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Isomerism in Complexes
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric isomers, in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH3)4Cl2]+ ion has two isomers (Figure 1) In the cis...
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If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
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Isomerism in Complexes
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Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
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Updated: Dec 6, 2025

Construction and Systematical Symmetric Studies of a Series of Supramolecular Clusters with Binary or Ternary Ammonium Triphenylacetates
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Ethylene Tetramerisation: A Structure-Selectivity Correlation.

Boitumelo F Makume1,2, Cedric W Holzapfel1, Munaka C Maumela1,2

  • 1Department of Chemical Sciences, University of Johannesburg, Kingsway Campus, Aucklandpark, 2006, Johannesburg, South Africa.

Chempluschem
|October 13, 2020
PubMed
Summary

Ligand structure significantly impacts ethylene oligomerization by-product formation. Increasing steric bulk on PNP ligands enhances 1-octene selectivity and catalyst activity while reducing unwanted by-products.

Keywords:
ethylene oligomerisationligandsreaction mechanismsselectivitystructure-activity relationships

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Area of Science:

  • Organometallic Chemistry
  • Catalysis
  • Polymer Science

Background:

  • Ethylene tetramerization to 1-octene is a key industrial process.
  • Understanding by-product formation in ethylene oligomerization is crucial for catalyst optimization.
  • The influence of ligand structure on selectivity is well-established, but by-product formation requires further investigation.

Purpose of the Study:

  • To correlate various PNP ligand structures with full product selectivity in ethylene oligomerization.
  • To investigate the impact of ligand steric bulk on catalyst activity and by-product formation.
  • To elucidate the mechanistic basis for ligand-controlled selectivity changes.

Main Methods:

  • Synthesis and characterization of a range of PNP ligands with varying N-substituent steric bulk.
  • Ethylene oligomerization experiments using these ligands with a suitable metal precursor.
  • Analysis of product selectivity (1-octene, C6 cyclics, C16+ by-products, 1-hexene, C10-14 fractions) using GC.
  • Mechanistic studies to explain observed selectivity trends.

Main Results:

  • Increasing steric bulk on PNP ligands shifts selectivity away from C6 cyclic and C16+ by-products, favoring 1-octene (up to ~70%).
  • Ortho-phenyl substitution on PNP ligands also induces similar selectivity shifts.
  • Increased ligand steric bulk enhances catalyst activity and reduces polymer formation by an order of magnitude.

Conclusions:

  • Ligand steric bulk is a critical factor in controlling selectivity and activity in ethylene oligomerization.
  • Steric bulk likely promotes the formation of active cationic catalytic species.
  • This study provides mechanistic insights into ligand design for selective ethylene oligomerization.