Synthesis, structure, and magnetic properties of a Mn(21) single-molecule magnet

The reaction of [Mn(3)O(O(2)CMe)(6)(py)(3)](ClO(4)) (1; 3Mn(III)) with [Mn(10)O(4)(OH)(2)(O(2)CMe)(8)(hmp)(8)](ClO(4))(4) (2; 10Mn(III)) in MeCN affords the new mixed-valent complex [Mn(21)O(14)(OH)(2)(O(2)CMe)(16)(hmp)(8)(pic)(2)(py)(H(2)O)](ClO(4))(4) (3; 3Mn(II)-18Mn(III); hmp(-) is the anion of 2-(hydroxymethyl)pyridine), with an average Mn oxidation state of +2.85. Complex 3.7MeCN crystallizes in the triclinic space group P. The structure consists of a low symmetry [Mn(21)(micro(4)-O)(4)(micro(3)-O)(12)(micro-O)(16)] core, with peripheral ligation provided by 16 MeCO(2)(-), 8 hmp(-), and 2 pic(-) groups and one molecule each of water and pyridine. The magnetic properties of 3 were investigated by both dc and ac magnetic susceptibility measurements. Fitting of dc magnetization data collected in the 0.1-0.8 T and 1.8-4.0 K ranges gave S = (17)/(2), D approximately -0.086 cm(-)(1), and g approximately 1.8, where S is the molecular spin of the Mn(21) complex and D is the axial zero-field splitting parameter. ac susceptibility studies in the 10-997 Hz frequency range reveal the presence of a frequency-dependent out-of-phase ac magnetic susceptibility (chi(M)' ') signal consistent with slow magnetization relaxation rates. Fitting of dc magnetization decay versus time data to the Arrhenius equation gave a value of the effective barrier to relaxation (U(eff)) of 13.2 K. Magnetization versus applied dc field sweeps exhibited hysteresis. Thus, complex 3 is a new member of the small but growing family of single-molecule magnets.     

Slow relaxation in a one-dimensional rational assembly of antiferromagnetically coupled [Mn4] single-molecule magnets

Four discrete Mn(III)/Mn(II) tetranuclear complexes with a double-cuboidal core, [Mn(4)(hmp)(6)(CH(3)CN)(2)(H(2)O)(4)](ClO(4))(4).2CH(3)CN (1), [Mn(4)(hmp)(6)(H(2)O)(4)](ClO(4))(4).2H(2)O (2), [Mn(4)(hmp)(6)(H(2)O)(2)(NO(3))(2)](ClO(4))(2).4H(2)O (3), and [Mn(4)(hmp)(6)(Hhmp)(2)](ClO(4))(4).2CH(3)CN (4), were synthesized by reaction of Hhmp (2-hydroxymethylpyridine) with Mn(ClO(4))(2).6H(2)O in the presence of tetraethylammonium hydroxide and subsequent addition of NaNO(3) (3) or an excess of Hhmp (4). Direct current (dc) magnetic measurements show that both Mn(2+)-Mn(3+) and Mn(3+)-Mn(3+) magnetic interactions are ferromagnetic in 1-3 leading to an S(T) = 9 ground state for the Mn(4) unit. Furthermore, these complexes are single-molecule magnets (SMMs) clearly showing both thermally activated and ground-state tunneling regimes. Slight changes in the [Mn(4)] core geometry result in an S(T) = 1 ground state in 4. A one-dimensional assembly of [Mn(4)] units, catena-{[Mn(4)(hmp)(6)(N(3))(2)](ClO(4))(2)} (5), was obtained in the same synthetic conditions with the subsequent addition of NaN(3). Double chairlike N(3)(-) bridges connect identical [Mn(4)] units into a chain arrangement. This material behaves as an Ising assembly of S(T) = 9 tetramers weakly antiferromagnetically coupled. Slow relaxation of the magnetization is observed at low temperature for the first time in an antiferromagnetic chain, following an activated behavior with Delta(tau)/k(B) = 47 K and tau(0) = 7 x 10(-)(11) s. The observation of this original thermally activated relaxation process is induced by finite-size effects and in particular by the noncompensation of spins in segments of odd-number units. Generalizing the known theories on the dynamic properties of polydisperse finite segments of antiferromagnetically coupled Ising spins, the theoretical expressions of the characteristic energy gaps Delta(xi) and Delta(tau) were estimated and successfully compared to the experimental values.     

Heptanuclear and decanuclear manganese complexes with the anion of 2-hydroxymethylpyridine

The synthesis and magnetic properties are reported of two new clusters [Mn(10)O(4)(OH)(2)(O(2)CMe)(8)(hmp)(8)](ClO(4))(4) (1) and [Mn(7)(OH)(3)(hmp)(9)Cl(3)](Cl)(ClO(4)) (2). Complex 1 was prepared by treatment of [Mn(3)O(O(2)CMe)(6)(py)(3)](ClO(4)) with 2-(hydroxymethyl)pyridine (hmpH) in CH(2)Cl(2), whereas 2 was obtained from the reaction of MnCl(2).4H(2)O, hmpH, and NBu(n)(4)MnO(4) in MeCN followed by recrystallization in the presence of NBu(n)(4)ClO(4). Complex 1.2py.10CH(2)Cl(2).2H(2)O crystallizes in the triclinic space group P1. The cation consists of 10 Mn(III) ions, 8 mu(3)-O(2)(-) ions, 2 mu(3)-OH(-) ions, 8 bridging acetates, and 8 bridging and chelating hmp(-) ligands. The hmp(-) ligands bridge through their O atoms in two ways: two with mu(3)-O atoms and six with mu(2)-O atoms. Complex 2.3CH(2)Cl(2).H(2)O crystallizes in the triclinic space group P1. The cation consists of four Mn(II) and three Mn(III) ions, arranged as a Mn(6) hexagon of alternating Mn(II) and Mn(III) ions surrounding a central Mn(II) ion. The remaining ligation is by three mu(3)-OH(-) ions, three terminal chloride ions, and nine bridging and chelating hmp(-) ligands. Six hmp(-) ligands contain mu(2)-O atoms and three contain mu(3)-O atoms. The Cl(-) anion is hydrogen-bonded to the three mu(3)-OH(-) ions. Variable-temperature direct current (dc) magnetic susceptibility data were collected for complex 1 in the 5.00-300 K range in a 5 kG applied field. The chi(M)T value gradually decreases from 17.87 cm(3) mol(-1) K at 300 K to 1.14 cm(3) mol(-1) K at 5.00 K, indicating an S = 0 ground state. The ground-state spin of complex 2 was established by magnetization measurements in the 0.5-3.0 T and 1.80-4.00 K ranges. Fitting of the data by matrix diagonalization, incorporating only axial anisotropy (DS(z)(2)), gave equally good fits with S = 10, g = 2.13, D = -0.14 cm(-1) and S = 11, g = 1.94, D = -0.11 cm(-1). Magnetization versus dc field scans down to 0.04 K reveal no hysteresis attributable to single-molecule magnetism behavior, only weak intermolecular interactions.     

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.     

Mn8 and Mn16 clusters from the use of 2-(hydroxymethyl)pyridine, and comparison with the products from bulkier chelates: a new high nuclearity single-molecule magnet

The synthesis, crystal structures, and magnetochemical characterization of two new Mn clusters [Mn(8)O(2)(O(2)CPh)(10)(hmp)(4)(MeOH)(2)] (1; 6Mn(II), 2Mn(III)) and [Mn(16)O(8)(OH)(2)(O(2)CPh)(12)(hmp)(10)(H(2)O)(2)](O(2)CPh)(2) (2; 6Mn(II), 10Mn(III)) are reported. They were obtained from the use of 2-(hydroxymethyl)pyridine (hmpH) under the same reaction conditions but differing in the presence or absence of added base. Thus, the reaction of hmpH with Mn(O(2)CPh)(2) in CH(2)Cl(2)/MeOH led to isolation of octanuclear complex 1, whereas the analogous reaction in the presence of NEt(3) gave hexadecanuclear complex 2. Complexes 1 and 2 possess either very rare or unprecedented core structures that are related to each other: that of 1 can be described as a linked pair of incomplete [Mn(4)O(3)] cubanes, while that of 2 consists of a linked pair of complete [Mn(4)O(4)] cubanes, on either side of which is attached a tetrahedral [Mn(4)(μ(4)-O)] unit. Solid-state direct current (dc) and alternating current (ac) magnetic susceptibility measurements on 1 and 2 establish that they possess S = 5 and 8 ground states, respectively. Complex 2 exhibits frequency-dependent out-of-phase (χ(M)") ac susceptibility signals at temperatures below 3 K suggestive of a single-molecule magnet (SMM). Magnetization versus applied dc field sweeps on single crystals of 2·10MeOH down to 0.04 K exhibited hysteresis, confirming 2 to be a new SMM. Comparison of the structure of 2 (Mn(16)) with Mn(12) or Mn(6) clusters previously obtained under the same reaction conditions but with two Me or two Ph groups, respectively, added next to the alkoxide O atom of hmp(-) indicate their influence on the nuclearity and structure of the products as being due to the overall bulk of the chelate plus the decreased ability of the O atom to bridge.     

Cationic Mn4 single-molecule magnet with a sterically isolated core

The synthesis, structure, and magnetic properties of a ligand-modified Mn(4) dicubane single-molecule magnet (SMM), [Mn(4)(Bet)(4)(mdea)(2)(mdeaH)(2)](BPh(4))(4), are presented, where the cationic SMM units are significantly separated from neighboring molecules in the crystal lattice. There are no cocrystallized solvate molecules, making it an ideal candidate for single-crystal magnetization hysteresis and high-frequency electron paramagnetic resonance studies. Increased control over intermolecular interactions in such materials is a crucial factor in the future application of SMMs.     

Structure and magnetic properties of the two-dimensional ferrimagnet (NEt4)[[Mn(salen)]2Fe(CN)6]: investigation of magnetic anisotropy on a single crystal

The title compound, (NEt(4))[[Mn(salen)](2)Fe(CN)(6)] (1), was synthesized via a 1:1 reaction of [Mn(salen)(H(2)O)]ClO(4) with (NEt(4))(3)[Fe(CN)(6)] in a methanol/ethanol medium (NEt(4)(+) = tetraethylammonium cation, salen(2)(-) = N,N'-ethylenebis(salicylidene)iminate). The two-dimensional layered structure of 1 was revealed by X-ray crystallographic analysis: 1 crystallizes in monoclinic space group P2(1)/c with cell dimensions of a = 12.3660(8) A, b = 15.311(1) A, c = 12.918(1) A, beta = 110.971(4) degrees, Z = 2 and is isostructural to the previously synthesized compound, (NEt(4))[[Mn(5-Clsalen)](2)Fe(CN)(6)] (5-Clsalen(2-) = N,N'-ethylenebis(5-chlorosalicylidene)iminate; Miyasaka, H.; Matsumoto, N.; Re, N.; Gallo, E.; Floriani, C. Inorg. Chem. 1997, 36, 670). The Mn ion is surrounded by an equatorial salen quadridentate ligand and two axial nitrogen atoms from the [Fe(CN)(6)](3-) unit, the four Fe[bond]CN groups of which coordinate to the Mn ions of [Mn(salen)](+) units, forming a two-dimensional network having [[bond]Mn[bond]NC[bond]Fe[bond]CN[bond]](4) cyclic repeating units. The network is spread over the bc-plane of the unit cell, and the layers are stacked along the a-axis. The countercation NEt(4)(+) is located between the layers. Compound 1 is a ferrimagnet with T(c) = 7.7 K and exhibits hysteresis with a remnant magnetization of 13.44 cm(3).mol(-1) (M/N mu(B) = 2.4) at zero field and a coercivity of 1000 Oe when the powder sample was measured at 1.9 K. Magnetic measurements of a direction-arranged single crystal were also carried out. The orientation of the crystallographic axes of a selected single crystal was determined by X-ray analysis, and magnetization was measured when an external field was applied in the a*, b, and c directions. The magnetization in the a* direction increased more easily than those in the b and c directions below the critical temperature. No hysteresis was observed only for the measurement in the a* direction, indicating the presence of strong structural anisotropy with potential anisotropy on Mn(III) ions.     

Single-molecule magnets: novel Mn(8) and Mn(9) carboxylate clusters containing an unusual pentadentate ligand derived from pyridine-2,6-dimethanol

The reactions of the Mn(III)(3) and Mn(II)Mn(III)(2) complexes [Mn(3)O(O(2)CEt)(6)(py)(3)][ClO(4)] and [Mn(3)O(O(2)CEt)(6)(py)(3)] with pyridine-2,6-dimethanol (pdmH(2)) afford the mixed-valence Mn(II)(6)Mn(III)(2) octanuclear complex [Mn(8)O(2)(py)(4)(O(2)CEt)(8)(L)(2)][ClO(4)](2) (1) and the Mn(II)(7)Mn(III)(2) enneanuclear complex [Mn(9)(O(2)CEt)(12)(pdm)(pdmH)(2)(L)(2)] (2), respectively. Both compounds contain a novel pentadentate ligand, the dianion of (6-hydroxymethylpyridin-2-yl)-(6-hydroxymethylpyridin-2-ylmethoxy)methanol (LH(2)), which is the hemiacetal formed in situ from the Mn-assisted oxidation of pdmH(2). Complex 1 crystallizes in the monoclinic space group P2(1)/n with the following cell parameters at -160 degrees C: a = 16.6942(5) A, b = 13.8473(4) A, c = 20.0766(6) A, beta = 99.880(1) degrees, V = 4572.27 A(3), and Z = 2, R (R(w)) = 4.78 (5.25). Complex 2.0.2MeCN crystallizes in the triclinic space group Ponemacr; with the following cell parameters at -157 degrees C: a = 12.1312(4) A, b = 18.8481(6) A, c = 23.2600(7) A, alpha = 78.6887(8) degrees, beta = 77.9596(8) degrees, gamma = 82.3176(8) degrees, V = 5076.45 A(3), and Z = 2, R (R(w)) = 4.12 (4.03). Both complexes are new structural types comprising distorted-cubane units linked together, albeit in two very different ways. In addition, complex 2 features three distinct binding modes for the chelating ligands derived from deprotonated pdmH(2). Complexes 1 and 2 were characterized by variable-temperature ac and dc magnetic susceptibility measurements and found to possess spin ground states of 0 and 11/2, respectively. Least-squares fitting of the reduced magnetization data gave S = 11/2, g = 2.0, and D = -0.11 cm(-1) for complex 2, where D is the axial zero-field splitting parameter. Direct current magnetization versus field studies on 2 at <1 K show hysteresis behavior at <0.3 K, establishing 2 as a new single-molecule magnet. Magnetization decay measurements gave an effective barrier to magnetization relaxation of U(eff) = 3.1 cm(-1) = 4.5 K.     

Studies of magnetic properties and HFEPR of octanuclear manganese single-molecule magnets

A new octanuclear manganese cluster [Mn(8)(Hpmide)(4)O(4)(EtCOO)(6)](ClO(4))(2) (1) is achieved by employing Hpmide as the ligand, and this paper examines the synthesis, X-ray structure, high-field electron paramagnetic resonance (HFEPR), magnetization hysteresis loops and magnetic susceptibilities. Complex 1 was prepared by two different methods, and hence, was crystallized in two space groups: P3(2)21 for 1a and P3(1)21 for 1b. Each molecule possesses four Mn(II) and four Mn(III) ions. The metal-oxo framework of complex 1 consists of three face-sharing cubes with manganese ions on alternate corners. The four Mn(III) cations have their Jahn-Teller elongation axes roughly parallel to the c axis of the crystal lattice. The dc magnetic susceptibility measurements reveal a spin-frustration effect in this compound. The ac magnetic susceptibilities, as well as the magnetization hysteresis measurements, clearly establish that complex 1a is a single-molecule-magnet (SMM) with a kinetic energy barrier (10.4 cm(-1)) for spin reversal. HFEPR further confirms that complex 1a is a new SMM with a magnetoanisotropy and quantized energy levels. However, interpretation of the complete set of measurements in terms of a well defined spin ground state is not possible due to the spin frustration.     

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     

New mixed-valent Mn clusters from the use of N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine (edteH4): Mn3, Mn4, Mn6, and Mn10

The syntheses, crystal structures, and magnetochemical characterization are reported for the new mixed-valent Mn clusters [Mn(2)(II)Mn(III)(O(2)CMe)(2)(edteH(2))(2)](ClO(4)) (1), [Mn(II)(2)Mn(III)(2)(edteH(2))(2)(hmp)(2)Cl(2)](Mn(II)Cl(4)) (2), [Mn(III)(6)O(2)(O(2)CBu(t))(6)(edteH)(2)(N(3))(2)] (3), [Na(2)Mn(III)(8)Mn(II)(2)O(4)(OMe)(2)(O(2)CEt)(6)(edte)(2)(N(3))(6)] (4), and (NEt(4))(2)[Mn(8)(III)Mn(2)(II)O(4)(OH)(2)-(O(2)CEt)(6)(edte)(2)(N(3))(6)](5), where edteH(4) is N,N,N',N'-tetrakis-(2-hydroxyethyl)ethylenediamine and hmpH is 2-(hydroxymethyl)pyridine. 1-5 resulted from a systematic exploration of the effect of different Mn sources, carboxylates, the presence of azide, and other conditions, on the Mn/edteH(4) reaction system. The core of 1 consists of a linear Mn(II)Mn(III)Mn(II) unit, whereas that of 2 is a planar Mn(4) rhombus within a [Mn(II)(2)Mn(III)(2)(μ(3)-OR)(2)] incomplete-dicubane unit. The core of 3 comprises a central [Mn(III)(4)(OR)(2)] incomplete-dicubane on either side of which is edge-fused a triangular [Mn(III)(3)(μ(3)-O)] unit. The cores of 4 and 5 are similar and consist of a central [Mn(II)(2)Mn(III)(2)(μ(3)-OR)(2)] incomplete-dicubane on either side of which is edge-fused a distorted [Mn(II)Mn(III)(3)(μ(3)-O)(2)(μ(3)-OR)(2)] cubane unit. Variable-temperature, solid-state direct current (dc) and alternating current (ac) magnetization studies were carried out on 1-5 in the 5.0-300 K range, and they established the complexes to have ground state spin values of S = 3 for 1, S = 9 for 2, and S = 4 for 3. The study of 3 provided an interesting caveat of potential pitfalls from particularly low-lying excited states. For 4 and 5, the ground state is in the S = 0-4 range, but its identification is precluded by a high density of low-lying excited states.     

Cyano-bridged bimetallic assemblies from hexacyanometalate, [M(CN)6](3-) (M = Mn(III) and Fe(III)), and [M(N4-macrocycle)]2+ (M=Fe(III), Ni(II) and Zn(II)) building blocks. Syntheses, multidimensional structures, and magnetic properties

Reactions between [M(N(4)-macrocycle)](2+) (M = Zn(II) and Ni(II); macrocycle ligands are either CTH = d,l-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane or cyclam = 1,4, 8, 11-tetrazaazaciclotetradecane) and [M(CN)(6)](3-) (M = Fe(III) and Mn(III)) give rise to cyano-bridged assemblies with 1D linear chain and 2D honeycomblike structures. The magnetic measurements on the 1D linear chain complex [Fe(cyclam)][Fe(CN)(6)].6H(2)O 1 points out its metamagnetic behavior, where the ferromagnetic interaction operates within the chain and the antiferromagnetic one between chains. The Neel temperature, T(N), is 5.5 K and the critical field at 2 K is 1 T. The unexpected ferromagnetic intrachain interaction can be rationalized on the basis of the axially elongated octahedral geometry of the low spin Fe(III) ion of the [Fe(cyclam)](3+) unit. The isostructural substitution of [Fe(CN)(6)](3-) by [Mn(CN)(6)](3-) in the previously reported complex [Ni(cyclam)](3)[Fe(CN)(6)](2).12H(2)O 2 leads to [Ni(cyclam)](3)[Mn(CN)(6)](2).16 H(2)O 3, which exhibits a corrugated 2D honeycomblike structure and a metamagnetic behavior with T(N) = 16 K and a critical field of 1 T. In the ferromagnetic phase (H > 1 T) this compound shows a very important coercitive field of 2900 G at 2 K. Compound [Ni(CTH)](3)[Fe(CN)(6)](2).13H(2)O 4, C(60)H(116)Fe(2)N(24)Ni(3)O(13), monoclinic, A 2/n, a = 20.462(7), b = 16.292(4), c = 27.262(7) A, beta = 101.29(4) degrees, Z = 4, also has a corrugated 2D honeycomblike structure and a ferromagnetic intralayer interaction, but, in contrast to 2 and 3, does not exhibit any magnetic ordering. This fact is likely due to the increase of the interlayer separation in this compound. ([Zn(cyclam)Fe(CN)(6)Zn(cyclam)] [Zn(cyclam)Fe(CN)(6)].22H(2)O.EtOH) 5, C(44)H(122)Fe(2)N(24)O(23)Zn(3), monoclinic, A 2/n, a = 14.5474(11), b = 37.056(2), c = 14.7173(13) A, beta = 93.94(1) degrees, Z = 4, presents an unique structure made of anionic linear chains containing alternating [Zn(cyclam)](2+) and [Fe(CN)(6)](3)(-) units and cationic trinuclear units [Zn(cyclam)Fe(CN)(6)Zn(cyclam)](+). Their magnetic properties agree well with those expected for two [Fe(CN)(6)](3-) units with spin-orbit coupling effect of the low spin iron(III) ions.     

Two-dimensional networks based on Mn4 complex linked by dicyanamide anion: from single-molecule magnet to classical magnet behavior

Three two-dimensional (2D) network compounds based on Mn(III)/Mn(II) tetranuclear single-molecule magnets (SMMs) connected by dicyanamide (dcn-) linkers have been synthesized: [Mn4(hmp)4(Hpdm)2(dcn)2](ClO4)2 x 2 H2O x 2 MeCN (2), [Mn4(hmp)4Br2(OMe)2(dcn)2] x 0.5 H2O x 2 THF (3), [Mn4(hmp)6(dcn)2](ClO4)2 (4), where Hhmp and H2pdm are 2-hydroxymethylpyridine and pyridine-2,6-dimethanol, respectively. The [Mn4]/dcn- system appears very versatile, but enables its chemistry to be rationalized by a fine-tune of the synthetic conditions. The double cuboidal [Mn4] unit is preserved in the whole family of compounds, despite strong modifications of its Mn(II) coordination sphere. The chemical control of the coordination number of dcn- on the Mn(II) sites has been the key to obtain the following series of compounds: a discrete cluster, [Mn4(hmp)6(NO3)2(dcn)2] x 2 MeCN (1), 2D networks (2, 3, and 4), and the previously reported 3D compound, [Mn4(hmp)4(mu3-OH)2][Mn(II)(dcn)6] x 2 MeCN x THF. Direct current magnetic measurements show that both Mn2+-Mn3+ and Mn3+-Mn3+ intra-[Mn4] magnetic interactions are ferromagnetic leading to an S(T) = 9 ground state for the [Mn4] unit. Despite the very similar 2D lattices in 2-4, the two kinds of orientation of the [Mn4] unit (i.e., angle variations between the two easy axes) lead to different magnetic properties ranging from SMM behavior for 2 and 1 to a long-range canted antiferromagnetic order for 4. Compound 3 is more complicated as the magnetic measurements strongly suggest the presence of a canted antiferromagnetic order below 2.1 K, although the magnetization slow relaxation is simultaneously observed. Heat capacity measurements confirm the long-range magnetic order in 4, while in 3, the critical behavior is frozen by the slow relaxation of the anisotropic [Mn4] units.     

Electronic state of a conducting single molecule magnet based on Mn-salen type and Ni-dithiolene complexes

The electrochemical oxidation of an acetone solution containing [Mn(III) (5-MeOsaltmen)(H(2)O)](2)(PF(6))(2) (5-MeOsaltmen(2-) = N,N'-(1,1,2,2-tetramethylethylene)bis(5-methoxysalicylideneiminate)) and (NBu(4))[Ni(dmit)(2)] (dmit(2-) = 2-thioxo-1,3-dithiole-4,5-dithiolate) afforded a hybrid material, [Mn(5-MeOsaltmen)(acetone)](2)[Ni(dmit)(2)](6) (1), in which [Mn(2)](2+) single-molecule magnets (SMMs) with an S(T) = 4 ground state and [Ni(dmit)(2)](n-) molecules in a charge-ordered state (n = 0 or 1) are assembled in a layer-by-layer structure. Compound 1 crystallizes in the triclinic space group P1 with an inversion center at the midpoint of the Mn···Mn dimer. The [Mn(2)](2+) unit has a typical nonplanar Mn(III) dimeric core and is structurally consistent with previously reported [Mn(2)] SMMs. The six [Ni(dmit)(2)](n-) (n = 0 or 1) units have a square-planar coordination geometry, and the charge ordering among them was assigned on the basis of ν(C═C) in IR reflectance spectra (1386, 1356, 1327, and 1296 cm(-1)). The [Mn(2)](2+) SMM and [Ni(dmit)(2)](n-) units aggregate independently to form hybrid frames. Electronic conductivity measurements revealed that 1 behaved as a semiconductor (ρ(rt) = 2.1 × 10(-1) Ω·cm(-1), E(a) = 97 meV) at ambient pressure and as an insulator at 1.7 GPa (ρ(1.7GPa) = 4.5 Ω·cm(-1), E(a) = 76 meV). Magnetic measurements indicated that the [Mn(2)](2+) units in 1 behaved as S(T) = 4 SMMs at low temperatures.     

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.     

Fast magnetization tunneling in tetranickel(II) single-molecule magnets

A series of Ni(4) cubane complexes with the composition [Ni(hmp)(ROH)Cl](4) complexes 1-4 where R= -CH(3) (complex 1), -CH(2)CH(3) (complex 2), -CH(2)CH(2)(C(4)H(9)) (complex 3), -CH(2)CH(2)CH(2)(C(6)H(11)) (complex 4), hmp(-) is the anion of 2-hydroxymethylpyridine, t-Buhmp(-) is the anion of 4-tert-butyl-2-hydroxymethylpyridine, and dmb is 3,3-dimethyl-1-butanol] and [Ni(hmp)(dmb)Br](4) (complex 5) and [Ni(t-Buhmp)(dmb)Cl](4) (complex 6) were prepared. All six complexes were characterized by dc magnetic susceptibility data to be ferromagnetically coupled to give an S = 4 ground state with significant magnetoanisotropy (D approximately equal to -0.6 cm(-1)). Magnetization hysteresis measurements carried out on single crystals of complexes 1-6 establish the single-molecule magnet (SMM) behavior of these complexes. The exchange bias observed in the magnetization hysteresis loops of complexes 1 and 2 is dramatically decreased to zero in complex 3, where the bulky dmb ligand is employed. Fast tunneling of magnetization is observed for the high-symmetry (S(4) site symmetry) Ni(4) complexes in the crystal of complex 3, and the tunneling rate can even be enhanced by destroying the S(4) site symmetry, as is the case for complex 4, where there are two crystallographically different Ni(4) molecules, one with C(2) and the other with C(1) site symmetry. Magnetic ordering temperatures due to intermolecular dipolar and magnetic exchange interactions were determined by means of very low-temperature ac susceptibility measurements; complex 1 orders at 1100 mK, complex 3 at 290 mK, complex 4 at approximately 80 mK, and complex 6 at <50 mK. This confirms that bulkier ligands correspond to more isolated molecules, and therefore, magnetic ordering occurs at lower temperatures for those complexes with the bulkiest ligands.     

Synthesis and characterization of a Mn22 single-molecule magnet and a [Mn22]n single-chain magnet

The reactions of [Mn12O12(O2CEt)16(H2O)4] with phenylphosphinic acid (PhHPO2H) in MeCN and MeCN/CH2Cl2 have led to isolation of [Mn22O12(O2CEt)22(O3PPh)8(H2O)8] (2) and [Mn22O12(O2CEt)20(O3PPh)8(O2PPhH)2(H2O)8]n (3), respectively, both containing PhPO3(2-) groups from in situ oxidation of PhHPO(2)(-). Complex 2 is molecular and consists of two Mn9 subunits linked by four additional Mn atoms. Complex 3 contains almost identical Mn22 units as 2, but they are linked into a one-dimensional chain structure. The Mn22 unit in both compounds is mixed-valence Mn(III)18, Mn(II)4. Solid-state, variable-temperature dc magnetic susceptibility and magnetization measurements were performed on vacuum-dried samples of 2 and 3, indicating dominant antiferromagnetic interactions. A good fit of low-temperature magnetization data for 2 could not be obtained because of problems associated with low-lying excited states, as expected for a high nuclearity complex containing Mn(II) atoms. An approximate fit using only data collected in small applied fields indicated an S = 7 or 8 ground state for 2. Solid-state ac susceptibility data established that the true ground state of 2 is S = 7 and that the connected Mn22 units of 3 are ferromagnetically coupled. Both 2 and 3 displayed weak out-of-phase ac signals indicative of slow magnetization relaxation. Single-crystal magnetization versus applied dc field scans exhibited hysteresis loops for both compounds, establishing them as new single-molecule and single-chain magnets, respectively. Complex 2 also showed steps in its hysteresis loops characteristic of quantum tunneling of magnetization, the highest nuclearity molecule to show such QTM steps. Arrhenius plots constructed from dc magnetization versus time decay plots gave effective barriers to magnetization relaxation (U(eff)) of 6 and 11 cm(-1) for 2 and 3, respectively.     

Encapsulation of cyanometalates by a tris-macrocyclic ligand tricopper(II) complex: syntheses, structural variation, and magnetic exchange coupling pathways

The reaction of [M(CN)(6)](3-) (M = Cr(3+), Mn(3+), Fe(3+), Co(3+)) and [M(CN)(8)](4-/3-) (M = Mo(4+/5+), W(4+/5+)) with the trinuclear copper(II) complex of 1,3,5-triazine-2,4,6-triyltris[3-(1,3,5,8,12-pentaazacyclotetradecane)] ([Cu(3)(L)](6+)) leads to partially encapsulated cyanometalates. With hexacyanometalate(III) complexes, [Cu(3)(L)](6+) forms the isostructural host-guest complexes [[[Cu(3)(L)(OH(2))(2)][M(CN)(6)](2)][M(CN)(6)]][M(CN)(6)]30 H(2)O with one bridging, two partially encapsulated, and one isolated [M(CN)(6)](3-) unit. The octacyanometalates of Mo(4+/5+) and W(4+/5+) are encapsulated by two tris-macrocyclic host units. Due to the stability of the +IV oxidation state of Mo and W, only assemblies with [M(CN)(8)](4-) were obtained. The Mo(4+) and W(4+) complexes were crystallized in two different structural forms: [[Cu(3)(L)(OH(2))](2)[Mo(CN)(8)]](NO(3))(8)15 H(2)O with a structural motif that involves isolated spherical [[Cu(3)(L)(OH(2))](2)[M(CN)(8)]](8+) ions and a "string-of-pearls" type of structure [[[Cu(3)(L)](2)[M(CN)(8)]][M(CN)(8)]](NO(3))(4) 20 H(2)O, with [M(CN)(8)](4-) ions that bridge the encapsulated octacyanometalates in a two-dimensional network. The magnetic exchange coupling between the various paramagnetic centers is characterized by temperature-dependent magnetic susceptibility and field-dependent magnetization data. Exchange between the CuCu pairs in the [Cu(3)(L)](6+) "ligand" is weakly antiferromagnetic. Ferromagnetic interactions are observed in the cyanometalate assemblies with Cr(3+), exchange coupling of Mn(3+) and Fe(3+) is very small, and the octacoordinate Mo(4+) and W(4+) systems have a closed-shell ground state.     

Coulombic aggregations of Mn(III) salen-type complexes and Keggin-type polyoxometalates: isolation of Mn2 single-molecule magnets

The reaction of Mn(III) salen-type complexes with di- and tetraanionic α-Keggin-type polyoxometalates (POMs) was performed, and three types of Coulombic aggregations containing Mn(III) out-of-plane dimeric units (abbreviated as [Mn(2)](2+)) that are potentially single-molecule magnets (SMMs) with an S(T) = 4 ground state were synthesized: [Mn(2)(5-MeOsaltmen)(2)(acetone)(2)][SW(12)O(40)] (1), [Mn(2)(salen)(2)(H(2)O)(2)](2)[SiW(12)O(40)] (2), and [Mn(5-Brsaltmen)(H(2)O)(acetone)](2)[{Mn(2)(5-Brsaltmen)(2)}(SiW(12)O(40))] (3), where 5-Rsaltmen(2-) = N,N'-(1,1,2,2-tetramethylethylene)bis(5-R-salicylideneiminate) with R = MeO (methoxy), Br (bromo) and salen(2-) = N,N'-ethylenebis(salicylideneiminate). Compound 1 with a dianionic POM, [SW(12)O(40)](2-), is composed of a 1:1 aggregating set of [Mn(2)](2+)/POM, and 2, with a tetraanionic POM, [SiW(12)O(40)](4-), is a 2:1 set. Compound 3 with [SiW(12)O(40)](4-) forms a unique 1D coordinating chain with a [-{Mn(2)}-POM-](2-) repeating unit, for which a hydrogen-bonded dimeric unit ([Mn(5-Brsaltmen)(H(2)O)(acetone)](2)(2+)) is present as a countercation. Independent of the formula ratio of [Mn(2)](2+)/POM, Mn(III) dimers and POM units in 1-3 form respective segregated columns along a direction of the unit cell, which make an alternate packing to separate evenly identical species in a crystal. The nearest intermolecular Mn···Mn distance is found in the order 2 < 3 < 1. The segregation of the [Mn(2)](2+) dimer resulted in interdimer distances long enough to effectively reduce the intermolecular magnetic interaction, in particular in 1 and 3. Consequently, an intrinsic property, SMM behavior, of Mn(III) dimers has been characterized in this system, even though the interdimer interactions are still crucial in the case of 2, where a long-range magnetic order competitively affects slow relaxation of the magnetization at low ac frequencies.