A family of dodecanuclear Mn11Ln single-molecule magnets

A series of four isostructural dodecanuclear complexes [Mn^III₉Mn^II₂Ln^III(O)₈(OH)(piv)₁₆(NO₃)(CH₃CN)]·xCH₃CN·yC₇H₁₆ (piv = pivalate; x = ½, y = ¾, Ln = Tb (1); x = 2, y = ½, Ln = Dy (2), Ho (3), and Y (4)) has been prepared for which the structural motif described as ‘a lanthanide ion nested in a large manganese shell’ is observed. All compounds show out-of-phase signals in their ac susceptibilities, and their single-molecule magnet behaviour was confirmed by single-crystal micro-SQUID studies of 1-3 which show hysteresis loops of molecular origin at T < 1.0 K. The SMM behaviour observed in compounds 1-3 is more pronounced than that for 4, which contains the diamagnetic Y^III ion. This is principally the result of ferromagnetic coupling between the paramagnetic anisotropic Ln^III ions (Tb^III, Dy^III and Ho^III) and the manganese shell, which enhances the total spin ground state of the complexes.     

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.     

A family of hexanuclear Mn(III) single-molecule magnets

In an attempt to employ salicylic acid (HOsalH), 2,6-dihydroxy benzoic acid {(HO)₂PhCO₂H}, and naphthalene-1,8-dicarboxylic acid {1,8-naph(CO₂H)₂} in Mn(III) salicylaldoximate chemistry as a means to alter the structural identity of the hexanucluear clusters usually obtained from this reaction system, we have isolated a family of hexanuclear Mn(III) complexes based on salicyladloxime (saoH₂) and 2-hydroxy-1-naphthaldehyde oxime (naphthsaoH₂). Five hexanuclear clusters, [Mn₆O₂(sao)₆(HOsal)₂(EtOH)₄]·EtOH (1·EtOH), [Mn₆O₂(sao)₆{1,8-naph(CO₂Me)(CO₂)}₂(MeOH)₆]·3MeOH (2·3MeOH), [Mn₆O₂(naphthsao)₆{1,8-naph(CO₂Et)(CO₂)}₂(EtOH)₆] (3·2MeOH), [Mn₆O₂(naphthsao)₆(MeCO₂)₂(EtOH)₄]·2H₂O (4·2H₂O), and [Mn₆O₂(naphthsao)₆{(HO)₂PhCO₂}₂(EtOH)₄]·4EtOH (5·4EtOH), have been synthesized and characterized by single-crystal X-ray crystallography. The magnetic properties of 3, 4, and 5 are discussed.     

A family of Mn16 single-molecule magnets from a reductive aggregation route

The synthesis and magnetic properties of three isostructural hexadecametallic manganese clusters [Mn(16)O(16)(OMe)(6)(O(2)CCH(2)Ph)(16)(MeOH)(6)] (1), [Mn(16)O(16)(OMe)(6)(O(2)CCH(2)Cl)(16)(MeOH)(6)] (2), and [Mn(16)O(16)(OMe)(6)(O(2)CCH(2)Br)(16)(MeOH)(6)] (3) are reported. The complexes were prepared by a reductive aggregation reaction involving phenylacetic acid, chloroacetic acid or bromoacetic acid, and NBu(n)()(4)MnO(4) in MeOH. Complex 1 crystallizes in the monoclinic space group C2/c and consists of 6 Mn(IV) and 10 Mn(III) ions held together by 14 mu(3)-O(2)(-), 2 mu-O(2)(-), 4 mu-MeO(-), and 2 mu-O(2)CCH(2)Ph(-) groups. The remaining 14 mu-O(2)CCH(2)Ph(-) ligands, 2 mu-MeO(-) groups, and 6 terminal MeOH molecules constitute the peripheral ligation in the complex. Variable-temperature, solid-state dc magnetic susceptibility measurements on 1-3 in the temperature range 5.0-300 K reveal that all three complexes are dominated by intramolecular antiferromagnetic exchange interactions. Low-lying excited states preclude an exact determination of the spin ground state for 1-3 by magnetization measurements. Alternating current susceptibility measurements at zero dc field in the temperature range 1.8-10 K and a 3.5 G ac field oscillating at frequencies in the 5-1488 Hz range display, at temperatures below 3 K, a nonzero, frequency-dependent chi(M)"signal for complexes 1-3, with the peak maxima lying at temperatures less than 1.8 K. Single-crystal magnetization versus dc field scans down to 0.04 K for complex 1 show hysteresis behavior at <1 K, establishing 1 as a new member of the SMM family. No clear steps characteristic of quantum tunneling of magnetization (QTM) were observed in the hysteresis loops.     

Single-crystal 55Mn NMR spectra of two Mn12 single-molecule magnets

The initial application is reported of single-crystal 55Mn NMR spectroscopy, and associated orientation dependence studies, to single-molecule magnets (SMMs). The studies were performed on two members of the Mn12 family of SMMs, [Mn12O12(O2CMe)16(H2O)4].2MeCO2H.4H2O (Mn12-Ac) and [Mn12O12(O2CCH2Br)16(H2O)4].4CH)Cl) (Mn12-BrAc). Single-crystal spectra give a dramatic improvement in the spectral resolution over oriented powder spectra, allowing the clear observation of quadrupolar splittings, the determination of quadrupole coupling parameters (e2qQ), and an assessment of the symmetry-lowering perturbation of the core of Mn12-Ac by hydrogen-bonding interactions with lattice solvate molecules of crystallization. The results emphasize the utility of single-crystal NMR studies to probe the cores of these magnetic molecules.     

A comparison between high-symmetry Mn12 single-molecule magnets in different ligand/solvent environments

We present angle-resolved high-frequency electron paramagnetic resonance (HFEPR) data collected for single-crystal samples of deuterated and undeuterated Mn₁₂-acetate. The spectra reveal fine structures associated with the various Mn₁₂ species corresponding to the different local solvent environments proposed by Cornia et al. [A. Cornia, R. Sessoli, L. Sorace, D. Gatteschi, A.L. Barra, C. Daiguebonne, Phys. Rev. Lett. 89 (2002) 257201], and recently confirmed by Hill et al. [S. Hill, R.S. Edwards, S.I. Jones, J.M. North, N.S. Dalal, Phys. Rev. Lett. 90 (2003) 217204] and del Barco et al. [E. del Barco, A.D. Kent, E.M. Rumberger, D.N. Hendrickson, G. Christou, Phys. Rev. Lett. 91 (2003) 047203]. Each of the fine structures exhibits a distinct dependence on the applied field orientation, thereby highlighting the discrete nature of the disorder. We then compare these data with spectra obtained for two recently discovered high-symmetry (S₄) analogs of Mn₁₂-Ac, differing only in their ligand and solvent structures. None of the highly reproducible fine-structures seen in the Mn₁₂-Ac samples are found for the new Mn₁₂ complexes, thus confirming the idea that the solvent structure significantly influences the QMT dynamics in Mn₁₂-Ac. Indeed, the HFEPR spectra for Mn₁₂-BrAc and Mn₁₂-tBuAc provide spectacular resolution, allowing for unique spectroscopic insights into high-symmetry giant spin SMMs.     

Cationic Mn12 single-molecule magnets and their polyoxometalate hybrid salts

A carboxy-substituted alkylammonium salt, namely, (4-carboxybenzyl)tributylammonium hexafluorophosphate, ZHPF(6), was prepared and used as incoming carboxylate ligand in a ligand-exchange reaction with [Mn(12)O(12)(O(2)CCH(3))(16)(H(2)O)(4)] (1) to afford a new Mn(12) single-molecule magnet (SMM), [Mn(12)O(12)(Z)(16)(H(2)O)(4)][PF(6)](16) (2), bearing 16 cationic units appended in the periphery. This compound behaves as a single-molecule magnet, exhibiting an out-of-phase ac magnetic susceptibility chi' '(M) signal that shows a single maximum in the 3.1-5.4 K temperature range. The frequency dependence of the maximum follows an Arrhenius law, with an effective energy barrier for reorientation of the spins U(eff) = 53 K. The reduced magnetization versus H/T data at different temperatures were fitted by using a Hamiltonian containing Zeeman, axial, and quartic zero-field splitting terms. The expected spin ground state S = 10 was found, and the least-squares fit afforded the following zero-field-splitting parameters: D = -0.44 cm(-1); B(4)(0) = 0.12 x 10(-4) cm(-1). Magnetization hysteresis loops were observed for 2, with a coercive field H(c) = 0.34 T. Complex 2 has been used as countercation in the preparation of a family of hybrid salts containing different polyoxometalate anions, [Mn(12)O(12)(Z)(16)(H(2)O)(4)][W(6)O(19)](8) (3), [Mn(12)O(12)(Z)(16)(H(2)O)(4)][PW(12)O(40)](16/3) (4), [Mn(12)O(12)(Z)(16)(H(2)O)(4)][(H(3)O)PW(11)O(39)Ni](4) (5), and [Mn(12)O(12)(Z)(16)(H(2)O)(4)][(H(3)O)PW(11)O(39)Co](4) (6). 3-6 exhibit typical magnetic hysteresis loops with higher coercive fields for the complexes containing diamagnetic polyanions: H(c) = 0.075 T (3), 0.046 T (4), 0.016 T (5), and 0.0075 T (6). However, the dynamics of the magnetic behavior below the blocking temperature is similar in all compounds. Broad frequency-dependent out-of-phase ac susceptibility signals are observed, presumably due to mixtures of different Jahn-Teller isomers. Their temperature dependence is also typical of an activated-energy process, with effective energy barriers in the 50 K range, irrespective of the nature of the polyoxoanion (diamagnetic, as in 3 and 4, or paramagnetic, as in 5 and 6). These findings seem to discard any influence of the polyoxometalate in the magnetic properties of the SMM.     

Theoretical insights into the magnetostructural correlations in Mn3-based single-molecule magnets

Density functional theory (DFT) and the valence bond configuration interaction (VBCI) model have been applied to the oximato-based Mn(III)(3)O single-molecule magnets (SMMs), allowing one to correlate the Mn(III)-Mn(III) exchange coupling energy (J) with the bridging geometry in terms of two structural angles: the Mn-O-N-Mn torsion angle (γ) and the Mn(3) out-of-plane shift of O (angle δθ). Using DFT, a two-dimensional (γ, δθ) energy surface of J is derived and shown to yield essentially good agreement with the reported J values deduced from magnetic susceptibility data on trigonal oximato-bridged Mn(3) SMMs. VBCI is used to understand and analyze the DFT results. It is shown that the exchange coupling in these systems is governed by a spin-polarization mechanism inducing a pronounced and dominating ferromagnetic exchange via the oximato bridge as opposed to kinetic exchange, which favors a weaker and antiferromagnetic exchange via the bridging oxide. In the light of these results, a discussion of the exchange coupling in the Mn(6) family of the SMM with a record demagnetization barrier is given.     

Grafting derivatives of Mn6 single-molecule magnets with high anisotropy energy barrier on Au111 surface

We study the magnetic properties of two new functionalized single-molecule magnets belonging to the Mn 6 family (general formula [Mn (III)6O2(R-sao)6(O2C-th)2L(4-6)], where R=H (1) or Et (2), HO2C-th=3-thiophene carboxylic acid, L=EtOH, H2O and saoH2 is salicylaldoxime) and their grafting on the Au(111) surface. Complex 1 exhibits spin ground-state S=4, as the result of ferromagnetic coupling between the two antiferromagnetic Mn (III) 3 triangles, while slight structural changes in complex 2, switch the dominant magnetic exchange interactions from anti- to ferromagnetic, enhancing the spin ground-state to S=12 and, consequently, the effective energy barrier for the relaxation of magnetization. Direct-current and alternating-current magnetic susceptibility measurements show that the functionalized complexes preserve the main magnetic properties of the corresponding not-functionalized Mn 6 clusters (i.e., total spin value and magnetic behavior as a function of temperature), though a reduction of the anisotropy barrier is observed in complex 2. For both complexes, the -O2C-th functionalization allows the direct grafting on Au(111) surface by liquid-phase deposition. X-ray photoemission spectroscopy demonstrates that the stoichiometry of the molecular cores is preserved after grafting. Scanning tunneling microscopy (STM) reveals a sub-monolayer distribution of isolated clusters with a slightly higher coverage for complex 1. The cluster stability in the STM images and the S-2p energy positions demonstrate, for both derivatives, the strength of the grafting with the gold surface.     

Single-molecule magnets: structural characterization, magnetic properties, and (19)F NMR spectroscopy of a Mn(12) family spanning three oxidation levels

The syntheses, crystal structures, and magnetic properties of [Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)] (2), (NMe(4))[Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)] (3), and (NMe(4))(2)[Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)] (4) are reported. Complex 2 displays quasi-reversible redox couples when examined by cyclic voltammetry in CH(2)Cl(2): one-electron reductions are observed at 0.64 and 0.30 V vs ferrocene. The reaction of complex 2 with 1 and 2 equiv of NMe(4)I yields the one- and two-electron reduced analogues, 3 and 4, respectively. Complexes 2.3CH(2)Cl(2), 3.4.5CH(2)Cl(2).(1)/(2)H(2)O, and 4.6C(7)H(8) crystallize in the triclinic P, monoclinic P2/c, and monoclinic C2/c space groups, respectively. The molecular structures are all very similar, consisting of a central [Mn(IV)O(4)] cubane surrounded by a nonplanar alternating ring of eight Mn and eight mu(3)-O(2)(-) ions. Peripheral ligation is provided by 16 bridging C(6)F(5)CO(2)(-) and 4 H(2)O ligands. Bond valence sum calculations establish that the added electrons in 3 and 4 are localized on former Mn(III) ions giving trapped-valence Mn(IV)(4)Mn(III)(7)Mn(II) and Mn(IV)(4)Mn(III)(6)Mn(II)(2) anions, respectively. (19)F NMR spectroscopy in CD(2)Cl(2) shows retention of the solid-state structure upon dissolution and detrapping of the added electrons in 3 and 4 among the outer ring of Mn ions on the (19)F NMR time scale. DC studies on dried microcrystalline samples of 2, 3, and 4.2.5C(7)H(8) restrained in eicosane in the 1.80-10.0 K and 1-70 kG ranges were fit to give S = 10, D = -0.40 cm(-)(1), g = 1.87, D/g = 0.21 cm(-)(1) for 2, S = 19/2, D = -0.34 cm(-)(1), g = 2.04, D/g = 0.17 cm(-)(1) for 3, and S = 10, D = -0.29 cm(-)(1), g = 2.05, D/g = 0.14 cm(-)(1) for 4, where D is the axial zero-field splitting parameter. The clusters exhibit out-of-phase AC susceptibility signals (chi(M)' ') indicative of slow magnetization relaxation in the 6-8 K range for 2, 4-6 K range for 3, and 2-4 K range for 4; the shift to lower temperatures reflects the decreasing magnetic anisotropy upon successive reduction and, hence, the decreasing energy barrier to magnetization relaxation. Relaxation rate vs T data obtained from chi(M)' ' vs AC oscillation frequency studies down to 1.8 K were combined with rate vs T data from DC magnetization decay vs time measurements at lower temperatures to generate an Arrhenius plot from which the effective barrier (U(eff)) to magnetization reversal was obtained; the U(eff) values are 59 K for 2, 49 and 21 K for the slower- and faster-relaxing species of 3, respectively, and 25 K for 4. Hysteresis loops obtained from single-crystal magnetization vs DC field scans are typical of single-molecule magnets with the coercivities increasing with decreasing T and increasing field sweep rate and containing steps caused by the quantum tunneling of magnetization (QTM). The step separations gave D/g values of 0.22 cm(-)(1) for 2, 0.15 and 0.042 cm(-)(1) for the slower- and faster-relaxing species of 3, and 0.15 cm(-)(1) for 4.     

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     

[Mn18]2+ and [Mn21]4+ single-molecule magnets

The synthesis and structural characterization of the two new Mn complexes [Mn₁₈O₁₄(O₂CMe)₁₈(hep)₄(hepH)₂(H₂O)₂](ClO₄)₂ (1) and [Mn₂₁O₁₆(O₂CMe)₁₆(hmp)₆(hmpH)₂(pic)₂(py)(H₂O)](ClO₄)₄ (3) are presented, together with a detailed study of their magnetic properties. Complex 1 possesses a ground-state spin of S=13, and the ground-state spin for 3 is estimated to be S=17/2 or 19/2. Both complexes 1 and 3 are new examples of single-molecule magnets (SMMs), displaying frequency-dependent out-of-phase AC signals, as well as magnetization vs. DC field hysteresis at temperatures below 1 K. Complex 1 straddles the classical/quantum interface by also displaying quantum tunneling of the magnetization (QTM).     

A family of enneanuclear iron(II) single-molecule magnets

Complexes [Fe₉(X)₂(O₂CMe)₈{(2‐py)₂CO₂}₄] (X^?=OH^? ( 1 ), N₃^? ( 2 ), and NCO^? ( 3 )) have been prepared by a route previously employed for the synthesis of analogous Co₉ and Ni₉ complexes, involving hydroxide substitution by pseudohalides (N₃^?, NCO^?). As indicated by DC magnetic susceptibility measurements, this substitution induced higher ferromagnetic couplings in complexes 2 and 3 , leading to higher ground spin states compared to that of 1 . Variable‐field experiments have shown that the ground state is not well isolated from excited states, as a result of which it cannot be unambiguously determined. AC susceptometry has revealed out‐of‐phase signals, which suggests that these complexes exhibit a slow relaxation of magnetization that follows Arrhenius behavior, as observed in single‐molecule magnets, with energy barriers of 41 K for 2 (τ₀=3.4×10^?12 s) and 44 K for 3 (τ₀=2.0×10^?11 s). Slow magnetic relaxation has also been observed by zero‐field ⁵⁷Fe Mössbauer spectroscopy. Characteristic integer‐spin electron paramagnetic resonance (EPR) signals have been observed at X‐band for 1 , whereas 2 and 3 were found to be EPR‐silent at this frequency. ¹H NMR spectrometry in CD₃CN has shown that complexes 1 – 3 are stable in solution.     

A ligand-field study of the ground spin-state magnetic anisotropy in a family of hexanuclear Mn(III) single-molecule magnets.

A ligand field analysis of two structurally related hexanuclear Mn(iii) coordination complexes reveals that the observed difference in their ground spin-state anisotropy originates from the difference in projection coefficients of the single-ion anisotropy to spin states of different total spin quantum-number, S, rather than the geometrical distortions of the metal ions. Furthermore we show that the single-ion second order anisotropy induces fourth and higher order anisotropy terms to the ground spin states of the studied systems, as a consequence of spin-state mixing effects due to the comparable magnitude of the single-ion second order anisotropy and the isotropic exchange parameters.     

Mn4 single-molecule magnets with a planar diamond core and S=9

The syntheses and magnetic properties are reported for three Mn₄ single-molecule magnets (SMMs): [Mn₄(hmp)₆(NO₃)₂(MeCN)₂](ClO₄)₂·2MeCN (3), [Mn₄(hmp)₆(NO₃)₄]·(MeCN) (4), and [Mn₄(hmp)₄(acac)₂(MeO)₂](ClO₄)₂·2MeOH (5). In each complex there is a planar diamond core of Mn^III ₂Mn^II ₂ ions. An analysis of the variable-temperature and variable-field magnetization data indicate that all three molecules have intramolecular ferromagnetic coupling and a S=9 ground state. The presence of a frequency-dependent alternating current susceptibility signal indicates a significant energy barrier between the spin-up and spin-down states for each of these three Mn^III ₂Mn^II ₂ complexes. The fact that these complexes are SMMs has been confirmed by the observation of hysteresis in the plot of magnetization versus magnetic field measured for single crystals of complexes 3 and 4. The hysteresis loops for both of these complexes exhibit steps characteristic of quantum tunneling of magnetization. Complex 4 shows its first step at zero field, whereas the first step for complex 3 is shifted to −0.10 T. This shift is attributable to weak intermolecular antiferromagnetic exchange interactions present for complex 3.