Synthesis and Surface Investigations of N-Substituted 2,5-Dithio-7

Article pubs.acs.org/JPCC

Synthesis and Surface Investigations of N-Substituted 2,5-Dithio-7azabicyclo[2.2.1]heptanes on Gold Surfaces Sharwatie Ramsaywack,† Sanela Martić,‡ Scott Milton,† Lisa Gates,† Andrew S Grant,*,† Mahmoud Labib,‡ Andreas Decken,§ and Heinz-Bernhard Kraatz*,‡ †

Department of Chemistry & Biochemistry, Mount Allison University, Sackville, NB, Canada, E4L 1G8 Department of Physical and Environmental Sciences, University of Toronto at Scarborough, Toronto, ON, Canada, M1C 1A4 § Department of Chemistry, The University of New Brunswick, Fredericton, NB, Canada, E3B 6E2 ‡

S Supporting Information *

ABSTRACT: The reaction of various primary amines and 2,5dihydroxy-1,4-dithiane in the presence of a catalytic amount of Mg(II) in distilled water provided a series of N-substituted 2,5dithia-7-azabicyclo[2.2.1]heptanes. The adsorption profiles of the sulfur-containing heterocycles on gold surfaces have been explored by time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), and electrochemistry. SIMS data indicated that these novel bicyclic sulfides interact with gold surfaces favorably, independent of the N-substitution, with minimal fragmentation. An XPS study revealed the three component core levels of S 2p with binding energies at 161, 162, and 163 eV, indicating a combination of the bound and unbound sulfur species. Using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), we found the efficient adsorption of heterocycles onto gold and the formation of densely packed films for alkyl and phenyl analogues. However, the adsorption and film packing properties were greatly compromised by an N-pyridyl substitution. The findings indicate that the surface behavior of N-substituted 2,5-dithia-7azabicyclo[2.2.1]heptanes varies with respect to the N-substitution and the nature of the substituent, suggesting that the adsorption profiles and the film packing of bicyclic sulfides on gold surfaces are highly dependent on the binding interface and the molecular orientation.

■

INTRODUCTION Self-assembling monolayers (SAMs) attract much attention due to their ability to control wetting adhesion, lubrication, and corrosion.1−5 More recently, they have also been recognized as suitable candidates for use in chemical and biochemical sensors.1,2 Probably, the most studied and well understood SAMs are those derived from alkanethiols (R−SH), dialkyl disulfides (R−S−S−R), and dialkyl sulfides (R−S−R) on gold surfaces.3,4 However, the variation in the film formation and properties greatly varies between the ligands. SAMs prepared from thiols or disulfides have similar structures and were found to adsorb on gold surfaces via dissociative adsorption (cleavage of S−H and S−S bonds), which leads to the formation of wellpacked and robust films.4,6 By contrast, the adsorption profiles of dialkyl sulfides on gold surfaces have been previously described to proceed without the cleavage of the S−C bond by using a range of analytical techniques including X-ray photoelectron, infrared, scanning tunneling microscopy and high-resolution electron energy loss spectroscopies, and timeof-flight secondary ion mass spectrometry (TOF-SIMS).7−12 The interactions of dialkyl sulfides with gold surfaces are based on the dative-type bonding through a lone pair of the sulfur atom in sulfides. While the sulfide films are characterized by a potentially compromised organization, robustness, and stability © 2012 American Chemical Society

compared to the thiols or disulfides, the advantages associated with sulfides make this class of compounds of interest.13−16 The sulfides exhibit greater stability and lower reactivity toward oxidation and nucleophilic attacks. The convenient synthesis and the inherent R−S−R structure of sulfides provide a convenient way toward developing new organosulfur targets through appropriate substitution (symmetrical or unsymmetrical) and for controlling the film composition on metal surfaces. In addition, the monolayers of some dialkyl sulfides have a very low defect density as compared to thiolates.11 Recent discovery of the two surface-bound enantiomers on gold surfaces, formed by a butyl methyl sulfide, indicates that the gold−sulfur interactions in sulfides may induce a chiral surface environment.17,18 Consequently, the potential use of dialkyl sulfides is in the area of surface functionalization and preparation of SAMs with interesting properties. Here, we explored a series of a new class of bicyclic sulfides to understand their interactions with gold surfaces and gauge their utility as precursors toward functional SAMs. In an effort to expand the repertoire of ligands available for the Received: January 26, 2012 Revised: March 15, 2012 Published: March 16, 2012 7886

dx.doi.org/10.1021/jp300866r | J. Phys. Chem. C 2012, 116, 7886−7896

The Journal of Physical Chemistry C

Article

modification of gold surfaces, the formation of SAMs from the neglected class of bicyclic sulfides, N-substituted 2,5-dithio-7azobicyclo[2.2.1]heptanes (2), was explored due to their intriguing bis-sulfide structures and synthetic availability. The ligands are characterized by the bicyclic structure composed of two sulfur atoms in a sew-saw conformation, capable of binding gold surface, and a N-substituent positioned away from the binding interface. The literature reports on the azobicyclo[2.2.1]heptanes include the synthesis from αmercaptoacetone and semicarbazide.19 Subsequent to this, there have been a few reports on the use of α-mercaptoacetone for the preparation of aza-bicyclo[2.2.1]heptanes that feature a more highly functionalized 1,4-dithiane ring.20 For example, Nsubstituted 2,5-dithia-7-azabicyclo[2.2.1]heptanes were made available by reacting aliphatic mercaptoketones with primary amines.21 Derivatives possessing a highly substituted 1,4dithiane core were reported by reacting mercaptoketones with primary amines as well as carbonyl reagents such as hydroxylamine, phenylhydrazine, and semicarbazide at low temperatures.22 Interestingly, the analogous reactions with mercaptoaldehyde have been, to the best of our knowledge, relatively unexplored. While various N-substituted 2,5-dithio-7azobicyclo[2.2.1]heptanes have been described in the literature,23 most recently by Nenajdenko,22 very little has been reported on the physical and chemical properties of these compounds. As a result of a fortuitous experiment, these compounds became readily available, and we became interested in their adsorption properties on gold surfaces aiming at exploring new organosulfur molecules as precursors to functional SAMs. In particular, series of azobicyclo[2.2.1]heptanes were chosen for this study due to the following: (i) ease of synthesis; (ii) stability under normal laboratory conditions; (iii) high solubility in common organic solvents; (iv) sulfur content; and (v) structural and electronic variation of N-substituents. Due to the presence of two sulfur atoms in the cyclic structure and the functional pending group at the bridging N-site, the azobicyclo[2.2.1]heptanes may find applications, as chemically stable surfaces, in the bioanalysis, materials sciences, and antifouling research. In this paper, we wish to report on the efficient one-step preparation of the bicyclic sulfides (2) containing variable substituents (Scheme 1). Investigations into the ability of several ligands to bind to gold surfaces were performed by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), time-of-flight secondary-ion mass spectrometry (TOF-SIMS), and X-ray photoelectron spectroscopy (XPS).

Scheme 1. Synthetic Scheme Towards Synthesis of NSubstituted 2,5-Dithio-7-azobicyclo[2.2.1]heptanes 2a

a

R = N-substitution ranging from alkyl to aryl functionalities (2a−2j).

tometer using θ and ω scans with a scan width of 0.3° and 30 s exposure times. The detector distance was 5 cm. All nonhydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms were found in Fourier difference maps and refined using isotropic displacement parameters. The data were reduced (SAINT)24 and corrected for absorption (SADABS).25 The structure was solved by direct methods and refined by full-matrix least-squares on F2(SHELXTL).26 The gold on silicon substrates (single-crystal Au(111) surface) was fabricated at the Nanofabrication facility (The University of Western Ontario, Canada) and used for the TOF-SIMS and XPS studies. Polycrystalline gold disk electrodes (0.02 cm2) were purchased from CHInstruments and were used for electrochemical studies. Despite the crystallographic differences between the two types of gold surfaces (polycrystalline gold disk electrodes and single-crystal Au(111) on silicon substrates), similar resistance to charge transfer trends in the EIS spectra was observed for the films of 2a−2c on the gold on silicon substrates and gold disk electrodes. Acetonitrile (Sigma Aldrich) was collected from the solvent distillation setup and degassed in Ar for 30 min. Absolute ethanol (Sigma Aldrich) was freshly distilled and degassed in Ar for 30 min prior to use. Electrode Cleaning and Film Preparation. For TOFSIMS and XPS experiments, all gold substrates were prepared by following sputtering procedure: titanium layer (7 nm) followed by gold layer (150 nm) onto silicon wafer. The gold wafer was cut into 1.0 × 0.5 cm pieces and cleaned as described. Gold substrates were first cleaned with piranha solution (3:1 % vv, concentrated H2SO4:H2O2; Caution: “piranha solution” reacts violently with organic materials and should be handled caref ully) for 5 min, then extensively rinsed with Milli-Q water, and finally sonicated in freshly distilled ethanol for 20 min. For electrochemical measurements, gold disk electrodes were cleaned in piranha solution for 5 min, then polished over alumina slurry (0.05 μm), rinsed with Milli-Q water, and sonicated in ethanol for 20 min. The films were prepared by incubating cleaned electrodes in 2 mM acetonitrile solution of compounds 2a−2c over 3 days at 5 or 37 °C. The gold electrodes were rinsed with acetonitrile and dried under a nitrogen stream prior to measurements.

■

EXPERIMENTAL METHODS General Information. Reagents were purchased as reagent grade from commercial suppliers and used without further purification. Anhydrous MgSO4 was used as the drying agent after aqueous workup. Evaporation and concentration in vacuo were done at water-aspirator pressure. Column chromatography: silica gel-60 (70−230 mesh). Thin Layer Chromatography (TLC): glass plates covered with silica gel-60 F254; visualization by UV light, I2 vapor, or KMnO4 stain. 1H and 13C NMR spectra were recorded at room temperature in CDCl3, and chemical shifts are determined relative to tetramethylsilane (TMS). Crystals of 2b were grown by slow evaporation of ethyl acetate at room temperature. Single crystals were coated with Paratone-N oil, mounted using a glass fiber, and frozen in the cold nitrogen stream of the goniometer. A hemisphere of data was collected on a Bruger AXS P4/SMART 1000 diffrac7887



Article

General Synthetic Method. To a suspension of 1,4dithiane-2,5-diol (1) (1.00 g, 6.57 mmol) in distilled water (dH2O, 30 mL) was added Mg(OAc)2·4H2O (0.0326 mmol, 0.007 g) dissolved in dH2O (2 mL). The resulting mixture is stirred at 40−50 °C in a warm water bath for ca. 15 min. To this mixture was added the respective amine in portions or dropwise. The reaction is monitored by TLC (partitioning an aliquot between ethyl acetate and water in a vial) for the disappearance for the amine (ca. 1−3 h). The product 2 was extracted with ethyl acetate (2 × 100 mL), dried with anhydrous MgSO4, and purified by column chromatography or recrystallization. 7-Decyl-2,5-dithia-7-aza-bicyclo[2.2.1]heptane (2a). Purification by column chromatography (silica gel, hexanes → hexanes/ethyl acetate 9:1) gave a yellow oil after removal of solvents. Yield = 63%. Rf = 0.35 (hexanes/ethyl acetate 9:1). 1H NMR (CDCl3, 270 MHz): 4.81 (d, J = 3.0 Hz, 2 H), 3.27 (dd, J = 9.7, 3.0 Hz, 2 H), 3.15 (d, J = 9.2 Hz, 2 H), 2.45−2.25 (m, 2 H), 1.54−1.41 (m, 2 H), 1.32−1.15 (m, 14 H), 0.85−0.80 (m, 3 H). 13C NMR (CDCl3, 67.5 MHz): 70.0, 48.7, 43.9, 31.9, 29.6, 29.5, 29.4, 28.8, 27.4, 22.7, 14.2. IR (neat) 2925, 2852 cm−1. ESI MS m/z 274.2 ([M + H]+, 100), 117.1 ([C4H6S2]2+, 2). 7-Benzyl-2,5-dithia-7-aza-bicyclo[2.2.1]heptane (2b). Purification by single-solvent recrystallization with ethyl acetate gave a white solid. Yield = 75%. Mp = 84−86 °C. Rf = 0.44 (hexanes/ethyl acetate 9:1). 1H NMR (CDCl3, 270 MHz): 7.37−7.30 (m, 5 H), 4.78 (d, J = 3.0 Hz, 2 H), 3.59 (s, 2 H), 3.37 (dd, J = 9.7, 3.0 Hz, 2 H), 3.22 (d, J = 9.4 Hz, 2 H). 13C NMR (CDCl3, 67.5 MHz): 137.5, 128.9, 128.7, 127.7, 69.8, 53.0, 44.0. IR (CH2Cl2) 3020, 1215 cm−1. ESI MS m/z 224.2 ([M + H]+, 100), 117.1 ([C4H6S2]2+, 6). 7-((Pyridin-2-yl)methyl)-2,5-dithia-7-aza-bicyclo[2.2.1]heptane (2c). Filtration through a short plug of silica gel (hexanes/ethyl acetate 1:1) gave a yellow solid after removal of solvents. Yield = 57%. Rf = 0.35 (hexanes/ethyl acetate 1:1). Mp = 62−64 °C. 1H NMR (CDCl3, 270 MHz): 8.58 (d, J = 4.0 Hz, 1 H), 7.70 (ddd, J = 7.7, 7.4, 1.6 Hz, 1 H), 7.50 (d, J = 7.7 Hz, 1 H), 7.22 (dd, J = 7.2, 5.0 Hz, 1 H), 4.90 (d, J = 3.0 Hz, 2 H), 3.84 (d, J = 13.9 Hz, 1 H), 3.70 (d, J = 13.9 Hz, 1 H), 3.45 (dd, J = 7.5, 3.0 Hz, 2 H), 3.26 (d, J = 9.7 Hz, 2 H). 13C NMR (CDCl3, 67.5 MHz): 157.6, 149.5, 136.9, 123.0, 122.6, 70.1, 54.7, 44.0. IR (CHCl3) 3019, 2980, 1216 cm−1. ESI MS m/z 225.1 ([M + H]+, 100), 117.1 ([C4H6S2]2+, 16). 7-Methyl-2,5-dithia-7-aza-bicyclo[2.2.1]heptanes (2d). Filtration through a short plug of silica gel (hexanes/ethyl acetate 2:1) gave a yellow oil after removal of solvents. Yield = 76%. Rf = 0.34 (hexanes/ethyl acetate 2:1). 1H NMR (CDCl3, 270 MHz): 4.79 (d, J = 3.2 Hz, 2 H), 3.39 (dd, J = 9.4, 3.0 Hz, 2 H), 3.22 (d, J = 9.5 Hz, 2 H), 2.37 (s, 3 H). 13C NMR (CDCl3, 67.5 MHz): 72.0, 43.7, 36.0. IR (neat) 2949, 2922, 2862, 2796 cm−1. ESI MS m/z 148.1 ([M + H]+, 100), 117.1 ([C4H6S2]2+, 31). 2-(2,5-Dithia-7-aza-bicyclo[2.2.1]heptan-7-yl)ethanol (2e). Purification by column chromatography (silica gel, ethyl acetate) gave a colorless oil after removal of solvents. Yield = 83%. Rf = 0.44 (1:2 hexanes/ethyl acetate). 1H NMR (CDCl3, 270 MHz): 4.96 (d, J = 3.0 Hz, 2 H), 3.74 (s, 2 H), 3.37 (dd, J = 9.7, 2.7 Hz, 2 H), 3.26 (d, J = 9.4 Hz, 2 H), 2.98 (s, 1 H), 2.77−2.68 (m, 1 H), 2.63−2.54 (m, 1 H). 13C NMR (CDCl3, 67.5 MHz): 70.3, 60.7, 50.9, 44.0. IR (neat) 3406, 2923, 2878,

Electrochemical Experiments. All electrochemical experiments including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out using a CHInstrument potentiostat 660B (Austin, TX). In a typical electrochemical experimental setup, a modified gold electrode was used as a working electrode, Ag/AgCl in 3 M KCl as the reference electrode, which was connected with the electrolyte via a salt bridge, and platinum wire as the counter electrode. All solution CV electrochemical measurements were carried out in the presence of 0.1 M tetrabutylammonium perchlorate and at a scan rate of 0.1 V s−1 in the potential range of −2 to 2 V. Electrochemical desorption studies were performed in 0.5 M KOH, at 100 mV s−1, and in −1.4 to 0 V potential range. The surface density, Γ, was calculated from the charge, Q, determined by integrating the redox peaks in the CV. Surface characterization studies were carried out by running the CV experiments in 5 mM solution of K4[Fe(CN)6]·3H2O and K3[Fe(CN)6]·3H2O in 0.1 M Milli-Q water (50 mM KNO3). All AC impedance spectra were acquired at the formal potential of the [Fe(CN)6]3−/4− redox couple (0.25 V vs Ag/AgCl) at 5 mV amplitude and in the 0.1 Hz to 100 kHz range. All impedance spectra are represented as Nyquist plots with real impedance (Z′) vs imaginary impedance (Z″) values. The experimental EIS data were fitted to an appropriate equivalent circuit by using the ZSimpWin 2.0 (EChem software). All electrochemical measurements were performed in duplicates with two different electrodes. TOF-SIMS Experiments. TOF-SIMS experiments were performed with TOF-SIMS IV (ION-TOF GmbH, Munster, Germany) which was equipped with a Bi liquid metal ion source. For all measurements, a 25 keV Bi3+ cluster primary ion beam with a pulse width of 12 ns was employed (target current of ∼1 pA). The cycle time for the processes of bombardment and detection of 100 μs (or 10 kHz) was used. A pulsed, lowenergy electron flood was used to neutralize sample charging. For each sample, spectra were collected from 128 × 128 pixels over an area of 500 × 500 μm for 60 s. The positive and negative secondary ions were extracted from the sample surface, mass separated, and detected via a reflectron-type of time-offlight analyzer, allowing parallel detection of ion fragments having a mass/charge ratio up to 900 within each cycle (100 μs). Positive and negative ion spectra were internally calibrated by using H+, H2+ and CH3+, and H−, C−, and CH− signals, respectively. Two spots per sample were analyzed using a random approach. X-ray Photoelectron Spectroscopy. The samples were analyzed by a Kratos Axis Ultra X-ray photoelectron spectrometer using a monochromatic Al Kα source (15 mA, 14 kV). The instrument was calibrated to give the binding energy (BE) of 83.96 eV for the Au 4f7/2 line for metallic gold, and the spectrometer dispersion was adjusted to give BE of 932.62 eV for the Cu 2p3/2 line of metallic copper. The Kratos charge neutralizer system was used on all samples. XPS can detect all elements except hydrogen and helium, probes the surface of the sample to a depth of 7−10 nm, and has detection limits ranging from 0.1 to 0.5 atomic percent depending on the element. Survey scan analyses were carried out with an analysis area of 300 × 700 μm and a pass energy of 160 eV. Highresolution analyses were carried out with an analysis area of 300 × 700 μm and a pass energy of 20 eV. Spectra have been charge corrected to the main line of the C 1s spectrum (adventitious carbon) set to 284.8 eV. Spectra were analyzed using CasaXPS software (version 2.3.14). 7888



Article

Figure 1. (A) ORTEP view of the 2b structure (50% probability level) with selected distances. (B) A view along the a axis of the packing diagram of 2b (sulfur (yellow), carbon (gray), and nitrogen (blue)). C(8)−S(1) = 1.8326(16) Å, C(9)−S(2) = 1.8259(16) Å, C(10)−S(2) = 1.8726(15) Å, C(11)−S(1) = 1.8336(18) Å, C(8)−S(1)−C(11) = 90.13°, C(10)−S(2)−C(9) = 90.87°, and S(1)···S(2) = 3.2110 Å.

2843 cm−1. ESI MS m/z 178.1 ([M + H]+, 100), 117.1 ([C4H6S2]2+, 22). 7-Phenyl-2,5-dithia-7-aza-bicyclo[2.2.1]heptanes (2f). Purification by using column chromatography (silica gel, hexanes/ethyl acetate 9:1) gave a white solid after removal of solvents. Yield = 31%. Rf = 0.49 (hexanes/ethyl acetate 9:1). Mp = 81−85 °C. 1H NMR (CDCl3, 270 MHz): 7.30−7.25 (m, 2 H), 7.03−6.95 (m, 3 H), 5.61 (d, J = 2.7 Hz), 3.38 (dd, J = 9.4 and 2.7 Hz, 2 H), 3.32 (d, J = 9.4 Hz, 2 H). 13C NMR (CDCl3, 67.5 MHz): 143.2, 129.5, 122.4, 117.5, 66.7, 44.6. IR (CHCl3) 3019, 2928, 1599, 1497, 1216 cm−1. ESI MS m/z 210.00 ([M + H]+, 100), 117.1 ([C4H6S2]2+, 28). Ethyl 2-(2,5-Dithia-7-aza-bicyclo[2.2.1]heptan-7-yl)acetate (2g). Purification by column chromatography (silica gel, hexanes/ethyl acetate 2:1) gave a colorless oil after removal of solvents. Yield = 63%. Rf = 0.35 (hexanes/ethyl acetate 2:1). 1 H NMR (CDCl3, 270 MHz): 5.05 (d, J = 3.0 Hz, 2 H), 4.21 (q, J = 7.2 Hz, 2 H), 3.26 (dd, J = 9.8, 3.0 Hz, 2 H), 3.30−3.21 (m, 4 H), 1.29 (t, J = 7.2 Hz, 3 H). 13C NMR (CDCl3, 67.5 MHz): 169.6, 70.3, 61.2, 49.9, 43.7, 14.2. IR (neat) 2979, 2927, 2866, 1741 cm−1. ESI MS m/z 220.1 ([M + H]+, 100), 117.1 ([C4H6S2]2+, 52). 2,5-Dithia-7-azabicyclo[2.2.1]heptan-7-ol (2h). Purification by column chromatography (silica gel, hexanes/ethyl acetate 2:1) gave a white solid after removal of solvents. Yield = 71%. Rf = 0.32 (hexanes/ethyl acetate 2:1). Mp = 108−110 °C. 1 H NMR (CDCl3, 270 MHz): 5.53 (s, 1 H), 4.97 (d, J = 3.5 Hz, 1 H), 4.69 (d, J = 3.5 Hz, 1 H), 3.69 (ddd, J = 9.8, 3.5, 0.8 Hz, 1 H), 3.39 (ddd, J = 9.0, 7.4, 3.5, 0.8 Hz, 1 H), 3.29 (d, J = 9.7 Hz, 1 H), 3.05 (d, J = 9.2 Hz, 1 H). 13C NMR (CDCl3, 67.5 MHz): 74.3, 70.4, 41.7, 38.1. IR (CH2Cl2) 3555, 3330, 2985, 2939, 1265 cm−1. ESI MS m/z 150.0 ([M + H]+, 100), 132.1 ([C4H6NS2]+, 12), 117.1 ([C4H6S2]2+, 39). 7-(Pyridin-2-yl)-2,5-dithia-7-aza-bicyclo[2.2.1]heptane (2i). Purification by column chromatography (silica gel, hexanes/ethyl acetate 1:1) gave a white solid after removal of solvents. Yield = 7%. Rf = 0.61 (hexanes/ethyl acetate 1:1). Mp = 106−108 °C. 1H NMR (CDCl3, 270 MHz): 8.25 (ddd, J =

4.8, 0.8, 0.7 Hz, 1 H), 7.57 (ddd, J = 7.9, 6.9, 1.9 Hz, 1 H), 6.88−6.81 (m, 2 H), 3.51 (dd, J = 9.5, 3.1 Hz, 2 H), 3.38 (d, J = 9.2 Hz, 2 H). 13C NMR (CDCl3, 67.5 MHz): 154.8, 148.6, 138.2, 117.2, 110.4, 64.7, 44.5. IR (CH2Cl2) 3054, 2986, 1593, 1472, 1435, 1265 cm−1. ESI MS m/z 211.0 ([M + H]+, 100), 117.1 ([C4H6S2]2+, 18). 7-(Prop-2-ynyl)-2,5-dithia-7-aza-bicyclo[2.2.1]heptane (2j). Purification by column chromatography (silica gel, 2:1 hexanes/ethyl acetate) gave a colorless oil after removal of solvents. Yield = 73%. Rf = 0.56 (2:1 hexanes/ethyl acetate). 1H NMR (CDCl3, 270 MHz): 4.90 (d, J = 3.0 Hz, 2 H), 3.31− 3.24 (m, 6 H), 2.27 (t, J = 2.5 Hz, 1 H). 13C NMR (CDCl3, 67.5 MHz): 79.0, 73.2, 69.6, 43.5, 38.4. IR (neat) 3285, 2973, 2923, 2869, 2835, 2122 cm−1. ESI MS m/z 172.1 ([M + H]+, 100), 117.1 ([C4H6S2]2+, 59).

■

RESULTS AND DISCUSSION Synthesis and Characterization of N-Substituted Azobicyclo[2.2.1]heptanes. While exploring the chemistry of 1,4-dithian-2,5-diol (1), the dimeric form of 2-mercaptoaldehyde, it was found that aza-bicyclo[2.2.1]heptanes are formed upon reaction with primary amines. As a result of an interest in 2,5-dihydroxy-1,4-dithiane we discovered a general one-step aqueous synthetic route, depicted in Scheme 1, to the class of N-substituted 2,5-dithio-7-azobicyclo[2.2.1]heptanes (2). The target compounds synthesized and studied in this work were prepared by using a method first reported by Asinger et al. with some modifications.20,21 Our revised method is an inexpensive, simple, one-step procedure that can be used to generate a library of compounds in good yields. A particularly attractive aspect of this method is that it is functional-group tolerant and can give rise to derivatives bearing ester, hydroxyl, aromatic, heteroaromatic, alkyl, benzylic, and propargylic functionalities on the nitrogen atom. The general reaction involves the treatment of primary amines (neat or as solution in methanol) and 2,5-dihydroxy1,4-dithiane (1) (1:1 mol equiv) in distilled water at 30 °C in the presence of catalytic Mg(OAc)2·4H2O (5 mol %). The one7889



Article

and the presence of two sulfur atoms per molecule, may make these derivatives more suitable candidates for monolayer formation on gold surfaces, as compared to other derivatives. Consequently, variable N-substitutions may allow for tailoring the structural and electronic factors toward self-assembled monolayers with desirable properties. In addition, the sea-saw conformation of dithiane ring positions the two sulfur atoms on the same side of the binding interface in a favorable arrangement for binding to gold surfaces. From the library of compounds synthesized, the N-substituted azobicyclo[2.2.1]heptanes 2a−2c were chosen for further surface investigations. Compound 2a contains a bicyclic dialkyl sulfide group and a long alkyl chain and is reminiscent of a typical alkanethiol. The N-aryl substitution allows for a direct head-to-head comparison of the N-alkyl analogue with aromatic ligands 2b and 2c which contain benzyl and pyridyl groups, respectively. The flexible methylene group in the aromatic ligands 2b and 2c may allow for a more efficient film formation over the rigid aromatic analogues. To minimize the misorientation associated with the additional interactions between ligands and gold surfaces, we have chosen compounds with exclusively sulfur groups. Compared to aliphatic derivatives, the aromatic ligands are prone to the π−π stacking interactions which may impart additional stability to the self-assembled monolayers. The interplay between the alkyl vs aromatic dithiols or disulfides has been previously investigated, and the adsorption profiles revealed that cyclic disulfides exhibit constrained chemisorption, due to the aromatic ring rigidity, while dithiols undergo facile chemisorption.29−31 However, the aromatic dithiols may provide a more robust film as compared to those derived from the alkanethiols of similar chain length. In addition, the nature of the sulfur−gold interactions in the cyclic sulfides containing two sulfur atoms as in N-substituted azobicyclo[2.2.1]heptanes is unknown. Hence, we investigated the interplay between the structural and electronic factors in the N-substituted azobicyclo[2.2.1]heptanes 2a−2c on gold surfaces for the first time. All three Nsubstituted azobicyclo[2.2.1]heptanes under study are characterized by two sulfur atoms in 1,4-positions which can potentially be used for formation of self-assembled monolayers on the gold surfaces, analogous to other alkanethiolates and dialkyl disulfides. The adsorption profiles of compounds 2a−2c on the gold surfaces were investigated electrochemically and by TOF-SIMS and XPS. A typical film formation was achieved by immersion of gold surfaces in 2 mM ligand solution in acetonitrile for 3 days at 5 °C to allow for gold−sulfur binding and efficient monolayer orientation. Following the film formation, the gold electrodes were rinsed with acetonitrile before electrochemical measurements. To evaluate the ability of these cyclic compounds to bind gold surfaces and undergo film formation, we have investigated their film properties electrochemically by CV and EIS. Cyclic voltammograms, presented in Figure 2A, depict gold surfaces with different modifications and are performed in the presence of a 5 mM [Fe(CN)6]3−/4− redox probe in Milli-Q water (50 mM KNO3). A bare gold electrode was included for comparison and shows a typical reversible behavior for the [Fe(CN)6]3−/4− redox pair for the unmodified gold surface. It is expected that the potential binding and film formation of compounds 2a−2c on gold surfaces may lead to the increased surface coverage and a current modulation. From CVs, it is evident that all three cyclic compounds adsorb on gold surfaces to varying degrees. Since 2b and 2c have similar structures, it is

step protocol resulted in acceptable yields of the bicyclic compounds as racemic mixtures within half an hour. Of particular interest, the 2,5-dihydroxy-1,4-dithiane is insoluble in water as are the bicyclic products. For example, in the case of benzyl amine the reaction proceeds from a white suspension of starting material 1,4-dithiane to a white suspension of bicyclic product (2b), as described by Sharpless et al.27 Not surprisingly, the dodecyl amine yields an oily product 2a. The reactions are easily run on a five gram scale in only 75 mL of water. On this scale and smaller, the products are most easily isolated via simple liquid−liquid extraction. More reactive amines, such as benzylamine and alkylamines, gave very good yields (63−75%). It is noteworthy that the formation of 2f was accompanied by side products with similar polarity, chromatographic behavior, and solubility which hindered purification and resulted in a lower isolated yield. The lowest isolated yield (7%) was observed for compound 2i and may be attributed to the fact that 2-aminopyridine is a weak base as well as a poor nucleophile for this reaction. These aza-bicyclo[2.2.1]heptane derivatives can be made on both analytical (1 g) and stored in the fridge for an extended period of time without showing any noticeable signs of decomposition. The bicyclic products 2a−2j were fully characterized by 1H and 13C NMR and mass spectrometry. The 1H NMR spectra of all but one of the N-substituted 2,5dithio-7-azobicyclo[2.2.1]heptanes show three peaks for the six protons in the bicyclic fragment of the molecule, indicating rapid pyramidal inversion about nitrogen, which incidentally does not result in interconversion of enantiomers. Only compound 2h, available from hydroxylamine hydrochloride, shows six distinct peaks in the 1H NMR spectrum and six for each proton in the bicyclic portion of the molecule. This is presumably a result of hydrogen bonding between the hydroxyl proton and the sulfur atom in the ring. This doubling-of-peaks effect is also evident in the 13C NMR spectrum of 2h. To ensure the identity of the bicyclic sulfides, single crystals of compound 2b were obtained by slow evaporation from ethyl acetate at room temperature. As shown in Figure 1, the X-ray analysis confirmed the bicyclic nature of the molecule and the relative orientation of the N-substituent. The structure also highlights the diastereotopic nature of the benzylic protons as revealed in the 1H NMR. In the molecular structure of the parent 1,4-dithiane reported by Marsh, the chair conformation was reported.28 By contrast, the introduction of the N-bicyclic motif as in 2b significantly affects the conformation of the dithiane ring leading to the seasaw structure. The structural constraints in N-substituted 2,5dithio-7-azobicyclo[2.2.1]heptane result in the variation associated with the bond lengths and angles relative to the unsubstituted 1,4-dithiane. While the structural parameters in the biyclic sulfide 2b are similar to those in the parent 1,4bithiane, there are several key differences. The unusually long bond length C(10)−S(2) = 1.8726(15) Å relative to the 1,4dithiane (1.8200 Å) and slightly smaller angles of C(8)−S(1)− C(11) and C(10)−S(2)−C(9) (90.13° and 90.87°) compared to the 1,4-dithiane (99.00°) were ascribed to the bicyclic nature of the molecule. The packing diagram along the crystallographic a axis in Figure 1B reveals that 2b forms noncovalent networks which are held together by weak C−H···S hydrogen bonds and weak π−π interactions. Electrochemical Surface Studies of N-Substituted Azobicyclo[2.2.1]heptanes on Gold Surfaces. The absence of sterically demanding groups on the bicyclic 1,4-dithiane core, 7890



Article

For investigation of the film resistance, EIS was used in the presence of the [Fe(CN)6]3−/4− redox pair. The advantage of the EIS over CV is that interfacial parameters describing capacities, resistances, and diffusion events can be observed explicitly; however, the main difficulty associated with this technique is in the choice of a suitable model of circuit elements that approximately describes the physical interface.32 We investigated the film properties of 2a−2c on gold surfaces by comparing the Nyquist plots of the impedance spectra as shown in Figure 2B. The complex impedance was presented as the sum of the real Z, Z′, and imaginary Z, Z″, components that originate mainly from the resistance and capacitance of the cell, respectively. A schematic representation of the circuit model used to describe a gold electrode-SAM using an adjacent electrolyte solution is shown in the inset of Figure 2B. The circuit was carefully selected to reflect the real electrochemical process and to enable fit, producing accurate values.33 The circuitry elements include an ohmic solution resistance, RS, which is the resistance between the SAM-modified gold electrode and the reference electrode. Therefore, the two electrodes were kept in the same position for each measurement, and all measurements were carried out using the same electrolyte concentration to minimize variations in RS value. The RS value is usually not affected by composition changes that occur at the electrode surface.34 The RS is in series with a circuit that represents the SAM/solution interface. This circuit comprises the resistance of the SAM, R1, which arises from the diffusion of ionic species through the monolayer possibly attributable to collapsed sites and pinholes. In addition, it also contains a finite length Warburg, W, and is in parallel with a constant phase element, CPE1, associated with the double layer and reflects the interface between the assembled film and the electrolyte solution. Both CPE1 and W correspond to a diffusion process of, typically, a solution redox species. Furthermore, CPE was selected to represent the double-layer capacitance because the polycrystalline gold electrode has surface irregularities and therefore behaves less ideally than a single-crystal surface. This circuit is in series with a second circuit that represents the gold electrode/SAM interface. The second series comprises of the charge transfer resistance, R2, in parallel with CPE2. The former represents the electron-transfer kinetics of the redox probe at the electrode surface, whereas the latter corresponds to a nonlinear capacitor.35 The diameter of the semicircle corresponds to the interfacial resistance at the electrode surface, R2, the value of which depends on the dielectric and insulating features of the surface layer. On the other hand, the low-frequency loop observed in Figure 2B can be attributed to the Warburg impedance, ZW, resulting from a diffusion-limited electrochemical process, presumably due to the conducting ion penetration.36,37 As shown in Table 1, the value of R2 was 1.04 Ω cm2, when the unmodified gold surface was used. The EIS spectrum of the bare gold electrode is characterized by the small resistance component and a

Figure 2. (A) Cyclic voltammograms of bare gold electrode and gold electrodes incubated in 2 mM solution of 2a−2c in acetonitrile. Electrochemical measurements were performed in a 5 mM solution of K4[Fe(CN)6]·3H2O and K3[Fe(CN)6]·3H2O in 0.1 M Milli-Q water (50 mM KNO3), 100 mV s−1 scan rate, and potential range −0.2 to 0.6 V. Ag/AgCl reference electrode, Pt wire auxiliary electrode, and gold working electrode. (B) Nyquist plots (−Z′ vs Z″) of impedance spectra for bare gold and gold electrodes incubated with compounds 2a−2c. Inset shows the equivalent circuit used to model the experimental data (symbols) to the fitting curves (solid).

expected that these analogues may behave differently from the aliphatic derivative 2a on the surface. However, CV of the gold surface, following the incubation with compound 2c, indicated only minimal current modulation from that of the bare gold surface. By contrast, the corresponding benzyl analogue 2b adsorbs to the surface more efficiently, judging from the significant reduction in current and the greater potential shift separation. In addition, an alkyl analogue 2a chemisorbs on the gold surfaces and results in blocking of the electrode, lowering the [Fe(CN)6]3−/4− current. The modulation of the CV signal of [Fe(CN)6]3−/4− is direct evidence of the compound’s ability to adsorb on gold surfaces and elucidates the film formation and quality. Our data indicate that the ability of N-substituted azobicyclo[2.2.1]heptanes to interact with gold follows the order 2b > 2a > 2c.

Table 1. Equivalent Circuit Element Values for the Unmodified Gold and Gold after Incubation with Compounds 2a−2c

Au 2a 2b 2c

RS

CPE1

R1

W

CPE2

(Ω cm2)

(μF cm2)

n1

(Ω cm2)

(μF0.5 cm2)

(μF cm2)

n2

(μF cm2)

0.04 0.01 0.01 0.09

26.82 3.41 26.08 23.19

0.96 1 1 0.97

3.61 0.46 5.99 10.7

7349 5473 2080 5270

51.06 29.12 17.74 16.53

1 0.91 0.92 0.98

1.04 343.7 544.6 4.17

7891

R2



Article

for electrodes previously immersed in 2 mM solutions of compounds 2a−2c. From the desorption process, we can obtain information regarding the quantity of the adsorbed sulfides on surfaces, film density, and homogeneity of the monolayers. The desorption potential was observed at −1.1 V versus Ag/AgCl for all three compounds. However, the desorption profiles of the aromatic sulfides were characterized by a sharp peak at −0.9 V which was absent in the case of an alkyl analogue. The potential at which the organosulfates desorbs depends on a range of factors, such as nature of the side arm, degree of ordering, and extent of intermolecular interactions within the films.39 The more positive potentials are typically associated with the loosely packed SAMs,40 which may explain the differences in the desorption potentials between aliphatic and aromatic sulfides. On the basis of the electrochemical desorption studies, the surface coverage, Γ, was estimated from the surface charge.41 The amount of charge was estimated by integrating the reduction peaks in CVs in the −1.2 and 0 V potential range. Clearly, films derived from compound 2c are characterized by the lowest coverage value (3.5 nmol cm−2), suggesting that compound 2c binds gold less efficiently and forms disordered or incomplete layers (Figure 3B). By contrast, the surface coverage values for the films of compounds 2a and 2b were estimated to be 7.2 and 7.6 nmol cm−2, respectively. These values are in agreement with the CV and EIS data presented above and suggest that the surface coverage plays an important role in addition to film packing and film density. It seems that all three compounds significantly adsorb onto gold; however, compound 2c forms a distinctly more disordered film than the other two conjugates. TOF-SIMS and XPS Studies of N-Substituted Azobicyclo[2.2.1]heptanes on Gold Surfaces. TOF-SIMS and XPS were used for the analysis of the adsorption profiles of the bicyclic sulfides. TOF-SIMS allows for measuring ion intensities coming off the surface following the ionic bombardment and is commonly used to characterize surfaces.42 In general, the positive and negative ion modes are indicative of the film composition and stability and have previously been used for the study of organosulfur films on gold surfaces.43 The films of compounds 2a−2c on gold surfaces were prepared as previously mentioned and were analyzed by TOF-SIMS in both modes. The negative ion mode spectra were collected for all three films, with the particular emphasis on a variety of ions of interest, including S− (m/z 31.9), AuS− (m/z 229), and AuS2− (m/z 259) ions, which are indicative of sulfur−gold interactions (Figure S22, Supporting Information). The intensities of the negative ions of interest, as estimated from mass spectra, were plotted versus film type and are presented in Figure 4A. In the case of all three ions, only slight differences were observed in ion intensities for 2a−2c, indicating similar surface coverage. A relative abundance of the S−, AuS−, and AuS2− ions in these films correlates well with our electrochemical desorption data and points to the relatively efficient adsorption by all three compounds. Unlike a negative ion mode, the positive ion mode TOF-SIMS is diagnostic of the stability of sulfide and the C−S bond cleavage. For example, the presence of the molecular ion and the gold-containing molecular ions are unequivocal evidence for the nondestructive adsorption of sulfides on gold.44,45 The low-weight region (m/z < 100) consists of numerous peaks due to the extensive fragmentation, and the gold peak (m/z 197) was present at a relatively low abundance. In the mass region between 20 and 100 m/z, the

diffusion-controlled process. Incubation of clean gold electrodes in solution of 2a, 2b, and 2c caused an increase in the charge transfer resistance to 343.7, 544.6, and 4.17 Ω cm2, respectively. A more efficient surface coverage, as seen by compound 2b, results in the greater resistance (Figure 2B). As the surface binding abilities of the compound decreases, so does the resistive component in the EIS spectra. The results indicate the ability of compound 2b to form a well-formed film on the electrode surface with few defects. On the other hand, compound 2c produced a noncompact film, which can be further evidenced by the presence of a low-frequency loop (Figure 2B). The alkyl analogue 2a binds the gold surface and produces a well-defined film, which may be expected for alkanethiols. The EIS adsorption profiles of target compounds were also investigated with respect to the incubation temperatures (5 and 37 °C). In general, the rate of desorption for impurities and solvent molecules is faster at elevated temperatures as they undergo facile displacement with the target adsorbate.38 At higher temperature, the films with greater coverage and lower defects were obtained for all three sulfides. We attributed the differences in the film formation among different compounds to the packing efficiencies and film densities. However, the surface coverage studies may indirectly elucidate the importance of the film formation and packing efficiencies by this class of compounds. For surface coverage estimation, we have employed the electrochemical desorption in alkaline solution at extremely negative potentials as depicted in Figure 3A. CVs were collected in 0.5 M KOH solution for bare gold electrode and

Figure 3. (A) Typical reduction CVs for a desorption process of gold surfaces before and after incubation with compounds 2a−2c. CVs are obtained in 0.5 M KOH solution and at 100 mV s−1, in the potential range of −1.4 to 0 V. (B) Plot of the surface coverage, Γ, versus film type calculated from the surface charge obtained from CVs. 7892



Article

Figure 4. (A) Plot of negative ion TOF-SIMS intensities of S−, AuS−, and AuS2− on gold surfaces after incubation with compounds 2a−2c and (B) partial positive ion mode TOF-SIMS spectra of gold surfaces after incubation with compounds 2a−2c showing [M−SH]+ and [M− SCH3]+ ions and their respective m/z values.

Figure 5. S 2p XPS spectra of gold surfaces after incubation with compounds 2a−2c. The relative component is depicted as the sum of S 2p2/3 and S 2p1/2 levels and color coded (I − black, II − blue, and III − red).

significant differences were observed between the ligands. The loss of CH2Ph (m/z 91) and CH2Py (m/z 92) groups was evident in the spectra of 2b and 2c, respectively. Interestingly, the loss of the side group was not observed in the spectrum of 2a. Overall, the intense [M−SH]+ and [M−SCH3]+ ions were observed in the positive ion mode spectra for all three compounds in Figure 4B. The loss from two sulfur groups was not evident. Notably, the molecular ion fragments and the goldcontaining molecular ions were only present at low abundance or completely absent, which may be due to the overall low abundance of the gold peak at m/z 197. TOF-SIMS results indicate that all three compounds adsorb onto gold and undergo some fragmentation under the given experimental conditions and may point to the possibility of the nondestructive adsorption of sulfides. XPS was used to characterize the chemisorbed sulfide films on gold surfaces to gain an understanding about the nature of the bonding interface and interactions with gold. To the best of our knowledge, XPS measurements on bicyclic sulfides on gold surfaces have not been reported previously. Figure 5 shows a series of S 2p XPS spectra of gold surfaces prepared by immersion in the 2 mM solution of the bicylic sulfides 2a−2c in acetonitrile. Theoretically, the S 2p spectra for an alkanethiol or dialkyl sulfide adsorbates are characterized by the presence of a doublet peak in ∼3:2 ratio due to the bound and unbound states of sulfur.46−48 Importantly, each state of sulfur should appear as a doublet in a 2:1 ratio due to the spin−orbit coupling of the 2p electrons; hence, the overall spectrum

should contain four peaks in total.49 The S 2p core levels of bicyclic sulfides on gold appear as triplets. Deconvolution of the S 2p spectra reveals the three sets of doublets for all three adsorbates indicating that three different components (I−III) are present. The binding energy of each component and their relative S 2p3/1 and S 2p1/2 values are presented in Table 2. Each doublet is composed of a larger peak at lower binding energy and a smaller peak at a higher energy, in a 2:1 ratio relative to each other, and with the spin−orbit splitting of 1.18 eV. The doublet (II) with the S 2p3/2 peak at 162.4 eV may be attributed to the S−Au bond, which has been previously reported.50 This component represents ∼20, 21, and 16% of the total S 2p spectra of 2a, 2b, and 2c, respectively. The second component (III), which is due to the unbound sulfur,51 with the S 2p3/2 peak at ∼163 eV, is present in all three films but dominates the spectrum of 2c (Figure 5). The unbound sulfur typically refers to the whole molecule being only physisorbed, but it may also represent one of the two sulfurs in bicyclic sulfide which is not engaged in the interactions with the gold surface. Unbound molecules may be incorporated into monolayers by π−π stacking interactions in the case of 2b and 2c, but the nonspecific adsorption on top of the monolayers may also take place for all three compounds. Hence, the components II and III may be ascribed to the presence of bound and unbound sulfur atoms, respectively, and have been previously seen for dialkyl sulfides.50 Given the presence of two sulfur atoms per molecule, one or both of those may contribute 7893



Article

Table 2. Summary of Core-Level Binding Energies from the XPS Spectra of Gold Surfaces after Incubation with Compounds 2a−2c (Peaks Were Identified from the Deconvoluted Spectra) binding energy (eV) XPS peaks S 2pa

C 1s N 1s

I II III

2a

2b

2c

161.47 (162.65) 46% 162.43 (163.61) 21% 163.77 (164.95) 33% 286.96 285.46 399.90

161.43 (162.61) 54% 162.47 (163.65) 21% 163.56 (164.74) 25% 286.59 285.09 399.52

161.45 (162.63) 28% 162.38 (163.56) 16% 163.69 (164.87) 56% 286.81 285.31 399.44

a

For each spectral component (I−III), the S 2p3/2 core levels are give. The corresponding S 2p1/2 are provided in parentheses. The percent area of each component is provided as well.

core-level XPS spectra were also obtained for the samples, and the binding energies are summarized in Table 2. The C 1s spectra for the prepared films of 2a−2c were dominated by the C−C/C−H/C−S peak at ∼285 eV. Notably, aliphatic and aromatic carbons could not be resolved from C 1s spectra. In all C 1s spectra, a peak at higher energy was observed at ∼286 eV, which is associated with the C−N resonance. The N 1s regions in all three spectra were characterized by a single peak at ∼399 eV arising from the bridging nitrogen in the molecular structure. Notably, the peaks due to the aromatic nitrogen in 2c or to the adsorbed nitrogen (∼400.5 eV) were not observed.58 To gain further understanding of the trends associated with these heterocyclic compounds, we performed the computation studies on compounds 2a−2c using the molecular mechanics energy minimization set with Spartan.59 The proposed models and their orientation on gold surfaces are shown in Figure 6

to bound states in the S 2p spectra. The third set of peaks (I) is located at the lower binding energy at 161.4 eV in the S 2p region and dominates the spectra of 2a (46%) and 2b (54%). The S 2p3/2 at 161.4 eV was recently reported for the cystamine and cystein films on gold surfaces and was ascribed to the formation of bilayers.52 The low binding energy of this component may not be associated with the formation of a bilayer or multilayer due to the unbound sulfur species which should appear at much higher binding energies.53 Rather, these species may appear due to the reorganization of the films on gold surfaces and the higher degree of sulfur coordination to gold atoms.54−56 The peak at 161 eV has been previously assigned to the atomic sulfur observed in the 4-mercaptopyridine following the C−S cleavage.54 It is unlikely that the component I is due to the atomic sulfur in our system since the film preparation was performed under mild conditions. Furthermore, the peak at 161 eV was also reported at the initial stages of film formation from thiols and for aromatic thiols or thiophenes and was assigned to another sulfur state without the molecular decomposition.55,56 The 161.3 eV S 2p3/2 peak may be due to the presence of the two neighboring sulfur atoms and a higher coordination number compared to a normal thiolate. We may assign the peak at ∼161 eV to the second type of sulfur−gold bond due to the hybridization change of sulfur from sp3 to sp or to the site differentiation by binding outside the ordered domains of the Au(111) surface.55,56 Presumably, the interactions through the sulfur lone pairs may also be attributed to multiple S 2p core levels. Hence, the spectral components I and II may be due to the bound sulfurs (either both or one sulfur atom at a time), while III is certainly related to the unbound sulfur or physisorbed molecules. To conclude, the film of compound 2c exhibits a much greater contribution from the component III (unbound sulfur) over 2a and 2b which may point to different interactions with gold surfaces, presumably through the aromatic nitrogen atom of 2c. The binding to gold through the nitrogen atom in 2c may also result in the formation of multilayers or more labile films which correlates well with our electrochemical data. Overall, the presence of the three S 2p3/2 core levels in our data may indicate that the sulfur−gold interactions are determined by the type of sulfur−gold interface, the tilt direction, the nature of the N-substituent, and the significant interactions between two sulfurs with next-nearest neighboring carbon or nitrogen atoms.57 Importantly, no oxidized sulfur species, such as sulfinites or sulfonates, in the 166−169 eV range were observed. The XPS survey scans of the sulfide films revealed the presence of C 1s and N 1s electrons with binding energies at ∼285 and 399 eV, respectively. High-resolution C 1s and N 1s

Figure 6. Illustration of compounds 2a−2c and the plausible adsorbate structures on gold surfaces via sulfur−gold interactions. The structures have been energy minimized, and the distances given have been estimated from the final structures.

and are characterized by a sea-saw cyclic conformation of the dithiane ring with both sulfur atoms interacting with the gold surface and extended substituent orientation pointing away from the surface. In our system, the sulfur lone pairs are used for binding to gold as previously reported.17,18 We have further estimated the length of the extended molecules using Spartan software and the molecular length of approximately when the compounds are bonded to the gold surface. The distances proposed for the aromatic conjugates are similar (9 Å); however, the orientation of the pyridyl relative to the phenyl rings is slightly altered. While the compound 2a adopts an extended orientation that is 19 Å away from the gold surface, compounds 2b and 2c are half the size. However, the alternative binding modes may exist, such as the interactions through a single sulfur and/or the aromatic nitrogen atom (as in 2c). 7894



Article

Alexander Scherer, and Eike Jahnke for their help with German to English translation and helpful discussions.

The X-ray crystal structure obtained for 2b revealed that this particular molecule adopts a conformation that is similar to that predicted by Spartan in the solid state. It is interesting to note that the average length of a molecule of 2b calculated by using the X-ray crystallographic data is ∼8 Å. This value is within close proximity to that obtained by using Spartan when a molecule of 2b adopts an extended orientation on the gold surface. This suggests that in both the solution and solid state molecules may adopt favorable conformations that allow them to act as bidentate ligands for binding to gold surfaces. However, the substitution of carbon by a nitrogen atom at the 2-position of the phenyl ring may have a detrimental effects on gold binding and the formation of the tightly packed films. Indeed, despite their structural similarities, conjugates 2b and 2c behave dramatically different on surfaces. In addition, the compound 2c may adsorb through a nitrogen atom in a side-on arrangement leading to more labile films. Hence, the differences associated with these compounds may be due to the orientation of the pending N-substituents and their effects on film packing.

■

(1) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151−256. (2) He, X. P.; Wang, X. W.; Jin, X. P.; Zhao, Hao; Shi, X. X.; Chen, G. R.; Long, Y. T. J. Am. Chem. Soc. 2011, 133, 3649−3657. (3) Ulman, A. Chem. Rev. 1996, 96, 1533−1554. (4) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. B.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103−1169. (5) Grunze, M.; Zharnikov, M. J. Phys.: Condens. Matter 2001, 13, 11133−11365. (6) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546−558. (7) Lee, H.; He, Z.; Hussey, C. L.; Mattern, D. L. Chem. Mater. 1998, 10, 4148−4153. (8) Zhong, C. J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518−525. (9) Beulen, M. W.; Huisman, B. H.; van der Heijden, P. A.; van Veggel, F. C. J. M.; Simons, M. G.; Biemond, E. F.; de Lange, P. J.; Reinhoudt, D. N. Langmuir 1996, 12, 6170−6172. (10) Noh, J.; Murase, T.; Nakajima, K.; Lee, H.; Hara, M. J. Phys. Chem. B 2000, 104, 7411−7416. (11) Bellisario, D. O.; Jewell, A. D.; Tierney, H. L.; Baber, A. E.; Sykes, E. C. H. J. Phys. Chem. C 2010, 114, 14583−14589. (12) Takiguchi, H.; Sato, K.; Ishida, T.; Abe, K.; Yase, K.; Tamada, K. Langmuir 2000, 16, 1703−1710. (13) Li, X. M.; de Jong, M. R.; Inoue, K.; Shinkai, S.; Huskens, J.; Reinhoudt, D. N. J. Mater. Chem. 2001, 11, 1919−1923. (14) Jung, C.; Dannenberger, O.; Xu, Y.; Buck, M.; Grunze, M. Langmuir 1998, 14, 1103−1107. (15) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1998, 4, 365−385. (16) Noh, J.; Makamura, F.; Kim, J.; Lee, H.; Hara, M. Mol. Cryst. Liq. Cryst. 2002, 377, 165−168. (17) Tierney, H. L.; Han, J. W.; Jewell, A. D.; Iski, E. V.; Baber, A. E.; Sholl, D. S.; Sykes, E. C. H. J. Phys. Chem. C 2011, 115, 897−901. (18) Maye, M. M.; Luo, J.; Lim, I. I. S.; Han, L.; Kariuki, N. N.; Rabinovich, D.l; Liu, T.; Zhong, C. J. J. Am. Chem. Soc. 2003, 125, 9906−9907. (19) Hromatka, O.; Haberl, R. Monatsh. Chem. 1954, 85, 1088−1096. (20) Asinger, F.; Offermanns, H. Angew. Chem., Int. Ed. 1967, 6, 907−919. (21) Asinger, F.; Saus, A.; Bähr-Wirtz, M. Liebigs Ann. Chem. 1979, 708−726. (22) Zhdanko, A. G.; Gulevich, A. V.; Nenajdenko, V. G. Tetrahedron 2009, 65, 4692−4702. (23) Barry, V. C.; Conalty, M. L.; McCormick, J. E.; McElhinney, R. S.; McInerney, M. R.; O’Sullivan, J. F. J. Med. Chem. 1970, 13, 421− 427. (24) SAINT 7.23A; Bruker AXS, Inc.: Madison, Wisconsin, USA, 2006. (25) Sheldrick, G. SADABS 2004; Bruker AXS, Inc.: Madison, Wisconsin, USA, 2004. (26) Sheldrick, G. M. SHELXTL. Acta Crystallogr. 2008, A64, 112− 122. (27) Narayan, S.; Muldoon, J.; Finn, M. G.; Fokin, V. V.; Kolb, H.; Sharpless, K. B. Angew. Chem., Int. Ed. 2005, 44, 3275−3279. (28) Marsh, R. E. Acta Crystallogr. 1955, 8, 91−94. (29) Garg, N.; Friedman, J. M.; Lee, T. R. Langmuir 2000, 16, 4266− 4271. (30) Garg, N.; Lee, T. R. Langmuir 1998, 14, 3815−3819. (31) Whitesides, G. M. Sci. Am. 1995, 9, 146−150. (32) Yan, F.; Sadik, O. A. J. Am. Chem. Soc. 2001, 123, 11335−11340. (33) Bai, D.; Hebersberger, B. M.; Jennings, G. K. J. Am. Chem. Soc. 2005, 127, 16486−16493. (34) Zhou, F.; Hu, H.; Yu, B.; Osborne, V. L.; Huck, W. T. S.; Liu, W. Anal. Chem. 2007, 79, 176−182. (35) Dijksma, M.; Boukamp, B. A.; Kamp, B.; van Bennekom, W. P. Langmuir 2002, 18, 3105−3112.

■

CONCLUSIONS In this work, we describe a simple, efficient, one-pot synthesis of a new class of bicyclic sulfides in water. Electrochemical studies indicated that several of the N-substituted azobicyclo[2.2.1]heptanes adsorb on the gold surfaces and form well-packed films. The aromatic derivative formed a more robust film over the aliphatic analogue, unless the heteroaromaticity was introduced. Our results show that the gold adsorption, film density, and film packing may be influenced by N-substitution of azobicyclo[2.2.1]heptanes. Molecular modeling and X-ray crystallography suggested that the azobicyclo[2.2.1]heptane framework adopts a conformation typical for a conventional bidentate chelating ligand on the gold surfaces, predominantly through sulfur−gold interactions. Experimentally, additional binding modes have been observed and point to the significant role of the N-substitution on the binding interface. To gain a better understanding of the selfassembly process for these molecules, we hope to study other derivatives which can undergo further functional group manipulation and explore the possibility of functionalizing the organic films following their formation. Investigations into the mechanism and efforts to expand the scope of this reaction are currently under way in our laboratories.

■

ASSOCIATED CONTENT

S Supporting Information *

1

H and 13C NMR spectra for new compounds, X-ray crystallographic data for 2b, TOF-SIMS, and XPS results. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (+)1 416 287 7278. Fax: (+)1 416 287 7279. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support for this work was provided by Mount Allison University and University of Toronto at Scarborough. We would also like to thank Andreas Waterloo, Drs. Frank Hampel, 7895



Article

(36) Pei, R.; Cheng, Z.; Wang, E.; Yang, X. Biosens. Bioelectron. 2001, 16, 355−361. (37) Baur, J.; Gondran, C.; Holzinger, M.; Defrancq, E.; Perrot, H.; Cosnier, S. Anal. Chem. 2010, 82, 1066−1072. (38) Kawasaki, M.; Sato, T.; Tanaka, T.; Takao, K. Langmuir 2000, 16, 1719−1728. (39) Kawaguchi, T.; Yasuda, H.; Shimazu, K.; Porter, M. D. Langmuir 2000, 16, 9830−9840. (40) Zhong, C. J.; Porter, M. D. J. Electroanal. Chem. 1997, 425, 147−153. (41) Lee, L. Y. S.; Lennox, R. B. Langmuir 2006, 23, 292−294. (42) Benninghoven, A. Surf. Sci. 1994, 299, 246−260. (43) Beulen, M. W. J.; Huisman, B. H.; van der Heijden, P. A.; van Veggel, F. C. J. M.; Simons, M. G.; Biemond, E. M. E. F.; de Lange, P. J.; Reinhoudt, D. N. Langmuir 1996, 12, 6170−6172. (44) Schierbaum, K. D.; Weiss, T.; Thoden van Velzen, E. U.; Engbersen, J. F.; Reinhoudt, D. N.; Goepel, W. Science 1994, 265, 1413−1415. (45) Mannini, M.; Sorace, L.; Gorini, L.; Piras, F. M.; Caneschi, A.; Magnani, A.; Menichetti, S.; Gatteschi, D. Langmuir 2007, 23, 2389− 2397. (46) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733−740. (47) Sun, F.; Grainger, D. W.; Castner, D. G.; Leach-Scampavia, D. K. Macromoelcules 1994, 27, 3053−3062. (48) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083−5086. (49) Salaneck, W. R.; Lipari, N. O.; Paston, A.; Zallen, R.; Liang, K. S. Phys. Rev. B 1975, 12, 1493−1500. (50) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Lanmguir 1998, 14, 5147−5153. (51) Zhubragel, C.; Deuper, C.; Schneider, F.; Neumann, M.; Grunze, M.; Schertel, A.; Woll, C. Chem. Phys. Lett. 1995, 238, 308− 312. (52) Wirde, M.; Gelius, U. Langmuir 1999, 15, 6370−6378. (53) Carey, R. I.; Folkers, J. P.; Whitesides, G. M. Langmuir 1994, 10, 2228−2234. (54) Lamp, B. D.; Hobara, D.; Porter, M. D.; Niki, K.; Cotton, T. M. Langmuir 1997, 13, 736−741. (55) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092−2096. (56) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799−6806. (57) Yang, Y. W.; Fan, J. L. J. Langmuir 2002, 18, 1157−1164. (58) Takiguchi, M.; Abe, Y.; Yoshida, Y.; Nakayama, Y.; Okazaki, M.; Akagawa, Y. Biomaterials 2003, 24, 1821−1824. (59) Spartan 06; Wavefunction Inc.: Pansilvenia, USA, 2006.

7896


Synthesis and Surface Investigations of N-Substituted 2,5-Dithio-7

Recommend Documents