Single-Molecule Magnets:  Jahn−Teller Isomerism and the Origin of Two Magnetization Relaxation Processes in Mn₁₂ Complexes

Abstract

Several single-molecule magnets with the composition [Mn12O12(O2CR)16(H2O)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 Mn12 complexes were prepared and structurally characterized, and the origin of the two magnetization relaxation processes was systematically examined. Different crystallographic forms of a Mn12 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 H2O and carboxylate ligands for the two crystallographically different forms with the same R substituent. The X-ray structure of cubic crystals of [Mn12O12(O2CEt)16(H2O)3]. 4H2O (space group P1) (complex 2a) has been reported previously. The more prevalent needle-form of [Mn12O12(O2CEt)16(H2O)3] (complex 2b) crystallizes in the monoclinic space group P2(1)/c, which at -170 degrees C has a = 16.462(7) A, b = 22.401(9) A, c = 20.766(9) A, beta = 103.85(2) degrees, and Z = 4. The arrangements of H2O and carboxylate ligands on the Mn12 molecule are different in the two crystal forms. The complex [Mn12O12-(O2)CC6H4-p-Cl)16(H2O)4].8CH2Cl2 (5) crystallizes in the monoclinic space group C2/c, which at -172 degrees C has a = 29.697(9) A, b = 17.708(4) A, c = 30.204(8) A, beta = 102.12(2) degrees, and 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. [Mn12O12(O2CC6H4-p-Me)16(H2O)4]. (HO2CC6H4-p-Me) (6) crystallizes in the monoclinic space group C2/c, which at 193 K has a = 40.4589(5) A, b = 18.2288(2) A, c = 26.5882(4) A, beta = 125.8359(2) degrees, and Z = 4. [Mn12O12(O2CC6H4-p-Me)16(H2O)4].3(H2O) (7) crystallizes in the monoclinic space group I2/a, which at 223 K has a = 29.2794(4) A, b = 32.2371(4) A, c = 29.8738(6) A, beta = 99.2650(10) degrees, and Z = 8. The Mn12 molecules in complexes 6 and 7 differ in their arrangements of the four bound H2O ligands. Complex 6 exhibits an out-of-phase ac peak (chi(M)' ') in the 2-3 K region, whereas the hydrate complex 7 has a chi(M)' ' signal in the 4-7 K region. In addition, however, in complex 6, one Mn(III) ion has an abnormal Jahn-Teller distortion axis oriented at an oxide ion, and thus 6 and 7 are Jahn-Teller isomers. This reduces the symmetry of the core of complex 6 compared with complex 7. Thus, complex 6 likely has a larger tunneling matrix element and this explains why this complex shows a chi(M)' ' signal in the 2-3 K region, whereas complex 7 has its chi(M)' ' peak in the 4-7 K region, i.e., the rate of tunneling of magnetization is greater in complex 6 than complex 7. Detailed 1H NMR experiments (2-D COSY and TOCSY) lead to the assignment of all proton resonances for the benzoate and p-methyl-benzoate Mn12 complexes and confirm the structural integrity of the (Mn12O12) complexes upon dissolution. In solution there is rapi...

Inelastic neutron scattering and frequency-domain magnetic resonance studies ofS=4andS=12Pieper1, Guidi2, Carretta3 et al. 2010Phys. Rev. B211160View full textAdd to dashboardCite

“…Thus, the highest-frequency line is assigned to the ͉12, Ϯ 12͘ → ͉12, Ϯ 11͘ transition, and so on. The lines are much narrower ͑11 eV FWHM͒ than those observed for other SMMs, e.g., 23 eV FWHM for Mn 12 Ac. The fit procedure showed that the lines are inhomogeneously broadened and best described by Gaussian line shapes.…”

Butterfly hysteresis and slow relaxation of the magnetization in (Et4N)3Fe2F9: manifestations of a single-molecule magnet

“…The prefactor τ 0 = 70 s obtained from the Arrhenius analysis of the relaxation dynamics at H z = 0 is unusually large, 7 − 11 orders of magnitude larger than in Mn 12 [1][2][3][4] and other SMMs [5,6,[8][9][10][11][12][13]. It is much larger than the timescale of ac magnetic susceptibility measurements, which are usually used to characterize and quantify the relaxation behavior in SMMs.…”

“…Chemists have joined forces with physicists in an attempt to fully characterize and understand the new phenomena and to develop strategies to design and prepare other members of this new family of single molecule magnets (SMMs). So far a number of further SMMs have been discovered, mostly by means of ac susceptibility, among them several manganese clusters, [8][9][10][11] as well as tetranuclear vanadium [12] and iron [13] complexes. All these compounds contain clusters with at least four paramagnetic centers.…”

Anisotropy barrier reduction in fast-relaxingMn12single-molecule magnets

“…Furthermore, the spacing between the QMT steps associated with the FR species (~0.4 T) is not all that dissimilar to those of the SR species (~0.45 T), suggesting that they possess similar uniaxial anisotropy constants D. [14][15][16] Subsequently, detailed synthetic work by Christou, Hendrickson and co-workers, involving ligand substitution and crystallization from a variety of solvents, resulted in the discovery of many different forms of Mn 12 possessing essentially the same neutral Mn 12 O 12 core. 11,12,[17][18][19][20][21][22][23][24][25][26][27] However, important differences have been inferred from X-ray and magnetic studies. Most notably, the various Mn 12 complexes can be grouped broadly into two categories, i.e.…”

“…In all cases where the FR species are obtained in single-crystal form, X-ray studies show that one or more of the Jahn-Teller (JT) elongation axes associated with the Mn III atoms are abnormally oriented 18,19,21,24,25,33 in comparison to the usual SR form of Mn 12 . 26,27,[34][35][36][37] This is illustrated in Fig.…”