Bis(imino)pyridine iron alkyls containing β-hydrogens: Synthesis, evaluation of kinetic stability, and decomposition pathways involving chelate participation

Ryan Trovitch, Emil Lobkovsky, Paul J. Chirik

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Abstract

Bis(imino)pyridine iron alkyl complexes bearing β-hydrogens, ( iPrPDI)FeR ((iPrPDI = 2,6-(2,6-iPr 2-C6H3N=CMe)2C5H 3N; R = Et, nBu, iBu, CH2 cycloC5H9; 1-R), were synthesized either by direct alkylation of (iPrPDI)FeCl (1-Cl) with the appropriate Grignard reagent or more typically by oxidative addition of the appropriate alkyl bromide to the iron bis(dinitrogen) complex, (iPrPDI)Fe(N 2)2 (1-(N2)2). In the latter method, the formal oxidative addition reaction produced (iPrPDI)FeBr (1-Br), along with the desired iron alkyl, 1-R. Elucidation of the electronic structure of 1-Br and related 1-R derivatives by magnetic measurements, structural studies and NMR spectroscopy established high spin ferrous compounds antiferromagnetically coupled to chelate radical anions. Thus, the formal oxidative process is bis(imino)pyridine ligand-based (one electron is formally removed from each chelate, not the iron) during oxidative addition. The kinetic stability of each 1-R compound was assayed in benzene-d6 solution and found to produce a mixture of the corresponding alkane and alkene. The kinetic stability of the iron alkyl complexes was inversely correlated with the number of β-hydrogens present. For example, the iron ethyl complex, 1-Et, underwent clean loss of ethane over the course of three hours, whereas the corresponding 1-iBu compound had a half-life of over 12 h under identical conditions. The mechanism of the decomposition was studied with a series of deuterium labeling experiments and support a pathway involving initial β-hydrogen elimination followed by cyclometalation of an isopropyl methyl group, demonstrating an overall transfer hydrogenation pathway. The relevance of such pathways to chain transfer in bis(imino)pyridine iron catalyzed olefin polymerization reactions is also presented.

Original languageEnglish
Pages (from-to)11631-11640
Number of pages10
JournalJournal of the American Chemical Society
Volume130
Issue number35
DOIs
Publication statusPublished - Sep 3 2008

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Pyridine
Hydrogen
Iron
Decomposition
Kinetics
Alkenes
Ferrous Compounds
Olefins
Iron Chelating Agents
Bearings (structural)
Alkanes
Ethane
Hydrogenation
Deuterium
Alkylation
Addition reactions
Benzene
Bromides
Polymerization
Magnetic variables measurement

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

@article{d2d8ee2666384ccfb60edc796462e7df,
title = "Bis(imino)pyridine iron alkyls containing β-hydrogens: Synthesis, evaluation of kinetic stability, and decomposition pathways involving chelate participation",
abstract = "Bis(imino)pyridine iron alkyl complexes bearing β-hydrogens, ( iPrPDI)FeR ((iPrPDI = 2,6-(2,6-iPr 2-C6H3N=CMe)2C5H 3N; R = Et, nBu, iBu, CH2 cycloC5H9; 1-R), were synthesized either by direct alkylation of (iPrPDI)FeCl (1-Cl) with the appropriate Grignard reagent or more typically by oxidative addition of the appropriate alkyl bromide to the iron bis(dinitrogen) complex, (iPrPDI)Fe(N 2)2 (1-(N2)2). In the latter method, the formal oxidative addition reaction produced (iPrPDI)FeBr (1-Br), along with the desired iron alkyl, 1-R. Elucidation of the electronic structure of 1-Br and related 1-R derivatives by magnetic measurements, structural studies and NMR spectroscopy established high spin ferrous compounds antiferromagnetically coupled to chelate radical anions. Thus, the formal oxidative process is bis(imino)pyridine ligand-based (one electron is formally removed from each chelate, not the iron) during oxidative addition. The kinetic stability of each 1-R compound was assayed in benzene-d6 solution and found to produce a mixture of the corresponding alkane and alkene. The kinetic stability of the iron alkyl complexes was inversely correlated with the number of β-hydrogens present. For example, the iron ethyl complex, 1-Et, underwent clean loss of ethane over the course of three hours, whereas the corresponding 1-iBu compound had a half-life of over 12 h under identical conditions. The mechanism of the decomposition was studied with a series of deuterium labeling experiments and support a pathway involving initial β-hydrogen elimination followed by cyclometalation of an isopropyl methyl group, demonstrating an overall transfer hydrogenation pathway. The relevance of such pathways to chain transfer in bis(imino)pyridine iron catalyzed olefin polymerization reactions is also presented.",
author = "Ryan Trovitch and Emil Lobkovsky and Chirik, {Paul J.}",
year = "2008",
month = "9",
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doi = "10.1021/ja803296f",
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journal = "Journal of the American Chemical Society",
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T1 - Bis(imino)pyridine iron alkyls containing β-hydrogens

T2 - Synthesis, evaluation of kinetic stability, and decomposition pathways involving chelate participation

AU - Trovitch, Ryan

AU - Lobkovsky, Emil

AU - Chirik, Paul J.

PY - 2008/9/3

Y1 - 2008/9/3

N2 - Bis(imino)pyridine iron alkyl complexes bearing β-hydrogens, ( iPrPDI)FeR ((iPrPDI = 2,6-(2,6-iPr 2-C6H3N=CMe)2C5H 3N; R = Et, nBu, iBu, CH2 cycloC5H9; 1-R), were synthesized either by direct alkylation of (iPrPDI)FeCl (1-Cl) with the appropriate Grignard reagent or more typically by oxidative addition of the appropriate alkyl bromide to the iron bis(dinitrogen) complex, (iPrPDI)Fe(N 2)2 (1-(N2)2). In the latter method, the formal oxidative addition reaction produced (iPrPDI)FeBr (1-Br), along with the desired iron alkyl, 1-R. Elucidation of the electronic structure of 1-Br and related 1-R derivatives by magnetic measurements, structural studies and NMR spectroscopy established high spin ferrous compounds antiferromagnetically coupled to chelate radical anions. Thus, the formal oxidative process is bis(imino)pyridine ligand-based (one electron is formally removed from each chelate, not the iron) during oxidative addition. The kinetic stability of each 1-R compound was assayed in benzene-d6 solution and found to produce a mixture of the corresponding alkane and alkene. The kinetic stability of the iron alkyl complexes was inversely correlated with the number of β-hydrogens present. For example, the iron ethyl complex, 1-Et, underwent clean loss of ethane over the course of three hours, whereas the corresponding 1-iBu compound had a half-life of over 12 h under identical conditions. The mechanism of the decomposition was studied with a series of deuterium labeling experiments and support a pathway involving initial β-hydrogen elimination followed by cyclometalation of an isopropyl methyl group, demonstrating an overall transfer hydrogenation pathway. The relevance of such pathways to chain transfer in bis(imino)pyridine iron catalyzed olefin polymerization reactions is also presented.

AB - Bis(imino)pyridine iron alkyl complexes bearing β-hydrogens, ( iPrPDI)FeR ((iPrPDI = 2,6-(2,6-iPr 2-C6H3N=CMe)2C5H 3N; R = Et, nBu, iBu, CH2 cycloC5H9; 1-R), were synthesized either by direct alkylation of (iPrPDI)FeCl (1-Cl) with the appropriate Grignard reagent or more typically by oxidative addition of the appropriate alkyl bromide to the iron bis(dinitrogen) complex, (iPrPDI)Fe(N 2)2 (1-(N2)2). In the latter method, the formal oxidative addition reaction produced (iPrPDI)FeBr (1-Br), along with the desired iron alkyl, 1-R. Elucidation of the electronic structure of 1-Br and related 1-R derivatives by magnetic measurements, structural studies and NMR spectroscopy established high spin ferrous compounds antiferromagnetically coupled to chelate radical anions. Thus, the formal oxidative process is bis(imino)pyridine ligand-based (one electron is formally removed from each chelate, not the iron) during oxidative addition. The kinetic stability of each 1-R compound was assayed in benzene-d6 solution and found to produce a mixture of the corresponding alkane and alkene. The kinetic stability of the iron alkyl complexes was inversely correlated with the number of β-hydrogens present. For example, the iron ethyl complex, 1-Et, underwent clean loss of ethane over the course of three hours, whereas the corresponding 1-iBu compound had a half-life of over 12 h under identical conditions. The mechanism of the decomposition was studied with a series of deuterium labeling experiments and support a pathway involving initial β-hydrogen elimination followed by cyclometalation of an isopropyl methyl group, demonstrating an overall transfer hydrogenation pathway. The relevance of such pathways to chain transfer in bis(imino)pyridine iron catalyzed olefin polymerization reactions is also presented.

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