Electrochemical and in-situ UV-visible-near-IR and FTIR spectroelectrochemical characterisation of the mixed-valence heteropolyanion PMo_(12)O_(40)~(n-) (n = 4, 5, 6, 7) in aprotic media

摘要

The mixed-valence heteropolyanion PMo_(12)O_(40)~(n-) (where n = 4, 5, 6, 7) in aprotic media was investigated by cyclic voltammetry and in-situ FTIR and UV-visible-near-IR spectroelectrochemical methods. A new optically transparent thin layer cell with adjustable optical path length was designed and characterised by thin layer cyclic voltammetry and double step chronoamperometry using ferrocene as a test redox system. The results indicate that this cell has small ohmic drop and good spectral response. The cell was used for both in-situ FTIR and UV-visible-near-IR spectroelectrochemical measurements. The cyclic voltammetric results indicate that the heteropolyanion PMo_(12)O_(40)~(3-) undergoes two reversible one-electron transfer reductions (first and second redox waves) and two quasi-reversible one-electron transfer reductions (third and fourth redox waves). The mixed-valence heteropolyanion PMo_(12)O_(40)~(n-) (where n = 4, 5, 6, 7) was formed after electroreduction. In-situ FTIR and UV-visible-near-IR spectroelectrochemical preliminary results indicate that the electronic structure of electrogenerated mixed-valence species PMo_(12)O_(40)~(4-) corresponds to the class II system in Robin and Day's classification of mixed-valence compounds. In -situ FTIR spectroelectrochemical studies also suggest that the bond energy of the Mo=O_d double bond and Mo-O_b-Mo and Mo-O_c-Mo bridge bonds was reduced after reduction of the original compound.