Mono- and Bi-Iron Chalcogenofumarato Complexes: Synthesis and Characterization

ABSTRACT New series of iron(II) chalcogenofumarato-based complexes has been successfully synthesized in good yields. The mono-iron complexes CpFe(CO)2ESCOCH=CHCOCl (E= S (1), Se (2)) were prepared from the reaction of the corresponding iron chalcogenides (µ-Ex)[CpFe(CO)2]2 (E= S; x = 2–4, E= Se; x = 1) with fumaryl chloride in a 1:1 molar ratio. The corresponding 2:1 (iron:ligand) reaction produced the bi-iron complexes [CpFe(CO)2ESCO]2(µ-CH=CH) (E= S (3), Se (4)). The complexes CpFe(CO)2ECOCH=CHCO2Et (E= S (5), Se (6)) were synthesized from the reaction of complexes 1 or 2 with EtOH or from the reaction of the chalcogenides with O-ethyl chlorofumarate ClCOCH=CHCO2Et. All complexes were characterized by UV-Vis, IR, 1H-, 13C{1H}-NMR spectroscopic techniques and the crystal structures of 1, 5 and 6 were further determined by single crystal X-ray diffraction measurements. Graphical abstract

ligand molar ratio, as shown in Scheme 1.The red metal chloride CpFe(CO) 2 Cl complex is also produced as a by-product of these reactions and separated by column chromatography.The reaction went by nucleophilic attack of the chalcogen lone pair on the carbonyl carbon resulting in a tetrahedral intermediate which lost chloride ion forming the chalcogenocarboxylato complexes.The other chalcogen atoms are lost as the elemental form.Mechanism of similar reactions has been reported. [44]omplexes 1-4 are air stable as solids and air sensitive in solution.The identity of these complexes were determined by IR, 1 H-, 13 C{ 1 H}-NMR spectroscopy and elemental analyses.In addition, the crystal structure of complex 1 was determined.As shown in Table 1, the IR spectra of the thiofumarates (KBr disk) showed two strong bands (1: 2038, 1996 cm −1 , 3: 2042, 1996 cm −1 ) attributed to the stretching frequencies of the two terminal carbonyl groups.These bands are at lower frequencies compared to those of selenofumarates (2: 2050, 1994 cm −1 , 4: 2048, 1998 cm −1 ).[35][36][37][38][39][40] The stretching band of the thiocarboxylato (SC=O) moiety appeared at 1582 cm −1 and 1583 cm −1 for complexes 1 and 3, respectively, while the corresponding selenofumarato (SeC=O) band was found at 1593 cm −1 for both 2 and 4. The higher wave number for the selenium containing compounds is due to weaker resonances of selenium lone pairs with the carbonyl group.The stretching band of the free acid chloride group (ClC=O) of 1 and 2 presented at 1757 and 1748 cm −1 , respectively.These bands are comparable to those found for CpFe(CO) 2 ECOArCOCl (1725-1775 cm −1 ). [41]The C=C vibration for complexes 1-4 appeared in the range of 1619-1685 cm −1 which is also comparable to that of CpFe(CO) 2 ECOCH=CR 2 (1637-1658 cm −1 ). [34,38]he 1 H-NMR spectra of complexes 1-4 are straightforward showing correct proton ratios and peak multiplicities for both the Cp and chalcogenofumarato ligands.In such complexes, a singlet in the range of 5.07-5.13ppm for the five equivalent Cp-protons is observed.[35][36][37][38][39][40] For the mono-iron complexes (1, 2), two broad singlets are shown for the two non-equivalent vinylic protons in the range of 6.35-7.88ppm while for the spectra of the dimers 3 and 4, the two vinylic protons are equivalent and displayed as a singlet in the range of 6.85-6.87ppm.A similar range (6.09-6.98 ppm) is determined for the vinylic chalcogenocarboxylates. [34,38] The 13  ppm.The assignment of these peaks is based on the reported data for the vinylic [34,38] and dimeric chalcogenocarboxylato complexes. [41]omplexes 1 and 2 have a free acid chloride group which converted to the ester derivatives by reaction with ethanol in the presence of a base like pyridine (Scheme 2).The ester derivatives (5 and 6) can also be produced from the reaction of the chalcogenides with ethyl fumaroyl chloride as shown in Scheme 2.
In their IR spectra (Table 1), 5 and 6 displayed two strong bands for the terminal carbonyls bonded to iron at 2033, 1994 cm −1 and 2025, 1986 cm −1 , respectively.These values are comparable to those of 1-4 complexes (vide supra).In addition, another three medium bands in the ranges 1701-1703, 1630-1633 and 1580-1603 cm −1 assignable to the ester carbonyl, the C=C and chalcogenofumarato moieties, respectively, were presented.
The 1 H-NMR spectra of complexes 5 and 6 showed a singlet at 5.05 and 4.97 ppm, respectively for the cyclopentadienyl ring protons.A typical triplet and doublet at high field with coupling constant of 4 Hz was presented for the ethyl Scheme 2. Synthesis of the iron chalcogenofumarato ester derivatives (5, 6).
group.Moreover, the two vinylic protons were presented as two doublets in the ranges of 6.44-6.49ppm and 7.21-7.29 ppm with coupling constant of 8 Hz.These values are similar to those of 1-4 and to the vinylic chalcogenocarboxylates. [34,38] The 13 C{ 1 H}-NMR spectra of these complexes displayed a singlet in the range of 84.7-85.2ppm for the Cp-carbons and a low-field peak in the range of 211.3-212.2ppm due to the terminal carbonyl carbons.The peaks in the ranges of 197.5-199.3 and 165.3 ppm may attributed to the EC=O and OC=O groups of the fumarato moieties, respectively.The vinylic carbons showed their presence in the spectra in the ranges of 122.7-123.9ppm and 142.1-143.8ppm, while the up field peaks were due to the carbons of the ethyl group.These chemical shifts are at comparable ranges to the corresponding values reported for similar iron systems. [34,38,44,45]

UV-Vis electronic spectra of 5 and 6
The UV-Vis absorption spectra of complexes 5 and 6, as representative examples, were measured in dichloromethane solution and are shown in Figure 6.The spectrum of the thiofumarato complex 5 displayed a band at 341 nm which is red shifted to 392 nm for 6.This absorption referred to metal to ligand charge-transfer transitions between Fe d-orbitals to π* orbital of the Cp ligand.The assignment of this band is based on those reported for iron chalcogeno-complexes. [34,38,44,46]

Conclusion
In conclusion, the reaction of fumaryl chloride with the iron chalcogenides can be controlled to give the mono-iron or bi-iron chalcogenofumarates.This indicates that fumaryl chloride behaves like aromatic acid chlorides toward the cyclopentadienyliron dicarbonyl chalcogenides.The mono-iron complexes with a free acid chloride group, can be transformed to the ethanoate derivatives by reacting with ethanol in basic medium.The geometries of the complexes were proved by spectroscopic and X-ray structural analysis.The different carbonyl groups presented in the complexes can be easily distinguished and assigned by IR spectroscopy.The complexes have distorted octahedral coordinated Fe atom and planar chalcogenofumarato moieties.

General
Manipulations were conducted under nitrogen atmosphere using standard Schlenk line techniques.Diethyl ether, n-hexane, toluene (sodium∕benzophenone) and dichloromethane (P 2 O 5 ) were purified by standard procedures.The compounds (µ-S x )[CpFe(CO) 2 ] 2 (x = 2-4) and (µ-Se)[CpFe(CO) 2 ] 2 were prepared by previously published procedures. [47,48]The following chemicals were used as received (Acros or Sigma-Aldrich): [CpFe(CO) 2 ] 2 , fumaryl chloride, O-ethyl fumaroyl chloride, sulfur and gray selenium (Se).Silica gel of particle size 0.063-0.200mm (70-230 mesh) was dried at 110°C and employed for column chromatography.All reaction steps were followed by thin layer chromatography (TLC).Infrared (FTIR) spectra were recorded with a Bruker, Tensor27 spectrometer in attenuated total reflection (ATR) configuration under nitrogen or as KBr disks.Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker-Avance 400 MHz spectrometer.Chemical shifts are given in ppm relative to CDCl 3 at 7.25 ppm for 1 H-NMR and CDCl 3 peak for 13 C{ 1 H}-NMR.UV-Vis spectra were recorded using UNICAM UV-Vis spectrometer in dichloromethane at room temperature.Elemental analyses were performed with a Leco CHNS-932 apparatus at the Institute for Organic and Macromolecular Chemistry of Jena University.

General procedure for the preparation of CpFe(CO) 2 ECOCH=CHCOCl (E= S (1), Se (2))
The iron chalcogenide (µ-E x )[CpFe(CO) 2 ] 2 (2.83 mmol) was dissolved in 100 mL of diethyl ether and fumaryl chloride (0.43 g, 2.83 mmol) was added.The resulting mixture was stirred overnight at room temperature.The volatiles were removed under vacuum and the residue was dissolved in 2 mL of diethyl ether and introduced to a silica gel column.Elution with a mixture of diethyl ether and n-hexane (1:3 volume ratio) gave an orange band, which was collected and identified as CpFe(CO) 2 ECOCH=CHCOCl, followed by a red band which was collected and identified as CpFe(CO) 2 Cl.The product was recrystallized from CH 2 Cl 2 /n-hexane at − 4°C.

General procedure for the preparation of [CpFe(CO) 2 ECO] 2 (µ-CH=CH) (E= S (3), Se (4))
A similar procedure to that shown in section 4.2 above is used here.The amount of the fumaryl chloride was (0.22 g, 1.42 mmol) and it was dropwise added to the iron solution over two hours.

General procedure for the preparation of CpFe(CO) 2 SCOCH=CHCO 2 Et (E= S (5), Se (6))
Method A: A similar procedure to that used for the preparation of 1 and 2 above is used here.The O-ethyl fumaroyl chloride (0.46 g, 2.83 mmol) is used.The product is eluted with a mixture of Et 2 O/n-hexane mixture (2:1 volume ratio).
Method B: Complex 1 or 2 (0.50 mmol) is dissolved in EtOH solution (50 mL) in the presence of few drops of pyridine.The mixture was heated to reflux for two hours.The volatiles were removed under vacuum and the residue was redissolved in minimum amount of CH 2 Cl 2 and introduced to a silica gel column.An orange band was eluted with Et 2 O/n-hexane (2:1 volume ratio).The volatiles were removed under vacuum and recrystallized from CH 2 Cl 2 /n-hexane.

X-ray structural analyses
Single crystal intensity data were collected on a Bruker-Nonius Kappa-CCD diffractometer equipped with a Mo-Kα IµS micro-focus source and an Apex2 CCD detector, at T = 120(2) K. Multi-scan absorption correction was applied to the intensity data. [49]The structures were solved with SHELXT-2018/3 [50] and refined by full-matrix least squares techniques on F 2 with SHELXL-2018/ 3, [51] using the Olex 1.2 environment. [52]Crystallographic data and refinement parameters for 1, 5 and 6 are presented in Table 3.The ester oxygen atom O5 is disordered over two orientations.The corresponding site occupancy factor of the major component was refined freely to 0.76 (for 5) and to 0.79 (for 6).A SADI restraint on the C=O bond lengths was used.
C{ 1 H}-NMR spectra of 1-4 revealed the resonances of the carbon atoms of the Cp-ring in the range of 84.5-85.0ppm and those of the terminal carbonyls in the range 212.8-214.8ppm.In addition, the spectra of the monomers, 1 and 2, presented the carbonyl group of the calcogenofumarato and of the acid chloride carbons in the ranges of 197.8-198.6 ppm and 163.4-165.3ppm, respectively while those of the two magnetically nonequivalent vinylic-carbon atoms were in the ranges of 121.4-127.3 and 137.0-143.1 ppm.The spectra of the dimers showed the chalcogenofumarato carbon in the range of 197.8-198.6 ppm and those of the vinylic carbons in the range of 166.9-167.6 chalcogenofumarato carbonyl bond length (1 = 1.221(2)Å, 5 = 1.229(2)Å, 6 = 1.219(5)Å hybridized as indicated by the Fe-E-CO bond angle (1 = 107.91(5)º, 5 = 107.2(4)º,6 = 103.95(5)º).

Figure 1 .
Figure 1.Molecular structure of CpFe(CO) 2 SCOCH=CHCOCl (1) in the crystal.The ellipsoids are drawn with 50% probability.Disorder of the COCl and Cp groups were omitted for clarity.The "A" in the atom labeling denotes the major component of a disordered group.

Figure 2 .
Figure 2. Molecular structure of CpFe(CO) 2 SCOCH=CHCO 2 Et (5) in the crystal.The ellipsoids are drawn with 50% probability.Disorder of the ester oxygen atom (O5) was omitted for clarity, the "A" in the atoms labeling denotes the major component of the disordered group.

Figures 4
Figures 4 and 5 showed the molecular packing of the unit cell of complexes 1 and 5, respectively.All of the three crystal structures contain well-separated molecules without any unusual short intermolecular contacts.

Figure 3 .
Figure 3. Molecular structure of CpFe(CO) 2 SeCH=CHCO 2 Et (6) in the crystal.The ellipsoids are drawn with 50% probability.Disorder of the ester oxygen atom (O5) was omitted for clarity.The "A" in the atoms labeling denotes the major component of the disordered group.

Figure 4 .
Figure 4. Representation of the contents of the primitive monoclinic unit cell of 1, viewed in a (100) projection.Hydrogen atoms and minor-occupancy fractions of disordered groups are omitted for clarity, coordination environment of the iron atoms highlighted as dark yellow tetrahedra.

Figure 5 .
Figure 5. Representation of the content of the C-centered monoclinic unit cell of 5, viewed in a (010) projection.Hydrogen atoms and minor-occupancy fractions of disordered groups are omitted for clarity, coordination environment of the iron atoms highlighted as dark yellow tetrahedra.The crystal structure of 6 is isotypic.

Table 2 .
Selected bond lengths and selected bond angles of complexes 1, 5 and 6.

Table 3 .
Crystallographic data and refinement parameters for 1, 5 and 6.The COCl and Cp groups are disordered over two orientations.The corresponding site occupancy factor of the major component was refined freely to 0.85 for COCl and to 0.62 for Cp.SADI and SIMU restraints were used for the disordered groups.