Stepwise on-surface dissymmetric reaction to construct binodal organometallic network

Dissymmetric reactions, which enable differentiated functionalization of equivalent sites within one molecule, have many potential applications in synthetic chemistry and materials science, but they are very challenging to achieve. Here, the dissymmetric reaction of 1,4-dibromo-2,5-diethynylbenzene (2Br-DEB) on Ag(111) is realized by using a stepwise activation strategy, leading to an ordered two-dimensional organometallic network containing both alkynyl–silver–alkynyl and alkynyl–silver–phenyl nodes. Scanning tunneling microscopy and density functional theory calculations are employed to explore the stepwise conversion of 2Br-DEB, which starts from the H-passivation of one Br-substituted site at 300 K in accompaniment with an intermolecular reaction to form one-dimensional organometallic chains containing alkynyl–silver–alkynyl nodes. Afterwards, the other equivalent Br-substituted site undergoes metalation reaction at 320–450 K, resulting in transformation of the chains into the binodal networks. These findings exemplify the achievement of the dissymmetric reaction and its practical application for controlled fabrications of complicated yet ordered nanostructures on a surface.


Optimized Models of Relevant Species and the C-Ag Bonding Energy in an ASA Dimer.
The geometries of three organometallic dimers (ASA, ASP and PSP) and three covalent dimers (AA, AP and PP) formed by 2Br-DEB in gas phase were optimized by density functional theory (DFT) 1,2 calculations with the Gaussain09 software 3 using B3LYP functional 4,5 . The 6-31g(d,p) basis set was employed for C and H atoms in all species, and LANL2DZ basis set with the corresponding effective core potential (ECP) 6-9 were used for Br and Ag atoms. The optimized models of these structures are shown in Supplementary Figure 1 with their dimensions marked. By using the same method, the geometry optimization and frequency calculations of a simplified ASA dimer formed by two alkynyl benzene radicals connected by one Ag atom were carried out. The energy of the C-Ag bond in the ASA dimer was calculated as below: where EASA dimer, EAg and Eradical correspondingly refer to the relaxed energy of the ASA dimer, silver atom and molecular radical. As a result, EC-Ag = -2.52 eV. All energies were reported by the addition of an unscaled zero-point energy (ZPE) correction.

Other Chemical Structures Proposed for the 2D Network.
In addition to the molecular structure of the hexagonal network shown in Fig. 5c Taking the length a of structure C-1 as an example, the intermolecular distance marked as a in C-1 is the distance between two molecules separated by two alkynyl-alkynyl (AA) connections.
As a result, a is calculated to be twice as the optimized intermolecular distance of an AA dimer, i.e., 0.95 nm, leading to a = 1.90 nm. The so-achieved calculated values of a, b, and c of the six proposed models are marked below each structure in Supplementary Figure 5c.
Comparisons between the calculated featured lengths of the proposed models and the measured values of the 2D network, as marked in Supplementary Figure 5a, indicate that none of the structures except OM-1 shows featured lengths that agree with the corresponding experimental measurements. Moreover, C-1 and C-2 display huge steric hinderance inside their triangular clusters due to the overlapped molecular groups, as highlighted by the red dashed circles in C-1 and C-2 models. As for structure OM-1, that is, the same as that illustrated by the model in Model for the 2D Structure with All Debromination Sites Involved in the PSP Nodes.
Supplementary Figure 9 presents the model of the 2D structure where all Br-substituted sites activated in the second step are involved in the PSP nodes. In this model, the ratio of the Ag atoms located at the hollow sites (light blue dots) to those at the bridge sites (dark blue dots) on Ag(111) is 1:2. This ratio in the hexagonal network becomes 2:1, as observed by STM. The occupancy of the hollow Ag atoms which are more stable in energy in the proposed structure is 33%, much lower than that (66%) in the experimentally observed 2D network, making the former less favorable in energy than the latter.