Synthesis, Electronic Structure, and Alkene Hydrosilylation Activity of Terpyridine and Bis(imino)pyridine Iron Dialkyl Complexes

Iron dialkyl complexes, [N-3]Fe(CH2SiMe3)(2), with three different classes of tridentate, nitrogen-based [N-3] ligands, aryl-substituted bis(imino)pyridines, terpyridine, and pyridine bis(oxazoline), have been synthesized and evaluated in the catalytic hydrosilylation of olefins with tertiary silanes. The 2,2':6',2-terpyridine (terpy) complex, (terpy)Fe(CH2SiMe3)(2), was prepared either via alkylation of (terpy)FeCl2 with LiCH2SiMe3 or by pyridine displacement from (pyridine)(2)Fe(CH2SiMe3)(2) by free terpyridine. The aryl-substituted bis(imino)pyridine compounds, ((PDI)-P-R)Fe(CH2SiMe3)(2) ((PDI)-P-R = 2,6-(2,6-R-2-C6H3N=CMe)(2)C5H3N), with smaller 2,6-dialkyl substituents (R = Et, Me) or a 2-Pr-i substituent (2-iPrPDI)Fe(CH2SiMe3)(2) (2-iprTDI = 2,6-(2-Pr-i-C6H4N=CMe)(2)C5H3N, are effective precursors (0.5 mol %) for the anti-Markovnikov hydrosilylation of 1-octene with (Me3SiO)(2)MeSiH and (EtO)(3)SiH over the course of 1 h at 60 degrees C. No hydrosilylation activity was observed with Et3SiH. The most hindered member of the series, ((PDI)-P-iPr)Fe(CH2SiMe3)(2), and the pyridine bis(oxazoline) iron compound, (R,R)-((iPr)Pybox)Fe(CH2SiMe3)(2) ((iPr)Pybox = 2,6-bis[isopropyl-2-oxazolin-2-yl]pyridine), were inactive for the hydrosilylation of 1-octene with all tertiary silanes studied. By contrast, the terpyridine precursor, (terpy)Fe(CH2SiMe3)(2), reached >95% conversion at 60 degrees C with Et3SiH and (Me3SiO)(2)MeSiH. In addition, the hydrosilylation of vinylcyclohexene oxide was accomplished in the presence of 1.0 mol % (terpy)Fe(CH2SiMe3)(2), demonstrating functional group compatibility unique to this compound that is absent from bis(imino)pyridine iron compounds. The electronic structures of all three classes of iron dialkyl compounds have been evaluated by a combination of X-ray diffraction, magnetochemistry, Mossbauer spectroscopy, and density functional theory calculations. All of the compounds are best described as high-spin iron(III) compounds with antiferromagnetic coupling to chelate radical anions.