Electrochemical reduction of N,N'-thiobisphthalimide and N,N'-dithiobisphthalimide: ejection of diatomic sulfur through an autocatalytic mechanism.

The electrochemical reduction of N,N'-dithiobisphthalimide and N,N'-thiobisphthalimide is investigated using electrochemical techniques and theoretical calculations. The results are rationalized using adequate electron transfer theories. The reduction leads to the ejection of diatomic sulfur and involves an interesting autocatalytic mechanism. This mechanism is dependent on the concentration of the initial compound and the cyclic voltammetric scan rate. The starting material is reduced both at the electrode and through homogeneous electron transfer from the produced sulfur. The initial electron transfer follows a stepwise mechanism involving the formation of the corresponding radical anion. This is supported by both the electrochemical data and the theoretical calculation results. The radical anion of the N,N'-dithiobisphthalimide dissociates through cleavage of the N-S chemical bond and not the S-S chemical bond. Application of the extension of the dissociative electron transfer theory to the dissociation of radical anions shows that the N-S chemical bond dissociates despite being stronger than the S-S chemical bond. This is due to the large difference in the oxidation potentials of the two potential anions (the phthalimidyl anion and the phthalimidyl thiyl anion). The electrochemical reduction of N,N'-thiobisphthalimide involves the intermediate formation of N,N'-dithiobisphthalimide and hence the autocatalytic process is less efficient.

The intriguing reaction of aromatic sulfonyl phthalimides with gold surfaces.

We report on a series of arene sulfonyl phthalimides which were prepared and used to modify polycrystalline gold and Au(111) gold surfaces. Three investigated compounds are the p-iodo-, the p-methoxy-, and the p-fluoro-benzenesulfonyl phthalimides. X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and scanning tunneling microscopy (STM) studies were used to characterize the modified surfaces. The XPS data show that all three investigated compounds decompose on gold surfaces. The decomposition leads to the adsorption of sulfur and ejection of the other groups except for the p-iodo compound, which also leads to the deposition of iodine. The cyclic voltammetry data confirm these results and show that high coverage values of deposited sulfur are obtained. High-resolution STM imaging showed a dynamic behaviour of sulfur on gold for all compounds. Movement of sulfur species on the Au(111) surface is observed. Various phases including a new 'zig-zag' pattern and a new 2 : 1 line pattern are presented. Sequential STM imaging also showed movement of one area of sulfur while another remains static. These results are important because (i) they provide direct experimental evidence that these hexavalent sulfur compounds react with gold surfaces breaking all sulfur chemical bonds, (ii) they show that sulfonyl phthalimides can be used as efficient precursors for the deposition of sulfur on gold, and (iii) very importantly they show the adlayer nature of the sulfur modified gold surface which has been a heavily debated question.

New insights into sulfur deposition on gold using dithiobisphthalimide as a new precursor.

Dithiobisphthalimide is used as a new precursor for the spontaneous deposition of sulfur on gold surfaces in acetonitrile. Characterization of the modified surfaces is achieved using X-ray photoelectron spectroscopy (XPS), electrochemistry and scanning tunneling microscopy (STM). The reported results indicate that the sulfur deposition is an efficient and fast process and that high coverages can be reached very quickly. Sequential high-resolution STM in air allows the direct observation, for the first time, of the mobility of the usually observed rectangular structures as individual units. It also shows the reversible association/dissociation of these rectangles. The nature of these structures is highly debated in the literature and the present work provides new insights into their nature through the use of a new sulfur precursor under non-traditional conditions. To explain our results we consider these structures as simple sulfur adlayers on the gold surface.

Application of the dissociative electron transfer theory and its extension to the case of in-cage interactions in the electrochemical reduction of arene sulfonyl chlorides.

Important aspects of the electrochemical reduction of a series of substituted arene sulfonyl chlorides are investigated. An interesting autocatalytic mechanism is encountered where the starting material is reduced both at the electrode and through homogeneous electron transfer from the resulting sulfinate anion. This is due to the homogenous electron transfer from the two-electron reduction produced anion (arene sulfinate) to the parent arene sulfonyl chloride. As a result, the reduction process and hence the generated final products depend on both the concentration of the substrate and the scan rate. A change is also observed in the reductive cleavage mechanism as a function of the substituent on the phenyl ring of the arene sulfonyl chloride. With 4-cyano and 4-nitrophenyl sulfonyl chlorides a "sticky" dissociative ET mechanism takes place where a concerted ET mechanism leads to the formation of a radical/anion cluster before decomposition. With other substituents (MeO, Me, H, Cl, and F) a "classical" dissociative ET is followed, where the ET and bond cleavage are simultaneous. The dissociative electron transfer theory, as well as its extension to the case of strong in-cage interactions between the produced fragments, along with gas phase chemical quantum calculations results helped us to rationalize both the observed change in the ET mechanism and the occurrence of the "sticky" dissociative ET mechanism. The radical/anion pair interactions have been determined both in solution as well as in the gas phase. The study also shows that despite the low magnitude of in-cage interactions in acetonitrile compared to the gas phase their existence strongly affects the dynamics of the involved reactions. It also shows that, as expected, these interactions are reinforced by the existence of strong electron-withdrawing substituents. The occurrence of an autocatalytic process and the existence of the radical/anion interaction may explain the differences previously observed in the reduction of these compounds in different media.

Sulfur multilayer formation on Au(111): new insights from the study of hexamethyldisilathiane.

Hexamethyldisilathiane was successfully used as a new precursor for the formation of S layers on Au and to study their interaction. Characterization of the S modified gold surface was done by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and scanning tunneling microscopy (STM). Key findings include the direct observation by STM of (i) coexistence of different phases, (ii) multiple sulfur layers formation, (ii) formation of rectangular structures not only on the adlayer but also on the top layer, and (iv) rectangular structure mobility on different layers. These results provide clear evidence regarding the adsorbate nature of the rectangular structures, solving a highly debated question.

A new eudesmane type sesquiterpene from Inula viscosa.

Investigation of Inula viscosa of Jordanian origin afforded the new sesquiterpene 1beta-hydroxyilicic acid in addition to the known 2beta-hydroxyilicic acid.

Dissociation of aryl sulfonyl phthalimide radical anions: relevance to the biological activity of aryl sulfonyl amides.

The reduction of two aryl sulfonyl phthalimides leads to the corresponding radical anions. Surprisingly, that of the nitro-derivative decomposes faster than that of the methyl derivative. A theoretical investigation along with application of the dissociative ET theory to the decomposition of radical anions allows rationalization of this unexpected behavior.

Gold catalyzed reduction of a hexavalent aromatic sulfonyl phthalimide to sulfur.

The reduction of N-(p-fluorobenzenesulfonyl)phthalimide on polycrystalline gold and Au(111) was studied. Our key finding is the decomposition of the compound on the surface, leaving only sulfur behind. This was supported by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and scanning tunneling microscopy (STM). The modification leads to observation by STM of well-known as well as new phases for the S modified Au(111) surface.

Regioselective bond cleavage in the dissociative electron transfer to benzyl thiocyanates: the role of radical/ion pair formation.

Important aspects of the electrochemical reduction of a series of substituted benzyl thiocyanates were investigated. A striking change in the reductive cleavage mechanism as a function of the substituent on the aryl ring of the benzyl thiocyanate was observed, and more importantly, a regioselective bond cleavage was encountered. A reductive alpha-cleavage (CH(2)-S bond) was seen for cyano and nitro-substituted benzyl thiocyanates leading to the formation of the corresponding nitro-substituted dibenzyls. With other substituents (CH(3)O, CH(3), H, Cl, and F), both the alpha (CH(2)-S) and the beta (S-CN) bonds could be cleaved as a result of an electrochemical reduction leading to the formation of the corresponding substituted monosulfides, disulfides, and toluenes. These final products are generated through either a protonation or a nucleophilic reaction of the two-electron reduction-produced anion on the parent molecule. The dissociative electron transfer theory and its extension to the formation/dissociation of radical anions, as well as its extension to the case of strong in-cage interactions between the produced fragments ("sticky" dissociative electron transfer (ET)), along with the theoretical calculation results helped rationalize (i) the observed change in the ET mechanism, (ii) the dissociation of the radical anion intermediates formed during the electrochemical reduction of the nitro-substituted benzyl thiocyanates, and more importantly (iii) the regioselective reductive bond cleavage.

A unique autocatalytic process and evidence for a concerted-stepwise mechanism transition in the dissociative electron-transfer reduction of aryl thiocyanates.

Electrochemical reduction of p-methyl-, p-methoxy-, and 3,5-dinitrophenyl thiocyanates as well as p-methyl- and p-methoxyphenyl disulfides was investigated in acetonitrile at an inert electrode. This series of compounds reveals a striking change in the reductive cleavage mechanism of the S-CN bond in thiocyanates as a function of the substituent on the aryl ring of the aryl thiocyanate. With nitro substituents, a stepwise mechanism, with an anion radical as the intermediate, takes place. When electron-donating groups (methyl and methoxy) are present, voltammetric as well as convolution analyses provide clear evidence for a transition between the concerted and stepwise mechanisms based on the magnitude of the transfer coefficient alpha. Moreover, a very interesting autocatalytic process is involved during the electrochemical reduction of these compounds. This process involves a nucleophilic substitution reaction on the initial aryl thiocyanate by the electrochemically generated arenethiolate ion. As a result of this unusual process, the electrochemical characteristics (peak potential and peak width) of the investigated series are concentration dependent.

Regioselective bond cleavage in the dissociative electron transfer to benzyl thiocyanates.

The electrochemical reduction of benzyl thiocyanate and p-nitrobenzyl thiocyanate was investigated in acetonitrile at an inert electrode. These two compounds reveal a change in the reductive cleavage mechanism, and more interestingly, they show a clear-cut example of a regioselective bond dissociation. Both phenomena may be understood on the basis of the dissociative ET theory and its extension to the formation/dissociation reactions of radical ions. While the effect of the standard oxidation potential of the leaving group seems to be predominant in understanding the change in the ET mechanism by changing the driving force, the regioselective cleavage is dictated by changes in the intrinsic barrier related to the nature of the substituent on the aryl moiety.