Electron Transfer to a Phosphomolybdate Monolayer on Glassy Carbon: Ambivalent Effect of Protonation

Abstract

The polyoxomolybdate hybrid TBA3[PMo11O39{Sn(C6H4)C≡C(C6H4)N2}] K(Mo)Sn[N2(+)] was prepared through Sonogashira-type coupling between TBA4[PMo11O39{Sn(C6H4)I}] K(Mo)Sn[I] and an excess of 3,3-diethyl-1-(4-ethynylphenyl)triaz-1-ene bearing a protected diazonium function, followed by its deprotection by the addition of trifluoroacetic acid (TFA). This enlarges the family of organic-inorganic polyoxomolybdate-based hybrids, which has been far less investigated than their related polyoxotungstates. The diazonium function allows for the electrochemical grafting on glassy carbon, and the K(Mo)Sn-modified electrode was further probed by cyclic voltammetry. The PMo11Sn core was found to be highly sensitive to protonation, and five bielectronic proton-coupled electron transfer processes were detected in the presence of an excess of TFA, thus corresponding to the injection of up to 10 electrons in the potential range between 0.15 and -0.45 V/SCE. The gain observed in the thermodynamic potentials is however detrimental to the apparent kinetics of the electron transfer, which drops from 500 s(-1) in the absence of acid to 12 s(-1) in the presence of an excess of TFA.

Tailor–made Covalent Organic‐Inorganic Polyoxometalate Hybrids: Versatile Platforms for the Elaboration of Functional Molecular Architectures

“…Importantly the potential shift upon the addition of acid is significantly greater for molybdates than for their tungstate analogues. This is attributed to the higher basicity of oxidized as well as reduced polyoxomolybdates compared to polyoxotungstates . We thus developed a series of POM hybrids displaying a wide range of electron accepting properties that can be further tuned by the careful control of the associated counter ions.…”

“…To tune the redox properties of the POMs, the analogous molybdate K Mo Sn [N 2 + ] was similarly attached to glassy carbon . It is more easily reducible than the corresponding tungstate as presented above but also very sensitive to the presence of adventitious protons (possibly remaining from the last step of its synthesis), which causes a broadening of the electrochemical waves and even a splitting of the reoxidation peak of the first process at high scan rates.…”

Thermodynamics, Electrode Kinetics, and Mechanistic Nuances Associated with the Voltammetric Reduction of Dissolved [n-Bu4N]4[PW11O39{Sn(C6H4)C≡C(C6H4)(N3C4H10)}] and a Surface-Confined Diazonium Derivative

“…Nano-sized metal oxoanions, known as polyoxometalates (POMs), are an important class of inorganic clusters that are usually constructed from early transition metals. Their ability to undergo an extensive series of electron transfer reactions when dissolved in solution or attached to electrode surfaces [1][2][3][4] has made these anions valuable in applications of electrochemistry in diverse fields such as electrocatalysis [5][6][7], electrosynthesis [8], biosensing [9,10] and energy storage [11,12].…”

“…Glassy carbon (GC) has been available as an electrode material [2,[13][14][15] for over half a century.…”

“…Much of the recent solution phase and surface confined POM electrochemistry has been reported at this electrode surface to improve selectivity in electroanalysis or to overcome slow electron-transfer kinetics found with use of conventional metal electrodes [16,17]. The main techniques for electrode modification are largely based on electrostatic [18][19][20] or covalent attachment [2,3,21,22].…”