PHOTOSYNTHETICA

Photosynthetica X:X | DOI: 10.32615/ps.2026.011

A perspective on why modeling the water-oxidizing complexin photosystem II remains functionally elusive

M.M. NAJAFPOUR^{1, 2}

¹ Department of Chemistry, Sharif University of Technology, 11155-9516 Tehran, Iran

² К. А. Timiryazev Institute of Plant Physiology RAS, Botanicheskaya Street 35, 127276 Moscow, Russia

The water-oxidation reaction (WOR) is a critical yet kinetically sluggish process, requiring a complex four-electron transformation. Manganese (Mn) is a premier candidate for WOR catalysts due to its low cost, low toxicity, and its fundamental role in natural photosynthesis. Consequently, extensive research has focused on designing Mn-based ligands and compounds. However, a significant challenge persists: harsh oxidizing conditions necessary for WOR often trigger unintended phase transformations. Spectroscopy reveals that many Mn complexes and salts - even biomimetic clusters - eventually degrade or rearrange into various Mn oxides. This typically occurs via a two-step mechanism where Mn ions leach into the electrolyte before redepositing as an oxide phase. Since the morphology and activity of the resulting oxide depend heavily on pH, ligands, and precursors, identifying the "true" catalyst remains difficult. These structural instabilities highlight the central hurdle in developing durable, well-defined Mn-based WOR systems.

Additional key words: catalyst reconstruction; catalyst stability; manganese; manganese oxides; precatalyst; true catalyst; water-oxidation reaction.

Received: February 21, 2026; Revised: April 9, 2026; Accepted: May 15, 2026; Prepublished online: May 28, 2026