Electroregulated Metal-Binding with a Crown Ether Tetrathiafulvalene Derivative: Toward Electrochemically Addressed Metal Cation Sponges

A redox responsive ligand incorporating the tetrathiafulvalene unit has been synthesized. The crystal structure of
the free ligand (Z)-1 (C20H30O5S8, triclinic P1̄, Z = 2, a = 9.087(6) Å, b = 11.637(7) Å, c = 14.370(8) Å, α =
65.54(3)°, β = 82.32(5)°, γ = 84.18(6)°, V = 1368 Å3) shows the redox-active tetrathiafulvalene core to be
essentially planar, which allows observation of two reversible one-electron processes upon electrochemical oxidation.
The efficiency of this system in the control of the reversible complexation/expulsion sequence of a metallic cation
(i.e., Ba2+) has been made possible thanks to a combination of (a) an unprecedented high coordination ability
among tetrathiafulvalene-based macrocycles as determined by LSI mass spectrometry (log K° = 3.5, NBA-matrix) as well as by solution investigations (1H NMR and cyclic voltammetry titration studies), which remarkably
converge to similar binding constant values (i.e., log K° = 4.2−4.3), and (b) reversible metal cation expulsion
upon electrochemical oxidation to the dicationic state. A channel-like solid-state structure is observed for the
Ba2+ complex (C20H30O5S8, Ba2+(CF3SO3)22-, (H2O)2, CD3CN, monoclinic C2/c, Z = 8, a = 45.66(1) Å, b =
8.897(5) Å, c = 23.124(8) Å, β = 105.54(4)°, V = 9050 Å3), which results from the segregated stacking mode
of the crown ether and the redox-active tetrathiafulvalene subunits, respectively.