Alkene hydrosilylation, the addition of a silicon hydride (Si-H) across a carbon-carbon double bond, is one of the largest-scale industrial applications of homogeneous catalysis and is used in the ...commercial production of numerous consumer goods. For decades, precious metals, principally compounds of platinum and rhodium, have been used as catalysts for this reaction class. Despite their widespread application, limitations such as high and volatile catalyst costs and competing side reactions have persisted. Here, we report that well-characterized molecular iron coordination compounds promote the selective anti-Markovnikov addition of sterically hindered, tertiary silanes to alkenes under mild conditions. These Earth-abundant base-metal catalysts, coordinated by optimized bis(imino) pyridine ligands, show promise for industrial application.
A new spin on polymers: The title cations comprise low‐spin CoII centers with neutral bis(imino)pyridine chelating ligands. These complexes serve as single‐component ethylene polymerization catalysts ...(see scheme) and offer insight into the mechanism of chain growth and catalyst deactivation, which occurs by forming inactive cationic bis(imino)pyridine cobalt complexes with a diethyl ether ligand.
The electronic structure of the diamagnetic pyridine imine enamide cobalt dinitrogen complex, (iPrPIEA)CoN2 (iPrPIEA = 2-(2,6-iPr2–C6H3NCMe)-6-(2,6-iPr2–C6H3NCCH2)C5H3N), was determined and is ...best described as a low-spin cobalt(II) complex antiferromagnetically coupled to an imine radical anion. Addition of potential radical sources such as NO, PhSSPh, or Ph3Cl resulted in C–C coupling at the enamide positions to form bimetallic cobalt compounds. Treatment with the smaller halocarbon, PhCH2Cl, again induced C–C coupling to form a bimetallic bis(imino)pyridine cobalt chloride product but also yielded a monomeric cobalt chloride product where the benzyl group added to the enamide carbon. Similar cooperative metal–ligand addition was observed upon treatment of (iPrPIEA)CoN2 with CH2CHCH2Br, which resulted in allylation of the enamide carbon. Reduction of Coupled-(iPrPDI)CoCl (Coupled-(iPrPDI)CoCl = 2-(2,6-iPr2–C6H3NCMe)-C5H3N-6-(2,6-iPr2–C6H3NCCH2−)CoCl2) with NaBEt3H led to quantitative formation of (iPrPIEA)CoN2, demonstrating the reversibility of the C–C bond forming reactions. The electronic structures of each of the bimetallic cobalt products were also elucidated by a combination of experimental and computational methods.
A series of planar aryl-substituted bis(imino)pyridine cobalt azide complexes were prepared and evaluated as synthetic precursors for the corresponding cobalt nitrido compounds. Thermolysis or ...photolysis of two examples resulted in intramolecular C−H activation of the benzylic positions of the aryl substituents. For the mesityl-substituted compound, C−H activation by the putative nitride resulted in formation of a neutral imine ligand and modification of the chelate by hydrogen transfer to the imine carbon.
Sodium amalgam reduction of the aryl-substituted bis(imino)pyridine cobalt dihalide complexes (ArPDI)CoCl2 and (iPrBPDI)CoCl2 (ArPDI = 2,6-(2,6-R2-C6H3NCMe)2C5H3N (R = iPr, Et, Me); iPrBPDI = ...2,6-(2,6-iPr2-C6H3NCPh)2C5H3N) in the presence of an N2 atmosphere furnished the corresponding neutral cobalt dinitrogen complexes (ArPDI)CoN2 and (iPrBPDI)CoN2. Magnetic measurements on these compounds establish doublet ground states. Two examples, (iPrPDI)CoN2 and (iPrBPDI)CoN2, were characterized by X-ray diffraction and exhibit metrical parameters consistent with one-electron chelate reduction and a Co(I) oxidation state. Accordingly, the toluene solution EPR spectrum of (iPrPDI)CoN2 at 23 °C exhibits an isotropic signal with a g value of 2.003 and hyperfine coupling constant of 8 × 10−4 cm−1 to the I = 7/2 59Co center, suggesting a principally bis(imino)pyridine-based SOMO. Additional one-electron reduction of (iPrPDI)CoN2 was accomplished by treatment with NaC10H8 in THF and yielded the cobalt dinitrogen anion (iPrPDI)CoN2−. DFT calculations on the series of cationic, neutral, and anionic bis(imino)pyridine cobalt dinitrogen compounds establish Co(I) centers in each case and a chelate-centered reduction in each of the sequential one-electron reduction steps. Frequency calculations successfully reproduce the experimentally determined NN infrared stretching frequencies and validate the computational methods. The electronic structures of the reduced cobalt dinitrogen complexes are evaluated in the broader context of bis(imino)pyridine base metal chemistry and the influence of the metal d electron configuration on the preference for closed-shell versus triplet diradical dianions.
The aryl-substituted bis(imino)pyridine cobalt methyl complex, (MesPDI)CoCH3 (MesPDI = 2,6-(2,4,6-Me3C6H2-NCMe)2C5H3N), promotes the catalytic dehydrogenative silylation of linear α-olefins to ...selectively form the corresponding allylsilanes with commercially relevant tertiary silanes such as (Me3SiO)2MeSiH and (EtO)3SiH. Dehydrogenative silylation of internal olefins such as cis- and trans-4-octene also exclusively produces the allylsilane with the silicon located at the terminus of the hydrocarbon chain, resulting in a highly selective base-metal-catalyzed method for the remote functionalization of C–H bonds with retention of unsaturation. The cobalt-catalyzed reactions also enable inexpensive α-olefins to serve as functional equivalents of the more valuable α, ω-dienes and offer a unique method for the cross-linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes 2,1-insertion of the alkene followed by selective β-hydrogen elimination from the carbon distal from the large tertiary silyl group and accounts for the observed selectivity for allylsilane formation.
Aryl-substituted bis(imino)pyridine iron dinitrogen complexes are active for the hydrosilylation of 1,2,4-trivinylcyclohexane with tertiary alkoxy silanes, a process used in the manufacture of low ...rolling resistance tires. The iron compounds exhibit unprecedented selectivity for the monohydrosilylation of the desired 4-alkene that far exceeds results obtained with commercially used platinum compounds.
Iron dialkyl complexes, N3Fe(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-C6H3NCMe)2C5H3N), with smaller 2,6-dialkyl substituents (R = Et, Me) or a 2- i Pr substituent (2‑iPrPDI)Fe(CH2SiMe3)2 (2‑iPrPDI = 2,6-(2- i Pr-C6H4NCMe)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-bisisopropyl-2-oxazolin-2-ylpyridine), 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.
The bis(phosphino)pyridine (PNP) cobalt( i ) methyl complex, ( iPr PNP)CoCH 3 is a rich platform for the oxidative addition of non-polar reagents such as H 2 , the C–H bonds of arenes and terminal ...alkynes. Rare examples of hexacoordinate cobalt( iii ) compounds including a trihydride, a bis(acetylide) hydride and a trimethyl complex have been isolated and two examples structurally characterized. These findings demonstrate that when placed in an appropriately strong ligand field, two-electron oxidative addition chemistry is possible with first row transition metals.
Einkomponenten‐Katalysatoren für die Ethylenpolymerisation stellen die im Titel bezeichneten Kationen aus Low‐spin‐CoII‐Zentren und neutralen Bis(imino)pyridin‐Chelatliganden dar (siehe Schema). Ihre ...Untersuchung gibt Einblick in die Mechanismen von Kettenwachstum und Katalysatordesaktivierung, die eine Folge der Bildung inaktiver kationischer Bis(imino)pyridin‐Cobalt‐Komplexe mit einem Diethylether‐Ligand ist.
