Phosphate‐Templated Encapsulation of a {Co(II) (4)O(4)} 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 PO(4) (3−) templated synthesis of three abundant‐metal‐based germanotungstate (GT) clusters Na(15)[Ge(4)PCo(4)(H(2)O)(2)W(24)O(94)] ⋅ 38H(2)O (Co(4)), Na(2.5)K(17.5)[Ge(3)PCo(9)(OH)(5)(H(2)O)(4)W(30)O(115)] ⋅ 45H(2)O (Co(9)), Na(6)K(16)[Ge(4)P(4)Co(20)(OH)(14)(H(2)O)(18)W(36)O(150)] ⋅ 61H(2)O (Co(20)) with non‐, quasi‐, or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn(4)Ca} oxygen evolving complex (OEC) in photosystem II was achieved. Under the conditions tested, all three GT‐scaffolds were active molecular WOCs, with Co(9) and Co(20) outperforming the well‐known Na(10)[Co(4)(H(2)O)(2)(PW(9)O(34))(2)] {Co(4)P(2)W(18)} 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) Co(9) 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 Co(9) was due to the higher energy of its highest occupied molecular orbital electrons as compared to Co(4).