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

The power of Fourier-transformed large amplitude alternating current voltammetry (FTACV) has been applied to parameterize the reduction of the phosphotungstate [PW11O39{Sn(C6H4)C C(C6H4)(N3C4H10)}](4-) polyoxometalate (POM) (K-Sn(W) [N3C4H10](4-/5-/6-) processes) at glassy carbon (GC), gold (Au), and platinum (Pt) electrodes as well as its GC surface-confined K-Sn(W)[-](4-)-grafted diazonium derivative in acetonitrile (0.10 M [n-Bu4N][PF6]). The thermodynamics (E-0) and heterogeneous electron-transfer kinetics (k(0) and alpha) were estimated using the Butler-Volmer relationship. FTACV provides access to significantly more detailed mechanistic information related to nonconformance to the theory than widely used DC voltammetric methods, especially with the more intricate surface-confined electrochemistry. Parameterization, the level of agreement, and systematic variations between experimental and simulated data were established by both an experimentercontrolled heuristic method and by a computationally efficient data optimization approach that employed parameter space searches restricted in scope by knowledge of the heuristically based estimations. The first electron transfer process for both acetonitrile-soluble K-Sn(W) [N3C4H10](4-) and surface-confined K-Sn(W)[-](4-) is always significantly faster than the second. The electrode dependence order is k(GC)(0) > k(Au)(0) > k(Pt)(0) for the K-Sn(W) [N3C4H10](4-/5-) process. The relatively slower electrode kinetics found for reduction of K-Sn(W) [N3C4H10](4-) as compared to some other monomeric Keggin POMs may be due to the long organic chain hindering the approach of the POM to the electrode surface, although differences in ion-pairing and other factors also may play a role. Subtle, but systematic, differences identified in comparisons of experimental and simulated voltammetry give rise to apparently data analysis method dependent parameterization and are discussed in terms of nuances not accommodated in the modeling. In the solution-phase voltammetry, data obtained by an electrochemical quartz crystal microbalance and other techniques are consistent with solid adhering to and modifying the electrode surface following reduction of K-Sn(W) [N3C4H10](4)(-) to K-Sn(W) [N3C4H10](5-). Kinetic and thermodynamic dispersions present in the heterogeneous K-Sn(W)[-](4-)grafted electrode are probable causes of nonideality detected in the surface-confined voltammetry of this material. Thus, FTACV gives valuable insights into what is needed to provide a more realistic description of the polyoxometalate/electrode interface in polyoxometalate electrochemistry by revealing subtle nuances that are often overlooked.