Detection of the Sn(III) Intermediate and the Mechanism of the Sn(IV)/Sn(II) Electroreduction Reaction in Bromide Media by Cyclic Voltammetry and Scanning Electrochemical Microscopy

Fast-scan cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) were used to investigate the reduction of Sn(IV) as the hexabromo complex ion in a 2 M HBr-4 M NaBr medium. CV at scan rates to 100 V/s and SECM indicated the reaction pathway involves ligand-coupled electron transfer via an ECEC-DISP process: (1) one-electron reduction of (SnBr62-)-Br-IV to (SnBr63-)-Br-III; (2) bromide dissociation of the reduced (SnBr63-)-Br-III to (SnBr52-)-Br-III; (3) disproportionation of the reduced 2Sn(III)Br(5)(2-) to (SnBr5-)-Br-IV and (SnBr53-)-Br-II; (4) one-electron reduction of (SnBr52-)-Br-III to (SnBr53-)-Br-II; (5) bromide dissociation from (SnBr5)-Br-II to (SnBr42-)-Br-II. The intermediate Sn(III) species was confirmed by SECM3-, where the Sn(III) generated at the Au tip was collected on a Au substrate in the tip generation/substrate collection mode when the distance between the tip and substrate was a few hundred nanometers.