SUPPORTING INFORMATION

Julia Agullo<sup>a</sup>, Sylvain Canesi<sup>a</sup>, Frank Schaper<sup>b</sup>, Mario Morin<sup>a</sup> and Daniel Bélanger<sup>a</sup>

UV spectra of 3-hydroxypyridine, 3-chloropyridine and a mixture of 3-aminopyridine and NaNO₂ in HCl. The UV spectrum recorded 60 min. after the addition of NaNO₂ in a solution containing 3-aminopyridine in 0.5 M HCl shows an adsorption band around 285 nm (Fig. S1). It was hypothesized that the adsorption band around 285 nm corresponds to a mixture of the diazonium cations generated and its hydrolyzed products. Figure S1 shows also UV spectra of 3-chloropyridine and 3-hydroxypyridine recorded in 0.5 M HCl which present one characteristic absorption band at 270 and 290 nm, respectively. The contribution of 3-chloropyridine in the mixture spectrum is slightly noticeable between 250 and 270 nm whereas the one of the 3-hydroxypyridine is observed above 270 nm. These results are in agreement with NMR analysis that demonstrates that the decomposition products are 3-hydroxypyridine and 3-chloropyridine (in minor proportions).

<footnote><sup>a</sup> Département de Chimie, Université du Québec à Montréal, C. P. 8888, succursale Centre-Ville, Montréal, Québec, H3C 3P8, Canada</footnote>

<footnote><sup>b</sup> Département de Chimie, Université de Montréal, C. P. 6128 succursale Centre-Ville, Montréal, Québec, H3C 3J7, Canada</footnote>
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Figure S1. UV spectra of 1 mM 3-aminopyridine + 1 mM NaNO₂ in a 0.5 M HCl recorded after reaction for 60 min. (solid line). The absorption spectra of 3-hydroxypyridine (---) and 3-chloropyridine (grey -- --) are also shown.

High-resolution mass spectrum of a solution containing a mixture of 3-aminopyridine and NaNO₂ in 0.5 M HCl recorded after 4 h. Figure S2 shows the high-resolution mass spectrum of a solution containing a mixture of 3-aminopyridine and NaNO₂ in 0.5 M HCl recorded after 4 h. Evidence for the presence of 3-hydroxypyridine is given by the peak for [C₅H₆NO]⁺ m+H⁺ / z = 96.0446, diff = 2.26 ppm.
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Figure S2. High-resolution mass-spectrum of 1 mM 3-aminopyridine + 1 mM NaNO₂ in a 0.5 M HCl after 4 h of reaction. 20 µL of the mixture dissolved in 500 µL of methanol was directly injected.

¹H NMR spectrum of 3-chloropyridine in DCl / D₂O. Figure S3 presents ¹H NMR spectrum of 3-chloropyridine in D₂O and in presence of deuterium chloride for a final concentration in DCl of 0.5 M. The spectrum presents four multiplets, with chemical shifts centered around 8.8, 8.6, 8.5 and 7.9 ppm, corresponding to protons on the aromatic ring of 3-chloropyridine.

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Figure S3. ¹H NMR spectra of 3-chloropyridine (30 M) in a 0.5 M DCl / D₂O solution.
Calculation of the rate constants for the formation and the decomposition of 3-diazopyridinium cations. Second order rate constants were determined with concentrations of 1 mM 3-aminopyridine and 1 mM NaNO₂ at 22°C. Disappearance of 3-aminopyridine (315 nm) and appearance of 3-diazopyridinium cations (285 nm) yielded identical rate constants. While other concentrations were not investigated in detail, reactions using 0.7 mM 3-aminopyridine and 1.3 mM NaNO₂ yielded comparable second-order rate constants. The rate constant k for the formation of 3-diazopyridinium cations was obtained by fitting the absorbance-time data from an average on 30 shots to an integrated second-order equation using OriginPro 8.0:

$$ \frac{1}{A - A_{\text{inf}}} = \frac{k}{\Delta\varepsilon \cdot \lambda} \cdot t + \frac{1}{A_0 - A_{\text{inf}}} \quad \text{and} \quad \Delta\varepsilon = \varepsilon_{\text{amine}} - \varepsilon_{\text{diazxo}} = \frac{A_0}{C_0 \cdot \lambda} - \frac{A_{\text{inf}}}{C_0 \cdot \lambda} $$

where A is the absorbance at a selected wavelength, A₀ is the initial absorbance of the 3-aminopyridine solution, Aₘₐₗ the absorbance at infinite time, εₐₘₐₗ and εₐₗₓ₀ are the molar extinction coefficients of the amine and the corresponding diazonium ion respectively and λ is the optical path length. The absorption of NaNO₂ in HCl is negligible for wavelength higher than 250 nm.

The concentration of the diazonium in the reaction mixtures was calculated from the integration of each NMR peaks at 22°C and from the assumption that the starting amine was mainly transformed into the diazonium ion. The observed rate constant kₐₗₓ₀ for the decomposition of the diazonium cations was estimated by fitting the absorbance-time data to an integrated first-order equation using OriginPro 8.0:

$$ \ln(C - C_{\text{inf}}) = -k_{\text{obs}} \cdot t + \ln(C_0 - C_{\text{inf}}) $$
where C is the concentration of the diazonium ions obtained from the NMR spectra at each time, C₀ the initial concentration of diazonium ions and Cₘf concentration of diazonium ions at infinite time.

*Calculation of the surface coverage of the glassy carbon electrode by the grafted layer.* The surface coverage of the electrode by the grafted layer was estimated from the pyridine N1s peak area / C1s peak area ratio.¹ The ratio which takes into account unprotonated and protonated pyridine nitrogen is 12.1 at. %. Assuming the simplest case of a carbon atom surface density equal to that of basal plane graphite (7.3 x 10⁻⁹ mol cm⁻²), the surface coverage was estimated at 8.8 x 10⁻¹⁰ mol cm⁻² using the following equation: $Γ = \frac{R}{C} \cdot 7.3 \cdot 10^{-9}$

where $\frac{R}{C}$ is the ratio of the total pyridine area under the N 1s peak with respect to the C 1s peak.

*Linear sweep voltammograms recorded after period of times ranging from 1 to 60 min. in a solution containing 10 mM of NaNO₂ in 0.5 M HCl.* Figure S4 shows the first linear sweep voltammogram for the nitrite reduction for different reaction times in acidic media. The onset of nitrite reduction occurred at around -0.3 V. The intensity of the reduction peak current around -0.6 V decreases continuously with the increase of the reaction time. The same trend is observed the diazonium reduction for different diazotization times.
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Figure S4. Linear sweep voltammograms recorded at a scan rate of 50 mV s⁻¹ for the reduction of an aqueous 10 mM of NaNO₂ / 0.5 M HCl solution. The initial potential is 0.6 V and only the first scan in the negative potential direction is shown. The reaction between NaNO₂ and HCl was carried out during period of times ranging from 1 to 60 min. prior to recording the linear sweep voltammogram. Inset: Cyclic voltammogram for the reduction of 10 mM of NaNO₂ after one min. of reaction in 0.5 M HCl recorded at a scan rate of 50 mV s⁻¹.

REFERENCE

(1) Liu, Y. C.; McCreery, R. L. *J. Am. Chem. Soc.* **1995**, *117*, 11254.