Magnetic Behavior of Heterometallic Wheels Having a [Mn(IV)6M2O9](10+) Core with M = Ca(2+) and Sr(2+)

Self-Assembled Heterometallic Complexes by Incorporation of Calcium or Strontium Ion into a Manganese(II) 12-Metallacrown-3 Framework Supported by a Tripodal Ligand with Pyridine-Carboxylate Motifs: Stability in Their Manganese(III) Oxidized Form

We report on the isolation of a new family of μ-carboxylato-bridged metallocrown (MC) compounds by self-assembly of the recently isolated hexadentate tris(2-pyridylmethyl)amine ligand tpada^2- incorporating two carboxylate units with metal cations. Twelve-membered MCs of manganese of the type 12-MC-3, namely, [{Mn^II(tpada)}₃(M)(H₂O)_n]²⁺ (Mn₃M) (M = Mn²⁺ (n = 0), Ca²⁺ (n = 1), or Sr²⁺ (n = 2)), were structurally characterized. The metallamacrocycles connectivity consisting in three -[Mn-O-C-O]- repeating units is provided by one carboxylate unit of the three tpada^2- ligands, while the second carboxylate coordinated a fourth cation in the central cavity of the MC, Mn²⁺ or an alkaline earth metal, Ca²⁺ or Sr²⁺. Mn₃Ca and {Mn₃Sr}₂ join the small family of heterometallic manganese-calcium complexes and even rarer manganese-strontium complexes as models of the OEC of photosystem II (PSII). A 8-MC-4 of strontium of the molecular wheel type with four -[Sr-O]- repeating unit was also isolated by self-assembly of the tpada^2- ligand with Sr²⁺. This complex, namely, [Sr(tpada)(OH₂)]₄ (Sr₄), does not incorporate any cation in the central cavity but instead four water molecules coordinated to each Sr²⁺. Electrochemical investigations coupled to UV-visible absorption and EPR spectroscopies as well as electrospray mass spectrometry reveal the stability of the 12-MC-3 tetranuclear structures in solution, both in the initial oxidation state, Mn^II₃M, as well as in the three-electrons oxidized state, Mn^III₃M. Indeed, the cyclic voltammogram of all these complexes exhibits three-successive reversible oxidation waves between +0.5 and +0.9 V corresponding to the successive one-electron oxidation of the Mn(II) ion into Mn(III) of the three {Mn(tpada)} units constituting the ring, which are fully maintained after bulk electrolysis.

Bismuth for Controlled Assembly/Disassembly of Transition-Metal Oxo Clusters, Defining Reaction Pathways in Inorganic Synthesis and Nature

Trivalent bismuth is a unique heavy p-block ion. It is highly insoluble in water, due to strong hydrolysis tendencies, and known for low toxicity. Its lone pair is structure-directing, providing framework materials with structural flexibility, leading to piezoelectric and multiferroic function. The flexibility it provides is also advantageous for dopants and vacancies, giving rise to conductivity, luminescence, color, and catalytic properties. We are exploiting Bi³⁺ in a completely different way, as a knob to "tune" the solubility and stability of transition-metal oxo clusters. The lone pair allows capping and isolation of metastable cluster forms for solid-state and solution characterization. With controlled release of the bismuth (via bismuth oxyhalide metathesis), the metal oxo clusters can be retained in aqueous solution, and we can track their reaction pathways and conversion to related metal oxyhydroxides. Here we present isolation of a bismuth-stabilized Mn^IV cluster, fully formulated [Mn^IV₆Bi₂KO₉(CH₃COO)₁₀(H₂O)₃(NO₃)₂] (Mn₆Bi₂). In addition to characterization by single-crystal X-ray diffraction, solution characterization in acetonitrile and acetonitrile-acetic acid by small-angle X-ray scattering (SAXS) and electrospray ionization mass spectrometry shows high stability and the tendency of Mn₆Bi₂ to link into chains by bridging the bismuth (and potassium) caps with nitrate and acetate ligands. On the other hand, the dissolution of Mn₆Bi₂ in water, with and without metathesis of the bismuth, leads to the precipitation of related oxyhydroxide phases, which we characterized by transmission electron microscopy (TEM), electron diffraction, and energy-dispersive spectroscopy, and the conversion pathway by SAXS. Without removal of bismuth, amorphous manganese/bismuth oxyhydroxides precipitate within a day. On the other hand, metathesis of BiOBr yields a solution containing soluble manganese oxyhydroxide prenucleation clusters that assemble and precipitate over 10 days. This allows tracking of the reaction pathway via SAXS. We observe one-dimensional growth of species, followed by the precipitation of nanocrystalline hollandite (identified by TEM). The hollandite is presumably templated by the K⁺, originally in the crystalline lattice of Mn₆Bi₂. In this Forum Article that combines new results and prospective, we compare these results to prior studies in which we first introduced the use of capping Bi³⁺ to stabilize reactive clusters, followed by destabilization to understand reaction pathways in synthesis and low-temperature geochemistry.

Artificial Mn4 Ca Clusters with Exchangeable Solvent Molecules Mimicking the Oxygen-Evolving Center in Photosynthesis

The natural Mn₄ Ca cluster in photosystem II serves as a blueprint to develop artificial water-splitting catalysts for the generation of solar fuel in artificial photosynthesis. Although significant advances have recently been achieved, it remains a great challenge to prepare robust artificial Mn₄ Ca clusters that precisely mimic the structure and function of the biological catalyst. Herein, we report the isolation and structural characterization of two Mn₄ CaO₄ complexes with polar solvent molecules, acetonitrile or N,N-dimethylformamide, which closely mimics the two water molecules on the calcium ion, as well as the oxidation states of the four manganese ions and the main geometric structure of the natural Mn₄ Ca cluster. These new artificial Mn₄ Ca complexes provide important chemical clues to understand the structure and mechanism of the biological system.

The catalytic activities and magnetic behaviours of rare μ3-chlorido and μ1,1,1-azido bridged defective dicubane tetranuclear Mn(ii) complexes

A μ₃-chlorido and a μ_1,1,1-azido bridged defective dicubane tetranuclear Mn(ii) complexes of polynucleating Mannich base ligand show significant catalytic oxidase activities and are antiferromagnetically coupled which is rationalized by DFT calculations.

Smoothing the single-crystal to single-crystal conversions of a two-dimensional metal–organic framework via the hetero-metal doping of the linear trimetallic secondary building unit

Doping of Co²⁺ in the linear Cd₃ cluster secondary building units lowers the single-crystal to single-crystal conversion reactivity of the resulting metal–organic framework.

Hollow Sr/Rh-codoped TiO2 photocatalyst for efficient sunlight-driven organic compound degradation

Sunlight-driven photocatalysis has emerged as a potential technology to address organic pollutant issues.