Tantalum–Polyhedral Oligosilsesquioxane Complexes as Structural Models and Functional Catalysts for Epoxidation

M-Ge-Si thermolytic molecular precursors and models for germanium-doped transition metal sites on silica

The synthesis, thermolysis, and surface organometallic chemistry of thermolytic molecular precursors based on a new germanosilicate ligand platform, -OGe[OSi(OtBu)3]3, is described. Use of this ligand is demonstrated with preparation of complexes containing the first-row transition metals Cr, Mn, and Fe. The thermolysis and grafting behavior of the synthesized complexes, Fe{OGe[OSi(OtBu)3]3}2 (FeGe), Mn{OGe[OSi(OtBu)3]3}2(THF)2 (MnGe) and Cr{OGe[OSi(OtBu)3]3}2(THF)2 (CrGe), was evaluated using a combination of thermogravimetric analysis; nuclear magnetic resonance (NMR), ultraviolet-visible (UV-Vis), and electron paramagnetic resonance (EPR) spectroscopies; and single-crystal X-ray diffraction (XRD). Grafting of the precursors onto SBA-15 mesoporous silica and subsequent calcination in air led to substantial changes in transition metal coordination environments and oxidation states, the implications of which are discussed in the context of low-coordinate and low oxidation state thermolytic molecular precursors.

Structure and Reactivity of Polynuclear Divalent Lanthanide Disiloxanediolate Complexes

Trinuclear molecular complexes of europium (II) and ytterbium(II) [Ln₃{(Ph₂SiO)₂O}₃(THF)₆], 1-Ln₃L₃ (Ln = Eu and Yb), supported by the dianionic tetraphenyl disiloxanediolate ligand, were synthesized via protonolysis of the [Ln{N(SiMe₃)₂}₂(THF)₂] complexes. In contrast, the reaction of [Sm{N(SiMe₃)₂}₂(THF)₂] with the (Ph₂SiOH)₂O ligand led to the isolation of the mixed-valent Sm(II)/Sm(III) complex [Sm₃{(Ph₂SiO)₂O}₃{N(SiMe₃)₂}(THF)₄], 2-Sm₃L₃, which was crystallographically characterized. The Eu(II) complex 1-Eu₃L₃ displays weak ferromagnetic coupling between the Eu(II) metal centers (J = 0.1035 cm^-1). The addition of 3 equiv of (Ph₂SiOK)₂O to 1-Eu₃L₃ resulted in the formation of the polynuclear Eu(II) dimer of dimers [K₄Eu₂{(Ph₂SiO)₂O}₄(Et₂O)₂]₂, 3-Eu₂L₄. Complexes 1-Ln₃L₃ (Ln = Eu and Yb) are stable in solution at room temperature, while 3-Eu₂L₄ shows higher reactivity and rapidly decomposes to give the mixed-valent Eu(II)/Eu(III) species [K₃Eu₂{(Ph₂SiO)₂O}₄], 4-Eu₂L₄. Complex 1-Yb₃L₃ affects the slow reductive disproportionation of carbon dioxide, but 1-Eu₃L₃ does not display any reactivity toward CO₂. However, the presence of one additional (Ph₂SiO^-)₂O per Eu(II) metal center in 3-Eu₂L₄ increases dramatically the reductive ability of the Eu(II) metal centers, affording the first example of carbon dioxide activation by an isolated divalent europium complex. The reduction of CO₂ by 3-Eu₂L₄ is immediate, and carbonate is formed selectively after the addition of a stoichiometric amount of CO_2.

Chiral molecular nanosilicas

Molecular nanoparticles including polyoxometalates, proteins, fullerenes and polyhedral oligosiloxane (POSS) are nanosized objects with atomic precision, among which POSS derivatives are the smallest nanosilicas. Incorporation of molecular nanoparticles into chiral aggregates either by chiral matrices or self-assembly allows for the transfer of supramolecular chirality, yet the construction of intrinsic chirality with atomic precision in discrete molecules remains a great challenge. In this work, we present a molecular folding strategy to construct giant POSS molecules with inherent chirality. Ferrocenyl diamino acids are conjugated by two or four POSS segments. Hydrogen bonding-driven folding of diamino acid arms into parallel β-sheets facilitates the chirality transfer from amino acids to ferrocene and POSS respectively, disregarding the flexible alkyl spacers. Single crystal X-ray structures, density functional theory (DFT) calculations, circular dichroism and vibrational circular dichroism spectroscopy clearly verify the preferential formation of one enantiomer containing chiral molecular nanosilicas. The chiral orientation and chiroptical properties of POSS show pronounced dependence on the substituents of α-amino acids, affording an alternative way to control the folding behavior and POSS chirality in addition to the absolute configuration of amino acids. Through the kinetic nanoprecipitation protocol, one-dimensional aggregation enables chirality transfer from the molecular scale to the micrometer scale, self-assembling into helices in accordance with the packing propensity of POSS in a crystal phase. This work, by illustrating the construction of chiral molecular nanosilicas, paves a new way to obtain discrete chiral molecular nanoparticles for potential chiroptical applications.

Zirconium and hafnium polyhedral oligosilsesquioxane complexes – green homogeneous catalysts in the formation of bio-derived ethers via a MPV/etherification reaction cascade

Newly designed POSS supported zirconium and hafnium isopropoxides are robust, durable and selective homogeneous catalysts for the conversion of hydroxymethylfurfural to bis(isopropoxymethyl)furane via MPV/etherification reaction with isopropanol as a green solvent/reagent.

A modified hybrid silsesquioxane/histidine composite for copper and zinc adsorption and it behavior in the electro-oxidation of ascorbic acid

Octa-(3-chloropropryl)silsesquioxane was chemically modified with histidine (SSQ-H) and characterized by spectroscopy in the infrared region (FT-IR), X-ray diffraction (XRD), X-ray Dispersive Energy Spectroscopy (EDX), Scanning Electron Microscopy (SEM). The analytical properties of SSQ-H were tested regarding of Cu²⁺ and Zn²⁺ adsorption and as an electrochemical sensor for the detection of ascorbic acid. The metal sorption results indicate that maximum amount of Cu²⁺ and Zn²⁺ adsorbed (Nf_max) were 1.58 × 10^-3 and 5.67 × 10^-4 mol g^-1, respectively. After Cu²⁺ and Zn²⁺ ion adsorption and interaction with potassium hexacyanoferrate (III), the investigated materials displayed electroactivity for ascorbic acid detection. The anodic peak currents responses of a graphite paste electrode containing CuHCFSSQ-H presented a linear response in the concentration range of 4.0 × 10^-4 to 4.0 × 10^-3 mol L^-1 for ascorbic acid detection. The limit of detection was of 2.99 × 10^-4 mol L^-1, with an amperometric sensitivity of 1.69 μA/mol L^-1. The intensity of the anodic peak currents of the graphite paste electrode containing ZnHCFSSQ-H in the concentration range of 9.0 × 10^-5 to 9.0 × 10^-4 mol L^-1 also displayed a linear response. The limit of detection was of 6.76 × 10^-5 mol L^-1, with an amperometric sensitivity of 0.0206 A/mol L^-1.

POSS nanostructures in catalysis

In this review we highlight the use of appealing POSS-based nanostructures for both homogeneous and heterogeneous catalytic applications.

Catalytic epoxidation of olefins in liquid phase over manganese based magnetic nanoparticles

Epoxidation of olefins stands out as a crucial class of reactions and is of great interest in academic research and industry due to the production of various important fine chemicals and intermediates. Manganese complexes have the potential to catalyze the epoxidation of olefins with high efficiency. Magnetic nanocatalysts have attracted significant attention for immobilizing homogeneous transition metal complexes. Easy separation by external magnetic fields, nontoxicity, and a core shell structure are the main advantages of magnetic nanocatalysts over other heterogeneous catalysts. The method of functionalizing magnetic nanoparticles and of anchoring homogeneous metal complexes has significant effects on catalytic performance. Therefore, a critical review of recent research progress on manganese complexes' immobilization on magnetic nanoparticles for liquid phase olefin epoxidation is necessary. In this work, magnetic nanoparticles are categorized according to their preparation procedures and structures. The physical/chemical properties, catalytic performance for olefin epoxidation, reusability and plausible reaction mechanisms will be discussed, in an attempt to unravel the structure-function relationship and to guide the future study of MNPs' design for olefin epoxidations.

A mononuclear tantalum catalyst with a peroxocarbonate ligand for olefin epoxidation in compressed CO2

A mononuclear tantalum complex bonded to a peroxocarbonate ligand has been proved to be particularly important in the epoxidation reactions.

Polymer immobilized tantalum(v)–amino acid complexes as selective and recyclable heterogeneous catalysts for oxidation of olefins and sulfides with aqueous H2O2

Polymer supported peroxotantalate based heterogeneous catalysts served as highly efficient, selective and recyclable catalysts for alkene epoxidation and sulfide oxidation with green oxidant aqueous H₂O₂ under mild reaction conditions.

New Ni4Na2-phenylgermsesquioxane architecture: synthesis, structure and slow dynamic behaviour

The first Ni(ii)-based germaniumsesquioxane cage compound [(PhGeO_1.5)₁₀(NiO)₄(NaO_0.5)₂] presents a unique sandwich-like structure and exhibits slow dynamics of the magnetization.

Surface organometallic chemistry in heterogeneous catalysis

Surface organometallic chemistry has been reviewed with a special focus on environmentally relevant transformations (C–H activation, CO₂conversion, oxidation).

Si10Cu6N4 Cage Hexacoppersilsesquioxanes Containing N Ligands: Synthesis, Structure, and High Catalytic Activity in Peroxide Oxidations

The synthesis, composition, and catalytic properties of a new family of hexanuclear Cu(II)-based phenylsilsesquioxanes are described here. Structural studies of 17 synthesized compounds revealed the general principle underlying their molecular topology: viz., a central metal oxide layer consisting of two Cu₃ trimers is coordinated by two cyclic [PhSiO_1.5]₅ siloxanolate ligands to form a skewed sandwich architecture with the composition [(PhSiO_1.5)₁₀(CuO)₆]²⁺. In addition to this O ligation by the siloxanolate rings, two opposite copper ions are additionally coordinated by the nitrogen atoms of corresponding N ligand(s), such as 2,2'-bipyridine (compounds 1-9), 1,10-phenanthroline (compounds 10-13), mixed 1,10-phenanthroline/2,2'-bipyridine (compound 14), or bathophenanthroline (compounds 15-17). Finally, the charge balance is maintained by two HO^- (compounds 1-7, 10-13, and 15-17), two H₃CO^- (compound 8), or two CH₃COO^- (compounds 9 and 14) anions. Complexes 1 and 10 exhibited a high activity in the oxidative amidation oxidation of alcohols. Compounds 1, 10, and 15 are very efficient homogeneous catalysts in the oxidation of alkanes and alcohols with peroxides.