Activation of [CrCl₃{R-SN(H)S-R}] Catalysts for Selective Trimerization of Ethene: A Freeze-Quench Cr K-Edge XAFS Study

Homogeneous chromium catalysts for the selective conversion of ethene to hex-1-ene are formed from Cr(III) reagents, aminothioether ligands of the type HN(CH2CH2SR)2, and aluminum reagents. In this study, the early activation steps are investigated by EPR, UV–visible, and Cr K-edge XAFS spectroscopy; rapid stopped-flow mixing and a freeze-quench allows good quality EXAFS analysis of a species formed in ∼1 s of reaction. This is shown to involve reduction to Cr(II) and deprotonation of a NH group of the auxiliary ligand. This 4-coordinate metal center may act as precursor for the coordination of ethene and subsequent selective oligomerization.