#POM 2025-03-03

多金属氧酸盐（POMs）又称多酸。

Polymetallic oxygenates are complex materials containing multiple metal atoms bridged by oxygen ligands. Examples include polyoxometalates (POMs) and metal-organic frameworks (MOFs). These systems exhibit diverse chemical properties (e.g., redox activity¹, catalytic behavior) due to their structural flexibility and metal-oxygen bonding dynamics.

Why Multiple Metal Centers Matter

Polymetallic oxygenates contain two or more metal atoms (e.g., Mo, W, V, Fe, Co) bridged by oxygen ligands. The presence of multiple metals introduces:

a. Cooperative Effects

• Synergistic Interactions: Different metals can work together to stabilize reactive intermediates or lower activation energies in catalytic cycles.
  Example: In polyoxometalates (POMs), a mix of metals (e.g., Mo and V) enables electron transfer between metal centers, enhancing redox activity.

• Diverse Oxidation States: Metals in varying oxidation states (e.g., Fe²⁺/Fe³⁺, Mo⁴⁺/Mo⁶⁺) allow flexible electron storage and transfer.

b. Structural Flexibility

• Tunable Geometry: Oxygen bridges (e.g., μ-oxo or μ-hydroxyl groups) connect metals into clusters (e.g., Keggin² or Wells-Dawson³ structures) or extended frameworks (e.g., metal-organic frameworks, MOFs).

  1. Tunable Geometry 可调几何形状

  What it means:

  "Tunable geometry" refers to the ability to design or adjust the shape and arrangement of atoms in a material. Think of it like building with LEGO blocks: you can rearrange the pieces to create different structures (e.g., a house vs. a spaceship), and each structure has unique properties.

  Why it matters:
  In polymetallic oxygenates, the geometry (shape) of the metal-oxygen clusters determines how the material behaves. For example:

  • A compact, rigid structure might be great for conducting electricity.⁴
  • A porous, open structure might trap pollutants or catalyze reactions.⁵

  How scientists tune geometry:
  By changing:

  • The types of metals used (e.g., iron vs. cobalt).
  • The number of oxygen atoms bridging the metals.
  • The reaction conditions (e.g., temperature, pH).

• Active Site Diversity: Multiple metals create distinct catalytic sites for multi-step reactions (e.g., oxidation and acid catalysis in POMs).

• Structural Features: Geometry (e.g., Keggin, Wells-Dawson structures), coordination environments, and oxidation states of metals.

• Electronic Properties: Bandgap, electron affinity, and charge transfer mechanisms.

• Synthesis & Stability: Conditions affecting formation and degradation.

Electronic Properties

Polymetallic oxygenate systems have delocalized electronic structures — electrons are not confined to one metal atom but move across the whole system.

• Metal-Metal Interactions: The d-electrons of different metal centers interact, which affects electrical conductivity and magnetic properties.
• Mixed-Valence Systems: Metals in different oxidation states (like Fe³⁺/Fe²⁺) create electron transfer pathways.
• Ligand⁶-to-Metal and Metal-to-Metal Charge Transfer (LMCT/MMCT): Electrons can hop between metals or from oxygen ligands to metal centers — crucial in catalytic and electronic applications. Example: POMs with W centers absorb UV light, enabling photocatalytic water splitting⁷.

Synthesis & Stability

a. Synthesis Strategies

b. Stability Challenges

• pH Sensitivity: Acidic/basic conditions can dissolve metal-oxygen bonds (e.g., POMs degrade at high pH).

• Thermal Stability: Decomposition at high temperatures limits use in industrial catalysis.

• Oxidative/Reductive Stability: Repeated redox cycles may degrade the structure.

c. Enhancing Stability

• Encapsulation: Embed clusters in silica or polymers⁸.

• Doping: Add stabilizing metals (e.g., Zr in MOFs improves thermal stability).

Key Examples

• For Keggin/Wells-Dawson: Focus on symmetry, metal-oxygen ratios, or cluster size.⁹
• For MOFs: Use pore size, linker flexibility, or metal electronegativity.¹⁰