A chromium precursor for the Phillips ethylene trimerization catalyst: (2-ethylhexanoate)2CrOHElectronic supplementary information (ESI) available: The CIF files for 6 and 8, IR and NMR charts. CCDC 1044532-1044534. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt01203a

The conventional Phillips ethylene trimerization catalyst prepared by reacting Cr(EH) 3 (EH = 2-ethylhexanoate), 2,5-dimethylpyrrole (Me 2 C 4 H 2 NH), Et 3 Al, and Et 2 AlCl in an aromatic hydrocarbon solvent was improved to obtain a congener composed of a new chromium precursor (EH) 2 CrOH, (Me 2 C 4 H 2 N)AlEt 2 , and Et 3 Al·ClAlEt 2 . Reaction of CrCl 3 with 3 equiv. Na(EH) in water did not generate Cr(EH) 3 , but unexpectedly produced (EH) 2 CrOH. In comparison with the erratic catalytic performance of the original Phillips system, due to the ill-defined nature of the Cr(EH) 3 source (16 or 6.8 × 10 6 g per mol-Cr h depending on the source), the improved system exhibited consistently high activity (54 × 10 6 g per mol-Cr h). Reaction of (EH) 2 CrOH with (Me 2 C 4 H 2 N)AlMe 2 ·OEt 2 afforded the dimeric Cr( ii )-complex ( 6 ) coordinated by (η 5 -Me 2 C 4 H 2 N)AlMe 2 (NC 4 H 2 Me 2 ) and μ 2 -κ 1 :η 2 -Me 2 C 4 H 2 N ligands. 6 provided highly active species when activated with Et 3 Al·ClAlEt 2 . The Phillips ethylene trimerization catalyst was improved to obtain a congener composed of a new chromium precursor (EH) 2 CrOH, (Me 2 C 4 H 2 N)AlEt 2 , and Et 3 Al·ClAlEt 2 .