Synthesis and electronic structure of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes: Evaluation of redox activity in single-component ethylene polymerization catalysts

A family of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes has been prepared, and their electronic and molecular structures have been established by a combination of X-ray diffraction, Mössbauer spectroscopy, magnelochemistry, and open-shell density functional theory. For the cationic complexes, [(^iPrPDI)Fe-R][BPh₄] (^iPrPDI = 2, 6-(2,6-ⁱPr₂-C₆H₃N=CMe)₂C₅H₃N; R = CH₂SiMe₃, CH₂CMe₃, or CH₃), which are known single-component ethylene polymerization catalysts, the data establish high spin ferrous compounds (S_Fe= 2) with neutral, redox-innocent bis(imino)pyridine chelates. One-electron reduction to the corresponding neutral alkyls, (^iPrPDI)Fe(CH₂SiMe₃) or (^iPrPDI)Fe(CH₂CMe₃), is chelatebased, resulting in a bis(imino)pyridine radical anion (S_pDI= V₂) antiferromagnetically coupled to a high spin ferrous ion (S_Fe= 2). The neutral neopentyl derivative was reduced by an additional electron and furnished the corresponding anion, [Li(Et₂O)₃][(^iPrPDI)Fe(CH₂CMe₃)N₂], with concomitant coordination of dinitrogen. The experimental and computational data establish that this S=O compound is best described as a low spin ferrous compound (S_Fe= O) with a closed-shell singlet bis(imino)pyridine dianion (Sprji = O), demonstrating that the reduction is ligand-based. The change in field strength of the bis(imino)pyridine coupled with the placement of the alkyl ligand into the apical position of the molecule induced a spin state change at the iron center from high to low spin. The relevance of the compounds and their electronic structures to olefin polymerization catalysis is also presented.