Size, Stoichiometry, Dimensionality, and Ca Doping of Manganese Oxide-Based Water Oxidation Clusters: An Oxyl/Hydroxy Mechanism for Oxygen-Oxygen Coupling

Cluster size dependent coordination of formate to free manganese oxide clusters

The interaction of free manganese oxide clusters, MnxOy+ (x = 1-9, y = 0-12), with formic acid was studied via infrared multiple-photon dissociation (IR-MPD) spectroscopy together with calculations using density functional theory (DFT). Clusters containing only one Mn atom, such as MnO2+ and MnO4+, bind formic acid as an intact molecule in both the cis- and trans-configuration. In contrast, all clusters containing two or more manganese atoms deprotonate the acid's hydroxyl group. The coordination of the resulting formate group is strongly cluster-size-dependent according to supporting DFT calculations for selected model systems. For Mn2O2+ the co-existence of two isomers with the formate bound in a bidentate bridging and chelating configurations, respectively, is found, whereas for Mn2O4+ the bidentate chelating configuration is preferred. In contrast, the bidentate bridging structure is energetically considerably more favorable for Mn4O4+. This binding motif stabilizes the 2D ring structure of the core of the Mn4O4+ cluster with respect to the 3D cubic geometry of the Mn4O4+ cluster core.

Water-Gas Shift Catalyzed by Iridium-Vanadium Oxide Clusters IrVO2- with Iridium in a Rare Oxidation State of -II

The discovery of compounds containing transition metals with an unusual and well-established oxidation state is vital to enrich our horizon on formal oxidation state. Herein, benefiting from the study of the water-gas shift reaction (CO + H₂O → CO₂ + H₂) mediated with the iridium-vanadium oxide cluster IrVO₂^-, the missing -II oxidation state of iridium was identified. The reactions were performed by using our newly developed double ion trap reactors that can spatially separate the addition of reactants and are characterized by mass spectrometry and quantum-chemical calculations. This finding makes an important step that all the proposed 13 oxidation states of iridium (+IX to -III) have been known. The iridium atom in the IrVO₂^- cluster features the Ir═V double bond and resembles chemically the coordinated oxygen atom. A reactivity study demonstrated that the flexible role switch of iridium between an oxygen-atom like (Ir^-IIVO₂^-) and a transition-metal-atom like behavior (Ir^+IIVO₃^-) in different species can drive the water-gas shift reaction in the gas phase under ambient conditions. This result parallels and well rationalizes the extraordinary reactivity of oxide-supported iridium single-atom catalysts in related condensed-phase reactions.

Spin-Gated Selectivity of the Water Oxidation Reaction Mediated by Free Pentameric CaxMn5-xO5+ Clusters

We report on the first preparation of isolated ligand-free CaMn₄O₅⁺ gas-phase clusters, as well as other pentameric Ca_xMn_5-xO₅⁺ (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 H₂¹⁸O 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.

Water Gas Shift Reaction Catalyzed by Rhodium-Manganese Oxide Cluster Anions

Fundamental understanding of the nature of active sites in real-life water gas shift (WGS) catalysts that can convert CO and H₂O into CO₂ and H₂ is crucial to engineer related catalysts performing under ambient conditions. Herein, we identified that the WGS reaction can be, in principle, catalyzed by rhodium-manganese oxide clusters Rh₂MnO_1,2^- in the gas phase at room temperature. This is the first example of the construction of such a potential catalysis in cluster science because it is challenging to discover clusters that can abstract the oxygen from H₂O and then supply the anchored oxygen to oxidize CO. The WGS reaction was characterized by mass spectrometry, photoelectron spectroscopy, and quantum-chemical calculations. The coordinated oxygen in Rh₂MnO_1,2^- is paramount for the generation of an electron-rich Mn⁺-Rh^- bond that is critical to capture and reduce H₂O and giving rise to a polarized Rh⁺-Rh^- bond that functions as the real redox center to drive the WGS reaction.