Synthesis of bis(imino)pyridine iron di- And monoalkyl complexes: Stability differences between FeCH₂SiMe₃ and FeCH ₂CMe₃ derivatives

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, CH ₂CMe₃; X = Cl, CH₂SiMe₃, CH ₂CMe₃) further demonstrated the relative stability of five-coordinate FeCH₂SiMe₃ versus FeCH₂CMe ₃ complexes. For the neopentyl compounds, ejection of a neopentyl radical or alkylation of the p-pyridine position of the chelate was observed.