Reaction of the ruthenium allenylidene complex [Ru]dCdCdCPh 2 (1, [Ru] = Cp(PPh 3 ) 2 Ru) with the propargylic Grignard reagent R-CtCCH 2 MgBr (R = CH 3 , CH 2 CH 3 , Ph) yielded a mixture of two acetylide complexes. The major products, [Ru]CtCCPh 2 C(R)dCdCH 2 (2a, R = CH 3 ; 2b, R = CH 2 CH 3 ; 2c, R = Ph), have an allenyne moiety, and the minor ones, [Ru]CtCCPh 2 CH 2 CtCR (3a, R = CH 3 ; 3b, R = CH 2 CH 3 ; 3c, R = Ph), have a diyne ligand. The reaction of similar propargyl Grignard reagent HCtCCH 2 MgBr with 1 yielded only the diyne complex 3d. Treatment of complexes 2a-2c with HBF 4 afforded the cyclization complexes 5a-5c, respectively, proceeding via a vinylidene intermediate. The cyclization of the allenyl and the vinylidene groups is accompanied with a phenyl group migration. Complex 5b is fully characterized by a single-crystal X-ray diffraction analysis. Similar cyclization of complexes 2a and 2b, catalyzed by a Au phosphine complex, gave the ruthenium vinylidene complexes 6a and 6b, respectively, with different selectivity from that of the protonation reaction. Au-catalyzed cyclization of the diyne complex 3d yielded 6d, which is fully characterized by a single-crystal X-ray diffraction analysis.

Self-assembly of benzenecarboxylic acids on well-defined noble metals has been intensively investigated using surface-sensitive techniques. However, most studies were focused on the formation of nanostructures via benzenetricarboxylic and benzenedicarboxylic acids, which are composed of multiple carboxylic acid functional groups in either the meta or para positions of the benzene ring, allowing the formation of long-range ordered molecular arrays through −COOH-mediated intermolecular hydrogen bonds. Two-dimensional nanostructures of benzoic acid molecules that are composed of a single carboxylic acid functional group on the phenyl ring at metal–electrolyte interfaces were rarely reported using scanning tunneling microscopy (STM) because there is only one carboxylic acid functional group for each benzoic acid available to form intermolecular hydrogen bonds, making it difficult to construct long-range ordered nanoarchitectures. In this work, we employed electrochemical scanning tunneling microscopy (EC-STM) in combination with electrochemical cyclic voltammetry (CV) techniques to explore the adsorption and phase formation of benzoic acids (BZAs) at Au(111)/electrolyte interfaces. Our experiments show how the electrolyte, molecular concentration, electrochemical potential, and co-adsorption of aqueous ions affect the adsorption and self-assembly of BZA molecules. It is found that the BZA molecules are not assembled into long-range ordered structures in the presence of a sulfuric acid electrolyte due to the strong competing co-adsorption of sulfate ions on a gold electrode. BZA molecules can form flat-oriented ordered adlayers in a perchloric acid electrolyte (containing weakly adsorbed ClO4 – ions) at a negatively charged surface only when the concentration of the molecular solution reaches above 6 mM. Below 6 mM, the CVs of BZA on Au(111) in 0.1 M HClO4 show only one pair of adsorption/desorption peaks. When the BZA concentration increases to 6 mM, the voltammogram exhibits three pairs of peaks, corresponding to the structural transformation of disordered phase [phase I, E sample (E S): −0.600 to −0.190 V], linear stripe pattern (phase II, E S: −0.190 to 0.108 V), zigzag pattern (phase III, E S: −0.108 to −0.066 V), and upright packing pattern (phase IV, E S: −0.066 to 0.300 V). These phases and molecular adlayers were revealed by STM in the four electrochemical potential regions. Effect of parameters (electrolyte ions, concentration, and electrochemical potential) explored in this study will provide valuable information for the formation of molecular adlayers, adsorption and self-assembly, materials, corrosion inhibition, and molecular devices.

Self-assembly provides unique routes to create supramolecular nanostructures at well-defined surfaces. In the present work, we employed scanning tunneling microscopy (STM) in combination with electrochemical techniques to explore the adsorption and phase formation of a series of aromatic carboxylic acids (ACAs) at Au(111)/0.1 M HClO4. Specific goals are to elucidate the roles of electrochemical potential and directional hydrogen-bonding on the structures and orientation of individual ACAs that form nanoarchitectures. ACAs are prototype materials for supramolecular self-assemblies via stereospecific hydrogen bonds between neighboring molecules. In this study, we mainly focus on a special ACA, terephthalic acid (TPA), which is almost insoluble in water, making the assembly of this molecule from aqueous solution challenging. Depending on the applied electric field, TPA molecules form distinctly different, highly ordered adlayers on Au(111) triggered by directional intermolecular hydrogen bonds. At low electrochemical potentials, TPA molecules are planar oriented, forming a potentially infinite hydrogen-bonded adlayer without any observed domain boundaries. The increase of the electrode potential triggers the deprotonation of one carboxylic acid functional group of TPA; additionally, this is accompanied by an orientation change of molecules from planar to perpendicular. In contrast, structural “defects” and multiple domain boundaries were found at this positively charged surface. The assembled nanostructures of TPA are compared with other ACAs (trimesic acid, benzoic acid, and isophthalic acid), and corresponding adsorption models were built for all molecular adlayers, showing that intermolecular hydrogen-bonding plays a determining role in the formation of two-dimensional ACA nanostructures.

We report a unique feature in an adsorption and molecular assembly discovered by classic electrochemical cyclic voltammetry (CV) and electrochemical scanning tunneling microscopy (EC-STM) techniques. With its aromatic carboxylic acid (ACA) composed of one phenyl ring and a single −COOH, benzoic acid (BZA) behaves in a drastically different manner than other ACAs. In this work, we systematically varied the BZA concentration and found that the number of current peaks in cyclic voltammograms (CVs) started to change from one to three at a concentration that we refer to as "critical phasetransition concentration (CPC)". Below the CPC, no ordered adlayer can be formed at a negatively charged Au(111) surface. Further, we discovered that the peak position in the CVs shifted as a function of solution concentration, resulting in larger peak separations between either anodic or cathodic peaks as concentration increased. The peak shifting and evolution are attributed to the nature of the BZA structure and the concentration-dependent assembly due to the lack of intermolecular H-bonds formed by the −COOH functional group in the BZAs, evidenced by high-resolution STM images and interpreted by the proposed adsorption models.

The effects of temperature and molecular concentration on the ordering of two-dimensional (2D) nanostructures have been investigated at the well-defined Au(111)–electrolyte interface. In comparison to the assembly of thiolated alkanes or hydrogen-bonded nonthiolated molecules, fabricating large aromatic thiolated molecules into a highly ordered adlayer on a surface remained a challenge. In this study, we demonstrated the importance of controlling the assembly conditions and procedures for the formation of ordered adlayers of redox-active viologen derivatives. The assembly conditions that were explored include the variation of molar concentration of assembly solutions, assembly time, and thermal annealing. We report that the optimal assembly conditions for creating highly ordered thiolated viologen derivatives on a Au(111)-(1 × 1) electrode surface are to limit the time in which the electrode is immersed in a deoxygenated 0.05 mM ethanolic viologen solution (preheated to 70 °C) to 45 s, followed by thermal annealing in absolute ethanol for 12 h. Highly ordered molecular adlayers were imaged by electrochemical scanning tunneling microscopy (STM), revealing the molecular packing of low-coverage adlayers. Furthermore, in situ STM combined with cyclic voltammetry (CV) allowed for the exploration of the structural transformation and potential limit of reductive and “oxidative” desorption of adlayers within the electrochemical potential range of the sample potential (E S) from −0.95 V to −0.10 V vs SCE.