5,6-Dihydrodibenzo[ c, e][1,2]azaphosphinine-Based PNP Ligands, Cr(0) Coordination, and Cr(III) Precatalysts for Ethylene Oligomerization

We elaborated a synthetic approach to a new family of PNP ligands that involves assembling sequential molecules based on the rigid 5,6-dihydrodibenzo[c,e][1,2]azaphosphinine backbone (−Z−). This approach allowed us to obtain PNP-type ligands of the general formula Ar2P–Z–Ar′ (Ar, Ar′ = Ph, o-MeOC6H4) with high yields and without the formation of PPN byproducts. Ar2P–Z–Ar′/Cr(acac)3 (acac = acetylacetonate) adducts activated by modified methylalumoxane (MMAO-12) effectively catalyzed ethylene oligomerization; the number of ortho-methoxyphenyl fragments n controls the direction of the process, from slow polymerization (n = 0) to rapid oligomerization (n = 3), thereby producing a mixture of 1-hexene and 1-octene in ∼2:1 ratio with very low formation of polyethylene (∼0.6%).