Activation of [CrCl3{PPh2N(iPr)PPh2}] for the selective oligomerisation of ethene: a Cr K-edge XAFS study

Stuart A. Bartlett, Jerome Moulin, Moniek Tromp, Gillian Reid, Andy J. Dent, Giannantonio Cibin, David S. McGuinness, John Evans

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Abstract

The activation of the ethene tetramerisation catalyst system based upon [CrCl3(THF)3] and N(iPr)(PPh2)2 has been investigated in situ via the reaction of [CrCl3{PPh2N(R)PPh2}(THF)] 1a (R = iPr) with excess AlMe3 in toluene. The Cr K-edge XAFS spectrum of the solution freeze quenched after 1 min reaction time indicated monomethylation of the metal with the resultant product being [CrClMe(ClAlCl3){PPh2N(R)PPh2}(THF)] 4a (R = iPr). After 5 minutes reaction time the XAFS spectra indicate that ∼50% of 4a had been converted to a Cr(ii) species, with the central core being high spin [CrCl2{PPh2N(R)PPh2}] 7a (R = iPr); a similar species, [CrClMe{PPh2N(R)PPh2}] 9a (R = iPr) was observed as its adduct with AlMe3 (10a) (R = iPr) when spectra were recorded on samples maintained at room temperature. Detailed analysis (EXAFS and XANES) indicated that 7a and 9a are stabilised by adduct formation of a Cr-Cl bond to the Lewis acids B(C6F5)3 and AlMe3, respectively. Modelling with DFT methods indicated that five-coordination was achieved, respectively by Cr-F (11a) and Cr-C (10a) interactions. In the presence of [Ph3C][Al{OC(tBuF)3}4], the Cr XAFS of the room temperature solution was inconsistent with the maintenance of a phosphine complex, but could be modelled with a site like [Cr2Me8]4- {Cr-Cr 2.01(2), Cr-C 2.14(4)}, thus demonstrating considerable variation in the effects of differing Lewis acids.

Original languageEnglish
Pages (from-to)6237-6246
Number of pages10
JournalCatalysis Science and Technology
Volume6
Issue number16
DOIs
Publication statusPublished - 21 Aug 2016

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Bartlett, S. A., Moulin, J., Tromp, M., Reid, G., Dent, A. J., Cibin, G., ... Evans, J. (2016). Activation of [CrCl3{PPh2N(iPr)PPh2}] for the selective oligomerisation of ethene: a Cr K-edge XAFS study. Catalysis Science and Technology, 6(16), 6237-6246. https://doi.org/10.1039/c6cy00902f