Correlation between Metal–Insulator Transition and Hydrogen-Bonding Network in the Organic Metal δ‑(BEDT-TTF)4[2,6-Anthracene-bis(sulfonate)]·(H2O)4

The sensitivity of electronic properties of organic conductors to minute modifications of their solid-state structure is investigated here within BEDT-TTF (ET) salts with organic bis-sulfonate anions, where specific hydrogen bonds between water molecules and sulfonate moieties are shown to dynamically control the organic slabs’ electronic structure. While the mixed-valence, 2,6-naphthalene-bis­(sulfonate) salt, (ET)4­(NBS)·​4H2O, exhibits a charge order state already at room temperature, the corresponding salt with the 2,6-anthracene-bis­(sulfonate) dianion, formulated as (ET)4­(ABS)·​4H2O, is metallic at RT and exhibits a metal–insulator transition at TMI = 85 K. The origin of the MI transition is revealed from a combination of temperature-dependent spectroscopic (Raman) measurements, X-ray structure elucidations (from 300 to 15 K), and theoretical investigations, demonstrating that the charge disproportionation observed below TMI is associated here with the progressive switching of bifurcated OH···O hydrogen bonds between the sulfonate moieties of the anion and the trapped water molecules. These movements within the anion layer are transmitted through weaker C–H···O interactions to the two A and B donor molecules, modifying the details of the overlap interactions within AA and BB pairs and opening a gap in the band structure.