Semiempirical Magnetostructural Correlation for High-Nuclearity MnIII-Oxo Complexes: Accommodation of Different Relative Jahn-Teller Axis Orientations.

The previous development of a magnetostructural correlation (MSC) for polynuclear FeIII/oxo clusters has now been extended to one for polynuclear MnIII/oxo clusters. A semiempirical model estimating each pairwise Mn2 exchange constant (Jij) from the Mn-O bond lengths and Mn-O-Mn angles has been formulated based on the angular overlap model. The extra complication, compared with the FeIII/oxo MSC, of different relative orientations of the Jahn-Teller distortion axes typical of high-spin MnIII in near-octahedral geometry was accommodated by developing a separate MSC variant for each possible situation. The final coefficients of the three MSC variants were determined by using reliable crystal structure data and experimentally determined Jij values from the literature. The estimated JMSC values from the new MnIII/oxo MSC have been employed to successfully rationalize the magnetic properties of a number of MnIII clusters in the nuclearity range Mn3-Mn10. These properties include relative spin vector alignments in the ground state, the presence of spin frustration effects, and the resulting overall ground state spin. In addition, the JMSC values can be used to simulate the direct-current magnetic susceptibility versus temperature data and provide realistic input values for fits of these data to minimize false-fit problems. A protocol for the use of the new MSC is also reported.

Synthetic Factors Determining the Curvature and Nuclearity of the Giant Mn70 and Mn84 Clusters with a Torus Structure of ∼4 nm Diameter.

New members of the Mn70 and Mn84 torus-like cluster family have been prepared from a hybrid comproportionation-alcoholysis reaction of [Mn12O12(O2CR)16(H2O)4] in alcohol in the presence of R'CO2H with NnBuMnO4 or MnII salts as initiators. Reactions using MeCO2H in nPrOH or nBuOH gave [Mn70O60(O2CMe)70(OnPr)20(nPrOH)18.5(H2O)21.5] (3) and [Mn70O60(O2CMe)70(OH)3(OnBu)17(nBuOH)7.5(H2O)32.5] (4), respectively, whereas EtCO2H in nPrOH gave [Mn84O72(O2CEt)84(OnPr)24(nPrOH)16(H2O)32] (5). They consist of alternating near-linear [Mn3(µ3-O)4]+ and distorted-cubane [Mn4(µ3-O)2(µ3-OR)2]6+ units bridged by syn,syn-µ-RCO2- and µ3-O2- groups and overall are [Mn14]5 and [Mn14]6 oligomers, the repeating unit containing two Mn3 and two Mn4 units. 3/4/5 possess external diameters (including organic ligands) of 4.0/4.1/4.6 nm, respectively, and crystallize as supramolecular nanotubes but with different packing arrangements. Considering all Mn70/Mn84 tori now available, we conclude that the Mn70 vs Mn84 nuclearity is determined by the relative bulk of the carboxylates vs the alkoxides, their increasing bulk favoring Mn84 and Mn70, respectively, with carboxylates larger than acetate giving Mn84. Alternating current (ac) magnetic susceptibility studies revealed frequency-dependent χ″M signals below ∼2.4 K, indicating 3-5 to be new members of the giant [Mn14]n torus family of giant single-molecule magnets (SMMs), in which Mn84 and Mn70 are the largest homometallic Mn/O clusters and SMMs to date.

Mononuclear Four-Coordinate Bis-Fluoride Bis-NHC Complexes of Chromium(II), Iron(II), and Cobalt(II).

The reaction between silylamido complexes of Cr(II), Fe(II), and Co(II) and IMes·2HF salt in the presence of IMes (IMes = 1,3-dimesitylimidazol-2-ylidene) led to isolation of Cr(IMes)2F2 (2-Cr), Fe(IMes)2F2 (2-Fe), and Co(IMes)2F2 (2-Co). X-ray structural studies revealed that 2-Cr adopts square planar geometry, while 2-Fe and 2-Co have distorted tetrahedral geometry. Magnetic susceptibility studies of 2-Cr, 2-Fe, and 2-Co were consistent with high-spin complexes, S = 2 for 2-Cr/2-Fe and S = 3/2 for 2-Co. We demonstrated that fluoride can be successfully exchanged for cyanide and azide using trimethylsilyl cyanide and trimethylsilyl azide (3-Fe and 4-Fe). DFT studies suggest that the preference of 2-Cr to adopt square planar geometry over tetrahedral is due to its d4 metal center, where four electrons fill the lower-lying d-orbitals.

Search for Toroidal Ground State and Magnetoelectric Effects in Molecular Spin Triangles with Antiferromagnetic Exchange.

Using first-principles methods and spin models, we investigate the magnetic properties of transition-metal trimers Cr3 and Cu3. We calculate exchange coupling constants and zero-field splitting parameters using density functional theory and, with these parameters, determine the ground spin state as well as thermodynamic properties via spin models. Results for Cr3 indicate uniaxial magnetic anisotropy with a magnetic easy axis aligned along the 3-fold rotational symmetry axis and a mostly isotropic exchange interaction. The Cu3 molecule lacks rotational symmetry and our results show strong antisymmetric interactions for three distinct exchange couplings within the molecule. We are able to reproduce experimental findings on magnetic susceptibility and magnetization of Cr3 with the first-principles spin-Hamiltonian parameters. Our results show no presence of a toroidal ordering of spins for Cr3 and a finite toroidal moment for Cu3 in the ground state. We apply an external electric field up to 0.08 V/Å to each system to reveal the field dependence of exchange coupling as magnetoelectric effects. Finally, we scan the parameter space of a spin Hamiltonian to gain insights into which parameters would lead to a sizable toroidal moment in such systems.

MnII/III and CeIII/IV Units Supported on an Octahedral Molecular Nanoparticle of CeO2.

The preparation of three new heterometallic clusters [Ce6Mn12O17(O2CPh)26] (1), [Ce10Mn14O24(O2CPh)32] (2), and [Ce23Mn20O48(OH)2(tbb)46(H2O)4](NO3)2 (3; tbb- = 4-tBu-benzoate) is reported. They all possess unprecedented structures with a common feature being the presence of an octahedral CeIV-oxo core: a Ce6 in 1, two edge-fused Ce6 giving a Ce10 bioctahedron in 2, or a larger Ce19 octahedron in 3. Complex 1 is the first Ce6 cluster with a central µ6-O2-. 2 and the cation of 3 are molecular nanoparticles of CeO2 (ceria) because they possess the fluorite structure of bulk ceria and are thus ultrasmall ceria nanoparticles in molecular form. The {Ce19O32} octahedral subunit of the cation of 3 had been predicted from density functional theory studies to be one of the stable fragments of the CeO2 lattice, but has never been previously synthesized in molecular chemistry. Around the Ce/O core of 1-3 is an incomplete monolayer of Mnn ions disposed as four Mn3, two Mn7, and four Mn5 units, respectively. This represents a clear structural similarity with composite (phase-separated) CeO2/MnOx mixtures where at high Ce:Mn ratios the Mn atoms segregate on the surface of CeO2 phases. Variable-temperature dc and ac magnetic susceptibility studies have revealed S = 2, S = 1/2, and S = 3/2 ground states for 1-3, respectively. Fitting of the 5.0-300 K dc data for 1 to a two-J model for an asymmetrical V-shaped Mn3 unit with no interaction between the end MnIII ions gave an excellent fit with the following values: J1 = 5.2(3) cm-1, J2 = -7.4(3) cm-1, and g = 1.96(2).

Magnetic Properties of High-Nuclearity Fex-oxo (x = 7, 22, 24) Clusters Analyzed by a Multipronged Experimental, Computational, and Magnetostructural Correlation Approach.

The synthesis, structure, and magnetic properties of three related iron(III)-oxo clusters are reported, [Fe7O3(O2CPh)9(mda)3(H2O)] (1), [Fe22O14(OH)3(O2CMe)21(mda)6](ClO4)2 (2), and [Fe24O15(OH)4(OEt)(O2CMe)21(mda)7](ClO4)2 (3), where mdaH2 is N-methyldiethanolamine. 1 was prepared from the reaction of [Fe3O(O2CPh)6(H2O)3](NO3) with mdaH2 in a 1:2 ratio in MeCN, whereas 2 and 3 were prepared from the reaction of FeCl3/NaO2CMe/mdaH2 in a 2:∼13:2 ratio and FeCl3/NaO2CMe/mdaH2/pyridine in a 2:∼13:2:25 ratio, respectively, both in EtOH. The core of 1 consists of a central octahedral FeIII ion held within a nonplanar Fe6 loop by three µ3-O2- and three µ2-RO- arms from the three mda2- chelates. The cores of the cations of 2 and 3 consist of an A:B:A three-layer topology, in which a central Fe6 (2) or Fe8 (3) layer B is sandwiched between two Fe8 layers A. The A layers structurally resemble 1 with the additional Fe added at the center to retain virtual C3 symmetry. The central Fe6 layer B of 2 consists of a {Fe4(µ4-O)2(µ3-OH)2}6+ cubane with an Fe on either side attached to cubane O2- ions, whereas that of 3 has the same cubane but with an {Fe3(µ3-O)(µ-OH)} unit attached on one side and a single Fe on the other. Variable-temperature dc and ac magnetic susceptibility studies revealed dominant antiferromagnetic coupling in all complexes leading to ground-state spins of S = 5/2 for 1 and S = 0 for 2 and 3. All Fe2 pairwise exchange parameters (Jij) for 1-3 were estimated by two independent methods: density functional theory (DFT) calculations using broken symmetry methods and a magnetostructural correlation previously developed for high-nuclearity FeIII/O complexes. The two approaches gave satisfyingly similar Jij values, and the latter allowed rationalization of the experimental ground states by identification of the spin frustration effects operative and the resultant relative spin vector alignments at each FeIII ion.

Synthesis, Structure, and Magnetic Properties of an Fe36 Dimethylarsinate Cluster: The Largest "Ferric Wheel".

The synthesis and characterization of a high-nuclearity FeIII/O/arsinate cluster is reported within the salt [Fe36O12(OH)6(O2AsMe2)63(O2CH)3(H2O)6](NO3)12 (1). The compound was prepared from the reaction of Fe(NO3)3·9H2O, dimethylarsinic acid (Me2AsO2H), and triethylamine in a 1:2:4 molar ratio in acetonitrile. The Fe36 cation of 1 is an unprecedented structural type consisting of nine Fe4 butterfly units of two types, three {FeIII4(µ3-O)2} units A, and six {FeIII4(µ3-O)(µ3-OH)} units B, linked by multiple bridging Me2AsO2- groups into an Fe36 triangular wheel/loop with C3 crystallographic and D3 virtual symmetry that looks like a guitar plectrum. The unusual structure has been rationalized on the basis of the different curvatures of units A and B, the presence of intra-Fe36 hydrogen bonding, and the tendency of Me2AsO2- groups to favor µ3-bridging modes. The cations stack into supramolecular nanotubes parallel to the crystallographic c axis and contain badly disordered solvent and NO3- anions. The cation of 1 is the highest-nuclearity "ferric wheel" to date and also the highest-nuclearity Fe/O cluster of any structural type with a single contiguous Fe/O core. Variable-temperature direct-current magnetic susceptibility data and alternating-current in-phase magnetic susceptibility data indicate that the cation of 1 possesses an S = 0 ground state and dominant antiferromagnetic interactions. The Fe2 pairwise Ji,j couplings were estimated by the combined use of a magnetostructural correlation for high-nuclearity FeIII/oxo clusters and density functional theory calculations using broken-symmetry methods and the Green's function approach. The three methods gave satisfyingly similar Ji,j values and allowed the identification of spin-frustration effects and the resulting relative spin-vector alignments and thus rationalization of the S = 0 ground state of the cation.

Validation of the Green's Function Approximation for the Calculation of Magnetic Exchange Couplings.

In this work, we assess the potential of the Green's function approximation to predict isotropic magnetic exchange couplings and to reproduce the standard broken-symmetry energy difference approach for transition metal complexes. To this end, we have selected a variety of heterodinuclear, homodinuclear, and polynuclear systems containing 3d transition metal centers and computed the couplings using both the Green's function and energy difference methods. The Green's function approach is shown to have mixed results for the cases tested. For dinuclear complexes with large strength couplings (≳50 cm-1), the Green's function method is unable to reliably reproduce the energy difference values. However, for weaker dinuclear couplings, the Green's function approach acceptably reproduces broken-symmetry energy difference couplings. In polynuclear cases, the Green's function approximation worked remarkably well, especially for FeIII complexes. On the other hand, for a NiII polynuclear complex, qualitatively wrong couplings are predicted. Overall, the evaluation of exchange couplings from local rigid magnetization rotations offers a powerful alternative to time-consuming energy differences methods for large polynuclear transition metal complexes, but to achieve a quantitative agreement, some improvements to the method are needed.

Dipole Switching by Intramolecular Electron Transfer in Single-Molecule Magnetic Complex [Mn12O12(O2CR)16(H2O)4].

We study intramolecular electron transfer in the single-molecule magnetic complex [Mn12O12(O2CR)16 (H2O)4] for R = -H, -CH3, -CHCl2, -C6H5, and -C6H4F ligands as a mechanism for switching of the molecular dipole moment. Energetics is obtained using the density functional theory (DFT) with onsite Coulomb energy correction (DFT + U). Lattice distortions are found to be critical for localizing an extra electron on one of the easy sites on the outer ring in which localized states can be stabilized. We find that the lowest-energy path for charge transfer is for the electron to go through the center via superexchange-mediated tunneling. The energy barrier for such a path ranges from 0.4 to 54 meV depending on the ligands and the isomeric form of the complex. The electric field strength needed to move the charge from one end to the other, thus reversing the dipole moment, is 0.01-0.04 V/Å.

The Role of the -OH Groups within Mn12 Clusters in Electrocatalytic Water Oxidation.

The formidable reactivity of the oxygen-evolving center near photosystem II is largely based on its protein environment that stabilizes it during catalysis. Inspired by this concept, the water-soluble Mn12 clusters Mn12 O12 (O2 CC6 H3 (OH)2 )16 (H2 O)4 (3,5DHMn12 ) and Mn12 O12 (O2 CC6 H3 (OH)3 )16 (H2 O)4 (3,4,5THMn12 ) were developed as efficient electrocatalysts for water oxidation. In this work, the role of the -OH groups in the electrocatalytic process was explored by describing the structural and electrocatalytic properties of two new Mn12 clusters, 3,4DHMn12 and 2,3DHMn12 , having one -OH group in the meta position relative to the benzoate-Mn moiety, and one at the para or ortho position, respectively. The Mn centers in 3,4DHMn12 were discovered to have lower oxidation potential compared with those in 2,3DHMn12 , and thus, 3,4DHMn12 can catalyze water oxidation with higher rate and TON than 2,3DHMn12 . Hence, the role of the -OH groups in the electrocatalysis was established, being involved in electronic stabilization of the Mn centers or in proton shuttling.

Expansion of the Family of Molecular Nanoparticles of Cerium Dioxide and Their Catalytic Scavenging of Hydroxyl Radicals.

The syntheses, crystal structures, and catalytic radical scavenging activity are reported for four new molecular clusters that have resulted from a bottom-up molecular approach to nanoscale CeO2. They are [Ce6O4(OH)4(dmb)12(H2O)4] (dmb- = 2,6-dimethoxybenzoate), [Ce16O17(OH)6(O2CPh)24(HO2CPh)4], [Ce19O18(OH)9(O2CPh)27(H2O)(py)3], and [Ce24O27(OH)9(O2CPh)30(py)4]. They represent a major expansion of our family of so-called "molecular nanoparticles" of this metal oxide to seven members, and their crystal structures confirm that their cores all possess the fluorite structure of bulk CeO2. In addition, they have allowed the identification of surface features such as the close location of multiple Ce3+ ions and organic ligand binding modes not seen previously. The ability of all seven members to catalytically scavenge reactive oxygen species has been investigated using HO• radicals, an important test reaction in the ceria nanoparticle biomedical literature, and most have been found to exhibit excellent antioxidant activities compared to traditional ceria nanoparticles, with their activity correlating inversely with their surface Ce3+ content.

Exchange-biased quantum tunnelling of magnetization in a [Mn3]2 dimer of single-molecule magnets with rare ferromagnetic inter-Mn3 coupling.

A covalently-linked dimer of two single-molecule magnets (SMMs), [Mn6O(O2CMe)6(1,3-ppmd)3](ClO4)2, has been synthesized from the reaction of [Mn3O(O2CMe)6(py)3](ClO4) with 1,3-phenylene-bis(pyridin-2-ylmethanone) dioxime (1,3-ppmdH2). It contains two [MnIII3O]+7 triangular units linked by three 1,3-ppmd2- groups into an [Mn3]2 dimer with D3 symmetry. Solid-state dc and ac magnetic susceptibility measurements showed that each Mn3 subunit retains its properties as an SMM with an S = 6 ground state. Magnetization vs. dc field sweeps on a single crystal reveal hysteresis loops below 1.3 K exhibiting exchange-biased quantum tunnelling of magnetization (QTM) steps with a bias field of +0.06 T. This is the first example of a dimer of SMMs showing a positive exchange bias of the QTM steps in the hysteresis loops, and it has therefore been subjected to a detailed analysis. Simulation of the loops determines that each Mn3 unit is exchange-coupled with its neighbour primarily through the 1,3-ppmd2- linkers, confirming a weak ferromagnetic inter-Mn3 interaction of J12≈ +6.5 mK (H = -2Jsi·sj convention). High-frequency EPR studies of a microcrystalline powder sample enable accurate determination of the zero-field splitting parameters of the uncoupled Mn3 SMMs, while also confirming the weak exchange interaction between the two SMMs within each [Mn3]2 dimer. The combined results emphasize the ability of designed covalent linkers to generate inter-SMM coupling of a particular sign and relative magnitude, and thus the ability of such linkers to modulate the quantum physics. As such, this work supports the feasibility of using designed covalent linkers to develop molecular oligomers of SMMs, or other magnetic molecules, as multi-qubit systems and/or other components of new quantum technologies.

Truly Monodisperse Molecular Nanoparticles of Cerium Dioxide of 2.4†nm dimensions: A {Ce100 O167 } Cluster.

Ultra-small nanoparticles of CeO2 obtained in molecular form, so-called molecular nanoparticles, have been limited to date to a family whose largest member is of nuclearity Ce40 with a {Ce40 O58 } core atom count. Herein we report that a synthetic procedure has been developed to the cation [Ce100 O149 (OH)18 (O2 CPh)60 (PhCO2 H)12 (H2 O)20 ]16+ , a member with a much higher Ce100 nuclearity and a {Ce100 O167 } core that is more akin to the smallest ceria nanoparticles. Its crystal structure reveals it to possess a 2.4 nm size and high D2d symmetry, and it has also allowed identification of core surface features including facet composition, the presence and location of Ce3+ and H+ (i.e. HO- ) ions, and the binding modes of the ligand monolayer of benzoate, benzoic acid, and water ligands.

Iron(III)-Oxo Cluster Chemistry with Dimethylarsinate Ligands: Structures, Magnetic Properties, and Computational Studies.

A program has been initiated to develop FeIII/oxo cluster chemistry with the "pseudocarboxylate" ligand dimethylarsinate (Me2AsO2-) for comparison with the well investigated FeIII/oxo/carboxylate cluster area. The synthesis and characterization of three polynuclear FeIII complexes are reported, [Fe12O4(O2CtBu)8(O2AsMe2)17(H2O)3]Cl3 (1), Na2[Fe12Na2O4(O2AsMe2)20(NO3)6(Me2AsO2H)2(H2O)4](NO3)6 (2), and [Fe3(O2AsMe2)6(Me2AsO2H)2(hqn)2](NO3) (3), where hqnH is 8-hydroxyquinoline. The Fe12 core of 1 is a type never previously encountered in FeIII carboxylate chemistry, consisting of two Fe6 units each of which comprises two {Fe3(µ3-O2-)} units bridged by three Me2AsO2- groups and linked into an Fe12 loop structure by two anti-anti η1:η1:µ Me2AsO2- groups, a bridging mode extremely rare with carboxylates. 2 also consists of two Fe6 units, differing in their ligation from those in 1, and this time linked together into a linear structure by a central {Na2(NO3)2} bridging unit. 3 is a linear Fe3 complex with no monatomic bridges between FeIII ions, a very rare situation in FeIII chemistry with any ligands and unprecedented in Fe carboxylate chemistry. The distinct differences observed in arsinate vs carboxylate ligation modes are rationalized largely based on the greater basicity of the former vs the latter. Variable-temperature dc and ac magnetic susceptibility data reveal all Fe2 pairwise interactions to be antiferromagnetic. For 1 and 2, the different Jij couplings were estimated by use of a magnetostructural correlation for high nuclearity FeIII-oxo clusters and by density functional theory calculations using broken symmetry methods, allowing identification of their relative spin vector alignments and thus rationalization of their S = 0 ground states. The Jij values were then used as input values to give excellent fits of the experimental χMT vs T data. For 3, the fits of the experimental χMT vs T data to the Van Vleck equation or with PHI gave a very weak J12 = -0.8(1) cm-1 (H = -2JSi·Sj convention) between adjacent FeIII ions and an S = 5/2 ground state. These initial FeIII arsinate complexes also provide structural parameters that help validate literature assignments of arsinate binding modes to iron oxide/hydroxide minerals as part of environmental concerns of using arsenic-containing herbicides in agriculture.

Long-Range Ferromagnetic Exchange Interactions Mediated by Mn-CeIV-Mn Superexchange Involving Empty 4f Orbitals.

Reactions involving reductive aggregation of MnO4- in methanol in the presence of CeIV and an excess of carboxylic acid have led to the synthesis of structurally related Ce/Mn clusters, [Ce3Mn5O8(OMe)(O2CBut)13(MeOH)] (1) and [Ce2Mn3O5(O2CPh)9(MeOH)3] (2), containing at least one {Mn2Ce2O4} cubane unit. The cores of both clusters contain Mnx units separated by three (1) or two (2) CeIV ions. Fits of variable-temperature, solid-state dc and ac magnetic susceptibility data reveal dominant ferromagnetic interactions within 1 and 2, resulting in the maximum S = 17/2 and S = 5 ground state spins, respectively, and thus suggesting significant ferromagnetic (F) interactions between the Mnx units that are ≥6 Å apart and separated by four intervening bonds through diamagnetic CeIV. Fits of magnetic susceptibility data also revealed unusual long-range F interactions, and this finding was further supported by high-field EPR measurements and simulations. Density functional theory calculations and a Wannier function analysis confirm long-range interactions and indicate a Mn-Ce-Mn superexchange pathway via Mn-d/Ce-f orbital overlap/hybridization.

Placental weight in relation to maternal and paternal preconception and prenatal urinary phthalate metabolite concentrations among subfertile couples.

INTRODUCTION: Phthalates are known reproductive toxicants that reduce placental and fetal weight in experimental animal studies. Although phthalate exposure has been associated with reduced birth weight in humans, there is limited epidemiologic evidence on whether the placenta is also affected. OBJECTIVE: To assess whether maternal and paternal preconception and prenatal urinary phthalate metabolite concentrations are associated with placental weight, and the birth weight: placental weight (BW:PW) ratio among singletons conceived by subfertile couples. METHODS: The present analysis included 132 mothers and 68 fathers, and their corresponding 132 singletons recruited in an academic hospital fertility center in Boston, Massachusetts. Urinary concentrations of eleven phthalate metabolites were measured and averaged in multiple paternal (n = 196) and maternal (n = 596) preconception, and maternal prenatal (n = 328) samples. Placental weight and birth weight (grams) were abstracted from delivery records, and the BW:PW was calculated. We estimated the association of natural log-phthalate metabolite concentrations across windows of exposure with placental weight and the BW:PW ratio using multivariable linear regression models, adjusting for a priori covariates. RESULTS: In adjusted models, each log-unit increase in paternal urinary concentrations of the sum of di-(2-ethylhexyl) phthalate (ΣDEHP) metabolites was associated with a 24 g (95% CI: -48, -1) decrease in placental weight. We also observed a significant negative association between maternal preconception monoethyl phthalate (MEP) metabolite concentrations and the BW:PW ratio (ß = -0.26; 95%CI: -0.49, -0.04). Additionally, each log-unit increase in prenatal MEP metabolite concentrations was associated with a 24 g (95% CI: -41, -7) decrease in placental weight. CONCLUSIONS: Our results suggest that certain paternal and maternal urinary phthalate metabolites may affect placental weight and the BW:PW ratio. However, given the small sample size within a subfertile cohort and the novelty of these findings, more studies are needed to confirm the present results.

Structural and Magnetic Variations in a Family of Isoskeletal, Oximate-Bridged {MnIV2 MIII } Complexes (MIII =Mn, Gd, Dy).

The self-assembly reaction of MnCl2 ⋅4H2 O, acenaphthenequinone dioxime (acndH2 ) and NEt3 has yielded an unprecedented, linear {MnIV2 MnIII } complex with an S=5 spin ground state and non-SMM behavior. The targeted replacement of the central MnIII ion with GdIII and DyIII ions has successfully increased the S and turned on the SMM dynamics without affecting the core structure and the nature of the magnetic exchange interactions.

Mn3 Single-Molecule Magnets and Mn6/Mn9 Clusters from the Use of Methyl 2-Pyridyl Ketone Oxime in Manganese Phosphinate and Phosphonate Chemistry.

The syntheses, structures, and magnetochemical properties are reported for five new Mn clusters: [MnIII3O(O2PPh2)3(mpko)3](ClO4) (1), [MnIII3O(O2PPh2)3(ppko)3](ClO4) (2) [MnIII6O2(OMe)4(O2PPh2)4(mpko)4](ClO4)2 (3), [MnIII8MnIIO6(O2CMe)7(O3PPh)2(mpko)3(H2O)] (4), and [MnIII2MnIIO(mpko)3(H2O)4(ClO4)2](ClO4) (5), where mpko- (or ppko-) is the anion of methyl (or phenyl) 2-pyridyl ketone oxime. 1 was obtained by carboxylate substitution on [MnIII3O(O2CMe)3(mpko)3](ClO4) by treatment with diphenylphosphinic acid (Ph2PO2H). The comproportionation reaction between Mn(ClO4)2 and NBun4MnO4 in the presence of Ph2PO2H and ppkoH in EtOH, or mpkoH in MeOH, led to 2 and 3, respectively. 4 was obtained as was 3, but with phenylphosphonic acid (PhPO3H2) instead of Ph2PO2H. 5 was obtained by aerial oxidation of Mn(ClO4)2 in the presence of mpkoH. 1 and 2 contain a triangular Mn3 core, 3 comprises the fusion of two Mn3 units of 1 by MeO- bridges, and 4 has a cagelike structure. 5 is similar to 1 in possessing a triangular core. Variable-temperature, solid-state direct-current (dc) and alternating-current (ac) magnetic data were collected on 1-5: 1 and 2 exhibit ferromagnetic Mn····Mn exchange interactions, S = 6 ground states, and are new single-molecule magnets (SMMs). 3-5 possess S = 4, 5/2, and 5/2 ground states, respectively, from dominant antiferromagnetic interactions. Fits of dc magnetization data in the 1.8-10.0 K and 10-70 kG ranges gave D and g values of: -0.29(2) cm-1 and 1.94(1) for 1, -0.38(2) cm-1 and 1.99(1) for 2, -0.29(2) cm-1 and 1.96(1) for 3, -1.26(4) cm-1 and 1.99(2) for 4, -1.41(4) cm-1 and 1.98(2) for 5, where D is the axial zero-field splitting parameter.

Three-Dimensional (3-D) Ferromagnetic Network of Mn12 Single-Molecule Magnets: Subtle Environmental Effects and Switching to Antiferromagnetic.

A new member of the Mn12 family of single-molecule magnets (SMMs) has been prepared and found to be the first of this family to give a 3-D ferromagnetic network. [Mn12O12(O2CC6H4-p-F)16(H2O)4] (2) was prepared by carboxylate substitution on the acetate derivative with p-F-benzoic acid and crystallizes as 2·8MeCN in space group I4̅2m with extensive formation of intermolecular C-H···F hydrogen-bonding. The latter leads to a combination of ferromagnetic (F) and antiferromagnetic (AF) interactions and an overall F network that gives a χMT value at low T that is abnormally high for an S = 10 ground state. 2·8MeCN undergoes solvent loss under vacuum to 2, with a decrease in unit-cell volume of 17%, primarily due to a 13% decrease in the c-axis. The χMT vs T plot for 2 indicates a switch to a net AF network. Exposure to air causes hydration to 2·3H2O, a concomitant increase in unit cell volume, and a switch back to a F network. The same conversion of 2·8MeCN to 2·3H2O can also be accomplished in one step rather than two steps, by leaving crystals of the former exposed to air at ambient temperature and pressure for 10 days, giving the same magnetic plots. Interestingly, the desolvation/solvation processes cause Jahn-Teller isomerism to occur, but the ratio of the faster-relaxing isomer to the normal slowly relaxing one does not change monotonically. Single-crystal micro-SQUID studies on 2·8MeCN show the expected magnetization hysteresis loops for a SMM and a small exchange-bias from the intermolecular interactions that is unexpectedly AF. Since the micro-SQUID study only identifies interactions along the easy-axis (z-axis) of the crystal, this is readily rationalized as due to the Jz components of the intermolecular interactions in 2·8MeCN being net AF. The combined results offer useful insights into the degree of sensitivity of the magnetic properties to small environmental perturbations.

Controlled Dimerization of Mn12 Single-Molecule Magnets.

Controlled dimerization of Mn12 single-molecule magnets (SMMs) was achieved via a synthetic route involving a competition between bridging and terminal ligands, namely, diols and alcohols. The reaction using a 1:1 ratio of the competing ligands resulted in the isolation of a new family of covalently linked dimers of Mn12 SMMs. This is the first step toward the controlled growth of SMM oligomeric arrays.