Phosphate Templated Encapsulation of a {CoII4O4} Cubane in Germanotungstates as Carbon-Free Homogeneous Water Oxidation Photocatalysts.

The ever-growing interest in sustainable energy sources leads to a search for an efficient, stable, and inexpensive homogeneous water oxidation catalyst (WOC). Herein, we report the PO43- templated synthesis of three abundant-metal-based germanotungstate (GT) clusters Na15[Ge4PCo4(H2O)2W24O94] • 38 H2O (Co4), Na2.5K17.5[Ge3PCo9(OH)5(H2O)4W30O115] • 45 H2O (Co9), Na6K16[Ge4P4Co20(OH)14(H2O)18W36O150] • 61 H2O (Co 20) with non-, quasi- or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn4Ca} oxygen evolving complex (OEC) in photosystem II. Under the conditions tested, all three GT-scaffolds are active molecular WOCs, with Co 9 and Co 20 outperforming the well-known Na10[Co4(H2O)2(PW9O34)2] {Co4P2W18} by a factor of 2 as shown by a direct comparison of their turnover numbers (TONs). With turnover numbers (TONs) up to 159.9 and a turnover frequency (TOF) of 0.608 s-1 Co9 currently represents the fastest Co-GT based WOC and the photoluminescence (PL) emission spectroscopy provides insights into its photocatalytic WOC mechanism. Cyclic voltammetry, dynamic light scattering (DLS), UV-vis-and IR-spectroscopy show recyclability and integrity of the catalysts under the applied conditions. The experimental results are supported by computational studies, which highlight that the facilitated oxidation of Co9 is due to the higher energy of its HOMO electrons as compared to Co4.