Angewandte Chemie International Edition
10.1002/anie.201705018
COMMUNICATION
Therefore, a plot of lnk versus 1/T would yield the slope of the
linear relationship, -E/R, and E for each step. Similar method has
been applied to measure the migration barrier for diffusion.[36,37]
The rate constant k can be determined by monitoring the
unconverted molecule ratio of before and after heating the sample
in time t (SI Experimental methods: Dynamics measurements,
Figure S4). The experimental plots of lnk versus 1/T are given in
the inset of Figure 3.
formation is much higher than that for the CI generation. Thus the
Ag-COIs adopt pathway 2. Since no CIs are observed when the
Ag-COIs are assembled, the ratio of the pre-exponential factors
or rate constants could not be experimentally measured.
Nevertheless, the self-assembly strategy does exert a strong
influence in the pre-exponential factors for both reaction
pathways.
Moreover, the energy barrier, Eu2 (1.23 eV), was about 0.13 eV
According to Figure 3, the first energy barrier in pathway 1, Eu1
a
higher than E (1.10 eV), leading to that the coupling reaction to
(
u stands for unassembled), for the conversion of Ag-COI into CI
was 0.88 ± 0.17 eV, and the second one, Eu2, for the conversion
of CI into QP, 1.23 ± 0.32 eV. In pathway 2, the energy barrier, E
a stands for assembled), for the direct conversion of Ag-COI into
the final product from the sparsely dispersed Ag-COIs takes place
at a higher temperature, i.e. 430 K, than that, i.e. 410 K, from the
assembled Ag-COIs at full coverage.
a
(
To conclude, the above experimentally measured reaction
energetics and dynamics for the initially formed Ag-COIs in
different existence states, either assembled or unassembled, at
surfaces can well explain reaction pathways steered by the
surface assembly strategy. In particular, the activation
temperatures for the organometallic intermediates to QP
reactions on all three surfaces studied, namely, Ag(111), Cu(111)
and Cu(100), were at least 10 ~ 30 K lower starting from the
unassembled Ag-COIs than that from the assembled ones. Our
experimental results clearly demonstrate that the surface reaction
pathways can be efficiently steered by the self-assembly strategy
which may be further employed to tweak reactions in surface
chemistry.
QP was deducted to be 1.10 ± 0.23 eV. Based on the intercepts
of the plots, the pre-exponential factor, A, was calculated: Au1 = 2
8
-1
8
-1
×
10 s , Au2 = 5×10 s and A
a
= 2×1011 s-1.
Acknowledgements
This work was jointly supported by NSFC (91527303, 21333001,
21261130090) and MOST (2017YFA0204702, 2013CB933400),
China. XS is grateful for the financial support of the Thousand
Talent Program for Young Outstanding Scientists of the Chinese
government.
Figure 3. Experimentally measured potential energy diagram for the Ullmann
coupling reactions in different pathways. In pathway 1, the CI appears prior to
the QP formation, while in pathway 2, the coupling product QP is directly
generated from organometallic intermediate, resulting in self-assembly steered
reaction pathways.
Keywords: Self-assembly Strategy• Surface Reaction Dynamics
•
Ullmann coupling• Scanning Tunnelling Microscopy
A general picture for the formation of the coupling products has
been achieved by theoretical calculations that surface-bonded
aryl species diffuse on surface, collide and combine together at a
suitable temperature.[38,39] Our experimental identification of the
CIs indicated that the surface- anchored aryl species retained at
surfaces after dissociation of the Ag-COIs. A possible scenario for
both pathways is that after the dissociation of the Ag-COIs, the
formed surface-anchored aryl species diffuse at surface and
encounter others in two possible manners. When sparsely
dispersed, the Ag-COIs can diffuse freely at surface, and the
energy barrier for the formation of the CIs is 0.22 eV lower than
that for direct generation of QP, which makes the rate constant of
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