The application of novel dendritic nickel catalysts in

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Catalysis Communications 9 (2008) 1612–1617 www.elsevier.com/locate/catcom

The application of novel dendritic nickel catalysts in the oligomerization of ethylene R. Malgas a, S.F. Mapolie b,*, S.O. Ojwach c, G.S. Smith d, J. Darkwa c b

a Chemistry Department, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa c Department of Chemistry, University of Johannesburg, P.O. Box-524, Auckland Park 2006, South Africa d Department of Chemistry, University of Cape Town, Private Bag, Rondebosch 7701, South Africa

Received 4 August 2007; received in revised form 30 December 2007; accepted 8 January 2008 Available online 15 January 2008

Abstract Multinuclear nickel complexes were derived from generation1 (G1) and generation 2 (G2) dendrimeric salicylaldimine ligands based polypropyleneimine dendrimers of the type, DAB-(NH2)n (n = 4 or 8, DAB = diaminobutane). These novel dendritic complexes were evaluated as catalyst precursors in the oligomerization of ethylene, using EtAlCl2 as an activator. The oligomerization results show that there is a clear dendritic effect, in that both the catalyst activity as well as selectivity is impacted by the dendrimer generation. Thus for example the second generation dendritic complex shows higher activity than the corresponding first generation complex. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Dendritic catalyst; Ethylene oligomerization; Nickel salicylaldimine complexes

1. Introduction Dendrimers are a class of three-dimensional macromolecules characterized by a central core and branches expanding to a periphery that becomes denser with increasing generation number. These molecules have a highly branched structure with a wide range of peripheral groups. Some of first examples of dendrimers were reported by Tomalia and Newkome, during the eighties [1,2]. Amongst the more common dendrimers are the polypropyleneimine dendrimers [1], the poly(amidoamine) dendrimers [3] and the poly(benzylether) dendrimers [4]. Dendrimers containing metals in the framework are known as metallodendrimers. Balzani et al. [5] and Newkome et al. [6] initiated the incorporation of transition metals into the framework of the dendrimers in the early 1990s [7]. Metals are usually introduced into the dendritic framework postsynthesis of the dendrimer. The metals can be located at

*

Corresponding author. Tel.: +27 21 8082721; fax: +27 21 8083849. E-mail address: [email protected] (S.F. Mapolie).

1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.01.009

various positions in the dendritic molecule viz. the terminal units, the branching points or at the core. Dendrimers have found a wide range of applications. These include the use as catalysts [8–10], biosensors [11,12], drug delivery agents [13], adhesives [14], magnetic resonance imaging agents [15] and high performance polymers [16]. One of the advantages of using catalysts based on metallodendrimers is the possibility of combining the best properties of homogeneous and heterogeneous catalyst in one system. Their stable structure and size render dendrimers more suitable for recycling than linear soluble polymersupported catalysts. For example dendrimer catalysts can potentially be separated from the products via ultrafiltration. Several mononuclear salicylaldimine and other nickel complexes have been reported to be effective catalysts in the oligomerization of ethylene [17–19]. However in many cases these catalysts are not very selective. Selectivity can often be achieved by tailoring the ligands of the metal catalyst [20]. Recently dendrimeric catalysts have gained growing popularity and have increasingly been studied in


R. Malgas et al. / Catalysis Communications 9 (2008) 1612–1617

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several catalytic processes including ethylene oligomerization processes [21,22]. This prompted us to incorporate salicylaldimine functionalities onto the periphery of the commercially available generation 1 and generation 2 polypropyleneimine dendrimers, DAB-(NH2)4 and DAB(NH2)8 and to subsequently complex these new dendrimeric ligands to nickel. The resultant multinuclear nickel complexes were applied as catalyst precursors in the oligomerization of ethylene.

and the mixture was stirred for 72 h at room temperature. After the reaction time, the solvent was removed under vacuum to afford a yellow oil. Dichloromethane (20 ml) was added to the product and the solution was washed with water (5 30 ml). The organic layer was dried over magnesium sulphate, filtered and the filtrate evaporated under reduced pressure to give the desired product as a yellow oil. Yield, 80%. ESI–MS, [M+H]+ = 1181. IR (nujol mull, NaCl cm 1): m(O–H) 3058, m(C@N) 1646 and m(C–O) 1279.

2. Experimental

2.3. Synthesis of generation 1 dendritic nickel complex, C1

All manipulations were performed under a nitrogen atmosphere using standard Schlenk techniques. Toluene was dried by refluxing over sodium/benzophenone. The DAB-(NH2)n dendrimers (n = 4 and 8) were obtained from SymoChem, the Netherlands and used without any further purification. Salicylaldehyde, nickel acetate tetrahydrate and ethylaluminium dichloride (25% in toluene) were purchased from Sigma Aldrich, and were used as received. Polymer grade ethylene was obtained from Afrox Ltd. Infrared spectra were recorded on a Perkin Elmer Paragon 1000 PC FT-IR spectrophotometer, using KBr pellets or as nujol mulls on NaCl plates. 1H NMR (200 MHz) and 13 C NMR (50.3 MHz) spectra were recorded on a Varian XR200 spectrometer, using tetramethylsilane as an internal standard. ESI Mass spectra were recorded at Stellenbosch University, South Africa, using a Waters API Q-TOF Ultima instrument in V-mode. The source temperature was 100 °C and the desolvation temperature was 350 °C. The capillary voltage used was 3.5 kV. Microanalysis was done at the University of Cape Town. Melting points were recorded on a Leitz Hot Stage 350. Gas chromatography analysis was carried out on a Varian CP-3800 using a HP PONA column. Dodecane was used as an internal standard. Gel permeation chromatography was performed on a Polymer Laboratories GPC220 instrument using polystyrene standards at the Polymer Science Institute at the University of Stellenbosch (South Africa).

To the 1st generation salicylaldimine ligand, L1 (0.5 g, 0.68 mmol) in ethanol (10 ml) in a round bottom flask was added Nickel acetate tetrahydrate (0.34 g, 1.4 mmol) and the reaction mixture was allowed to stir under reflux for 24 h forming a green precipitate.. The precipitate was filtered off by vacuum filtration and washed extensively with ethanol to afford C1 as a green solid. Yield = 85%. ESI–MS, [M+H]+ = 848. IR (KBr cm 1): m(C@N) 1628 and m(C–O) 1324, m.p. 275–278 °C. Anal. calcd for (C54H52N6Ni2O4): C, 62.18; H, 6.08; N, 9.74. Found: C, 62.18; H, 6.15; N, 9.9%. 2.4. Synthesis of generation 2 dendritic nickel complex, C2 To a solution of the DAB-G2 salicylaldimine ligand, L2 (0.35 g, 2.2 mmol) in ethanol (10 ml) was added nickel acetate tetrahydrate (0.23 g, 9.1 mmol) and the mixture stirred under reflux for 24 h. The solvent was evaporated via rotary evaporation to give a green residue. The residue was then dissolved in dichloromethane (15 ml) and the solution was filtered by gravity and dried to afford complex C2 as a green solid. Yield, (80%). ESI–MS, [M+H]+ = 1833. IR (KBr cm 1): m(C@N) 1632 and m(C–O) 1344, m.p. 210–215 °C. Anal. calcd for (C96H120N14Ni4O8 2CH2Cl2): C, 58.77; H, 6.24; N, 9.79. Found: C, 58.24; H, 6.25; N, 9.48%. 2.5. General ethylene oligomerization procedure

2.1. Synthesis of generation 1 salicylaldimine ligand, L1 To a solution of DAB-(NH2)4 (0.50 g, 1.58 mmol) in toluene (10 ml) in a Schlenk tube, was added salicylaldehyde (0.77 ml, 6.3 mmol) and the mixture was allowed to stir for 72 h at room temperature. The solvent was evaporated under vacuum to give a yellow oil. Recrystallization from a dichloromethane/hexane mixture at 4 °C for 72 h resulted in the formation of a yellow precipitate which was filtered off under vacuum and dried. Yield (90%). ESI–MS, [M+H]+ = 733. IR (KBr cm 1): m(O–H) 2924, m(C@N) 1632 and m(C–O) 1284; m.p. 66–68 °C. 2.2. Synthesis of generation 1 salicylaldimine ligand, L2 To a solution of DAB-(NH2)8 (1.6 g, 2.1 mmol) in toluene (10 ml) was added salicylaldehyde (2.4 ml, 16.4 mmol)

The ethylene oligomerization reactions were carried out in a 300 ml steel autoclave equipped with an overhead stirrer and internal cooling coil. The autoclave was charged with the reagents in a nitrogen-purged glove box. The appropriate amount of catalyst corresponding to 5 lmol of nickel was suspended in dry toluene (50 ml) in a stainless steel PARR reactor. The required amount of EtAlCl2 was added to the solution using a glass syringe. The reactor was sealed and removed from the glove-box. The reactor was flushed with dry nitrogen three times. The ethylene pressure was then set at 5 atm. and maintained at this pressure throughout the oligomerization process by constantly feeding the gas. The reaction was conducted at room temperature for 1 h. Unreacted ethylene was vented from the reactor at the end of the reaction time. A sample was withdrawn and then examined by gas chromatography. The


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remaining reaction mixture was then quenched by adding 10 ml of acidified ethanol. The solvent was evaporated under vacuum and the mass of product determined.

The IR spectra of both ligands show peaks in the m(C@N) region indicating that the aldehyde had condensed with the amino groups on the periphery of the DAB(NH2)n polyamine dendrimers to form the corresponding imines. The broad bands in the range 3400–3300 cm 1 are due to the m(O–H) of the salicylaldimine units. The broadness of these bands are indicative of extensive Hbonding. The 1H NMR spectra of both salicylaldimine ligands show proton signals for the internal branches of the dendrimer d 1.41–d 3.60 ppm, the proton attached to the imine group d 8.3 ppm and the protons from the aryl rings of the salicylaldimine units d 6.88–d 7.32 ppm. The ligands were also characterized by elemental analysis, with both giving acceptable results.

3. Results and discussion 3.1. Ligand synthesis The synthesis of generation 1 (L1) and generation 2 (L2) dendrimeric salicylaldimine ligands was performed as previously described by Smith et al. [23]. The ligands were synthesized via Schiff base condensation of the diaminobutane polypropyleneimine dendrimer (DAB-(NH2)n) (n = 4 or 8) with salicylaldehyde (Scheme 1).

H2N

NH2 OH N

N

O

H

NH2

H2N

HO OH N N

L1

N N HO OH N N

Ni(OAc)2

O

N N Ni

N

Ni

N O

O

C1

N N

Scheme 1.

O



3.2. Complex synthesis

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Table 1 Microanalysis results of ligands and complexes

The nickel complexes, C1 and C2 were prepared by the reaction of the ligands L1 and L2 with nickel acetate tetrahydrate as the metal precursor in refluxing ethanol. Both complexes were isolated as green solids in high yields of 80% and 85% respectively. Fig. 1 shows the structure of the generation 2 nickel complex. The IR spectra of the nickel complexes show new bands around 1628 cm 1 and 1324 cm 1 for the m(C@N), and the m(C–O) stretching frequencies, respectively. These peaks had shifted when compared to analogous bands in the ligand spectra. This confirms the complexation of the ligand to metal. The m(O–H) bands in the region 3400– 3300 cm 1 originally seen in the spectra of the ligands but absent in that of the complexes, indicate that coordination occurred via the O atom of the ligand. Both nickel complexes were paramagnetic thus the 1H NMR spectra gave very broad peaks and was not amenable to meaningful interpretation. However, other analytical techniques such as elemental analysis and mass spectrometry confirmed the structures of the complexes. The microanalysis results for the complexes are shown in Table 1. The calculated data corresponds well to the experimental results, confirming the structure of the complexes synthesized. The complexes were also fairly stable thermally with both C1 and C2 only decomposing at temperatures exceeding 200 °C. 3.3. Ethylene oligomerization Ethylene oligomerization reactions were carried out by activation of complexes, C1 and C2 with methylaluminox-

Calculated%

Found%

C

H

N

C

H

N

L1 L2

72.1 71.9

7.7 8.03

11.47 12.21

71.9 70.87

7.9 8.28

11.49 11.76

C1 C2

62.18 58.77a

6.08 6.24

9.74 9.79

62.18 58.04

6.15 6.25

9.9 7.08

a

Solvent inclusion, two moles of dichloromethane.

ane (MAO), EtAlCl2 and Et2AlCl as co-catalysts. Of the three co-catalysts only EtAlCl2 showed activity under the reaction conditions employed and was hence used for further catalytic investigations. The reaction conditions were kept constant except for the Al:Ni ratios which ranged from 20:1 to 3000:1. All reactions were performed at 5 atm of ethylene and at room temperature. The metal concentration was kept constant in all reactions irrespective of the catalyst precursor employed. The activities of the generation 1 (C1) and the generation 2 (C2) complexes expressed as turn over frequencies (TOF) are given in Table 2. The oligomeric products were weighed after evaporation of the solvent. C1 showed optimum catalyst activity at an Al:Ni ratio of 500:1 while C2 showed optimum activity at an Al:Ni ratio of 2000:1. The overall activity of the generation 2 catalyst is significantly higher than that of the generation 1 catalyst. It should however be noted that the optimum activity for the generation 2 catalyst is obtained at Al concentrations that are four fold greater than those used in reactions of the generation 1 catalyst. A possible reason for this difference in behaviour of the two catalysts in respect of the amount of co-catalyst used, could be due

N

N

O

O Ni

Ni N

N

O

N

N

N N

N

N

N

O

O

N

Ni

O

Ni N

N

C2 Fig. 1. The structure of generation 2 salicylaldimine nickel complex, C2.

O O


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Table 2 Catalyst activity of the dendrimeric generation 1 and generation 2 Ni complexes Entry 1 2 3 4 5 6 7 8 9

Al:Ni 20 50 100 200 500 1000 2000 2500 3000

a b

TOFa C1 a

120 556a 1088a 1400a 2600b 2000a 1600a – –

TOFa C2 42a 106a 436a 718a 798a 2086a 4020b 2720a 1972a

TOF – kg of total products produced per mol of nickel per hour. Optimum activity.

to differences in the architecture of the two dendrimeric catalysts. The generation 1 catalyst, C1 has two tertiary amine units within its internal structure, while the generation 2 catalyst, C2 has six tertiary amine units. In both cases these tertiary amines, being Lewis base sites, are potential positions for the binding of the Lewis acidic organoaluminium co-catalyst. It is thus thought that the co-catalyst, EtAlCl2, first binds to these Lewis basic sites before it becomes involved in the activation of the metal centre. It is well known that N-donor molecules form adducts with Lewis acidic Al complexes [24–26]. Since the generation 2 catalyst has more N-donor sites it will react with a larger amount of the aluminium alkyl than the generation 1 catalyst. It therefore requires larger amounts of co-catalyst before the optimum activity is reached for the G2 system. The activity of the generation 2 catalyst is much lower than that of the generation 1 catalyst when Al:Ni ratios lower than the optimum ratio are employed. Our results reflect a similar trend to that observed in the case of the dendrimeric carbosilane nickel complexes reported by de Jesus et al. [21]. In their case, an increase in activity for ethylene oligomerization was observed as the dendrimer generation increased from G1 to G3. In these carbosilane dendrimers, the activity expressed as TOF ranged from 293 to 957 for G1–G3 catalysts. Furthermore these workers also observed the formation of polyethylene in addition to the oligomers obtained. In the case of our catalysts we do not observe any polymer formation, even at relatively high Al:Ni ratios which normally favours polymer production. In addition, our dendrimeric nickel complexes show relatively high activity for ethylene oligomerization when compared to the carbosilane nickel dendrimers reported by de Jesus et al. [21]. Furthermore the oligomerization activities displayed by our catalysts compare favourably to some mononuclear ethylene oligomerization nickel catalyst activities in literature [27,28]. The oligomers obtained from our G1 catalyst, C1 were analyzed using gas chromatography (GC). The GC analysis was done on the residues after evaporation of the solvent. This was done after we confirmed that no low boiling point oligomers were present in the reaction med-

ium. The GC results are tabulated in Table 3. From the results it would appear that at low levels of Al, the chain transfer process from the active Ni centre to the Al is much more rapid than chain growth. This results in relatively shorter oligomer chains being formed. At these low Al concentrations, largely C10 and C12 oligomers were obtained, with C10 being more dominant. At very high levels of Al longer chain oligomers were obtained. GC analysis was also performed on reaction mixtures obtained from reactions using the generation 2 catalyst, C2. No short chain oligomers were observed in these samples. The only products isolated from the generation 2 catalyst were thick and waxy materials which were yellow-brown in colour. The reaction products were analyzed by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as a solvent and against polystyrene standards. The GPC results for the products obtained from the generation 2 catalyst are given in Table 4. The molecular weight (Mw) of the products ranged from 72 900 g/mol to 94 600 g/mol. The PDI for the low molecular weight polymers obtained using catalyst C2 ranges from 1.57 to 1.99. These PDI values are indicative of a relatively uniform distribution of molecular weight of the products. In addition these PDI values are lower than those reported by Zhang et al. whose nickel catalysts produced oligomers which had PDI values ranging from 2.24 to 6.67 [27]. Carbosilane based dendrimeric nickel complexes synthesized by de Jesus et al. produced oligomers with the PDIs ranging from 2.77 to 20.12 for their generation 1–generation 3 complexes [21]. Thus it can be seen that our dendritic catalysts produced materials with fairly uniform molecular weight distribution. It should also be noted that the PDI values seem to be independent of Al:Ni ratio. This is not unexpected as co-catalyst concentration is not known to impact significantly on molecular weight distribution. Table 3 GC results for products obtained using complex C1 as catalyst Al:Ni ratio

%C10

%C12

%C14

%C16+

20 50 100 200 500 1000 2000

59 58 17 7 1 – –

41 33 57 64 46 – –

– 5 15 12 10 – –

– 4 11 18 43 100 100

Table 4 GPC results for products obtained using C2 as catalyst Al:Ni ratio 20 50 100 200 500 1000

Mw (g/mol) 4

8.05 10 7.29 104 7.96 104 8.92 104 7.59 104 9.46 104

PDI (Mw/Mn) 1.59 1.99 1.87 1.54 1.49 1.57



From our results we observe that our two dendrimer systems give very different types of oligomeric products. Oligomerization using the generation 1 dendrimer complex, C1 yields short chain oligomers whereas those obtained from the generation 2 dendrimer catalyst, C2 resulted in longer chain products with molecular weights up to 94 600 g/mol. It appears that in the case of the generation 2 catalyst, the rate of the b-hydride elimination process is slowed down relative to that of the generation 1 case. This could possibly be due to the fact that the generation 2 catalyst, C2 experiences greater steric crowding around the active centre. This would hinder b-hydride transfer from the growing alkyl chain to the metal centre. The retardation of b-hydride transfer should result in longer chained materials. We are currently further investigating this by systematic modification of the dendrimers by introducing bulky substituents on the salicylaldimine rings. Our results on this will be reported in due course. 4. Conclusions We have successfully prepared two new dendritic nickel catalysts (C1 and C2). Both catalysts are highly active in ethylene oligomerization. There is a clear dendritic effect in terms of both the activity and nature of products formed. The generation 2 catalyst, exhibited higher activity than the generation 1 catalyst albeit at higher Al:Ni ratios. The type of oligomers formed is also dependent on the nature of the catalyst. The generation 1 catalyst formed short chain oligomers within the C10–C20 diesel range, while C2 produces higher molecular weight products which were waxy in nature. Acknowledgements This work was supported by the NRF/DST Centre of Excellence in Catalysis. The authors also wish to acknowledge the research committee of the University of the Western Cape for additional financial support. References [1] D.A. Tomalia, H. Baker, J. Dewald, M. Hall, G. Kallos, S. Martin, J. Roeck, J. Ryder, P. Smith, Polym. J. 17 (1985) 117.

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