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Topology‐Selective Ullmann Coupling on Metal Surfaces by Precursor Design and Adsorbate‐Substrate Interaction: Towards the Control over Polymer versus Macrocycle Formation
Author(s) -
Fan Qitang,
Gottfried J. Michael
Publication year - 2019
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201900493
Subject(s) - trimer , radical , topology (electrical circuits) , substrate (aquarium) , chemistry , steric effects , polymerization , bent molecular geometry , coupling reaction , polymer , photochemistry , stereochemistry , polymer chemistry , dimer , catalysis , organic chemistry , oceanography , mathematics , combinatorics , geology
Abstract In polymerization reactions, controlling the formation of open‐chain versus cyclic product topologies is necessary because of the different properties of polymer chains and macrocycles. Here, we report a topology‐selective Ullmann coupling on metal surfaces with control of the ring/chain competition. The precursor employed is 4,4′′‐dibromo‐ ortho ‐terphenyl (DBOTP), which features a 60° bent feature and polymerizes into zigzag polyphenylene chains on both Au(111) and Ag(111) surface via Ullmann coupling. However, the cyclotrimerization of the precursor occurs only on Ag(111) but not Au(111). It is proposed that the cyclotrimerization reaction on Au(111) is blocked, because the necessary C−C coupling of two carbon radicals with different vertical heights is unfavored. Such height difference stems from the intrinsic steric repulsion between the two ortho ‐substituted phenyl groups. On Ag(111), the stronger adsorbate‐substrate interaction reduces the extent of the tilting of the phenyl group, resulting in a smaller height difference of the carbon radicals and consequently in the increased probability for the formation of the cyclic trimer.