Identifying Reactive Intermediates in the Ullmann Coupling Reaction by Scanning Tunneling Microscopy and Spectroscopy

Abstract

We present an atomic-scale study of substituent effects in the Ullmann coupling reaction on Cu{111} using low-temperature scanning tunneling microscopy and spectroscopy. We have observed fluorophenyl intermediates and phenyl intermediates as well as biphenyl products on Cu{111} after exposure to 4-fluoro-1-bromobenzene (p-FC(6)H(4)Br) and bromobenzene (C(6)H(5)Br), respectively. When p-FC(6)H(4)Br dissociatively chemisorbs at 298 K on Cu{111}, the relatively weakly bound Br dissociates, and fluorophenyl intermediates are formed. These intermediates couple to form 4,4'-difluorobiphenyl and desorb at temperatures below 370 K. However, by cooling the substrate to low temperature (4 K), we have observed unreacted fluorophenyl intermediates distributed randomly on terraces and at step edges of the Cu{111} surface. Alternatively, at similar coverages of C(6)H(5)Br, we have observed biphenyl distributed on terraces and step edges. In each case, Br adatoms were randomly distributed on the surface. Chemical identification of fluorophenyl and phenyl intermediates and biphenyl products was achieved by vibrational spectroscopy via inelastic tunneling spectroscopy. The strongest vibrational mode in the phenyl species disappears when the tilted intermediates couple to form biphenyl products. We infer that the surface normal component of the dipole moment is important in determining the transition strength in inelastic electron tunneling spectroscopy.

Assembling molecular Sierpiński triangle fractals

“…After deposition, the sample was instantaneously cooled by liquid helium or liquid nitrogen to avoid cleavage of the C-Br bond, which may take place at room temperature 30,31 . STM measurements were performed at 78 K, 63 K or 4.4 K with a Unisoku low-temperature STM (USM-1200S) housed in an ultrahigh vacuum chamber with a base pressure of <2 × 10 −10 mbar.…”

Electrons, Photons, and Force: Quantitative Single-Molecule Measurements from Physics to Biology

“…178,179 Selection rules for single-molecule IETS are not completely understood but appear to involve a propensity for longitudinal modes normal to the surface. 176,180 …”

“…Single-molecule tunneling spectroscopy at low temperatures allows identification of Br and fluorophenyl (F−Ph) intermediates in an Ullman coupling reaction on a Cu{111} surface and measurement of the C−H out-of-plane bending mode in the fluorophenyl group. Adapted from ref (180). …”

Oberflächengestützte Synthese von Hyperbenzol‐Nanogehegen

“…Die Synthese von kleinen Molekülen auf Oberflächen mit der Ullmann-Reaktion ist mçglich, wie anhand von Biphenyl gezeigt werden konnte. [22][23][24] Die einzelnen Reaktionsschritte kçnnen dabei sogar mit einem Rastertunnelmikroskop (RTM) kontrolliert werden. [25] Versuche, große cyclische Moleküle auf Oberflä-chen mithilfe der Ullmann-Reaktion zu synthetisieren, waren bisher jedoch nicht erfolgreich; [26] lediglich Sexiphenylen wurde als Nebenprodukt bei der Synthese von Oligophenylenketten beobachtet.…”