Triazacyclohexane chromium triflate complexes as precursors for the catalytic selective olefin trimerisation and its investigation by mass spectrometry

Article abstract of DOI:10.1016/j.poly.2020.114572

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.