Triazacyclohexane Chromium Triflate Complexes as Precursors for the Catalytic Selective Olefin Trimerisation and its Investigation by Mass Spectrometry
| Autor: | Samaphon Chunawat, Shahram Mihan, Alexander G.N. Coxon, Callum J. Heron, Gabriele Kociok-Köhn, Catherine L. Lyall, Shaun Reeksting, Randolf D. Köhn |
|---|---|
| Jazyk: | angličtina |
| Rok vydání: | 2020 |
| Předmět: |
Chromium
Selective olefin trimerisation Substituent chemistry.chemical_element Trimer Alkylation 010402 general chemistry 01 natural sciences Medicinal chemistry Catalysis Coordination complex Inorganic Chemistry chemistry.chemical_compound Materials Chemistry Electrospray mass spectrometry Physical and Theoretical Chemistry chemistry.chemical_classification Olefin fiber 010405 organic chemistry 0104 chemical sciences Coordination chemistry chemistry Trifluoromethanesulfonate |
| Zdroj: | Köhn, R, Coxon, A, Chunawat, S, Heron, C, Mihan, S, Lyall, C, Reeksting, S & Kociok-Kohn, G 2020, ' Triazacyclohexane Chromium Triflate Complexes as Precursors for the Catalytic Selective Olefin Trimerisation and its Investigation by Mass Spectrometry ', Polyhedron, vol. 185, 114572 . https://doi.org/10.1016/j.poly.2020.114572 |
| Popis: | A novel set of linear α-olefin trimerisation pre-catalysts is presented. The R3TACCr(OTf)3 (R3TAC = 1,3,5-trialkyl-1,3,5-triazacyclohexane) complexes produce an active system after facile activation with less than 10 equivalents of trialkylaluminium. Isomerisation was observed in many cases, which is proposed to occur via the π-allyl complex mechanism, though this was minimised by optimisation of the reaction conditions. The pre-catalysts can be synthesised from R3TACCrCl3 in neat TfOH or by addition of R3TAC to Cr(OTf)3 or better Cr(OTf)3(THF)3, the synthesis and structure of which is presented here. The use of this highly defined system allowed the identification of 2-methyl-1-hexene as a side product of activation with AlMe3, in agreement with the proposed metallacyclic mechanism. Isomer distribution of the trimer product is similar to R3TACCrCl3/MAO activated systems and depends mostly on the ligand substituent R. ESI mass spectrometry of an ortho-difluorobenzene solution of the activated catalyst was analysed at different stages of reaction. A series of signals was observed that matched those expected for cationic chromium species predicted by the metallacyclic mechanism. In particular, [R3TACCrMe2]+ was observed to form immediately after alkylation with AlMe3, while [R3TACCr(olefin)n]+ (n = 0, 1, 2, 3, 4 and olefin = 1-hexene or 1-octene) formed after addition of 1-hexene. Absence of any detected tetramer and observation of [R3TACCr(olefin)4]+ leads to the conclusion that a metallacycloheptane-olefin complex may lead to β-H abstraction rather than insertion and may even be required to avoid formation of [R3TACCr(olefin)1]+ which is observed in only very small amounts indicating a low stability. Chromium(I) complexes [R3TACCr(arene)]+ with the arene solvent can also be observed and their signal intensity indicate a relative stability of o-C6H4F2 < cymene < benzene < PhF, toluene < p-xylene. |
| Databáze: | OpenAIRE |
| Externí odkaz: |
|