New Iminophosphonamide Chromium(II) Complexes as Highly Active Polymer-Free Ethylene Oligomerization Catalysts

The reactions of the deprotonated form of cis-{[(mu-N)(t-Bu)](2)PN(H) (o-OMeC6H4)](2)} (a) with either CrCl2(THF)(2) or CrCl3(THF)(3) afforded the corresponding dimeric Cr(II) and monomeric Cr(III) complexes {cis-[(mu-N)(t-Bu)](2)[PN (o-MeOC6H4)](2)Cr}(2) (1a) and cis-[(mu-N)(t-Bu)](2)[PN-2-MeOC6H4](2)CrCl (2a). By replacing the ligand's o-OMeC6H4 groups with less-crowded CH2CH2N(i-Pr)(2) functionalities and reacting the deprotonated form of cis-{[(mu-N)(t-Bu)](2)[PN(H)CH2CH2N(i-Pr)(2))](2)} (b) with CrCl2(THF)(2), the dimetallic, divalent {cis-[(mu-N)(t-Bu)](2)[PNCH2CH2N(i-Pr)(2))](2)Cr}(2) (1b) was obtained in crystalline form. Upon activation with MAO, both la and 1b afforded highly active, polyethylene-free ethylene oligomerization catalysts. Complex 2a is instead catalytically inactive.