantum tunneling of magnetization.

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: 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...

The origin of the second relaxation process in the [Mn12O12(O2CR)16(H2O)4] single-molecule magnets: ‘Jahn–Teller isomerism’ in the [Mn12O12] core

The origin of the second, faster relaxation process in Mn12 molecules has been identified as a different relative orientation of the Jahn–Teller elongation axes of the MnIII ions, which we have termed ‘Jahn–Teller isomerism’.