Although the {CaMn4O5} oxygen evolving complex (OEC) of photosystem II is a major paradigm for water oxidation catalyst (WOC) development, the comprehensive translation of its key features into active molecular WOCs remains challenging. The [Co(II)3Ln(hmp)4(OAc)5H2O] ({Co(II)3Ln(OR)4}; Ln = Ho-Yb, hmp = 2-(hydroxymethyl)pyridine) cubane WOC series is introduced as a new springboard to address crucial design parameters, ranging from nuclearity and redox-inactive promoters to operational stability and ligand exchange properties. The {Co(II)3Ln(OR)4} cubanes promote bioinspired WOC design by newly combining Ln(3+) centers as redox-inactive Ca(2+) analogues with flexible aqua-/acetate ligands into active and stable WOCs (max. TON/TOF values of 211/9 s(-1)). Furthermore, they open up the important family of 3d-4f complexes for photocatalytic applications. The stability of the {Co(II)3Ln(OR)4} WOCs under photocatalytic conditions is demonstrated with a comprehensive analytical strategy including trace metal analyses and solution-based X-ray absorption spectroscopy (XAS) investigations. The productive influence of the Ln(3+) centers is linked to favorable ligand mobility, and the experimental trends are substantiated with Born-Oppenheimer molecular dynamics studies.

We introduce the novel Co 4 O 4 complex [Co II 4 (hmp) 4 (μ-OAc) 2 (μ 2 -OAc) 2 (H 2 O) 2 ] (1) (hmp = 2-(hydroxymethyl)pyridine) as the first Co(II)-based cubane water oxidation catalyst. Monodentate acetate and aqua ligands lend the flexible environment of 1 closest resemblance to photosystem II among its tetranuclear mimics to date. Visible-light-driven catalytic activity of 1 increases with pH value through aqua ligand deprotonation. The Co(II) core combines robustness and stability with flexibility through a new type of water-oxidation mechanism via mobile ligands.

Cobalt carbodiimide emerges as a heterogeneous non-oxidic water oxidation catalyst prototype with high dual photochemical and electrocatalytic activity.

COVER ARTICLEPatzke et al. Photocatalytic water oxidation with cobalt-containing tungstobismutates: tuning the metal core www.rsc.org/catalysis A new type of tungstobismutate water oxidation catalyst (WOC) with a disordered Co|W core, [{Co(H 2 O) 3 } 2 {CoBi 2 W 19 O 66 (OH) 4 }] 10− (1) was tested for visible-light-driven performance and compared to a series of isostructural Co-and Mn-containing polyoxometalates with variable transition metal contents, ([Co 2.5 (H 2 O) 6 {Bi 2 W 19.5 O 66 (OH) 4 }] 8− (2) and [Mn 1.5 (H 2 O) 6 {Bi 2 W 20.5 O 68 (OH) 2 }] 6− (3)). All compounds were structurally characterized, and no indications for significant decomposition under catalytic conditions for visible-light-driven water oxidation ([Ru(bpy) 3 ] 2+ as photosensitizer (PS) and S 2 O 8 2− as electron acceptor in different buffer systems)were found. For the first time, subtle differences in the core disorder patterns of isostructural POM-WOCs were revealed to be decisive for the catalytic activity of (1) (maximum TON of 21 with 97% oxygen yield for 115 μM(1)). Performance comparison of the POM series sheds new light on the structure-activity relationships for targeted POM-WOC construction. Indeed, the Co disorder differences between (1) and (2) exclusively affect the sterically more accessible external site of the two crystallographically independent Co core positions which has a 25% higher Co occupancy in (1). This points to a stereoselective reaction pattern for the tetranuclear POM core of WOC (1) which might open up novel construction strategies for the economic redesign of sandwich-type POM-WOCs. In parallel, we demonstrate for the POM series (1)-(3) that electrochemical measurements under catalytic conditions are a promising and convenient pre-screening strategy for WOC activity. Furthermore, POM/PS complex formation of (1) with [Ru(bpy) 3 ] 2+ is investigated in detail, and the different roles of Mn-and Co-centers in POM-WOC synthesis are compared. ; Fax: +41 44 635 6802; Tel: +41 44 635 4691 † Electronic supplementary information (ESI) available: Crystallographic data (ICSD 426302, 426303 and 426304) and BVS calculations for all compounds, powder X-ray diffraction patterns, FT-IR and UV/Vis spectra, TG measurements and cyclic voltammograms, experimental details for catalytic water oxidation, FT-IR and TG data for POM/PS complexes. See octahedron which is orientationally disordered. 11 Small molybdate clusters, such as [CoMo 6 O 24 H 6 ] 3− and [Co 2 Mo 10 O 38 H 4 ] 6− , have also been identified as promising WOCs, thus demonstrating that neither a critical POM size nor multi-site metal cores are mandatory for catalytic activity. 4k,12 Not only the key requirements for POM-WOC shell construction still remain unclear to a large extent, but the type and oxidation state of the POM belt atoms require further systematic SAR explorations as well. Ni-POMs serve as an example: whereas the [Ni 5 (OH) 6 (OH 2 ) 3 (Si 2 W 18 O 66 )] 12− polyanion with a clamshellshaped geometry exhibits WOC activity, 7 the Ni-analogue of the ...

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