Determination of the protonation state of [H3PW11O39]4− and the stability constant of [Ag(H2O)(H3PW11O39)]3− in aqueous solution

Abstract By employing elemental analysis, 31P NMR, pH, and conductivity measurements, the protonated state of lacunary heteropolyoxotungstophosphates in aqueous solution, {PW11O39}, is determined to be [H3PW11O39]4−. Using it as ligand, a complex of [Ag(H2O)(H3PW11O39)]3− is formed. An electrochemical cell is designed as follows: (−) Hg, Hg2Cl2 [Ag(H2O)(H3PW11O39)]3− (aq) | Ag(s) (+) (salt bridge is saturated KNO3 solution). By measuring the electromotive force of the cell, the stability constant of [Ag(H2O)(H3PW11O39)]3− in aqueous solution is determined to be 4.34 × 103 (25 °C).


Introduction
Lacunary polyoxometalates (POMs) are a class of inorganic oxygen-containing ligands which can form stable complexes with metal ions [1,2]. In recent years, homogenous molecular water oxidation catalysts of cobalt and ruthenium-containing POM complexes have attracted attention because they possess high stability toward oxidative degradation [3]. Their stability in aqueous solution has been of concern, especially for cobalt ion complexes [4]. Usually, the stability of water oxidation catalysts has been studied through qualitative technologies such as NMR, UV, dynamic light scattering, and capillary electrophoresis [5]. Therefore, it is significant to study the stability quantitatively for understanding the catalytic oxidation mechanism.
In the 1960s, Baker and Pope reported a general structural category of heteropolyoxometalates, formulated as [H n M +m O 6 X +x O 4 W 11 O 30 ] - (14−x-m-n) [6], the structure of which is a modification of the well-known plays important roles in both the transmission of electron and proton, and the redox performance of Ag + is improved. However, there was a concern about the stability of [Ag(H 2 O) L] 3− in the catalysis reaction. Although its stability has been investigated by NMR, UV-visible absorption spectra, dynamic light scattering, and cyclic voltammetry, it is still necessary to understand its stability quantitatively with the stability constant of complexes. Based on the work, the stability constant of [Ag(H 2 O)L] 3− in aqueous solution is studied.

General methods and materials
All reagents and chemicals were purchased from commercial sources and used without purification. The elemental analyses of K, P, and W were performed on an ICP-PRODIgY analyzer. The IR spectrum was recorded using KBr pellets on a TeNSOR27 Bruker AXS spectrometer from 4000 to 400 cm −1 . Tg analysis was performed on a Pyris Diamond Tg/DTA thermal analyzer at a heating rate of 10 °C min −1 in air. X-ray powder diffraction data were collected on a D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å). 31 P NMR was performed in a Bruker AVANCe500 spectrometer with a heavy water lock. Conductivity was measured in a DDS-11A conductivity meter. The pH was determined in a pHS-3C precise pH meter. electromotive force (e.M.F.) measurement was carried out in a UJ-25 type potential difference meter with saturated calomel electrode and silver electrode as negative and positive electrodes, respectively. All experiments were carried out in ultrapure water with conductivity of 1.6 us cm −1 .

Determination of the stability constant of [Ag(H 2 O)L] 3− with E.M.F.
As reported [14], the following ionization reaction occurs when K 3 [H 3 AgPW 11 O 39 ]·12H 2 O is dissolved in water: As shown in figure 1(b The stability constant K a is deduced as follows: An electrochemical cell is designed (salt bridge is saturated KNO 3 solution) as follows: The electrode reactions: (1) where φ calomel = 0.2412 V, φ θ (Ag+/Ag) = 0.7991 V [17], and Ag + = Ag + [Ag + ] ( Ag + represents the activity coefficient of Ag + ). Therefore, For [Ag(H 2 O)L] 3− solution, γ ± (γ ± represents the average activity coefficient in the dilute solution, the activity coefficient of silver ions Ag + can replace γ ± approximately) at concentrations of 1.6, 1.2, 1.0, 0.8, 0.6, 0.4, and 0.2 mM is calculated through the Debye-Hückel limiting equation (see table 5 and Supplemental Material). The e.M.F. of the cell was determined and the data are listed in table 5. The concentrations of dissociated Ag + can be obtained (see table 5) according to equation (9). Then, K c can be obtained by formula (4).
To eliminate the influence of the ionic strength (I) on the stability constant and obtain the activity stability constant K a , a plot of K c against I is drawn (figure 2). By extrapolating to the ionic strength of 0, K a of [Ag(H 2 O)L] 3− can be obtained [18] as 4.34 × 10 3 at 25 °C.

Conclusion
The

Disclosure statement
No potential conflict of interest was reported by the authors.