The previously described electrochemical procedure was used to obtain aminyl radical complexes 1+–3+ [42]. ESR spectroscopy was employed to examine the interaction between aminyl radical complexes and MEA. The platinum complex 3+ was selected as the model compound for this investigation due to the presence of the 195Pt isotope with spin ½, which can serve as a marker for spectroscopic studies. The ESR spectrum of the DMF solution of 3+ (black spectrum in Fig. 3) shows a signal with a g-factor of 2.080. The signal coupling originates from the nitrogen, the two phosphorus atoms, and the eight hydrogens of the pincer ligand. Another type of paramagnetic species is also observed, which is attributed to the complexes of 195Pt with a coupling of 90.4 G. At room temperature, the intensity of the ESR signal of the aminyl radical complex 3+ decreases with the addition of increasing amounts of MEA, indicating a reduction of the complex (Fig. 3). The addition of 0.1 equiv. of MEA in form of 5.0 mM solution in DMF reduces the signal intensity by 15%, while 0.25 and 0.4 equiv. reduce it by 44% and 70%, respectively. The signal disappears completely with the addition of 1 equivalent of MEA. Taking into consideration the gradual dilution of complex 3+ solution, it can be concluded that one equivalent of MEA reacts with one equivalent of 3+ (under ESR experiment conditions), indicating one-electron oxidation of the former. No new signals were observed, suggesting that the aminyl radical formed upon the reaction of [(PNP)PtCl]+ with MEA immediately undergoes further transformations (vide infra).
31P{1H} NMR spectroscopy was used to investigate the fate of the complex 3+ after the addition of MEA. Thus, the 31P{1H} NMR spectrum of the neutral complex 3 exhibits a signal at 41.66 ppm with characteristic 195Pt satellites with a spin-spin coupling of 2645 Hz (Fig. 4a). Oxidation of 3 leads to NMR silent solution of 3+ (Fig. 4b). The addition of excess MEA to the NMR tube containing 3+ species regenerated the starting complex 3 as evidenced by the presence of a signal at 42.18 ppm accompanied by 195Pt satellites (Fig. 4c). The insignificant shift of the signal in the spectrum could be attributed to the presence of MEA and its oxidation products. In this way, complexes 1–3 meet the requirements typically expected of a redox mediator or catalyst. The substances exhibit a reversible redox couple and both the oxidized and reduced forms are stable in solution.
Thus, all amido complexes were tested as electrocatalysts for MEA oxidation process. The electrocatalytic experiments were performed in DMF with [n-Bu4N][BF4] (0.1 M) as supporting electrolyte. The cyclic voltammograms were measured for the reaction mixtures of 1–3 with MEA where concentrations of the latter were systematically increased until the icat/ip ratio remained constant reaching the substrate-independent region (Fig. 5). It is important to note that MEA is electrochemically inactive in this region, as shown by the black curves in Fig. 3. The CVs represent the reversible oxidation process responsible for the formation of aminyl radicals 1+–3+ in the absence of any MEA. However, when the concentration of MEA is increased up to 0.25 mM, the oxidation peak loses its reversibility for all complexes while the current of the peak increases. The efficiency of electrocatalytic MEA oxidation can be estimated via the ratio of the maximum catalytic current (icat) to the peak current (ip) in the presence and absence of amine, respectively. This way the palladium complex 2 appears the most effective catalyst in the series with an icat/ip value of 6.7, which is one and a half as active as 3 and three times more active than 1 (Fig. 6).
Controlled potential electrolysis (CPE) was performed in order to reveal MEA oxidation products. Thus, the CPE of 0.5 mM solution of complex 3 in DMF in the presence of [n-Bu4N][BF4] (0.1 M) and MEA (0.5 M) was conducted at 0.23 V (vs. Fc+/Fc) in H-type electrochemical cell with glassy-carbon as working and counter electrode and a silver wire as pseudo-reference electrode. GC-MS and ESI-MS analysis of the reaction mixture suggest the formation of glycolic acid as a main product of this interaction, which is usually coupled with ammonia release (Scheme 1) [24].
With these results in hand, it is possible to propose the mechanism of this catalysis, which involves the electrooxidation of complex 1–3 with the formation of the aminyl radical species 1+ -3+ (Fig. 2). The interaction of 1+–3+ with MEA leads to the regeneration of 1–3 and to the formation of the aminium radical a (Fig. 1). The latter undergoes deprotonation step leading to the imine radical b, which is prone to second electron abstraction and deprotonation, resulting in the formation of imine c. Imine c can react sequentially with residual water from the solvent and oxygen to produce ammonia and glycolic acid.