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• 0   Introduction

  Much attention has been focused on the synthesis of a series of biscyclopentadienyl carbonyl ruthenium complexes in recent decades, which mainly include non-bridged^[1-2], singly bridged^[3-8] and doubly bridged^[9-10] biscyclopentadienyl complexes. Bridged bis(cyclopentadienyl) ligands have been extensively studied as frameworks for dinuclear metal complexes that are resistant to fragmentation and maintain two metal centers in close proximity even after metal-metal bond cleavage^[11-13]. Especially, rhenium carbonyl complexes have been studied as catalysts for many reactions due to their catalytic activity. When compared to mononuclear rhenium complexes, bridged dicyclopentadienyl dirhenium analogues, in which the bridging ligand binds two reactive metal centers, may promote the distinctive chemical reactivity and catalytic properties. However, only a few examples of Friedel-Crafts reactions catalyzed by rhenium carbonyl complexes have been reported up to now^[14-16]. Recently, Our group have reported the synthesis and catalytic activity of three monobridged bis(cyclopentadienyl)rhenium carbonyl complexes^[17], showing that these rhenium carbonyl complexes have catalytic activity for Friedel-Crafts alkylation reactions. To develop a deeper understanding of the structures and reactivity of bridged bis(cyclopentadienyl)rhenium carbonyl complexes, here in this paper we selected two bridged ligand precursors (C₅Me₄H)E(C₅Me₄H) (E=C₆H₄, (C₆H₄)₂), in which both two bridging groups have planer structure, and expected to see the catalytic reactivity of both p-phenylene-and p-biphenylene-bridged bis(cyclopentadienyl)rhenium carbonyl complexes.

  1   Experimental

  1.1   General considerations

  All procedures were performed under an argon atmosphere by using standard Schlenk techniques. Solvents were distilled from appropriate drying agents under nitrogen atmosphere. Elemental analyses of C and H were performed with a Vario EL Ⅲ elemental analyzer. The IR spectra were recorded as KBr disks on a Thermo Fisher is50 spectrometer. Gas chromatograms were recorded with an Agilent 6820 gas chromatograph. ¹H and ¹³C NMR spectra were recorded on a Bruker AV Ⅲ-500 (or 600) instrument in CDCl₃. The ligand precursors (C₅Me₄H)E(C₅Me₄H) (E=C₆H₄, (C₆H₄)₂) were synthesized according to the literature^[18-20].

  1.2   Synthesis of complex 1

  A solution of free ligand (C₅Me₄H)C₆H₄(C₅Me₄H) (0.19 g, 0.6 mmol) and Re₂(CO)₁₀ (0.2 g, 0.3 mmol) in mesitylene (15 mL) was refluxed for 48 h. After removal of solvent, the residue was placed on an alumina column. Elution with petroleum ether/CH₂Cl₂ (2:1, V/V) developed a colorless band, which was collected, and after concentration, afforded (C₆H₄)[(η⁵-C₅Me₄)Re(CO)₃]₂ (1) as a white solid. Yield: 52.1% (0.134 g). m.p. 316 ℃; Anal. Calcd. for C₃₀H₂₈O₆Re₂(%): C, 42.05; H, 3.29. Found(%): C, 42.32; H, 3.13. ¹H NMR (CDCl₃, 500 MHz): δ 2.16 (s, 12H, C₅Me₄), 2.25 (s, 12H, C₅Me₄), 7.28 (s, 4H, C₆H₄). ¹³C NMR (CDCl₃, 125 MHz): δ 10.9, 11.4, 97.7, 102.2, 104.0, 131.8, 132.4, 197.37. IR (KBr, cm^-1): 1 900(s), 2 002(s).

  1.3   Synthesis of complex 2

  Using a procedure similar to that described above, ligand precursor (C₅Me₄H)(C₆H₄)₂(C₅Me₄H) was reacted with Re₂(CO)₁₀ in refluxing mesitylene for 48 h. After chromatography with petroleum ether/CH₂Cl₂, (C₆H₄)₂)[(η⁵-C₅Me₄)Re(CO)₃]₂ (2) was obtained as white crystals. Yield: 77.8% (0.217 g). m.p. 313 ℃; Anal. Calcd. for C₃₆H₃₂O₆Re₂(%): C, 46.34; H, 3.46. Found(%): C, 45.98; H, 3.59. ¹H NMR (CDCl₃, 500 MHz): δ 2.17 (s, 12H, C₅Me₄), 2.26 (s, 12H, C₅Me₄), 7.38 (d, 4H, J=8.0 Hz, C₆H₄), 7.61 (d, 4H, J=8.5 Hz, C₆H₄). ¹³C NMR (CDCl₃, 125 MHz): δ 10.9, 11.3, 97.7, 102.1, 104.4, 127.0, 131.4, 133.0, 139.8, 197.4. IR (KBr, cm^-1): 1 903(s), 2 012(s).

  1.4   Crystallographic analysis

  Crystallographic data for complexes 1 and 2 were collected at 298 K on a Bruker AXS SMART 1000 CCD diffractometer with Mo Kα radiation (λ=0.071 073 nm) using the φ-ω scan technique. The crystal structures were solved by direct method and refined on F² by full-matrix least-squares technique using the SHELXL-97 program package^[21]. All non-hydrogen atoms were found from the Fourier difference maps refined anisotropically, hydrogen atoms were included in calculated positions riding on the parent atoms and refined with fixed thermal parameters. Crystallographic data and experimental details for structural analysis of the complexes are summarized in Table 1.

  Table 1.  Crystal data and structure refinement parameters for complexes 1 and 2

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  CCDC: 1506755, 1; 1506754, 2.

  1.5   General procedure for catalytic tests

  The catalytic reactions were carried out under an argon atmosphere with magnetic stirring. The required complexes (0.02 mmol) was mixed with 1, 2-dichloroethane (3.5 mL) in a 25 mL round-bottom flask at room temperature. Aromatic compounds and acylation reagents were added by syringe. The reaction mixture was heated at 80 ℃ for 24 h. After cooling to room temperature, the solid catalyst was separated from the reaction mixture by filtration. The solvent was removed by rotary evaporation, and the residue was purified by Al₂O₃ column chromatography, eluting with petroleum ether and ethyl acetate to give a white solid.

  2   Results and discussion

  2.1   Preparation of complexes 1 and 2

  Reactions of ligand precursors (C₅Me₄H)E(C₅Me₄H) (E=C₆H₄, (C₆H₄)₂) with Re₂(CO)₁₀ in refluxing mesitylene for 48 h afforded the corresponding complexes (E)[(η⁵-C₅Me₄)Re(CO)₃]₂ (E=C₆H₄ (1), (C₆H₄)₂ (2)) in yield of 52% and 78%, respectively (Scheme 1). The IR spectra of complexes 1 and 2 all exhibited only terminal carbonyl bands (1: 1 900, 2 002 cm^-1; 2: 1 903, 2 012 cm^-1). The ¹H NMR spectra of 1 and 2 all displayed two groups of singlets for the four methyl protons, indicating the symmetrical structure in solution, in addition, complex 1 showed a singlet at δ 7.28 for the phenylene protons and complex 2 showed two doublets at δ 7.38 and 7.61 for the biphenylene protons.

  Scheme 1.  Synthesis of complexes 1 and 2

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  2.2   Crystal structures of complexes 1 and 2

  Slow evaporation of the solvent from the complexes in hexane-CH₂Cl₂ solution gave single crystals 1 and 2 suitable for X-ray diffraction. Selected bond parameters are presented in Table 2 and their structures are depicted in Fig. 1 and 2, respectively.

  Table 2.  Selected bond distances (nm) and angles (°) for complexes 1 and 2

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  Figure 1.  Molecular structure of complex 1

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  Figure 2.  Molecular structure of complex 2

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  Single-crystal X-ray diffraction analysis reveals that 1 and 2 crystallize in the triclinic space group P1 Complexes 1 and 2 are monobridged bis(cyclopentadienyl)rhenium carbonyl complexes and similar to each other. Both structures show that the molecule consists of two [(η⁵-C₅Me₄)Re(CO)₃] moieties linked by a single bridge, with each rhenium atom is η⁵-coordinated to the cyclopentadienyl ring and three terminal CO ligands. Two Re(CO)₃ units are located on the opposite site of the monobridged ligand and anti to each other, the Re atoms exhibit a three-legged piano-stool geometry. Re-Re bonds are not observed in both complexes. Complex 1 has a symmetric structure, the molecule consists of two [(η⁵-C₅Me₄)Re(CO)₃] moieties linked by a p-phenylene-bridge. Complex 2 also has similar structure, consisting of two [(η⁵-C₅Me₄)Re(CO)₃] moieties linked by a p-biphenylene-bridge. The dihedral angle between the two Cp ring planes is 0° and the torsion angle Re1-Cp(centroid)…Cp(centroid)-Re1^ⅰ is -180°, indicating two Cp rings are parallel to each other. For complexes 1 and 2, the distances of Re1-CEN and Re1^ⅰ-CEN (CEN means the centroid of the cyclopentadienyl ring) are equal (Table 2). The Re-CEN distance in 1 is 0.195 6 nm and the Re-CEN distance in 2 is 0.194 6 nm, which compare very well with those values in the analogues [(η⁵-C₅H₄)₂C(CH₂)₅][Re(CO)₃]₂ (0.195 7 and 0.196 0 nm) and [(η⁵-C₅H₄)₂ Si(CH₃)₂][Re(CO)₃]₂ (0.194 6 and 0.195 2 nm)^[17]. From above data, we can conclude that the different ligand bridges in these complexes have no obvious effect on their structures.

  2.3   Catalysis of Friedel-Crafts acylation reactions

  In order to test the catalytic capability in Friedel -Crafts acylation reactions (Scheme 2~6) catalyzed by two complexes, considering factors such as the reaction time, yield, economic considerations, the following experimental conditions were chosen for next work: reaction time 24 h; 1, 2-dichloroethane as solvent; 80 ℃; the molar ratio of aromatic substrates and acylation reagents was 1:3; the amount of catalyst was 1.0%(n/n) (substrate as reference).

  Scheme2.  Complex 1 or 2 catalyzed Friedel-Crafts acylation reaction of anisole with benzoyl chloride

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  Scheme3.  Complex 1 or 2 catalyzed Friedel-Crafts acylation reactions of anisole and methylbenzene with acyl chlorides

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  Scheme4.  Complex 1 or 2 catalyzed Friedel-Crafts acylation reactions of anisole and methylbenzene with acetic anhydride

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  Scheme5.  Complex 1 or 2 catalyzed Friedel-Crafts acylation reactions of acyl chlorides

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  Scheme6.  Complex 1 or 2 catalyzed Friedel-Crafts acylation reactions of acetic anhydride

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  With the above studies, the yields(%) were found to vary with aromatic substrates with different acylation reagents, the catalytic results of complex 1 and 2 are shown in Table 3. Friedel-Crafts acylation reactions are electrophilic substitution reactions, however halogen element and carbonyl formed p-π conjugation in acylating agent, thus the acylating agents were difficult to lose halogen element to form carbocation. Benzoyl chloride, phenylacetyl chloride, cinnamoyl chloride and cyclohexanecarboxylic acid chloride could be used as acylation reagents in these reactions and the corresponding products were obtained with high selectivity for the para-products without detection of di-substituted in all cases, suggesting that the catalytic reaction has high regioselectivity. The order of increasing reactivity was found to be: 4-bromoanisole < 4-methyl anisole < methylbenzene < 2-bromoanisole < anisole < 2-methylanisole, which was consistent with the characteristics of the aromatic electrophilic substitution mechanism. Overall, both complexes gave similar results, showing that the different ligand bridges have only a small influence on the catalytic behavior.

  Table 3.  Catalyzed Friedel-Crafts acylation reaction of aromatic substrates with different acylation reagents

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  3   Conclusions

  Two new bridged bis(cyclopentadienyl)rhenium carbonyl complexes (E)[(η⁵-C₅Me₄)Re(CO)₃]₂ (E=C₆H₄ (1), (C₆H₄)₂ (2)) have been synthesized and structurally characterized. Friedel-Crafts reactions of aromatic substrates with acylation reagents showed that two complexes have catalytic activity. Compared with traditional catalysts, these complexes have significant practical advantages, namely lower amounts of catalyst, mild reaction conditions, and high selectivity. Further studies to elucidate the reaction mechanism and expand the synthetic utility of these catalysts are in progress.

  Supporting information is available at http://www.wjhxxb.cn

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• 

  Scheme 1  Synthesis of complexes 1 and 2

  E=C₆H₄ (1), (C₆H₄)₂ (2)

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  Figure 1  Molecular structure of complex 1

  Ellipsoids correspond to 30% probability; Hydrogen atoms are omitted for clarity; Symmetry codes: ^ⅰ 1-x, 1-y, 1-z

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  Figure 2  Molecular structure of complex 2

  Ellipsoids correspond to 30% probability; Hydrogen atoms are omitted for clarity; Symmetry codes: ^ⅰ 1-x, -y, 1-z

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  Scheme2  Complex 1 or 2 catalyzed Friedel-Crafts acylation reaction of anisole with benzoyl chloride

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  Scheme3  Complex 1 or 2 catalyzed Friedel-Crafts acylation reactions of anisole and methylbenzene with acyl chlorides

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  Scheme4  Complex 1 or 2 catalyzed Friedel-Crafts acylation reactions of anisole and methylbenzene with acetic anhydride

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  Scheme5  Complex 1 or 2 catalyzed Friedel-Crafts acylation reactions of acyl chlorides

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  Scheme6  Complex 1 or 2 catalyzed Friedel-Crafts acylation reactions of acetic anhydride

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  Table 1.  Crystal data and structure refinement parameters for complexes 1 and 2

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  Table 2.  Selected bond distances (nm) and angles (°) for complexes 1 and 2

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  Table 3.  Catalyzed Friedel-Crafts acylation reaction of aromatic substrates with different acylation reagents

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