Electronic Effects on the Reactivity of Cross-Conjugated Carbonyl Systems with Fe 2 (CO) 9

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Introduction
The α,β-unsatured carbonyl compounds react with Fe 2 (CO) 9 forming η 2 -[Fe(CO) 4 ] and the most thermodynamically stable η 4 -[Fe(CO) 3 ] complexes, 1 which have shown different applications such as transfer agent of Fe(CO) 3 , 2 protecting group, 3 stereodirector group, 4 etc.Recently, we have described how reactivity changes drastically when heteroatoms (nitrogen and sulfur) in β-position are located on the system leading to novel an unexpected dinuclear complexes. 5In this subject, we have reported that the reactivity of cross-conjugated carbonyl systems β-positioned by sulfur atoms towards Fe 2 (CO) 9 , principally produce partial and total C-S bond activation in addition to the formation of η 4 -[Fe(CO) 3 ] complexes (Scheme 1). 6As a part of our constant interest in the understanding of the coordination patterns of this kind of systems, we herein report the study of electronic effects induced by p-substituents on the phenyl ring on the mechanism of reaction of cross-conjugated carbonyls

Results
We have synthesized a series of ligands including electron-withdrawing and electron-donating groups on para-position of aromatic ring (Scheme 2).These ligands were totally characterized by the conventional spectroscopic techniques (MS, IR, 1 H and 13 C NMR) and they have some similarities in their 1 H and 13 C NMR spectra such as, a multiple signal around 3.4 ppm that corresponds to the methylene groups and one simple signal around 6.8 ppm assigned to the HCCSS in 1 H NMR. In 13 C NMR, they exhibit two signals in 35 and 39 ppm for the methylene groups of the dithiolane system, the corresponding signal of HCCSS shifted at 112 ppm, the CSS around 165 ppm, and the CO group at 185 ppm.The IR spectra of these ligands show a weak narrow band around 1630 cm -1 assigned to CO group, a strong fine band in 1500 cm -1 , attributed to C=C bond, in addition to the respective bands for each ligand.The band intensities for the CO and C=C groups indicate that an S...O interaction through to C-C double bond is present as reported elsewhere. 7he structural arrangement for 3 was fully established by a single-crystal X-ray diffraction analysis (Figure 1).The PhCH=CHCO moiety presents a s-trans conformation.The ylidenbutenone backbone of the ligand is quasi-planar with the phenyl ring tilted 18.6° out of main plane.The bond distance C1-C11 in comparison with C3-C4 bond is longer [0.069Å] (Table1) and the distance [S1---O1, 2.705Å] is shorter than the ∑ of Van der Waals radii.These features indicate that an interaction between the sulfur and oxygen atoms through the conjugated system is present, in agreement with the obtained results from IR technique and with described in literature. 6,7The 1,3dithiolane ring adopts a half-chair conformation with C 2 axe over C11.Additionally, this moiety presents disorder generating two conformers in 46/54 ratio, only the major contributors are shown in Figure 1.
anhydrous THF led to the formation of four comple (Scheme 3), being 8 the major product, evidencing well-known reductive character of Fe 2 (CO) 9 . 8The η 2 -[Fe(CO) 4 ] (2a) and κ N -[Fe(CO) 4 ] (9) complexes w obtained in a very low yield and the complex 7 obtained as a result of both C-S bond activation in ligand in agreement with our previous studies. 6T complex has been obtained by other methods. 9hen the coordination reactions were carried out w ligands 3 and 4 (Scheme 4), the formation of f 2 and 4a) and η 4 -[Fe(CO) 3 ] (3b and 4b) complexes were obtained as orange crystalline solids.These complexes displayed in IR spectra 3 bands around 2000 cm -1 that correspond to ν(M-CO) region.In MS the corresponding peaks assigned to the fragments [M + -28] were observed.
The 1 H NMR spectra for these complexes exhibit the peaks corresponding to the AB system shifted upfield, indicating the coordination of the iron complex to the C=C π-system.
The chemical shift of the other signals was very similar to that of the free-ligands.The 13 C NMR spectra of these complexes show that the corresponding signals to the C=C double bond are shifted to higher frequencies (∆δ around 67). Signals around 200 and 208 ppm were assigned for M-CO group.The most salient difference between η 2 -[Fe(CO) 4 ] and η 4 -[Fe(CO) 4 ] complexes is that for η 4 -[Fe(CO) 3 ] complexes (3b and 4b), the ketone group signal was shifted from 184 ppm at 155 ppm and this fact indicates the coordination of CO group to iron atom.The X-ray diffraction analysis for 4b (Figure 2), showed that the [Fe(CO) 3 ] fragment was bonded in η 4 -coordination mode to the enone moiety O4-C5-C6-C7, thus the coordination polyhedron can be best described as a and the C3-C4 double bond in the equatorial positions, with the O4 oxygen atom and the remaining CO group in apical positions with a bond angle of 164.5(2)º (Table 2).This arrangement is in agreement with other η 4 -[Fe(CO) 3 ] complexes described in literature. 10One O4---S1 interaction (2.757 Å) was also present and was in the same order than with the obtained crystallographic data from 3. The 1,3-dithiolane ring adopts an envelope conformation with C16(C16B) as flap, the crystalline structure showed disorder in this moiety leading two conformers in 77/23 ratio, only the major contributor are shown in Figure 2.
Additionally, the dinuclear complexes 3c and 4c were obtained in moderate yields.They have the same structural arrangement as the complex 1c, previously reported. 6,11he IR spectra of these compounds show the typical bands of ν(M-CO) in 2058 and 2017 cm -1 .The 1 H NMR spectra of 3c and 4c exhibit changes in the assigned signals to the methylene groups, which shifted upfield with respect to ligands.In 13 C NMR spectra, as key signals, we observed that the alpha CH and the carbon attached to sulfur and iron atoms were shifted to higher frequencies (12 and 25 ppm respectively).In both reactions, complex 7 was The reaction with ligands 5 and 6 lead to the formation of the η 4 -[Fe(CO) 3 ] complexes (5b and 6b) in low yield (Scheme 5), being the major products the dinuclear complexes 5c and 6c, these complexes displayed a similar structural arrangement to those of 1c, 3c and 4c vide supra.
In both experiments, complex 7 was also obtained in low yield and for the ligand 6, the κ N -[Fe(CO) 4 ] complex (10)  was additionally obtained in traces.For 6b, an additionally σ-N coordination of [Fe(CO) 4 ] fragment towards amine group was evidenced by 1 H and 13 C NMR, as well as by a peak in 599 m/z in MS.
We have carried out some experiments exploring different condition reaction (Table 4), which evidenced that the coordination reactions were sensitive to the temperature and the stability of the complexes.Thus, the η 2 -[Fe(CO) 4 ] complexes are intermediates in the forma of most thermodynamic stable η 4 -[Fe(CO) 3 ] comple whose formation depends strongly on the electro characteristics of the C-C double bond. 12The dinuc complexes 3c-6c are the intermediates in the formatio complex 7, resulting from the total C-S bond activatio the corresponding ligands, as we have previou proposed. 6Screening the reactivity of Fe 2 (CO) 9 toward this kind of systems, the chemical shift of C-3 on 13 C NMR have been analyzed using correlation with Hammett type substituent constant (σ p ) 13 (Table 5) giving a lineal correlation as shown in Figure 3.
The positive value of (ρ) indicates that the electronic effect of the group on the aromatic ring increased the electronic density over C3 as a long distance β effect (Scheme 6). 16s a result of this study, and the several possibility of coordination ability of Fe(0) towards cross-conjugated compounds, we have observed that [Fe(CO) 4 ] fragment has a preference to react with electronic deficient double bound in agreement with G. Reichenbach, 17 who proposed that the [Fe(CO) 4 ] fragment is an electron-relaxing.Thus, when there are electron-withdrawing groups on the aromatic ring, the reaction produce the η 2 -[Fe(CO) 4 ] and η 4 -[Fe(CO) 4 ] complexes as the main compounds, in agreement with the Hammett plot.
Nevertheless, with electron-donating groups (Scheme 5), it might be expected that the transmission of electronic effects should increased by mesomeric effect over C3=C4 and then, the reaction proceeds differently leading as major products the dinuclear complexes resulting from the

Conclusion
In this work, we have evidenced that the reactivity of Fe 2 (CO) 9 with cross-conjugated carbonyl ligands depends on the electronic density of the heterodienic moiety, thus, electron-withdrawing groups promote the coordination reaction towards the formation of η 2 -[Fe(CO) 4 ] and η 4 -[Fe(CO) 3 ] complexes, while electron-donor groups aim the reactivity of Fe 2 (CO) 9 towards the heterodienic moiety β-positioned by sulfur atoms leading the formation of dinuclear iron complexes, coming from the partial and total carbon-sulfur bonds activation.Experimental 1 H NMR and 13 C NMR spectra were recorded on a JEOL 300 spectrometer, using CDCl 3 , (CD 3 ) 2 SO and C 6 D 6 as solvents and TMS as internal reference.IR spectra were performed on a Perkin-Elmer 283 B or 1420 spectrometer.The FAB spectra were obtained on a JEOL JMS SX 102A mass spectrometer operated at an accelerating voltage of 10 Kv.Samples were desorbed from a nitrobenzyl alcohol matrix using a 6 kev Xenon atoms.The electronic impact (EI) ionization mass spectra were acquired on a JEOL JMS-AX505 HA Mass spectrometer operated in the positive ion mode.The acquisition conditions were ion source temperature 230 °C, ionization energy 70 eV, emission current 0.14 mA and ionization current 100 mA.Mass measurements in FAB are performed at 10000 resolution using electrical field scans and the polyethylene glycol ions as reference material.Melting points were measured using a Mel-Temp II apparatus and are uncorrected.All reagents were obtained from commercial suppliers and used as received.Reactions were performed under nitrogen atmosphere in carefully dried glassware.THF and ether were distilled from sodium-benzophenone under argon and/or nitrogen atmosphere.Column chromatography was performed with Merck silica gel (70-230 mesh) or alumina using ethyl acetate: hexane in different ratios as eluent.

Synthesis of ligands
To a solution of 5 mmol of 1-(1,3-dithiolane-2-yliden) 6 alcoholic solution of NaOH (5.5 mmol) at 0 The solution of corresponding aldehyde (5 mmol) in 5 mL ethanol was added and the reaction was keeping magnetic stirring during 18 h at room temperature.solvent was eliminated for distillation at reduced press then Ethyl acetate (50 mL) and water (2 x 50 mL) were added, extracting the organic phase, drying with Na 2 and removing the volatile in vacuo.The reaction mix was purified by column chromatography on alumina, u hexane/AcOEt in different ratios as eluent.

X-ray crystal structure determinations of compounds 3 and 4b
Data collection and refinement parameters are summarized in Table 3.The diffraction data for 3 and 4b were collected on a Bruker Smart Apex CCD diffractometer with MoKα radiation, λ = 0.71063 Å.Each data set was corrected for Lorentz and polarization effects and analytical absorption corrections based on face indexed were applied.The structures were solved by direct methods 18 and each structure was refined by full-matrix least squares on F 2 using all data with the all non-hydrogen atoms assigned anisotropic displacement parameters and hydrogen atoms bound to carbon atoms inserted at calculated position with isotropic temperature factor 1.2 times the Uiso of the parent carbon atom.The program used in the final refinements was SHELXL-97. 19Selected bond lengths and bond angles are shown in Tables 1-2.

Table 3 .
Summary of Crystal data, data collection, and refinement details

Table 5 .
Chemical shift on13C NMR from C3 and σ p