{Fe6O2}-Based Assembly of a Tetradecanuclear Iron Nanocluster

The tetradecanuclear FeIII pivalate nanocluster [Fe14O10(OH)4(Piv)18], comprising a new type of metal oxide framework, has been solvothermally synthesized from a hexanuclear iron pivalate precursor in dichlormethane/acetonitrile solution. Magnetic measurements indicate the presence of very strong antiferromagnetic interactions in the cluster core.


Introduction
Polynuclear iron coordination clusters have received considerable attention since the discovery that they can serve as models for protein active sites [1] and exhibit interesting magnetic properties including slow magnetization relaxation and quantum tunneling phenomena [2][3][4][5][6][7][8][9][10][11][12][13][14]. Tremendous efforts have been devoted to the development of new synthetic routes to increase the nuclearity of such clusters. One synthetic strategy is based on serendipitous self-assembly of ferric salts and simple ligands, which allows for a balance between the preferred coordination geometry of the metal ions and OPEN ACCESS the coordination mode of the ligand [15]. The vast majority of single molecular magnets have been made in such a serendipitous manner. A second approach involves reactions of small metal cages (typically oxo-centered homo-and heterotrinuclear complexes) in solution (including the use of non-coordinating, high-boiling solvents such as decane or tetradecane) or at high temperature in the solid state. A survey of compounds derived in this manner indicates that such compounds can eliminate weakly bound solvent molecules or ligands, giving higher nuclearity metal clusters by oligomerization of the initial metal cores [16][17][18][19]. A third, very useful synthetic path includes the use of hydrolysis or alcoholysis reactions in the presence of carboxylate groups [20][21]. In the forth synthetic strand, hydro-and solvothermal heating under higher pressure as well as a microwave irradiation has also proven useful to induce aggregation of smaller units into larger clusters, with several reported precedents [22][23]. Other approaches to high-nuclearity or high-symmetry Fe n polytopes utilize diamagnetic polyoxometalate scaffolding structures [24][25].
The infrared spectrum of 1 displays strong and broad bands in the 1590-1547 cm -1 region and at 1429 cm -1 , arising from asymmetric and symmetric stretching vibrations of the coordinated carboxylate groups of the pivalate ligands, respectively. The CH asymmetric and symmetric stretch vibrations for tert-butyl groups of pivalates are observed in the 2962-2872 cm -1 region, along with a strong single band at 1485 cm -1 and a doublet at 1379-1362 cm -1 , which correspond to asymmetric and symmetric bending vibrations for methyl groups, respectively. The weak bands at 3633 and 3610 cm -1 are (OH) vibrations.
Its neutral state and the highly lipophilic cluster surface of 1, nearly completely defined by the tert-butyl groups, renders the cluster soluble in e.g., dichloromethane or tetrahydrofurane but insoluble in water.

X-ray Structure
Cluster 1 crystallizes in the hexagonal space group R-3c with a = 15.6604 (17), b = 15.6604 (17), c = 96.907 (13) Å. An ORTEP drawing of the asymmetric unit in 1 with the labeling scheme and the complete molecular structure of 1 are shown in Figure 1. Selected bond distances and angles of 1 are listed in Table 1. The structure of 1 consists of 14 Fe III atoms (there are only three crystallographically independent Fe atoms, Fe1, Fe2 and Fe3), which are connected by three  4 -oxo atoms, eight  3 -oxo atoms and three  2 -hydroxo atoms forming a [Fe 14 3 ] core, as shown in Figure 2 (Table 1). A pair of pivalate ligands additionally connects the outer Fe atoms (Fe1 and its equivalents) in each tetrahedron. This Fe 8 arrangement resembles a propeller with three tilted blades. The three  2 -OH atoms (O3, O3A, O3B) ( Figure 3) link the blades of the propeller. The outer trinuclear groups are bridged to the octanuclear iron layer via  3 -O positions (O1 and its equivalents) and three pivalate ligands, resulting in three additional Fe 3 trinuclear units with central  3 -O atoms (O1 and its symmetry equivalents) and one bridging pivalate. As for the Fe···Fe distances within the triangle, the Fe2···Fe1 distance of 2.962(7) Å is significantly shorter than the Fe1···Fe3 (3.464(1) Å) and Fe2···Fe3 (3.376(1) Å) distances.    Bond valence sum (BVS) calculations for the Fe atoms indicate an oxidation state +III for all iron centers (see Table 2). The total cationic charge (+42) is only partially compensated by the 18 pivalate groups and the 14 oxygen positions (total anionic charge: -46). Electroneutrality of 1 requires formulation of the cluster with four protons, three of which are indicated to reside on the three O3 atoms, for which a bond valence sum value of 1.12 is found ( Table 2). Given that no counterions can be found, we postulate that the fourth proton is disordered over other oxygen positions, resulting in a total of four OH groups in the neutral cluster 1.

Magnetic Properties
The presence of all-Fe III ions (high-spin 3d 5 , 6 A 1g ground state for octahedral environments) in 1 simplifies the magnetochemical interpretation as they represent pure spin-5/2 centers. Figure 4 represents the temperature dependence of  m T (at 0.1 Tesla) and the isothermal molar magnetization M m (at 2 K) as a function of the external field. The  m T vs. T graph indicates strong dominating antiferromagnetic intramolecular exchange, as expected from the multiple µ-O and µ-OCO exchange pathways in 1. A plot of the temperature dependence of the reciprocal susceptibility does not approach linear behavior up to 290 K, therefore no Curie-Weiss parameters can be derived. At room temperature,  m T approaches a value of 11.86 cm 3 K mol -1 , which is much smaller than the spin-only value for 14 isolated Fe III ions considering an average Zeeman factor g = 2.0, that is 61.25 cm 3 K mol -1 . With decreasing temperature,  m T decreases and reaches a shoulder at ca. T = 10 K with  m T  5.1 cm 3 K mol -1 . This value is higher than the spin-only value obtained for one isolated Fe III ion, i.e., 4.375 cm 3 K mol -1 ; at lower temperatures, an onset of a steeper decrease in  m T is apparent. The inset in Figure 4 shows the field dependence of the molecular magnetization cluster up to 5.0 T at 2.0 K. The blue curve is the best fit of the data to the Brillouin function Least squares fitting indicates that at 2.0 K the magnetic state of 1 can be represented by S = 5/2 with the corresponding g factor equal to 2.0.

Materials and Physical Measurements
All reactions were carried out under aerobic conditions using commercial grade solvents.  [17,24] were synthesized as described elsewhere. The infrared spectra were recorded on a Perkin-Elmer Spectrum One spectrometer using KBr pellets in the region 4000-400 cm -1 . Magnetic susceptibility measurements were performed in a Quantum Design MPMS-5XL SQUID magnetometer as a function of field (0.1 to 5.0 Tesla) and temperature (2.0 to 290 K). Experimental data were corrected for the sample holder (PTFE tubes) and diamagnetic contributions calculated from tabulated values; χ dia (1) = -1.514 × 10 3 cm 3 mol -1 .

X-ray Crystallography
Single-crystal X-ray studies were carried out using a Bruker APEX CCD diffractometer employing graphite-monochromated Mo K radiation (λ = 0.71073 Å) at 150 K. Details of the crystal, data collection and refinement parameters are in Table 2. The structures were solved by direct methods and refined by full-matrix least squares on weighted |F| 2 values for all reflections using the SHELX program suite. The non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms could not be located in difference Fourier maps and were placed in fixed, idealized positions and refined as rigidly bonded to the corresponding atom.

Conclusions
We have demonstrated how a combination of different synthesis approaches can yield novel polynuclear iron oxide carboxylate cluster structures, as exemplified by a tetradecanuclear Fe III pivalate nanocluster. The cluster comprises a unique {Fe 14 O 14 } core fragment consisting of a condensed {Fe 8 O 6 } building unit in which eight iron atoms are connected by three  4 -oxygen atoms and three  2 -hydroxy atoms, and two triangular {Fe 3 O} building units attached to the bottom and top of the {Fe 8 O 5 } unit via six  3 -oxygen atoms. Eighteen bridging pivalate ligands at the periphery of the molecule additionally stabilize the metal oxide core. Preliminary magnetic susceptibility measurements indicate strong antiferromagnetic exchange caused by both the bridging oxo and carboxylate groups.
Given the well-documented redox activity of archetypal [Fe 3 (µ 3 -O)(µ-RCOO) 6 ] n+ -type trimers, we expect 1 to also show redox activity, potentially localized at the two outer {Fe 3 (µ 3 -O)(µ-Piv) 3 } trimers but possibly also in its central {Fe 8 O 5 } fragment. Therefore, potential mixed-valent Fe(II/III) derivatives of 1 should in principle exist. In follow-up work, we aim to study the electrochemistry of 1 and probe the synthesis of mixed-valent derivatives by bulk electrolysis as well as by using Fe(II/III) trimer clusters as co-reagents.