Ethylene Trimerization with Mixed-Donor Ligand (N,P,S) Chromium Complexes:  Effect of Ligand Structure on Activity and Selectivity

Methylaluminoxane (MAO)-activated chromium(III) complexes of tridentate PNP and SNS ligands of the form (R2PCH2CH2)2NR‘ (R = alkyl, aryl; R‘ = H, Me, benzyl) and (RSCH2CH2)2NH (R = alkyl) have been prepared and tested for the trimerization of ethylene to 1-hexene. The effect of P or S donor substitution, nitrogen substitution, and chelate ring size has been examined. Sterically compact and basic P or S groups lead to the highest activities and selectivities, while a secondary amine central donor is crucial for high activity, raising the possibility that deprotonation of this group occurs during catalyst formation. Expanding the chelate ring size by introduction of a propyl spacer on one side of the SNS ligand gives EtSC3H6N(H)C2H4SEt, which leads to a trimerization catalyst with lower activity. The scope of S-donor-based ligands for ethylene trimerization was further studied by preparing and testing PSP and SPS tridentates of CrIII. These complexes give rise to active oligomerization systems that give product distributions characteristic of both linear chain growth and selective trimerization.