Concomitant Use of Tetrathiafulvalene and 7,7,8,8-Tetracyanoquinodimethane within the Skeletons of Metal-Organic Frameworks: Structures, Magnetism, and Electrochemistry.

Abstract

In search of multifunctional metal-organic frameworks (MOFs), redox-active donors and acceptors, namely, tetrathiafulvalene (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ), were concomitantly used as skeletal components with diamagnetic metal nodes (Cd and Zn) to construct unique framework materials. Six isostructural frameworks were synthesized by diffusion of metal salts, TTF(py)4, and either paramagnetic Li(TCNQ) or diamagnetic H2TCNQ. They were characterized by single-crystal X-ray diffraction and FT-IR and UV-vis-NIR spectroscopy, and their physical properties were studied, including two postsynthetic modifications involving crystal-to-crystal transformations following a solid-solution reaction with I2. The highly colored crystals of two isostructural Zn and Cd frameworks contain undulating Cd-TTF(py)4 layers entwined with TCNQ in a chicken-wire net as part of the skeleton of the MOF as well as TCNQ intercalated within the channels, while nitrate anions are occluded within the cavities formed by the pyridine moieties. Reaction with I2 replaces each intercalated TCNQ•- within the channels with I3-. The optical properties and the electron paramagnetic resonance (EPR) spectra indicate the presence of only radical TCNQ•- in the parent compounds, while the magnetic susceptibilities enabled an estimation of the amount of TCNQ•- ( S = 1/2) leading to almost paramagnetic behavior. Solid-state electrochemistry provides evidence of several one-electron redox states corresponding to the electroactive cores.