Probing the early activation mechanism of olefin oligomerisation catalysis using molybdenum, scandium and chromium halide complexes

Abstract

The efficient catalytic conversion of small molecules into more complex species by clean, energy efficient processes is of high commercial demand. The selective trimerisation and tetramerisation of ethene to produce the linear alpha-olefins (LAOs) 1-hexene and 1-octene, respectively, are of major significance due to the importance in the production of linear low-density polyethylene (LLDPE). The activation of [MX3(L)] (M = Mo, Sc, Cr; X = Cl, Br*; L = tridentate ligands with S3, N3, SNS and PNP donor sets) by AlMe3, based around the industrially important [CrCl3(L)] catalysts for selective oligomerisation of alkenes, has been investigated by K-edge X-ray absorption (XAS), UV–Visible and NMR spectroscopies. Time-resolved stopped-flow Mo XAS with UV-Vis spectroscopy, in combination with a newly developed anaerobic freeze-quench approach, established the complete alkylation of the Mo centres and a slower, stepwise sequence for [MoBr3(L)]. Sc K-edge XAS with 45Sc and 27Al NMR measurements revealed a possible ethene polymerisation pathway via a chloro bridged alumino scandium species. The novel freeze-quench approach, which can trap reaction solutions within 1 second of mixing to allow long data acquisition, was applied to the industrially important [CrCl3(SNS)] and [CrCl3(PNP)] complexes to yield a four coordinate [CrIICl(SNS)] species and five coordinate [CrIICl2Me(PNP)] upon reaction with AlMe3. This demonstrates the power of the freeze-quench technique in identifying new reactive and short lived intermediates within homogeneous catalysis.
*Bromides only investigated on molybdenum.

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ORCID for Gillian Reid: orcid.org/0000-0001-5349-3468

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