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  Continuing our investigation on the amino, imino-functionalized indolyl ancillary ligand system, we have synthesized europium(Ⅲ) amido complex having 2-(phenyliminomethine)ⁱndoly ligands^[21].Encouraged by above-mentioned results, we further investigated the reactivity of the indolyl europium(Ⅲ) amide with formamidines, as shown in Scheme 1.According to our experimental results, when treating of europium(Ⅲ) complex containing imino-functionalized indolyl groups and N(SiMe₃)₂ group as ancillary ligands with formamidines, the attack occurs primarily onto the Eu-N bonds between indolyl ligand and central metal.Moreover, the results also made it clear that the reactivity of europium(Ⅲ) amido complex having the functionalized indolyl ligands and the structures of the resultants are significantly dependent on the steric hindrance of formamidines.Herein, we will report these results and structural studies of new complexes.

  Recently, we have reported the reactivity of 2-(2, 6-diisopropylphenylaminomethine)ⁱndolyl europium(Ⅱ) amido complex with formamidines having different spatial hindrance afforded novel complexes incorpor-ating indolyl ligands bonded with metal in novel μ-η³:η¹:η¹ and μ-η²:η¹:η¹ bonding manners^[19], respectively.In addition, we have also reported the reactivities of rare-earth metal monoalkyl complexes incorporating bidentate indolyl ligands with PhSiH₃ or amidine, and have obtained the first example of trivalent rare-earth metal complexes having an amido-functionalized indolyl ligand bonded to a metal in the μ-η⁶:η¹:η¹ fashions^[20].

  

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  During the course of lanthanide chemistry studies, novel reactivity of rare-earth metal complexes has been one of the most important research fields^[1-3], because such kinds of reactions are usually the basis for lots of important catalytic procedures, organic transformations and proposed pathway for the formation of lanthanide compounds.To date, much effort has been devoted to study the reactivity of rare-earth metal complexes^[4-15], which involves a number of small molecular substrates including H₂^[4], CO^[5], CO₂^[6-9], alkane^[10], alkene^{[8, 11]}, alkyne^[11], amine^[9], carbodiimide^[12], silane^{[4, 13]}, phosphine^[14], nitrile^[7-8], isocyanate^[8-9], ami-dine^[13], carbonyl compound^{[7, 11]}, heterocyclic compound^[11] and so on^[15].Among them, amidines, used as ligands or substrates for reactivity studies^[16-18], have drawn much attention due to being easily prepared and tuned sterically and electronically.Amidines have played an important role in the investigation on the influence of spatial hindrance and electronic effect on the synthesis, bonding modes, structures and performances of rare-earth metal complexes.Thus, reactivity studies between indolyl rare-earth metal amido complexes with formamidines are still the interesting work.

  1    Experimental

  1.1    Materials and methods

  All manipulations of air- and moisture-sensitive materials were performed using Schlenk techniques or in a glovebox under an atmosphere of purified argon.Toluene and hexane were refluxed and freshly distilled over sodium benzophenone ketyl under argon prior to use.[2-(C₆H₅NH=CH)C₈H₅N]₂Eu[N(SiMe₃)₂] (1)^[21] and 2, 6-R²C₆H₃N=CHNH(C₆H₃R_2-2, 6) (R=ⁱPr (2), Me (3))^[22] were prepared according to the literature methods.Elemental analyses data were obtained on a Perkin-Elmer 2400 Series Ⅱ elemental analyzer and analyses were carried out in the microanalytical laboratory of Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences.IR spectra were run on a Shimadzu FTIR-8400s spectrometer (KBr pellet).Melting points were determined in sealed capillaries and are uncorrected.

  1.2    Syntheses of complexes

  1.3    X-ray crystallographic analyses

  Single crystals of complexes 4 and 5 suitable for X-ray diffraction studies were sealed in thin-walled glass capillaries under argon.Diffraction was performed on a Bruker SMART CCD area detector diffractometer using graphite-monochromated Mo Kα radiation (λ=0.071 073 nm).An empirical absorption correction was applied using the SADABS program^[23].All structures were solved by direct methods, completed by subsequent difference Fourier syntheses, refined anisotropically for all nonhydrogen atoms by full-matrix least-squares calculations on F ² using the SHELXTL program package^[24]. All hydrogen atoms were refined using a riding model.A summary of the crystallographic data and selected experimental information are listed in Table 1, while the selected bond lengths and angles are summarized in Table 2.

  Table2. Selected bond lengths (nm) and angles (°) for complexes 4 and 5

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  Table2. Selected bond lengths (nm) and angles (°) for complexes 4 and 5
  Table1. Crystallographic data and structure refinement for complexes 4 and 5

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  Table1. Crystallographic data and structure refinement for complexes 4 and 5

  CCDC: 1496184, 4; 1496185, 5.

  1.2.1    Synthesis of [η¹:η¹-2-(C₆H₅NH=CH)C₈H₅N]Eu [(η³-2, 6-ⁱPr₂C₆H₃)N=CHN(C₆H₃ⁱPr_2-2, 6)] [N(SiMe₃)₂](4)

  A Schleck flask was charged with complex 1 (0.90 g, 1.20 mmol), 2, 6-ⁱPr₂C₆H₃N=CHNH(C₆H₃ⁱPr_2-2, 6) (2) (0.43 g, 1.20 mmol) and toluene (30 mL).The reaction mixture was stirred at 70 ℃ for 16 h.The red-brown solution was evaporated to dryness and extracted with n-hexane (14 mL).Orange-red crystals that were suitable for X-ray diffraction were obtained upon crystallization from hexane at 0 ℃ after a few days (0.59 g, 55%).m.p.187 ℃.Anal.Calcd.for C₄₆H₆₅N₅Si₂Eu(%): C, 61.65; H, 7.31; N, 7.81.Found(%): C, 61.16; H, 7.63; N, 8.22.IR (KBr pellet, cm^-1): 2 959 (w), 2 865 (w), 1 660 (s), 1 614 (s), 1 589 (s), 1 483 (m), 1 445 (m), 1 423 (m), 1 298 (w), 1 194 (w), 1 065 (w), 957 (w), 908 (w), 872 (w), 800 (m), 767 (m), 753 (m), 737 (m), 695 (s).

  1.2.2    Synthesis of [(η³-2, 6-Me₂C₆H₃)N=CHN(C₆H₃Me_2-2, 6)]₂Eu[N(SiMe₃)₂](5)

  The synthesis of complex 5 was same to that of complex 4 except that 2, 6-Me₂C₆H₃N=CHNH(C₆H₃Me_2-2, 6) (3) (0.60 g, 2.40 mmol) was used instead of com-pound 2.Yield: 0.55 g, 56%.m.p.205 ℃.Anal.Calcd.for C₄₀H₅₈N₅Si₂Eu(%): C, 58.80; H, 7.16; N, 8.57.Found(%): C, 58.38; H, 7.42; N, 8.68.IR (KBr pellet, cm^-1): 2 961 (w), 2 854 (w), 1 661 (s), 1 613 (s), 1 589 (s), 1 483 (m), 1 444 (m), 1 425 (m), 1 298 (w), 1 195 (w), 1 066 (w), 955 (w), 906 (w), 870 (w), 799 (m), 765 (m), 750 (m), 737 (m), 696 (s).

  2    Results and discussion

  

  Figure 1. Molecular structure of 4 with 30% thermal ellipsoids

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  Figure 2. Molecular structure of 5 with 30% thermal ellipsoids

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  Complex 4 crystallizes in a triclinic crystal system with space group P1.X-ray analyses revealed that 4 is five-coordinated mononuclear complex.As shown in Fig.1, the central Eu³⁺ is coordinated by a bidentate indolyl ligand in η¹:η¹ hapticities, an formamidinato ligand in an η³ fashion and N₃)₂ ligand adopting a distorted trigonal bipyramid configuration similar to that of complex 1^[21].The C(28)-N(3) length of 0.132 0(8) nm and C(28)-N(4) of 0.131 0(7) nm has become average and close to the value between carbon-carbon length of single bond and that of double bond, showing the characteristic of delocalized bond and indicating that the formamidinato ligand is bonded to the Eu3+ with an η³ manner.Among the Eu-N lengths in complex 4, the Eu(1)-N(2) length of 0.251 0(6) nm of the appendant arm is significantly longer than the others (Eu(1)-N(1) 0.234 3(6) nm, Eu(1)-N(3) 0.240 2(5) nm, Eu(1)-N(4) 0.243 6(5) nm, Eu(1)-N(5) 0.225 0(5) nm), and close to the corresponding value of 0.252 5(3) nm in complex 1 and that of 0.256 5(2) nm in complex [2-(^tBuN=CH)C₈H₅N]Eu[N₃)₂]2^[21], indicating coor-dination nature of the N₂ atom.The bond angles of N(5)-Eu(1)-N(1), N(5)-Eu(1)-N(3), N(1)-Eu(1)-N(3), N(1)-Eu(1)-N(4) and N(5)-Eu(1)-N(2) are respectively 110.20(19)°, 113.13(19)°, 104.65(19)°, 111.82(18)° and 96.80(19)°.

  As shown in Scheme 1, reaction of europium complex 1 with the sterically bulky formamidine [(2, 6-ⁱPr₂C₆H₃)NCHNH(C₆H₃iPr_2-2, 6)] (2) gave europium complex formulated as [η¹:η¹-2-(C₆H₅NH=CH)C₈H₅N]Eu[(η³-2, 6-ⁱPr₂C₆H₃)N=CHN(C₆H₃iPr_2-2, 6)][N(SiMe₃)₂] (4), containing an indolyl ligand and an formamidinato ligand via ligand exchange process.Treatment of europium complex 1 with the less bulky formamidine 2, 6-Me₂C₆H₃N=CHNH(C₆H₃Me_2-2, 6) (3) afforded euro-pium complex formulated as [(η³-2, 6-Me₂C₆H₃)N=CHN(C₆H₃Me_2-2, 6)]2Eu[N₃)₂] (5), only incorporating formamidinato ligands and N₃)₂. Complexes 4 and 5 are extremely sensitive to air and moisture.They were characterized by spectroscopic and elemental analyses, and their structures were further elucidated by X-ray diffraction study.The structures of 4 and 5 are individually displayed in Fig.1 and 2, while selected bond lengths and angles are given in Table 2.

  Complex 5, crystallizing in a monoclinic crystal system with space group C2/c, is also five-coordinated.Differently, the coordination environment of the central metal is completed by two formamidino groups and one -N₃)₂ ligand as a centrosymmetric tetragonal pyramid arrangement, in which N3 atom of -N₃)₂ group occupies the vertex, as shown in Fig.2. Given that the C(9)-N(1)ⁱ, N(1)-C(9)ⁱ lengths of 0.131 3(3) nm, C(9)-N(2)ⁱ of 0.132 0(3) nm and N(2)- C(9)ⁱ of 0.132 1(3) nm, the bonding mode of formamidino ligand bonded to Eu3+ is described as an η³ fashion.The Eu(1)-N(1)ⁱ length of 0.240 16(19) nm or Eu(1)- N(2)ⁱ length of 0.242 4(2) nm is in accordance with the corresponding Eu(1)-N(1) length of 0.240 16(19) nm or Eu(1)-N(2) length of 0.242 4(2) nm, indicating a centrosymmetric geometry.Moreover, the centrosym-metric structure is also deduced by the bond angles of N(3)-Eu(1)-N(2)ⁱ of 103.84(5)°, N(3)-Eu(1)-N(1)ⁱ of 122.32(5)°, N(3)-Eu(1)-N(1) of 122.32(5)° and N(3)- Eu(1)-N(2) of 103.84(5)°.Thus, the result suggested that imino-functionalized indolyl ligands in complex 1 are completely replaced by the less bulky formamidino groups, while the geometry of the resultant complex 5 is obviously different from that of complex 1.

  3    Conclusions

  In summary, we have studied the reactivity of europium(Ⅲ) amido complex having 2-(phenylimino-methine)ⁱndoly ligands [2-(C₆H₅NH=CH)C₈H₅N]2Eu[N₃)₂] (1) with formamidines.Reaction of 1 with the sterically bulky formamidine [(2, 6-ⁱPr₂C₆H₃)NCHNH(C₆H₃iPr_2-2, 6)] or the less bulky formamidine 2, 6-Me₂C₆H₃N=CHNH(C₆H₃Me_2-2, 6) gave complexes [η¹:η¹-2-(C₆H₅NH=CH)C₈H₅N]Eu[(η³-2, 6-ⁱPr₂C₆H₃)N = CHN(C₆H₃iPr_2-2, 6)][N₃)₂] (4) and [(η³-2, 6-Me₂C₆H₃)N=CHN(C₆H₃Me_2-2, 6)]2Eu[N₃)₂] (5), respectively.The results showed that the steric hindrance of organic molecular substrates have significantly effects on the reactivity of rare-earth metal amido complexes containing imino-functionalized indolyl ligands and the structures of the resultant new lanthanide complexes.Further study on the reactivity of the rare-earth metal compounds with the related indolyl ligands is in progress in our laboratory and will be reported soon.

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  Figure 1  Molecular structure of 4 with 30% thermal ellipsoids

  All hydrogen atoms are omitted for clarity

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  Figure 2  Molecular structure of 5 with 30% thermal ellipsoids

  All hydrogen atoms are omitted for clarity; Symmetry codes: ⁱ -x+1,y,-z+1/2

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  Table 1.  Crystallographic data and structure refinement for complexes 4 and 5

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  Table 2.  Selected bond lengths (nm) and angles (°) for complexes 4 and 5

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