N−N Bond Cleavage in N-Nitrosoarylamines - American Chemical

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J. Phys. Chem. C 2008, 112, 12928–12935

N-N Bond Cleavage in N-Nitrosoarylamines Louise A. Evans,† Marija Petrovic,‡ Milan Antonijevic,*,‡ Charlotte Wiles,† Paul Watts,† and Jay Wadhawan*,† Department of Physical Sciences (Chemistry), The UniVersity of Hull, Cottingham Road, Kingston-upon-Hull, HU6 7RX, United Kingdom, and Technical Faculty Bor, UniVersity of Belgrade, Serbia ReceiVed: October 30, 2007; ReVised Manuscript ReceiVed: June 14, 2008

The rupture of N-N bonds in three specific N-nitrosoarylamines (N,N′-dimethyl-N,N′-dinitroso-paraphenylenediamine, Cupferron, and N-nitrosodiphenylamine) upon electrochemical oxidation in acetonitrile solution is investigated. In two cases, viz., Cupferron and N-nitrosodiphenylamine, self-inhibition via partial blocking of the electrode due to adsorption is observed. In the case of the bis(nitroso)arylamine, waveshape analysis using the Marcus theory of steady-state voltammetry at microelectrodes suggests that the oxidation involves the formation of a dication before N-N bond cleavage or a mechanism of the ECEC type. 1. Introduction This paper is concerned with the possibility of electrogeneration of nitric oxide resulting from oxidative cleavage of N-N bonds in acetonitrile solution. The small released molecule was chosen since it is a known protagonist in biology and biochemical pathways, playing an often complex role in chemotaxis and chemokinesis,1 as well as being recognized to be pivotal in vascular regulation, in neuronal communication, and as a cytotoxic agent in the immune system. The solvent, acetonitrile, was selected for its fast (0.2 ps) longitudinal relaxation time,2a and the electrode, platinum, chosen merely because it is a readily available material which possess a wide anodic potential window in acetonitrile. The choice of system studied for the electrochemical investigations reported herein, the N-nitrosoarylamines (see Scheme 1), is well-known as having weak N-N bonds, with cleaving being reported under electrolysis,3 photochemical irradiation,4 or thermal activation.5 The main problem with employing photochemical dissociation is that the reverse process could proceed at a significant rate, leading to excitation followed by deactivation, so that the overall picture is one of inefficient bond rupture. In contrast, the approach favored herein, electrolysis,6,7 may be an efficient alternative, owing to the relative ease of modulating the applied driving force required to encourage a “one-way” reaction. Moreover, the effect of the charge transfer is to modify, and sometimes grossly rearrange, the solvation structure (and/or the internal structure) of the reacting species, in addition to effecting the bond cleavage. This leads to the view that the dissociative electron transfersthe cleavage of a σ-bond by virtue of electron transfer between a redox-active species (acceptor or donor) and an electrode (donor or acceptor, respectively)sis an elegant, chemically clean, and mild method to achieve the desired goal. Furthermore, the ability of the Marcus-Hush theory and its various adaptations and variations2 to rationalize the kinetics of the electron transfer quantitatively in terms of simple concepts such as reorganization energies, reaction driving force, and the * Authors to whom all correspondence should be addressed. E-mail: [email protected]. Tel.: +44 (0) 1482 466 354. Fax: +44 (0) 1482 466 416. Web: http://www.hull.ac.uk/chemistry/wadhawan. E-mail: [email protected]. Tel.: +381 30 424 555. Fax: +381 30 421 078. † The University of Hull. ‡ University of Belgrade.

degree of electronic coupling between reagents and electrode, convoluted with the power of electrochemical techniques such as voltammetry, to deduce homogeneous and heterogeneous reaction kinetics,8 enables a dynamic and precise realization of the spatial and temporal coordinates of the electro-generated chemical stimulant under any arbitrary system-defined material transport regime.9 Nevertheless, the main drawback with employing electrochemical methods for the systems described herein is that electro-release of NO+ rather than nitric oxide is a concern. This paper seeks to address this issue with reference to three compounds: the ammonium salt of N-nitrosophenylhydroxylamine (Cupferron), N-nitrosodiphenylamine, and the bis(Nnitroso)phenylenediamine depicted in Scheme 1. 2. Experimental 2.1. Chemical Reagents. All electrochemical experiments were undertaken in acetonitrile solvent (HPLC grade, Fisher Scientific, UK) in the presence of 0.1 M tetrabutylammonium perchlorate (TBAP, electrochemical grade, Fluka) as supporting electrolyte. These materials were used as received, without any further distillation steps (in the case of acetonitrile). Ferrocene (Aldrich, UK), Cupferron (Acros Organics, UK), and Nnitrosodiphenylamine (TCI Europe) were purchased in the highest commercially avaliable grade and used without further purification. Other chemicals used for the syntheses outlined below were purchased in the purest commercially available grade from the Sigma-Aldrich company. N,N′-Dimethyl-N,N′-dinitroso-para-phenylenediamine (DMDNPD) was synthesized following the works by Fujimori et al.4g and Pacheco et al.4h–j Briefly, the synthesis followed the reaction sequence illustrated in Scheme 1b. In a first step, N-(4formylaminophenyl)-formamide was synthesized from 1,4diaminobenzene as follows. Acetic formic anhydride was prepared via the dropwise addition of formic acid (11.16 mL) to acetic anhydride (22.73 mL) at 0 °C, followed by gentle heating (60 °C) for 3 h, prior to the addition of 1,4-phenylenediamine (5.00 g) in tetrahydrofuran (20 mL); the resulting purple reaction mixture was stirred at room temperature overnight. Removal of the reaction solvent in vacuo afforded a purple solid which was filtered under suction and washed with hexane (50 mL), followed by dichloromethane (50 mL), to afford N-(4-

10.1021/jp710473a CCC: $40.75  2008 American Chemical Society Published on Web 07/31/2008

N-N Bond Cleavage in N-Nitrosoarylamines

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SCHEME 1: (a) N-Nitrosoamines Considered in this Studya and (b) Preparation of DMDNPD Employed Herein

a One bis(nitroso) species, N,N′-dimethyl-N,N′-dinitroso-para-phenylenediamine (DMDNPD) (i), and two mono(nitroso) compounds, Cupferron (note only the electroactive anion is drawn here) (ii) and N-nitrosodiphenylamine (iii).

formylaminophenyl)-formamide as a lilac solid (7.39 g, 97% yield). 1H NMR characterization of this latter compound revealed it to exist in tautomeric forms (50% in deuterated dimethylsulfoxide,10 d-DMSO) with the following characteristics: δH(400 MHz, d-DMSO) 2.40 (0.25 H, brs, OH), 3.29 (0.75 H, s, NH), 7.06 (0.75 H, d, J 8.6, ArH), 7.44 (0.5 H, m, ArH), 8.14 (0.25 H, s, C(OH)H), 8.62 (0.75 H, d, J 8.6, ArH), 10.05 (0.75 H, s, COH). δC(100 MHz, d-DMSO): 118.3 (1.0 × CH), 119.6 (0.5 × CH), 120.2 (0.5 × CH), 133.9 (0.75 × C0NH), 134.0 (0.25 × C0N ) COH), 159.3 (0.75 × CO), 162.4 (0.25 × CN). ESI: m/z 164 (+H 165). The notation here and subsequently refers to the molecular ion, with the M + 1 form indicated in parentheses. N,N′-Dimethyl-para-phenylenediamine was synthesized in a second step as detailed below. N-(4-Formylaminophenyl)formamide (1.00 g) was dissolved in THF (20 mL) and the solution cooled to 0 °C, prior to the dropwise addition of the borane:dimethylsulfide complex (BMS, 6.1 mL of a commercial solution purchased from Fluka, UK, provided in dimethylsulfide). After the initial vigorous reaction ceased, the resulting reaction mixture was heated to reflux for 3 h. The reaction mixture was subsequently cooled to 0 °C, and methanol (20 mL) was added and stirred for 1 h. The reaction mixture was subsequently acidified (pH 1) using sulfuric acid and again refluxed for 1 h. After cooling, the reaction mixture concentrated in vacuo to afford a purple oil which was washed with aqueous NaOH and extracted into dichloromethane (3 × 50 mL). The combined organic extracts were dried over MgSO4 and concentrated in vacuo to afford a burgundy oil which was purified by column chromatography (eluted 12% ethyl acetate in hexane) to afford a burgundy oil (0.72 g, 87.2% yield), which exhibited the following characteristics. δH(400 MHz, CDCl3/TMS) 2.79 (6 H, s, 2 × CH3), 3.75 (2 H, br s, 2 × NH), 6.65 (4 H, s, 4 × ArH). δC(100 MHz, CDCl3/TMS) 31.0 (2 × CH3), 114.3 (4 × CH), 140.5 (2 × C0). ESI: m/z 136 (+H 137). The target product was then obtained in a final step as follows. Sodium nitrite (4.06 g) was added to a solution of N,N′dimethyl-para-phenylenediamine (1.00 g) in deionized and doubly filtered water (20 mL) of resistivity not less than 18

MΩ cm, taken from an Elgastat machine (Vivendi, UK) and stirred at room temperature overnight. The reaction mixture was subsequently acidified using aqueous HCl and diluted with water (30 mL), and the product was extracted into dichloromethane (3 × 50 mL). The combined organic extracts were subsequently dried over MgSO4 and concentrated in vacuo to afford a beige solid (1.28 g, 89.6% yield) with the following properties. δH(400 MHz, CDCl3/TMS)4g 3.47 (6 H, s, 2 × N(CH3)NO), 7.68 (4H, s, 4 × ArH). δC(100 MHz, CDCl3/TMS) 31.2 (2 × NCH3), 119.8 (4 × CH), 141.1 (2 × C0N). ESI: m/z 194 (+H 195). 2.2. Instrumentation. For all electrochemical measurements, a commercially available potentiostat (Autolab, PGSTAT30, Eco-Chemie, The Netherlands) was utilized, controlled by a Pentium IV processor computer. In all cases, a three-electrode system was employed, with a platinum wire counter electrode and a chloride-saturated silver/silver chloride reference electrode (Bioanalytical Systems, UK) or a saturated calomel electrode (SCE, Radiometer, France) or a silver wire pseudo reference electrode used. All electrochemical experiments were undertaken at a thermostatted temperature of 294 ( 2 K, under an impurity-free nitrogen atmosphere (BOC Gases, Ltd., Guildford, Surrey, UK), using solutions that had been rigorously purged with nitrogen prior to undertaking any voltammetric measurement. For the experiments undertaken using an aqueous-based reference electrode, liquid junction potentials7i were not corrected for the data analysis. The disk working electrodes employed were either platinum or gold with the following dimensions: 3.0 mm diameter, Metrohm, Switzerland, 1.0 mm and 0.5 mm diameter, fabricated in-house using standard procedures, 25 and 10 µm (diameter), constructed in-house. These electrodes were polished using progressively finer grades of carborundum paper (Presi, France), followed by gentle lapping on a wet-napped polishing cloth (Presi, France) using a 0.3 µm alumina slurry. Reproducibility of the experiments was ensured by rinsing the electrodes between scans with nitric acid (10 vol. %, Fisher Scientific, UK), so as to remove any adventitious adsorbates from the electrode surface, for which deionized and doubly filtered water taken from an Elgstat system

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Evans et al. of resistivity not less than 18 MΩ cm was used. The diameters of the microdisc electrodes were electrochemically ascertained using ferrocene in acetonitrile and TBAP (0.1 M) and the diffusion coefficient11 of 2.3 × 10-5 cm2/s. All data are reported here without performing Frumkin corrections.12 Potentiostatic bulk electrolysis was undertaken in a single, stirred reaction vessel at the transport-limited oxidation potential. The voltammetry of adsorbed N-nitrosodiphenylamine was achieved via soaking gold disk electrodes (fabricated using standard procedures in-house and with a diameter of 0.5 mm) in a 1.0 mM solution of N-nitrosdiphenylamine in acetonitrile for a period of six hours, after which they were washed in fresh acetonitrile prior to undertaking voltammetry in actetonitrile/ 0.1 M TBAP.

Figure 1. (a) Cyclic voltammograms (showing five consecutive cycles, T ) 296 K) for the oxidation of 0.97 mM DMDNPD in 0.1 M Bu4NClO4/MeCN at a 3.0 mm diameter platinum electrode at a scan rate of 0.1 V/s. The peaks identified using roman numeral labels are discussed in the text. The arrow indicates the direction of the initial voltage sweep. (b) Steady-state voltammogram for the oxidation of 0.5 mM DMDNPD in 0.1 M Bu4NClO4/MeCN at a 10.0 µm diameter platinum electrode. The arrow indicates the direction of the initial voltage sweep. (c) Plot illustrating the apparent nonvariance of the peak current for DMDNPD oxidation normalized with respect to the DMDNPD MeCN solution concentration (c0), scan rate (V), and transfer coefficient (β), with the experimental time scale. The inset illustrates Randles-Sevic´k plots for the oxidation of DMDNPD of concentration c0 ) 0.56 mM (x), 0.97 mM (+), or 1.96 mM (O). (d) Illustration of the shift in the peak oxidation potentials observed in cyclic voltammograms with the experimental time scale. Note that the data represent experiments undertaken at a variety of concentrations of DMDNPD.

3. Results and Discussion 3.1. Voltammetry of N,N′-Dimethyl-N,N′-dinitroso-paraphenylenediamine (DMDNPD) in Acetonitrile Solution. Figure 1a illustrates typical cyclic voltammograms at a scan rate (V) of 0.2 V/s for the oxidation of DMDNPD in acetonirile. Only a single, sharp, irreversible oxidation wave (peak I) is observed with peak potentials in the range 1.55-1.64 V vs Ag|AgCl|Cl-. On the reverse sweep, two new, electrochemically reversible, one-electron waves (peaks II′ and III′) appear with Emid ) 1/2(EpOx + EpRed) ) 0.71 V vs Ag|AgCl|Cl- (∆Epp ) 62.5 mV at 0.2 V/s) and Emid ) 0.21 V vs Ag|AgCl|Cl- (∆Epp ) 68.7 mV at 0.2 V/s). Figure 1b illustrates a steady-state voltammogram for the oxidation of DMDNPD in acetonitrile. Again, only a single, irreversible wave is observed. Experiments with different concentrations of DMDNPD, c0, in the range 0.5 e c0/mM e 2.0 revealed entirely similar voltammetric features, suggesting that the follow-on reaction causing chemical irreversibility is likely first-order. Taken together with the occurrence of a single oxidation wave, in contrast with the normally observed redox behavior of tetraN-substituted-para-pheneylenediamines,13 it suggests a twoelectron oxidation of DMDNPD. In order to determine the number of electrons transferred, microelectrode chronoamperomograms were recorderd at various electrode sizes and DMDNPD concentrations (data not shown). These were analyzed using conventional procedures,8b viz. by comparing them with the Shoup-Szabo theoretical expression for the temporal dependence (dummy index j) of the current, iterating through the number of electrons and diffusion coefficient until an optimum fit, as observed by a minimum in the parameter (1/∑j)∑(iexptl - itheory )/iexptl . j j j j The two-electron nature of the primary peak does not appear to be due to reaction between the electrolytically formed species with the starting materialsplots of the peak oxidative current (ipOx) normalized by solution concentration of DMDNPD (c0) and the square root of the voltage scan rate weighted by the symmetry coefficient for oxidation (β, vide infra), i.e., iOx p / c0
βV, were independent of the scan rates employed for the experiments (Figure 1c), as anticipated by the Randles-Sevcˇik equation for a simple, n-electron, electrochemically irreversible heterogeneous charge transfer event8


RTFV

iirrev ) 0.4958n√n′βFSc0√D p

(1)

In the above, n′ is the number of electrons that are transferred in the rate-limiting reaction step, S the electrode area, and D the diffusion coefficient for the electroactive reagent (DMDNPD). Moreover, the concentration-normalized peak oxidative current is independent of the solution concentration of DMD-

N-N Bond Cleavage in N-Nitrosoarylamines

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SCHEME 2: Proposed Reaction Mechanism for the Oxidation of DMDNPDa

a Note HS is a hydrogen atom source, likely the supporting electrolyte (see text), and that the products depicted are both readily amenable for further oxidation (homogeneous or heterogeneous).

NPD, except at the fastest scan rates employed (2.0 V/s), where the small deviations are in line with that anticipated for Ohmic losses within the system.14 For the experimental timescales employed, the heterogeneous electron transfer is obserVed to be slow, with the oxidative cyclovoltammetric peak shifting to more positive potentials by 67.6 ( 10.1 mV per decadic change in the voltage scan rate (q.V. Figure 1d, for which the Pearson’s Product Moment correlation coefficient was 0.921). This value is larger than that anticipated for Nernstian electron transfer, which is confirmed by examining the peak widths which vary in the range 40 e (Ep - Ep/2)/mV e 80 for the scan rates considered, suggesting that 0.3 e β e 0.6 [β ) 1.857(RT/nF(Ep - Ep/2))]. In order to deduce the electrochemical reaction pathway, potentiostatic electrolysis was undertaken. The passage of twoFaradays-per-mole of DMDNPD caused the yellow solution to turn dark brown. This is not surprising since NO+ is known to

Figure 2. Cyclic voltammograms (showing five consecutive cycles, T ) 296 K) for the oxidation of 2.0 mM Cupferron in 0.1 M Bu4NClO4/ MeCN at a 3.0 mm diameter platinum electrode at a scan rate of 0.1 V/s. The arrow indicates the direction of the initial voltage sweep.

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SCHEME 3: Mechanism for the Oxidation of Cupferrona

a (a) The only mechanism reported in the literature. (b) The proposed stepwise pathway involving mixed surface adsorption and diffusion of the material to the electrode surface prior to electron transfer.

form intensely colored charge-transfer complexes with a variety of arenes.16a Isolation and subsequent analysis of the resulting products proved difficult, indicating a reaction mechanism complicated by several reaction products. Scheme 2 illustrates several routes for the two-electron oxidation of DMDNPD, based on the literature-reported value of the •NO|NO+ redox potential (1.51 V vs standard hydrogen electrode)16 at platinum electrodes in acetonitrile. To determine whether the N-N bonds are cleaved in a concerted or sequential manner with the heterogeneous electron transfer, steady-state microelectrode voltammograms were treated as described in S1 of the Supporting Information to obtain variations of the heterogeneous rate constant (khet) with applied potential. Such plots are curved and enable the deduction of the symmetry factor

β)-

(

∂∆G+ RT ∂ln khet 1 n(E - E0) ) 1 ) 2 ∂∆G0 nF ∂E 4∆G+0

)

(2)

where all symbols have their usual meanings, and the intrinsic activation barrier (∆G+ 0 ) is measured in eV. Plots of β vs E are linear, enabling the calculation of 4∆G+ 0 ) 1.2 ( 0.1 eV from the gradient and E0 ) 1.3 ( 0.2 V vs Ag|AgCl|Cl- (deduced when β ) 1/2). The latter is consistent with the formation of +NO. Since the reorganization energy for the formation of the radical cation of N,N,N′,N′-tetramethyl-para-phenylenediamine is17 ca. 0.42 eV and the continuum models of solvation energy vary as the square of the charge change, the experimental data are consistent with the formation of a dication with charge centered on the NO moieties, followed by fast NO+ release, or via an ECEC mechanism in which both chemical cleavages are fast and the second oxidation occurs faster than the first. Given the likelihood of forming NO+ in the bis(nitrosamine), it is pertinent to enquire whether other nitrosamines exhibit such

behaviors. Accordingly, following the pioneering work by Kolthoff,3a Lawless and Hawley,3c and Wang,3g,i demonstrating that Cupferron (N-nitrosophenylhydroxylamine) is able to release nitric oxide upon electrolysis, the molecular mechanism was next probed. 3.2. Voltammetry of Ammonium N-Nitrosophenylhydroxylamine (Cupferron) in Acetonitrile Solution. Figure 2 illustrates five consecutive voltammteric sweeps for the oxidation of Cupferron in acetonitrile at 0.1 V/s. A single, broad, irreversible oxidation peak is observed at 0.55 V vs Ag|AgCl|Cl-. The reduction wave at -0.35 V vs Ag|AgCl|Clis the reduction of the nitroso functional group of Cupferron.3b Repetitive scanning causes a slight shift toward more positive potentials in the peak oxidative current, with a concomitant decrease in the magnitude of the peak current. The reproduction of the voltammetry without extensive cleaning of the electrode using nitric acid was particularly difficult, suggesting the occurrence of adsorptive blocking of the electrode surface. Consistent with this interpretation is the rather peculiar current voltage tracing for the fifth scan in Figure 2, where the current on the reverse scan initially augments before dropping. The oxidation of Cupferron, previously studied in aqueous media by Lawless and Hawley,3c results in the release of nitric oxide from the compound via cleavage of the N-N single bond with the removal of a single electron (n ) 1.0), although the route (stepwise or concerted) was not determined (see Scheme 3). Although the behavior of the Cupferron voltammetry described in that earlier work was undertaken in an aqueous system, the similarities of the characteristics illustrated herein in acetontrile solutions suggests that it is not unfeasible that the oxidation peak at 0.55 V vs Ag|AgCl|Cl- is due to the release of nitric oxide. Lawless and Hawley provided evidence, later confirmed by Wang,3g that •NO is generated during the oxidation

N-N Bond Cleavage in N-Nitrosoarylamines

Figure 3. (a) Cyclic voltammograms (showing five consecutive cycles, T ) 296 K) for the oxidation of 1.0 mM N-nitrosodiphenylamine in 0.1 M Bu4NClO4/MeCN at a 3.0 mm diameter platinum electrode at a scan rate of 0.5 V/s. The arrow indicates the direction of the initial voltage sweep. The inset illustrates the variation of the peak oxidation potential as a function of the scan rate, illustrating the anticipated anodic shift as the time scale decreases. (b) Cyclic voltammogram (at scan rate of 1.0 V/s) illustrating the surface electrochemistry of Nnitrosodiphenylamine adsorbed onto a 3.0 mm diameter gold electrode, obtained in 0.1 M Bu4NClO4/MeCN solution containing none of the target compound. Note the presence of the reverse wave indicates a reduced degree of cleavage, thereby confirming the presence of a stepwise reaction pathway. The arrow indicates the direction of the initial voltage sweep.

of Cupferron by noticing further oxidation waves (at much higher potentials) corresponding to the oxidation of •NO to first nitrite then nitrate. In the acetonitrile solutions employed, it was observed that •NO is not oxidized at the electrode at the potentials considered during these experiments. Irrespective of the pathway (stepwise or concerted), the experimental data suggest that the reaction mechanism is further complicated by adsorptive phenomena. Experiments designed to elucidate this latter phenomenon, using ferricyanide reduction18 as a marker for the degree at which adsorptive “blocking” of the electrode occurs,19 revealed that neither Cupferron nor the aromatic byproduct of the electrolysis, nitrosobenzene, foul the electrode even after 1.5 h of soaking in an acetonitrile solution containing millimolar Cupferron or nitrosobenzene. Rather, above the point of zero charge (pzc) of polycrystalline platinum (determined by Fawcett20 to be ca. +0.1 V vs Ag|AgCl|Cl- in acetonitrile21 solution containing 0.1 M tetraethlyammonium perchlorate), anionic Cupferron adsorbs onto the surface, thereby “self-inhibiting” the reaction, as evidenced by the ferricyanide voltammetry resembling that of a partially blocked electrode.22 Nevertheless, the fact that an adsorbed species (of fractional coverage θ) is found at the electrode surface and the fact that the peak current scales with the square root of the scan rate (data not shown, nor analyzed in further detail, owing to the uncertainty in the electrode area), characteristic of diffusion of

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12933 material to the electrode surface, suggest that to drive the oxidation of Cupferron a greater overpotential has to be applied than would otherwise be required, since the heterogeneous kinetics are effectively “decelerated” to ks0(1 - θ) compared with those observed at a “clean” surface, ks0. Considering the stepwise/concerted dichotomy, that a greater driving force is applied to induce the same degree of Faradaic efficiency, suggests the view that the partial blocking of the electrode surface is a new and alternative route9 to drive the mechanistic pathway to a sequential pathway, so that a reactive intermediate is electrochemically generated. Accordingly, Tafel analysis of the voltammetric data (see S2 Supporting Information) affords symmetry coefficients of ca. 0.3 with only slight variation over 3 orders of magnitude in time scale. Such values for β are characteristic of the stepwise process, which is not altogether unrealistic given the degree of reorganization that is required for electrolysis of the adsorbed anion into neutral products. It follows that the oxidation of other N-nitrosoamines could also follow a stepwise pathway when controlled by adsorption. Accordingly, the focus is next turned to the examination of a variation of this generic model by investigating the electrochemical oxidation of N-nitrosodiphenylamine. 3.3. Voltammetry of N-Nitrosodiphenylamine in Acetonitrile Solution. Figure 3a shows voltammograms for the oxidation of N-nitrosodiphenylamine in acetonitrile. Only a single oxidation peak is observed at 1.85 V vs SCE, with no immediate reverse peak obtained, even at the fastest scan rates employed (2.0 V/s). Repetitive scanning causes a slight shift in peak oxidation potential to more positive values and a major concurrent decrease in peak oxidative current. Also noteworthy is the progressive broadness of the peaks with consecutive scans. These data, taken together, suggest the oxidation is irreversible and that the voltammetry may be affected by adsorption. On the basis of the photochemical decomposition of Nnitrosodiphenylamine, the noted standard reduction potential for NO+, and the suggestion from § 3.2 that adsorption may favor a sequential route, the reaction mechanism may be described as depicted in Scheme 4, in which a single heterogeneous electron transfer event generates a cation radical which subsequently cleaves to form NO+. The marked decrease of the peak current on successive scans as illustrated in Figure 3a is unusual. Such behavior has been observed before by Save´ant and co-workers24,25 in which a reactive intermediate “self-inhibits” the reaction by bonding to the surface, so that the outcome of the electrolysis is an electro-grafted surface. Such interpretations are consistent with the observed voltammograms. Evidence to confirm the stepwise reaction route was sought from Tafel plots. The oxidation wave was analyzed in the upper and lower quartile potentials, and the data are reported in S3 (see Supporting Information) with the gradient from the plots of the potential against the logarithm of the current at each scan rate providing values for the transfer coefficient. Indeed, for this case, the symmetry factor changes with the experimental time scale, going from ca. 0.4 at long timescales to 0.2 at more rapid experimental durations. These values do indeed suggest a sequential mechanism provided the electron transfer is the slow step. The probing of the dynamics of this latter mechanism was attempted using steady-state voltammetry at both microdisc electrodes and rotating disk electrodes. However, the observed plots were peak-shaped rather than sigmoidal (data not shown), suggesting that the N-nitrosodiphenylamine adsorbs to the electrode surface prior to the oxidation. Accordingly the oxidation of adsorbed N-nitrosodiphenylamine was investigated.

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SCHEME 4: Observed Stepwise Mechanism for the Oxidation of N-Nitrosodiphenylamine, with a Significant Adsorbed Componenta

a

Note the possibility (not indicated here) for the electro-grafting of the oxidized intermediate with the electrode surface.

Previous work has demonstrated that nitrobenzenes in general have an affinity for gold surfaces and readily adsorb.26,27 Thus, gold electrodes were employed. The voltammetric behavior of these N-nitrosodiphenylamine-modified gold electrodes in 0.1 M TBAP + MeCN solution at 1.0 V/s is depicted in Figure 3b. A single broad and reversible pair of “bell-shaped” oxidation and reduction peaks are observed at Emid ) 1/2(EpOx + EpRed) ) 1.0 V vs SCE. The peak-to-peak potential separation of ca. 0.0 mV is ideal in spite of the broak waves and demonstrates the ready and stepwise conversion of neutral starting material into radical cations. Analysis of the integrated charge under the oxidation or reduction waves indicates a coverage density of ca. 1011-1010 molecules/cm2, suggesting the formation of a passivating monolayer. 4. Conclusion The electrochemical oxidation of N-nitrosoarylamines has been considered. Although Cupferron enables the electrosynthesis of nitric oxide, the other two compounds, N-nitrosodiphenylamine and the bis(nitroso)phenylenediamine, appear to produce NO+. This latter species is highly oxidizing. Work is currently in progress in this laboratory to harness this source of NO+ as a reagent for controlled nitrosoation. Acknowledgment. L.A.E. thanks EPSRC for a DTA studentship. M.P. thanks the Serbian Government for a training scholarship. C.W. and P.W. thank EPSRC for financial support. M.A. thanks the Serbian Ministry of Science (project number: 142 012). We thank Richard Bourne for undertaking several preliminary experiments. J.W. gratefully acknowledges The University of Hull for financial support. Supporting Information Available: S1-S3: Methods for determining heterogeneous electron transfer rate constants from steady-state microelectrode data and Tafel analysis of the

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J. Phys. Chem. C, Vol. 112, No. 33, 2008 12935 (18) Although ferricyanide is employed due to its perceived outersphere electron transfer characteristics, it should be noted that Weaver and coworkers2a have observed an adsorbed Fe-CN stretching occurring at the surface of metal electrodes, in particular gold. In other words, the use of ferricyanide is an excellent marker for the occurrence of adsorptive phenomena. (19) (a) Amatore, C.; Save´ant, J.-M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39. (b) Brookes, B. A.; Davies, T. J.; Fisher, A. C.; Evans, R. G.; Wilkins, S. J.; Yunus, K.; Wadhawan, J.; Compton, R. G. J. Phys. Chem. B 2003, 107, 1616. (c) Davies, T. J.; Brookes, B. A.; Fisher, A. C.; Yunus, K.; Wilkins, S. J.; Greene, P. R.; Wadhawan, J.; Compton, R. G. J. Phys. Chem. B 2003, 107, 6431. (20) Marinkovic´, N. S.; Hecht, M.; Loring, J. S.; Fawcett, W. R. Electrochim. Acta 1996, 41, 641. (21) The 300 mV difference observed between the reported pzc and the experimental data presented herein can be attributed to differences due to the chemical identity of the supporting cation, the degree of polycrystallinity of the electrodes employed in the study herein and in that of Fawcett20 and most probably due to noncalculated differences due to the liquid junction potentials present in the work described herein. (22) Further evidence of the occurrence of adsorption of the starting materials was inferred from two further experiments in which the rate of material transport to the electrode surface was augmented compared with the cyclovoltammetric case described. In the first, rotating-disc experiments conducted exhibited sigmoidal voltammograms in which only a single wave was observable, with half-wave potential (E1/2) in the range 0.41 e E1/2/V vs Ag|AgCl|Cl- e 0.44, dependent on both the rotation speed and the (irreproducible) degree of mechanical polishing of the electrode surface. The second set of experiments investigated the oxidation of 1.0 mM Cupferron at both a 25.0 and 10.0 µm platinum disc electrode at 298 K. Analysis of the potential parameters of the wave suggested initially slow kinetics followed by faster kinetics in the upper quartile of the wave. These data are in agreement with the view that the oxidation of Cupferron is “selfinhibiting”, with significant adsorption affecting the initial kinetics of the mechanism. Additional evidence of this was the failure to model the steadystate voltammetry at the micro-disc electrodes using the approach developed by Bard and Mirkin.23 (23) (a) Mirkin, M. V.; Bard, A. J. Anal. Chem. 1992, 64, 2293. The protocols outlined therein are based on theory developed in ref 23b. (b) Oldham, K. B.; Myland, J. C.; Zoski, C. G.; Bond, A. M. J. Electroanal. Chem. 1989, 270, 79. (c) Welford, P. J.; Brookes, B. A.; Wadhawan, J.; McPeak, H. B.; Hahn, C. E. W.; Compton, R. G. J. Phys. Chem. B 2001, 105, 5253. (24) (a) Delamas, M.; Hitmi, R.; Pinson, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1992, 114, 5883. (b) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1997, 119, 201. (c) Downard, A. J. Electroanalysis 2000, 12, 1085. (d) Pinson, J.; Podvorica, F. Chem. Soc. ReV. 2005, 34, 429. (25) (a) Bhugun, I.; Save´ant, J.-M. J. Electroanal. Chem. 1995, 395, 127. (b) Bhugun, I.; Save´ant, J.-M. J. Electroanal. Chem. 1996, 408, 50. (26) (a) Wadhawan, J.; Davies, T. J.; Clegg, A. D.; Lawrence, N. S.; Ball, J. C.; Klymenko, O. V.; Rees, N. V.; Bethell, D.; Woolfall, M. P.; France, R. R.; Compton, R. G. J. Electroanal. Chem. 2002, 533, 33. (b) Macfie, G.; Wadhawan, J.; Compton, R. G. J. Electroanal. Chem. 2001, 510, 120. (27) Koopmann, R.; Gerisher, H. Ber. Bunsenges. Phys. Chem. 1966, 70, 127.

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