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Treatment of the lithium salt of t-BuN(H)PPh2 in THF with CrCl2 (THF)(2) afforded a dinuclear, [(t-BuNPPh2)Cr-2(mu-t-BuNPPh2)(3)]center dot(toluene)(1.5) (1), and a tetrameric cluster, [(t-BuNPPh2)Cr(mu-t-BuNPPh2)(2)Cr(mu-Cl)](2)center dot(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, [{(mu-AlMe3)(t-BuNPPh2)}(2)Cr]center dot(toluene) (2), [{(mu-AlMe2Cl)(t-BuNPPh2)}(2)Cr]center dot(toluene) (3), and [{(mu-AlEt2Cl)(t-BuNPPh2)}(2)Cr]center dot(toluene) (4), containing organo-aluminum residues. Similarly, reaction of 5 with AlMe3 gave [{(mu-AlMe2)(t-BuNPPh2)(2)}Cr(mu-Cl)(2)center dot(toluene)(1.9) (6), also characterized by an X-ray crystal structure. Finally, the trivalent complex [(t-BuNPPh2)(3)Cr] (7) was readily prepared via reaction of the lithiated ligand with CrCl3(THF)(3). Upon treatment with AlMe3 or Et2AlCl, complexes 2 and [{(mu-AlEtCl2)(t-BuNPPh2)}(2)Cr]center dot(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.

Reaction of RN(H)PBr(Ph)(2)N(H)R [R = t-Bu, i-Pr, Ph] with two equivalents of n-BuLi followed by reaction with CrCl2(THF)(2) afforded the divalent chromium complexes [(t-Bu)NP(Ph)(2)-N(t-Bu)]Cr(mu(2)-Cl)(2)Li(THF)(2) (1), (i-Pr)NP(Ph)(2)N(i-Pr)]Cr(mu(2)-Cl)(2)Li(THF)(2) (2), and [{[(Ph)-NP(Ph)(2)N(Ph)]Cr}(2)(mu(2)-Cl)(3)][Li(DME)(3)] (3). The trivalent analogue of 1, {[(t-Bu)NP(Ph)(2)-N-(t-Bu)]Cr(mu(2)-Cl)(3)(mu(2)-Cl)(2)}{Li (THF)(2)} (4), was :obtained in a similar rnahner via treatment of the double deprotonated ligand with CrC13(THP)3. Both-reactions of the -divalent Lortriva/ent 4 with AlMe3 yielded the trivalent and {[(t-Bu)NP(Ph)(2)N(t-Bu)](2)Cr}{Me3Al)(2)Cl)}center dot toluene (5) Upon activation With MAO 1-3 produced produced unprecedented and potentially; useful catalytic systems for nonselective ethylene oligomerization devoid of polymer. Divalent clearly the species responsible for the catalytic behavior, ruling out that nonselective oligomerization proceeds via a redox metallacycle mechanism. The absence of polymer in combination with the record activity makes 1 competitive with the best performing industrially used systems.