Synthesis and ethylene polymerization catalysis of mono(cyclopentadienyl)lanthanide compounds with the pyrazinamide ligand

Journal of Alloys and Compounds 344 (2002) 92–95

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Synthesis and ethylene polymerization catalysis of mono(cyclopentadienyl)lanthanide compounds with the pyrazinamide ligand ´ Renata Diana Miotti a , Alessandra de Souza Maia b , Icaro Sampaio Paulino c , Ulf Schuchardt c , b, Wanda de Oliveira * a

´ FIEO, Av. Franz Voegeli, 300, CEP 06020 -190, Osasco, SP, Brazil UNIFIEO, Centro Universitario b ˜ Paulo, C.P. 26077, 05599 -970, Sao ˜ Paulo, SP, Brazil ´ , Universidade de Sao Instituto de Quımica c ´ , Universidade Estadual de Campinas, C.P. 6154, 13083 -970, Campinas, SP, Brazil Instituto de Quımica

Abstract In this work the synthesis of mono(cyclopentadienyl)lanthanide compounds containing methanesulfonate anion and pyrazinamide ligand is presented. The compounds were characterized by elemental analyses, complexometric titration with EDTA, thermal analyses, and vibrational spectra in the infrared region. In a preliminary catalytic study these compounds were active in ethylene polymerization when MAO was used as cocatalyst producing low crystalline polyethylene.  2002 Elsevier Science B.V. All rights reserved. Keywords: Organolanthanides; Methanesulfonates; Cyclopentadienyl

1. Introduction As a result of Ziegler and Natta’s discoveries [1], a variety of new plastic products and elastomers were created. Besides, the new catalytic systems then denominating, Ziegler–Natta systems, caused a revolution in the knowledge of the properties and structures of polymers, polymerization processes and in organometallic chemistry. The synthesis of polyethylene of high density, in soft conditions (low temperature and pressure), had been developed from the works of Ziegler and Natta, being used as catalysts for transition metal halides with alkylaluminum [1]. In 1980, Sinn and Kaminsky discovered a new catalytic system based on metallocenic compounds and methylaluminoxan [2], that presented high catalytic activity in the polymerization of ethylene in soft conditions, being up to 100 times more active than the classic Ziegler–Natta systems. In the last two decades an extensive study has been developed in the field of the organolanthanide compounds

*Corresponding author. Tel.: 155-11-3818-3847. E-mail address: [email protected] (W. de Oliveira).

containing the cyclopentadienyl group [3], due its applications in polymerization of olefins, in particular ethylene. In an attempt to contribute to the application of organolanthanides as catalysts for olefin polymerization, we report the synthesis of mono(cyclopentadienyl)lanthanide compounds LnCp(MS) 2 (PzA) 2 (Ln5Nd, Sm, Eu, Tb, Cp 2 5cyclopentadienyl, MS 2 5CH 3 SO 32 5methanesulfonate and PzA5pyrazinamide). These compounds were characterized by elemental analysis, thermal analyses and infrared spectra. The catalytic activity of such compounds was verified in ethylene polymerization. The organolanthanide compounds are usually obtained by reaction between anhydrous salts and cyclopentadienylsodium, followed by the introduction of ligands [4]. The use of anhydrous salts is essential for the formation of the organolanthanide compounds, as these compounds are extremely sensitive to air and moisture. To eliminate the difficult step of dehydration salts, in the present work the mono(cyclopentadienyl)lanthanide compounds LnCp(MS) 2 (PzA) 2 (Ln5Nd, Sm, Eu and Tb) were obtained by reaction of intermediate coordination compounds [Ln(MS) 3 (PzA) 4 ] with NaCp instead of the reaction between anhydrous lanthanide methanesulfonates and NaCp followed by reaction of organolanthanide compounds formed with the PzA ligand.

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00315-8

R.D. Miotti et al. / Journal of Alloys and Compounds 344 (2002) 92–95

2. Experimental details All manipulations were performed under prepurified argon. Solvents were dried by standard techniques and thoroughly deoxygenated before use. The hydrated lanthanide methanesulfonates Ln(MS) 3 ?xH 2 O (Ln5Nd, Sm, Eu and Tb) were prepared from lanthanide basic carbonates and methanesulfonic acid [5]. Elemental analyses (%C, ´ %H, %N and %S) were performed by Central Analıtica IQ-USP. %Ln was determined by complexometric titration with EDTA [6]. Thermal analyses were recorded on a Shimadzu Thermogravimetric Analyzer-TGA-50 by heating 2–5 mg samples in a platinum pan in air (20 8C / min, 50.0 ml / min) from room temperature to 900 8C. Infrared spectra were recorded on an FTIR-BOMEM MB-102, from 4000 to 200 cm 21 , using Nujol or Fluorolube mulls between cesium iodide windows. Differential scanning calorimetry (DSC) measurements were performed on a DSC 2910 TA Instrument. The samples were heated from room temperature to 200 8C at a heating rate of 10 8C / min. The melting temperature values (T m ) and the heat of fusion (DHf ) were taken from the second heating curve. The degree of crystallinity was calculated from DHf , using the equation: w CH 5DHf 3100 / 288 [7].

2.1. Synthesis and characterization of compounds 2.1.1. [ Ln( MS)3 ( PzA)4 ] A solution of pyrazinamide (4 mmol) in methanol was added to a solution of hydrated lanthanide methanesulfonate Ln(MS) 3 ?H 2 O (Ln5Nd, Sm, Eu and Tb) (1 mmol) in methanol and the resulting mixture was stirred for 2 h at room temperature. The mixture was allowed to evaporate slowly at room temperature until dryness and then the anhydrous compounds [Ln(MS) 3 (PzA) 4 ] were obtained as light blue (Ln5Nd), light yellow (Ln5Sm) or white (Ln5 Eu and Tb) solids. Yield 75%. Anal. Calc. For [Nd(MS) 3 (PzA) 4 ]: C, 29.96; H, 3.17; N, 18.23; Nd, 15.64. Found: C, 29.63; H, 3.28; N, 18.67; Nd, 15.08; For [Sm(MS) 3 (PzA) 4 ]: C, 29.76; H, 3.15; N, 18.10; Sm, 16.20. Found: C, 29.55; H, 3.70; N, 18.38; Sm, 15.90; For [Eu(MS) 3 (PzA) 4 ]: C, 29.71; H, 3.14; N, 18.07; Eu, 16.34. Found: C, 29.62; H, 3.38; N, 18.27; Eu, 16.38; For [Tb(MS) 3 (PzA) 4 ]: C, 29.49; H, 3.12; N, 17.94; Tb, 16.96. Found: C, 29.17; H, 3.76; N, 17.54; Tb, 17.07. 2.1.2. LnCp( MS)2 ( PzA)2 These compounds were prepared by reaction between CpNa (3 mmol) in tetrahydrofuran and [Ln(MS) 3 (PzA) 4 ] (1 mmol). This mixture was stirred for ca. 24 h, followed by removal of THF under vacuum. Ethanol was added to the mixture followed by stirring for ca. 24 h. The mixture was then allowed to stand overnight. The ethanol solution was removed by filtration and the remaining residue was dried under vacuum to give amorphous beige solids. Yield 70%. Anal. Calc. For NdCp(MS) 2 (PzA) 2 : C, 31.62; H,

93

3.28; N, 13.01; S, 9.93; Nd, 22.33. Found: C, 31.08; H, 3.49; N, 13.07; S, 9.66; Nd, 21.99; For SmCp(MS) 2 (PzA) 2 : C, 31.32; H, 3.25; N, 12.89; S, 9.84; Sm, 23.07. Found: C, 31.55; H, 3.46; N, 12.78; S, 9.43; Sm, 23.10; For EuCp(MS) 2 (PzA) 2 : C, 31.24; H, 3.24; N, 12.86; S, 9.84; Eu, 23.25. Found: C, 31.27; H, 3.46; N, 12.47; S, 9.30; Eu, 23.17; For TbCp(MS) 2 (PzA) 2 : C, 30.91; H, 3.20; N, 12.72; S, 9.71; Tb, 24.06. Found: C, 30.76; H, 3.37; N, 12.53; S, 9.46; Tb, 23.75.

2.2. Catalytic polymerization of ethylene The polymerization experiments were carried out in a ¨ Buchi autoclave at 70 8C and 3 bar of ethylene using 2.0 mg of organolanthanide compound and 2.0–2.5 ml of MAO (10% in toluene) in 50 ml of toluene.

3. Results and discussion The synthetic route used here for the preparation of the organolanthanide compounds was simple and eliminated a hard step that generally is necessary when synthesizing such compounds: the dehydration of lanthanide salts. The intermediate anhydrous coordination compounds were obtained by a simple method, adding the pyrazinamide ligand to the hydrated lanthanide salts. The organolanthanide compounds were obtained as amorphous solids by the reaction of coordination compounds with cyclopentadienylsodium. The analytical data (presented in Section 2) indicated that organolanthanide compounds can be formulated as LnCp(MS) 2 (PzA) 2 (Ln5Nd, Sm, Eu, Tb). These compounds are practically insoluble in acetonitrile, ethanol, methanol, nitromethane and tetrahydrofuran. Due to insolubility of the organolanthanide amorphous compounds obtained they could not be better characterized by tech1 13 niques such as H or C NMR or single-crystal XRD. The thermal analysis studies were based on TG / DTG techniques. The compounds lost weight gradually with increasing temperature from ca. 30 to 850 8C. The process of thermal decomposition for the organolanthanide compounds LnCp(MS) 2 (PzA) 2 (Ln5Nd, Sm, Eu and Tb) are almost the same and at this temperature Ln 2 O 2 SO 4 (Ln5 Nd, Sm, Eu and Tb) is formed. For these compounds only one peak defines the decomposition of the pyrazinamide. By knowing the weight loss percentage and the thermal decomposition product, it was possible to calculate the molecular weight of the compounds (Table 1) that were in agreement with the expected for the synthesized compounds based on elemental analyses (presented in Section 2). Infrared spectra of the compounds LnCp(MS) 2 (PzA) 2 (Ln5Nd, Sm, Eu and Tb) (Table 2) show negative shifts of n (C=O) at 1680 cm 21 in free pyrazinamide to 1611–

R.D. Miotti et al. / Journal of Alloys and Compounds 344 (2002) 92–95

94 Table 1 Results of the thermal analysis studies Ln

Weight lost (%)

m ( initial ) (mg)

m (residue) (mg)

Residue

MM calcd. (g / mol)

MM exp. (g / mol)

Nd a Sm a Eu a Tb a

68.64 65.16 67.96 67.34

1.231 3.299 3.179 3.229

0.386 1.030 1.019 1.054

Nd 2 O 2 SO 4 Sm 2 O 2 SO 4 Eu 2 O 2 SO 4 Tb 2 O 2 SO 4

645 652 653 660

664 686 674 683

a

Compound of formula LnCpMS 2 (PzA) 2 .

Table 2 Infrared frequencies (cm 21 ) of compounds LnCp(MS) 2 (PzA) 2 Nd

Sm

Eu

Tb

Assignment

3420 s 3260 s 2720 m 2660 w 1641 m 1570 s–1520 s 1260 s–1200 s 1060 s 1040 m 795 m 640 w–440 m 1das (SO 3 )1ds (SO 3 )

3435 m 3277 m 2734 m 2687 w 1656 m 1578 m–1515 m 1234 s–1198 s 1066 m 1047 m 781 m 636 w–433 m

3412 m 3279 w 2721 m 2662 w 1641 m 1559 m–1515 m 1250 s–1191 s 1059 s 1044 m 794 m 647 w–426 m

3422 m 3270 w 2725 m 2673 w 1652 m 1578 m–1521 m 1261 s–1195 s 1061 m 1047 m 785 m 662 w–431 m

nas NH (PzA) ns NH (PzA) ns (CH 3 ) MS ns (CH) Cp n C=O (PzA) Ring vibration PzA nas (SO) MS ns (SO) MS1g (CH) (Cp) d (CH) Cp ns (CS) MS1 nas (CH) (Cp) Ring vibration PzA

s, strong; m, medium; w, weak; sh, shoulder.

1614 cm 21 in organolanthanide compounds, indicating coordination of the pyrazinamide to the lanthanide ion via the oxygen of the carbonyl group [8–10]. The presence of bands at 794, 1050, 1075 and 2670 cm 21 , assigned, under C5v local symmetry, to A 1 and E 1 out of plane wagging, E 1 (C–H) in plane wagging, E 1 (C– C) ring breathing and E 1 (C–H) stretching modes, respectively, indicated s-centered coordination of the cyclopentadienyl anion to the lanthanide(III) ions, with ionic character [11]. The MS 2 anion has a C3v point group symmetry and may act as a non-coordinated or a mono-, bi- or tridentate ligand. The H 3 CSO 32 group shows strong bands, which have been assigned [12] as anti-symmetric and symmetric SO 2 3 modes in the spectrum region between 1300 and 1050 cm 21 . The coordination to the anion (mono-, bi- or tridentate) distorts and reduces the C3v symmetry [13]. The splitting (ca. 55 cm 21 ) observed in vibration attributed to nas (SO) of the MS 2 anion was interpreted in terms of a lowering of its C3v symmetry and that the MS 2 anions were coordinated to the lanthanide ions through the oxygen atoms. The similar coordination modes of the pyrazinamide ligands to the lanthanide ions indicated by infrared spectra analysis was also suggested by thermogravimetry results. The compounds LnCp(MS) 2 (PzA) 2 (Ln5Nd, Sm, Eu, Tb) presented catalytic activities of 6.4–5.1 gPE mmol Ln 21 h 21 bar 21 at 70 8C, with Al / Ln ratios of ca. 2000,

independently of the lanthanide ions present. The compounds were active in polymerization when MAO was used as cocatalyst. This may imply that a cationic species formed by the reaction of MAO with the organolanthanide compound is responsible for catalytic activity. These activities are significantly lower than those obtained using zirconocene catalysts [14] but are similar to the catalytic systems formed by organolanthanide compounds and MAO as cocatalyst, as the system LnBr 2 CpPzA / MAO (Ln5Nd, Sm), which presented catalytic activities of 4.3 gPE mmol Nd 21 h 21 bar 21 and 4.6 gPE mmol Sm 21 h 21 bar 21 [15]. Infrared absorption has been found to be a valuable tool in the study of the polyethylene structure. Theory and experimental evidence from crystalline hydrocarbons suggest that in polyethylene the absorption band at 730 cm 21 arises from the crystalline regions only and the 721 cm 21 band arises from both crystalline and the amorphous regions [16]. Infrared spectra of the produced polyethylene show: (a) bands at 1473 and 1462 cm 21 assigned to deformation of the methylene groups; (b) bands at 730 and 719 cm 21 assigned to rocking of the methylene groups, indicating that the polyethylene obtained has crystalline and amorphous regions. These infrared data are supported by melting points at 126 8C (DHf 559 J / g) for the obtained polyethylene by DCS measurements, which showed 20% crystalline product.

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4. Conclusions

References

A simple method for the synthesis of mono(cyclopentadienyl)lanthanide compounds was presented. The organolanthanide compounds LnCp(MS) 2 (PzA) 2 (Ln5Nd, Sm, Eu, Tb) have been prepared in good yields by the reaction of cyclopentadienylsodium and [Ln(MS) 3 (PzA) 4 ] in appropriate molar ratio and show catalytic activities of 6.4–5.1 gPE mmol Ln 21 h 21 bar 21 with ethylene polymerization at 70 8C and the polyethylene obtained presents low crystallinity (20%).

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Acknowledgements We thank CNPq (Conselho Nacional de Pesquisa) and ˜ de Amparo a` Pesquisa do Estado de FAPESP (Fundac¸ao ˜ Paulo) for financial support. Sao