Quantum tunneling of magnetization in a new [Mn18]2+ single-molecule magnet with s = 13

The reaction between 2-(hydroxyethyl)pyridine (hepH) and a 2:1 molar mixture of [Mn3O(O2CMe)6(py)3](ClO4) and [Mn3O(O2CMe)6(py)3](py) in MeCN leads to isolation of [Mn18O14(O2CMe)18(hep)4(hepH)2(H2O)2](ClO4)2 (1) in 10% yield. The complex is 2MnII,16MnIII and consists of a Mn4O6 central unit to either side of which is attached a Mn7O9 unit. Magnetization data collected in the 2.0-4.0 K and 20-50 kG ranges were fit to yield S = 13, g = 1.86, and D = -0.13 cm-1 = -0.19 K, where D is the axial zero-field splitting parameter. AC susceptibility studies in the 0.04-4.0 K range at frequencies up to 996 Hz display out-of-phase (chiM' ') signals, indicative of a single-molecule magnet (SMM). Magnetization vs applied DC field scans exhibit hysteresis at <1.0 K, confirming 1 to be a SMM. DC magnetization decay data were collected on both a microcrystalline sample and a single crystal, and the combined data were used to construct an Arrhenius plot. Between 3.50 and 0.50 K, the relaxation rate is temperature-dependent with an effective barrier to relaxation (Ueff) of 14.8 cm-1 = 21.3 K. Below ca. 0.25 K, the relaxation rate is temperature-independent at 1.3 x 10-8 s-1, indicative of quantum tunneling of magnetization (QTM) between the lowest energy Ms = +/-13 levels of the S = 13 state. Complex 1 is both the largest spin and highest nuclearity SMM to exhibit QTM.     

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.     

Large Mn25 single-molecule magnet with spin S = 51/2: magnetic and high-frequency electron paramagnetic resonance spectroscopic characterization of a giant spin state

The synthesis and structural, spectroscopic, and magnetic characterization of a Mn25 coordination cluster with a large ground-state spin of S = 51/2 are reported. Reaction of MnCl2 with pyridine-2,6-dimethanol (pdmH2) and NaN3 in MeCN/MeOH gives the mixed valence cluster [Mn25O18(OH)2(N3)12(pdm)6(pdmH)6]Cl2 (1; 6Mn(II), 18Mn(III), Mn(IV)), which has a barrel-like cage structure. Variable temperature direct current (dc) magnetic susceptibility data were collected in the 1.8-300 K temperature range in a 0.1 T field. Variable-temperature and -field magnetization (M) data were collected in the 1.8-4.0 K and 0.1-7 T ranges and fit by matrix diagonalization assuming only the ground state is occupied at these temperatures. The fit parameters were S = 51/2, D = -0.020(2) cm(-1), and g = 1.87(3), where D is the axial zero-field splitting parameter. Alternating current (ac) susceptibility measurements in the 1.8-8.0 K range and a 3.5 G ac field oscillating at frequencies in the 50-1500 Hz range revealed a frequency-dependent out-of-phase (chi(M)') signal below 3 K, suggesting 1 to be a single-molecule magnet (SMM). This was confirmed by magnetization vs dc field sweeps, which exhibited hysteresis loops but with no clear steps characteristic of resonant quantum tunneling of magnetization (QTM). However, magnetization decay data below 1 K were collected and used to construct an Arrhenius plot, and the fit of the thermally activated region above approximately 0.5 K gave U(eff)/k = 12 K, where U(eff) is the effective relaxation barrier. The g value and the magnitude and sign of the D value were independently confirmed by detailed high-frequency electron paramagnetic resonance (HFEPR) spectroscopy on polycrystalline samples. The combined studies confirm both the high ground-state spin S = 51/2 of complex 1 and that it is a SMM that, in addition, exhibits QTM.     

Mixed-valence MnIIIMnIV clusters [Mn7O8(O2SePh)8(O2CMe)(H2O)] and [Mn7O8(O2SePh)9(H2O)]: single-chain magnets exhibiting quantum tunneling of magnetization

The syntheses, structures, and magnetic properties of two new Mn7 complexes containing phenylseleninate ligands are reported. [Mn7O8(O2SePh)8(O2CMe)(H2O)] (1) and [Mn7O8(O2SePh)9(H2O)] (2) were both prepared by the reaction of 18 equiv of benzeneseleninic acid (PhSeO2H) with [Mn12O12(O2CMe)16(H2O)4] in MeCN. Complex 1 x 6MeCN crystallizes in the triclinic space group P, and complex 2 x 2CH2Cl2 crystallizes in the monoclinic space group P2(1)/m. Both compounds possess an unprecedented [Mn7O8]9+ core comprising a central [MnIII3(micro3-O)4]+ unit attached to [MnIV2(micro-O)2]4+ and [MnIV2(micro-O)(micro3-O)]4+ units on either side. In each cluster, the PhSeO2- groups function as bridging ligands between adjacent Mn centers. The structure reveals strong Se.O intermolecular contacts between Mn7 units to give a one-dimensional chain structure, with weak interchain interactions. Solid-state DC magnetic susceptibility measurements of complexes 1 and 2 reveal that they have very similar properties, and detailed studies on 1 by AC susceptibility measurements confirm an S = 2 ground-state spin value. In addition, out-of-phase AC signals are observed, suggesting slow magnetization relaxation. Magnetization versus DC field sweeps down to 0.04 K reveals hysteresis loops, but the temperature dependence of the coercivity is not what is expected of a single-molecule magnet. Instead, the behavior is due to single-chain magnetism, albeit with weak antiferromagnetic interactions between the chains, with the barrier to relaxation arising from a combination of molecular anisotropy and ferromagnetic intermolecular exchange interactions mediated by the Se...O contacts. An Arrhenius plot was constructed from the magnetization versus time decay data. The thermally activated region at > 0.5 K gave an effective relaxation barrier (Ueff) of 14.2 K. Below approximately 0.1 K, the relaxation is independent of temperature, which is characteristic of magnetization quantum tunneling through the anisotropy barrier. These Mn7 compounds are thus the first single-chain magnets to comprise polynuclear metal clusters and also the first for which the temperature-independent relaxation characteristic of tunneling has been identified. The work also emphasizes that out-of-phase AC signals for ostensibly molecular compounds are not sufficient proof by themselves of a single-molecule magnet.     

[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)).     

"Switching on" the properties of single-molecule magnetism in triangular manganese(III) complexes

The reaction between oxide-centered, triangular [MnIII3O(O2CR)6(py)3](ClO4) (R = Me (1), Et (2), Ph (3)) compounds and methyl 2-pyridyl ketone oxime (mpkoH) affords a new family of Mn/carboxylato/oximato complexes, [MnIII3O(O2CR)3(mpko)3](ClO4) [R = Me (4), Et (5), and Ph (6)]. As in 1-3, the cations of 4-6 contain an [MnIII3(mu3-O)]7+ triangular core, but with each Mn2 edge now bridged by an eta1:eta1:mu-RCO2- and an eta1:eta1:eta1:mu-mpko- group. The tridentate binding mode of the latter causes a buckling of the formerly planar [MnIII3(mu3-O)]7+ core, resulting in a relative twisting of the three MnIII octahedra and the central O2- ion now lying approximately 0.3 A above the Mn3 plane. This structural distortion leads to ferromagnetic exchange interactions within the molecule and a resulting S = 6 ground state. Fits of dc magnetization data for 4-6 collected in the 1.8-10.0 K and 10-70 kG ranges confirmed S = 6 ground states, and gave the following D and g values: -0.34 cm(-1) and 1.92 for 4, -0.34 cm(-1) and 1.93 for 5, and -0.35 cm(-1) and 1.99 for 6, where D is the axial zero-field splitting (anisotropy) parameter. Complexes 4-6 all exhibit frequency-dependent out-of-phase (chi" M) ac susceptibility signals suggesting them possibly to be single-molecule magnets (SMMs). Relaxation rate vs T data for complex 4 down to 1.8 K obtained from the chi" M vs T studies were supplemented with rate vs T data measured to 0.04 K via magnetization vs time decay studies, and these were used to construct Arrhenius plots from which was obtained the effective barrier to relaxation (Ueff) of 10.9 K. Magnetization vs dc field sweeps on single-crystals of 4.3CH2Cl2 displayed hysteresis loops exhibiting steps due to quantum tunneling of magnetization (QTM). The loops were essentially temperature-independent below approximately 0.3 K, indicating only ground-state QTM between the lowest-lying Ms = +/-6 levels. Complexes 4-6 are thus confirmed as the first triangular SMMs. High-frequency EPR spectra of single crystals of 4.3CH2Cl2 have provided precise spin Hamiltonian parameters, giving D = -0.3 cm(-1), B40 = -3 x 10(-5) cm(-1), and g = 2.00. The spectra also suggest a significant transverse anisotropy of E > or = 0.015 cm(-1). The combined work demonstrates the feasibility that structural distortions of a magnetic core imposed by peripheral ligands can "switch on" the properties of an SMM.     

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.     

Single-molecule magnetism properties of the first strontium-manganese cluster [SrMn14O11(OMe)3(O2CPh)18(MeCN)2

The preparation and properties of the first strontium-manganese molecular complex are described. The reaction of (NBu(n)4)[Mn4O2(O2CPh)9(H 2O)] (4Mn(III)) with Sr(ClO4)2 in MeCN/MeOH led to the isolation of [SrMn14O11(OMe)3(O2CPh)18(MeCN)2] ( 1; 13Mn(III), Mn(II)). The structure of 1 consists of two [Mn4O3(OMe)] cubane units attached to a central, near-planar, trinuclear [Mn3O4] unit, to which are also attached a Mn and a Sr above the plane and a [Mn2O(OMe)] rhomb below the plane. Peripheral ligation is provided by 18 bridging benzoate and two terminal MeCN groups. Variable-temperature and -field dc magnetization (M) data were collected in the 1.8-10 K and 0.1-4.0 T ranges and fit by matrix diagonalization methods to give S = 9/2, D = -0.50(5) cm(-1), and g = 1.88(10), where S is the ground-state spin and D is the axial zero-field splitting parameter. Magnetization versus dc field sweeps at various temperatures and scan rates exhibited hysteresis loops, confirming 1 to be a new single-molecule magnet. Because complex 1 is the initial molecular example of intimately associated Mn and Sr atoms, Sr EXAFS studies have been performed for the first time on a synthetic Sr-containing molecule. This has also allowed comparisons with the EXAFS data on the Sr-substituted water oxidizing complex (WOC) of Photosystem II (PS II), which contains a SrMn4 complex.     

Single-molecule magnets: preparation and properties of low symmetry [Mn4O3(O2CPh-R)4(dbm)3] complexes with S = 9/2

The preparation and properties of [Mn(4)O(3)(O(2)CPh-R)(4)(dbm)(3)] (R = H, p-Me, p-OMe, and o-Cl; dbm(-) is the anion of dibenzoylmethane) single-molecule magnets (SMMs) with virtual C(S) symmetry are reported. They were prepared by controlled potential electrolysis in 26-80% yields. The structures comprise a distorted-cubane core of virtual C(S) symmetry, in contrast to the other, more common complexes of this type with virtual C(3)(V) symmetry. Solid-state magnetic susceptibility data establish the complexes have S = 9/2 ground-state spins, and ac susceptibility studies indicate they are single-molecule magnets (SMMs). Magnetization vs dc field sweeps below 1.00 K reveal hysteresis loops confirming a SMM, with a very large step at zero applied field diagnostic of fast quantum tunneling of magnetization (QTM) through the anisotropy barrier. The fast QTM rate suggested a significant rhombic ZFS parameter E, as expected from the low (virtual C(S)) symmetry. This was confirmed by high-frequency electron paramagnetic resonance spectroscopy on polycrystalline and single-crystal studies. The results confirm the importance of symmetry on the QTM rates.     

Template synthesis and single-molecule magnetism properties of a complex with spin S = 16 and a [Mn8O8]8+ saddle-like core

The compound [CeIVMnIII8O8(O2CMe)12(H2O)4].4H2O (1.4H2O) has been obtained from a template synthesis involving the reaction of the chain polymer {[MnIII(OH)(O2CMe)2] .(MeCO2H).(H2O)}n (3) with Ce(IV). Compound 1 contains a MnIII8 loop inside which is held the Ce(IV) ion by the bridging oxide ions. Magnetization and magnetic susceptibility studies establish that 1 has an S = 16 spin ground state, the largest yet for a Mn cluster, and displays the slow magnetization relaxation and hysteresis behavior of a single-molecule magnet (SMM). It is thus the highest spin Mn SMM discovered to date.     

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.     

Mn7O5(OR)2(O2CPh)9(terpy)] (R = Me, CH2Ph) complexes with a fused cubane/butterfly core and an S = 6 ground-state spin

Two new heptanuclear Mn clusters, [Mn7O5(OMe)2(O2CPh)9(terpy)] (1) and [Mn7O5(OCH2Ph)2(O2CPh)9(terpy)] (2), were prepared from the partial alcoholysis of the trinuclear complex [Mn3O(O2CPh)6(py)2(H2O)] (3) in the presence of terpy (terpy = 2,2':6',2' '-terpyridine). Complexes 1 and 2 crystallize in the triclinic P and the orthorhombic Pbca space groups, respectively. The clusters are both mixed valent, containing three Mn oxidation states: MnIV, 5MnIII, and MnII. The Mn ions are held together by nine doubly bridging benzoates, four mu3-O2- ions, one mu5-O2- ion, and either two mu-MeO- (1) or two mu-PhCH2O- (2) groups. The single terpy chelate in each complex is attached to the MnII ion. The core topology is novel and very unusual, comprising a cubane and a butterfly unit fused by sharing a MnIII and the mu5-O2- ion. Solid-state dc and ac magnetic susceptibility studies establish that complexes 1 and 2 both possess an S = 6 ground-state spin. Fits of variable-temperature and -field magnetization data gave S = 6, g = 1.88, and D = -0.21 cm-1 for 1 and S = 6, g = 1.86, and D = -0.18 cm-1 for 2. Single-crystal magnetization vs dc field scans down to 0.1 K for 2 show only very little hysteresis at 0.1 K.     

Single-molecule magnets: site-specific ligand abstraction from [Mn12O12(O2CR)16(H2O)4] and the preparation and properties of [Mn12O12(NO3)4(O2CCH2Bu(t))12(H2O)4

Site-selective carboxylate abstraction has been achieved from [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)] complexes by treatment with HNO(3) in MeCN. The reaction of the R = Ph or CH(2)Bu(t)() complexes with 4 equiv of HNO(3) gives [Mn(12)O(12)(NO(3))(4)(O(2)CR)(12)(H(2)O)(4)] (R = CH(2)Bu(t) (6) or Ph (7)) in analytical purity. Complex 6.MeNO(2) crystallizes in monoclinic space group C2/c with the following cell parameters at -168 degrees C: a = 21.280(5), b = 34.430(8), c = 33.023(8) A, beta = 104.61(1) degrees, V = 23413 A, and Z = 8. The four NO(3)(-) groups are not disordered and are bound in bridging modes at axial positions formerly occupied by bridging carboxylate groups. (1)H NMR spectroscopy in CD(2)Cl(2) and CDCl(3) shows retention of the solid-state structure on dissolution in these solvents. DC magnetic susceptibility (chi(M)) and magnetization (M) studies have been carried out in the 2.00-300 K and 1.0-7.0 T ranges. Fits of M/Nmu(B) versus H/T plots gave S = 10, g = 1.92, and D = -0.40 cm(-1), where D is the axial zero-field splitting parameter. AC magnetic susceptibility studies on 6 have been performed in the 1.70-10.0 K range in a 3.5 Oe field oscillating at frequencies up to 1500 Hz. Out-of-phase magnetic susceptibility (chi(M)' ') signals were observed in the 4.00-8.00 K range which were frequency-dependent. Thus, 6 displays the slow magnetization relaxation diagnostic of a single-molecule magnet (SMM). The data were fit to the Arrhenius law, and this gave the effective barrier to relaxation (U(eff)) of 50.0 cm(-1) (72.0 K) and a pre-exponential (1/tau(0)) of 1.9 x 10(8) s(-1). Complex 6 also shows hysteresis in magnetization versus DC field scans, and the hysteresis loops show steps at regular intervals of magnetic field, the diagnostic evidence of field-tuned quantum tunneling of magnetization. High-frequency EPR (HFEPR) spectroscopy on oriented crystals of complex 6 shows resonances assigned to transitions between zero-field split M(s) states of the S = 10 ground state. Fitting of the data gave S = 10, g = 1.99, D = -0.46 cm(-1), and B(4)(0) = -2.0 x 10(-5), where B(4)(0) is the quartic zero-field coefficient. The combined results demonstrate that replacement of four carboxylate groups with NO(3)(-) groups leads to insignificant perturbation of the magnetic properties of the Mn(12) complex. Complex 6 should now be a useful starting point for further reactivity studies, taking advantage of the good leaving group properties of the NO(3)(-) ligands.     

A large [Mn10Na]4 loop of four linked Mn10 loops

A large [Mn10Na]4 loop-of-loops aggregate was prepared from the use of 1,3-propanediol (pdH2) in manganese carboxylate chemistry. It is constructed from four [Mn10(mu3-O)2(O2CMe)13(pd)6(py)2]- loops linked through Na+ ions and exhibits a saddlelike topology. Magnetic characterization showed that the Mn10 loop has an S approximately 4 ground-state spin and displays frequency-dependent in-phase and out-of-phase alternating current signals and also hysteresis loops that, however, are not typical of single-molecule magnets because of the existence of intermolecular interactions between the Mn10 units.     

The properties of the [Mn12O12(O2CR)16(H2O)4] single-molecule magnets in truly axial symmetry: [Mn12O12(O2CCH2Br)16(H2O)4].4CH2Cl2

Detailed studies are reported of a Mn(12) single-molecule magnet (SMM) in truly axial (tetragonal) symmetry. The complex is [Mn(12)O(12)(O(2)CCH(2)Br)(16)(H(2)O)(4)].4CH(2)Cl(2) (2.4CH(2)Cl(2) or Mn(12)-BrAc), obtained by the standard carboxylate substitution method. The complex has an S = 10 ground state, typical of the Mn(12) family, and displays frequency-dependent out-of-phase AC susceptibility signals and hysteresis in single-crystal magnetization vs applied DC field sweeps. Single-crystal high-frequency EPR spectra in frequencies up to 360 GHz exhibit narrow signals that are not overlapping multiplets, in contrast to [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)].2MeCO(2)H.4H(2)O (1 or Mn(12)-Ac), which also crystallizes in an axial (tetragonal) space group but which now is recognized to consist of a mixture of six hydrogen-bonded isomers in the crystal and thus gives multiple, inhomogeneously broadened EPR signals. Similarly, single-crystal (55)Mn NMR spectra on Mn(12)-BrAc display much sharper signals than a single crystal of Mn(12)-Ac, and this allows one Mn(III) signal to show an almost baseline-resolved quintet from quadrupolar splitting ((55)Mn, I = 5/2, 100%), allowing quadrupole coupling parameters (e(2)qQ) to be determined. In addition, it was found that crushing crystals of Mn(12)-BrAc into a microcrystalline powder causes severe broadening and shifts of the NMR resonances, emphasizing the superiority of single-crystal studies. The combined results establish that Mn(12)-BrAc is far superior to Mn(12)-Ac for the study of the intrinsic properties of the Mn(12) family of SMMs in axial symmetry, and for the search for new phenomena such as quantum interference effects caused by higher-order (>2nd-order) transverse terms in the spin Hamiltonian.     

Single-molecule magnets: a large Mn30 molecular nanomagnet exhibiting quantum tunneling of magnetization

The largest single-molecule magnet (SMM) to date has been prepared and studied. Recrystallization of known [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(H(2)O)(4)] (1; 8Mn(III), 4Mn(IV)) from CH(2)Cl(2)/MeNO(2) causes its conversion to [Mn(30)O(24)(OH)(8)(O(2)CCH(2)Bu(t))(32)(H(2)O)(2)(MeNO(2))(4)] (2; 3Mn(II), 26Mn(III), Mn(IV)). The structure of 2 consists of a central, near-linear [Mn(4)O(6)] backbone, to either side of which are attached two [Mn(13)O(9)(OH)(4)] units. Peripheral ligation around the resulting [Mn(30)O(24)(OH)(8)] core is by 32 Bu(t)CH(2)CO(2)(-), 2 H(2)O, and 4 MeNO(2) groups. The molecule has crystallographically imposed C(2) symmetry. Variable-temperature and -field magnetization (M) data were collected in the 1.8-4.0 K and 0.1-0.4 T ranges and fit by matrix diagonalization assuming only the ground state is occupied at these temperatures. The fit parameters were S = 5, D = -0.51 cm(-1) = -0.73 K, and g = 2.00, where D is the axial zero-field splitting parameter. AC susceptibility measurements in the 1.8-7.0 K range in a zero DC field and a 3.5 G AC field oscillating at frequencies in the 50-997 Hz range revealed a frequency-dependent out-of-phase (chi(M)') signal below 3 K, indicating 2 to be a single-molecule magnet (SMM), the largest yet obtained. Magnetization versus DC field sweeps show hysteresis loops but no clear steps characteristic of quantum tunneling of magnetization (QTM). However, magnetization decay data below 1 K were collected and used to construct an Arrhenius plot that revealed temperature-independent relaxation below 0.3 K. The fit of the thermally activated region above approximately 0.5 K gave U(eff)/k = 15 K, where U(eff) is the effective relaxation barrier. Resonant QTM was confirmed from the appearance of a "quantum hole" when the recent quantum hole digging method was employed. The combined results demonstrate that SMMs can be prepared that are significantly larger than any known to date and that this new, large Mn(30) complex still demonstrates quantum behavior.     

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.