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

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{PPh2N(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){PPh2N(R)PPh2}(THF)] 4a (R= i 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= i Pr); a similar species, [CrClMe{PPh2N(R)PPh2}] 9a (R= i Pr) was observed as its adduct with AlMe3 (10a) (R= i Pr) when spectra were recorded on samples maintained a 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, respectiveley. 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( 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 [Cr2Me8] 4-{Cr-Cr 2.01 (2), Cr-C 2.14(4)}, thus demonstrating considerable variation in the effects of differing Lewis acids.

PNPN‐H in Comparison to other PNP, PNPN and NPNPN Ligands for the Chromium Catalyzed Selective Ethylene Oligomerization

“…The formation of dinuclear Cr(II) species like [Cr 2 Me 8 ] 4− was discussed by McGuinnes and co‐workers in a Cr−K‐edge XAFS study during the activation of [CrCl 3 {Ph 2 PN( i Pr)PPh 2 }] for the selective oligomerization of ethylene . The same group published DFT mechanistic studies on the ethylene tri‐ and tetramerization with Cr/PNP ligands regarding single versus double insertion pathways .…”

“…For this reason a comparison of different ligands regarding the formation of tri‐ and tetramers is meaningful. Very recently isotopic labelling and other experiments were reported concerning selectivity of 1‐hexene and 1‐octene formation in the ethylene oligomerization with the result that di‐ or multinuclear Cr species are not the active species in the mechanism . Nevertheless, there some results from substitution of sterically overcrowded PNPN−H and PNPNP ligands provide hints that large substituents lead to a shift from tri‐ to tetramerization.…”

Role of the ligand and activator in selective Cr–PNP ethene tri- and tetramerization catalysts – a spectroscopic study

“…In a freeze-quench XAS study, we observed the formation of ((C 6 H 5 ) 2 P) 2 N iPr CrMeCl in the reaction between ((C 6 H 5 ) 2 P) 2 N iPr )CrCl 3 (THF) and AlMe 3 . 14,15 In another study, Bercaw and coworkers performed an EPR and UV-vis study of a CrCl 3 complex containing the PNP ligand shown in Scheme 1a, functionalized with solubilizing groups. 16 Upon reaction of the complex with MMAO, a variety of Cr I , Cr III and EPR-silent complexes (dinuclear Cr I , or Cr II ) were identified.…”

Investigating the Active Species in a [(R‐SN(H)S‐R)CrCl3] Ethene Trimerization System: Mononuclear or Dinuclear?

“…Recently, we have developed a freeze‐quench XAS technique which allows for the trapping of reactive complexes . Freeze‐quench XAS is required to study these ethene trimerization systems due to catalyst deactivation occurring on the same timescale as a high‐quality EXAFS data acquisition times (several hours).…”

“…[12] Recently, we have developed a freeze-quench XAS technique which allows for the trapping of reactive complexes. [18][19][20] Freeze-quench XAS is required to study these ethene trimerization systems due to catalyst deactivation occurring on the same timescale as a high-quality EXAFS data acquisition times (several hours). The freeze-quench methodology allows for 'trapping' of reaction intermediates at different points in time, i. e. obtaining time-resolution, while freezing these intermediates allows long measurement times required for high quality data.…”