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Direct electrochemical P(V) to P(III) reduction of phosphine oxide facilitated by triaryl borates Joseph S. Elias, Cyrille Costentin, and Daniel G. Nocera J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07149 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 2, 2018
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Journal of the American Chemical Society
Direct electrochemical P(V) to P(III) reduction of phosphine oxide facilitated by triaryl borates Joseph S. Elias,† Cyrille Costentin,§ ,†,* and Daniel G. Nocera†,* †
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138
Laboratoire d’Electrochimie Moléculaire, Unité Mixte de Recherche Université – CNRS 7591, Université Paris Diderot, Sorbonne Paris Cité, 75205 Paris Cedex 13, France §
ABSTRACT: Triaryl borate Lewis acids facilitate the direct two-electron reduction of the P(V) center of triphenylphosphine oxide (TPPO) to the P(III) center of triphenylphosphine at faradaic efficiencies of 37%. Insight from direct P(V) to P(III) reduction is provided from cyclic voltammetry. The electrochemical reduction of TPPO proceeds through an unusual ECrECi mechanism in which the breaking of the phosphoryl bond in a two-electron-reduced association complex with the triaryl borate is rate-determining. The rate and faradaic efficiency for TPPO reduction are tuned by judicious choice of substituents on triaryl borate, with tris(4-methoxyphenyl) borate demonstrating the highest for both. These results suggest that an attractive route towards the room-temperature reduction of phosphate for phosphorus reclamation is greatly facilitated by the stabilization of reduced phosphate intermediates through their association with Lewis acids.
Introduction
selectively reduce the phosphine oxide back to the parent phosphine. 13 The most successful strategies often involve the use of an excess of silyl halide – most notably trimethylsilyl chloride (TMSCl) – which is reactive toward attack by a nucleophilic, activated phosphoryl bond to produce an equivalent of siloxane per two equivalents of silyl halide under reducing potentials. 14,15 The resulting siloxane is thermodynamically stable, and the silyl halide can only be regenerated through the energy-intensive carbothermal reduction of SiO2 to elemental Si followed by reaction with the appropriate alkyl halide. Similarly, other reported strategies often involve the use of excess reagents (such as AlCl3, 16,17 or P(OPh)3 18) that serve as sinks for O2– after the cleavage of the phosphoryl bond, producing byproducts (Al2O3, O=P(OPh)3) that can only be regenerated through high-energy routes. Herein we report that triaryl borate Lewis acids facilitate the electrochemical reduction of triphenylphosphine oxide (TPPO) to PPh3 at glassy carbon electrodes with faradaic efficiencies of 37%. Cyclic voltammetry reveals as a main pathway a unique ECrECi reaction mechanism in which the breaking of the phosphoryl bond is rate-determining. As a result of the slow P=O bond-breaking step, the two-electron-reduced association complex Ph3P–O– B(OAr)32– disproportionates to give the four-electron product PHPh2 at nearly identical faradaic efficiencies at glassy carbon electrodes, a reaction that may be suppressed at gold surfaces. The overall faradaic efficiency for TPPO reduction can be tuned by judicious choice of aryl substituents on the Lewis acid, with tris(4methoxyphenyl) borate (B(OC6H4OMe)3) giving the highest faradaic efficiency. The electrochemical reduction of TPPO with such Lewis acids proceeds with the formation of coordinativelysaturated aryl borate species, which can be regenerated to B(OAr)3 through hydrolysis and subsequent condensation with ArOH and appropriate volumes of toluene. The methodology reported herein for the reduction of TPPO bypasses the use of high-energy density
Phosphorus is an essential element for life and holds tremendous importance in the purview of commodity chemical synthesis. As of 2018, the increase in the world consumption of phosphorus outpaces global population growth by nearly 23%, with the top two producers of phosphate rock – China and the United States – on track to deplete their reserves by the middle of the twenty-first century. 1 Mined phosphate rock is typically reduced to white phosphorus (P4) used in fine chemical synthesis, where it is subsequently oxidized to P(III) halide or sulfide intermediates before further modification. Although the oxidation of P4 is downhill (ΔH0rxn = –304 kcal mol–1), the reduction of phosphate rock is considerably endothermic (ΔH0rxn = 700 kcal mol–1) making the overall process both energy intensive and inefficient. The direct, two-electron reduction of P(V) oxoacids to their synthetically-useful P(III) counterparts, therefore, is of considerable interest since this reaction is governed by a smaller thermodynamic penalty (ΔG0rxn = 68 kcal mol–1 at pH 7). A fundamental challenge facing the reduction of P(V) compounds is the selective cleavage of the phosphoryl (P=O) bond, which has multiple-bond character and a high bond-dissociation energy (110 kcal mol–1 for H3PO). 2 Thus, from both energy and environmental standpoints of sustainability, 3 it is critical to develop direct methods that can reclaim, recycle, and repurpose phosphorus compounds from phosphates. Phosphines make up an important class of ligands in organometallic chemistry and they are ubiquitous reagents in the Appel, 4– 6 Mitsunobu, 7,8 Staudinger, 9,10 and Wittig reactions, 11,12 where the thermodynamic preference to form the phosphoryl bond drives these transformations. As the resulting P(V) phosphine oxide byproduct is generally discarded after separation, recent efforts have been directed toward developing strategies to
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Table 1. Electrochemical reduction of TPPO with B(OAr)3.
B(OAr)3 none B(OPh)3 B(OC6H4OMe)3
F.E. (PPh3)/% a 0 23(10) 37(5)
F.E. (PHPh2)/% a 0 21(4) 40(7)
Faradaic efficiencies were averaged over four points during 30 min potentiostatic bulk electrolysis. The standard deviations are given in the parentheses.
a
Figure 1. Cyclic voltammetry of triphenylphosphine oxide (1 mM) with the titration of B(OC6H4OMe)3. Experiments were carried out in 0.1 M TBAClO4/MeCN at a glassy carbon electrode at 0.25 V s–1.
PHPh2, is observed with comparable faradaic efficiencies (Table 1, Figure S3, and Table S2). The appearance of the four-electron reduced product PHPh2 cannot arise from PPh3 reduction because the standard reduction potential for PPh3 is 155 mV more negative19 than that for TPPO reduction. In support of this contention, electrochemical measurements were carried out in 1.0 mM PPh3 with 5 equiv of B(OAr)3. The CVs (Figure S7) indicate reductive waves with peak currents sufficiently negative of –3.0 V. No reduced phosphorus species were detected by 31P NMR after bulk electrolysis (Figures S8), even when carried out for 6 h. Additionally, no organophosphinous acid esters, such as Ph2POBu, are detected as products in the bulk electrolysis. Such products have been reported previously to arise from the reaction of electrochemically-generated TPPO•– with the tetrabutylammonium cation in acetonitrile solutions. 20 Along these lines, bulk electrolysis in the absence of B(OAr)3 resulted in resonances at 14.2, 13.4 and 10.1 ppm that we attribute to organophosphinic (O=PR2OH) and organophosphonic (O=PR(OH)2) acids and their esters (Figure S4), which arise from the cleavage of the P–C bond. Benzene, as a side product of P–C bond cleavage, is also detected in the absence of B(OAr)3 (Figure S4C). Importantly, the Lewis acid is required for phosphoryl bond cleavage as neither PPh3 nor PHPh2 are detected without B(OAr)3. Analysis of the bulk electrolysis products by 11B NMR spectroscopy (Figure S5) indicated the presence of B(OC6H4OMe)3 (broad resonance at 16 ppm) in addition to the appearance of sharp resonances at 1.5–0.4 ppm that we attribute to tetrahedral borate species. Specifically, a set of peaks at 1.52, 0.53, and 0.43 ppm appears after 5 min (0.5 C) at the expense of a pair of peaks at 1.02 and 0.96 ppm. We assign the peaks at 1.02, 0.96, and 0.43 ppm to reduced TPPO/B(OC6H4OMe)3 intermediates while the growth of the features at 1.52 and 0.53 ppm correspond to the accumulation of tetrahedral borate byproducts such as BOH(OC6H4OMe)3– or [(MeOC6H4O)3B]2O2–. Bulk electrolysis in the absence of TPPO (Figure S6) leads to the decomposition of B(OC6H4OMe)3 and the appearance of a single new resonance at 0.89 ppm, thereby excluding the possibility of the direct reduction of B(OC6H4OMe)3 during TPPO reduction. Decomposition products are also observed for B(OAr)3 equivalencies < 5. Bulk electrolysis performed with 2 equiv of B(OC6H4OMe)3 did not afford any reduced phosphorus products (Figure S9). Inspection of byproducts by 11B NMR reveals evidence for the reductive decomposition of the triaryl borate under these conditions, with a peak at 1.52 ppm growing in
reagents and suggests a general strategy for the electrochemical reduction of thermodynamically stable X=O (X = main group element) bonds.
Results and Discussion Bulk electrolyses of TPPO with B(OAr)3. Figure 1 shows the cyclic voltammograms (CVs) of a 1 mM acetonitrile solution of TPPO in 0.1 M TBAClO4 upon the addition of the Lewis acid B(OC6H4OMe)3. In the absence of any Lewis acid (black), the CV of TPPO exhibits a single wave at E° = –2.95V vs. Fc/Fc+. The peak separation (60 mV) indicates a reversible, one-electron process that has previously been attributed to the generation of the TPPO anion radical (TPPO•–). 19 The addition of substoichiometric amounts of B(OC6H4OMe)3 leads to the increase of the cathodic component with the concurrent decrease of the anodic component until the wave becomes completely irreversible with excess B(OC6H4OMe)3. Notably, the cathodic peak potential is unshifted with the titration of the Lewis acid. Further addition of B(OC6H4OMe)3 increases the cathodic peak current but to a less dramatic extent and also induces an anodic shift of the wave (Figure S1). Plots of the cathodic peak current, normalized to the peak current of pure TPPO, exhibit two regimes (Figure S2C). Over the regime of 0 – 1 mM B(OC6H4OMe)3, addition of the Lewis acid leads to a ~2 electron reduction of TPPO that is highly dependent on the concentration of the Lewis acid, whereas in the regime >1 mM B(OC6H4OMe)3, the cathodic current increases with a lesser dependence on the concentration of the borate. Additional features in the CV appear at more positive potentials after 1 equiv of B(OC6H4OMe)3 is added (Figure S1), suggesting multiple electrochemical processes. Bulk electrolyses were carried out potentiostatically at Eapp = –3.0 V in a three-compartment cell in order to determine the products of the electrochemical reduction of TPPO in the presence of B(OC6H4OMe)3. For these experiments, a glassy carbon rod (6 mm Ø) working electrode was separated by a glass frit from a platinum mesh counter electrode, which was separated by a Vycor frit from a Ag/Ag+ pseudo-reference electrode. The volume of the solution in the working electrode compartment was 5.0 mL. Full conversion of 1 mM TPPO to PPh3 corresponds to 0.965 C of charge passed. An excess of 5 equiv of B(OC6H4OMe)3 is required to observe, by 31P NMR spectroscopy, reduced phosphorus species. The two-electron P(III) product, PPh3, is generated at a faradaic efficiency of 37%; additionally, the four-electron reduced product,
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Journal of the American Chemical Society Scheme 1. The ECrECi mechanism for TPPO reduction with triaryl borates.
Figure 2. Scan rate dependence for cyclic voltammograms of TPPO (1 mM) with 0.75 mM B(OC6H4OMe)3. CVs were carried out in 0.1 M TBAClO4/MeCN at a glassy carbon electrode.
intensity at the expense of peaks at 0.96 and 0.43 ppm (Figure S9C). A similar result is obtained with 5 equiv of aryl borate (Figure S6). Importantly, no peaks are detected at 0.53 or 1.02 ppm, leading us to conclude that the peak that grows in at 1.52 ppm corresponds to an off-pathway decomposition product of the aryl borate. Similarly, we conclude that the resonances at 1.02, 0.96 and 0.43 ppm correspond to intermediates along the pathway for TPPO reduction while the peak at 0.53 ppm is likely the tetrahedral borate byproduct of TPPO reduction. As indicated by Tables 1 and S2, ~20% of the charge passed is unaccounted for in bulk electrolysis with 5 equiv of B(OC6H4OMe)3 that we attribute to current consumed by B(OAr)3 decomposition along the pathway of TPPO reduction. Mechanistic studies. In order to interrogate the mechanism of TPPO reduction by B(OC6H4OMe)3, we examined the scan rate dependence of the cyclic voltammetric response at low concentrations of Lewis acid (
