A 3D Cu-Phosphonate Metal-Organic Framework with Cu-Coordination-Enhanced Electrical Conductivity
Herein, we report on the synthesis and electronic properties of the phosphonate metal-organic framework (MOF) Cu<sub>3</sub>(H<sub>5</sub>-MTPPA)<sub>2</sub> (H<sub>8</sub>-MTPPA = methane tetra-<i>p</i>-phenylphosphonic acid). This MOF, termed TUB1, has a unique one-dimensional inorganic building unit composed of square planar and distorted trigonal bipyramidal copper atoms, and a calculated BET surface area of 766.2 m<sup>2</sup>/g. The formation of TUB1 reduces the band gap of the linker from 4.2 eV to 2.4 eV. DFT calculations reveal two spin-dependent gaps of 2.60 eV and 0.48 eV for the alpha and beta spins, respectively, and that the lowest unoccupied crystal orbital<b> </b>for both gaps predominantly resides on the square planar copper atoms. Single-crystal conductivity measurements on TUB1 crystals yield an average electrical conductance of 53.2 ± 29.5 S/m. Although the H<sub>8</sub>-MTPPA linker blocks the extended conjugation in TUB1, we show that the presence of the square planar copper atoms in the inorganic building unit promotes the electrical conductivity.