Synthesis and Reactivity of the Bis(phenylimino)pyridine Iron Bis(dinitrogen) Complex: Catalyst Deactivation Via Arene Coordination
In pursuit of highly active iron-based catalysts for bond-forming reactions, the phenyl-substituted bis(phenylimino)pyridine iron bis(dinitrogen) complex, (iPrPhPDI)Fe(N2)2 (iPrPhPDI = 2,6-(2,6-(CHMe2)2C6H3N=CC6H5)2C5H3N), was prepared by sodium amalgam reduction of the ferrous dichloride precursor under four atmospheres of dinitrogen. The bis(dinitrogen) compound displayed catalytic productivity for the hydrogenation and hydrosilation of 1-hexene superior to that of the methyl-substituted analog, (iPrPDI)Fe(N2)2 (iPrPDI = 2,6-(2,6-(CHMe2)2C6H3N=CCH3)2C5H3N). However, the catalytic productivity with more hindered substrates, such as cyclohexene and (R)-(+)-limonene, was inferior. The diminished catalytic productivity with these substrates precipitated from competitive deactivation via irreversible formation of n6-aryl and -phenyl complexes unobserved in the chemistry of (iPrPDI)Fe(N2)2. Dissolution of (iPrPhPDI)Fe(N2)2 in coordinating solvents such as THF or cyclohexene prompted exclusive formation of the n6-phenyl derivative, whereas dissolution in non-coordinating solvents such as pentane, ether, and mesitylene afforded solely the n6-aryl compound. A family of bis(imino)pyridine ligands bearing alkylimino-substituents (rather than arylimino-substituents) was also synthesized and complexed to ferrous dibromide. Conversion to the corresponding dicarbonyl compounds was affected by sodium amalgam reduction under four atmospheres of carbon monoxide. Electronic studies of these alkylimino-substituted bis(imino)pyridine iron dicarbonyls demonstrated the electron-donating character of the ligands relative to their arylimino-substituted counterparts. Initial attempts to isolate effective precatalysts for C-H bond-forming reactions bearing the alkylimino-substituted bis(imino)pyridine ligand were unsuccessful. However, through the course of these attempts, a new precatalyst bearing the arylimino-substituted EtPDI ligand (EtPDI = (2,6-(2,6-Et2C6H3N=CMe)2C5H3N) was isolated by sodium amalgam reduction in the presence of excess 1,3-butadiene. The catalytic productivity of this butadiene complex for hydrogenation of simple olefins was assayed. An induction period was identified, and its origins examined.
Iron; Bis(imino)pyridine; Iron Catalysis; Arene Coordination; dinitrogen
dissertation or thesis