American Chemical Society
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Reactivity of Chromium Complexes of a Bis(imino)pyridine Ligand:  Highly Active Ethylene Polymerization Catalysts Carrying the Metal in a Formally Low Oxidation State

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journal contribution
posted on 2020-04-03, 14:23 authored by Indu Vidyaratne, Jennifer Scott, Sandro Gambarotta, Robbert Duchateau
A divalent chromium complex of bis(imino)pyridine, {2,6-[2,6-(i-Pr)2PhNC(CH3)]2(C5H3N)}CrCl2 (1), was prepared with the aim of studying its reactivity with alkylating agents. Upon treatment with MeLi, the metal center was both reduced and alkylated, forming {2,6-[2,6-(i-Pr)2PhNC(CH3)]2(C5H3N)}CrMe(μ-Me)Li(THF)3 (2). Complex 1 is also conveniently reduced with either NaH or metallic sodium to give the new species {2,6-[2,6-(i-Pr)2PhNC(CH3)]2(C5H3N)}CrCl (3). Despite the appearance of the metal center in a rare monovalent oxidation state, the square-planar geometry of the Cr atom suggests that the metal is most likely divalent, with the electron housed in the ligand π* orbital. When it is activated with MAO, complex 3 is a very and even more active catalyst for the polymerization of ethylene than either the −CrCl2 or −CrCl3 derivative of this ligand system and yet produces polymers with similar properties. Subsequent reactivity studies of complex 3 have allowed the isolation of several products. Reaction with either LiCH2Si(CH3)3 or MeLi resulted in deprotonation of one of the methyl groups on the ligand backbone, forming {2-[2,6-(i-Pr)2PhNC(CH3)]-6-[2,6-(i-Pr)2PhNCCH2](C5H3N)}Cr(THF) (4) and {2-[2,6-(i-Pr)2PhNC(CH3)]-6-[2,6-(i-Pr)2PhNCCH2](C5H3N)}Cr(μ-Me)Li(THF)3 (5), respectively. On the other hand, alkylation with AlMe3 allowed the successful preparation of another organochromium species, {2,6-[2,6-(i-Pr)2PhNC(CH3)]2(C5H3N)}CrCH3 (7), along with small amounts of the byproduct {2,6-[2,6-(i-Pr)2PhNC(CH3)]2(C5H3N)}Cr(μ-Cl)2Al(CH3)2 (6). Interestingly, complex 7, which also has the deceiving connectivity of a monovalent species, displays an even greater activity for ethylene polymerization than all of the other species reported herein, again producing a polymer with nearly identical characteristics. Activation with IBAO revealed a deactivation pathway similar to that observed with the FeCl2 system. In this case, the stronger reducing power of IBAO resulted in the usual reduction not only of the ligand backbone but also of the metal center. As a result of the metal reduction, partial transmetalation of the ligand system occurred, with formation of [η4-{2,6-[2,6-(i-Pr)2PhNC(CH3)]2(C5H3N)}Al2(i-Bu)3(μ-Cl)]Cr-(η6-C7H8) (8). By being catalytically inactive, the partly transmetalated 8 suggests that ligand demetalation is a possible catalyst deactivation pathway.