Activation of [CrCl3{PPh2N(iPr)PPh2}] for the selective oligomerisation of ethene: a Cr K-edge XAFS study12. Issue 16 (25th May 2016)

• Record Type: Journal Article
• Authors: Bartlett, Stuart A.; Moulin, Jerome; Tromp, Moniek; Reid, Gillian; Dent, Andy J.; Cibin, Giannantonio; McGuinness, David S.; Evans, John
• Is Part Of: Catalysis science & technology. Volume 6:Issue 16(2016)
• Page Start: 6237
• Page End: 6246
• Publication Date: 2016-05-25
• Subjects: Catalysis -- Periodicals; 541.395
• DOI: 10.1039/c6cy00902f
• Languages: English
• ISSNs: 2044-4753

Abstract

Abstract : Freeze-quench XAFS reveals initial methylation of the Cr(iii ) precursor prior to reduction to Lewis-acid dependent Cr(ii ) species. Abstract : The activation of the ethene tetramerisation catalyst system based upon [CrCl3 (THF)3 ] and N( i Pr)(PPh2 )2 has been investigated in situ via the reaction of [CrCl3 {PPh2 N(R)PPh2 }(THF)]1a (R = i Pr) 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 ){PPh2 N(R)PPh2 }(THF)]4a (R = i Pr). After 5 minutes reaction time the XAFS spectra indicate that ∼50% of4a had been converted to a Cr(ii ) species, with the central core being high spin [CrCl2 {PPh2 N(R)PPh2 }]7a (R = i Pr); a similar species, [CrClMe{PPh2 N(R)PPh2 }]9a (R = i Pr) was observed as its adduct with AlMe3 (10a ) (R = i Pr) when spectra were recorded on samples maintained at room temperature. Detailed analysis (EXAFS and XANES) indicated that7a and9a are stabilised by adduct formation of a Cr–Cl bond to the Lewis acids B(C6 F5 )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 [Ph3 C][Al{OC( t Bu F )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 [Cr2 Me8 ] 4− {Cr–Cr 2.01(2), Cr–C 2.14(4)}, thus demonstrating considerable variation in the effects of differing Lewis acids.