FeIII Centers Research Articles

Syntheses, Structures, and Magnetic Properties of a Novel mer-[(bbp)FeIII(CN)3]2– Building Block (bbp: bis(2-benzimidazolyl)pyridine dianion) and Its Related Heterobimetallic Fe(III)–Ni(II) Complexes

A new symmetrical tricyanide building block mer-[Fe(bbp)(CN)3](2-) [1; bbp = bis(2-benzimidazolyl)pyridine dianion] has been prepared and structurally and magnetically characterized. It forms a new low-spin meridionally capped {Fe(III)L(CN)3} fragment with the tridentate bbp ligand. The reaction of 1 with Ni(II) salts in the presence of various ancillary ligands affords several new cyanido-bridged complexes: a trinuclear complex {[Ni(ntb)(MeOH)]2[Fe(bbp)(CN)3][ClO4]2}·2MeOH (2), a tetranuclear compound {[Ni(tren)]2[Fe(bbp)(CN)3]2}·7MeOH (3), and a one-dimensional heterobimetallic system: {[Ni(dpd)2]2[Fe(bbp)(CN)3]2}·9MeOH·3H2O (4) [ntb = tris(2-benzimidazolylmethyl)amine, tren = tris(2-aminoethyl)amine, and dpd = 2,2-dimethyl-1,3-propanediamine]. The structural data shows that 2 is a linear complex in which a central Fe(III) ion links two adjacent Ni(II) ions via axial cyanides, while 3 is a molecular square that contains cyanido-bridged Ni(II) and Fe(III) ions at alternate corners. Complex 4 is a one-dimensional system that is composed of alternating cyanido-bridged Ni(II) and Fe(III) centers. Compounds 2-4 display extensive hydrogen bonding and moderately strong π-π stacking interactions in the solid state. Magnetic studies show that ferromagnetic exchange is operative within the Fe(III)LS(μ-CN)Ni(II) units of 2-4.

Efficient and Selective Oxidation of Primary and Secondary Alcohols Using an Iron(III)/Phenanthroline Complex: Structural Studies and Catalytic Activity

AbstractAn efficient catalytic system for the oxidation of alcohols has been developed by using iron(III) catalyst [Fe(phen)2Cl2]NO3 (1) (phen = 1,10‐phenanthroline) and H2O2 as terminal oxidant. A series of primary and secondary alcohols were oxidized into aldehydes and ketones in good yields and excellent selectivities after a short reaction time. The mononuclear iron(III) complex [Fe(phen)2Cl2]NO3 was characterized by several independent methods. The X‐ray structure shows distorted octahedral geometry around the FeIII center, which is in a high‐spin state (S = 5/2) according to Mössbauer study.

Syntheses, crystal structure, spectroscopic, redox and magnetic properties of oxo- and carboxylato-bridged polynuclear iron(III) complexes with phenolate- and pyridine-substituted benzimidazole ligands

Using the same trinuclear [Fe3O]+7 core having oxo and carboxylate bridges, two different types of polynuclear iron(III) complexes have been synthesized. The bidentate ligands, (2-hydroxyphenyl)benzimidazole (HL1) and (2-pyridyl)benzimidazole (L2), produce the dinuclear and tetranuclear complexes [Fe2(L1)4(C6H5COO)]NO3·3H2O (1) and [Fe4O2(C6H5COO)7(L2)2]NO3·H2O·CH3CN (2·CH3CN), respectively. Complex 1 crystallizes in the monoclinic space group P2(1)/c, while 2·CH3CN crystallizes in the monoclinic space group C2/c. In 1, the two hexa-coordinated iron(III) centers are bridged by the two phenolate oxygens of the ligand HL1. The tetranuclear entity 2 consists of a [Fe4(μ3-O)2]8+ unit comprising four FeIII centers with a “butterfly” arrangement. Each pair of iron(III) centers occupy the “body” or “hinge” and “wing-tip” sites, respectively. The Fe2IIIFe2III complex 2 undergoes two stepwise one electron reductions at E1/2=−0.625V and −0.11V, while 1 displays an irreversible reduction wave at EP·C=−0.3V. Variable-temperature magnetic susceptibility measurements have been carried out for 1 and 2 in the temperature range 2–300K. Complex 1 exhibits a very weak Fe⋯Fe exchange interaction, J=−0.24(1)cm−1 (H=−J(S1·S2)). In complex 2, moderate antiferromagnetic exchange interactions occur among the four high-spin FeIII centers. The exchange coupling constant Jbb (body–body interaction) is indeterminate due to the prevailing spin frustration, but the ‘wing-body’ antiferromagnetic interaction (Jwb) is evaluated as −73(3) cm−1, using the spin Hamiltonion model H=−J1(S1·S2+S2·S3+S3·S4+S4·S1)−J2(S2·S4).

V-shaped Fe3III and linear FeII/FeIII/FeII complexes supported by phenyl-pyridine-2-yl-methanone oxime ligand: Solvothermal syntheses, structures and magnetic property

Under solvothermal conditions, one polynuclear FeIII complex of formula [Fe3{(py)C(ph)O}4Cl4][FeCl4] (1) was synthesized from reaction of phenyl-pyridine-2-yl-methanone oxime (py)C(ph)NOH with FeCl3, and one FeII/FeIII/FeII complex of composition [Fe3{(py)C(ph)NO}6][CH3COO] (2) was prepared from treatment of (py)C(ph)NOH with Fe(OAc)2. The most interesting synthetic feature of complex 1 is the in situ transformation of (py)C(ph)NOH into the (py)C(ph)O− ligand. Complexes 1 and 2 were characterized by X-ray single crystal diffraction, IR spectroscopy, and elemental analysis. X-ray analysis revealed that complex 1 contains a V-shaped FeIII core with two terminal FeIII atoms displaying distorted square-pyramidal geometries and one central FeIII ion exhibiting octahedron geometry. Complex 2 is a mixed-valence linear FeII/FeIII/FeII complex with all the three metal centers displaying distorted octahedron geometries. The low-temperature magnetic susceptibility measurement for the solid sample of 1 revealed the weak antiferromagnetic FeIII…FeIII interactions. Theoretical calculations based on density functional theory were performed using the crystal structure of complex 1.

Synthesis and complexes of an N4 Schiff-base macrocycle derived from 2,2′-iminobisbenzaldehyde

An N(4) tetradentate [1 + 1] Schiff base metal free macrocycle HL was prepared, by 1 : 1 condensation of 2,2'-iminobisbenzaldehyde (1) and diethylenetriamine, and characterised. Seven mononuclear complexes, [Zn(II)L(py)](BF(4)) (2), [Cu(II)L](BF(4))]·H(2)O (3), [Ni(II)L](BF(4))·H(2)O (4), [Co(II)L](BF(4))]·H(2)O (5), Fe(III)L(BF(4))(2)·2H(2)O·MeCN (6), [Co(III)L(NCS)(2)]·0.3py (7) and [Fe(III)L(NCS)(2)] (8), of L(-) are reported. The Cu(II) and Ni(II) complexes were prepared by a template approach whereas the others were accessed by metallation of pre-formed HL. The X-ray crystal structure determinations show that [Cu(II)L](BF(4)) and [Ni(II)L](BF(4)) feature square planar N(4) coordinated Cu(II) and Ni(II) centres, respectively, whereas [Fe(III)L(NCS)(2)]·NO(2)Me features an octahedral N(6) coordinated Fe(III) centre (two NCS anions bound axially) and the Zn(II) complex, which crystallised as 2{[Zn(II)L(py)](BF(4))}·py, features square pyramidal Zn(II) ions (a pyridine molecule bound axially). In all cases the N(4) macrocycle is bound equatorially to the metal ion. Cyclic voltammograms of the soluble BF(4) complexes, 2-5, were carried out in MeCN vs. 0.01 mol L(-1) AgNO(3)/Ag and revealed multiple, mostly irreversible or quasi-reversible, redox processes. The Zn(II) complex 2 exhibited two irreversible oxidation processes and one irreversible reduction process, all of which are ligand-centered. The Ni(II) complex 4 showed a process with a weak return wave at E(m) = +0.57 V (ΔE = 0.05 V). Interestingly, after controlled potential coulometry experiments on 2, 3 and 4 (at +0.48, +0.61 and +0.71 V which transferred 1.2, 1.0 and 1.6 e(-) equiv. per complex, respectively), a new reversible or quasi-reversible process was obtained, with a lower potential than beforehand (E(m) (ΔE)/V = +0.16 (0.08), +0.31 (0.13) and +0.45 (0.11) respectively).

The Fe-catalyzed oxidation of aroyl hydrazones to aroyl hydrazines: mechanistic insight to a remarkable reaction

The FeIII-catalyzed oxidation of pyridinecarbaldehyde isonicotinoyl hydrazone to isonicotinoyl picolinoyl hydrazine in aqueous solution has been studied from a kinetico-mechanistic perspective. The reaction is more complex than one would anticipate from the known simple and reproducible preparative procedures. A series of very fast ligand coordination reactions to the FeIII center are followed by slower –N=CH– bond hydrolysis and rapid ligand-to-metal electron transfer coupled to H• elimination. These processes lead to a partially oxidized ligand mixture with FeII coordinated to unreacted hydrazone. The slow decomposition of this FeII-hydrazone complex in aerobic alkaline solution produces ferric hydroxide as a new source of FeIII to sustain the hydrazone (–CO–NH–N=CH–) to hydrazine (CO–NH–NH–CO–) oxidation.

Ferromagnetic Assemblies Composed of MnIII Salen-Type Complexes and a Tripodal FeIII Complex with an Imidazolate Bridge

Abstract The use of a low-spin FeIII complex bearing three imidazolate groups in a reaction with MnIII salen-type complexes provided two different imidazolate-bridged MnIII–FeIII assemblies, a dinuclear complex and a one-dimensional chain, dependent on the salen ligand and counter ion used. Both compounds exhibit a ferromagnetic exchange between MnIII and FeIII centers via the imidazolate linkage.

Unconventional Spin Crossover in Dinuclear and Trinuclear Iron(III) Complexes with Cyanido and Metallacyanido Bridges

AbstractA nonsymmetrical triamine, 1,6‐diamino‐4‐azahexane, was Schiff‐condensed with (X‐substituted) o‐salicylaldehyde to yield pentadentate ligands X‐L5: salpet and MeBu‐salpet. These ligands form mononuclear, dinuclear, and trinuclear FeIII complexes, whose structures were determined by single‐crystal X‐ray analysis. Of the mononuclear complexes, [FeIII(salpet)Cl] and [FeIII(MeBu‐salpet)Cl] are high spin (S = 5/2), whereas [FeIII(salpet)CN]·MeOH is low spin (S = 1/2). The dinuclear and trinuclear complexes show a kind ofthermally induced spin crossover. The dinuclear complex [L5FeIII(CN)FeIIIL5](ClO4)·2H2O (L5 = salpet) is a mixed‐spin assembly: the C‐coordinated FeIII center is low spin (L) and the N‐coordinated FeIII center is high spin (H) at low temperature; an antiferromagnetic interaction occurs between them. This LH reference state is mixed with the LL one. Upon heating, the system shows an increasing content of the HH state. Also, the dinuclear complex [L5FeIII(CN)FeIIIL5](BPh4)·2MeCN (L5 = MeBu‐salpet) exhibits a spin transition between LH and HH spin pairs. The mixed‐valence trinuclear complex [L5FeIII{FeII(CN)5(NO)}FeIIIL5]·0.5MeOH·3.75H2O (L5 = salpet) shows spin crossover with a residual high‐spin fraction at liquid He temperature owing to the LL + LH ground state. The metallacyanido‐bridged complex [L5FeIII{Ni(CN)4}FeIIIL5]·2MeOH (L5 = MeBu‐salpet) contains a high‐spin pair, HH, over the whole temperature interval with a ferromagnetic exchange interaction. A theoretical model was outlined that allows simultaneous fitting of all available experimental data (magnetic susceptibility, magnetization, high‐spin mole fraction obtained from the Mössbauer spectra) on a common set of parameters. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Synthesis, structure and magnetic properties of one chiral heptanuclear Fe cluster: {Fe[(FeL)2(μ2-OH)]3(μ2-OH)6} · 13H2O

One chiral heptanuclear FeIII cluster, {Fe[(FeL)2(μ2-OH)]3(μ2-OH)6}·13H2O (1) (H2L=N-(2-hydroxybenzyl)-l-histidine), has been synthesized and characterized structurally and magnetically. Complex 1 crystallizes in the chiral space group C2. The heptanuclear FeIII cluster of 1 has a propeller-like [Fe7(μ2-OH)9]12+ core, with the peripheral ligation provided by six tetradentate L2− ligands. The configurations of the C- and the N-centers of L2− is (S,R) and the central atom Fe4 is chiral Λ configuration. All FeIII centers are hexacoordinated, displaying a distorted octahedral coordination sphere. Variable temperature magnetic studies showed complex 1 is antiferromagnetic with C=14.27cm3Kmol−1 and θ=−27.56K. Magnetisation and susceptibility measurements show that cluster 1 exhibits a ground spin state of S=3.

Combination of Lacunary Polyoxometalates and High-Nuclear Transition-Metal Clusters under Hydrothermal Conditions. 5. A Novel Tetrameric Cluster of [{FeIIFeIII12(μ3-OH)12(μ4-PO4)4}(B-α-PW9O34)4]22-

A novel tetrameric cluster, (enH2)3.5H15[{FeIIFeIII12(mu3-OH)12(mu4-PO4)4}(B-alpha-PW9O34)4].83H2O (1; en = ethylenediamine), has been hydrothermally synthesized and characterized by IR, optical diffuse reflectance, thermogravimetric analysis, and single-crystal X-ray diffraction. The skeleton of 1 is composed of four tri-FeIII-substituted [FeIII3(mu3-OH)3(B-alpha-PW9O34)]3- Keggin moieties fused together through a FeII2O2 unit and four mu4-PO4 bridges, resulting in a unique tetramer with C2 symmetry. Magnetic measurements indicate that 1 reveals the antiferromagnetic interactions between FeII and FeIII centers.

Activation of nitrite ion by biomimetic iron(III) complexes

The rate of reaction of NO 2 − ion with various FeIII porphyrins in the presence of PPh3 is shown to depend on the redox potential of the FeIII center. There is a linear relationship between the ease of reduction of the FeIII to FeII and the kinetics for the formation of the FeII porphyrin nitrosyl adduct, with concomitant oxidation of PPh3 to PPh3O. Cyclic voltammograms show reversible one-electron reductions that can be ascribed to the FeIII/FeII couple ranging from E1/2 = −343 to −145 mV (versus Ag/AgCl). The order of increasing half-wave reduction potentials for the FeIII/FeII porphyrin redox centers studied is octaethylporphyrin > etioporphyrin I > deuteroporphyrin IX dimethyl ester > protoporphyrin IX dimethyl ester > α,β,γ,δ-tetraphenylporphyrin. This sequence of redox potentials complements the pseudo first-order kinetics ( ${k_{\rm obs}=2.2\times10^{-3}}$ to ${13\times10^{-3}}$ m s −1) for the oxidation of PPh3 and subsequent FeII porphyrin nitrosyl adduct formation. The rates of reaction of biomimetic FeIII porphyrins with NO 2 − ion demonstrate how metal center redox properties are influenced by the surrounding ligand. In this paper we have elucidated a possible mechanistic control for the rate of this reaction.

Ammonium bis{3-anilinocarbonyl-1-[(5-chloro-2-oxidophenyl)diazenyl]-2-naphtholato}ferrate(III) acetone solvate

The title compound, (NH4)[Fe(C23H24ClN3O3)2]·C3H6O, is a charge-control agent used in electrophotography. The central FeIII atom is coordinated by four O atoms and two N atoms of two symmetry-independent ligands. The metallic complex has no crystallographic symmetry.

Ammonium bis{3-anilinocarbonyl-1-[(5-chloro-2-oxidophenyl)diazenyl]-2-naphtholato}ferrate(III) methanol solvate

The title compound, (NH4)[Fe(C23H24ClN3O3)2]·C3H6O, is a charge-control agent used in electrophotography. The central FeIII atom is coordinated by four O atoms and two N atoms of two symmetry-independent ligands. The metallic complex has no crystallographic symmetry.

Cyano-Bridged FeIII2CuII3and FeIII4NiII4Complexes: Syntheses, Structures, and Magnetic Properties

Two tricyanometallate precursors, (Bu4N)[(Tp4Bo)Fe(CN)3].H2O.2MeCN (1) and (Bu4N)[(pzTp)Fe(CN)3] (2) [Bu4N+ = tetrabutylammonium cation; Tp4Bo = tris(indazolyl)hydroborate; pzTp = tetrakis(pyrazolyl)borate], with a low-spin FeIII center have been synthesized and characterized. The reactions of 1 or 2 with [Cu(Me3tacn)(H2O)2](ClO4)2 (Me3tacn = N,N',N' '-trimethyl-1,4,7-triazacyclononane) afford two pentanuclear cyano-bridged clusters, [(Tp4Bo)2(Me3tacn)3Cu3Fe2(CN)6](ClO4)4.5H2O (3) and [(pzTp)2(Me3tacn)3Cu3Fe2(CN)6](ClO4)4.4H2O (4), respectively. Assembly reactions between 2 and [Ni(phen)(CH3OH)4](ClO4)2 (phen = 1,10-phenanthroline) or Zn(OAc)2.2H2O afford a molecular box [(pzTp)4(phen)4Ni4Fe4(CH3OH)4(CN)12](ClO4)4.4H2O (5) and a rectangular cluster [(pzTp)2Zn2Fe2(OAc)2(H2O)2(CN)6] (6). Their molecular structures were determined by single-crystal X-ray diffraction. In complexes 1 and 2, the central FeIII ions are coordinated by three cyanide carbon atoms and three nitrogen atoms of Tp4Bo- or pzTp-. Both complexes 3 and 4 show a trigonal-bipyramidal geometry, in which [(L)Fe(CN)3]- units occupy the apical positions and are linked through cyanide to [Cu(Me3tacn)]2+ units situated in the equatorial plane. Complex 5 possesses a cubic arrangement of eight metal irons linked through edge-spanning cyanide bridges, while complex 6 shows Zn2Fe2(CN)4 rectangular structure, in which FeIII and ZnII ions are alternately bridged by the cyanide groups. Intramolecular ferromagnetic couplings are observed for complexes 3-5, and they have S = 5/2, 5/2, and 6 ground states and appreciable magnetic anisotropies with negative D values equal to -0.49, -2.39, and -0.39 cm-1, respectively.

Mechanistic Studies on the Activation of NO by Iron and Cobalt Complexes (Eur. J. Inorg. Chem. 6/2007)

AbstractThe cover picture shows how mechanistic insight into the activation of NO by biological and model metal complexes can be gained through volume profile analysis. The presented volume profile describes the binding of NO to the high‐spin, five‐coordinate FeIII centre in cytochrome P450 in the presence of camphor to form a low‐spin (diamagnetic), six‐coordinate FeII–NO+ product. Details are presented in the Microreview by R. van Eldik et al. on p. 773 ff.

Odd-Numbered FeIII Complexes: Synthesis, Molecular Structure, Reactivity, and Magnetic Properties

Three isostructural disklike heptanuclear FeIII compounds of the general formula [FeIII7(mu3-O)3(L)3(mu-O2CCMe3)6(eta1-O2CCMe3)3(H2O)3], where L represents a di- or triethanolamine moiety, display a three-blade propeller topology, with the central Fe atom representing the axle or axis of the propeller. This motif corresponds to the theoretical model of a frustrated Heisenberg star, which is one of the very few solvable models in the area of frustrated quantum-spin systems and can, furthermore, be converted to an octanuclear cage for the case where L is triethanolamine to give [FeIII8(mu4O)3(mu4-tea)(teaH)3(O2CCMe3)6(N3)3].1/2MeCN.1/2H2O or [FeIII8(mu4O)3(mu4-tea)(teaH)3(O2CCMe3)6(SCN)3].2MeCN when treated with excess NaN3 or NH4SCN, respectively. The core structure is formally derived from that of the heptanuclear compounds by the replacement of the three aqua ligands by an {Fe(tea)} moiety, so that the 3-fold axis of the propeller is now defined by two central FeIII atoms. Magnetic studies on two of the heptanulcear compounds established unequivocally S = 5/2 spin ground state for these complexes, consistent with overall antiferromagnetic interactions between the constituent FeIII ions.

Deprotonations and Charges of Well-Defined {Mo72Fe30} Nanoacids Simply Stepwise Tuned by pH Allow Control/Variation of Related Self-Assembly Processes

The solution behavior of the largest inorganic acid known thus far, the neutral, spherical iron/molybdenum/oxide nanocluster {Mo72Fe30} ([triple bond{(MoVI) MoVI5}12FeIII30 1a), including the pH-controlled deprotonation, is reported. The acidic properties are due to the 30 peripheral, weakly acidic FeIII(H2O) groups that form a unique Archimedean solid with all edges and dihedral angles being equal, the icosidodecahedron, and therefore an "isotropic" surface. Interestingly, the aqueous solutions are stable even for months because of the inertness of the spherical solutes and the presence of the hard FeIII and MoVI centers. The stability can be nicely proven by the very characteristic Raman spectrum showing, because of the (approximately) icosahedral symmetry, only a few lines. Whereas the {Mo72Fe30} clusters exist as discrete, almost neutral, molecules in aqueous solution at pH < 2.9, they get deprotonated and self-associate into single-layer blackberry-type structures at higher pH while the assembly process (i.e., the size of the final species) can be controlled by the pH values; this allows the deliberate generation of differently sized nanoparticles, a long-term goal in nanoscience. The average hydrodynamic radius (Rh) of the self-assembled structures decreases monotonically with increasing number of charges on the {Mo72Fe30} macroanions (from approximately 45 nm at pH approximately 3.0 to approximately 15 nm at pH approximately 6.6), as studied by laser light scattering and TEM techniques. The {Mo72Fe30} macroions with high-stability tunable charges/surfaces, equal shape, and masses provide models for the understanding of more complex polyelectrolyte solutions while the controllable association and dissociation reported here of the assembled soft magnetic materials with tuneable sizes could be interesting for practical applications.

Biliverdine-Based Metalloradicals: Sterically Enhanced Noninnocence

This is a first density functional theory survey of transition-metal biliverdines (Blv), where we have chosen to focus on key Mn, Fe, Co, and Cu complexes. According to the calculations, the complexes are invariably noninnocent, featuring Blv*2- ligand radicals. In this, biliverdine complexes resemble metallocorroles, but the parallels are only approximate. Briefly, metallobiliverdines exhibit a much greater tendency to adopt noninnocent electronic structures than analogous metallocorroles. The O...O nonbonded contacts in biliverdines apparently preclude the formation of short metal-N bonds that, in turn, could stabilize high-valent metal ions. Thus, while most copper corroles (Cor) exhibit diamagnetic CuIII ground states, copper biliverdines are clearly Cu(II)Blv*2- species. In the same spirit, while chloroiron corroles are best described as FeIII(S = 3/2)Cor*2-,the analogous biliverdine derivative seems best described as Fe(III)(S = 5/2)Blv*2-, i.e., featuring a high-spin FeIII center with long (>2.0 A) Fe-N bond distances. Overall, the results highlight the important role that steric effects may play in modulating the electronic structures and the potentially noninnocent nature of transition-metal complexes.

Symmetry-Based Magnetic Anisotropy in the Trigonal Bipyramidal Cluster [Tp2(Me3tacn)3Cu3Fe2(CN)6]4+

Reaction of [(Me3tacn)Cu(H2O)2]2+ (Me3tacn = N,N',N' '-trimethyl-1,4,7-triazacyclononane) with [TpFe(CN)3]- (Tp- = hydrotris(pyrazolyl)borate) in a mixture of ethanol and acetonitrile affords the pentanuclear cluster [Tp2(Me3tacn)3Cu3Fe2(CN)6]4+. Single-crystal X-ray analysis reveals a trigonal bipyramidal structure featuring a D3h-symmetry core in which two opposing FeIII (S = 1/2) centers are linked through cyanide bridges to an equatorial triangle of three CuII (S = 1/2) centers. Fits to variable-temperature dc magnetic susceptibility data are consistent with ferromagnetic coupling to give an S = 5/2 ground state, while fits to low-temperature magnetization data indicate the presence of a large axial zero-field splitting (D = -5.7 cm-1). Frequency dependence observed in the ac magnetic susceptibility data confirms single-molecule magnet behavior, with an effective spin reversal barrier of Ueff = 16 cm-1. When compared with the much lower anisotropy barrier previously observed for the face-centered cubic cluster [Tp8(H2O)6Cu6Fe8(CN)6]4+, the results demonstrate the enormous influence of the geometry in which a given set of metal ions are arranged.

Inducing Molecular Growth in an {Mo57Fe6}-type Nanocluster: Synthesis, Structure, and Properties of $$\{\hbox{Mo}_{57}\hbox{(Mo)}_{2}\hbox{Fe}^{\rm III}_{6}\}$$ *

The reaction of the polyoxomolybdate anion $$[\hbox{Mo}_{32}^{\rm VI}\hbox{(MoNO)}_{4}\hbox{O}_{108}(\hbox{H}_{2}\hbox{O})_{16}]^{12-} =\{\hbox{Mo}_{36}\hbox{(NO)}_{4}\}$$ with FeII salts and hydrazine sulfate yields the cluster anion $$[\hbox{Mo}^{\rm V/VI}_{51}(\hbox{Mo}^{\rm VI}\hbox{O})_{2}\hbox{Fe}^{\rm III}_{6}\hbox{(MoNO)}_{6}\hbox{O}_{176}\hbox{(OH)}_{3} (\hbox{H}_{2}\hbox{O})_{22}]^{15-}$$ (1a), isolated as its corresponding ammonium salt (NH4)15 1a · 36H2O (1) (space group P63/mmc, a=b=23.607(5) A, c=26.767(6) A, 4076 unique reflections, 293 parameters). Despite the absence of oxygen and the reducing conditions the product comprises only FeIII centers as determined by a combination of spectroscopic, magnetic and crystallographic analysis. Reduction of peripheral Mo centers causes the aggregation of two additional [MoVIO]4+ groups onto the archetypal {Mo57M6} structure. The single-crystal X-ray determined structure of 1 is reported.