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

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(C5H3N)Fe(CH2SiMe3)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-CH2SiMe3(C5H3N)Fe(CH2SiMe3) (1), and at the imine C atom, 2-[2,6-(i-Pr)2PhN=C(CH3)]-6-[2,6-(i-Pr)2PhNC(CH3)(CH2 SiMe3)](C5H3N)Fe(CH2SiMe3) (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(C5H3N)Fe(mu-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)2C6H5]N=C(CH3)(C5H3N)[2,6-(i-Pr)26H5]N=CCH2Fe(CH2SiMe3)]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(C5H3N)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.