Spin-Gated Selectivity of the Water Oxidation Reaction Mediated by Free Pentameric Ca_xMn_5–xO₅⁺ Clusters

Abstract

We report on the first preparation of isolated ligand-free CaMn4O5 + gas-phase clusters, as well as other pentameric Ca x Mn5–x O5 + (x = 0–4) clusters with varying Ca contents, which serve as molecular models of the natural CaMn4O5 inorganic cluster in photosystem II. Ion trap reactivity studies with D2O and H2 18O reveal a pronounced cluster composition-dependent ability to mediate the oxidation of water to hydrogen peroxide. First-principles density functional theory simulations elucidate the mechanism of water oxidation, proceeding via formation of a terminal oxyl radical followed by oxyl/hydroxy (O/OH) coupling. The critical coupling reaction step entails a single electron transfer from the oxyl radical to the accommodating cluster core with a concurrent O/OH coupling forming an adsorbed OOH intermediate group. The spin-conserving electron transfer step takes place when the spin of the transferred electron is aligned with the spins of the d-electrons of the Mn atoms in the cuboidal high-spin cluster isomer. The d-electrons provide a ferromagnetically ordered environment that facilitates the spin-gated selective electron transfer process, resulting in parallel-spin-exchange stabilization and a lowered transition state barrier for the coupling reaction involving the frontier orbitals of the oxyl and hydroxy reactant intermediates.

Cluster size dependent coordination of formate to free manganese oxide clusters

“…The particular structural fluxionality of Mn 4 O 4 + has previously been described with respect to water adsorption, deprotonation, and oxidation. 29,32,33 Based on these observations, further structural changes can be expected upon adsorption of multiple ligands, which might eventually also lead to the stabilization of the cubic structure and a change of its activity in the water oxidation reaction. This will be subject of further investigations.…”

“…However, the binding of the resulting formate group is strongly cluster-size-dependent and can differ from the bridging m-oxo configuration typically found for Mn 2 O 2 -and Mn 4 O 4 -based molecular catalysts. The consequences of cluster fluxionality [32][33][34] and differences from the ligation with acetic acid 35,36 are discussed.…”

“…[20][21][22][23][24][25][26] In our project, we investigate gas-phase clusters consisting of elements also contained in the natural OEC, i.e., Mn, Ca, and O. In the first steps of this approach, we have prepared bare, non-ligated manganese [27][28][29][30] and calcium-manganese oxide clusters [31][32][33] of different size and composition, and studied their water activation and oxidation properties via reactivity measurements and via infrared spectroscopy. We now aim to increase the model system's complexity by attaching small carboxylic acids mimicking a ligand environment that could potentially stabilize the cluster structure and/or serve as proton/electron transfer media.…”

Vibrational spectroscopy of free di-manganese oxide cluster complexes with di-hydrogen

“…Such conceptual insight could guide the future design of novel artificial water oxidation catalysts. Towards this goal, we pursue a hierarchical modelling strategy by employing free manganese oxide [13][14][15][16][17][18] and calcium-manganese oxide clusters [18][19][20] of well-controlled and sequentially increasing complexity. Such studies have a joint theoretical and experimental character, where most experiments were done utilizing ion-molecule reactions in radiofrequency ion traps.…”

Room-Temperature Conversion of Methane to Methanediol by [FeO2]+

“…Thus, gas-phase ion–molecule reactions of isolated clusters with well-defined compositions, structures, and initial states have been employed as ideal models to aid the design of artificial catalysts − or interrogate the intrinsic chemical reactivity of existing catalysts; , the molecular-level details of the reactions can be obtained. − There exist a few studies on methane activation by gas-phase iron–oxo species, such as [FeO] + − and [Fe­(O)­OH] + . However, it remains to be explored how a high-valent iron–oxo species, e.g., [FeO 2 ] + , − performs regarding methane activation.…”