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, the PO43- templated synthesis of three abundant-metal-based germanotungstate (GT) clusters Na15 [Ge4 PCo4 (H2 O)2 W24 O94 ] ⋅ 38H2 O (Co4 ), Na2.5 K17.5 [Ge3 PCo9 (OH)5 (H2 O)4 W30 O115 ] ⋅ 45H2 O (Co9 ), Na6 K16 [Ge4 P4 Co20 (OH)14 (H2 O)18 W36 O150 ] ⋅ 61H2 O (Co20 ) with non-, quasi-, or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn4 Ca} oxygen evolving complex (OEC) in photosystem II was achieved. Under the conditions tested, all three GT-scaffolds were active molecular WOCs, with Co9 and Co20 outperforming the well-known Na10 [Co4 (H2 O)2 (PW9 O34 )2 ] {Co4 P2 W18 } by a factor of 2 as shown by a direct comparison of their turnover numbers (TONs). With TONs up to 159.9 and a turnover frequency of 0.608 s-1 Co9 currently represents the fastest Co-GT-based WOC, and photoluminescence emission spectroscopy provided insights into its photocatalytic WOC mechanism. Cyclic voltammetry, dynamic light scattering, UV/Vis and IR spectroscopy showed recyclability and integrity of the catalysts under the applied conditions. The experimental results were supported by computational studies, which highlighted that the facilitated oxidation of Co9 was due to the higher energy of its highest occupied molecular orbital electrons as compared to Co4 .