Metal versus ligand alkylation in the reactivity of the (bis-iminopyridinato)Fe catalyst

Abstract

The alkylation of the Brookhart-Gibson {2,6-[2,6-(i-Pr)2PhN= C(CH3)]2(C5H3N)} FeCl2 precatalyst with 2 equiv of LiCH2Si(CH3)3 led to the isolation of several catalytically very active products depending on the reaction conditions. The expected dialkylated species {2,6-[2,6-(i-Pr) 2PhN=C(CH3)]2}(C5H 3N)Fe(CH2-SiMe3)2 (2) was indeed the major component of the reaction mixture. However, other species in which alkylation occurred at the pyridine ring ortho position, {2,6-[2,6-(i-Pr) 2PhN=C(CH3)]2-2-CH2SiMe 3}-(C5H3N)Fe(CH2SiMe3) (1), and at the imine C atom, {2-[2,6-(i-Pr)2PhN=C(CH 3)]-6-[2,6-(i-Pr)2PhNC(CH3)-(CH2 SiMe3)](C5H3N)}Fe(CH2SiMe 3) (3), have also been isolated and fully characterized. In addition, deprotonation of the methyl-imino functions and formation of a new divalent Fe catalyst {[2,6-[2,6-(i-Pr)2PhN-C=(CH2)]2(C 5H3N)}Fe(μ-Cl)Li(THF)3 (4) also occurred depending on the reaction conditions. In turn, the formation of 4 might trigger the reductive coupling of two units through the methyl-carbon wings. This process resulted in the one-electron reduction of the metal center, affording a dinuclear Fe(I) alkyl catalyst {[{[2,6-(i-Pr)2C6H 5]N=C(CH3)}(C5H3N){[2,6-(i-Pr) 2C6H5]N=CCH2}Fe(CH 2SiMe3)]}2 (5). Different from other metal derivatives, complex 5 could not be prepared from the monodeprotonated version of the ligand. Its reaction with a mixture of FeCl2 and RLi afforded instead [{2,6-[2,6-(i-Pr)2PhN-C=(CH2)]2(C 5H3N)}FeCH2Si(CH3) 3]-[Li(THF)4] (6) which is also catalytically active. All of these high-spin species have been shown to have high catalytic activity for olefin polymerization, producing polymers of two distinct natures, depending on the formal oxidation state of the metal center. © 2005 American Chemical Society.