Unexpected mixed valence iron ( II , III ) μ 3-oxo furfuryl complex with wheel motif isolated when preparing iron ( II ) arene complexes

A series of new compounds including 2-({[(4-(propoan-2-yl)benzyl]Oxy}methyl)tetrahydrofuran ligand were tested in order to link auxiliary and labile ligands to iron arene complexes. The given reaction of benzyletherfurane ligand with Fe (II) sources resulted to the isolation of unexpected mixed valence μ3oxo-Fe6 II Fe III compound, which has been fully characterized by x-ray crystallography. The resulting complex differs from the similar compound known in the literature.


INTRODUCTION
Stoechiometrical and functional organometallic materials based on molecular building blocks represent an increasingly important area of research in contemporary materials applied chemistry.Due to their constitutional order, such compounds have always attracted much attention in a variety of applications.These applications include electrochemical sensors (Balzani et al., 2006;Daniel et al., 2003aDaniel et al., , 2004)), redox catalysts (Kim et al., 2003), and molecular Batteries (Astruc et al., 2002;Nlate et al., 2000).In some occasion they act as dendrimer [ rs  (Balzani et al., 2006; Daniel et al., 2003a; Daniel et al.,  2003b).Also, their properties can be modulated by the size and nature of external branching groups (Oshio et al., 2003(Oshio et al., , 2009)).On other hand, the use of iron clusters as models for biological systems such as the iron-storage protein ferritin has been also exploited as well as their use as nanoscale magnetic particles, the so-called Single Molecule Magnets SMMs (Oshio et al., 2003(Oshio et al., , 2009)).

(Bischloro)bistetrahydrofuranIron(II)
A Schlenk tube was charged with 1.0 g (7.9 mmol) of anhydrous FeCl2 and 20 ml of THF under argon.The obtained suspension was heated to reflux for 8 h.Then the mixture was allowed to cool down to Room Temperature (RT) and the upper yellowish solution was filtered.After evaporation of solvent and drying under vacuum, 1.5 g (68% yield) of pale yellow solid was obtained.The isolated product is air sensitive, it turns redbrown if in contact with air.UV -Vis (THF, 200-800 nm): 335.IR (KBr film, 4000-400 cm -

2-({[4-(propoan-2-yl)benzyl]Oxy}methyl)tetrahydrofuran 4
To a two-necked flask equipped with a bubbler was charged with 140 mg, (2.1 mmol) of NaH (60% in mineral oil washed three times with pentane and dried under vacuum), then a solution of tetrahydrofurn-2-yl-methanol (0.22 g, 2.1 mmol) in terahydrofuran (10 ml) was added dropwise over a period of 15 min.The solution was left to stir until gas evolution ceased followed by addition of KI (0.08 g, 0.5 mmol), benzylbromide 7 (0.50 g, 2.3 mmol) dissolved in 1 ml of THF over a period of 15 min.The reaction mixture was stirred overnight and then mixed with 5 ml of water and saturated NaCl solution (5 ml).The aqueous phase was extracted three times with CH2Cl2 and the combined organic phases were washed with 20 ml of water then dried over MgSO4.After evaporation of the solvent, the obtained yellow oil product was purified by flash column chromatography (diethylether/pentane (1:1), Rf = 0.7) affording 0.5 g (92%) of 53 as a pale-yellow liquid.

RESULTS AND DISCUSSION
Recently, Raemy reported a possible existence of N 2 Fe II (toluene) complex 2 resulting from the treatment of FeCl 2 in toluene under permanent nitrogen bubbling trough the reaction mixture (Raemy, 2003).This discovery is important since Fe-N 2 complexes play an important role in biological and industrial nitrogen derivatization (Delfino et al., 2010;Bothe et al., 2010).The UV-Vis spectrum of this material in toluene shows two maxima at 358 and 315 nm, respectively.This double band was taken as confirmation for the formation of the proposed compound.Additionally, when the UV-Vis spectrum was taken in THF, the peak of absorption at 358 nm was slightly shifted to a higher value of 363 nm.This bathochromic shift was assumed to be provoked by an exchange of the labile nitrogen ligand with a THF fragment 3 (Scheme 1).
All attempts to isolate these compounds failed and the use of NMR and MS experiments were unsuccessful; indeed the product decomposes during the measurements.It is important to note that when the reaction was carried out under continuous argon bubbling conditions, the UV-Vis spectra of obtained material show only singular weak absorption.Since the isolation and characterization of the presumed FeCl 2 (THF)(toluene) compound 3 was not possible, it was of interest to develop a new type of ligand containing both strong and weak donor groups.The choice was directed to a bidentate ligand in which the tetrahydrofurane moiety was attached to the aryl group via an ether link resulting in the chelate-or tweezers-like benzyletherfuran ligand 4, thus the aryl linkage would prevent the dissociation of the weakly coordinated furan from the metal in complex 5 by the chelation effect (Scheme 2).
Furthermore, the hemilabile arm of such ligands will provides the capability to do reversible dissociation from the metal center.Such a dynamic behaviour will produce vacant coordination sites that allow complexation of substrates during the catalytic cycle.Simultaneouslly the strong donor moiety remains attached to the metal center.To our knowledge there is no report in literature concerning complexes of Fe(II) with both chelating aryl and dangling donor tetrahydrofurane groups.
Arene Fe(II) chloride complexes was found also to be excellent precatalysts for hydrogenation of olefins, oligoand polymerization of acetylenes after activation with LiAlH 4 , BuLi or DIBAH.However, this require the presence of an additional labile ligand such as ethylene (Figure 3), (Raemy, 2003).
In the MS(ESI+) spectrum, the peak at m/z 594(100%) was attributed to complex 9 or 9a where FeCl 2 is linked with two ligands 4 via the aryl functions or the ether functions respectively.The peak at m/z 360 (40%) corresponds to the expected complex 5 or the ether complex 5a.We assumed that both complexes exist in solution as Fe(II) species.The cationic Fe(III) species observed in the mass spectrum are probably formed under the spragring conditions of electrospray ionisation, known to be oxidizing in the positive mode.The UV-Vis spectrum in THF shows three maxima at 364, 308 and 256 nm, respectively.These absorbances are in the same range of those observed by Raemy (Raemy, 2003) in his nitrogen-aryl or allybenzylether Fe(II)chloride complexes (Figure 3).The UV-Vis spectrum of the benzyletherfuran 4 is dominated by two absorptions in the UV region of the spectra (220 and 263 nm), which is characteristic for -* transitions.
According to the reported studies of Lambert et al. (1999) on similar substituted iron-arene complexes and of Velusamy et al. (2003) on iron-Phenolate complexes, the absorbance in the range of 300-400 nm could be attributed to a metal to ligand charge transfer or ligand to metal charge transfer (MLCT or LMCT).In our case, the metal to ligand charge transfer (MLCT) is found at 364 nm and the position of this band demonstrates the high-spin state of complex 5.However, the values observed here are lower as those normally observed in others systems such as iron oxo-or amido-pyridine complexes, which usually display a MLCT band in the visible region (ca.410 nm) in polar solvent such as tetrahydrofurane, methanol or acetonitrile (Bernal et al., 1995).Moreover, there is no d-d transition observed in the spectrum of 5.These transitions are reported to appear near the infrared region of the spectrum.
Compared to the free ligand, the absorptions in the infrared show slight shifts to the low frequencies for the C-O-C ether stretching at 1042 cm -1 instead of 1088 cm -1 ; three new different absorption appear at 1601 (very strong), 920 and 820 cm -1 , respectively.The 1 H-NMR spectrum of complex 5 is also typical for a high-spin ferrous compound and is difficult to be interpreted because of the absence of distinct signals (Figures 4 and  5).
However, the signals corresponding to the alkyl-protons are easily identifiable via the integration of the peaks situated at 1.3 and 3.0ppm respectively giving six and one protons attributed to the isopropyl moiety on the aromatic group (figure 5).The remained peaks, whose integrals do not correspond to the numbers of the protons, could be assigned to the benzyl and furfuryl protons.
In the 13 C-NMR spectrum of 5 (Figure 6), it is interesting to note that only the expected peak located at  138.2 ppm and attributed to the carbon bearing benzylic linker can be identified, all the other aromatic peaks are only less influenced by the paramagnetism of the metal center, that is in contrast to the furfuryl carbon signals located between  64 to 76 ppm.
The previous observation can lead to formulate the hypothesis according to which in the complex 5 the aromatic ring is not complexed directly with the metal atom, the connection will be done somewhat via the oxygen atoms 9a.Consequently, only the carbon bearing benzyl linker, which is near to the complexed oxygen atom, is influenced by this paramagnetic effect of iron.Another direct consequence of this formulation is that the oxygen atom connected to the iron becomes very acidic and, could thus be stabilized by losing the aromatic moiety as isopropylbenzylalcohol compound, when some water traces exist in solution.This mechanism could explain the formation of surprising multinuclear iron complex 1 (Figure 1).Crystallization attempts in a mixture of toluene and dichloromethane allow the isolation of  green hexagonal crystals.The X-ray analysis yields the unexpected oxo-polymetallic complex 1, probably as a result of decomposition of complex 5 or 9 via ether cleavage induced by Fe(III) impurities (Figures 6, 7 and  8).
The starting ligand 2-({[4-(propoan-2yl)benzyl]Oxy}methyl)tetrahydrofurane 4 has lost the benzyl moiety, only the tetrahydrofuran-2-ylmethanolate part remains attached to the iron centers in 1.The tetrahydrofuran-2-ylmethanolate ligand has two possible coordination sites, namely the alkoxy and the cyclic oxygen.As per description of Labat and Oshio for similar compounds, the core can be regarded as a wheel with crystallographically imposed S 6 symmetry, in which the central iron atom Fe1 is in distorted octahedral environment attached to six other iron atoms Fe 2 through six hydroxyl ligands in a µ 3 -oxo mode.Furthermore, each external iron is hexacoordinated with one chloride atom, two other positions are occupied by hydroxyl groups and the two remaining sites are engaged with oxygen atoms of tetrahydrofuran-2-ylmethanolate molecules.Each of the six exocyclic oxygen atoms of the tetrahydrofuran-2ylmethanolate moiety is shared by two-peripheral iron atoms Fe2 in a µ 2 -oxo mode, while each cyclic oxygen atom complexes only one Fe2 center.Selected bond distances and angles are given in the Table 1 with Fe1-O1 bond lengths in the range of 2.095(2) to 2.176(1)Å, these values are comparable with those reported for similar compounds (Figures 2 and 3).Table 1.Selected bond distances and angles of complex 1. ligands are oriented orthogonal to the plane of the iron avoiding steric interactions with the tetrahydrofuran-2ylmethanolate ligands.
The counter ion is clearly Fe(III) Fe-Cl distances in the range of 2.167 to 2.183Å and allows no other interpretation, which oblige to the oxo-heptametallic core double positive charge (+2).On the assumption that all µ 3 -oxo ligands are hydroxyl groups and all tetrahydrofuran-2-yl-methanol are deprotonated alkoxy ligands, the seven iron centers mostly possess a Fe 1 II Fe 6 III configuration.Unfortunately, we were unable to obtain sufficient material for measurement such as Magnetic susceptibility or Mössbauer Spectroscopy.We performed MS (ESI+, FAB and MALDI) experiments in order to find out if there are hydrogen atoms on all the bridging µ-oxo ligands, the obtained spectras does not afford peaks in favour of an expected structure.

Conclusions
Assumptions about the ability of Fe(II) allyletheraryle bidentate complexes to carry out the catalytic hydrogenation and polymerization of olefins, guided us to successfully prepare new iron benzyletherfuran complex 5. Unfortunately, the complex failed to perform polymerization either with butadiene or with ethylene, under mild reaction conditions (RT, 2 bar for ethylene).However, we were able to isolate a new hepta-core iron complex resulting probably from the degradation of 5.
Scheme 5. Synthesis of complex 5 in THF.
The O-Fe1-O cis-angles are 80.23(10)° and 100.69(10)°, but the O-Fe1-O trans arrangement is 180°.As shown in Figures 1 and 6, the six tetrahydrofuran-2ylmethanolate rings are dangling oriented from each other to mitigate the steric interaction around the external iron atoms.Such steric hindrance may rationalize the facile cleavage of the oxo-bridge of original 2-({[4-(propoan-2-yl)benzyl]Oxy}methyl) tetrahydrofuran ligand with weak proton sources such as H 2 O.The chloro