Mixed-Valence Cu^I /Cu^III Metal-Organic Frameworks with Non-innocent Ligand for Multielectron Transfer

Area of Science:

• Materials Science
• Inorganic Chemistry
• Solid-State Chemistry

Background:

• Metal-organic frameworks (MOFs) are crystalline materials constructed from metal ions and organic linkers.
• MOFs offer tunable properties for applications in catalysis, gas storage, and electronics.
• Exploring novel MOF structures with unique electronic properties is crucial for advancing materials science.

Purpose of the Study:

• To synthesize and characterize novel three-dimensional copper-benzoquinoid metal-organic frameworks (MOFs).
• To investigate the electronic structure and properties, particularly the presence of mixed valency and formal Cu(III) species.
• To understand the relationship between the mixed-valent metal centers, redox-non-innocent ligands, and the resulting semiconducting behavior.

Main Methods:

• Synthesis of two novel MOFs: [Cu4L3]n and [Cu4L3·Cu(iq)3]n, where LH4 = 1,4-dicyano-2,3,5,6-tetrahydroxybenzene and iq = isoquinoline.
• Utilized spectroscopic techniques to probe the electronic structure and coordination environment.
• Employed computational studies to analyze the mixed valency and charge transfer mechanisms.

Main Results:

• Successfully synthesized and characterized two novel 3D copper-benzoquinoid MOFs.
• Revealed unprecedented mixed valency, featuring formal Cu(I) and Cu(III) oxidation states, a first for MOFs.
• Observed strong long-range electronic delocalization and semiconducting behavior attributed to enhanced metal-ligand orbital overlap and through-bond charge transfer.
• Demonstrated the significance of the pore environment and the redox-non-innocent ligand in facilitating multielectron transfer.

Conclusions:

• The discovery of formal Cu(III) in MOFs opens new avenues for designing advanced electronic materials.
• The interplay between mixed-valent copper ions, the specific ligand, and the framework's porosity dictates the material's semiconducting properties.
• These findings underscore the potential of tailored MOFs for applications requiring efficient multielectron transfer and unique electrochemical characteristics.