Isomeric Forms of [Mn 12 O 12 (O 2 CR) 16 H 2 O) 4 ] Complexes Showing Different Magnetization Relaxation Processes (original) (raw)
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Secondary magnetic relaxations in Mn12 complexes
Journal of Magnetism and Magnetic Materials, 2000
We present new experiments on the zero-"eld magnetic relaxation in Mn acetate and Mn 2-Cl benzoate. By dynamical magnetic measurements, this study shows that a small portion of few percent of the clusters does not undergo the main relaxation process and that they relax faster in the temperature region of 2 K. The e!ective relaxation times of these secondary processes follow an Arrhenius law with energy barrier of approximately 23 and 30 K for the acetate and the 2-Cl benzoate, respectively.
Inorganic Chemistry, 2001
Several single-molecule magnets with the composition [Mn 12 O 12 (O 2 CR) 16 (H 2 O) x ] (x = 3 or 4) exhibit two out-of-phase ac magnetic susceptibility signals, one in the 4-7 K region and the other in the 2-3 K region. New Mn 12 complexes were prepared and structurally characterized and the origin of the two magnetization relaxation processes was systematically examined. Different crystallographic forms of a Mn 12 complex with a given R substituent exist, where the two forms have different compositions of solvent molecules of crystallization and this results in two different arrangements of bound H 2 O and carboxylate ligands for the two crystallographically different forms with the same R substituent. The X-ray structure of cubic crystals of [Mn 12 O 12 (O 2 CEt) 16 (H 2 O) 3 ] · 4H 2 O (space group 1 P ) (complex 2a) has been reported previously. The more prevalent needle-form of [Mn 12 O 12 (O 2 CEt) 16 (H 2 O) 3 ] (complex 2b) crystallizes in the monoclinic space group P2 1 /c, which at -170 °C has a = 16.462(7) Å, b = 2 22.401(9) Å, c = 20.766(9) Å, β = 103.85(2)°, and Z = 4. The arrangements of H 2 O and carboxylate ligands on the Mn 12 molecule are different in the two crystal forms. The complex [Mn 12 O 12 (O 2 CC 6 H 4 -p-Cl) 16 (H 2 O) 4 ] · 8CH 2 Cl 2 (5) crystallizes in the monoclinic space group C2/c, which at -172 °C has a = 29.697(9) Å, b = 17.708(4) Å, c = 30.204(8) Å, β = 102.12(2)°a nd Z = 4. The ac susceptibility data for complex 5 show that it has out-of-phase signals in both the 2-3 K and the 4-7 K ranges. X-ray structures are also reported for two isomeric forms of the p-methylbenzoate complex. [Mn 12 O 12 (O 2 CC 6 H 4 -p-Me) 16 (H 2 O) 4 ] · (HO 2 CC 6 H 4 -p-Me) (6) crystallizes in the monoclinic space group C2/c, which at 193 K has a = 40.4589(5) Å, b = 18.2288(2) Å, c = 26.5882(4) Å, β = 125.8359(2)° , and Z = 4. [Mn 12 O 12 (O 2 CC 6 H 4 -p-Me) 16 (H 2 O) 4 ] · 3(H 2 O) (7) crystallizes in the monoclinic space group I2/a, which at 223 K has a = 29.2794(4) Å, b = 32.2371(4) Å, c = 29.8738(6) Å, β = 99.2650(10)° , and Z = 8. The Mn 12 molecules in complexes 6 and 7 differ in their arrangements of the four bound H 2 O ligands.
Solid State Communications, 1999
The Mn cluster complexes Mn 12 (RCOO) 16 (H 2 O) 4 O 12 with R = CH 3 and 2ClPh (hereafter referred to as Mn 12 acetate and Mn 12 2-Cl benzoate, respectively) exhibit macroscopic magnetic quantum tunneling. Dynamical magnetic measurements indicate for both samples that the anisotropy energy barrier is nearly the same and that the relaxation times have sharp minima at the same resonant fields. However, we show that for both compounds a minority portion of the clusters does not undergo this main relaxation process. From ac-susceptibility experiments, we report evidence of a continuous distribution of faster relaxation times, giving rise to a second relaxation regime. The zero-field effective relaxation times of such a distribution follow an Arrhenius law with energy barrier of approximately 23 and 30 K for the acetate and the benzoate, respectively. In the case of Mn 12 2-Cl benzoate, the field dependence of the effective relaxation time points towards the existence of spin tunneling with H z = 0 being a resonant field. The existence of a broad distribution of energy barriers is corroborated in magnetization experiments by the observation of logarithmic relaxation at short times.
Single-Molecule Magnets: A Reductive Aggregation Route to New Types of Mn 12 Complexes
Inorganic Chemistry, 2005
Three dodecanuclear Mn clusters [Mn 12 O 10 (OMe) 3 (OH)(O 2 CC 6 H 3 F 2 ) 16 (MeOH) 2 ]‚8MeOH (1), [Mn 12 O 10 (OMe) 4 (O 2 -CBu t ) 16 (MeOH) 2 ] (2), and [Mn 12 O 12 (O 2 CBu t ) (MeOH) 4 ] (3) synthesized by reductive aggregation reactions are reported. Clusters 1 and 2 possess a central alkoxide-bridged planar Mn 4 topology, whereas 3 is a new high-symmetry member of the normal Mn 12 family. Complexes 1 and 2 crystallize in the monoclinic space groups C2/c and P2 1 /n, respectively. Both consist of four Mn IV and eight Mn III ions held together by 10 µ 3 -O 2ions, and either (i) one µ-OHand three µ-MeOgroups for 1 or (ii) four µ-MeOgroups for 2. Complex 3 crystallizes in the orthorhombic space group Aba2 and possesses the normal Mn 12 structure but with terminal MeOH molecules. The cyclic voltammogram (CV) of 1 exhibits no reversible redox processes. Variable-temperature, solid-state dc and ac magnetic susceptibility measurements on 1 and 2 reveal that they possess S ) 5 and 9 ground states, respectively. In addition, ac susceptibility measurements on complex 1 in a zero dc field in the temperature range 1.8−10 K and in a 3.5 G ac field oscillating at frequencies in the 5−1488 Hz range display a nonzero frequency-dependent out-of-phase ( M ′′) signal at temperatures below 3 K, with the peak maxima lying at temperatures below 1.8 K. For complex 2, two frequency dependent M ′′ signals are seen, one in the higher temperature range of 3−5 K and a second at lower temperatures with its peak maxima at temperatures below 1.8 K. Single-crystal magnetization vs dc field scans down to 0.04 K for 1‚8MeOH and 2 show hysteresis behavior at <1 K, confirming that both complexes are new examples of SMMs. Gatteschi, D.; Caneschi, A.; Novak, M. A. Nature 1993, 365, 141. (b) Sessoli R.; Ysai, H. -L.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N.
Journal of the American Chemical Society, 2003
The complex [Mn12O12(O2CCHCl2)16(H2O)4] (2) in MeCN exhibits three quasi-reversible oneelectron reduction processes at significantly higher potentials than [Mn12O12(O2CMe)16(H2O)4] (1). This has allowed the two-electron reduced version of 2 to be generated and isolated. Reaction of 2 with one and two equivalents of PPh4I led to isolation of (PPh4)[Mn12O12(O2CCHCl2)16(H2O)4] (3) and (PPh4)2[Mn12O12(O2-CCHCl2)16(H2O)4] (4), respectively. The latter represents a new isolated oxidation level of the Mn12 family of single-molecule magnets (SMMs). Crystallization from CH2Cl2/hexanes yields a mixture of two crystal forms, 4‚4CH2Cl2‚H2O (4a) and 4‚6CH2Cl2 (4b), both of which have been structurally characterized as triclinic and monoclinic, respectively. The molecular structures are very similar, with the added electrons localized on former Mn(III) ions to give a trapped-valence 2Mn(II), 6Mn(III), 4Mn(IV) oxidation state description. Dried solid analyzed as unsolvated 4. 1 H NMR spectral data in CD2Cl2 confirm that 4 retains its solid-state structure in solution. Bulk DC magnetization data for dried 4 in the 1.80-4.00 K and 10-70 kG ranges were fit to give S ) 10, D ) -0.275 cm -1 , g ) 2.00 and |D|/g ) 0.14 cm -1 , where D is the axial zero-field splitting (anisotropy) parameter. Complexes 4a and 4b give resolvable frequency-dependent outof-phase ( M′′) signals in AC susceptibility studies resulting from the magnetization relaxation of SMMs. Relaxation rate vs T data to 1.8 K obtained from the M′′ vs temperature studies were supplemented with rate vs T data measured to lower temperatures via magnetization vs time decay data, and these were fit to the Arrhenius equation to give the effective barrier to relaxation (Ueff). The Ueff values are 18.5 and 30.3 K for 4a and 4b, respectively. A similar analysis for dried 4 using AC data gave Ueff ) 32 K. Magnetization vs DC field sweeps on single crystals of 4a and 4b gave hysteresis loops containing steps due to quantum tunneling of magnetization (QTM). The step separations yielded |D|/g values of 0.087 and 0.14 cm -1 for 4a and 4b, respectively, suggesting that the differences in Ueff are primarily caused by changes to D. The combined work demonstrates the sensitivity of the magnetic properties of these new [Mn12] 2-SMMs to subtle differences in their environment as determined by the precise packing, solvent molecules, and overall crystal symmetry (space group) and represents an important caveat to workers in the field. magnetism and in finding other compounds that exhibit similar properties. Indeed, a few more manganese clusters, 2-4 as well as vanadium, 5 iron, 6 cobalt, 7 and nickel clusters 8 have been found to possess the necessary properties to function as singlemolecule magnets (SMMs). However, of the SMMs known to date, the [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ] (R ) various) family possesses the best structural and electronic properties for this (1) (a) Sessoli, R.; Ysai, H.-L.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N.
Polyhedron, 2001
The preparation and physical characterization are reported for the single-molecule magnet salts A[Mn 12 O 12 (O 2 CC 6 H 4 F-(-o)) 16 (H 2 O) 4 ] (A + =PPh 4 + (2a), Fe(C 5 Me 5 ) 2 + (2b), and Co(C 5 Me 5 ) 2 + (2c)). The effects of the magnetic cation on the magnetization relaxation behavior of the [Mn 12 ] − anions are investigated. All complexes exhibit out-of-phase ac magnetic susceptibility (¦ M ) signals in the 4.8-5.1 K range at 1 kHz ac frequency. The temperature of the ¦ M peaks is frequency dependent, as expected for a single-molecule magnet. From Arrhenius plots of the frequency dependence of the temperature of the ¦ M peaks, the effective energy barriers U eff for changing the magnetization direction from spin ''up'' to spin ''down'' were estimated to be in the 52-57 K range. Magnetization hysteresis loops were observed for all the complexes studied. They show clear hysteresis loops with steps, indicating the effect of the magnetic cation on the magnetization relaxation of the anionic [Mn 12 ] − complex is rather small. The least-squares fittings of variable-field magnetization data show the ground state of complex 2a is best described as S= 21/2 with g =1.96 and D = −0.56 K, while complexes 2b and 2c have S= 19/2 ground states. The fitting parameters are g=1.96 and D = −0.54 K for complex 2b and g= 1.95 and D= −0.57 K for complex 2c. These analyses show the magnetic cation has essentially no effect on the ground state spin or on the parameters g and D for the [Mn 12 ] − anion.
Evidence for resonant tunneling of magnetization in Mn-12 acetate complex
Physical Review B, 1997
We have measured the dc magnetization at low temperatures of tetragonal crystals of Mn 12 acetate complex ͓Mn 12 O 12 ͑CH 3 COO͒ 16 ͑H 2 O͒ 4 ͔, a material composed of a large ͑Avogadro's͒ number of identical magnetic molecules, each of spin 10. Exchange coupling between Mn ions within each molecule is very strong, while the interaction between molecules is negligible. A large, uniaxial anisotropy ͑ϳ60 K͒ gives rise to a doubly degenerate ground state corresponding to spin projections of Ϯ10 along the easy axis ͑c axis͒; hysteretic behavior is found below a blocking temperature T b ϳ3 K. Based on measurements of oriented crystallites at temperatures between 1.7 and 3.2 K, we report strong evidence for resonant tunneling of the magnetization: periodic steps in the hysteresis loop, and periodic marked increases in the magnetic relaxation rate at the magnetic fields corresponding to these steps. A total of seven increases in the relaxation rate were found within the temperature range of our experiments with a period of 0.46 T; we suggest that many more such steps would be found at lower temperatures. We attribute these observations to thermally assisted resonant tunneling of the magnetization and propose a detailed model to account for our results. ͓S0163-1829͑97͒00709-1͔ BACKGROUND