Mixed Ni/Cu-sandwich dawson polyoxotungstates: Electrochemistry and DFT insights

• Mixed Ni/Cu Dawson POMs synthesised with identical frameworks but swapped metal sites • CV and EQCM reveal distinct Cu(II)/Cu(I)/Cu(0) pathways depending on metal ordering • Ni–Cu electronic synergy modulates W(VI) redox waves and enhances Cu reducibility • XPS confirms internal vs external Cu and Ni positions in the sandwich cluster • DFT explains site-dependent redox behaviour and supports experimental assignments Given the established electrocatalytic potential of Ni(II)- and Cu(II)-containing tungsten-based POMs, the primary objective of this work was to create a unique molecular architecture combining both active metal centres in the two Dawson sandwich POMs, [(CuOH 2 ) 2 Ni 2 (As 2 W 15 O 56 ) 2 ] 16− and [(NiOH 2 ) 2 Cu 2 (As 2 W 15 O 56 ) 2 ] 16− . This aims to exploit potential synergistic effects for enhanced catalytic performance and to deepen the understanding of redox mechanisms in these mixed-metal systems. A detailed electrochemical study of the two POMs in solution, compared to their homometallic analogues containing only Ni(II) or only Cu(II) centres, was conducted using cyclic voltammetry (CV), also coupled to quartz crystal microbalance in order to monitor Cu deposition/desorption phenomena, and controlled potential coulometry, enabling unambiguous identification and differentiation. X-ray photoelectron spectroscopy (XPS) unequivocally corroborated these findings. Furthermore, density functional theory (DFT) calculations provide additional insights that are relevant to rationalise the electrochemical measurements. The molecular orbital sequence, energies and shapes provide relevant supporting information to the experimental facts. Combined evidence establishes that Na/Ni compounds maintain the W-only electrochemical fingerprint, whereas new CV features appear when Cu(II) is part of the structure, modulated by the rest of the equatorial composition. Protonation has been explored and its effects on the redox chemistry examined at the theoretical level.