Polymetallic clusters of iron(III) with derivatised salicylaldoximes

[Fe(µ2-OH)6]3- Linked Fe3O Triads: Mössbauer Evidence for Trigonal µ3-O2- or µ3-OH- Groups in Bridged versus Unbridged Complexes

The syntheses, coordination chemistry, and Mössbauer spectroscopy of hepta-iron(III) complexes using derivatised salicylaldoxime ligands from two categories; namely, 'single-headed' (H2L) and 'double-headed' (H4L) salicylaldoximes are described. All compounds presented here share a [Fe3-µ3-O] core in which the iron(III) ions are µ3-hydroxo-bridged in the complex C1 and µ3-oxo-bridged in C2 and C3. Each compound consists of 2 × [Fe3-µ3-O] triads that are linked via a central [Fe(µ2-OH)6]3- ion. In addition to the charge balance and microanalytical evidence, Mössbauer measurements support the fact that the triads in C1 are µ3-OH bridged and are µ3-O bridged in C2 and C3.

[Fe15]: a frustrated, centred tetrakis hexahedron

The combination of two different Fe^III salts in a solvothermal reaction with triethanolamine results in the formation of a high symmetry [FeIII15] cluster whose structure conforms to a centred, tetrakis hexahedron.

Synthesis and Characterization of Symmetrically versus Unsymmetrically Proton-Bridged Hexa-Iron Clusters

Syntheses and magnetic and structural characterization of hexa-iron complexes of derivatized salicylaldoximes are discussed. Complexation of Fe(BF₄)₂·6H₂O with each ligand (H₂ L1 and H₄ L2) in a methanolic-pyridine solution resulted in hexa-iron compounds (C1 and C2, respectively), which each contain two near-parallel metal triangles of [Fe₃-μ₃-O], linked by six fluoride bridges and stabilized by a hydrogen-bonded proton between the μ₃-O groups. Within each metal triangle of C2, Fe(III) ions are connected via the amine "straps" of (H₄ L2-2H). Variable-temperature magnetic susceptibility and Mössbauer data of C1 and C2 indicate the presence of dominant antiferromagnetic interactions between the high-spin (S = 5/2) Fe(III) centers. For C1, two quadrupole doublets are observed at room temperature and 5 K, consistent with structural data from which discrete but disordered [Fe₃-μ₃-O] and [Fe₃-μ₃-OH] species were inferred. For C2, a single sharp quadrupole doublet with splitting intermediate between those determined for C1 was observed, consistent with the symmetric [Fe₃-μ₃-O···H···μ₃-O-Fe₃] species inferred crystallographically from the very short μ₃-O···μ₃-O separation. The differences in the physical properties of the complexes, as seen in the Mössbauer, X-ray, and magnetic data, are attributed to the conformational flexibility imparted by the nature of the linkages between the closely related ligands.

Atomically Precise Lanthanide-Iron-Oxo Clusters Featuring the ϵ-Keggin Ion

Atomically precise molecular metal-oxo clusters provide ideal models to understand metal oxide surfaces, self-assembly, and form-function relationships. Devising strategies for synthesis and isolation of these molecular forms remains a challenge. Here, the synthesis of four Ln-Fe oxo clusters that feature the ϵ-{Fe₁₃ } Keggin cluster in their core is reported. The {Fe₁₃ } metal-oxo cluster motif is the building block of two important iron oxyhydroxyide phases in nature and technology, ferrihydrite (as the δ-isomer) and magnetite (the ϵ-isomer). The reported ϵ-{Fe₁₃ } Keggin isomer as an isolated molecule provides the opportunity to study the formation of ferrihydrite and magnetite from this building unit. The four currently reported isostructural lanthanide-iron-oxo clusters are fully formulated [Y₁₂ Fe₃₃ (TEOA)₁₂ (Hyp)₆ (μ₃ -OH)₂₀ (μ₄ -O)₂₈ (H₂ O)₁₂ ](ClO₄ )₂₃ ⋅50 H₂ O (1, Y₁₂ Fe₃₃ ), [Gd₁₂ Fe₃₃ (TEOA)₁₂ (Hyp)₆ (μ₃ -OH)₂₀ (μ₄ -O)₃₂ (H₂ O)₁₂ ](ClO₄ )₁₅ ⋅50 H₂ O (2, Gd₁₂ Fe₃₃ ) and [Ln₁₆ Fe₂₉ (TEOA)₁₂ (Hyp)₆ (μ₃ -OH)₂₄ (μ₄ -O)₂₈ (H₂ O)₁₆ ](ClO₄ )₁₆ (NO₃ )₃ ⋅n H₂ O (Ln=Y for 3, Y₁₆ Fe₂₉ , n=37 and Ln=Gd for 4, Gd₁₆ Fe₂₉ n=25; Hyp=trans-4-Hydroxyl-l-proline and TEOA=triethanolamine). The next metal layer surrounding the ϵ-{Fe₁₃ } core within these clusters exhibits a similar arrangement as the magnetite lattice, and Fe and Ln can occupy the same positions. This provides the opportunity to construct a family of compounds and optimize magnetic exchange in these molecules through composition tuning. Small-angle X-ray scattering (SAXS) and high-resolution electrospray ionization mass spectrometry (HRESI-MS) show that these clusters are stable upon dissolution in both water and organic solvents, as a first step to performing further chemistry towards building magnetic arrays or investigating ferrihydrite and magnetite assembly from pre-nucleation clusters.

Theoretical Studies on Hexanuclear [M3(μ3-O/OH)]2 (M = Fe(III), Mn(III), and Ni(II)) Clusters: Magnetic Exchange, Magnetic Anisotropy, and Magneto-Structural Correlations

Controlling spin Hamiltonian parameters such as magnetic exchange and magnetic anisotropy of polynuclear clusters is of great interest in the area of single molecule magnets (SMMs). Among large polynuclear clusters, hexanuclear clusters offer the best compromise in terms of size as they are often rigid, solution stable, and chemically amenable. The {M₆O₂} core is one of the common architectures known for many hexanuclear clusters and generally reported to possess a diamagnetic S_T = 0 spin ground state, barring a few exceptions. In these clusters, there are several open questions that are poorly understood: (a) What controls the nature of magnetic exchange, which in turn dictates the ground state spin values? (b) For clusters possessing a nonzero spin ground state, what dictates the magnetic anisotropy? Here, using density functional methods, we have attempted to shed light on these two question by evaluating the exchange coupling constants in [Fe₆^IIIO₂(OH)₂{(C₄N₂H₂SMe)₂C(OH)O}₂( ^tBuCO₂)₁₀] (1), [Fe₆^III(O)₂(O₂CH₂)(O₂CCH₂ ^tBu)₁₂(py)₂] (2), [Fe₆^III(O₂)(O)₂(O₂CCMe₃)₁₂(py)₂] (3), [Fe^III₆O₃(O₂CMe)₉(OEt)₂(bpy)₂]ClO₄ (4), [Mn^III₆O₂(O₂CH₂)(O₂CPe ^t)₁₁(HO₂CPe ^t)₂(O₂CMe)] (5), and [Ni^II₆(OH)₄(O₂C ^tBu)₈( ^tBuCO₂H)₄] (6) complexes. We have estimated all the eight near-neighbor exchange coupling constants in these clusters. Our calculations not only agree with the experimental results but also offer insight on the origin of the spin ground state. Extensive magneto-structural correlations developed by varying M-O-M angles and M-O distances reveal that J values are extremely sensitive to small structural distortions. Correlations developed indicate that both the parameters are important for Fe(III), but for Mn(III) and Ni(II), the angles were found to play a dominant role. Quite interestingly, the computed zero-field splitting parameter D _S=5 of complex 1 reveals that the exchange contribution to the anisotropy controls the sign of the ground state D value-an observation which differs from the general perception that the ground state D is controlled by the single-ion zero-field splitting parameter.

Binding of ligands containing carbonyl and phenol groups to iron(iii): new Fe6, Fe10 and Fe12 coordination clusters

The initial use of ligands 2'-hydroxyacetophenone (HL¹), 2-hydroxybenzophenone (HL²) and 2,2'-dihydroxybenzophenone (H₂L³) in iron(iii) chemistry is described. The syntheses and crystal structures are reported for five iron(iii) clusters: [Fe₁₀O₄(OMe)₁₄(L¹)₆(MeOH)₂](NO₃)₂·3MeOH (1·3MeOH), [Fe₁₂O₄(OH)(OMe)₁₇(L¹)₈](ClO₄)₂·2H₂O (2·2H₂O), [Fe₁₀O₄(OMe)₁₄Cl₄(L²)₄(MeOH)₂] (3), [Fe₁₀O₄(OMe)₁₄(L²)₆(py)₂](ClO₄)₂·MeOH (4·MeOH), where py = pyridine, and [Fe₆O₂(OEt)₆(O₂CMe)₂(L³)₂(HL³)₂] (5). The molecular structures of the decanuclear clusters 1, 3 and 4 are organized around a {Fe₁₀(μ₄-O)₄(μ₃-OMe)₂(μ-OMe)₁₂}⁸⁺ core consisting of ten {Fe₃O₄} face-sharing defective cubane units. The core of 2 consists of a {Fe₁₂(μ₄-O)₄(μ₃-OMe)₄(μ-OH)(μ-OMe)₁₃}¹⁰⁺ unit composed of twelve {Fe₃O₄} face-sharing defective cubanes. The ligands (L¹)^- and (L²)^- in 1-4 adopt the O,O'-bidentate chelating coordination mode and their roles are to terminate the further aggregation of the Fe^III/O^2-/RO^- cores. Complex 5 contains the {Fe₆(μ₄-O)₂(μ-OEt)₆(μ-O_carbonyl)₂}⁴⁺ core, where the μ-O_carbonyl atoms are the bridging carbonyl oxygens of the two η¹:η²:η¹:μ (L³)^2- ligands; the (HL³)^- groups behave as O_phenolate, O_carbonyl-bidentate chelating ligands with the neutral hydroxyl group being unbound to the Fe^III atoms. The core is composed of four {Fe₃O₄} face-sharing defective cubanes. The Fe^III atoms in 1-5 are all six-coordinate with distorted octahedral geometries. The IR spectra of the complexes are discussed in terms of the known coordination modes of the ligands and the ionic character of nitrates and perchlorates. Variable-temperature magnetic susceptibility and variable-field magnetization measurements establish that 2, 3 and 5 have S = 3, 0 and 5 ground states, respectively. The susceptibility data for 5 were fitted using a 3-J model indicating the simultaneous presence of both antiferromagnetic and ferromagnetic Fe^IIIFe^III exchange interactions. Known magnetostructural correlations in oxido-bridged iron(iii) systems have been applied to rationalise the magnetic behaviour of the three clusters. The results of the present study demonstrate the utility of HL¹, HL² and H₂L³ in the stabilisation of robust iron(iii)/oxido/alkoxido clusters.

Nine members of a family of nine-membered cyclic coordination clusters; Fe6Ln3 wheels (Ln = Gd to Lu and Y)

We report a family of isostructural nonanuclear Fe^III–Ln^III cyclic coordination clusters, [FeIII6LnIII3(μ-OMe)₉(vanox)₆ (benzoate)₆], with a planar core structure built up from three [Fe₂Ln] units to give a giant Ln₃ triangle embedded into an Fe₆ ring.

A self-assembled octanuclear complex bearing the uncommon close-packed {Fe4Mn4(μ4-O)4(μ-O)4} molecular core

An Fe/Mn Schiff base complex with an unprecedented metal core type for a heterometallic compound, has been prepared by the one-pot direct synthesis method and magnetostructurally characterized.

Kravtsov V, Ellern A, Kögerler P, Baca SG. Iron(iii) carboxylate/aminoalcohol coordination clusters with propeller-shaped Fe8 cores: approaching reasonable exchange energies

A new approach has been used to determine reasonable exchange energies in a series of new octanuclear propeller-shaped Fe(iii) clusters that feature distorted hexagonal-bipyramidal Fe₈ polytopes of isotropic spin-5/2 centers.

A novel alkaline earth strontium(II) coordination polymer: poly[hexaaquatetrakis(μ5-pyridine-2,6-dicarboxylato)bis(μ4-pyridine-2,6-dicarboxylato)(μ7-sulfato)heptastrontium(II)]

The title compound, [Sr7(C7H3NO4)6(SO4)(H2O)6]n, has been synthesized by an ionothermal method using the ionic liquid 1-ethyl-3-methylimidazolium ([Emim]Br) as solvent, and characterized by elemental analysis, energy-dispersive X-ray spectroscopy, IR and single-crystal X-ray diffraction. The structure of the compound can be viewed as a three-dimensional coordination polymer composed of Sr(2+) cations, pyridine-2,6-dicarboxylate anions, sulfate anions and water molecules. The compound not only exhibits a three-dimensional structure with a unique coordination mode of the sulfate anion, but also features the first example of a heptanuclear strontium(II) coordination polymer. The structure is further stabilized by O-H···O hydrogen bonds and π-π stacking interactions.

Linking [M(III)3] triangles with "double-headed" phenolic oximes

Strapping two salicylaldoxime units together with aliphatic α,Ω-aminomethyl links in the 3-position gives ligands which allow the assembly of the polynuclear complexes [Fe(7)O(2)(OH)(6)(H(2)L1)(3)(py)(6)](BF(4))(5)·6H(2)O·14MeOH (1·6H(2)O·14MeOH), [Fe(6)O(OH)(7)(H(2)L2)(3)](BF(4))(3)·4H(2)O·9MeOH (2·4H(2)O·9MeOH) and [Mn(6)O(2)(OH)(2)(H(2)L1)(3)(py)(4)(MeCN)(2)](BF(4))(5)(NO(3))·3MeCN·H(2)O·5py (3·3MeCN·H(2)O·5py). In each case the metallic skeleton of the cluster is based on a trigonal prism in which two [M(III)(3)O] triangles are tethered together via three helically twisted double-headed oximes. The latter are present as H(2)L(2-) in which the oximic and phenolic O-atoms are deprotonated and the amino N-atoms protonated, with the oxime moieties bridging across the edges of the metal triangles. Both the identity of the metal ion and the length of the straps connecting the salicylaldoxime units have a major impact on the nuclearity and topology of the resultant cluster, with, perhaps counter-intuitively, the longer straps producing the "smallest" molecules.

Exploring the no-man's land between molecular nanomagnets and magnetic nanoparticles

The comparison of the structural and magnetic properties of molecular nanomagnets (MNM) and magnetic nanoparticles (MNP) can be instructive to get a deeper understanding of the magnetic behavior on the intermediate scale between molecular and bulk objects. In this respect iron oxo based clusters are particularly interesting, since they provide an increasing number of molecular systems with sizes close to that of iron oxide MNP. In this Minireview we report a survey of literature data aimed at improving our understanding of the emergence of MNP properties from MNM ones.

Hexa- and octanuclear iron(III) salicylaldoxime clusters

The syntheses, structures and magnetic properties of six iron complexes stabilised with the derivatised salicylaldoxime ligands Me-saoH(2) (2-hydroxyethanone oxime) and Et-saoH(2) (2-hydroxypropiophenone oxime) are discussed. The four hexanuclear and two octanuclear complexes of formulae [Fe(8)O(2)(OMe)(4)(Me-sao)(6)Br(4)(py)(4)]·2Et(2)O·MeOH (1·2Et(2)O·MeOH), [Fe(8)O(2)(OMe)(3.85)(N(3))(4.15)(Me-sao)(6)(py)(2)] (2), [Fe(6)O(2)(O(2)CPh-4-NO(2))(4)(Me-sao)(2)(OMe)(4)Cl(2)(py)(2)] (3), [Fe(6)O(2)(O(2)CPh-4-NO(2))(4)(Et-sao)(2)(OMe)(4)Cl(2)(py)(2)]·2Et(2)O·MeOH (4·2Et(2)O·MeOH), [HNEt(3)](2)[Fe(6)O(2)(Me-sao)(4)(SO(4))(2)(OMe)(4)(MeOH)(2)] (5) and [HNEt(3)](2)[Fe(6)O(2)(Et-sao)(4)(SO(4))(2)(OMe)(4)(MeOH)(2)] (6) all are built from a series of edge-sharing [Fe(4)(μ(4)-O)](10+) tetrahedra. Complexes 1 and 2 display a new μ(4)-coordination mode of the oxime ligand and join a small group of Fe-phenolic oxime complexes with nuclearity greater than six.

Discrete tetrairon(III) cluster exhibiting a square-planar Fe4(mu4-O) core: structural and magnetic properties

The aerobic reaction of the Schiff-base ligand N-(benzimidazol-2-yl)salicylaldimine (Hbisi) with iron(II) perchlorate in methanol leads to the formation of the remarkable coordination compound [Fe(4)(mu(4)-O)(mu-MeO)(4)(bisi)(4)](ClO(4))(2) x 4 MeOH (1), whose single-crystal X-ray structure reveals the presence of a discrete Fe(III)(4)(mu(4)-O) core. Magnetic and Mossbauer studies both show that the exchange interaction within the square tetranuclear iron(III) unit is dominated by the central bridging mu(4)-oxido ligand, the involvement of the mu-methoxido bridges being negligible.

Building Fe(III) clusters with derivatised salicylaldoximes

The syntheses, structures and magnetic properties of nine new iron complexes containing salicylaldoxime (saoH(2)) or derivatised salicylaldoximes (R-saoH(2)), [Fe(3)O(OMe)(Ph-sao)(2)Cl(2)(py)(3)].2MeOH (1.2MeOH), [Fe(3)O(OMe)(Ph-sao)(2)Br(2)(py)(3)].Et(2)O (2.Et(2)O), [Fe(4)(Ph-sao)(4)F(4)(py)(4)].1.5MeOH (3.1.5MeOH), [Fe(6)O(2)(OH)(2)(Et-sao)(2)(Et-saoH)(2)(O(2)CPh)(6)] (4), [HNEt(3)](2)[Fe(6)O(2)(OH)(2)(Et-sao)(4)(O(2)CPh(Me)(2))(6)].2MeCN (5.2MeCN), [Fe(6)O(2)(O(2)CPh)(10)(3-(t)But-5-NO(2)-sao)(2)(H(2)O)(2)].2MeCN (6.2MeCN), [Fe(6)O(2)(O(2)CCH(2)Ph)(10)(3-(t)But-sao)(2)(H(2)O)(2)].5MeCN (7.5MeCN), {[Fe(6)Na(3)O(OH)(4)(Me-sao)(6)(OMe)(3)(H(2)O)(3)(MeOH)(6)].MeOH}n (8.MeOH) and [HNEt(3)](2)[Fe(12)Na(4)O(2)(OH)(8)(sao)(12)(OMe)(6)(MeOH)(10)] (9) are discussed. The predominant building block appears to be the triangular [Fe(3)O(R-sao)(3)](+) species which can self-assemble into more elaborate arrays depending on reaction conditions. An interesting observation is that the R-saoH(-)/R-sao(2-) ligand system tends to adopt coordination modes similar to carboxylates. The most unusual molecule is the [Fe(4)F(4)] molecular square, 3. While Cl(-) and Br(-) appear to act only as terminal ligands, the F(-) ions bridge making a telling impact on molecular structure and topology.

Cation and anion selectivity of zwitterionic salicylaldoxime metal salt extractants

3-Dialkylaminomethyl substituted salicylaldoximes are efficient metal salt extractants, and, in contrast to related "salen"-based reagents, are sufficiently stable to acid hydrolysis to allow commercial application in base metal recovery. Crystal structures show that metal salts are bound by a zwitterionic form of the reagents, with copper(II) nitrate, tetrafluoroborate and trifluoroacetate forming [Cu()(2)X(2)] assemblies in a tritopic arrangement with a trans-disposition of the anions outwith the coordination sphere. Copper(II) chloride, bromide and zinc(II) chloride form 1:1 assemblies, [Cu()X(2)], with the halides in the inner coordination sphere of the metal, leading to high chloride selectivity and very good mass transport efficiencies of CuCl(2). Introduction of the anion-binding sites into the salicylaldoxime extractants changes their cation selectivities; the ligands co-extract small amounts of Fe(III) along with Cu(II) from mixed metal aqueous feed solutions, an issue which will need to be addressed prior to industrial application.