Synthesis and crystal structure of a-diimine iron(II) complex of [Ph-N=C(Me)-C(Me)=N-Ph]FeBr₂

Abstract The complex [Ph-N=C(Me)-C(Me)=N-Ph]FeBr₂ (1) has been synthesized via the reaction of FeBr₂ with equivalent of Ph-N=C(Me)-C(Me)=N-Ph in THF at room temperature in ca. 93% yield, supported by elemental analysis and X-ray crystal determination. The center iron atom is coordinated to two nitrogen atoms of a-diimine ligands and two bromine atoms to form a distorted tetrahedral geometry.
Keywords Iron complex, a-Diimine ligand, Crystal structure, Synthesis

Received March 16, 2008; Supported by National Natural Science Foundation of China (No. 20772089) and the Key Laboratory of Organic Synthesis of Jiangsu Province.

1. INTRODUCTION
During the past few decades, a-diimine derivatives are among the most versatile and thoroughly studied groups of ligands in late transition metal-based organometallic chemistry.^{[1, 2]} Particularly, iron-based complexes bearing a-diimine ligands have recently drawn much attention in atom transfer radical polymerization (ATRP)^[3]. Up to date, a series of a-diimine Fe(II) complexes of (N,N)FeX₂ (where [N,N] = RN=C(R')-C(R')=NR, R = aryl and alkyl, R'= H and alkyl) have been synthesized, structural characterization and used as catalysts for ATRP of styrene^[3,4,6,7,8] and methyl methacrylate^[3,5]. The substituent on the nitrogen atom of a-diimine ligands exerted great influence on the catalytic behaviors of those iron complexes. For example, the bulky arylimine stabilized iron catalysts, i.e. R= 2,6-diisopropylphenyl and mesityl, gave rise to the catalytic chain transfer polymerization (CCT) of styrene, whereas the alkylimine derivatives afforded ATRP of styrene^[3]. It is suggested that a steric effect could be used as the switch in the polymerization mechanism from CCT to ATRP, and less crowded coordination sphere around the iron atom was of benefit to the ATRP mechanism. With a view to gaining a detailed understanding of the steric effect of a-diimine ligands, herein we synthesized and structural characterized a novel a-diimine Fe(II) complex of [Ph-N=C(Me)-C(Me)=N-Ph]FeBr₂ (1), which lacks the ortho substituent on the aryl ring of a-diimine ligand. Very recently, a analogue iron complex of [Ph-N=C(Me)-C(Me)=N-Ph]FeBr₂ has been reported in brief as a catalyst for the ATRP of styrene without X-ray structure analysis^[3].

2. EXPERIMENTAL
2.1. General procedures
All manipulations were performed under pure argon with rigorous exclusion of air and moisture using standard Schlenk techniques. Solvents were distilled from Na/benzophenone ketyl under pure argon prior to use. Ph-N=C(Me)-C(Me)=N-Ph^[9] was prepared according to literature procedure. Elemental analysis was performed by direct combustion on a Carlo-Erba EA-1110 instrument.
2.2. Synthesis of [Ph-N=C(Me)-C(Me)=N-Ph]FeBr₂ (1)                  
By the modified literature procedure^[10], a 100 mL round-bottomed flask was charged with 1.20 g (5.1 mmol) of Ph-N=C(Me)-C(Me)=N-Ph and 60 mL of THF. With stirring, 0.84 g (3.9 mmol) of FeBr₂ was added at 25ºC. The dark green suspension was stirred for 18 h and filtered through Celite, and the resulting solid was extracted with an additional 100 mL of THF. The solvent was removed in vacuo, and the residue was washed with toluene. Removal of the toluene in vacuo afforded the product as brown solid in yield of 93% (2.17 g). Anal. Calcd for C₁₆H₁₆Br₂FeN₂: C, 42.52; H, 3.57; N, 6.20. Found: C, 42.89; H, 3.18; N, 6.59.
2.3 X-ray crystallography                  
Crystalline sample of 1 was grown from THF at –10ºC under Ar atmosphere. A dark-red needle crystal with dimension of 0.48 mm × 0.40 mm × 0.30 mm was mounted in a thin-walled glass capillary for X-ray structural analysis. Diffraction data were collected on a standard Bruker-axs SMART CCD area detector equipped with a graphite-monochromatized MoKa radiation (l= 0.71073 Å) at 223(2) K using the w scan mode in the range of 3.08 <q< 25.33°, and a multi-scan absorption was applied to the intensity data. A total of 15716 reflections were collected, of which 3136 (R_int = 0.0594) were independent and used. The frame was integrated with the Bruker-axs SAINT program. Data were corrected for absorption with the SADABS program. The structure was solved by direct methods and refined by full-matrix least-squares procedures based on F² (Bruker-axs, SHELXTL). All non-hydrogen atoms were refined with anisotropic displacement coefficients. Hydrogen atoms were determined with theoretical calculation and refined isotropically. The structures were solved and refined using SHELXS-97 (Sheldrick, 1990) and SHELXL-97 (Sheldrick, 1997) programs, respectively. The final R = 0.0719 and wR = 0.1386 (w = 1/[s²(F_o²) + (0.0741P)² + 0.0000P], where P = (F_o² + 2F_c²)/3 for 2761 observed reflections with I > 2s(I). S = 1.082, (D /s)_max = 0.000, (Dr)_max = 0.912 and (Dr)_min = -0.853 e/Å.

3. RESULTS AND DISCUSSION            
[Ph-N=C(Me)-C(Me)=N-Ph]FeBr₂ (1) was readily synthesized via a modified method by Dieck^[10]. Thus, the treatment of FeBr₂ with Ph-N=C(Me)-C(Me)=N-Ph in THF (Scheme 1) resulted the target product 1 in high yields. The complex 1 is isolated as air stable microcrystallines. Crystals of 1 suitable for an X-ray structure determination were grown from THF at -10ºC.

Scheme 1

    The proposed structure for 1 was first supported by elemental analysis. The further characterization for its molecular structure was obtained by X-ray analysis.
    The structure of the title compound is shown in Fig. 1. The crystallographic data and analysis parameters are shown in Table 1. Selected bond lengths and angles are listed in Table 2.

Fig. 1. Molecular structure of the title complex (1)

Table 1 Crystal data and structure analysis parameters

Table 2 Selected bond lengths (Å) and bond angles (º)

    X-ray structure analysis reveals that the title complex crystallized in orthorhombic system and has a monomeric structure in P212121 space group. This complex has no crystallographic symmetry factor due to the torsional twist of the two Ph rings from each other. The dihedral angle between the two Ph rings is ca. 22º. The fact that complex 1 being unsymmetry is quite different from those of reported bulky arylimine derivatives, i.e. C_s symmetry for 2,6-diisopropylphenylimine^[5] or mesitylimine^[3] derivatives. This difference might be due to the lack of the ortho substituent on the phenyl ring in 1, which leading to a more labile N-C_phenyl bond to form a less crowded array of the two Ph rings in 1. The iron atom is bonded to two nitrogen atoms of a-diimine ligands and two bromine atoms to form a distorted tetrahedral geometry [angles in the range of 77.6(2)-123.38(5)º] with the FeBr₂ and FeN₂ planes being nearly orthogonal (ca. 86º) to each other. The five-membered chelate ring was essentially planar (to within 0.005 Å), which is similar to tert-butylimine derivative^[4], but quite different to the 2,6-diisopropylphenylimine derivative (the chelate ring is folded about the N···N vector by ca. 20°, the iron atom lying 0.58 Å out of the C₂N₂ plane) ^[5]. The coplanarity of the iron atom with the chelate ring is expected to give rise to an open geometry around the metal center^[5].
    The relatively short C=N bond lengths of 1.286(8) and 1.307(9) Å and the relatively long C1-C2 bond lengths of 1.506(1) Å indicate that there is no noticeable bond delocalization in the five-membered chelate ring of complex 1. This pattern is very similar to those observed in the the tert-butylimine derivative^[4] and bulky arylimine derivatives^{[3, 5]}. Besides, the bond lengths observed here, including those to the iron atom, i.e. the Fe-N bond and the Fe-Br bond, are very close to the reported values^[3,5]. For example, the Fe-N bond lengths of 2.082(6) and 2.106(6) Å almost fall in the range previously reported values of 2.098(8)-2.132(8) Å presented in 2,6-diisopropylphenylimine,^[5] mesitylimine^[3] and tert-butylimine^[3] derivatives. The Fe-Br bond lengths of 2.365(1) and 2.364(1) Å are almost the same as these values of 2.367(1) Å present in tert-butylimine analogue^[3].
    In conclusion, the present work provides an opportunity to examine the steric constraints around the iron atom by comparison of the structural data of 1 with its bulky arylimine derivatives or tert-butylimine analogue. The X-ray structure analysis reveals that the present modification in the ortho substituent on the aryl ring has an obvious effect on the geometry around the iron atom although exerts a little effect on the relative bond lengths presented in 1. The lack of ortho substituent on the phenyl ring leads a more open geometry around the iron atom, which is expected to make the center metal more accessible and allow an ATRP to dominate^[5].

4. CONCLUTION
In summary, we have successfully synthesized a new a-diimine Fe(II) complex [Ph-N=C(Me)-C(Me)=N-Ph]FeBr₂ 1 in high yield, and characterized its structural feature by X-ray crystallography. In comparison with the reported results^[3,4,6,7,8], the present work provides a direct evidence to elucidate that the no substituent on the aryl ring of a-diimine ligands make a little difference on the molecular structure of the a-diimine-based iron complexes. The further investigation focus on its catalytic activity and modification is proceeding in our laboratory.

REFERENCES
[1] Ittel, S. D.; Johnson, L. K.; Brookhart, M.. Chem. Rev. 2000, 100: 1169.
[2] Gibson, V. G.; Spitzmesser, S. K. Chem. Rev. 2003, 103: 283.
[3] O'Reilly, R. K.; Shaver, M. P.; Gibson, V. C.; White, A. J. P. Organometallics 2007, 40: 7441, and the references cited therein.
[4] Gibson, V. C.; O’Reilly, R. K.; Reed, W.; Wass, D. F.; White,A. J. P.; Williams, D. J. Chem.Commun. 2002, 1850.
[5] Gibson, V. C.; O’Reilly, R. K.; Wass, D. F.; White,A. J. P.; Williams, D. J. Macromolecules 2003, 36: 2591.
[6] Shaver, M. P.; Laura, E. N.; Allan, H. S.; Gibson, V. C. Angew. Chem. 2006, 118: 1263.
[7] Shaver, M. P.; Laura, E. N.; Allan, H. S.; Gibson, V. C. Organometallics 2007, 26: 4725.
[8] Laura, E. N.; Allan, H. S.; Shaver, M. P.; Andrew J. P.; White.; Gibson, V. C. Inorg. Chem. 2007, 46: 8963.
[9] Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 2000, 122: 6686.
[10] Tom Dieck, H.; Diercks, R. Angew. Chem., Int. Ed. Engl. 1983, 22: 778.

a -二亚胺铁配合物的合成与晶体结构
陶雪平，孙宏枚，沈琪，张勇
(江苏省有机合成重点实验室，苏州大学化学化工学院，苏州 215123，中国)
摘要 通过a-二亚胺和FeBr₂的1：1（摩尔比）反应，我们以高收率合成了a-二亚胺铁配合物1。配合物1通过了元素分析和X-衍射等表征。结构测定表明，中心金属原子Fe与a-二亚胺上的两个氮原子和两个溴原子配位，形成了一个略扭曲的四面体。本文的研究结果表明a-二亚胺芳基上的取代基对相应铁配合物的组成和结构有一定的影响。
关键词 铁配合物、a-二亚胺、晶体结构、合成

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