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

Abstract

A divalent chromium complex of bis(imino)pyridine, {2,6-[2,6-(i-Pr) 2 PhN C(CH 3 )] 2 (C 5 H 3 N)}CrCl 2 ( 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) 2 PhN C(CH 3 )] 2 (C 5 H 3 N)}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) 2 PhN C(CH 3 )] 2 (C 5 H 3 N)}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 −CrCl 2 or −CrCl 3 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 LiCH 2 Si(CH 3 ) 3 or MeLi resulted in deprotonation of one of the methyl groups on the ligand backbone, forming {2-[2,6-(i-Pr) 2 PhN C(CH 3 )]-6-[2,6-(i-Pr) 2 PhNC CH 2 ](C 5 H 3 N)}Cr(THF) ( 4 ) and {2-[2,6-(i-Pr) 2 PhN C(CH 3 )]-6-[2,6-(i-Pr) 2 PhNC CH 2 ](C 5 H 3 N)}Cr(μ-Me)Li(THF) 3 ( 5 ), respectively. On the other hand, alkylation with AlMe 3 allowed the successful preparation of another organochromium species, {2,6-[2,6-(i-Pr) 2 PhN C(CH 3 )] 2 (C 5 H 3 N)}CrCH 3 ( 7 ), along with small amounts of the byproduct {2,6-[2,6-(i-Pr) 2 PhN C(CH 3 )] 2 (C 5 H 3 N)}Cr(μ-Cl) 2 Al(CH 3 ) 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 FeCl 2 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 [η 4 -{2,6-[2,6-(i-Pr) 2 PhN C(CH 3 )] 2 (C 5 H 3 N)}Al 2 (i-Bu) 3 (μ-Cl)]Cr-(η 6 -C 7 H 8 ) ( 8 ). By being catalytically inactive, the partly transmetalated 8 suggests that ligand demetalation is a possible catalyst deactivation pathway.