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

Iron dialkyl complexes, [N3]­Fe­(CH2SiMe3)2, with three different classes of tridentate,
nitrogen-based “[N3]” 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)2Fe­(CH2SiMe3)2 by free terpyridine. The aryl-substituted bis­(imino)­pyridine compounds,
(RPDI)­Fe­(CH2SiMe3)2 (RPDI = 2,6-(2,6-R2-C6H3NCMe)2C5H3N), with smaller 2,6-dialkyl substituents
(R = Et, Me) or a 2-iPr substituent (2‑iPrPDI)­Fe­(CH2SiMe3)2 (2‑iPrPDI
= 2,6-(2-iPr-C6H4NCMe)2C5H3N, are effective
precursors (0.5 mol %) for the anti-Markovnikov hydrosilylation of
1-octene with (Me3SiO)2MeSiH and (EtO)3SiH over the course of 1 h at 60 °C. No hydrosilylation activity
was observed with Et3SiH. The most hindered member of the
series, (iPrPDI)­Fe­(CH2SiMe3)2, and the pyridine bis­(oxazoline) iron compound,
(R,R)-(iPrPybox)­Fe­(CH2SiMe3)2 (iPrPybox = 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 °C with Et3SiH and (Me3SiO)2MeSiH. 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, Mössbauer 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.