Preparation and Characterization of a Reduced Chromium Complex via Vinyl Oxidative Coupling: Formation of a Self-Activating Catalyst for Selective Ethylene Trimerization

Reaction of the divalent [(t-Bu)NP(Ph)2N(t-Bu)]CrCl2Li(THF)2 (1) with 1 equiv of vinyl Grignard (CH2CH)MgCl reproducibly afforded the triangulo {π-[(t-Bu)N−P(Ph)2−N(t-Bu)]Cr}2(μ,μ′,η4,η4′-C4H4){σ-[(t-Bu)N−P(Ph)2−N(t-Bu)]Cr} (2) containing a σ-/π-bonded butadiene-diyl unit. The diene-diyl moiety was generated by an oxidative coupling and deprotonation of two vinyl anions. The crystal structure revealed that of the three chromium atoms, each bearing one NPN ligand, two are perpendicularly bonded to the two sides of the π-system of the butadiene-diyl residue in a sort of inverted sandwich type of structure. The third is instead coplanar with the doubly deprotonated C4 unit and σ-bonded to the two terminal carbon atoms. Despite the appearance as a Cr(II)/Cr(I) mixed valence species, DFT calculations have revealed that the structure of 2 consists of three divalent chromium atoms, while the additional electron resides on the π-system of the bridging organic residue. Complex 2 behaves as a single component selective catalyst for ethylene trimerization.