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

Abstract

Iron dialkyl complexes, [N 3 ]Fe(CH 2 SiMe 3 ) 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-(CH 2 SiMe 3 ) 2 , was prepared either via alkylation of (terpy)-FeCl 2 with LiCH 2 SiMe 3 or by pyridine displacement from (pyridine) 2 Fe(CH 2 SiMe 3 ) 2 by free terpyridine. The arylsubstituted bis(imino)pyridine compounds, ( R PDI)Fe(CH 2 SiMe 3 ) 2 ( R PDI = 2,6-(2,6-R 2 -C 6 H 3 NCMe) 2 C 5 H 3 N), with smaller 2,6-dialkyl substituents (R = Et, Me) or a 2-i Pr substituent ( 2-iPr PDI)Fe(CH 2 SiMe 3 ) 2 ( 2-iPr PDI = 2,6-(2-i Pr-C 6 H 4 N CMe) 2 C 5 H 3 N, are effective precursors (0.5 mol %) for the anti-Markovnikov hydrosilylation of 1-octene with (Me 3 SiO) 2 MeSiH and (EtO) 3 SiH over the course of 1 h at 60 °C. No hydrosilylation activity was observed with Et 3 SiH. The most hindered member of the series, ( iPr PDI)Fe(CH 2 SiMe 3 ) 2 , and the pyridine bis(oxazoline) iron compound, (R,R)-( iPr Pybox)Fe(CH 2 SiMe 3 ) 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(CH 2 SiMe 3 ) 2 , reached >95% conversion at 60 °C with Et 3 SiH and (Me 3 SiO) 2 MeSiH. In addition, the hydrosilylation of vinylcyclohexene oxide was accomplished in the presence of 1.0 mol % (terpy)Fe(CH 2 SiMe 3 ) 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.

Catalytic Hydrogenation Activity and Electronic Structure Determination of Bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes

“…Aryl-substituted bis(imino)pyridine iron dinitrogen complexes have proven to be effective pre-catalysts for alkene hydrogenation, 18,19 hydrosilylation, 18,20,21 cycloaddition 22,23,24 and hydroboration 25,26 reactions. In many cases, the activity and selectivity of these compounds rivals or surpasses established precious metal catalysts.…”

Computational insights into the mechanism of iron carbonyl-catalyzed ethylene hydrosilylation or dehydrogenative silylation

“…In this regard, noteworthy efforts have been devoted to the development of efficient and selective reaction of hydrosilanes to olefins employing a wide variety of transition metals [3][4][5][6][7][8][9][10]. Since iron is much less expensive and environmentally friendly compared to other transition metals like Rh, Pt and Ru [11], iron-based catalysts for hydrosilylation have received considerable interests [12][13][14][15][16][17][18][19].…”

Iron‐Catalysed Chemo‐, Regio‐, and Stereoselective Hydrosilylation of Alkenes and Alkynes using a Bench‐Stable Iron(II) Pre‐Catalyst

“…[6] To date, hydrosilylation has been dominated by precious metal catalysts such as platinum, palladium, ruthenium and rhodium, however these catalysts can suffer from competitive alkene isomerisation, dehydrosilylation reactions, and incompatibility with amino-substituted olefins. [10] Chirik has reported that bisA C H T U N G T R E N N U N G (imino)pyridine iron(0) bis(dinitrogen) complexes, [11] and terpyridine and bis-A C H T U N G T R E N N U N G (imino)pyridine iron(II) dialkyl complexes, [12] are precatalysts for the hydrosilylation of alkenes and alkynes under mild conditions. Despite the high activity, only limited functional group tolerance has been disclosed, and the pre-catalysts are air-and moisturesensitive.…”