Some Theoretical and Experimental Insights on the Mechanistic

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Some theoretical and experimental insights on the mechanistic routes leading to the spontaneous grafting of gold surfaces by diazonium salts Avni Berisha, Catherine Combellas, Frédéric Kanoufi, Philippe Decorse, Nihal Oturan, Jérôme Médard, Mahamadou Seydou, François Maurel, and Jean Pinson Langmuir, Just Accepted Manuscript • Publication Date (Web): 02 Jun 2017 Downloaded from http://pubs.acs.org on June 3, 2017

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Some theoretical and experimental insights on the mechanistic routes leading to the spontaneous grafting of gold surfaces by diazonium salts Avni Berisha†‡*, Catherine Combellas†, Frédéric Kanoufi†, Philippe Decorse†, Nihal Oturan§, Jérôme Médard†, Mahamadou Seydou†, François Maurel†, Jean Pinson†*. †Sorbonne Paris Cité, Université Paris Diderot, ITODYS, UMR 7086 CNRS, 15 rue J-A de Baïf, 75013 Paris, France. ‡ University of Prishtina, Chemistry Department of Natural Sciences Faculty, rr.“Nëna Tereze”nr. 5, 10000 Prishtina, Kosovo. § Université Paris-Est, Laboratoire Géomatériaux et Environnement, EA 4508, UPEM, 5 Bd Descartes, 77454 Marne-la-Vallée cedex 2, France

ABSTRACT. The spontaneous grafting of diazonium salts on gold may involve the carbocation obtained by heterolytic dediazonation and not necessarily the radical, as usually observed on reducing surfaces. The mechanism is addressed based on DFT calculations and experiments carried out under conditions where the carbocation and the radical are produced selectively. The calculations indicate that the driving force of the reaction leading from a gold cluster, used as a gold model surface, and the carbocation to the modified cluster is higher than that of the analogous reaction starting from the radical. The experiments performed under conditions of heterolytic dediazonation show the formation of thin films on the surface of gold. The grafting of a carbocation is therefore possible but a mechanism where the cleavage of the Ar-N bond is catalyzed by the surface of gold cannot be excluded.

1. INTRODUCTION Surface modification by aryldiazonium cations1-3 ArN2+ has evolved to a standard method that permits to tune the surface properties (hydrophily/phoby, electron transfer,

catalysis, …) by

attaching organic groups (from small organic molecules to proteins, metal complexes...). Such surfaces can be used for sensing devices, energy storage, composite materials, organic electronics3… Diazonium surface modification provides covalent bonds between the surface and the organic film, which yields very stable layers. The surface-organic layer bond has been characterized by different experimental methods (XPS, ToF-SIMS, SERS, Mossbauer spectroscopy, etc.).2,3 In addition, diazonium grafting represents the only means of grafting metals, semiconductors, oxides, and polymers starting from the same molecule,, a feature that is not observable with other precursors (thiols,4 alkenes,5 silanes,6 phosphonic acids,7 etc.). This grafting reaction can be triggered by means of electrochemistry,2 photochemistry,8 microwave irradiation,9 ultrasonication,10 reducing reagents,11 but also spontaneouly on reducing surfaces (iron, zinc, carbon),12 and even on non-reducing surfaces such as gold13-16 (Scheme 1).

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+ S + Ar-N2 +1e

S

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Ar

S

Ar Ar

Ar

Ar

S: Surface

Scheme 1. Electrografting of diazonium salts showing the formation of a first layer and then of multilayers.

In the mechanism of the electrochemical reduction of diazonium salts, the intermediate formation an aryl radical has been confirmed by many methods: i) Electron Paramagnetic Resonance, directly17 or after trapping with nitrones18 or the 2-methyl-2-nitrosopropane dimer15,19 or ii) characterization of the compound obtained after trapping the 4-nitrophenyl radical by 2-diphenyl-1-picrylhydrazyl,20 iii) formation of phenyl and diphenyl mercury compounds when reduction is performed on mercury pools.21 Moreover, since the height of the reduction wave observed by polarography in aqueous acidic media corresponds to a 1e- transfer, there is no doubt about the intermediacy of an aryl radical during the electrochemical reduction of diazonium salts.22 The same radical intermediate is likely to be involved when the reduction takes place spontaneously on reductive metals such as iron or zinc23. However, during the spontaneous grafting of Au surfaces the mechanism is not so clear; indeed gold is not reducing enough (E°(Au3+/Au) = 1.18 V/SCE and E°(Au+/Au) = 1.44 V/SCE) to reduce the diazonium cation. Therefore an issue has to be addressed: is there a different mechanism specific to the spontaneous grafting on gold? Based on the work of Bravo-Díaz and coworkers, we will briefly examine the different species that are present in a diazonium solution depending on the acidity of the medium.24-27, In ACN the diazonium cation is present, but in an aqueous medium diazonium cation, diazohydroxyde and diazoate species can be present depending on pH. In aqueous acidic (pH < 4) solutions, in the dark, and in the absence of reductants, arenediazonium ions, ArN2+ decompose spontaneously through the rate-limiting formation of the extremely unstable aryl cation. However, in weak acidic and alkaline solutions, ArN2+ reacts with H2O at the terminal nitrogen to give azo adducts such as ArN2OH -

(diazohydroxide) and with OH- at higher pH to give ArN2O (diazoate) that are in equilibrium with the parent ArN2+. These reactions are summarized in Scheme 2 along with some pKas. R-Ar-N2+ + H2O

K1

K2 R-Ar-N2-OH

R-Ar-N2-OH + H+ R-Ar-N2-O- + H+

R = NO2

pK1 = 5.24

R = CH3

pK1 = 5.72

Scheme 2. Acid base equilibria of diazonium salts. 2


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Different results are reported in the literature concerning the spontaneous grafting of diazonium salts on gold and the type of bonding observed; we have sorted these investigations based on the medium in which they have been performed; as we have seen above, the species present in solution are different depending on the solvent and the acidity. In aqueous acidic media, during the spontaneous grafting of a gold surface by 4nitrobenzenediazonium (DNO2), a jump of the open circuit potential was observed after the addition of the diazonium salt. This was interpreted as an electron transfer from the gold surface to the DNO2 cation altogether with positive charge accumulation on the substrate as the film grows; the accumulated charge discharges through a process tentatively assigned to the oxidation of adventitious impurities in solution. Finally, grafting of the diazonium cation occurs by electron transfer from gold.13 By analyzing the SERS, DFT and XPS spectra of spontaneously grafted gold nanostructures the aryl moiety was found to be bonded to the gold surface through Au-N=N-bonds when starting from unsubstituted, 4-decyl, 4- aminoethylbenzenediazonium salts but through Au-C bonds in the case of the 4-carboxybenzenediazonium salt. Therefore the grafting mechanism is sensitive to the substituent14 under conditions where an heterolytic cleavage provides the cation.24-27 In unbuffered water where the diazohydroxide is the predominant species, a very detailed comparative analysis of the spontaneous grafting on Ni and Au surfaces showed that three components in the N1s XPS signal were observed after the spontaneous grafting of DNO2 on gold: i) a peak at 405.8 eV corresponding to the NO2 group, ii) a broad peak at 400 eV assigned to N=N bonds and iii) a distinct contribution at 397.5 eV assigned to gold nitride, Au-N. The authors suggested that the spontaneous attachment of aryl groups on gold takes place both via an aromatic carbocation to give Au-aryl bonds and by direct attack of the diazonium salt to give Au-N=Nbonds.15 Both radicals and carbocations can be formed in unbuffered water, the homolytic dediazonation being much faster than the heterolytic one.24-27 In this medium, the grafting of DNO2 onto citrate capped gold nanorods was shown to occur through Au-C bonds. However, the possible involvement of an aryl carbocation was not discussed.16 Therefore both the Au-C and Au-N=N- bonds have been observed when dediazonation was performed from the diazonium cation and the diazohydroxide, and both a radical and a carbocation have been advocated as the reactive species. It is not so surprising that the positively charged 3


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diazonium cation reacts with the electron rich gold surface; this reaction is less likely from the diazohydroxide and the negatively charged diazoate. Note that the intermediacy of benzylic carbocations has been previously proposed during the oxidative electrografting of arylmethyl carboxylates.28 1

+

Ar- N

N

2

Ar+ Ar

.

?

Ar Au

3 1: Heterolytic cleavage 2: Homolytic cleavage 3: Direct attack

Ar

Au

Ar-N=N

of the diazonium cation

Scheme 3. Which are the species that react on the gold surface in the course of spontaneous grafting?

In this paper, we examine the possible grafting of an aryl carbocation, an aryl radical and a diazonium cation by determining the most energetically favorable reaction pathway through first principle calculations and through selective experimental formation of the radical or the carbocation (Scheme 3). As the starting diazonium salt we have chosen 4-methylbenzenediazonium (tolyldiazonium salt, DMe) with an electron donating methyl group (σpara = -0.17, σ : Hammett parameter) and 4-nitrobenzenediazonium (DNO2) with an electron withdrawing group (σpara = 0.78) that respectively favor and disfavor the stability of the carbocation. Benzenenediazonium (DH) was used for comparison in the calculations. This mechanistic investigation is indeed of some importance; would the aromatic carbocation react on surfaces, new avenues would open to the grafting of surfaces; up to now, only radicals (obtained from diazonium salts, amines, hydrazines, organomagnesium compounds, carboxylates) have been shown to react on surfaces.2

2. MATERIALS AND METHODS Computational Methods. All calculations were performed using the Gaussian09 suite of programs.29 Optimizations were performed using the B3LYP density functional methods, in combination with the 6-31g(d) basis sets for gold atoms the LanL2DZ basis set was used. The Polarizable Continuum Model (PCM) using the integral equation formalism variant (IEFPCM) was used to compute the calculations in the solvent.30,31 All energy minima were characterized by performing a vibrational analysis to ensure the lack of imaginary frequencies.

4


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Materials. 4-methylbenzenediazonium (DMe) terafluoroborate was synthesized from 4-toluidine in aqueous acidic solution in the presence of NaNO2. NMR (400 MHz, DMSOd6): δ 8.55 (d, 2H, 2 and 6- CH), 7.80 (d 2H, 3 and 5-CH), 2.58 (s, 3H, CH3). 4-nitrobenzene diazonium (DNO2) terafluoroborate was obtained from Sigma-Aldrich. Substrates. 1x1 cm2 gold coated silicon wafers (Aldrich, 100 nm coating) (Au) were cleaned with concentrated H2SO4 at room temperature, and rinsed under sonication for 10 min in Milli-Q water. Pt was deposited by Atomic Layer Deposition on mica. Functionalization of Au Plates. With DMe. A cleaned Au plate was placed in a buffered EtOH/H2O 80/20 v/v solution (10 mL, pH 1 or 9). 206 mg of DMe (100 mM) were introduced. The vials were thermostated at 60°. After for 17 h 30 min (pH 1) or 90 min (pH 9), the plates were withdrawn and rinsed in water under sonication for 3 min, then in ethanol (puriss. ≥ 99.8 %) for 3 min and dried under argon. The solutions were analyzed by a GC-mass LaChrom High Pressure Liquid Chromatograph equipped with a RP18 column eluted with MeOH/H2O 70/30 containing 1% acetic acid at a 1 mL min-1. The final solutions (c= 10 mM) were also evaporated and the residue dissolved in CDCl3 the 1H NMR spectrum was recorded with a Bruker AC 400 spectrometer at 400 MHz. With DNO2. Two solutions were prepared from distilled water + 0.1 M HCl + 10 mM DNO2: either as it is or in a mixture with 40% MeOH (v/v). A cleaned Au plate was placed in each of the solutions, which were left at 50° C for different periods of time to establish the kinetics of grafting. The plates were rinsed and the final solutions analyzed as above. Different concentrations were used for DMe and DNO2; they were chosen in order to obtain significant IR spectra of the Au modified surfaces. IRRAS. IRRAS spectra of the modified plates were recorded using a purged (low CO2, dry air) Jasco FT/IR-6100 Fourier Transform Infra Red Spectrometer equipped with MCT (mercury-cadmiumtelluride) detector. For each spectrum, 1000 scans were accumulated with a spectral resolution of 4 cm-1. The background recorded before each spectrum was that of a cleaned substrate. XPS. XPS spectra were recorded using a Thermo VG Scientific ESCALAB 250 system fitted with a micro-focused, monochromatic Al Kα X-ray source (1486.6 eV) and a magnetic lens, which increases the electron acceptance angle and hence the sensitivity. The pass energy was set at 150 and 40 eV for the survey and the narrow regions, respectively. The Avantage Software, version 4.67, was used for digital acquisition and data processing. The spectra were calibrated against C1s set at 285 eV. Ellipsometry. Thicknesses of the films on Au and Pt were measured with a mono wavelength ellipsometer Sentech SE400. The following values were taken for Au: ns = 0.19, ks = 3.33 and ns = 5


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1.55 and ks = 1.82 for Pt. These values were measured on cleaned surfaces before using the plates for grafting. The film thicknesses were determined from the same plates after modification, taking ns = 1.46, ks = 0 for the layer.

3. RESULTS 3.1. DFT Calculations. DFT simulations of the chemical bond formed between aryl groups and different materials has been reported.32,33 The calculated binding energies were shown to be dependent on the material, ranging from the lowest value on Au (111), 24.0 kcal/mol, to the largest one on Si(111)H, 70.0 kcal/mol. The angle of the phenyl group on the surface also depends on the substrate. We compared the dissociation energies for a phenyl or a 4-tolyl group covalently grafted to a 19atom gold cluster (Au19-Ph, Au19-Tol); the dissociation of these modified clusters occurs either through a homolytic (Bond Dissociation Energy) or a heterolytic pathway (heterolytic dissociation energy) leading respectively to a phenyl (or 4-tolyl) radical or carbocation and the gold cluster34. These dissociation energies were compared with those of a thiol (Au19-S-Ph) on gold as Self Assembled Monolayers are very often used for the surface modification of gold. A higher dissociation energy indicates a higher driving force for the reaction going from the radical, the carbocation, or the diazonium cation to the grafted cluster. The dissociation energy was first computed for the cleavage of the benzenediazonium to a phenyl carbocation and dinitrogen in the gas phase and in water (respectively 37.7 and 35.7 kcal.mol-1). The energy for the heterolytic cleavage of the phenyl diazonium cation is relatively low and is comparable with that of an easy cleavable molecule such as iodine (35.5 kcal/mol).35 From this value it is possible to obtain a rough estimate (neglecting entropic effects and solvation changes) of the energy for the homolytic cleavage by taking into account the redox potential of the Ph+ / Ph. redox couple (Ph+ + 1e- = Ph., E° = + 1.80 V/NHE) that has be calculated in ACN.36 One obtains -5.8 kcal/ mol for the reaction Ph-N=N+ + 1e- = Ph. + N2. The dissociation energies for the Au19-Ph and Au19-Tol clusters were calculated in the vacuum and in water (using in each case isoelectronic systems) according to either a heterolytic or a homolytic cleavage (Scheme 4 and Figure 1). The main result is that the energy needed for a heterolytic cleavage is much higher than for a homolytic one whether in the gas phase or in water (the difference is 23.4 and 37.1 kcal/mol in water 6


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for phenyl and tolyl groups respectively). In the case of the heterolytic cleavage, the high value of the dissociation energy calculated for the gas phase is comparable with that of the Au-C bond obtained for the Au(I)–alkynyl complexes (115.53 kcal/mol)37. In the gas phase, the dissociation energy for the homolytic cleavage of a phenyl group is 10 kcal/mol higher than that obtained previously using a different calculation method (24.0 kcal/mol)32 and the stabilization of the grafted cluster in the water system is evidenced by a gain of ~ 4 kcal/mol compared with the value obtained in the gas phase.

Scheme 4. Homolytic and heterolytic cleavage of the Au19–Ph modified cluster.

Figure 1. Computed dissociation energies for homolytic and heterolytic bond cleavage in the case of Au19-Ph grafted clusters with a comparison to thiolate-Au bond.

As a comparison, the calculated dissociation energy for the heterolytic cleavage of Au19-S-Ph in water is 40.0 kcal mol-1, significantly smaller that for the diazonium derived modified cluster; this is in agreement with the somewhat lesser stability of Au–thiol surfaces compared with Au-aryl surfaces.38 The dissociation energy for Au-S in organometallic clusters is 57.6 kcal mol-1, which is slightly higher than the Au-Au bond (54.5 kcal mol-1) in the Au2 cluster (determined experimentally in the gas phase).39 Generally the strength of the Au-thiolate bond (alkyl or aryl thiols) is accepted to be in the region of 40-50 kcal mol-1.40 This strong interaction of thiols with the gold surface, as evidenced through ab initio dynamic simulations, is responsible for pulling the gold atom out from the surface, finally breaking the Au-Au bond.41 A similar change of the gold atom position after bonding was evidenced during the geometry optimization of Au19-Ph (Figure 2). As the calculated dissociation energy of Au19-Ph is higher than that of Au19-S-Ph such pulling out of the gold atom is also expected for an aryl modified gold 7


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cluster. Attaching an aryl group to the surface of Au19 strongly changes its geometry. The bond distances between neighboring gold atoms change after the grafting: in the bare cluster d(Au8-Au3) = d(Au8-Au9) = 2.89 Å, while after grafting to Au8 d(Au8-Au3) = 4.05 Å, d(Au8-Au9) = 3.99 Å, indicating a similar effect to that observed in the case of thiols.40,41 The dihedral angles for top atoms (and therefore the phenyl groups) are also modified : (Au16-Au9-Au3-Au8) is 4.6° for Au19 but 20.8° in Au19-Ph; in the case of Au19-N=N-Ph this dihedral angle remains fairly unchanged (3.3°), whereas in the case of Au19-S-Ph it is 25.4°.

Figure 2. Optimized structures (side and top view / gas phase) for: A. Au19 cluster and B. Au19-Ph grafted clusters.

We also simulated the direct grafting of the benzenediazonium salt on the gold surface that has been observed by XPS and Raman spectroscopies as detailed in the introduction.14,15 The dissociation energy of Au19-N=N-Ph (7.91 kcal mol-1 in water) is much smaller than that of Au19-Ph or -Tol. During the optimization step (Figure 3) this azophenyl moiety changes its bonding position from Au8 in the initial step to the edge gold atom Au9. This behavior can be attributed to: i) some mobility of this moiety (similar to that of thiols) although no such evidence can be found in the literature2,3,41 (only STM would be able to observe such a change) or ii) a simple adsorption of this species due to its low dissociation energy value, meaning this is not a thermodynamically favorable form of grafting. After the grafting d(Au8-Au9) = 2.94 Å remains almost unchanged by comparison to the Au-Ph grafting mode and the dihedral angle (Au16-Au9-Au3-Au8) is 3.3° close to that in Au19, pointing out insignificant interactions of the azophenyl moiety with gold. It could be possible that this Au19-N=N-Ph modified cluster be the result of a kinetically controlled reaction. This 8


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dissociation energy is in agreement with the observation of Au-N=N- bonds by XPS15 and its low value is supported by the observation of the lower stability of such spontaneously grafted films to extended sonication in ACN by comparison with electrografted films.13

Figure 3. Optimized structure for the Au19-N=N-Ph modified cluster. A. Initial and final positions of the aryl group in the Au-19 cluster. B,C. Two different views of the modified cluster.

As a conclusion of these calculations i) the grafting of an aryl group on a gold cluster is clearly evidenced, ii) the heterolytic cleavage of Au19-Ph in water is disfavored by 24.0 kcal/mol compared to the homolytic cleavage, which means that the driving force for the reverse reaction: Ph+ + Au19- = .

Au19-Ph is larger than that of the analogous reaction Ph + Au19 = Au19-Ph; therefore, in the presence of a carbocation and a radical Au19-Ph will react preferentially with the carbocation and, iii) regarding the direct grafting of an azophenyl bond the dissociation of Au19-N=N-Ph is quite easy and therefore the stability of this species is low. 3.2. Experimental Evidences. We have tried to corroborate the DFT simulations with experimental evidences for the presence/absence of carbocations as intermediates in the spontaneous grafting of diazonium salts on gold. The experiments are based on a previously published investigation in a homogeneous phase that demonstrates that during the dediazonation of the diazoether, TolN=N-OEt -obtained by dissolving 4-tolyldiazonium (c = 0.1 mM) in 80/20 EtOH/H2O, different products are obtained depending on pH.24 At pH < 2 a mixture of 4methylphenol and 4-methylphenylethylether is obtained as a result of a heterolytic dediazonation and the nucleophilic attack of water and ethanol on the carbocation, while at pH > 5 in the presence of ethanol, toluene is obtained that derives from a homolytic dediazonation and hydrogen atom abstraction from ethanol. To find out whether carbocations or radicals are responsible for the spontaneous grafting on gold, gold plates were immersed inside the reacting mixture (DMe in 80/20 EtOH/H2O at different pH) and analyzed by IRRAS and XPS to examine if grafting occurred or not. Gold plates were dipped into two 80/20 EtOH/H2O solutions of DMe (c = 100 mM, a rather high 9


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concentration in order to obtain a significant aromatic IRRAS signal) at pH 1 or 9 for 17 h 30 min or 90 min respectively at 60°C (the half-life of the first order reaction is about 50 times smaller at pH 9 than at pH 1).24 After careful rinsing in ethanol under sonication, the gold plates (Au-MepH1 and AuMepH9) were analyzed by IRRRAS and XPS. We also analyzed the final solutions by HPLC and 1H NMR to check that our results are in agreement with those in reference 24. Note that the calculations were performed with water as the solvent and the experiments in water/alcohol mixtures. The IRRAS spectrum of Au-MepH9 is much more intense than that of Au-MepH1 (Figure 4). For AuMepH9, between 3100 and 2700 cm-1 CH stretching vibrations are observed at 2952, 2925, 2869 and 2854 cm-1 that can be assigned to the methyl signature of the tolyl groups grafted on the surface (by comparison 2946, 2920, 2875 cm-1 for toluene42). In the 1800-1400 cm-1 region, a strong C=O band is observed at 1711 cm-1 and the signature of the tolyl ring at 1605, 1512 and 1463 cm-1 (by comparison 1605, 1524 and 1461 cm-1 for toluene42). The origin of the carbonyl band that results from an H-atom abstraction from ethanol by the tolyl radical is discussed in the Supporting Information. For Au-MepH1 there is no significant signal between 3100 and 2700 cm-1 and only a very weak spectrum between 1800 and 1400 cm-1. 2925

0,015

pH 9 2854 2952 2869 0,005

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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pH 1 3100

3000

2900

2800

2700

-0,005 0,04

0,03 1711

pH 9

0,02

0,01

1605 pH 1

1512

1463

0

1800

1700

1600

Wavenumber

1500

1400

(cm-1)

Figure 4. IRRAS spectra of Au-MepH1 (c= 100 mM, 17 h 30 min reaction time) and Au-MepH9 (c = 100 mM, 90 min reaction time).

10


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The XPS survey spectra of both Au-MepH1 and Au-MepH9 show the presence of Au4f, C1s, O1s and N1s (see Table SI1 in Supporting Information). The observation of Au4f (27.1 %) in Au-MepH1 indicates a thin film (5.6 ± 0.7 nm by ellipsometry), on the contrary for Au-MepH9, Au4f is very small (< 0.1%) indicating a thicker film in agreement with ellipsometry (47 ± 15 nm, probably including holes that permit to observe Au). The high resolution N1s spectrum (Figure 5) of both Au-MepH1 and Au-MepH9 can be deconvoluted with a main peak at ~ 400 eV and two minor peaks at 401.7 and ~398.0 eV. The peak at 400 eV is assigned to azo bonds on the gold surface and inside the film43 and to some contamination. The peak at ~398 eV has been assigned to Au-N where the nitrogen atom is directly bonded to gold and corresponds to the direct attack of the diazonium on gold,15 while the peak at 401.7 eV correspond to protonated nitrogen (N1s(401.7 eV)/C1s = 5.8x10-3 for Au-MepH1 and 1.5x10-3 for Au-MepH9). By changing the concentration of DMe from 10 to 100 mM, the thickness of the Au-MepH1 film increases only from 4.5 ± 0.4 nm to 5.6 ± 0.7 nm while that of Au-MepH9 changes from 5 ± 1 nm to 47 ± 15 nm. 93000 Counts Au-Me pH1 91000

89000

87000 395 38000

397

399

401

403

405

Counts Au-Me pH9

37000

36000

35000 395

397

399

401

403

405

Binding Energy (eV)

Figure 5. High resolution N1s XPS spectra of Au-MepH1 (c= 100 mM, 17 h 30 reaction time) and Au-MepH9 (c = 100 mM, 90 min reaction time).

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The reaction solutions (c = 10 mM) were analyzed by HPLC. At pH1 the main product is CH3-C6H4OH (95%), in agreement with previous results in solution.24 At pH9 toluene (0.5%), CH3-C6H4-OH (14%) and unidentified aromatic products (~60%) -some of which precipitate- are observed by NMR (see Supporting Information). The formation of dimers and oligomers at the expense of toluene is due to the higher concentration of our grafting experiments by comparison with that of reference 24 (x105) where the yield of toluene was ~90%. Higher concentrations favor second order reactions and the formation of higher molecular weight products. The selective homolytic or heterolytic dediazonation of DNO2 in solution (c = 0.1 mM) has also been described.25 In pure acidic water, a heterolytic dediazonation is observed that yields 100% of nitrophenol, while in acidic 40% MeOH, a homolytic dediazonation gives ~100% nitrobenzene. We have reproduced these experiments in the presence of a gold plate (Au-NO2[MeOH0] and AuNO2[MeOH40]) with higher concentration of DNO2 (c = 10 mM) to obtain significant layers. The reaction time was 24 h, which corresponds approximately to one half reaction time in pure acidic water and ~ 60 reaction times in 40% MeOH. The IRRAS spectra of the modified plates are presented in Figure 6. Both spectra present the signature of the nitrophenyl group (1600, 1527, 1352 cm-1, by comparison 1603, 1527, 1347 for nitrobenzene) but the spectrum of Au-NO2[MeOH40] is ~3 times more intense than that of AuNO2[MeOH0]. 0,03

1352

1527

Absorbance a.u

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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a

0,02

0,01

1600

b

0

1700

1600

-0,01

1500

1400

1300

1200

Wavenumber cm-1

Figure 6. IRRAS spectra of (a) Au-NO2[MeOH40] and (b) Au-NO2[MeOH0] (c= 10 mM, 24 h reaction time).

The XPS survey spectra of Au-NO2[MeOH40] and Au-NO2[MeOH0] (Figure SI2 in Supporting Information) show the presence of Au4f, C1s, O1s and N1s. Au4f is 6.8 % for Au-NO2[MeOH40] and 14.9 % for Au-NO2[MeOH0], in agreement with the thicknesses of the films (8.0 and 3.8 nm respectively) and the above IRRAS spectra. 12


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The high resolution N1s spectra (Figure 7) show the presence of two main components at 400 and 406 eV; the former is assigned to N=N bonds at the surface of gold and inside the film, while the latter is characteristic of the NO2 group. For both spectra, the 400 eV peak can be deconvoluted into three components at ~398.8, 400.0 and ~401.4 eV, which correspond as for DMe, respectively, to Au-N, N=N bonds and protonated nitrogen.

Counts Au-NO2 [MeOH0]

100000

95000

90000 395

397

399

401

403

405

407

409

403

405

407

409

95000

Counts 90000

Au-NO2 [MeOH40]

85000

80000

75000

395

397

399

401

Binding Energy (eV)

Figure 7. High resolution N1s XPS spectra of Au-NO2[MeOH0] and Au-NO2[MeOH40], (c = 10 mM, 24 h reaction time).

The reaction solutions (c = 10 mM) were analyzed by HPLC. In pure water, nitrophenol (41%) was detected (no nitrobenzene) together with unidentified higher mass products. In 40% MeOH, nitrobenzene (94%) was identified. The kinetics of the grafting reaction of DNO2 (c = 10 mM) at 0 and 40% MeOH was examined to compare with the known kinetics of the formation of the carbocation and the radical (first order reaction with t1/2 = 1383 and 23 min, respectively).25 Figure 8 presents the thickness of the layer (a similar curve is obtained by plotting the intensity of the -NO2 IR band as a function of time, Figure SI3 in Supporting Information). The thickness rises for 1 h and then remains nearly constant for Au13


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NO2[MeOH0], while for Au-NO2[MeOH40] it stabilizes after ~ 5 h. Such kinetics are very different from the first order formation of the carbocation and the radical reported above25 indicating that the dediazonation is not the rate determining step of the grafting reaction. In order to find if there is a specific behavior of gold, the same reaction was reproduced in the absence of MeOH (c = 10 mM) on platinum, another non reductive substrate (Pt-NO2[MeOH0]). After 24 h under the same conditions as above, a film (1.9 ± 0.8 nm) was formed, the spectrum of which is similar in position and intensity (Figure SI4 in Supporting Information) to that obtained on gold, indicating that the reaction is not specific to gold.

9

a

8

Thickness (nm)

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7 6 5

b

4 3 2 1 0 0

5

10

15

20

25

30

Time (h)

Figure 8. Kinetics of spontaneous grafting of DNO2 in an acidic medium in the presence of a) 40% MeOH , b) 0 MeOH.

4. DISCUSSION In the case of DMe, DFT simulations were performed for Au19-Ph, Au19-Tol and Au19-S-Ph clusters. The simulated homolytic or heterolytic cleavage of Au19-Ph and Au19-Tol indicates that the carbocation is much higher in energy than the radical; therefore the driving force for the reaction .

Ar+ + Au19- = Au19-Ar is larger than that of Ar + Au19 = Au19-Ar. The driving force should be even larger starting from the 4-nitrophenyl carbocation (the difference in energy between the 4nitrophenyl and 4-methylphenyl carbocations is 16.2 kcal mol-1 44). Experimentally at pH 9 in a EtOH/H2O mixture, the transient diazoether CH3-C6H4-N=N-O-Et, formed by reaction of DMe with ethanol,25 undergoes a solvolytic radical dediazonation. This radical may undergo two reactions i) in solution it abstracts a hydrogen atom from ethanol to give toluene; such aromatic compounds are typical of the Gomberg-Bachman reaction which provides a number of dimers and oligomers upon homolytic dediazonation in a basic medium45 and, ii) on the surface it forms a Au-MepH9 film (47 nm), that was characterized by IR and XPS. The formation of multilayers 14


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at the surface and oligomers in solution is in agreement with the radical character of this reaction.43 At pH 1 the thermal decomposition of CH3-C6H4-N=N+ provides a carbocation that reacts with water to give CH3-C6H4-OH. On the surface, ellipsometry indicates the formation of a thinner Au-MepH1 film (5.5 nm), the thickness of which does not vary significantly with the concentration of the starting diazonium. Therefore, at pH 1 the experiments are in favor of the grafting of a carbocation to the gold surface and the growth of a film through a zero order reaction of the carbocation on the previously grafted groups (the mechanism should be similar to that of the Friedel Crafts reaction) ; since the layer thickness remains moderate compared to that obtained at pH 9 with a radical reaction, the reaction of a radical on a grafted aryl group is faster than the reaction of an aryl carbocation on the same aryl group. Starting from DNO2 in 40% MeOH the formation of a 8.0 nm thick film of nitrophenyl groups is observed, similar to the films obtained by electrochemistry upon reaction of radicals with the surface of gold; in solution nitrobenzene is obtained by H-abstraction by the nitrophenyl radical. But in the absence of MeOH, where a carbocation is formed that provides nitrophenol in the solution, a film is also observed; it is constituted of nitrophenyl groups as shown by IRRAS and XPS but somewhat thinner (3.8 nm). Therefore the energetically disfavored carbocation reacts with gold to give a monolayer; further formation of multilayers results from the same mechanism that leads to high mass products in solution. When the reactions are performed in unbuffered water the diazohydroxide is the predominant species in equilibrium with the diazonium cation and the diazoate.15 The grafting reaction would then occur from the homolytic cleavage of the diazohydroxide, which is faster than the heterolytic cleavage of the diazonium salt. The grafting kinetics of Figure 8 can be interpreted as the formation of the radical or the carbocation depending on the medium; only a small fraction of the latter species reacts on the surface with kinetics independent of the formation of both intermediates. However, a different mechanism leading to the spontaneous formation of Au-C bonds without a radical or a carbocation intermediate should also be considered. It involves the cleavage of the diazonium cation or the diazohydroxide on the surface of gold, this decomposition being catalyzed by the gold surface (Scheme 5).

15


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I-C12H25 Au

I ethanol

C12H25 Au

. Ar + OH + N2

Ar-N=N-OH Au

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water

Au

Scheme 5. Grafting mechanism of an alkyl group on Au starting from alkyliodide50 and proposed mechanism for the grafting of an aryl starting from an aryldiazohydroxide.

Similar reactions have already been observed. Under ultra-high vacuum alkyl halides undergo a dissociative chemisorption on metallic surfaces, including Cu(110), Ag(111 and 110), Al(111 and 100). For example, 1-iodo (or 1-iodo,2-methyl) propane undergo a C−I bond scission of the adsorbed 1-iodopropane and grafting of the propyl group and iodine on the surface.46 Since the metal−C bond is stable on the surface up to∼450 K the propyl group is strongly bonded to the surface. On a smooth Au(111) surface, phenylacetylene and iodobenzene react under vacuum to yield the homo-coupling products, diphenyldiacetylene and biphenyl, and the Sonogashira cross coupling product, diphenylacetylene, owing to the cleavage of the C-I bond.47 The same reactions are observed in DMF catalyzed by gold nanoparticles.48,49 In EtOH, the cleavage of a C-I bond has also been observed on gold in the presence of 1-iodooctane, and results in the grafting of Au−C8H17 and Au−I.50 As the C-I bond (~56.2 kcal mol-1 for iodobutane) is somewhat stronger than the C-N2+ bond (see above) a similar reaction could take place from the diazonium cation or the diazohydroxide catalyzed by the surface of gold and involves a C-N cleavage with release of dinitrogen. Since a similar grafting is observed on Pt, this reaction should also occur on Pt. Finally the direct attack of the diazonium salt on gold would provide Au-N=N-Ar as indicated by the XPS spectra15 but the calculation indicates that such groups are only weakly bonded to gold.

4. CONCLUSIONS DFT calculations and selective experimental production of carbocations and radicals indicate that: i) as previously observed in multiple instances, aryl radicals react with the surface of gold and with the first grafted layer leading to nanometer films; ii) carbocations can also react on gold but the growth of the film is slower than with radicals and the films are thinner; iii) a direct attack of the diazonium cation on gold is possible but the Au-(N=N-Ar) bond is much weaker than the Au-Ar bond. Finally, a 16


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direct cleavage of the Au-N bond catalyzed by the gold surface should also be considered. Further investigations should determine the relative importance of gold catalysis and carbocation reactions and exploit the use of carbocations to modify gold and other surfaces.

ACKNOWLEDGMENTS. We are grateful to Pascal Doppelt for the gift of the Pt substrate.

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TOC Graphic

Ar+ + Au + Ar-N

N

?

.

Ar

Au N N

18


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Some Theoretical and Experimental Insights on the Mechanistic

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