Organic compounds of sulphur that contain the group -S- linked to two hydrocarbons are known as thioethers. This alternative name to sulphides comes from the fact that they are analogues of ethers in which the oxygen is replaced by a sulphur; thioethers are generally more reactive than ethers, though. The formation of thioether structures increases the mechanical strength, improves energy storage and charge conductance, favours light absorption, enables tunable electroluminescence emitters, and favours (bio)-catalytic activities, such as those observed in enzymes or proteins and macro-cycles. Sulphur-containing polycyclic aromatic hydrocarbons such as thioethers are thus fundamental for a variety of applications such as nanoelectronics, optoelectronics, catalysis, biology, or pharmaceutics. They are typically obtained by the addition of unsaturated compounds that form a cyclic adduct, a reaction known as cycloaddition.
The metal-catalyzed cycloaddition of S-rich molecules faces, however, greater difficulties than other reactions. The strength and coordination of this chalcogen atom with traditional metal catalysts, such as Au, Ni, or Pd, easily induce its detachment from the organic structure, eventually poisoning the catalyst surface. Consequently, the on-surface synthesis of such polymers is usually limited to the C−C coupling of prefunctionalized molecular aryls containing O, N, or S atoms, while the direct involvement of chalcogen atoms in intermolecular covalent bonds remains challenging.
Now, a team of researchers shows 1 that the synthesis of thioether polymers by −C−S− intermolecular coupling can be successfully achieved on the surface of Au(111), despite the affinity of sulphur with gold. The team combined experimental and theoretical methodology allowed the identification of four reaction steps through which both terminal groups of the pristine molecules must undergo to polymerize.
Using scanning probe techniques at low temperatures and density functional theory (DFT) calculations, the researchers describe the formation of thioether bonds by characterizing the thermal activation of functional groups of 4′-bromo-4-mercaptobi-phenyl (Br-MBP) molecules adsorbed on this metal surface.
The double functionalization of the Br-MBP precursors with a Br and a sulfhydryl terminal group is likely fundamental for mastering the described difficulties in the C−S etherification. On its own, each of these two molecular terminations follows two of the most extensively characterized reaction schemes that lead to strong chemical bonding. Their simultaneous presence on the same reaction template, however, unexpectedly modifies these paths.
The researchers find that, in order to form the thioether polymer and to overcome the competitive formation of C−C bonds, two reaction steps, the dehalogenation, and dissociation of the S−Au bond, must occur simultaneously. Thus, the proximity of both molecular ending groups and likely the simultaneity of two of the four identified reaction steps are at the origin of this modified reaction mechanism, as the team speculatively hypothesized.
Clearly, the Br-MBP molecules′ thioetherification requires the dissociation of the strong Au-sulfate bond, which is not achieved by thermal annealing alone. Similarly, the extensively characterized C−C coupling that is frequently reported upon the annealing of Br-functional groups is outweighed by the exclusive synthesis of C−S bonds.
The team details the electronic properties of the phenyl−sulphur bond and the polymer as a function of the ligand length. Altogether, these findings provide new insight into the synthesis of these structures, whose electronic properties can be tuned based on their ligand’s length.
Author: César Tomé López is a science writer and the editor of Mapping Ignorance
Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.