Synthesis and electrocatalytic reactivity for water oxidation of two cerium complexes

Synthetic Mn3Ce2O5-Cluster Mimicking the Oxygen-Evolving Center in Photosynthesis

The photosynthetic oxygen-evolving center (OEC) is a unique Mn4CaO5-cluster that catalyses water splitting into electrons, protons, and dioxygen. Precisely structural and functional mimicking of the OEC is a long-standing challenge and pressingly needed for understanding the structure-function relationship and catalytic mechanism of O-O bond formation. Herein we report two simple and robust artificial Mn3Ce2O5-complexes that display a remarkable structural similarity to the OEC in regarding of the ten-atom core (five metal ions and five oxygen bridges) and the alkyl carboxylate peripheral ligands. This Mn3Ce2O5-cluster can catalyse the water-splitting reaction on the surface of ITO electrode. These results clearly show that cerium can structurally and functionally replace both calcium and manganese in the cluster. Mass spectroscopic measurements demonstrate that the oxide bridges in the cluster are exchangeable and can be rapidly replaced by the isotopic oxygen of H2 18O in acetonitrile solution, which supports that the oxide bridge(s) may serve as the active site for the formation of O-O bond during the water-splitting reaction. These results would contribute to our understanding of the structure-reactivity relationship of both natural and artificial clusters and shed new light on the development of efficient water-splitting catalysts in artificial photosynthesis.

MnII/III and CeIII/IV Units Supported on an Octahedral Molecular Nanoparticle of CeO2

The preparation of three new heterometallic clusters [Ce₆Mn₁₂O₁₇(O₂CPh)₂₆] (1), [Ce₁₀Mn₁₄O₂₄(O₂CPh)₃₂] (2), and [Ce₂₃Mn₂₀O₄₈(OH)₂(tbb)₄₆(H₂O)₄](NO₃)₂ (3; tbb^- = 4-^tBu-benzoate) is reported. They all possess unprecedented structures with a common feature being the presence of an octahedral Ce^IV-oxo core: a Ce₆ in 1, two edge-fused Ce₆ giving a Ce₁₀ bioctahedron in 2, or a larger Ce₁₉ octahedron in 3. Complex 1 is the first Ce₆ cluster with a central μ₆-O^2-. 2 and the cation of 3 are molecular nanoparticles of CeO₂ (ceria) because they possess the fluorite structure of bulk ceria and are thus ultrasmall ceria nanoparticles in molecular form. The {Ce₁₉O₃₂} octahedral subunit of the cation of 3 had been predicted from density functional theory studies to be one of the stable fragments of the CeO₂ lattice, but has never been previously synthesized in molecular chemistry. Around the Ce/O core of 1-3 is an incomplete monolayer of Mn_n ions disposed as four Mn₃, two Mn₇, and four Mn₅ units, respectively. This represents a clear structural similarity with composite (phase-separated) CeO₂/MnO_x mixtures where at high Ce:Mn ratios the Mn atoms segregate on the surface of CeO₂ phases. Variable-temperature dc and ac magnetic susceptibility studies have revealed S = 2, S = ¹/₂, and S = ³/₂ ground states for 1-3, respectively. Fitting of the 5.0-300 K dc data for 1 to a two-J model for an asymmetrical V-shaped Mn₃ unit with no interaction between the end Mn^III ions gave an excellent fit with the following values: J₁ = 5.2(3) cm^-1, J₂ = -7.4(3) cm^-1, and g = 1.96(2).

Long-Range Ferromagnetic Exchange Interactions Mediated by Mn-CeIV-Mn Superexchange Involving Empty 4f Orbitals

Reactions involving reductive aggregation of MnO₄^- in methanol in the presence of Ce^IV and an excess of carboxylic acid have led to the synthesis of structurally related Ce/Mn clusters, [Ce₃Mn₅O₈(OMe)(O₂CBu^t)₁₃(MeOH)] (1) and [Ce₂Mn₃O₅(O₂CPh)₉(MeOH)₃] (2), containing at least one {Mn₂Ce₂O₄} cubane unit. The cores of both clusters contain Mn_x units separated by three (1) or two (2) Ce^IV ions. Fits of variable-temperature, solid-state dc and ac magnetic susceptibility data reveal dominant ferromagnetic interactions within 1 and 2, resulting in the maximum S = ¹⁷/₂ and S = 5 ground state spins, respectively, and thus suggesting significant ferromagnetic (F) interactions between the Mn_x units that are ≥6 Å apart and separated by four intervening bonds through diamagnetic Ce^IV. Fits of magnetic susceptibility data also revealed unusual long-range F interactions, and this finding was further supported by high-field EPR measurements and simulations. Density functional theory calculations and a Wannier function analysis confirm long-range interactions and indicate a Mn-Ce-Mn superexchange pathway via Mn-d/Ce-f orbital overlap/hybridization.