Highly conductive three-dimensional metal organic frameworks from small in situ generated ligands

Abstract

Four isostructural formate based electrically conductive metal organic frameworks namely, [H₂N(CH₃)₂][M(HCO₂)₃] (M = Mn; Mn-F, M = Co; Co-F, M = Ni; Ni-F, and M = Zn; Zn-F), were synthesized with simple and cost effective methods. The in situ generated formate ion was attributed to decomposition of DMF, which was used as the solvent of the reactions, under high pressure and temperature conditions. Single crystal X-ray diffraction analysis reveals that MOFs also contain in-situ generated dimethyl ammonium cations inside their pores to maintain the charge neutrality of the framework. The as-synthesized MOFs exhibit impressive room temperature electrical conductivity compared to the MOFs reported so far. The conductivity is attributed to the charge (in situ generated) flow along the pores of the MOFs and electron flow through the metal–ligand bond owing to the metal d-orbital and ligand p-orbital overlap. All the MOFs are semiconducting in nature and their conductivities increase with temperature. Mn-F exhibits conductivity as high as 47.846 S cm⁻¹ at 50 °C which was the highest among the conductivities of the four reported MOFs. Upon removal of guests from the pores the room temperature electrical conductivity of all the frameworks was improved except for Co-F. The formation of highly mobile hydronium ions upon removal of guests may be one of the reasons for improvement in the conductivity of the aforementioned de-guested MOFs. By the theoretical evaluation of the bonds of the MOFs, through bond conductivity is significantly determined by the number of high spin electrons in the metal d-orbitals.