Keys for the Existence of Stable Dimers of Bis-tetrathiafulvalene (bis-TTF)-Functionalized Molecular Clips Presenting [TTF]^•+···[TTF]^•+ Long, Multicenter Bonds at Room Temperature

A systematic theoretical and computational investigation is performed to determine the keys governing the existence, in acetonitrile solutions, of dimers of bis-tetrathiafulvalene (bis-TTF)-functionalized diphenylglycoluril molecular clips (clip₂ⁿ⁺) that are stable at room temperature for n ≤ 4. Although the experimental structure of these dimers in solution is unknown, electronic absorption studies suggest that they have [TTF]^l+···[TTF]^m+ interactions that are preserved at room temperature (note that when l = m = 1 these interactions become long, multicenter bonds). In good agreement with the interpretation of the experimental spectroscopic data, all clip₂ⁿ⁺ dimers whose charge is ≤4 present an optimum geometry that, in all cases, has three short interfragment [TTF]^l+···[TTF]^m+ interactions. The computed ΔG(298 K) for these optimum structures matches the available experimental data on the stability of these dimers. Such optimum geometry, combined with the zwitterionic character of the electron distribution in monomers and dimers (most of the net positive charge is equally distributed among the TTF groups, while a 1– au charge is located in the central fused five-membered rings) allows the formation of a maximum of two long, multicenter [TTF]^•+···[TTF]^•+ bonds when all TTF groups host a 1+ au of charge, as in clip₂⁴⁺. However, these long, multicenter bonds alone do not account for the stability of clip₂ⁿ⁺ dimers at room temperature. Instead, the studies carried out here trace the origin of their stability to (1) the zwitterionic character of their charge distribution, (2) the proper geometrical shape of the interacting monomers, which allows the intercalation of their arms, thus making possible the simultaneous formation of two short contacts, both involving the positively charged TTF group of one monomer and the negatively charged central ring of the other, (3) the simultaneous presence of three short contacts among the TTF groups in the optimum geometry of the clip₂ⁿ⁺ dimers, which become two long, multicenter bonds and one van der Waals interaction when the four TTF groups host a 1+ charge, and (4) the net stabilizing effect of the solvent.