Origins of electrical conductivity in 3D iron-tetrazole-based metal–organic frameworks

Abstract

Electrically conductive metal–organic frameworks (MOFs) have emerged as materials for energy conversion and storage, with the advantages of intrinsic porosity and high tunability. One promising strategy to design conducting MOFs is the use of electroactive ligands, combined with mixed-valence phenomena promoted by redox-accessible inorganic pairs. In this regard, a porous material based on a nitrogenated ligand (benzeneditetrazole, BDT) and the transition metal cation Fe(II), Fe₂(BDT)₃, holds a record conductivity among 3D MOFs. Its efficient charge transport was ascribed to the –(Fe–N–N–)_∞ network, opening the door to the design of novel conducting materials based on that scaffold. We present a theoretical investigation of the charge-transport properties for the record Fe₂(BDT)₃ polymorph, and extend the study to two analogous polymorphs with the same chemical composition but different crystal symmetry. Density functional theory calculations of the electronic band structure reveal the presence of alternative transport channels with high electronic delocalization along the π-conjugated ditetrazole ligand in combination to the iron–nitrogen chain. Our results demonstrate that ligand protonation distribution mandates charge-transport efficiency, and defines a different hole/electron conduction pathway for each polymorph. We thus propose a new strategy to enhance conductivity in porous materials based on protic ligands through engineering of protonic ordering. A detailed analysis of the partial Fe(II) oxidation to Fe(III) confirms insertion of empty Fe(III)-d energy levels within the bandgap with a small energy penalty, thus allowing enhancement of the electronic properties of the material through mixed-valence phenomena. This work provides insights into the factors influencing charge transport in MOFs, guiding the design and discovery of advanced porous conductors for next-generation applications.