Christos Lampropoulos

Inducing Single-Molecule Magnetism in a Family of Loop-of-Loops Aggregates: Heterometallic Mn40Na4 Clusters and the Homometallic Mn44 Analogue

The syntheses, crystal structures, and magnetic properties of a new family of heterometallic Mn(40)Na(4) and homometallic Mn(44) loop-of-loops aggregates are reported. The reactions of [Mn(3)O(O(2)CMe)(6)(py)(3)]·py with 1,3-propanediol (pdH(2)) and 2-methyl-1,3-propanediol (mpdH(2)) in the presence of NaN(3) afforded [Mn(10)Na(μ(3)-O)(2)(O(2)CMe)(13)(pd)(6)(py)(2)](4) (1)(4) and [Mn(10)Na(μ(3)-O)(2)(O(2)CMe)(13)(mpd)(6)(py)(H(2)O)](4) (2)(4), respectively. Mn(40)Na(4) complexes (1)(4) and (2)(4) consist of four Mn(10) loops linked through Na(+) ions to give a supramolecular aggregate with a saddle-like topology. Magnetic characterization of compound (1)(4) showed that each Mn(10) loop has an S = 4 ground-state spin and displays frequency-dependent in-phase and out-of-phase ac susceptibility signals. It also exhibits hysteresis loops that, however, are not typical of single-molecule magnets (SMMs) due to the existence of interloop interactions between the neighboring Mn(10) units of (1)(4) through the diamagnetic Na(+) ions, and also intermolecular interactions between different Mn(40)Na(4) aggregates. The magnetically discrete Mn(44) analogue was targeted with high priority and finally prepared from the reaction of [Mn(3)O(O(2)CMe)(6)(py)(3)]·py with pdH(2) in the presence of Mn(ClO(4))(2)·6H(2)O. The loop-of-loops structure of [Mn(44)(μ(3)-O)(8)(O(2)CMe)(52)(pd)(24)(py)(8)](ClO(4))(OH)(3) (3) is essentially identical to those of (1)(4) and (2)(4), with the most significant difference being that the four Na(+) ions of (1)(4) and (2)(4) have been replaced with Mn(2+) ions. Compound 3 is thus best described magnetically as a Mn(44) cluster. In accord with this description and the stronger exchange coupling between the four Mn(10) loops expected through the connecting Mn(2+) ions, magnetic susceptibility measurements revealed that 3 has an S = 6 ground-state spin and displays frequency-dependent in-phase and out-of-phase ac signals. Magnetization vs dc field sweeps on single-crystals of 3 displayed scan rate- and temperature-dependent hysteresis loops confirming that complex 3 is a new SMM, and is thus the second largest Mn cluster and SMM reported to date.

Binding of Higher Alcohols onto Mn 12 Single-Molecule Magnets(SMMs): Access to the Highest Barrier Mn 12 SMM

Two new members of the Mn(12) family of single-molecule magnets (SMMs), [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(Bu(t)OH)(H(2)O)(3)].2Bu(t)OH (3.2Bu(t)OH) and [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(C(5)H(11)OH)(4)] (4) (C(5)H(11)OH is 1-pentanol), are reported. They were synthesized from [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)].2MeCO(2)H.4H(2)O (1) by carboxylate substitution and crystallization from the appropriate alcohol-containing solvent. Complexes 3 and 4 are new members of the recently established [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(solv)(4)] (solv = H(2)O, alcohols) family of SMMs. Only one bulky Bu(t)OH can be accommodated into 3, and even this causes significant distortion of the [Mn(12)O(12)] core. Variable-temperature, solid-state alternating current (AC) magnetization studies were carried out on complexes 3 and 4, and they established that both possess an S = 10 ground state spin and are SMMs. However, the magnetic behavior of the two compounds was found to be significantly different, with 4 showing out-of-phase AC peaks at higher temperatures than 3. High-frequency electron paramagnetic resonance (HFEPR) studies were carried out on single crystals of 3.2Bu(t)OH and 4, and these revealed that the axial zero-field splitting constant, D, is very different for the two compounds. Furthermore, it was established that 4 is the Mn(12) SMM with the highest kinetic barrier (U(eff)) to date. The results reveal alcohol substitution as an additional and convenient means to affect the magnetization relaxation barrier of the Mn(12) SMMs without major change to the ligation or oxidation state.

A nontwisted, ferromagnetically coupled Mn(III)3O triangular complex from the use of 2,6-bis(hydroxymethyl)-p-cresol.

The reaction between Mn(O(2)CMe)(2) x 4 H(2)O and hmcH(3) [hmcH(3) = 2,6-bis(hydroxymethyl)-p-cresol] in CH(2)Cl(2) in the presence of NEt(3) affords the Mn(III)(3) complex [NEt(3)(CH(2)Cl)](2)[Mn(3)O(hmcH)(3)(hmcH(2))(3)] (1). The anion of 1 contains a [Mn(III)(3)(mu(3)-O)](7+) triangular core, with the central O(2-) ion lying above the Mn(3) plane. The complex is ferromagnetically coupled with a resulting S = 6 ground state.

A caveat for single-molecule magnetism: non-linear Arrhenius plots.

Single-molecule magnets (SMMs) are molecules that, below a certain temperature (TB), function as individual nanoscale magnetic particles, exhibiting magnetization hysteresis loops. As such, they represent an alternative and molecular (bottom-up) route to nanomagnetism, complementing the top-down approach to traditional magnetic nanomaterials. SMMs also exhibit fascinating quantum behavior such as quantum tunneling of the magnetization (QTM) and quantum phase interference (QPI), showing that they are truly mesoscale entities straddling the classical/quantum divide. The barrier causing slow magnetization relaxation arises from a combination of a large ground-state spin (S) and easy-axis anisotropy (negative zerofield splitting parameter, D). The most studied SMMs are the [Mn12O12 ACHTUNGTRENNUNG(O2CR)16 ACHTUNGTRENNUNG(H2O)4] family with S=10 ground states, and their derivatives, while in recent years others have been discovered. Alternating current (ac) magnetic susceptibility studies are a convenient method of assessing whether a compound might be an SMM; frequency-dependent out-of-phase (c’’M) signals are indicative of the superparamagnet-like properties of an SMM. The variation in signal position with ac frequency can then be used as a source of rate vs T kinetic data, because the c’’M peak maximum is the temperature at which the angular frequency of the oscillating field equals the rate (1/t, where t is the relaxation lifetime) of spin vector reversal. This allows construction of a ln ACHTUNGTRENNUNG(1/t) vs 1/T plot based on the Arrhenius relationship given in Equation (1), the behavior expected of a thermally-activated process over a single barrier :

Synthesis, structure, and spectroscopic and magnetic characterization of [Mn12O12(O2CCH2But)16(MeOH)4]·MeOH, a Mn12 single-molecule magnet with true axial symmetry.

The synthesis and properties are reported of a rare example of a Mn(12) single-molecule magnet (SMM) in truly axial symmetry (tetragonal, I4). [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(MeOH)(4)]·MeOH (3·MeOH) was synthesized by carboxylate substitution on [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)]·2MeCO(2)H·4H(2)O (1). The complex was found to possess an S = 10 ground state, as is typical for the Mn(12) family, and displayed both frequency-dependent out-of-phase AC susceptibility signals and hysteresis loops in single-crystal magnetization vs DC field sweeps. The loops also exhibited quantum tunneling of magnetization steps at periodic field values. Single-crystal, high-frequency electron paramagnetic resonance spectra on 3·MeOH using frequencies up to 360 GHz revealed perceptibly sharper signals than for 1. Moreover, careful studies as a function of the magnetic field orientation did not reveal any satellite peaks, as observed for 1, suggesting that the crystals of 3 are homogeneous and do not contain multiple Mn(12) environments. In the single-crystal (55)Mn NMR spectrum in zero applied field, three well-resolved peaks were observed, which yielded hyperfine and quadrupole splitting at three distinct sites. However, observation of a slight asymmetry in the Mn(4+) peak was detectable, suggesting a possible decrease in the local symmetry of the Mn(4+) site. Spin-lattice (T(1)) relaxation studies were performed on single crystals of 3·MeOH down to 400 mK in an effort to approach the quantum tunneling regime, and fitting of the data using multiple functions was employed. The present work and other recent studies continue to emphasize that the new generation of truly high-symmetry Mn(12) complexes are better models for thorough investigation of the physical properties of SMMs than their predecessors such as 1.

On-chip SQUID measurements in the presence of high magnetic fields

We report a low temperature measurement technique and the magnetization data of a quantum molecular spin, by implementing an on-chip SQUID technique. This technique enables SQUID magnetometry in high magnetic fields, up to 7 T. The main challenges and the calibration process are detailed. The measurement protocol is used to observe quantum tunneling jumps of the S = 10 molecular magnet, Mn(12)-tBuAc. The effect of a transverse field on the tunneling splitting for this molecular system is addressed as well.

Crystal Lattice Desolvation Effects On The Magnetic Quantum Tunneling Of Single-Molecule Magnets

High-frequency electron paramagnetic resonance (HFEPR) and alternating current (ac) susceptibility measurements are reported for a new high-symmetry

Manganese/Cerium Clusters Spanning a Range of Oxidation Levels and CeMn8, Ce2Mn4, and Ce6Mn4 Nuclearities: Structural, Magnetic, and EPR Properties

{\text{Mn}}_{12}

α-Benzoin Oxime in Higher Oxidation State 3d Metal Cluster Chemistry: Structural and Magnetic Study of a New MnIII9 Complex

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Synthesis, magnetism, and high-frequency EPR spectroscopy of a family of mixed-valent cuboctahedral Mn13 complexes with 1,8-naphthalenedicarboxylate ligands.

[{\text{Mn}}_{12}{\text{O}}_{12}{({\text{O}}_{2}{\text{CCH}}_{3})}_{16}{({\text{CH}}_{3}\text{OH})}_{4}]\ensuremath{\cdot}{\text{CH}}_{3}\text{OH}