Hydrogen-bonded maleate counter anions in tetrathiafulvalene crystals

Abstract

Intramolecular and/or intermolecular hydrogen-bonded maleate (HMA⁻) monoanions were introduced into cation radical salts of tetrathiafulvalene (TTF), forming three kinds of crystals: (TTF⁺)(H2MA)(HMA⁻) (1), (TTF⁺)₃(HMA⁻)₃(CH₃OH)(H₂O)_0.5 (2), and (TTF⁺)₃(TTF⁰)₅(HMA⁻)₃ (3). In crystals 1 and 2, the monovalent TTF⁺ cation formed a strong π-dimer of (TTF⁺)₂ and a π-trimer of (TTF⁺)₃, respectively. In crystal 3, a non-uniform π-stack of TTF⁰ and TTF⁺ was formed, which was further surrounded by neutral TTF through S⋯S and S⋯π interactions, forming a cylindrical TTF⁰ array around the π-stack. A charge-separated electronic state of (TTF⁺)₃(TTF⁰)₅(HMA⁻)₃ in crystal 3 was observed in the Raman spectra. Crystal 3 showed a semiconducting temperature dependent behavior with a room temperature conductivity of 3 × 10⁻³ S cm⁻¹, which was six orders of magnitude higher than those of crystals 1 and 2. The protonated state of the (H2MA)(HMA⁻) anion in crystal 1 was representative of an (HMA⁻⋯H⁺⋯HMA⁻) dimer, where the bridging proton between the two HMA⁻ anions was located at the midpoint between the two oxygen sites. A very strong intramolecular symmetrical O–H⋯O⁻ hydrogen-bonding interaction was observed in crystals 1–3 according to potential energy calculations of proton coordination in the O–H⋯O⁻ hydrogen bonding.