Mixed addenda polyoxometalates by cooperative self-assembly and modulation of their optoelectronic properties

Abstract

Orbital engineering through the cooperative effect of different transition metals (TMs) is a powerful and versatile approach for modulating the chemistry of polyoxometalates (POMs), either by introducing novel POM structures or by designing POM hybrids for more effective catalytic applications. Here, we present a cooperative mixed-metal strategy for the synthesis of mixed-addenda (Mo/W) sandwich POMs with varying compositions, denoted by the general formula [(TM_i)₂(TM_e)₂(H₂O)₂(XMo_xW_9−xO₃₄)₂]ⁿ⁻ (TM_e(2+) = Fe/Co/Ni/Zn and TM_i(3+) = Mn/Fe, X = Zn/Co/Fe). Using this cooperative mix-metal strategy, overall, we report 24 new POMs, including 8 mixed-addenda, 12 W-based sandwich POMs, and 4 POM-based 1-D coordination frameworks. Structural analyses reveal that Mo-addenda incorporation into the POM framework, alongside W (Mo/W), is strongly influenced by the variation of the transition metal composition at the sandwich core, their oxidation states, and the pH of the reaction media. Electrospray ionization mass spectrometry (ESI-MS) and energy dispersive X-ray (EDAX) analysis confirm the detailed POM compositions, while UV-vis spectroscopy and complementary density functional theory (DFT) analysis provide insights into orbital engineering via distinctive charge transfer processes. Theoretical and electrochemical studies demonstrate that electron transfer modulation occurs through both mixed-addenda incorporation and mixed-metal substitution at the sandwich position. This is further elucidated by enhanced oxygen evolution (OER) activity, where the cooperative mixed-metal and mixed-addenda POMs exhibit significantly improved performance, with an overpotential of 500 mV at 1 mA cm⁻², compared to 570 mV in a pH 7.1 buffer. Additionally, this cooperative mixed-metal, mixed-addenda strategy extends to the formation of 1-D polyoxometalate coordination frameworks (POMCFs), where the oxidation state of precursor metals plays a vital role in determining the overall structural attributes.