GUO Li-Ping SONG Ren-Yuan

a (Anhui Provincial Engineering Laboratory of Silicon-based Materials, Bengbu University, Bengbu233030, China)

b (The Key Laboratory of Functional Molecular Solids,Ministry of Education, Anhui Normal University, Wuhu241000, China)

ABSTRACT Rare-earth metal dialkyl complexes [1-Bn-3-(DippN=CH)C8H4N]RE(CH2SiMe3)2(thf)2 (Dipp =2,6-iPr2C6H3, RE = Y (1) and Er (2)) were prepared through the cyclometalation reactions of the N-Bn-3-imino-functionalized indolyl ligand 1-Bn-3-(DippN=CH)C8H5N with one equivalent of rare-earth metal trialkyl precursors. The structures of compounds 1 and 2 were confirmed by X-ray crystal analyses and characterized by elemental analysis, IR, NMR spectroscopy wherein applicable. In the presence of cocatalysts, these rare-earth metal dialkyl complexes initiated isoprene polymerization with a high activity (95% conversion of 2000 equivalent of isoprene in 360 min), producing polymers with high regioselectivity (1,4-polymers up to 91%).

Keywords: indolyl ligand, rare-earth metal dialkyl complex, isoprene, polymerization;

1 INTRODUCTION

In the past decades rare-earth metal dialkyl complexes have attracted much attention for their high reactivity in organic synthesis and excellent catalytic activity and selectivity for olefin polymerization[1,2]. Ligands have played a very important role in stabilizing these rare-earth metal dialkyl complexes for their easy decomposition to the corresponding monoalkyl complexesvialigands redistribution. To date, Cp (Cp = cyclopentadienyl)[3],β-diketiminato[4], Tp (Tp = tris(pyrazolyl)borate)[5]and amidiante[6]with the advantage of easy modulation of the steric and electronic properties have been successfully used in the development of rare-earth metal dialkyl complexes. Indole and its derivatives have been widely used as ligand platform in coordination chemistry to facilitate the discovery of new multihapto-binding modes[7]. Previously, we have reported the reaction of 1-alkyl-3-imino functionalized indolyl ligands with lithium reagents or rare-earth trialkyl precursors[8]. The rare-earth metal monoalkyl complexes obtained by the reactions of corresponding ligands with rare-earth metal trialkyl precursors in the molar ratio of 2:1 have shown high activity and selectivity in isoprene polymerization. In this paper,we describe the synthesis of two rare-earth metal dialkyl complexes [1-Bn-3-(DippN=CH)C8H4N]RE(CH2SiMe3)2(thf)2(RE = Y (1) and Er (2)), focusing on their crystal structures and catalytic activity toward isoprene polymerization.

2 EXPERIMENTAL

2. 1 Instruments and reagents

All syntheses and manipulations of air- and moisturesensitive materials were carried out under an atmosphere of argon using standard Schlenk techniques or in an argon-filled glovebox. THF, toluene and hexane were refluxed and distilled over sodium benzophenone ketyl under argon prior to use unless otherwise noted. Dichloromethane and chlorobenzene were distilled over CaH2under argon before use. Benzyl chloride was dried by stirring with P2O5for 48 hours and distilled under reduced pressure prior to use.B(C6F5)3, [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4] were purchased from STREM. AlMe3, AlEt3and AliBu3were commercial from Sigma-Aldrich and used as received.Isoprene was available from TCI, dried with CaH2, and distilled before use. 1-Benzyl-3-indolealdehyde and the ligand were prepared following the literature procedures[8,9].Elemental analyses data were obtained on a Perkin-Elmer Model 2400 Series II elemental analyzer.1H NMR and13C NMR spectra of the compounds were recorded on a Bruker AV-500 NMR spectrometer (500 MHz for1H; 125 MHz for13C) in C6D6for compound 1 and in CDCl3for polyisoprene.Chemical shifts (δ) were reported in ppm, andJvalues in Hz.IR spectra were recorded on a Shimadzu Model FTIR-8400s spectrometer (KBr pellet). Gel permeation chromatography(GPC) analyses of the polymer samples were carried out at 30 ℃ using THF as an eluent on a Waters-2414 instrument and calibrated using monodispersed polystyrene standards at a flow rate of 1.0 mL·min-1.

2. 2 Syntheses of compounds 1 and 2

The general procedure for the synthesis of compounds 1 and 2 is shown in Scheme 1. To a toluene (15 mL) solution of HL (0.394 g, 1.0 mmol) was added a toluene (5 mL) solution of Y(CH2SiMe3)2(thf)2(0.49 g, 1.0 mmol). The mixture was stirred for 3 h at room temperature, and the color of the solution changed from pistachio to yellow in the process. The solvent was evaporated under reduced pressure. The residue was extracted withn-hexane (15 mL). Yellow crystals were obtained at 0 ℃ for several days (0.6 g, 75% yield).1H NMR (500 MHz, C6D6):δ–0.35 (s, 4H, CH2SiMe3), 0.22 (s,18H, CH2SiMe3), 1.10 (d,J= 5.0 Hz, 6H, CHMe2), 1.16 (m,8H,β-CH2THF), 1.22 (d,J= 5.0 Hz, 6H, CHMe2), 3.25 (m,J= 5.0 Hz, 2H, CHMe2), 3.58 (m, 8H,α-CH2THF), 5.71 (s,2H, CH2C6H5), 6.95～7.01 (m, 4H), 7.07～7.13 (m, 7H),7.51 (d,J= 10.0 Hz, 1H), 8.53 (s, 1H, CH=N).13C NMR(125 MHz, C6D6):δ4.5, 23.0, 25.2, 26.0, 28.6, 31.9, 54.9,70.4, 111.8, 116.5, 121.5, 121.7, 123.8, 125.9, 127.0, 127.2,128.7, 130.7, 138.8, 141.5, 141.9, 149.3, 168.5, 206.5 (d,JY–C= 37.5 Hz, 2-indolyl). Calcd. (%) for C44H67N2O2Si2Y: C,65.97; H, 8.43; N, 3.50. Found (%): C, 65.67; H, 8.23; N,3.43. IR (KBr pellets, cm-1):v2956 (s), 2866 (s), 1620 (w),1535 (w), 1460 (s), 1423 (s), 1381 (s), 1361 (s), 1247 (s),1161 (s), 1045 (s), 860 (w), 744 (w), 694 (s).

Scheme 1. Preparation of the rare-earth metal dialkyl compounds 1 and 2

Compound 2 was obtained as yellow crystals in 72% yield by the treatment of Er(CH2SiMe3)(thf)2(0.57 g, 1.0 mmol)with HL (0.394 g, 1.0 mmol) following procedure similar to the prepartion of 1. The color of the solution changed from pink to yellow during the reaction. Calcd. (%) for C44H67N2O2Si2Er: C, 60.09; H, 7.68; N, 3.19. Found (%): C,59.74; H, 7.66; N, 2.95. IR (KBr pellets, cm-1):v2958 (w),2864 (s), 1627 (w), 1543 (s), 1465 (s), 1388 (s), 1361 (s),1253 (s), 1161 (s), 1033 (s), 854 (s), 746 (s).

2. 3 Crystal data collection and structure determination

A suitable crystal of compound 1 or 2 was each mounted in a sealed capillary. Diffraction data collection was performed on a Bruker SMART APEX II CCD area detector diffractometer equipped with graphite-monochromatic MoKαradiation (λ= 0.71073 ?) with anω-φscan mode at 293(2) K.For compound 1, a total of 39346 reflections with 10637 unique ones (Rint= 0.0581) were collected in the ranges of 1.88≤θ≤27.58°, –25≤h≤27, –13≤k≤12 and –28≤l≤28,of which 6380 were observed withI> 2σ(I). For 2, in the ranges of 1.88≤θ≤27.74°, –27≤h≤27, –13≤k≤13 and–28≤l≤26, 38870 total reflections were collected with 10733 unique ones (Rint= 0.0319), of which 9036 were observed withI> 2σ(I). An empirical absorption correction was applied using the SADABS program[10]. Both structures were solved by direct methods, completed by subsequent difference Fourier syntheses, and refined anisotropically for all non-hydrogen atoms by full-matrix least-squares calculations onF2using the SHELXTL program package[11].The hydrogen atom coordinates were calculated with SHELXTL by using an appropriate riding model with varied thermal parameters. The residual electron densities were of no chemical significance.

2. 4 Isoprene polymerization

The procedures for the isoprene polymerization catalyzed by compounds 1 and 2 were similar, and a typical polymerization procedure is given below. A 50 mL Schlenk flask equipped with a magnetic stirring bar was charged a desired amount of solvent, the rare-earth metal complex, borate, alkyl aluminium and isoprene. Then the mixture was stirred vigorously for the desired time, during which an increase of viscosity was observed. The reaction mixture was quenched by the addition of 30 mL of acidified methanol. The polymer was coagulated, washed with methanol twice and finally dried under vacuum to a constant weight.

3 RESULTS AND DISCUSSION

Reactions of the ligand 1-Bn-3-(DIPP=CH)C8H5N with one equivalent of rare-earth metal trialkyl precursor in toluene afforded the carbonσ-bonded indolyl supported rare-earth metal dialkyl compounds 1 and 2 throughsp2C–H activation in good yields. Compounds 1 and 2 were fully characterized by spectroscopic methods and elemental analyses. The structures were determined by single-crystal X-ray diffraction. These complexes are soluble in hexane,toluene and THF. The disappearance of proton at the 2-indolyl position (atδ6.89 in C6D6for free ligand) in1H NMR spectra of the diamagnetic yttrium compound 1 provedsp2C–H activation. In addition, the signals centered at 206.5 ppm in the13C NMR spectra are assigned to the resonance of carbon atom at the 2 position of the indolyl ligand coupled to the yttrium nucleus withJY–C= 37.5 Hz, which is smaller than that found in the corresponding monoalkyl compound supported by the same ligand (JY–C= 50.0 Hz)[8a]. In the spectrum of compound 1, the signal at high field –0.35 ppm is assigned to the methylene protons of the Y-CH2SiMe3groups. The protons of theMe2CH groups show two doublets at 1.10 and 1.22 ppm, respectively, with theJvalue to be 5.0 Hz.

Single crystals of compounds 1 and 2 suitable for X-ray diffraction study were obtained by cooling the concentrated hexane solution at 0 ℃. The crystal structures of compounds 1 and 2 with atom numbering are shown in Figs. 1 and 2,respectively, and the selected bond lengths and bond angles are listed in Table 1. X-ray diffraction revealed that 1 and 2 are isomorphous and crystallize in the monoclinicP21/cspace group and adopt monomeric structures. The rare-earth ions of compounds 1 and 2 are in a distorted octahedral coordination environment, with one THF oxygen atom and carbon atom of CH2SiMe3group occupying the axial positions, while the other oxygen atom of THF molecule and carbon atom of CH2SiMe3group as well as carbon atom of indolyl moiety and the nitrogen atom of imino group lie in the equatorial positions. The RE–C(sp2) (2.491(3) ? for 1 and 2.477(3) ? for 2) are much longer than RE–C(sp3) (av.2.399(3) ? for 1 andav.2.383(3) ? for 2) and comparable to the values observed in six-coordination yttrium complexes[C4H2S-2-CH2N-(2,6-iPr2C6H3)]YCH2SiMe3(thf)3(2.423(3)?)[12]and [N2EtThYCH2SiMe3(thf)2] [N2EtThH = C5H3N-2-CMe2NH(2,6-iPr2C6H3)-6-(2-EtC4H2S)] (2.482(2) ?)[13]if the difference of Ln3+ionic radii is counted (for six-coordinate ionic radii: Y3+: 0.900 ?; Er3+: 0.890 ?)[14]. The atoms C(1), C(2), C(16), N(2) and RE formed a five-membered ring through cyclometalation reaction and are almost coplanar with the mean deviations of 0.0635 and 0.0636 ? for compounds 1 and 2, respectively. The bite angles of C(1)–RE–N(2) (70.53(8)° for 1 and 70.79(9)° for 2) and the angles of the axial C(29)–RE–O(2) (173.84(8)° for 1 and 173.68(9)° for 2) both indicate the geometry of the rare-earth centers deviation from ideal octahedral configuration.

Fig.1. ORTEP diagram of compound 1 with thermal ellipsoids at 30% probability level. Hydrogen atoms are omitted for clarity

Fig.2. ORTEP diagram of compound 2 with thermal ellipsoids at 30% probability level. Hydrogen atoms are omitted for clarity

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

Rare-earth metal monolakyl complexes suppoted by N-protected-3-imino indolyl ligands exhibited high activity and excellent regio- and stereoselectiy for isoprene polymerization[8a]. The catalytic activities of compounds 1 and 2 toward isoprene polymerization were investigated.None of the above rare-earth dialkyl complexes could inititae the polymerization of isoprene. After the abstration of one CH2SiMe3group by adding one equivalent of[Ph3C][B(C6F5)4], the forming cationic rare-earth metal monoalky complex also could not initiate the polymerization of isoprene. It might be the reason of two residual THF molecules around the metal center blocking the coordination of olefin. The ternary system of 1/[Ph3C][B(C6F5)4]/AlR3could initiate the polymerization. The catalytic activity and selectivity were affected by the aluminum alkyls. When the more steric bulkyl AliBu3was used, quantitative polyisoprene was obtained in 120 min with 91% 1,4-content and high molecular weigh (Entry 4). A previous report showed that the solvent has a significant inpact on the group 3 metal cationic species, which in turn influences the activity and selectivity of the olefin polymerization[15]. In this accout, compound 1 for isoprene polymerization was test in different solvents.When the polymerization was carried out in C6H5CH3or C6H5Cl, the 1,4-selectivity decreases to 67%, although the catalytic activity was enhanced (Entries 6 and 7). The results are different from that found in N-alkyl-3-imino indolyl supported rare-earth monoalkyl system, in which solvents CH2Cl2and C6H5Cl exhibited high activity and selectivity than C6H5CH3. The organic borate also affected the activity and selectivity of the system. When [PhNMe2H][B(C6F5)4]was used, the acitivity of the system declined although the selctivity was preserved (Entry 5). In cas of B(C6F5)3, the ternary system showed no catalytic activity for isoprene polymerization (Entry 1). The Er analogue 2 showed a similar activity to that of yttrium complex 1 with 90% 1,4-selectivty(Entry 8). Furthermore, the catalyst system 1/[Ph3C][B(C6F5)4]/AliBu3could initiate isoprene polymerization with different monomers to initiate ratios. On increasing the monomer to complex ratio from 1000 to 2000,the molecular weight of the resulting polymers increaes from 11.7 × 104to 23.0 × 104with almost unchanged regioselectivity (Entry 9).

Table 2. Isoprene Polymerization by the Title Compoundsa