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Issue 16, 2016
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Activation of [CrCl3{PPh2N(iPr)PPh2}] for the selective oligomerisation of ethene: a Cr K-edge XAFS study

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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.

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

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Publication details

The article was received on 25 Apr 2016, accepted on 17 May 2016 and first published on 18 May 2016


Article type: Paper
DOI: 10.1039/C6CY00902F
Citation: Catal. Sci. Technol., 2016,6, 6237-6246
  • Open access: Creative Commons BY license
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    Activation of [CrCl3{PPh2N(iPr)PPh2}] for the selective oligomerisation of ethene: a Cr K-edge XAFS study

    S. A. Bartlett, J. Moulin, M. Tromp, G. Reid, A. J. Dent, G. Cibin, D. S. McGuinness and J. Evans, Catal. Sci. Technol., 2016, 6, 6237
    DOI: 10.1039/C6CY00902F

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