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CATIONIC COMPLEXES [Ni(η5-C5H4R)(CH3CN)(NHC)]+(PF6)-. SYNTHESIS AND CATALYTIC ACTIVITY IN STYRENE POLYMERIZATION

Joanna Conder

Abstract

Introduction The aim of this thesis was to synthesize cationic half-sandwich nickel complexes of the general formula [Ni(η5-C5H4R)(CH3CN)(NHC)]+(PF6)- and structurally characterize them by 1H and 13C NMR spectroscopy, electrospray mass spectroscopy and by single-crystal X-ray diffraction study. The catalytic activity of this cationic complexes was examined in styrene polymerization in the presence of methyl alumoxane (MAO) and compared to the results previously obtained for their chloride analogues. The research work was also focused on the characterization of polystyrene (13C NMR, GPC) and on this grounds evaluation whether cationic mechanism recently proposed for chloride nickel complexes is justified. The results and discussion The series of the neutral complexes [Ni(η5-C5H4R)Cl(NHC)] were prepared according to the literature procedure by reacting nickelocene with an appropriate imidazolium chloride in THF. I have synthesized complexes 1a with SIMes and 1b with IMes ligands on my own. Complexes 1c with SIPr ligand and 1d bearing allyl-substituted cyclopentadienyl ligand were prepared earlier at ZKiChM. Treatment of the neutral nickel complexes in acetonitrile with KPF6 (25˚C, 60 min) resulted in the formation of cationic complexes [Ni(η5-C5H4R)(CH3CN)(NHC)]+(PF6)- 5a-d. All final products were isolated by removing the solvent, washing with hexane, dried over high-vacuum and completely characterized (1H, 13C NMR, ESI-MS). In addition, the molecular structure of complex 5c was determined by single-crystal X-ray diffraction. The catalytic activity of the cationic nickel complexes 5a - d was tested in styrene polymerization according to the general formula presented in the Scheme 1. The reactions were carried out at 50˚C in toluene for 3 h ([Ni] = 1.30 mmol/L, styrene/Ni = 1000, Al/Ni = 100). The highest catalytic activity (0.065 kg polymer / 1g nickel) was observed for complex 5a. For complex 5b the polymer yield was lower and reached 0.035 kg polymer / 1g nickel. In contrast, using the more sterically hindered complex 5c bearing SIPr ligand resulted in the significant decrease of polymer yield up to 0.019 kg polymer / 1g nickel. Similar result was achieved when complex 5d with allyl-substituted cyclopentadienyl ligand was used. 14 n n MAO Ni complex 5a-d, toluene PF6 - Ni+ N C H3C N N 5a PF6 - Ni+ N C H3C N N 5b PF6 - Ni+ N C H3C N N 5c Ni+ N C H3C N N 5d PF6 - Scheme 1. Styrene polymerization The structure of the resulting polystyrene was elucidated by 13C NMR spectroscopy and GPC chromatography. 13C NMR spectra showed that the polymers obtained in catalytic system [Ni(η5-C5H4R)(CH3CN)(NHC)]+(PF6)-/MAO were atactic. Furthermore, MALDI-TOF MS spectra showed that during the reaction styrene oligomers were also formed. In addition, two control experiments were carried out: without MAO and without cationic nickel complex. In both cases no polystyrene was obtained. To get some further insight into the lack of the activity of [Ni(η5-C5H4R)(CH3CN)(NHC)]+(PF6)- without MAO I have tried to substitute acetonitrile ligand for a styrene molecule. The course of the reaction was monitored by 1H NMR spectra. According to the resulting NMR spectra no structural changes were observed. I have also tried to generate the cationic spieces [Ni(η5-C5H4R)(NHC)(styrene)]+ by reacting complex 1b with KPF6 or AgBF4 directly in styrene. Expected π-complex of [Ni(η5-C5H4R)(NHC)]+ with styrene was not formed under the reaction conditions. Conclusion In summary, I have obtained a series of cationic complexes [Ni(η5-C5H4R)(CH3CN)(NHC)]+(PF6)- in good yields (62 -75%). I have checked that cationic complexes are active in styrene polymerization in the presence of MAO. The correlation between the catalytic activity of the cationic complex type used in polymerization is very similar to the tendency observed for the chloride analogues. This results suggest that styrene polymerizes according to the cationic mechanism in the studied system.
Record ID
WUTb7ea0d063a8a40259112fbd4295263ff
Diploma type
Master of Science
Author
Joanna Conder Joanna Conder,, Undefined Affiliation
Title in Polish
KATIONOWE KOMPLEKSY [Ni(η5-C5H4R)(CH3CN)(NHC)]+(PF6)-. SYNTEZA I AKTYWNOŚĆ KATALITYCZNA W POLIMERYZACJI STYRENU
Supervisor
Włodzimierz Buchowicz (FC/DCOC) Włodzimierz Buchowicz,, Department Of Catalysis And Organometallic Chemistry (FC/DCOC)Faculty of Chemistry (FC)
Certifying unit
Faculty of Chemistry (FC)
Affiliation unit
Department Of Catalysis And Organometallic Chemistry (FC/DCOC)
Study subject / specialization
, Technologia Chemiczna
Language
(pl) Polish
Status
Finished
Defense Date
28-06-2012
Issue date (year)
2012
Keywords in Polish
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Keywords in English
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Abstract in Polish
urn:pw-repo:WUTb7ea0d063a8a40259112fbd4295263ff