Activation of [CrCl3{PPh2N(Pr-i)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(Pr-i)(PPh2)(2) has been investigated in situ via the reaction of [CrCl3{PPh2N(R)PPh2}(THF)] 1a (R = Pr-i) 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-i). After 5 minutes reaction time the XAFS spectra indicate that similar to 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-i); a similar species, [CrClMe{PPh2N(R)PPh2}] 9a (R = Pr-i) was observed as its adduct with AlMe3 (10a) (R = Pr-i) 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-t(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.