Role of Multiple Vanadium Centers on Redox Buffering and Rates of Polyvanadomolybdate-Cu(II)-Catalyzed Aerobic Oxidations

A recent report established that the tetrabutylammonium (TBA) salt of hexavanadopolymolybdate TBA4H5[PMo6V6O40] (PV6Mo6) serves as the redox buffer with Cu(II) as a co-catalyst for aerobic deodorization of thiols in acetonitrile. Here, we document the profound impact of vanadium atom number (x = 0–4 and 6) in TBA salts of PVxMo12–xO40(3+x)– (PVMo) on this multicomponent catalytic system. The PVMo cyclic voltammetric peaks from 0 to −2000 mV vs Fc/Fc+ under catalytic conditions (acetonitrile, ambient T) are assigned and clarify that the redox buffering capability of the PVMo/Cu catalytic system derives from the number of steps, the number of electrons transferred each step, and the potential ranges of each step. All PVMo are reduced by varying numbers of electrons, from 1 to 6, in different reaction conditions. Significantly, PVMo with x ≤ 3 not only has much lower activity than when x > 3 (for example, the turnover frequencies (TOF) of PV3Mo9 and PV4Mo8 are 8.9 and 48 s–1, respectively) but also, unlike the latter, cannot maintain steady reduction states when the Mo atoms in these polyoxometalate (POMs) are also reduced. Stopped-flow kinetics measurements reveal that Mo atoms in Keggin PVMo exhibit much slower electron transfer rates than V atoms. There are two kinetic arguments: (a) In acetonitrile, the first formal potential of PMo12 is more positive than that of PVMo11 (−236 and −405 mV vs Fc/Fc+); however, the initial reduction rates are 1.06 × 10−4 s−1 and 0.036 s–1 for PMo12 and PVMo11, respectively. (b) In aqueous sulfate buffer (pH = 2), a two-step kinetics is observed for PVMo11 and PV2Mo10, where the first and second steps are assigned to reduction of the V and Mo centers, respectively. Since fast and reversible electron transfers are key for the redox buffering behavior, the slower electron transfer kinetics of Mo preclude these centers functioning in redox buffering that maintains the solution potential. We conclude that PVMo with more vanadium atoms allows the POM to undergo more and faster redox changes, which enables the POM to function as a redox buffer dictating far higher catalytic activity.

S2 electrolysis until the current dropped to <10% of the initial value, then aliquots were withdrawn, and the UV-Vis spectra were recorded under Ar. The electrolysis was then resumed at the more negative potential as listed in Table S1-S5. Rotating disk electrode (RDE) voltammetry and square pulse wave voltammetry (SWV) were conducted on a Wavedriver 10 potentiostat/galvanostat (Pine Research Instrumentation). For both experiments, the standard three electrode setup was used with a 3-mm diameter glassy carbon disk working electrode, a Ag/Ag + (0.01 M AgNO3 in CH3CN) reference electrode, and a platinum wire counter electrode. The rotation speed from 500-3000 RPM was controlled by a Model AFMSRCE ring-disk electrode system (Pine Research Instrumentation).
For 31 P NMR spectra, PVMo11 has a single peak at -4.31 ppm that proves its purity. PV2Mo10 has a peak at -4.31 ppm indicating the PVMo11 component and a broad peak that split to -4.54 and -4.60 ppm which is assigned to PV2Mo10 and PV3Mo9 components. For PV3Mo9, in addition to the peaks that have essentially the same chemical shifts as for PV2Mo10, multiple peaks more positive than -4ppm may be assigned to PV4Mo8 components. PV4Mo8 and PV6Mo6 all show multiple peaks that cannot be clearly assigned indicating the many components and positional isomers present. It is well-establishd that heterpolyacids, H3+xPVxMo12-xO40 (3+x)-, when x>1, are mixtures of positional isomers and components with different x. 6 The 31 P NMR data in this work shows that the TBA salts of PVMo in acetonitrile are isomeric mixtures.

RSH oxidation and measurement of the varying PVMo reduction states
2-Mercaptoethanol was used as an exemplary substrate for probing the aerobic thiol oxidation, eq 1 in the text, where RSH is 2-mercaptoethanol. The mechanism of the PV6Mo6/Cu system was S3 thoroughly studied in previous work. 5 This article focuses on the impact of the number of vanadium atoms (x = 0-4, and 6) in PVxMo12-xO40 (3+x)-(PVMo). The RSH concentration was quantified using Ellan's reagent (5,5-dithiobis(2-nitrobenzoic acid) (DTNB)). 7 In a typical reaction, 0.1 mL of DTNB solution (5 mg/mL in methanol) was added to a 5 mL pH = 7.4 phosphate buffer solution (50 mM). This solution was first used as the blank for UV-vis measurements. Then, a 10 µL aliquot of the reaction solution was added and the absorbance at 412 nm was followed and the RSH concentration calculated.
In a typical RSH oxidation reaction, POM (0.1 mM), Cu(ClO4)2 (0.8 mM) and 2-mercaptoethanol (30 mM) were stirred in acetonitrile in a heavy-wall glass pressure vessel in an air-conditioned room at 25±2 o C. Aliquots of the solution were withdrawn every several minutes and monitored by UV-vis spectra as described above.  Figure S21).

Stopped-Flow Measurements
A stopped-flow UV-vis spectrometer was used to monitor the rates of PVnMo12-nO40 (3+n)reduction  Table S1 below. [c] Ending current ratio is defined as the final current at the end of the bulk electrolysis over the initial current at the beginning of the bulk electrolysis at the specific potential.  Table S2 below.   Table S3 below.

Quantifying the speciation of reduced POMs
Here we use PVMo11 as an example to calculate the POM distribution in different reduction states.
However, the same procedure was used for the other PVMo. This distribution depends on the chemical solution potential, E, as described by eq S1, where Ei is the standard reduction potential of a (PVMo11)i / (PVMo11)i+1 couple measured electrochemically The calculated apparent reduction state of POM, napp (average), is given in eq S2, and the results are given in Figure S15.