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

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.