Solvent effects on the adsorption and self-organization of Mn12 on Au(111)

A sulfur-containing single molecule magnet, [Mn12O12(O2CC6H4SCH3)16(H2O)4], was assembled from solution on a Au(111) surface affording both submonolayer and monolayer coverages. The adsorbate morphology and the degree of coverage were inspected by scanning tunneling microscopy (STM), while X-ray photoelectron spectroscopy (XPS) allowed the determination of the chemical nature of the adsorbate on a qualitative and quantitative basis. The properties of the adsorbates were found to be strongly dependent on the solvent used to dissolve the magnetic complex. In particular, systems prepared from tetrahydrofuran solutions gave arrays of isolated and partially ordered clusters on the gold substrate, while samples prepared from dichloromethane exhibited a homogeneous monolayer coverage of the whole Au(111) surface. These findings are relevant to the optimization of magnetic addressing of single molecule magnets on surfaces.     

Single-molecule magnets: a new approach to investigate the electronic structure of Mn12 molecules by scanning tunneling spectroscopy

A new approach to the deposition of Mn12 single-molecule magnet monolayers on the functionalized Au(111) surface optimized for the investigation by means of scanning tunneling spectroscopy was developed. To demonstrate this method, the new Mn12 complex [Mn12O12(O2CC6H4F)16(EtOH)4].4.4CHCl3 was synthesized and characterized. In MALDI-TOF mass spectra the isotopic distribution of the molecular ion peak of the latter complex was revealed. The complex was grafted to Au(111) surfaces via two different short conducting linker molecules. The Mn12 molecules deposited on the functionalized surface were characterized by means of scanning tunneling microscopy showing homogeneous monolayers of highest quality. Scanning tunneling spectroscopy measurements over a wider energy range compared with previous results could be performed because of the optimized Au(111) surface functionalization. Furthermore, the results substantiate the general suitability of short acidic linker molecules for the preparation of Mn12 monolayers via ligand exchange and represent a crucial step toward addressing the magnetic properties of individual Mn12 single-molecule magnets.     

Magnetic structure variation in manganese-oxide clusters

Negative-ion photoelectron spectroscopy and ab initio simulations are used to study the variation in magnetic structure in Mn(x)O(y) (x = 3, 4[semicolon] y = 1, 2) clusters. The ferrimagnetic and antiferromagnetic ground-state structures of Mn(x)O(y) are 0.16-1.20 eV lower in energy than their ferromagnetic isomers. The presence of oxygen thus stabilizes low-spin isomers relative to the preferred high-spin ordering of bare Mn(3) and Mn(4). Each cluster has a preferred overall magnetic moment, and no evidence is seen of competing states with different spin multiplicities. However, non-degenerate isomags, which possess the same spin multiplicity but different arrangements of local moments, do contribute additional features and peak broadening in the photoelectron spectra. Proper accounting for all possible isomags is shown to be critical for accurate computational prediction of the spectra.     

Single-molecule magnets: structure and properties of [Mn18O14(O2CMe)18(hep)4(hepH)2(H2O)2](ClO4)2 with spin S = 13

The reaction of 2-(hydroxyethyl)pyridine (hepH) with a 2:1 molar mixture of [Mn3O(O2CMe)6(py)3]ClO4 and [Mn3O(O2CMe)6(py)3] in MeCN afforded the new mixed-valent (16Mn(III), 2Mn(II)), octadecanuclear complex [Mn18O14(O2CMe)18(hep)4(hepH)2(H2O)2](ClO4)2 (1) in 20% yield. Complex 1 crystallizes in the triclinic space group P. Direct current magnetic susceptibility studies in a 1.0 T field in the 5.0-300 K range, and variable-temperature variable-field dc magnetization studies in the 2.0-4.0 K and 2.0-5.0 T ranges were obtained on polycrystalline samples. Fitting of magnetization data established that complex 1 possesses a ground-state spin of S = 13 and D = -0.18 K. This was confirmed by the value of the in-phase ac magnetic susceptibility signal. Below 3 K, the complex exhibits a frequency-dependent drop in the in-phase signal, and a concomitant increase in the out-of-phase signal, consistent with slow magnetization relaxation on the ac time scale. This suggests the complex is a single-molecule magnet (SMM), and this was confirmed by hysteresis loops below 1 K in magnetization versus dc field sweeps on a single crystal. Alternating current and direct current magnetization data were combined to yield an Arrhenius plot from which was obtained the effective barrier (U(eff)) for magnetization reversal of 21.3 K. Below 0.2 K, the relaxation becomes temperature-independent, consistent with relaxation only by quantum tunneling of the magnetization (QTM) through the anisotropy barrier via the lowest-energy MS = +/-13 levels of the S = 13 spin manifold. Complex 1 is thus the SMM with the largest ground-state spin to display QTM.     

Contribution of spin and anisotropy to single molecule magnet behavior in a family of bell-shaped Mn11Ln2 coordination clusters

The synthesis, structures, and magnetic properties of a family of isostructural "bell-shaped" heterometallic coordination clusters [Mn(III)(9)Mn(II)(2)La(III)(2)(μ(4)-O)(7)(μ(3)-O)(μ(3)-OH)(2)(piv)(10.8)(O(2)CC(4)H(3)O)(6.2)(NO(3))(2)(OH(2))(1.5)(MeCN)(0.5)]·12CH(3)CN·2H(2)O (1) and [Mn(III)(9)Mn(II)(2)Ln(2)(μ(4)-O)(7)(μ (3)-O)(μ(3)-OH)(2)(piv)(10.6)(O(2)CC(4)H(3)O)(6.4)(NO(3))(2)(OH(2))]·nCH(3)CN·H(2)O (Ln = Pr(III), n = 8 (2); Ln = Nd(III), n = 10 (3); Ln = Eu(III), n = 17 (4); Ln = Gd(III), n = 13 (5); piv = pivalate) are reported. The complexes were obtained from the reaction of [Mn(III)(2)Mn(II)(4)O(2)(piv)(10)(4-Me-py)(2.5)(pivH)(1.5)] and Ln(NO(3))(3)·6H(2)O in the presence of 2-furan-carboxylic acid (C(4)H(3)OCOOH) in CH(3)CN. Compounds 1-5 are isomorphous, crystallizing in the triclinic space group P1 with Z = 2. The Mn(III) and Mn(II) centers together form the shell of the bell, while the two Ln(III) centers can be regarded as the bell's clapper. The magnetic properties of 1-4 reveal dominant antiferromagnetic interactions between the magnetic centers leading to small spin ground states; while those of 5 indicate similar antiferromagnetic interactions between the manganese ions but with unusually strong ferromagnetic interactions between the Gd(III) ions leading to a large overall spin ground state of S = 11-12. While ac and dc magnetic measurements confirmed that Mn(11)Gd(2) (5) is a single-molecule magnet (SMM) showing hysteresis loops at low temperatures, compounds 1-4 do not show any slow relaxation of the magnetization, indicating that the S = 7 spin of the ferromagnetic Gd(2) unit in 5 is a necessary contribution to its SMM behavior.     

Toward a magnetostructural correlation for a family of Mn6 SMMs

We have structurally and magnetically characterized a total of 12 complexes based on the Single-Molecule Magnet (SMM) [MnIII6O2(sao)6(O2CH)2(MeOH) 4] (1) (where sao2- is the dianion of salicylaldoxime or 2-hydroxybenzaldeyhyde oxime) that display analogous structural cores but remarkably different magnetic behaviors. Via the use of derivatized oxime ligands and bulky carboxylates we show that it is possible to deliberately increase the value of the spin ground state of the complexes [Mn6O2(Me-sao)6(O2CCPh3)2(EtOH)4] (2), [Mn6O2(Et-sao)6(O2CCMe3)2(EtOH)5] (3), [Mn6O2(Et-sao)6(O2CPh2OPh)2(EtOH)4] (4), [Mn6O2(Et-sao)6(O2CPh4OPh)2(EtOH)4(H2O)2] (5), [Mn6O2(Me-sao)6(O2CPhBr)2(EtOH)6] (6), [Mn6O2(Et-sao)6(O2CPh)2(EtOH)4(H2O)2] (7), [Mn6O2(Et-sao)6{O2CPh(Me)2}2(EtOH)6] (8), [Mn6O2(Et-sao)6(O2C11H15)2(EtOH)6] (9), [Mn6O2(Me-sao)6(O2C-th)2(EtOH)4(H2O)2] (10), [Mn6O2(Et-sao)6(O2CPhMe)2(EtOH)4(H2O)2] (11), and [Mn6O2(Et-sao)6(O2C12H17)2(EtOH)4(H2O)2] (12) (Et-saoH2 = 2-hydroxypropiophenone oxime, Me-saoH2 = 2-hydroxyethanone oxime, HO2CCPh3 = triphenylacetic acid, HO2CCMe3 = pivalic acid, HO2CPh2OPh = 2-phenoxybenzoic acid, HO2CPh4OPh = 4-phenoxybenzoic acid, HO2CPhBr = 4-bromobenzoic acid, HO2CPh(Me)2 = 3,5-dimethylbenzoic acid, HO2C11H15 = adamantane carboxylic acid, HO2C-th = 3-thiophene carboxylic acid, HO2CPhMe = 4-methylbenzoic acid, and HO2C12H17 = adamantane acetic acid) in a stepwise fashion from S = 4 to S = 12 and, in-so-doing, enhance the energy barrier for magnetization reorientation to record levels. The change from antiferromagnetic to ferromagnetic exchange stems from the "twisting" or "puckering" of the (-Mn-N-O-)3 ring, as evidenced by the changes in the Mn-N-O-Mn torsion angles.     

Largest mixed transition metal/actinide cluster: a bimetallic Mn/Th complex with a [Mn10Th6O22(OH)2]18+ core

A high-nuclearity mixed transition metal/actinide complex has been prepared from the reaction of a Mn(III)4 complex with Th(NO3)4 in MeCN/MeOH. The complex [Th6Mn10O22(OH)2(O2CPh)16(NO3)2-(H2O)8] is the largest such complex to date and the first Th/Mn species. It is rich in oxide groups, which stabilize all of the metals in the high Th(IV) and Mn(IV) oxidation levels. Magnetic characterization establishes that the complex has an S = 3 ground-state spin value.     

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.     

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.     

New polynuclear manganese clusters from the use of the hydrophobic carboxylate ligand 2,2-dimethylbutyrate

The syntheses, structures, and magnetic properties are reported of [Mn12O12(O2CPe(t))16(MeOH)4] (4), [Mn6O2(O2CH2)(O2CPe(t))11(HO2CPe(t))2(O2CMe)] (5), [Mn9O6(OH)(CO3)(O2CPe(t))12(H2O)2] (6), and [Mn4O2(O2CPe(t))6(bpy)2] (7, bpy = 2,2'-bipyridine), where Pe(t) = tert-pentyl (Pe(t)CO2H = 2,2-dimethylbutyric acid). These complexes were all prepared from reactions of [Mn12O12(O2CPe(t))16(H2O)4] (3) in CH2Cl2. Complex 4 x 2MeCN crystallizes in the triclinic space group P1 and contains a central [Mn(IV)4O4] cubane core that is surrounded by a nonplanar ring of eight alternating Mn(III) and eight mu3-O(2-) ions. This is only the third Mn12 complex in which the four bound water molecules have been replaced by other ligands, in this case MeOH. Complex 5 x (1/2)CH2Cl2 crystallizes in the monoclinic space group P2(1)/c and contains two [Mn3(mu3-O)]7+ units linked at two of their apexes by two Pe(t)CO2(-) ligands and one mu4-CH2O2(2-) bridge. The complex is a new structural type in Mn chemistry, and also contains only the third example of a gem-diolate unit bridging four metal ions. Complex 6 x H2O x Pe(t)CO2H crystallizes in the orthorhombic space group Cmc2(1) and possesses a [Mn(III)9(mu3-O)6(mu-OH)(mu3-CO3)]12+ core. The molecule contains a mu3-CO3(2-) ion, the first example in a discrete Mn complex. Complex 7 x 2H2O crystallizes in the monoclinic space group P2(1)/c and contains a known [Mn(III)2Mn(II)2(mu3-O)2]6+ core that can be considered as two edge-sharing, triangular [Mn3O] units. Additionally, the synthesis and magnetic properties of a new enneanuclear cluster of formula [Mn9O7(O2CCH2Bu(t))13(THF)2] (8, THF = tetrahydrofuran) are reported. The molecule was obtained by the reaction of [Mn12O12(O2CCH2Bu(t))16(H2O)4] (2) with THF. Complexes 2 and 4 display quasireversible redox couples when examined by cyclic voltammetry in CH2Cl2; oxidations are observed at -0.07 V (2) and -0.21 V (4) vs ferrocene. The magnetic properties of complexes 4-8 have been studied by direct current (DC) and alternating current (AC) magnetic susceptibility techniques. The ground-state spin of 4 was established by magnetization measurements in the 1.80-4.00 K and 0.5-7 T ranges. Fitting of the reduced magnetization data by full matrix diagonalization, incorporating a full powder average and including only axial anisotropy, gave S = 10, g = 2.0(1), and D = -0.39(10) cm(-1). The complex exhibits two frequency-dependent out-of-phase AC susceptibility signals (chi(M)') indicative of slow magnetization relaxation. An Arrhenius plot obtained from chi(M)' vs T data gave an effective energy barrier to relaxation (U(eff)) of 62 and 35 K for the slower and faster relaxing species, respectively. These studies suggest that complex 4 is a single-molecule magnet (SMM). DC susceptibility studies on complexes 5-8 display overall antiferromagnetic behavior and indicate ground-state spin values of S < or = 2. AC susceptibility studies at < 10 K confirm these small values and indicate the population of low-lying excited states even at these low temperatures. This supports the small ground-state spin values to be due to spin frustration effects.     

An oximate-based hexanuclear mixed-valence Mn(III)4Mn(II)2 edge-sharing bitetrahedral core with an St = 5 spin ground state

The synthesis, structures and magnetic properties of two hexanuclear Mn(6) clusters are reported: Mn(6)(mu(4)-O)(2)(dapdo)(2)(dapdoH)(4)(mu(2)-OH)(2)](ClO(4))(2).6MeCN (.6MeCN) and [Mn(6)(mu(4)-O)(2)(dapdo)(2)(dapdoH)(4)(mu(2)-OCH(3))(2)](ClO(4))(2).2Et(2)O (.2Et(2)O) [dapdo(2-) is the dianion of 2,6-diacetylpyridine dioxime and dapdoH(-) is the monoanion of the aforesaid dioxime ligand]. Both complexes are mixed-valent with two Mn(II) and four Mn(III) atoms disposed in an edge-sharing bitetrahedral core. Both complexes and display the same [Mn(III)(4)Mn(II)(2)(mu(4)-O)(2)(mu(2)-OR)(2)](10+) core in which R = H for and R = Me for . The [Mn(III)(4)Mn(II)(2)] core is rather uncommon compared to the reported [Mn(III)(2)Mn(II)(4)] core in the literature. DC magnetic susceptibility measurements on and reveal the presence of competing exchange interactions resulting in an S(t) = 5 ground spin state. The magnetic behavior of the compounds indicates antiferromagnetic coupling between the manganese(iii) centers, whereas the coupling between the manganese(iii) and manganese(ii) is weakly antiferromagnetic or ferromagnetic depending on the bridging environments. Finally the interaction between the manganese(ii) centers from the two fused tetrahedra is weakly ferromagnetic in nature stabilizing S(t) = 5 ground spin state in compounds and .     

Isolated heterometallic Cr7Ni rings grafted on Au(111) surface

A study of the deposition of heterometallic antiferromagnetically coupled rings onto gold surfaces is reported. Two new {Cr7Ni} rings, [NH2nPr2][Cr7NiF8(3-tpc)16] (1) (where 3-tpc=3-thiophenecarboxylate) and [nBuNH2CH2CH2SH] [Cr7NiF8(O2CtBu)16] (2) have been made and structurally characterized. They have been deposited from the liquid phase on Au(111) and the adsorbed molecules compared by means of scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS). In both cases a two-dimensional distribution of individually accessible {Cr7Ni} heterometallic rings on the gold surface has been obtained, exploiting the direct grafting of sulfur-functionalized clusters. There is a competition between the chemisorption of the {Cr7Ni} clusters and a thiolic self-assembled monolayer (SAM) formed by free ligands. In 2, the presence of a single sulfur ligand should force the molecule to graft with the ring axis normal to the surface. The cluster stability in the STM images and the S-2p energy positions demonstrate, for both functionalizations, the strength of the grafting with the gold surface.     

Studies of a linear single-molecule magnet

Reaction of the dinuclear complex [Mn2O2(bpy)4](ClO4)3 with H3cht (cis,cis-l,3,5-cyclohexanetriol) in MeCN produces the complex [Mn3(Hcht)2(bpy)4](ClO4)3.Et2O.2MeCN (1.Et2O.2MeCN). Dc magnetic susceptibility measurements reveal the existence of weak ferromagnetic exchange between the three Mn ions, leading to a spin ground state of S = 7, with D = -0.23 cm(-1). W-Band (94 GHz) EPR measurements on restrained powdered crystalline samples confirm the S = 7 ground state and determine the ground state zero-field splitting (ZFS) parameters of D = -0.14 cm(-1) and B4(0)= +1.5 x 10(-5) cm(-1). The apparent 4th order behaviour is due to a breakdown of the strong exchange limit approximation (J approximately d, the single-ion ZFS). Single crystal dc relaxation decay and hysteresis loop measurements reveal the molecule to have an appreciable energy barrier to magnetization relaxation, displaying low temperature sweep rate and temperature-dependent hysteresis loops. Density functional studies confirm the ferromagnetic exchange coupling between the Mn ions.     

Polynuclear manganese complexes with the dicarboxylate ligand m-phenylenedipropionate: a hexanuclear mixed-valence (3Mn(III), 3Mn(IV)) complex

The dicarboxylate group m-phenylenedipropionate (mpdp(2)(-)) has been used for the synthesis of four new Mn compounds of different nuclearities and oxidation states: [Mn(2)O(mpdp)(bpy)(2)(H(2)O)(MeCN)](ClO(4))(2) (3), [Mn(3)O(mpdp)(3)(py)(3)](ClO(4)) (4), [Mn(3)O(mpdp)(3)(py)(3)] (5), and [Mn(6)O(7)(mpdp)(3)(bpy)(3)](ClO(4)) (6). Compound 3 (2Mn(III)) contains a [Mn(2)(micro-O)](4+) core, whereas 5 (Mn(II), 2Mn(III)) and 4 (3Mn(III)) contain the [Mn(3)(micro(3)-O)](6+,7+) core, respectively. In all three compounds, the mpdp(2)(-) ligand is flexible enough to adopt the sites occupied by two monocarboxylates in structurally related compounds, without noticeable distortion of the cores. Variable-temperature magnetic susceptibility studies establish that 3 and 5 have ground-state spin values of S = 0 and S = 1/2, respectively. Compound 6 is a highly unusual 3Mn(III), 3Mn(IV) trapped-valent compound, and it is also a new structural type, with six Mn atoms disposed in a distorted trigonal antiprismatic topology. Its electronic structure has been explored by variable-temperature measurements of its dc magnetic susceptibility, magnetization vs field response, and EPR spectrum. The magnetic data indicate that it possesses an S = 3/2 ground state with an axial zero-field splitting parameter of D = -0.79 cm(-)(1), and this conclusion is supported by the EPR data. The combined results demonstrate the ligating flexibility of the mpdp(2)(-) ligand and its usefulness in the synthesis of a variety of Mn(x) species.     

Accidentally on purpose: construction of a ferromagnetic, oxime-based [Mn(III)2] dimer

The serendipitous self-assembly of the complex [Mn(III)(2)Zn(II)(2)(Ph-sao)(2)(Ph-saoH)(4)(hmp)(2)] (1),whose magnetic core consists solely of two symmetry equivalent Mn(iii) ions linked by two symmetry equivalent -N-O- moieties, provides a relatively simple model complex with which to study the magneto-structural relationship in oxime-bridged Mn(III) cluster compounds. Dc magnetic susceptibility measurements reveal ferromagnetic (J = +2.2 cm(-1)) exchange resulting in an S = 4 ground state. Magnetisation measurements performed at low temperatures and high fields reveal the presence of significant anisotropy, with ac measurements confirming slow relaxation of the magnetisation and Single-Molecule Magnetism behaviour. Simulations of high field, high frequency EPR data reveal a single ion anisotropy, D((Mn(III))) = -3.83 cm(-1). DFT studies on a simplified model complex of 1 reveal a pronounced dependence of the exchange coupling on the relative twisting of the oxime moiety with respect to the metal ion positions, as suggested previously in more complicated [Mn(III)(3)] and [Mn(III)(6)] clusters.     

Wheel-shaped [Mn12] single-molecule magnets

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

Role of the orbitally degenerate Mn(III) ions in the single-molecule magnet behavior of the cyanide cluster ([MnII(tmphen)2]3[Mn(III)(CN)6]2) (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline)

We report a new theoretical model that accounts for the unusual magnetic properties of the cyanide cluster ([MnII(tmphen)2]3[MnIII(CN)6]2) (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline). The model takes into account (1) the spin-orbit interaction, (2) the trigonal component of the crystal field acting on the ground-state cubic (3)T(1) terms of the apical Mn(III) ions, and (3) the isotropic contribution to the exchange interaction between Mn(III) and Mn(II) ions. The ground state of the cluster was shown to be the state with the total angular momentum projection |M(J)| = 15/2; the energies of the low-lying levels obtained from this treatment increase with decreasing |M(J)| values, a situation that leads to a barrier for the reversal of magnetization (U(eff) approximately 30 cm(-1)). The new model explains the recently discovered single-molecule magnet behavior of the ([MnII(tmphen)2]3[MnIII(CN)6]2)in contrast to the traditional approach that takes into account only the ground-state spin (S) and a negative zero-field splitting parameter (D(S) < 0).     

High nuclearity single-molecule magnets: a mixed-valence Mn26 cluster containing the di-2-pyridylketone diolate dianion

The employment of the dianion (dpkd(2-)) of the gem-diol form of di-2-pyridylketone (dpk) as a tetradentate chelate in manganese chemistry is reported, and the synthesis, crystal structure, and magnetochemical characterization of [Mn26O16(OMe)12(dpkd)12(MeOH)6](OH)6 x solv (3 x solv) are described. The reaction of Mn(ClO4)2 x 6 H2O, dpk, NaOMe, and NEt3 (2:1:4:2) in MeCN/MeOH affords complex 3, which possesses a rare metal topology and is mixed-valence (4 Mn(II), 22 Mn(III)). The complicated [Mn26(mu4-O)10(mu3-O)6(mu3-OMe)12(mu-OR)12](18+) core of 3 consists of an internal Mn(III)16 cage of adjacent Mn4 tetrahedra surrounded by an external Mn(II)4Mn(III)6 shell. The latter is held together by the alkoxide arms of twelve eta(1):eta(2):eta(1):eta(1):mu3 dpkd(2-) groups. Variable-temperature, solid-state direct current (dc), and alternating current (ac) magnetization studies were carried out on 3 in the 1.8-300 K range. Complex 3 is predominantly antiferromagnetically coupled with a resulting S = 6 ground state, a conclusion confirmed by the in-phase (chi'(M)) ac susceptibility data. The observation of out-of-phase (chi'(M)) ac susceptibility signals suggested that 3 might be a single-molecule magnet, and this was confirmed by single-crystal magnetization vs dc field sweeps that exhibited hysteresis, the diagnostic property of a magnet. Combined ac chi'(M) and magnetization decay vs time data collected below 1.1 K were used to construct an Arrhenius plot; the fit of the thermally activated region above approximately 0.1 K gave U(eff) = 30 K, where U(eff) is the effective relaxation barrier. At lower temperatures, the complex exhibits temperature-independent relaxation, characteristic of ground-state quantum tunneling of magnetization between the lowest-lying M(s) = +/-6 levels. The combined work demonstrates the ligating flexibility of dipyridyl-diolate chelates and their usefulness in the synthesis of polynuclear Mn(x) clusters with interesting magnetic properties, without requiring the co-presence of carboxylate ligands.     

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.     

Low-energy isomer identification, structural evolution, and magnetic properties in manganese-doped gold clusters MnAu(n) (n = 1-16)

The size-dependent electronic, structural, and magnetic properties of Mn-doped gold clusters have been systematically investigated by using relativistic all-electron density functional theory with generalized gradient approximation. A number of new isomers are obtained for neutral MnAu(n) (n = 1-16) clusters to probe the structural evolution. The two-dimensional (2D) to three-dimensional (3D) transition occurs in the size range n = 7-10 with manifest structure competitions. From size n = 13 to n = 16, the MnAu(n) prefers a gold cage structure with Mn atom locating at the center. The relative stabilities of the ground-state MnAu(n) clusters show a pronounced odd-even oscillation with the number of Au atoms. The magnetic moments of MnAu(n) clusters vary from 3 μ(B) to 6 μ(B) with the different cluster size, suggesting that nonmagnetic Au(n) clusters can serve as a flexible host to tailor the dopant's magnetism, which has potential applications in new nanomaterials with tunable magnetic properties.