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Hosseinmardi, Soosan; Scheurer, Andreas; Heinemann, Frank W.; Kuepper, Karsten; Senft, Laura; Waldschmidt, Pablo; Ivanovic-Burmazovic, Ivana and Meyer, Karsten (2021): Evaluation of Manganese Cubanoid Clusters for Water Oxidation Catalysis: From Well-Defined Molecular Coordination Complexes to Catalytically Active Amorphous Films. In: Chemsuschem, Vol. 14, No. 21: pp. 4741-4751

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With a view to developing multimetallic molecular catalysts that mimic the oxygen-evolving catalyst (OEC) in Nature's photosystem II, the synthesis of various dicubanoid manganese clusters is described and their catalytic activity investigated for water oxidation in basic, aqueous solution. Pyridinemethanol-based ligands are known to support polynuclear and cubanoid structures in manganese coordination chemistry. The chelators 2,6-pyridinedimethanol (H2L1) and 6-methyl-2-pyridinemethanol (HL2) were chosen to yield polynuclear manganese complexes;namely, the tetranuclear defective dicubanes [(Mn2Mn2III)-Mn-II(HL1)(4)(OAc)(4)(OMe)(2)] and [(Mn2Mn2III)-Mn-II(HL1)(6)(OAc)(2)] (OAc)(2).2 H2O, as well as the octanuclear-dicubanoid [(Mn6Mn2III)-Mn-II(L-2)(4)(O)(2)(OAc)(10)(HOMe/OH2)(2)].3MeOH.MeCN. In freshly prepared solutions, polynuclear species were detected by electrospray ionization mass spectrometry, whereas X-band electron paramagnetic resonance studies in dilute, liquid solution suggested the presence of divalent mononuclear Mn species with g values of 2. However, the magnetochemical investigation of the complexes' solutions by the Evans technique confirmed a haphazard combination of manganese coordination complexes, from mononuclear to polynuclear species. Subsequently, the newly synthesized and characterized manganese molecular complexes were employed as precursors to prepare electrode-deposited films in a buffer-free solution to evaluate and compare their stability and catalytic activity for water oxidation electrocatalysis.

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