Electrically conductive metal-organic frameworks (MOFs) have been emerged as one of the most promising electronic materials of the 21st century, due to their unparalleled synthetic simplicity, structural tunability, and functional versatility. With much of the recent efforts focused on developing electrically conducting MOFs based on electron-rich aromatic ligands, such as hexa-coordinating aromatic compounds and tetratopic tetrathiafulvalene derivatives that can support either through-bond or through-space charge delocalization, π-electron-deficient (i.e., π-acidic) N-heteroaromatic ligands with soft coordination sites have been largely overlooked. Encouraged by its unique dual capabilities to bind both transition metal ions and anions, herein, we introduce for the first time a highly π-acidic 1,4,5,8,9,12-hexaazatriphenylene-2,3,6,7,10,11-heaxacarbonitrile (HATHCN) ligand to construct a novel three-dimensional (3D) MOF [Ag2(HATHCN)(CF3SO3)2], in which each ligand coordinates six Ag+ ions with its four bidentate core-N atoms and four CN groups and engages four TfO– anions in anion–π (i.e., O-lone-pair/π) interaction with the electron-deficient heteroaromatic core. The MOF features staircase-like covalent chains made of alternating Ag+ cations and HATHCN ligands that can potentially support through-bond charge movement and distinct anion–π interactions between TfO– anions and HATHCN ligands that enhances the electron density of the latter, endowing them a paramagnetic radical anion character. The computational studies revealed significant overlap between the Ag+-d and ligand-p (C, N, and O) orbitals in the valence band maximum and conduction band minimum located around the Fermi level, confirming the possibility of through-bond charge transport, while the fairly narrow electronic bandgap (Eg ~ 1.35 eV) bodes well for its electrically conductive nature. Commensurately, the in-situ pressed MOF pellets displayed an impressive electrical conductivity (σ = 7.3 Χ 10–4 S/cm at 293 K), outperforming the vast majority of 3D MOF made of electron-rich aromatic ligands. This study demonstrated for the first time that the anion–π interaction and through-bond charge transport capabilities of a π-acidic N-heteroaromatic ligand could lead to exceptionally high electrical conductivity and narrow electronic bang gap in a 3D MOF.

Electrically conductive 3D metal-organic framework featuring π-acidic hexaazatriphenylene hexacarbonitrile ligands with anion-π interaction and efficient charge-transport capabilities

ACS Spring Meeting 2021

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