Activation of [ CrCl 3 { PPh 2 N ( iPr ) PPh 2 } ] for the selective oligomerisation of ethene : a Cr K - edge XAFS study † ‡ Stuart

The activation of the ethene tetramerisation catalyst system based upon [CrCl3(THF)3] and N( Pr)(PPh2)2 has been investigated in situ via the reaction of [CrCl3{PPh2N(R)PPh2}(THF)] 1a (R = 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){PPh2N(R)PPh2}(THF)] 4a (R = Pr). 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 = Pr); a similar species, [CrClMe{PPh2N(R)PPh2}] 9a (R = Pr) was observed as its adduct with AlMe3 (10a) (R = Pr) 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( Bu)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.