Implications of Charge and Heteroatom Dopants on the\nThermodynamics and Kinetics of Redox Reactions in Keggin-Type Polyoxometalates

The utilization of\npolyoxometalate-based materials is largely dictated\nby their redox properties. Detailed understanding of the thermodynamic\nand kinetic efficiency of charge transfer is therefore essential to\nthe development of polyoxometalate-based systems for target applications.\nToward this end, we report electrochemical studies of a series of\nheteroatom-doped Keggin-type polyoxotungstate clusters [PW<sub>12</sub>O<sub>40</sub>]<sup>3–</sup> (<b>PW</b><sub><b>12</b></sub>), [VW<sub>12</sub>O<sub>40</sub>]<sup>3–</sup> (<b>V</b><sub><b>in</b></sub><b>W</b><sub><b>12</b></sub>), [P(VW<sub>11</sub>)O<sub>40</sub>]<sup>4–</sup> (<b>PV</b><sub><b>out</b></sub><b>W</b><sub><b>11</b></sub>), and [V(VW<sub>11</sub>)O<sub>40</sub>]<sup>4–</sup> (<b>V</b><sub><b>in</b></sub><b>V</b><sub><b>out</b></sub><b>W</b><sub><b>11</b></sub>) to elucidate\nthe role of the identity and spatial location of heteroatoms and overall\ncluster charge on the rate constants of electron transfer and redox\nreaction entropies. Electrochemical analyses of the polyoxotungstates\nreveal that the kinetics of electron transfer for W-based redox processes\nchange as a function of the redox activity of the heteroatom, whereas\nthe spatial location of the heteroatom dopant does not significantly\nimpact the electrokinetics. Variable temperature cyclic voltammetry\nmeasurements in organic solutions containing noncoordinating electrolyte\nions establish that redox reaction entropies are primarily dictated\nby the overall charge of the clusters. Specifically, the redox entropy\nexhibits a good linear relationship with the dielectric continuum\nfunction <i>Z</i><sub>ox</sub><sup>2</sup> – <i>Z</i><sub>red</sub><sup>2</sup> (<i>Z</i><sub>ox</sub> = charge of oxidized species, <i>Z</i><sub>red</sub> =\ncharge of reduced species). Finally, our experimental data do not\nshow a prominent correlation between the kinetics of electron transfer\nand redox entropy, implying that the charge-transfer kinetics are\nnot solely governed by structural reorganization. Taken together,\nthese results highlight how structural and electronic parameters can\ninfluence the kinetics and thermodynamics of charge transfer in polyoxotungstates\nand provide insights into the design of polyoxometalate compounds\nwith target redox properties.