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.     

Chiral single-molecule magnets: a partial Mn(III) supertetrahedron from achiral components

A [Mn(III)(9)] partial supertetrahedron is a Single-Molecule Magnet (SMM) with an energy barrier to magnetisation reversal of ~30 K and represents the first chiral SMM obtained from achiral starting materials.     

Molecular wheels: new Mn12 complexes as single-molecule magnets

The preparation, structure and magnetic properties of three new wheel-shaped dodecanuclear manganese complexes, [Mn12(Adea)8(CH3COO)14] x 7 CH3CN (1 x 7CH3CN), [Mn12(Edea)8(CH3CH2COO)14] (2) and [Mn12(Edea)8(CH3COO)2(CH3CH2COO)12] (3), are reported, where Adea(2-) and Edea(2-) are dianions of the N-allyl diethanolamine and the N-ethyl diethanolamine ligands, respectively. Each complex has six Mn(II) and six Mn(III) ions alternating in a wheel-shaped topology, with eight n-substituted diethanolamine dianions. All variable-temperature direct current (DC) magnetic susceptibility data were collected in 1, 0.1, or 0.01 T fields and in the 1.8-300 K temperature range. Heat capacity data, collected in applied fields of 0-9 T and in the 1.8-100 K temperature range, indicate the absence of a phase-transition due to long-range magnetic ordering for 1 and 3. Variable-temperature, variable-field DC magnetic susceptibility data were obtained in the 1.8-10 K and 0.1-5 T ranges. All complexes show out-of-phase signals in the AC susceptibility measurements, collected in a 50-997 Hz frequency range and in a 1.8-4.6 K temperature range. Extrapolation to 0 K of the in-phase AC susceptibility data collected at 50 Hz indicates an S = 7 ground state for 1, 2, and 3. Magnetization hysteresis data were collected on a single crystal of 1 in the 0.27-0.9 K range and on single crystals of 2 and 3 in the 0.1-0.9 K temperature range. Discrete steps in the magnetization curves associated with resonant quantum tunneling of magnetization (QTM) confirm these complexes to be single-molecule magnets. The appearance of extra QTM resonances on the magnetic hysteresis of 1 is a result of a weak coupling between two Mn ions at opposite ends of the wheel, dividing the molecule into two ferromagnetic exchange-coupled S = 7/2 halves. The absence of these features on 2 and 3, which behave as rigid spin S = 7 units, is a consequence of different interatomic distances.     

Multiple-decker phthalocyaninato Tb(III) single-molecule magnets and Y(III) complexes for next generation devices

A new magnetic relaxation phenomenon for an Ising dimer of a Tb-phthalocyaninato triple-decker SMM Tb₂(obPc)₃ (1) is reported. In Argand plots, the magnetic relaxation splits from a one-component system into a two-component system (temperature-independent and temperature-dependent regimes) in a dc magnetic field. There was clear evidence that the magnetic relaxation mechanisms for the Tb³⁺ dimer depended heavily on the temperature and the dc magnetic field. The relationships among the molecular structure, ligand field, ground state, and SMM properties in a direct current (dc) magnetic field are discussed. Furthermore, in order to investigate the stability of the complexes in vacuum evaporation (dry) process and the control of their surface morphology after transferring to a surface, we studied the lanthanoid-phthalocyaninato triple-decker molecule Y₂Pc₃ deposited on a Au(1 1 1) surface using a low-temperature scanning tunneling microscope. It is important to both understand and control the quantum properties of Ln-Pc multiple-decker SMMs with an external field and the monolayer or multi-layer structures on a substrate for next generation devices, such as magnetic information storage.     

Hexanuclear manganese(III) single-molecule magnets from derivatized salicylamidoximes

Four novel hexanuclear manganese(III) complexes based on derivatized salicylamidoximes, [Mn^III₆(μ₃-O)₂(O₂CPh)₂(Me₂N-sao)₆(EtOH)₄] (1), [Mn^III₆(μ₃-O)₂(O₂CPh)₂(Me₂N-sao)₆(ⁱPrOH)₄] (2), [Mn^III₆(μ₃-O)₂(O₂CPh)₂(Et₂N-sao)₆(EtOH)₄] (3) and [Mn^III₆(μ₃-O)₂(O₂CPh)₂(Et₂N-sao)₆(ⁱPrOH)₄] (4) (Me₂N-Hsao = dimethylsalicylamidoxime; Et₂N-Hsao = diethylsalicylamidoxime), have been prepared and characterized. Single-crystal X-ray diffraction allows one to determine that 1·2CHCl₃ and 4 crystallize in the triclinic system with space group P(–1), whereas 3 crystallizes in the monoclinic system with space group P2₁/n. dc and ac magnetic susceptibility measurements of 1-4 reveal ferromagnetic coupling between Mn(III) metal ions and single-molecule magnet behaviour. The anisotropy barriers are 56, 52, 71 and 59 K for 1, 2, 3 and 4, respectively.     

The search for 3d-4f single-molecule magnets: synthesis, structure and magnetic properties of a [Mn(III)2Dy(III)2] cluster

Tetranuclear [Mn(III)2Ln(III)2] complexes formed using tripodal ligands display frequency dependent out-of-phase ac susceptibility signals and temperature and sweep rate dependent hysteresis loops.     

Using Ln(III)[15-MCCuII((N)(S)-pheHA(-5)]3+ complexes to construct chiral single-molecule magnets and chains of single-molecule magnets

The {DyIII[15-MC-CuII(N)(S)-pheHA(-5)]}3+ complex displays slow magnetic relaxation behavior in a frozen solution at low temperature, whereas the analogous HoIII structure does not exhibit similar behavior.     

Copper(II)-terbium(III) single-molecule magnets linked by photochromic ligands

Two assemblies composed of single-molecule magnets (SMMs) linked by photochromic ligands, [Cu(II)(2)Tb(III)(2)(L)(2)(NO(3))(2)(dae-o)(2)]·2(n-BuOH) (1) and {[Cu(II)Tb(III)(L)(n-BuOH)(0.5)](2)(dae-c)(3)}·5(DMF)·4(n-BuOH)·2(H(2)O) (2), were synthesized by reacting the SMM [Cu(II)Tb(III)(L)(NO(3))(3)] (H(2)L = 1,3-bis((3-methoxysalicylidene)amino)propane) and photochromic molecules, H(2)dae-o and H(2)dae-c, which are open- and closed-ring isomers of 1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluoropentene (H(2)dae), respectively. 1 has a tetranuclear ring-like structure comprised of two [CuTb] units and two dae-o(2-) ligands. On the other hand, 2 has a one-dimensional ladder-type structure involving the [CuTb] and dae-c(2-) units in a 3?:?2 ratio. Magnetic studies revealed that 1 and 2 had ferromagnetic interactions between the Cu(II) and Tb(III) ions and that both compounds exhibited frequency dependence of ac susceptibilities owing to freezing the magnetization of the [CuTb] SMM. Upon irradiation with ultraviolet light and visible light, an absorption band at ～600 nm changed, indicating that photochromic reactions involving the dae(2-) ligands occurred. After irradiation, the magnetic behaviour of 1 did not change, whereas magnetic behaviour of 2 changed, due to the modification of intermolecular environment.     

[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).     

Quantum tunneling and quantum phase interference in a [Mn(II)2Mn(III)2] single-molecule magnet

[Mn4(hmp)6(H2O)2(NO3)2](NO3)2.2.5H2O (1) has been synthesized from the reaction of 2-hydroxymethylpyridine (Hhmp) with Mn(NO3)2.4H2O in the presence of tetraethylammonium hydroxide. 1 crystallizes in the triclinic P space group with two crystallographically independent centrosymmetrical [Mn4(hmp)6(H2O)2(NO3)2]2+ complexes in the packing structure. Four Mn ions are arranged in a double-cuboidal fashion where outer Mn2+ are heptacoordinated and inner Mn3+ are hexacoordinated. dc magnetic measurements show that both Mn2+...Mn3+ and Mn3+...Mn3+ interactions are ferromagnetic with J(wb)/k(B) = +0.80(5) K, and J(bb)/k(B) = +7.1(1) K, respectively, leading to an S(T) = 9 ground state. Combined ac and dc measurements reveal the single-molecule magnet (SMM) behavior of 1 with both thermally activated and ground-state tunneling regimes, including quantum phase interference. In the thermally activated regime, the characteristic relaxation time (tau) of the system follows an Arrhenius law with tau0 = 6.7 x 10(-)(9) s and delta(eff)/k(B) = 20.9 K. Below 0.34 K, tau saturates indicating that the quantum tunneling of the magnetization becomes the dominant relaxation process as expected for SMMs. Down to 0.04 K, field dependence of the magnetization measured using the mu-SQUID technique shows the presence of very weak inter-SMM interactions (zJ'/k(B) approximately -1.5 x 10(-3) K) and allows an estimation of D/k(B) at -0.35 K. Quantum phase interference has been used to confirm the D value and to estimate the transverse anisotropic parameter to E/k(B) = +0.083 K and the ground-state tunnel splitting delta(LZ) = 3 x 10(-7) K at H(trans) = 0 Oe. These results rationalize the observed tunneling time (tau(QTM)) and the effective energy barrier (delta(eff)).     

A novel class of tetrairon(III) single-molecule magnets with graphene-binding groups

Tripods of general formula R’–O–CH₂C(CH₂OH)₃ are excellent site-specific ligands for the preparation of functionalized Fe₄ single-molecule magnets. Herein, we describe the synthesis and characterization of two novel complexes designed to bind graphene surfaces, in which the R group consists of an alkyl spacer –(CH₂)_n– (n = 6 and 10) and a terminal pyrenyl moiety. The site-specific ligand substitution on [Fe₄(OMe)₆(dpm)₆] (Hdpm = dipivaloylmethane) with the new tripods has been studied with ²H NMR on isotopically-enriched samples, revealing that, once formed, these clusters are stable in solution over long timescales. It was not possible to isolate the new compounds as crystalline solids, nevertheless they were chemically characterized by elemental analysis and ¹H NMR. The presence of the pyrenyl ending groups prompted us to investigate the effect of metal complexation on fluorescence, and a full pyrene-to-iron cluster excitation energy transfer was observed. The analysis of the magnetic behaviour revealed an S = 5 ground spin state with a negative zero-field splitting parameter D = ?0.42 cm^?1.     

Raising the spin-reversal barrier in cyano-bridged single-molecule magnets: linear Mn(III)(2)M(III)(CN)(6) (M = Cr, Fe) species incorporating [(5-Brsalen)Mn](+) units

Reactions between K(3)[M(CN)(6)] and [Mn(5-Brsalen)(H(2)O)(2)](+) (5-Brsalen = N,N'-ethylenebis(5-bromosalicylidene)aminato dianion) in a mixture of methanol and water afford the compounds K[(5-Brsalen)(2)(H(2)O)(2)Mn(2)M(CN)(6)].2H(2)O, with M = Cr (1) or Fe (2). The two compounds are isostructural, each containing a molecular cluster with a linear Mn(III)-NC-M(III)-CN-Mn(III) core and tetragonally elongated coordination about the Mn(III) centers. Magnetic data indicate the presence of weak exchange interactions within the clusters, giving rise to ground states of S = (5)/(2) and (9)/(2) with significant zero-field splitting. Despite the proximity of spin-excited states, ac susceptibility data reveal frequency-dependent out-of-phase signals characteristic of single-molecule magnets with spin-reversal barriers of U(eff) = 16 and 25 cm(-)(1), respectively.     

Microwave-assisted synthesis of a hexanuclear Mn(III) single-molecule magnet

The use of microwave heating has improved the reaction rate, enhanced the yield of an inorganic cluster complex, and allowed for the high-temperature/high-pressure synthesis of a Mn(III) single-molecule magnet.     

Carboxylate-free Mn(III)2Ln(III)2 (Ln = lanthanide) and Mn(III)2Y(III)2 complexes from the use of (2-hydroxymethyl)pyridine: analysis of spin frustration effects

The initial employment of 2-(hydroxymethyl)pyridine for the synthesis of Mn/Ln (Ln = lanthanide) and Mn/Y clusters, in the absence of an ancillary organic ligand, has afforded a family of tetranuclear [Mn(III)(2)M(III)(2)(OH)(2)(NO(3))(4)(hmp)(4)(H(2)O)(4)](NO(3))(2) (M = Dy, 1; Tb, 2; Gd, 3; Y; 4) anionic compounds. 1-4 possess a planar butterfly (or rhombus) core and are rare examples of carboxylate-free Mn/Ln and Mn/Y clusters. Variable-temperature dc and ac studies established that 1 and 2, which contain highly anisotropic Ln(III) atoms, exhibit slow relaxation of their magnetization vector. Fitting of the obtained magnetization (M) versus field (H) and temperature (T) data for 3 by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) showed the ground state to be S = 3. Complex 4 has an S = 0 ground state. Fitting of the magnetic susceptibility data collected in the 5-300 K range for 3 and 4 to the appropriate van Vleck equations revealed, as expected, extremely weak antiferromagnetic interactions between the paramagnetic ions; for 3, J(1) = -0.16(2) cm(-1) and J(2) = -0.12(1) cm(-1) for the Mn(III)···Mn(III) and Mn(III)···Gd(III) interactions, respectively. The S = 3 ground state of 3 has been rationalized on the basis of the spin frustration pattern in the molecule. For 4, J = -0.75(3) cm(-1) for the Mn(III)···Mn(III) interaction. Spin frustration effects in 3 have been quantitatively analyzed for all possible combinations of sign of J(1) and J(2).     

Tetracoordinate cobalt(II) complexes with neocuproine: single-molecule magnets with potential biological activity

Interaction of two mononuclear tetracoordinate complexes [Co(dmphen)Br₂] and [Co(dmphen)I₂] (dmphen = 2,9-dimethyl-1,10-phenanthroline), which have been recently reported to behave as single-molecule magnets (SMMs) in an applied external field, with calf thymus (CT) DNA in solution was studied by spectral methods (UV–Vis, fluorescence, and circular dichroism). Results indicate that both complexes along with their chlorido analogue [Co(dmphen)Cl₂] are able to bind with the CT DNA via intercalation, with the values of Stern–Volmer constants obtained from the linear quenching plot in range of 1.86 × 10⁴–2.11 × 10⁴ M^?1. Furthermore, Topoisomerase I inhibition studies suggest that all three complexes exhibit inhibition activity at concentrations of 45 μM.     

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.