A Gas‐Phase Ca_nMn_4−nO₄⁺ Cluster Model for the Oxygen‐Evolving Complex of Photosystem II

Abstract

One of the fundamental processes in nature, the oxidation of water, is catalyzed by a small CaMn3O4⋅MnO cluster located in photosystem II (PS II). Now, the first successful preparation of a series of isolated ligand‐free tetrameric CanMn4−nO4+ (n=0–4) cluster ions is reported, which are employed as structural models for the catalytically active site of PS II. Gas‐phase reactivity experiments with D2O and H218O in an ion trap reveal the facile deprotonation of multiple water molecules via hydroxylation of the cluster oxo bridges for all investigated clusters. However, only the mono‐calcium cluster CaMn3O4+ is observed to oxidize water via elimination of hydrogen peroxide. First‐principles density functional theory (DFT) calculations elucidate mechanistic details of the deprotonation and oxidation reactions mediated by CaMn3O4+ as well as the role of calcium.

Charge separation and successive reconfigurations of electronic and protonic states in a water-splitting catalytic cycle with the Mn4CaO5 cluster. On the mechanism of water splitting in PSII

“…As for water-splitting in photosystem II (PSII), complete understanding of the catalytic cycle with the Mn 4 CaO 5 cluster being the unique catalyst is one of the ultimate goals. [1][2][3][4][5][6][7][8][9][10] Yet there are still quantum mechanical mysteries involved despite many experimental and theoretical studies. For technological applications of photoinduced watersplitting (see for example ref.…”

“…3,6 However, it is natural to presuppose the presence of hydroxy groups under the circumstance where the Cl À ions exist nearby the cluster. 2 In fact, Landman et al 9,25 have experimentally discussed the presence of a m-hydroxo bridge even in gas phase Mn oxo clusters such as Ca n Mn 4Àn O 4…”

Doubly-Charged Negative Ions as Novel Tunable Catalysts: Graphene and Fullerene Molecules Versus Atomic Metals

“…The importance of the need to understand the fundamental functional roles of the various constituents of current complicated catalysts is demonstrated in a recent investigation of water oxidation catalyzed by the oxygen-evolving complex of photosystem II [1]. These authors also pursue the development of new artificial water oxidation catalysts for artificial photosynthetic water oxidation [2].…”

“…The fundamental mechanism underlying negative-ion catalysis involves anionic molecular complex formation in the transition state (TS), with the atomic/molecular negative ion breaking up the hydrogen bond strength. The oxidation reaction to be catalyzed that is considered here is 2 H 2 O + O 2 → 2 H 2 O 2 , as opposed to that discussed in [1], 2 H 2 O → H 2 O 2 + H 2 . Here, we first introduce the anionic catalyst, Au − , to break up the hydrogen-bond strength in the water and then follow with the addition of O 2 to form the desired H 2 O 2 .…”

“…Preliminary DFT TS calculations found Pu 2− , Pa 2− , Sn 2− and La 2− to be the best catalysts among the investigated anions in reducing considerably the energy barrier. In [1], the role of the Ca 2+ was elucidated as well. Previously, cationic systems were discovered to increase the TS energy barriers in the synthesis of peroxide from H 2 O [28].…”