Exploiting Cooperative Catalysis for the On‐Surface Synthesis of Linear Heteroaromatic Polymers via Selective C–H Activation

Abstract

Regiospecific CÀ H activation is a promising approach to achieve extended polymers with tailored structures. While a recent on-surface synthetic approach has enabled regioselective homocoupling of heteroaromatic molecules, only small oligomers have been achieved. Herein, selective CÀ H activation for dehydrogenative CÀ C couplings of hexaazatriphenylene by Scholl reaction is reported for the first time. By combining low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM), we revealed the formation of one-dimensional polymers with a double-chain structure. The details of the growth process are rationalized by density functional theory (DFT) calculations, pointing out a cooperative catalytic action of Na and Ag adatoms in steering the CÀ H selectivity for the polymerization.

Selective Activation of Aromatic C–H Bonds Catalyzed by Single Gold Atoms at Room Temperature

“…The organometallic DCA–Au–DCA structure is qualitatively different from previously observed 2D MOFs consisting of DCA molecules coordinated with metal atoms via their cyano groups. − Moreover, the formation of the organometallic DCA–Au–DCA motif requires highly regioselective cleavage of the position A aromatic C–H bonds to enable the C–Au–C covalent bonding. Such C–H bonds have large dissociation enthalpies, ,, Δ H , meaning their cleavage seldom occurs at RT, and they often suffer from poor regioselectivity. ,,,, For a gas-phase DCA molecule for instance, we calculated Δ H values of 112.1 and 113.2 kcal/mol (4.86 and 4.91 eV) via DFT for the aromatic C–H bonds at positions A and B, respectively (see Figure a and SI Figure S7).…”

“…52−58 Moreover, the formation of the organometallic DCA−Au−DCA motif requires highly regioselective cleavage of the position A aromatic C−H bonds to enable the C−Au−C covalent bonding. Such C−H bonds have large dissociation enthalpies, 60,68,69 ΔH, meaning their cleavage seldom occurs at RT, 70 and they often suffer from poor regioselectivity. 25,38,60,68,69 For a gas-phase DCA molecule for instance, we calculated ΔH values of 112.1 and 113.2 kcal/mol (4.86 and 4.91 eV) via DFT for the aromatic C−H bonds at positions A and B, respectively (see Figure 1a and SI Figure S7).…”

“…We claim that this significant difference between positions A and B explains the experimentally observed regioselectivity of the C−H dissociation. The 19 kcal/mol barrier represents a significant reduction relative to comparable C−H activation reactions with barriers of 30−40 kcal/mol (1.3−1.8 eV), experimentally occurring in a temperature window of 400−700 K. 60,68,69,72 Furthermore, other onsurface reactions with barriers of ∼20 kcal/mol (∼0.87 eV) have been observed at RT, 71,73 supporting the interpretation of our experimental results as RT C−H activation.…”

Unusual Scaffold Rearrangement in Polyaromatic Hydrocarbons Driven by Concerted Action of Single Gold Atoms on a Gold Surface

“…In such QM/MM calculations, the molecule and Au adatoms were involved in the QM region, while the underlying surface was treated in the MM region. Previous studies [15,51] have shown that this QM/MM approach provides a reasonably accurate description of the chemical reactions involving adatoms.…”

Multiple molecular interactions between alkyl groups and dissociated bromine atoms on Ag(111)

“…Understanding the behaviour of the atomic halogen atoms in the molecular self-assembly may be beneficial to control the reaction pathway since a target coupling reaction can be induced with the reactants aligned by the halogen adatoms. 41,42 To the best of our knowledge, the condensation by the co-existing alkyl-alkyl interaction [43][44][45][46][47] and hydrogen bonding with dissociated bromine atoms 30,36 on the surface has not yet been investigated in detail under UHV at low temperature.…”