Synthesis of Bis(imino)pyridine Iron Di- and Monoalkyl Complexes: Stability Differences between FeCH2SiMe3and FeCH2CMe3Derivatives

Abstract

The scope of aryl-substituted bis(imino)pyridine iron dihalide alkylation and arylation reactions has been explored with LiCH2SiMe3, LiCH2CMe3, and LiPh. Dialkylation was only observed when the 2,6-diisopropylaryl-substituted iron complex was treated with 2 equiv of LiCH2SiMe3. In all other cases, reductive alkylation to form four-coordinate bis(imino)pyridine iron monoalkyl complexes occurred. One of these products, (EtPDI)FeCH2CMe3 [EtPDI = 2,6-(2,6-Et2C6H3N═CMe)2C6H3N)], has been crystallographically characterized and shown to exhibit a distorted square planar geometry arising from the steric disposition of the alkyl group. Five-coordinate bis(imino)pyridine iron aryl“ate” complexes bearing weakly activated dinitrogen ligands were isolated from treatment of the corresponding dihalide with 3 equiv of aryllithium. The tolyl derivative was characterized by X-ray diffraction, and a distorted square pyramidal structure was established where the dinitrogen ligand completes the idealized basal plane. Ligand substitution reactions where the terdentate bis(imino)pyridine is used to displace pyridine from py2Fe(R)X (R = CH2SiMe3, CH2CMe3; X = Cl, CH2SiMe3, CH2CMe3) further demonstrated the relative stability of five-coordinate FeCH2SiMe3 versus FeCH2CMe3 complexes. For the neopentyl compounds, ejection of a neopentyl radical or alkylation of the p-pyridine position of the chelate was observed.