Switchable Chromium(II) Complexes of a Chelating Amidophosphine (N−P) for Selective and Nonselective Ethylene Oligomerization

Treatment of the lithium salt of t-BuN(H)PPh2 in THF with CrCl2·(THF)2 afforded a dinuclear, [(t-BuNPPh2)Cr2(μ-t-BuNPPh2)3]·(toluene)1.5 (1), and a tetrameric cluster, [(t-BuNPPh2)Cr(μ-t-BuNPPh2)2Cr(μ-Cl)]2·(toluene)2 (5), depending on the reagents' stoichiometric ratio. Complex 1 is dimeric with a long intermetallic distance and the ligands adopting an asymmetric and distorted bridging-chelating bonding mode. Complex 5 is instead a symmetry-generated tetramer with two identical dimetallic units, each closely related in geometry to 1, linked by two bridging chlorine atoms. The reactions of 1 with alkyl aluminum activators afforded a series of divalent complexes, [{(μ-AlMe3)(t-BuNPPh2)}2Cr]·(toluene) (2), [{(μ-AlMe2Cl)(t-BuNPPh2)}2Cr]·(toluene) (3), and [{(μ-AlEt2Cl)(t-BuNPPh2)}2Cr]·(toluene) (4), containing organo-aluminum residues. Similarly, reaction of 5 with AlMe3 gave [{(μ-AlMe2)(t-BuNPPh2)2}Cr(μ-Cl)]2·(toluene)1.9 (6), also characterized by an X-ray crystal structure. Finally, the trivalent complex [(t-BuNPPh2)3Cr] (7) was readily prepared via reaction of the lithiated ligand with CrCl3(THF)3. Upon treatment with AlMe3 or Et2AlCl, complexes 2 and [{(μ-AlEtCl2)(t-BuNPPh2)}2Cr]·(toluene) (8) were isolated and fully characterized. In turn, this indicated that reduction to the divalent state is the primary stage of the activation process of the trivalent species. The catalytic behavior of all of these complexes has been assessed in the presence and absence of cocatalyst and with different solvents. The result showed a pronounced solvent effect, allowing switching from nonselective oligomerization to selective trimerization.