Polyhedral Oligomeric Silsesquioxane-Encapsulating Amorphous Palladium Nanoclusters as Catalysts for Heck Reactions

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Oil foams stabilized by POSS/organosilica particle assemblies: application for aerobic oxidation of aromatic alcohols

A novel amphiphilic polyhedral oligomeric silsesquioxane (POSS) with surfactant-like behavior was synthesized. By combining this new POSS, used as a frother, with surface-active catalytic organosilica particles, used as a stabilizer, we designed a dual particle system able to generate foams in pure organic solvents. Tunable foamability and foam stability were achieved in a variety of organic solvents by simply adjusting the POSS concentration. As a result, the catalytic activity was drastically boosted in the aerobic oxidation of pure aromatic alcohols under 1 bar O₂ pressure. Particles were conveniently recycled with high foamability and the catalytic efficiency was maintained for at least 7 consecutive runs.

Langmuir and Langmuir Blodgett films of zinc oxide (ZnO) nanocrystals coated with polyhedral oligomeric silsesquioxanes (POSS)

HYPOTHESIS

The type and properties of ligands capping nanoparticles affect the characteristics of corresponding Langmuir and Langmuir-Blodgett films. When ligands are firmly anchored to the surface, as in zinc oxide nanocrystallites (ZnO NCs), compression at the air/water interface might cause ligands interdigitation and then the formation of supra-structures. Here, we evaluate how the introduction of bulky ligands, namely polyhedral oligomeric silsesquioxanes (POSSs), influences the self-assembly of POSS@ZnO NCs and the properties of corresponding thin films.

EXPERIMENTS

ZnO NCs capped with asymmetrical POSS derivatives are prepared via a one-pot two-step self-supporting organometallic (OSSOM) method. POSS@ZnO NCs are characterized by employing STEM, DLS, TGA, NMR, IR, UV-VIS, and photoluminescence spectroscopy. Changes in surface pressure, surface potential, and morphology (using BAM) are recorded upon compression at the air/water interface. Films transferred onto solid substrates are examined utilizing XRR and AFM.

FINDINGS

All studied POSS@ZnO NCs form stable Langmuir films. POSSs prevent the interdigitation of ligands capping neighboring NCs. Thus, POSS@ZnO NCs films resemble those composed of classical amphiphiles but without any visible structural source of amphiphilicity. We suggest that the core provides enough hydrophilicity to anchor NCs at the air/water interface. POSS ligands provide enough hydrophobicity for the NCs not to disperse into the subphase upon compression.

Multinuclear Clusters of Manganese and Lithium with Silsesquioxane-Derived Ligands: Synthesis and Ligand Rearrangement by Dioxygen- and Base-Mediated Si-O Bond Cleavage

The synthesis of manganese cluster complexes templated by polyhedral oligomeric silsesquioxane-derived ligands is described. Mn^II₃(Ph₇Si₇O₁₂)₂Pyr₄ (1) and Mn^II₄(Ph₄Si₄O₈)₂(Bpy)₂(Py)₂ (3) are prepared by replacement of the amide ligands of Mn(NR₂)₂ (R = SiMe₃) via ligand protolysis by the acidic proton of the respective silsesquioxane-derived silanols. Complex 1 is shown to undergo ligand rearrangement by reaction with O₂, which results in oxidation of the cluster to a mixed Mn^II/III cluster, concomitant with cleavage of the Si-O bonds of the ligand, releasing a [Ph₂Si-O]⁺ unit, opening a new ligating siloxide group, and resulting in the formation of Mn₃(Ph₆Si₆O₁₁)₂Pyr₄ (2). The ligand framework of 1 can also be perturbed by a base. The addition of LiOH/BuLi delivers a soluble equivalent of Li₂O to 1, resulting in cleavage of the Si-O bonds and linkage of the resulting exposed silicon atoms by the new oxide, giving a linked ligand variant that templates a Li₂Mn₃ cluster, Mn₃Li₂(Ph₇Si₇O₁₂OPh₇Si₇O₁₂)DMF₅Pyr (4). These systems are characterized by single-crystal X-ray diffraction, absorption spectroscopy, Fourier transform infrared, cyclic voltammetry, and CHN combustion analysis. Mechanistic implications for the Si-O bond cleavage events are discussed.

POSS nanostructures in catalysis

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

Xantphos-Capped Pd(II) and Pt(II) Macrocycles of Aryldithiolates: Structural Variation and Catalysis in C-C Coupling Reaction

The self-assembly of Xantphos-capped M(OTf)₂ (M = cis-[M'(Xantphos)]²⁺; M' = Pd, Pt) with bridging ligands 1,4-benezenedithiol or 4,4'-biphenyldithiol has been investigated. The reactions have yielded complexes [M{S(C₆H₄) _nSH}]₂(OTf)₂ (I) and [M₂{S(C₆H₄) _nS}]₂(OTf)₄ (II) ( n = 1 or 2). The equilibrium between I and II has been established in platinum complexes for n = 2, whereas the analogous Pd complex exclusively exist as II. These results are different from our previously reported dppe or triethyl phosphine-capped complexes which showed only type II. The same reaction with 1,3-benezenedithiol lead to the complex [M₂(SC₆H₄SSC₆H₄S)](OTf)₂ (III), containing a S-S bond between two thiolate ligands, formed via a complex of type I in solution. Characterization of the complexes was accomplished by NMR spectroscopy, UV-vis spectroscopy and mass spectrometry, and X-ray crystallography. Density functional calculations were performed to estimate the relative stability of three types of complexes. The palladium complexes are excellent catalysts in Suzuki C-C cross coupling reactions under mild conditions, and can be reused eight times without losing significant yield. The activity of the Pd catalysts derived from three dithiol ligand follows opposite trend of the stability as III > II > I. The comparative catalytic activity of the tetranuclear Pd complexes (II) of bis-phosphines of varied bite angles, including the structurally characterized [Pd₂(dppf)₂(SC₁₂H₈S)]₂(OTf)₄ has also been demonstrated.

Quaternized POSS modified rGO-supported Pd nanoparticles as a highly efficient catalyst for reduction and Suzuki coupling reactions

Hydrophilic QPOSS modified rGO nanosheets are fabricated as a robust catalyst support of PdNPs for reduction and Suzuki coupling reactions.

Highly Efficient Catalytic Hydrogen Evolution from Ammonia Borane Using the Synergistic Effect of Crystallinity and Size of Noble-Metal-Free Nanoparticles Supported by Porous Metal-Organic Frameworks

A series of nonprecious metal nanoparticles (NPs) supported by metal-organic framework MIL-101 were synthesized using four methods and their catalytic performance on hydrogen evolution from ammonia borane (NH₃BH₃) was studied. The results showed that the crystalline Co NPs with size of 4.5-8.5 and 14.5-24.5 nm had low activities featuring the total turnover frequency (TOF) values of 9.9 and 4.5 mol_H2 mol_cat^-1 min^-1, respectively. In contrast, the amorphous Co NPs with size of 1.6-2.6 and 13.5-24.5 nm exhibited high activities featuring the total TOF values of 51.4 and 22.3 mol_H2 mol_cat^-1 min^-1, respectively. The remarkably different activities could be ascribed to the different crystallinity and size of Co NPs in the catalysts. Moreover, the ultrasound-assisted in situ method was also successfully applied to bimetallic systems, and MIL-101-supported amorphous CuCo, FeCo and NiCo NPs had the catalytic activities with total TOF values of 51.7, 50.8, and 44.3 mol_H2 mol_cat^-1 min^-1, respectively, which were the highest in the values of the reported non-noble metal Co-based catalysts. The present approach, namely, using the synergistic effect of crystallinity and size of metal NPs, may offer a new prospect for high-performance and low-cost nanocatalysts.

Facile synthesis of novel hybrid POSS biomolecules via “Click” reactions

A novel alkyne-terminated cubic-octameric POSS was synthesised in high yield and click chemistry has been used to attach bio-oligomers.

Effects of biomineralization peptide topology on the structure and catalytic activity of Pd nanomaterials

Three dimensional porous structures of Pd were formed through a designed peptide with precisely defined topological features. The hierarchical materials exhibited excellent catalytic performance in the reduction of nitrophenol isomers with preference for the meta isomer.