One-dimensional chain of tetranuclear manganese single-molecule magnets

A one-dimensional chain of interconnected single-molecule magnets (SMMs) is obtained that consists of [Mn(4)(hmp)(6)](4+) units bridged by chloride ions. Slow magnetization relaxation is evident in the AC susceptibility data and in magnetization hysteresis measurements for [Mn(4)(hmp)(6)Cl(2)](n)(ClO(4))(2)(n). The magnetization hysteresis loops for this complex are similar to those for an SMM and show significant coercive field and steps at regular magnetic intervals. Spin-canted antiferromagnetic coupling due to misalignment of easy axes of neighboring Mn(4) units is also observed for this complex.     

A supramolecular aggregate of four exchange-biased single-molecule magnets

The reaction between 3-phenyl-1,5-bis(pyridin-2-yl)pentane-1,5-dione dioxime (pdpdH(2)) and triangular [Mn(III)(3)O(O(2)CMe)(py)(3)](ClO(4)) (1) affords [Mn(12)O(4)(O(2)CMe)(12)(pdpd)(6))](ClO(4))(4) (3). Complex 3 has a rectangular shape and consists of four [Mn(III)(3)O](7+) triangular units linked covalently by the dioximate ligands into a supramolecular [Mn(3)](4) tetramer. Solid-state dc and ac magnetic susceptibility measurements revealed that [Mn(3)](4) contains four Mn(3) single-molecule magnets (SMMs), each with an S = 6 ground state. Magnetization versus dc-field sweeps on a single crystal gave hysteresis loops below 1 K that exhibited exchange-biased quantum tunneling of magnetization steps, confirming 3 to be a supramolecular aggregate of four weakly exchange-coupled SMM units.     

One-dimensional coordination polymers of antiferromagnetically-coupled [Mn4] single-molecule magnets

Reactions of the rhombic [MnII2 MnIII2 (hmp)6]4+ complex in acetonitrile with simple carboxylate ligands yield (i) three new isolated [Mn4] complexes, namely [Mn4(hmp)6(CH3COO)2(H2O)2](ClO4)2.4H2O (1), [Mn4(hmp)6(CCl3COO)2(H2O)2](ClO4)2 (2) and [Mn4(hmp)6(C6H5COO)2(H2O)2](ClO4)2.4CH3CN.2H2O (3) in the presence of either bulky carboxylate or of an excess of Mn(II) source; and (ii) two 1D arrangements of [Mn4] complexes connected through double syn-syn carboxylate bridges when using acetate and chloroacetate, namely {[Mn4(hmp)6(CH3COO)2](ClO4)2.H2O}n (4) and {[Mn4(hmp)6(ClCH2COO)2](ClO4)2.2H2O}n (5). The assembly of such building blocks can thus be controlled by an adequate choice of the bridging anion. As expected, the isolated [Mn4] complexes behave as Single-Molecule Magnets as shown by the study of their static and dynamic magnetic properties. Detailed magnetic studies both on polycrystalline samples and single crystals show that the chain compounds are isolated antiferromagnetic chains. The slow relaxation of their staggered magnetization has been studied thanks to finite-size effects induced by the intrinsic defects of the material     

Heterometallic cubane single-molecule magnets

Two new heterometallic cubane molecules have been synthesized. High-frequency electron paramagnetic resonance and magnetization measurements indicate that [Mn(3)Ni(hmp)(3)O(N(3))(3)(C(7)H(5)O(2))(3)] (1) displays a well-isolated S = 5 ground state (DeltaE > 120 K), with g = 2.0, D = -0.23 cm(-1), and ferromagnetic Mn-Mn exchange interactions competing with antiferromagnetic Ni-Mn interactions. [Mn(3)Zn(hmp)(3)O(N(3))(3)(C(3)H(5)O(2))(3)] (2) possesses a S = 6 ground state (DeltaE > 105 K), with g = 2.0, D = -0.14 cm(-1), and ferromagnetic Mn-Mn exchange interactions. Magnetization vs magnetic field data for oriented single crystals of 1 and 2 indicate that both complexes are single-molecule magnets.     

Mixed-valence tetranuclear manganese single-molecule magnets

The preparations, X-ray structures, and detailed physical characterizations are presented for two new mixed-valence tetranuclear manganese complexes that function as single-molecule magnets (SMM's): [Mn4(hmp)6Br2(H2O)2]Br2-4H2O (2) and [Mn4(6-me-hmp)6Cl4]-4H2O (3), where hmp(-) is the anion of 2-hydroxymethylpyridine and 6-me-hmp(-) is the anion of 6-methyl-2-hydroxymethylpyridine. Complex 2-4H2O crystallizes in the space group P2(1)/c, with cell dimensions at -160 degrees C of a = 10.907(0) A, b = 15.788(0) A, c = 13.941(0) A, beta = 101.21(0) degrees, and Z = 2. The cation lies on an inversion center and consists of a planar Mn4 rhombus that is mixed-valence, Mn2(III)Mn2(II). The hmp(-) ligands function as bidentate ligands and as the only bridging ligands in 2-4H2O. Complex 3-4H2O crystallizes in the monoclinic space group C2/c, with cell dimensions at -160 degrees C of a = 17.0852(4) A, b = 20.8781(5) A, c = 14.835(3) A, beta = 90.5485(8) degrees, and Z = 4. This neutral complex also has a mixed-valence Mn2(III)Mn2(II) composition and is best described as having four manganese ions arranged in a bent chain. A mu2-oxygen atom of the 6-me-hmp(-) anion bridges between the manganese ions; the Cl(-) ligands are terminal. Variable-field magnetization and high-frequency and -field EPR (HFEPR) data indicate that complex 2-4H2O has a S = 9 ground state whereas complex 3.4H(2)O has S = 0 ground state. Fine structure patterns are seen in the HFEPR spectra, and in the case of 2.4H(2)O it was possible to simulate the fine structure assuming S = 9 with the parameters g = 1.999, axial zero-field splitting of D/k(B) = -0.498 K, quartic longitudinal zero-field splitting of B4(omicron)/k(B) = 1.72 x 10(-5) K, and rhombic zero-field splitting of E/k(B) = 0.124 K. Complex 2-4H2O exhibits a frequency-dependent out-of-phase AC magnetic susceptibility signal, clearly indicating that this complex functions as a SMM. The AC susceptibility data for complex 2-4H2O were measured in the 0.05-4.0 K range and when fit to the Arrhenius law, gave an activation energy of DeltaE = 15.8 K for the reversal of magnetization. This DeltaE value is to be compared to the potential-energy barrier height of U/k(B) = absolute value DSz(2) = 40.3 K calculated for 2-4H2O.     

Wheel-shaped Mn16 single-molecule magnets

The syntheses, structures, and magnetic properties of two new single-stranded hexadecanuclear manganese wheels [Mn16(CH3COO)8(CH3CH2CH2COO)8(teaH)12] x 10 MeCN (1 x 10 MeCN) and [Mn16((CH3)2CHCOO)16(teaH)12] x 4 CHCl3 (2 x 4 CHCl3), where teaH(2-) is the dianion of triethanolamine, are reported. 1 crystallizes in the tetragonal I4(1)/a space group [a = b = 33.519(4) A and c = 16.659(2) A]. 2 crystallizes in the monoclinic C2/c space group [a = 21.473(5), b = 26.819(6), c = 35.186(7), and beta = 93.447(5) degrees]. Both complexes consist of 8 Mn(II) and 8 Mn(III) ions alternating in a wheel-shaped topology with 12 monoprotonated triethanolamine ligands. Variable-temperature direct current (DC) magnetic susceptibility data were collected in 1 T, 0.1 and 0.01 T fields, and in the 1.8-300 K temperature range for 1 and 2. Variable-temperature variable-field DC magnetic susceptibility data were obtained in the 1.8-10 K and 0.1-5 T ranges and least-squares fitting of these reduced magnetization versus H/T data indicates a S = 13 ground-state for 1 and 2. Single-crystal magnetization hysteresis measurements were performed in a 0.04-1 K temperature range for complex 2. Hysteresis loops were observed that showed a temperature dependence, which indicates that 2 exhibits magnetization relaxation and is a SMM. Both 1 and 2 show frequency-dependent out-of-phase signals in the AC susceptibility measurements, collected in a temperature range of 1.8-5 K and in the frequency range of 50-10,000 Hz. Extrapolation of the in-phase component of the AC susceptibility data to 0 K indicates an S = 12 ground state for 1 and an S = 11 ground-state for 2. Complex 1 has the highest-spin ground state reported to date for a single-stranded manganese wheel and is likely to be an SMM based on a frequency-dependent out-of-phase signal in the AC susceptibility. The AC susceptibility as well as magnetization hysteresis data for 2 confirm that this species is an SMM.     

Isolated single-molecule magnets on native gold

The incorporation of thioether groups in the structure of a Mn12 single-molecule magnet, [Mn12(O12)(L)16(H2O)4] with L = 4-(methylthio)benzoate, is a successful route to the deposition of well-separated clusters on native gold surfaces and to the addressing of individual molecules by scanning tunnelling microscopy.     

Search for new iron single-molecule magnets

The synthesis, X-ray structure, and magnetic properties of a trinuclear iron complex with the formulation [Fe₃O₂Cl₂(4,7-Me-phen)₆](BF₄)₃ (complex 1) are reported. DC magnetic susceptibility measurements show the Fe atoms are antiferromagnetically coupled, yielding an S=5/2 ground state. An investigation as to whether complex 1 exhibits the properties associated with single-molecule magnetism was undertaken. Detailed high frequency EPR experiments were carried out to determine the spin Hamiltonian parameters associated with the S=5/2 spin ground state. Analysis of the temperature dependence of the transitions seen with the magnetic field oriented along the easy axis (z axis) of the Fe₃ complex confirm that the molecule has a positive D value. A fit of the frequency dependence of the resonances afforded the following spin Hamiltonian parameters: S=5/2, g_z=1.95, g_x=g_y=2.00, D=0.844 cm⁻¹, E=±0.117 cm⁻¹, and B₄ ⁰=−2.7×10⁻⁴ cm⁻¹. Low temperature magnetization versus magnetic field data confirm that complex 1 has no barrier towards magnetization reversal. Thus, it is concluded that, due to the positive D-value, complex 1 is not a single-molecule magnet.     

Two frustrated, bitetrahedral single-molecule magnets

Two unusual mixed-valent {MnIII6MnII} bitetrahedra display frustrated magnetic exchange, leading to S = 13/2 +/- 1 and 11/2 +/- 1 ground states and slow magnetization relaxation.     

Mononuclear lanthanide single-molecule magnets based on polyoxometalates

[ErW10O36]9- is the first polyoxometalate behaving as a single-molecule magnet (SMM). It shows frequency-dependent out-of-phase magnetization and a thermally activated single relaxation process with an effective barrier of 55.8 K. This single lanthanide ion polyoxometalate is the inorganic analogue of the bis(phthalocyaninato)lanthanide SMMs, both exhibiting very similar ligand field symmetries around the lanthanide ion (idealized D4d). It is chemically stable and offers new avenues for organization and processing of single-molecule magnets. Furthermore, it can be made free from nuclear spins and opens the possibility to be used for studies of decoherence on unimolecular qubits.     

Wheel-shaped [Mn12] single-molecule magnets

The reaction of [Mn(12)O(12)(O(2)CCH(3))(16)(H(2)O)(4)].4H(2)O.2CH(3)COOH with n-methyldiethanol amine (H(2)mdea), n-ethyldiethanol amine (H(2)edea), or n-butyldiethanol amine (H(2)bdea) leads to the formation of wheel-shaped Mn(III)(6)Mn(II)(6) complexes with the general formula [Mn(12)(R)(O(2)CCH(3))(14)] (1, R = mdea; 2, R = edea; and 3, R = bdea). Complex 1 crystallizes in the triclinic space group P1, whereas complex 3 crystallizes in the monoclinic space group C(2/c). Complex 1a has the same molecular structure as complex 1 but crystallizes in the monoclinic space group P2(1/n). Complex 3a has the same molecular structure as complex 3 but crystallizes in the triclinic space group P1. Variable-temperature magnetic susceptibility data collected for complexes 1, 2, and 3 indicate that antiferromagnetic exchange interactions are present. The spin ground states of complexes 1, 2, and 3 were determined by fitting variable-field magnetization data collected in the 2-5 K temperature range. Fitting of these data yielded the spin ground-state parameters of S = 8, g = 2.0, and D = -0.47 cm(-1) for complex 1; S = 8, g = 2.0, and D = -0.49 cm(-1) for complex 2; and S = 8, g = 2, and D = -0.37 cm(-1) for complex 3. The ac magnetic susceptibility data were measured for complexes 1, 2, and 3 at temperatures between 1.8 and 10 K with a 3 G ac field oscillating in the range 50-1000 Hz. Slow kinetics of magnetization reversal relative to the frequency of the oscillating ac field were observed as frequency-dependent out-of-phase peaks for complexes 1, 2, and 3, and it can be concluded that these three complexes are single-molecule magnets.     

Exchange bias in Ni4 single-molecule magnets

The syntheses and physical properties are reported for three single-molecule magnets (SMMs) with the composition [Ni(hmp)(ROH)Cl]₄, where R is CH₃ (complex 1), CH₂CH₃ (complex 2) or CH₂CH₂C(CH₃)₃ (complex 3) and hmp⁻ is the monoanion of 2-hydroxymethylpyridine. The core of each complex is a distorted cube formed by four Ni^II ions and four alkoxide hmp⁻ oxygen atoms at alternating corners. Ferromagnetic exchange interactions give a S=4 ground state. Single crystal high-frequency EPR spectra clearly indicate that each of the complexes has a S=4 ground state and that there is negative magnetoanisotropy, where D is negative for the axial zero-field splitting DŜ_z². Magnetization versus magnetic field measurements made on single crystals with a micro-SQUID magnetometer indicate these Ni₄ complexes are SMMs. Exchange bias is seen in the magnetization hysteresis loops for complexes 1 and 2.     

Heterospin single-molecule magnets with extra-large anisotropic barrier

The mixtures of Co(X-hfpip)₂; X = I and H, and bisdiazo-dipyridine ligands, D2py₂(TBA), in 1:1 ratios gave the discrete cobalt complexes, 1 and 2, respectively. The molecular structures for 1 and 2 revealed by X-ray crystallography were cyclic 2:2 cobalt complexes formulated as [(Co(X-hfpip)₂)₂(D2py₂(TBA))₂], in which the cobalt units were compressed octahedra. After irradiation of the microcrystalline samples, the resulting Co-carbene complexes, 1c and 2c, showed SMM behaviors exhibiting slow magnetic relaxations. In dc and ac magnetic susceptibility experiments, the activation barrier, U_eff, for reorientation of the magnetic moment were estimated to be 139 and 135 K for 1c and 2c, respectively, and the hysteresis loops of the magnetization (the coercive force, H_c, and 26 and 15 kOe at 1.9 K for 1c and 2c, respectively) were observed. In addition, the values of the quantum tunneling time, τ_Q, were determined to be 1.1 × 10⁵ and 5.4 × 10⁵ s (t_1/2 = 21 and 104 h) for 1c and 2c, respectively, below 2.5 K.     

Metalloligands for designing single-molecule and single-chain magnets

2-Hydroxy-N-{2-[2-hydroxyethyl)amino]ethyl}benzamide (L1H?) and 2-hydroxy-N-{2-[2-hydroxybutyl)amino]ethyl}benzamide (L2H?) ligands coordinate to copper ions to give anionic metalloligands, L1Cu? and L2Cu?, after deprotonation of their amide, alcohol and phenol functions. In presence of ancillary ligands as diketones, these metalloligands react with lanthanide salts to yield tetranuclear [Cu-Ln]? complexes while in presence of lanthanide salts alone, L1Cu? gives 1D chains of tetranuclear entities linked through the Ln ions. The micro-SQUID data evidence the SMM behavior of the tetranuclear Tb complexes and the SCM behavior of the Tb chain derivative.     

High-spin molecules with magnetic anisotropy toward single-molecule magnets

High-spin molecules with easy-axis magnetic anisotropy show slow magnetic relaxation of spin-flipping along the axis of magnetic anisotropy and are called single-molecule magnets (SMMs). SMMs behave as molecular-size permanent magnets at low temperature and magnetic relaxation occurs by quantum tunneling processes; such molecules are promising candidates for use in quantum devices. We first discuss intramolecular ferromagnetic interactions for preparing high-spin molecules. Second, we determine the magnetic anisotropy for single metal ions with d(n) configurations and discuss how molecular anisotropy arises from single-ion anisotropy of the assembled component metal ions.     

Polyoxometalate-supported 3d-4f heterometallic single-molecule magnets

The reactions of [CuTbL(Schiff)(H(2)O)(3)Cl(2)]Cl complexes with A- or B-type Anderson polyoxoanions lead to new polyoxometalate-supported 3d-4f heterometallic systems with single-molecule-magnet behavior.     

New Mn12 single-molecule magnets from edge-sharing bioctahedra

Dodecanuclear Mn clusters based on edge-sharing bioctahedra display the scan-rate- and temperature-dependent hysteresis loops that are indicative of single-molecule magnetism behaviour.