Solid-state Magnetic Susceptibility Studies Research Articles

Crystal designing and magneto-chemical characterization of unusual nickel(II) MOFs

Here, we have reported crystal designing and magneto-chemical characterization of two new complexes [Ni(3,5-dnba)2(hmp)2] (1) and [Ni3(Py)8(µ-CH3COO)4](I3)2(2) employed by 3,5-dinitro benzoic acid (dnba), 2-Pyridine methanol (hmpH) and pyridine. Complex 1 consists of an unusual NiII octahedral MOF-like monomeric structure, whereas 2 comprises of planar three Ni asymmetric “distorted hexagonal planar” units linked by alternating syn, syn, anti, a (μ-O2CCH3) and syn, syn, (μ-O2CCH3)2 units. Variable-temperature, solid-state magnetic susceptibility studies in the 3.0–300 K temperature range in a 0.33 T (333 Hz) direct-current (dc) magnetic field reveal the paramagnetic and ferromagnetic behavior with calculated ground-state spin (S) values are 1 and 3/2 of complex 1 and 2, respectively. Interesting, complex 2 have weak antiferromagnetic intra-Ni3 exchange couplings. Alternating current magnetic susceptibility studies, both the complex did not belong to the SMM category.

Structural diversity in polymeric and discrete complexes constructed by divalent transition metals and unsymmetrical quasi semirigid pyridinecarboxylate isomers

Nine polymeric and discrete complexes including a couple of genuine supramolecular isomers have been successfully prepared from divalent transition metal ions (Mn(II), Co(II), Ni(II), Cu(II), and Zn(II)) and unsymmetrical quasi semirigid pyridinecarboxylate isomers, InMe-n-py (n = 2, 3, 4). Structural analyses reveal that the unsymmetrical quasi semirigid ligands manage the metal(II) ions to form discrete (0D) armed metallocycle and dinuclear paddlewheel molecule, 1D ribbon, simple 2D rhombus (4,4) grid, 2D chiral wavy square (4,4) grid, and 2D double-edged layer with decorated (4,4) topology. Noncovalent hydrogen bonds and aromatic stacking interactions are found to be accessorial secondary interactions that are helpful for the extension and stabilization of the final inorganic−organic hybrid supramolecular networks of metal(II) complexes. Positional isomerism and conformational diversity of the unsymmetrical quasi semirigid ligands and the coordination preference of divalent metal ions play key roles in dominating structure variation and network complexity. Thermogravimetric analysis of mass loss showed that the frameworks of these complexes are thermally stable up to ca. 220–400 °C. Variable-temperature, solid-state magnetic susceptibility studies suggest that Mn(II) complexes 1 and 2 exhibit antiferromagnetic exchange coupling.

Pyrrolyl-based pincer complexes of iron – Synthesis and electronic structure

Four- and five-coordinate high-spin (S=2) Fe(II) complexes bearing the pyrrolyl-derived tBuPNP pincer ligand [tBuPNP=2,5-(tBu2PCH2)2(C4H2N)] have been prepared. All three complexes were characterized by X-ray diffraction, zero-field 57Fe Mössbauer spectroscopy and solid-state magnetic susceptibility studies. Furthermore, the experimentally determined Mössbauer parameters of compounds 4–6 can be modeled satisfactorily by density functional theory (DFT) computations at the B97D level of theory. Introduction of the sterically demanding bis(trimethylsilyl)amido moiety induces significant distortions from ideal tetrahedral geometry in [(tBuPNP)FeN(SiMe3)2] (4), featuring an Fe(II) high-spin state with large negative axial zero-field splitting and slow paramagnetic relaxation at low temperature, as evidenced by measurements of the solid-state magnetic susceptibility and zero-field 57Fe Mössbauer spectroscopy. Replacing the chlorido ligand in [(tBuPNP)FeCl] (3) by the bidentate acetylacetonato (acac) ligand enforces a trigonal–bipyramidal structure in [(tBuPNP)Fe(acac)] (5), in which the Fe(II) atom also adopts a high-spin configuration. Reaction of 3 with adamantyl azide allowed us to confirm the inherent lability of the phosphine coordination, since the azide inserts into the FeP bond to form [(tBuPNPN3Adamantyl)FeCl] (6), in which the tBuPNP ligand is transformed to a phosphino-pyrrolyl-phosphazide ligand framework (tBuPNPN3Adamantyl). This motif can be considered as a trapped Staudinger intermediate along the reaction pathway of free phosphines with organic azides to form phosphinimines. Bidentate binding of the phosphazide sidearm in κN:κN-fashion to the Fe(II) atom enforces an s-anti coordination of the P1–N2–N3–N4 moiety, which raises the barrier for the thermodynamically favorable N2 elimination and allows this intermediate to be trapped. Complex 6 contains a high-spin Fe(II) atom with a square-pyramidal structure.

Structural Diversities in Heterometallic Mn-Ca Cluster Chemistry from the Use of Salicylhydroxamic Acid: {MnIII4Ca2}, {MnII/III6Ca2}, {MnIII/IV8Ca}, and {MnIII8Ca2} Complexes with Relevance to Both High- and Low-Valent States of the Oxygen-Evolving Complex.

One-pot reactions between the [Mn3O(O2CPh)6(py)x]+/0 triangular precursors and either CaBr2·xH2O or CaCl2·6H2O, in the presence of salicylhydroxamic acid (shaH2), have afforded the heterometallic complexes [MnIII4Ca2(O2CPh)4(shi)4(H2O)3(Me2CO)] (1) and (pyH)[MnII2MnIII4Ca2Cl2(O2CPh)7(shi)4(py)4] (2), respectively, in good yields. Further reactions but using a more flexible synthetic scheme comprising the Mn(NO3)2·4H2O/Ca(NO3)2·4H2O and Mn(O2CPh)2·2H2O/Ca(ClO4)2·4H2O "metal blends" and shaH2, in the presence of external base NEt3, led to the new complexes (NHEt3)2[MnIII4MnIV4Ca(OEt)2(shi)10(EtOH)2] (3) and (NHEt3)4[MnIII8Ca2(CO3)4(shi)8] (4), respectively. In all reported compounds, the anion of the tetradentate (N,O,O,O)-chelating/bridging ligand salicylhydroxime (shi3-), resulting from the in situ metal-ion-assisted amide-iminol tautomerism of shaH2, was found to bridge both Mn and Ca atoms. Complexes 1-4 exhibit a variety of different structures, metal stoichiometries, and Mn oxidation-state descriptions; 1 possesses an overall octahedral metal arrangement, 2 can be described as a Mn4Ca2 octahedron bound to an additional Mn2 unit, 3 consists of a Mn8 "ring" surrounding a CaII atom, and 4 adopts a rectangular cuboidal motif of eight Mn atoms accommodating two CaII atoms. Solid-state direct-current magnetic susceptibility studies revealed the presence of predominant antiferromagnetic exchange interactions between the Mn centers, leading to S = 0 spin ground-state values for all complexes. From a bioinorganic chemistry perspective, the reported compounds may demonstrate some relevance to both high-valent scheme (3) and lower-oxidation-level species (1, 2, and 4) of the catalytic cycle of the oxygen-evolving complex.

Closing the Gap: Preparation and Characterization of the First Half-Open and Open Manganocene Complexes

The first preparations of half-open and open manganocenes were accomplished. Treatment of KPdl′ (Pdl′ = 2,4-(Me3C)2C5H5)) with [{(η5-Cp”)Mn(thf)(μ-I)}2] (Cp″ = 1,2,4-(Me3C)3C5H2) and MnI2(thf)2 resulted in the formation of [(η5-Cp″)Mn(Pdl′)] (2) and [(Pdl′)2Mn] (4), respectively. Both compounds adopt a high-spin (S = 5/2) ground state. Maximum spin states are rather unusual for pentadienyl complexes, since these ligands generally stabilize transition metal complexes in their low-spin state. In addition, the electronic structure of 2 was compared to its closed analogue [(η5-Cp″)Mn(η5-Cp′)] (Cp′ = 1,3-(Me3C)2C5H3), which also adopts the high-spin configuration because of steric hindrance destabilizing the electronically more favorable low-spin state. Reaction of KPdl′ with [(C5H5)2Mn] yields the trimetallic compound [{(η5-Pdl′)Mn(η5-C5H5)]}2Mn] (5) concomitant with the formation of 2,4,7,9-tetra-tert-butyl-1,3,7,9-decatetraene (Pdl′2), indicating reduction of two Mn atoms. Solid-state magnetic susceptibility studies and density functional theory computations suggest that the electronic structure in 5 is best described as two diamagnetic (S = 0) [(η5-Pdl′)Mn(η5-C5H5)]− Mn(I) anions, each being coordinated to a central Mn(II) cation with a high-spin (S = 5/2) configuration.

MOF-like supramolecular network of Mn3 single-molecule magnets formed by extensive π–π stacking

[Mn3O(dhb)3(mpko)3](ClO4) (4) has been obtained from the carboxylate substitution reaction of triangular [Mn3O(O2CMe)3(mpko)3](ClO4) (mpkoH=methyl(2-pyridyl)ketone oxime) with 3,5-dihydroxybenzoic acid (dhbH). Complex 4 possesses an equilateral triangle [Mn3O]7+ core with the dhb− and mpko− ligands on opposite sides of the Mn3 plane. All dhb− and mpko− groups are involved in π–π stacking interactions with those on neighboring Mn3 molecules to give a 3-D supramolecular network resembling a metal–organic framework (MOF), with periodic voids resulting from a repeating unit comprising an [Mn3]8 rhombohedron. Variable-temperature, solid-state magnetic susceptibility studies in the 5.0–300K temperature range reveal the predominance of antiferromagnetic inter-Mn3 exchange couplings. Alternating current magnetic susceptibility studies on 4 show that the single-molecule magnet (SMM) behavior characteristic of [Mn3O(O2CR)3(mpko)3]+ complexes has been lost. This work demonstrates that constructing MOF-like networks from SMMs by weak interactions is feasible but that even these can introduce significant inter-SMM exchange interactions that weaken or destroy the SMM properties.

Emissive {Mn4(III)Ca} clusters with square pyramidal topologies: syntheses and structural, spectroscopic, and physicochemical characterization.

The one-pot reactions between Mn(ClO4)2·6H2O, Ca(ClO4)2·4H2O, and the potentially tetradentate (N,O,O,O) chelating/bridging ligand salicylhydroxime (shiH3), resulting from the in situ metal ion-assisted amide-iminol tautomerism of salicylhydroxamic acid in the presence of various fluorescence carboxylate groups and base NEt3, afford a family of structurally similar {Mn4Ca} clusters with distorted square pyramidal topology. The reported complexes (NHEt3)2[Mn4Ca(L1)4(shi)4] (1), (NHEt3)2[Mn4Ca(L2)4(shi)4] (2), (NHEt3)5[Mn4Ca(L2)4(shi)4(shiH2)2](ClO4) (3), and (NHEt3)2[Mn4Ca(L3)4(shi)4] (4) contain a similar [Mn4Ca(μ-NO)4](10+) core of four Mn(III) atoms at the square base and a Ca(II) atom occupying the apical site. Peripheral ligation about the core is provided by four η(1):η(1):μ carboxylate groups of the anions of 2-naphthoic acid (L1(-)), 9-anthracenecarboxylic acid (L2(-)), and 1-pyrenecarboxylic acid (L3(-)). Solid-state direct current magnetic susceptibility studies revealed the presence of predominant antiferromagnetic exchange interactions between the 4 Mn(III) centers, which were primarily quantified by using a simple 1-J fit model to give S = 0 spin ground states with low-lying excited states close in energy to the ground state. Solution studies in solvent MeCN were carried out on all complexes and confirmed their structural integrity. Cyclic voltammetry studies showed a similar well-defined reversible oxidation and an irreversible reduction for all complexes, thus establishing their redox potency and electrochemical efficiency. Emission studies in solution proved the optical activity of all compounds, with the observed "blue" emission peaks attributed to the π-rich chromophores of the organic fluorescence ligands. The combined results demonstrate the ability of shiH3 and fluorescence carboxylates to yield new heterometallic Mn/Ca clusters with (i) the same Mn/Ca ratio as the oxygen-evolving complex of Photosystem II, (ii) structural stability in solution, and (iii) a pronounced redox and optical activity.

A Mn4 single-molecule magnet with the defective-dicubane structure from the use of pyrenecarboxylic acid

The reaction between Mn(pc)2·4H2O (pcH is 1-pyrenecarboxylic acid), N-methyldiethanolamine (mdaH2), and NEt3 in a 2:2:3 molar ratio in CH2Cl2 or CHCl3 gives the tetranuclear complex [Mn4(pc)4(mda)2(mdaH)2], isolated as the ·2CH2Cl2 (3) and ·2CHCl3 (4) solvates, respectively. Only crystals of 4 were of sufficient quality for single-crystal X-ray crystallography. Complex 4 possesses a [MnII2MnIII2] core with the Mn ions arranged in a planar rhombus that can be described as two Mn3 triangles fused at one edge. Additional monoatomic bridging by mda2− or mdaH−μ- and μ3-O atoms on each edge and at the center of the Mn3 triangular units gives a defective-dicubane core structure. Ligation is completed by two bridging and two monodentate pc− groups, each of which is involved in π–π stacking interactions with those on neighboring Mn4 molecules to give a 2D network. Variable-temperature solid-state magnetic susceptibility studies of 3 and 4 in the temperature range 5.0–300K. Various fitting and simulation models were employed in analyzing the data for 3, which shows no evidence of significant intermolecular interactions, and it was concluded that it contains three symmetry-inequivalent exchange interactions, J=+2.34cm−1, J′=+7.70cm−1, J″=−1.33cm−1 and g=1.99, where J′ is the MnIII⋯MnIII interaction, and J and J″ are the MnII⋯MnIII interactions. This indicates an S=8 ground state with a very low-lying an S=9 excited state. This conclusion was supported by fits of reduced magnetization data, which gave an S=8 ground state with axial zero-field splitting parameter D=−0.19(1)cm−1, and g=1.94(1). Ac susceptibility studies on 3 show that it is a single-molecule magnet (SMM). Complex 4 shows different magnetic behavior from 3 due to the intermolecular interactions.

Complexes of manganese, iron and cobalt with sterically demanding indenyl ligands

A series of manganese, iron and cobalt complexes bearing sterically demanding 1,3-disubstituted indenyl ligands, 1,3-(Me(3)C)(2)C(9)H(5) (Ind(tBu)) (1) and 1,3-(C(6)H(11))(2)C(9)H(5) (Ind(cHexyl)) (2), has been prepared. These complexes have been fully characterised by various spectroscopic techniques, elemental analysis, and X-ray diffraction experiments. In addition the electronic and steric properties of these ligands have been evaluated. Although the cone angles and electronic properties are similar to 1,2,4-(Me(3)C)(3)C(5)H(2) (Cp'), indenyl iron half-sandwich complexes are only stable at low temperature. This has been demonstrated for 1-FeI using suitable trapping experiments such as CO or NaCp' addition to yield 1-Fe(CO)(2)I and 1-FeCp', respectively. Overall the metal-ligand bonds in these indenyl compounds are weaker than in the corresponding cyclopentadienyl derivatives. In addition, the bis(indenyl)manganese complexes, 1-Mn and 2-Mn, are high-spin, as established by solid state magnetic susceptibility studies in the temperature range 2-300 K.

Ferromagnetic heteronuclear {Fe4(Er,Lu)2} cyclic coordination clusters based on ferric wheels

A new family of hexanuclear ferric-lanthanide [Fe(III)(4)Ln(III)(2)(Htea)(4)(Piv)(6)(N(3))(4)] clusters, where Ln(III) = Er(1) and Lu(2) is reported. Variable temperature solid-state magnetic susceptibility studies of 1 and 2 reveal the presence of ferromagnetic (1) or competing anti- and ferromagnetic exchange interactions (2) between the constituent Fe(III) ions.

Structure, Magnetism and Theory of a Family of Nonanuclear CuII5LnIII4–Triethanolamine Clusters Displaying Single‐Molecule Magnet Behaviour

Synthesis, crystal structures and magnetic studies are reported for four new heterometallic Cu(II)-Ln(III) clusters. The reaction of Cu(NO(3))(2)·3H(2)O with triethanolamine (teaH(3)), pivalic acid, triethylamine and Ln(NO(3))(3)·6H(2)O (Ln=Gd, Tb, Dy and Ho) results in the formation of four isostructural nonanuclear complexes of general formula [Cu(II)(5)Ln(III)(4)O(2)(teaH)(4){O(2)CC(CH(3))(3)}(2)(NO(3))(4)(OMe)(4)]·2MeOH·2Et(2)O [Ln=Gd (1), Tb (2), Dy (3) and Ho (4)]. The metal core of each cluster is made up of four face- and vertex-sharing tetrahedral units. Solid-state DC magnetic susceptibility studies reveal competing anti- and ferromagnetic interactions within each cluster leading to large-spin ground states for 1-4. Solid-state AC magnetic susceptibility studies show frequency-dependent out-of-phase (χ''(M)) signals for 2-4 below 4 K, suggestive of single-molecule magnet behaviour. Ab initio calculations on one of the anisotropic examples (3) provided a rare set of J values for Dy-Cu and Cu-Cu exchange interactions (Dy-Dy zero), some ferro- and some antiferromagnetic in character, that explain its magnetic behaviour.

Triangular Ni II2Ln III and Ni II2Y III complexes derived from di-2-pyridyl ketone: Synthesis, structures and magnetic properties

The synthetic investigation of the Ni II/M(NO 3) 3·6H 2O/di-2-pyridyl ketone [(py) 2CO] tertiary reaction system in EtOH has yielded triangular Ni 2M cationic complexes (M = lanthanide, Y). The reaction between Ln(NO 3) 3·6H 2O, Ni(ClO 4) 2·6H 2O, (py) 2CO and base (1:3:3:3) in EtOH under gentle heating gave the isostructural complexes [Ni 2Ln{(py) 2C(OEt)(O)} 3{(py) 2C(OH)(O)}(NO 3)(H 2O)](ClO 4) 2 (Ln = Gd, 2; Ln = Tb, 3) in high yields. The ligands (py) 2C(OEt)(O) − and (py) 2C(OH)(O) − are the monoanions of the hemiketal and gem-diol derivatives of (py) 2CO, respectively, formed in situ in the presence of the metal ions. The cations of 2 and 3 consist of one 8-coordinate Ln III and two distorted octahedral Ni II atoms in an essentially isosceles, triangular arrangement capped by a central μ 3-Ο − atom of the unique 3.3011 (Harris notation) (py) 2C(OH)(O) − ligand. Each metal–metal edge is bridged by the deprotonated O atom of one 2.2011 (py) 2C(OEt)(O) − ligand. The isostructural complexes [Ni 2M{(py) 2C(OEt)(O)} 4(NO 3)(H 2O)] 2[M(NO 3) 5](ClO 4) 2 (M = Y, 4 ; M = Tb, 5 ; M = Dy, 6) were prepared by the 1:1 reaction of the mononuclear “metalloligand” [Ni(O 2CMe){(py) 2CO}{(py) 2C(OH) 2}](ClO 4) ( 1) and M(NO 3) 3·6H 2O in EtOH under mild heating in moderate to good yields. The structures of the dications of 4– 6 are similar to those in 2 and 3, the only difference being the replacement of the unique 3.3011 (py) 2C(OH)(O) − ligand of the latter by one 3.3011 (py) 2C(OEt)(O) − group in the former. The Y III, Tb III and Dy III atoms in [M(NO 3) 5] 2− are coordinated by five bidentate chelating nitrato groups. Characteristic IR bands of the complexes are discussed in terms of the known structures and the coordination modes of the ligands. Variable temperature, solid-state direct current magnetic susceptibility and magnetization studies were carried out on dried samples of 2– 4. The data indicate ferromagnetic Ni⋯Ni and Ni⋯Gd exchange interactions, and an S T = 11/2 ground state for 2. Complex 3 is characterized by a high-spin ground state while the ferromagnetic Ni⋯Ni interaction for 2 is independently supported by the study of 4. No out-of-phase, alternating current susceptibility signals have been detected for 3 that would be indicative of SMM behavior.

Polynuclear Cation−Cation Complexes of Pentavalent Uranyl: Relating Stability and Magnetic Properties to Structure

Reaction of {[UO2Pys][mu-KI2Py2]}n (1) with 2 equiv of potassium dibenzoylmethanate (Kdbm) in pyridine or acetonitrile affords, respectively, the corresponding tetranuclear complexes of pentavalent uranyl ([UO2(dbm),2]2[mu-K(Py)2]2[mu8-K(Py)]}2I2 x Py2 (2) (in 70% yield) and {[UO2(dbm)2]2[mu-K(MeCN)2][mu8-K]}2 (3) (in 40% yield) in which four UO2+ are mutually coordinated (T-shaped "cation-cation" interaction). The X-ray structures of 2 and 3 show also the presence of, respectively, six and four potassium cations involved in UO2+...K+ interactions. Reaction of 2 with an excess of 18-crown-6 (18C6) affords the dimeric complex [UO2(dbm)2K(18C6)]2 (4) presenting a diamond-shaped interaction between two UO2+ groups, in 45% yield. 1H and PFGSTE diffusion NMR spectroscopy of 2 and 3 in pyridine show unambiguously the presence of UO2+...UO2+ and UO2+...K+ interactions (tetrametallic species) in solution, which leads to a rapid (7 days) disproportionation of pentavalent uranyl to afford [U(dbm)4] and [UO2(dbm)2] species. The UO2+...K+ interaction plays an important synergistic role in the stabilization of the UO2+...UO2+ interactions. Accordingly, the lower affinity of (K(18C6))+ for the uranyl(V) oxygen in complex 4 results in a lower number of coordinated K+ and therefore in a weakened UP2+...UO2+ interaction. The UO24+...UO2+ interactions is completely disrupted in dmso or in the presence of Kdbm, preventing disproportionation of pentavalent uranyl. Solid-state variable-temperature magnetic susceptibility studies showed the unambiguous presence of antiferromagnetic coupling between the two oxo-bridged uranium centers of complex 4, with the appearance of a maximum in chi versus T at approximately 5 K. The different behavior of the tetrameric complex 3, which probably involves a magnetic coupling occurring at lower temperature, can be ascribed to the different geometric arrangement of the interacting uranyl(V) groups.

Preparation and Reactions of Base-Free Bis(1,3-di-tert-butylcyclopentadienyl)titanium, Cp′2Ti, and Related Compounds

The titanium(III) metallocene derivatives [1,3-(Me3C)2C5H3]2TiX, Cp′2TiX, X = Cl, Me, H, OH, are prepared and shown to be monomeric with a d1 electron configuration by solid state magnetic susceptibility studies over the temperature range 5–300 K. Reduction of the chloride by potassium amalgam in an argon atmosphere yields the base-free titanocene Cp′2Ti, which is a spin triplet over the temperature range 5–300 K. In contrast, reduction in a nitrogen atmosphere gives the dimetal metallocene Cp′2Ti(N2)TiCp′2, which shows a singlet–triplet equilibrium over the temperature range 5–300 K, which can be modeled by Heisenberg coupling with −2J = 210 cm−1. The base-free titanocene reacts reversibly with H2 or C2H4 to give Cp′2TiH2 and Cp′2Ti(C2H4), respectively. Both adducts are characterized by X-ray crystallography. The base-free titanocene reacts irreversibly with PhC≡CPh or N2O to give Cp′2Ti(PhC≡CPh) and Cp′4Ti2(μ-O), which are characterized by X-ray crystallography. In contrast Cp′2Ti(C2H4) reacts with N2O to give the bisoxo-bridged dimetal derivative Cp′4Ti2(μ-O)2.

Single-Molecule Magnets: A Family of MnIII/CeIV Complexes with a [Mn8CeO8]12+ Core

Four heterometallic, enneanuclear Mn8Ce clusters [Mn8CeO8(O2CMe)12(H2O)4] (4), [Mn8CeO8(O2CMe)12(py)4] (5), [Mn8CeO8(O2CPh)12(MeCN)4] [Mn8CeO8(O2CPh)12(dioxane)4] (6), and [Mn8CeO8(O2CCHPh2)12(H2O)4] (7) have been prepared by various methods. Their cores are essentially isostructural and comprise a nonplanar, saddlelike [MnIII8O8]8+ loop containing a central CeIV ion attached to the eight micro3-O2- ions. Peripheral ligation around the [Mn8CeO8]12+ core is provided by eight micro- and four micro3-O2CR- groups. Terminal ligation on four MnIII atoms is provided by H2O in 4 and 7, pyridine in 5, and MeCN/dioxane in 6. Solid-state magnetic susceptibility studies, fits of dc magnetization vs field and temperature data, and in-phase ac susceptibility studies in a zero dc field have established that complexes 4, 5, and 7 possess S=16, S=4 or 5, and S=6+/-1 spin ground states, respectively, but in all cases there are very low-lying excited states. The large variation in the ground-state spins for this isostructural family is rationalized as due to a combination of weak exchange interactions between the constituent MnIII atoms, and the presence of both nearest-neighbor and next-nearest-interactions of comparable magnitudes. Magnetization vs applied dc field sweeps on single crystals of 4.4H2O and 7.4H2O.3MeCN.2CH2Cl2 down to 0.04 K have established that these two complexes are new single-molecule magnets (SMMs). The former also shows an exchange-bias, a perturbation of its single-molecule properties from very weak intermolecular interactions mediated by hydrogen-bonding interactions with lattice-water molecules of crystallization.

Mixed 3d/4d and 3d/4f metal clusters: Tetranuclear [formula omitted] and [formula omitted] complexes, and the first Fe/4f single-molecule magnets

The synthesis, structure and magnetic properties are reported of Fe 2Ln 2, Fe 2Y 2, Mn 2Yb 2 and Mn 2Y 2 complexes. The compound [Fe 2Ho 2(OH) 2(teaH) 2(O 2CPh) 4(NO 3) 2] ( 1), where teaH 3 is triethanolamine, was obtained from the reaction of eight equivalents of teaH 3 with one equivalent each of Ho(NO 3) 3 and [Fe 3O(O 2CPh) 6(H 2O) 3](O 2CPh) in MeCN/MeOH. The use instead of Dy(NO 3) 3 or Tb(NO 3) 3 leads to the structurally similar products [Fe 2Dy 2(OH) 2(teaH) 2(O 2CPh) 6] ( 2) and [Fe 2Tb 2(OH) 2(teaH) 2(O 2CPh) 6] ( 3). The compounds [Fe 2Y 2(pdmH) 6Cl 4]Cl 2 ( 4) and isostructural [Fe 2Ho 2(pdmH) 6Cl 4]Cl 2 ( 5), where pdmH 2 is pyridine-2,6-dimethanol, were prepared from the reaction of four equivalents of pdmH 2 with two equivalents of YCl 3 or HoCl 3, respectively, and FeCl 2 in MeOH. The compounds [Mn 2Y 2O 2(O 2CPh) 6(OMe) 4(MeOH) 4] ( 6) and [Mn 2Yb 2O 2(O 2CPh) 6(OMe) 4(MeOH) 4] ( 7) were prepared by methanolysis of ( NBu 4 n ) - [ Mn 4 O 2 ( O 2 CPh ) 9 ( H 2 O ) ] in the presence of Yb(NO 3) 3 or Y(NO 3) 3, respectively. Complexes 1– 3, 6 and 7 contain a ‘defect dicubane’ M 4 core (including the first mixed 3d/4f examples of this type to contain Fe III or Mn IV), whereas 4 and 5 have a U-shaped core. Variable-temperature solid-state magnetic susceptibility studies of 1– 6 in the temperature range 5.00–300 K were carried out, and for all the complexes predominantly antiferromagnetic exchange interactions between the metal centers was observed. Magnetization versus applied field sweeps on single crystals of 1 and 2 at low temperature reveal hysteresis loops, confirming these species to be new examples of single-molecule magnets (SMMs). Thus, complexes 1 and 2 are the first examples of Fe/4f SMMs.

Crystallography and magnetism of 3-(N-tert-butyl-N-aminoxyl)benzoic acid

3-(N-tert-Butyl-N-aminoxyl)benzoic acid is a stable red solid that crystallizes to form chains of nitroxide to aryl CH contacts, with the pairs of such antiparallel chains joined by complementary carboxylic acid two-point hydrogen bonds. Although it strongly adsorbs to silica gel, it does not show solution phase ESR evidence of discrete triplet state radical pairing. Solid-state magnetic susceptibility studies show antiferromagnetic downturn at lower temperatures, with a change in slope at about 10 K in Curie–Weiss plots. The results suggest either a phase change at low temperatures, or the presence of multiple competing exchange interactions.

New hexanuclear and dodecanuclear Fe(III) clusters with carboxylate and alkoxide-based ligands from cluster aggregation reactions

The syntheses, structures and magnetic properties are reported of three new iron(III) clusters [Fe 6(μ 3-O) 2(O 2CH 2)(O 2CCH 2Bu t ) 12(py) 2] ( 1), [Fe 6(μ 3-O) 2(O 2CCH 2Cl) 6(moe) 6](NO 3) 2 ( 2) and [Fe 12(μ 3-O) 8(μ-OMe) 2(O 2CPh) 12(pam) 6] ( 3). Complex 1 was synthesised from the reaction of [Fe 3O(O 2CCH 2Bu t ) 6(H 2O) 3](O 2CCH 2Bu t ) with pyridine in MeCN. Complex 2 was prepared by treatment of [Fe 3O(O 2CCH 2Cl) 6(H 2O) 3](O 2CCH 2Cl) and Fe(NO 3) 3 with methoxyethanol (moeH) in MeCN, and complex 3 was obtained from the reaction of [Fe 3O(O 2CPh) 6(H 2O) 3](O 2CPh), syn-2-pyridinealdoxime (pamH) and NaOMe in MeCN. All three compounds can be structurally described as representing different ways of linking or fusing oxide-centered [Fe 3(μ 3-O)] 7+ triangular units. Thus, the Fe 6 core of 1 comprises two such units bridged at one of their edges to give a twisted boat conformation, whereas the Fe 6 core of the cation of 2 comprises two [Fe 3(μ 3-O)] 7+ units face-to-face in a staggered manner to give an Fe 6 octahedron. The more complicated Fe 12 core of 3 contains a central Fe 6 unit that can be described in various ways, one of which is as four edge-sharing [Fe 3(μ 3-O)] 7+ triangular units and two additional [Fe 3(μ 3-O)] 7+ units attached to the flanks. One particular additional point of interest in 1 is the occurrence of a rare methanediolate group formed in situ and bound in an unusual μ 4-bridging mode. Variable-temperature solid-state magnetic susceptibility studies on complexes 1– 3 in the temperature range 5.00–300 K reveal that the complexes all possess S = 0 ground state spins, consistent with the expected antiferromagnetic exchange interactions between the constituent iron(III) ions.

A new class of oxo-bridged high-valent hexamanganese clusters supported by sterically hindered carboxylate ligands.

A new class of oxo-bridged high-valent hexamanganese (Mn6) clusters containing a novel (Mn6O8)6+ core, [MnIV(4)MnIII2(mu-O)4(mu3-O)4(dmb)6(O2CR)2]4+ (where dmb=4,4'-dimethyl-2,2'-bipyridine, and RCO2=2,6-di(p-tolyl)benzoate (Ar(Tol)CO2-) (3) or 2,6-di(4-tert-butylphenyl)benzoate (Ar(4-tBuPh)CO2-) (4)), was synthesized using sterically hindered m-terphenyl-derived carboxylate ligands. These complexes can be synthesized by oxidizing the MnII mononuclear complexes, [Mn(dmb)2(OH2)(O2CR)]+ (where RCO2=Ar(Tol)CO2- (1) or Ar(4-tBuPh)CO2- (2)) with (n-Bu4N)MnO4, by direct Mn(II) + Mn(VII) in situ comproportionation reactions, or by ligand substitution on the dinuclear manganese (III,IV) or (IV,IV) complexes, [(Mn2(mu-O)2(dmb)4)](3+/4+). The compound [MnIV4MnIII2(mu-O)4(mu3-O)4(dmb)6(Ar(Tol)CO2)2](OTf)4 [3(OTf)4] crystallizes in the monoclinic space group P2(1)/n, with the cell parameters a=15.447(1) A, b=15.077(2) A, c=27.703(2) A, beta=91.68(2) degrees, V=6449.3(6) A3, and Z=2. The X-ray structure reveals that there are three different bridging modes for the oxo groups: mu, "pyramidal" mu3, and "T-shaped" mu3. Solid-state variable temperature magnetic susceptibility studies suggest that the Mn centers are net antiferromagnetically coupled to yield a diamagnetic ST=0 ground spin state with a large number of low-lying, thermally accessible states with ST>0. 1H NMR spectra were recorded for both Mn6 clusters and selected resonances assigned. The electronic and redox properties of these complexes along with the effect of the presence of the bulky carboxylate ligands are also described here.

Preparation and properties of new Fe6 and Fe8 clusters of iron(iii) with tripodal ligands

The syntheses, structures and magnetic properties are reported of new iron(III) complexes with Fe6 wheel and Fe8 cage topologies, and containing tripodal ligands. The compound [Fe8O3(O2CPh)9(tea)(teaH)3]·MeCN (1·MeCN), where teaH3 is triethanolamine, was obtained from the reaction of two equivalents of teaH3 with [Fe3O(O2CPh)6(H2O)3](O2CPh) in MeCN. Similarly, the compound [Fe6(bic)6]·4H2O·4MeOH (2·4H2O·4MeOH), where bicH3 is bicine, was prepared by the analogous reaction but with two equivalents of bicine. Variable-temperature solid-state magnetic susceptibility studies of 1 and 2 in the temperature range 5.00–300 K reveal that both complexes possess S = 0 ground state spins, consistent with the expected antiferromagnetic exchange interactions between the constituent iron(III) ions.