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

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(^tBu^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 [Cr₂Me₈]^4- {Cr-Cr 2.01(2), Cr-C 2.14(4)}, thus demonstrating considerable variation in the effects of differing Lewis acids.