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The Mn 4 CaO 5(6) cluster in photosystem II catalyzes water splitting through the S i state cycle (i = 0-4). Molecular O 2 is formed and the natural catalyst is reset during the final S 3  → (S 4 ) → S 0 transition. Only recently experimental breakthroughs have emerged for this transition but without explicit information on the S 0 -state reconstitution, thus the progression after O 2 release remains elusive. In this report, our molecular dynamics simulations combined with density functional calculations suggest a likely missing link for closing the cycle, i.e., restoring the first catalytic state. Specifically, the formation of closed-cubane intermediates with all hexa-coordinate Mn is observed, which would undergo proton release, water dissociation, and ligand transfer to produce the open-cubane structure of the S 0 state. Thereby, we theoretically identify the previously unknown structural isomerism in the S 0 state that acts as the origin of the proposed structural flexibility prevailing in the cycle, which may be functionally important for nature's water oxidation catalysis., (© 2024. The Author(s).)

Most functional groups, especially those consisting of the abundant elements of organic matter-carbon, nitrogen and oxygen-have been extensively studied and only very few remain speculative due to their high intrinsic reactivity. In contrast to the well-explored chemistry of diazoalkanes (R 2 C=N 2 ), diazoalkenes (R 2 C=C=N 2 ) have been postulated in several organic transformations, but remain elusive long-sought intermediates. Here, we present a room-temperature stable diazoalkene, utilizing a dinitrogen transfer from nitrous oxide. This functional group shows dual-site nucleophilicity (C and N atoms) and features a bent C-C-N entity (124°) and a long N-N bond together with a remarkable low infrared absorption (1,944 cm -1 ). Substitution of N 2 by an isocyanide leads to a vinylidene ketenimine. Furthermore, photochemically triggered loss of dinitrogen might proceed through a transient triplet vinylidene. We anticipate the existence of a stable diazoalkene functional group to pave an exciting avenue into the chemistry of low-valent carbon and unsaturated carbenes.