Evaluation of Manganese Cubanoid Clusters for Water Oxidation Catalysis: From Well-Defined Molecular Coordination Complexes to Catalytically Active Amorphous Films

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.

Automated summary

The synthesis and catalytic activity of various dicubanoid manganese clusters for water oxidation in basic, aqueous solution are investigated. Different polynuclear manganese complexes were prepared using pyridinemethanol-based ligands, showing a variety of manganese coordination species in solution. These complexes were then utilized to prepare electrode-deposited films for water oxidation electrocatalysis, with varying stabilities and activities observed.