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

Abstract

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, [((iPr)PDI)Fe-R][BPh(4)] ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)N═CMe)(2)C(5)H(3)N; R = CH(2)SiMe(3), CH(2)CMe(3), or CH(3)), 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, ((iPr)PDI)Fe(CH(2)SiMe(3)) or ((iPr)PDI)Fe(CH(2)CMe(3)), 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(Et(2)O)(3)][((iPr)PDI)Fe(CH(2)CMe(3))N(2)], 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.

Synthetic Approaches to (smif)2Ti (smif = 1,3-di-(2-pyridyl)-2-azaallyl) Reveal Redox Non-Innocence and C–C Bond-Formation

“…In the context of Wieghardt’s and Chirik’s analyses, 36–38 the pyridine-imine is at least mono-reduced, and a case can be made for the dianionic form. A normal titanium-pyridine d(Ti-N4) of 2.1120(12) Å, and a titanium-imine d(Ti-N5) of 2.0231(12) Å, are also observed, and the latter is considerably shorter than the aforementioned Ti-N aza linkage of the smif ligand.…”

Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction

“…A similar complex is synthesized by addition of neopentyllithium to (PDI)Fe(N 2 ) 2 to form [(PDI)Fe(CH 2 t 2 Bu)](μ-N 2 )Li(OEt 2 ) 3 ( 7 ). This complex has a slightly more activated N 2 ligand, with ν NN = 1948 cm −1 [69]. An interesting study of the reduction of (PDI)FeCl 2 with NaH by Scott et al showed a diverse array of products depending on the amount of reductant used [95].…”

Methyl Complexes of the Transition Metals

“…77 The group of Chirik has studied a series of cationic, neutral, and anionic bis(imino)pyridine (PDI) iron alkyl complexes by a combination of techniques including X-Ray diffraction, Mössbauer spectroscopy, and DFT in order to gain insight into the redox activity of single-component (without MAO or other cocatalyst) ethylene polymerization catalysts. 78 The previous complexes are examples of low-electron-count iron compounds. Bulky tris(pyrazolyl)borate ligands (Tp) have also been used to stabilise 14-electron tetracoordinated complexes, including TpFeCH3 (Scheme 11).…”