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

摘要

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, magnetochemistry, 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 chelate-based, resulting in a bis(imino)pyridine radical anion (S_PDI = 1/₂) 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 = 0 compound is best described as a low spin ferrous compound (S_Fe = 0) with a closed-shell singlet bis(imino)pyridine dianion (S_PDI = 0), 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.