Unusually long, multicenter, cation(δ+)···anion(δ-) bonding observed for several polymorphs of [TTF][TCNE]

The α, β, and δ polymorphs of [TTF][TCNE] (TTF=tetrathiafulvalene; TCNE=tetracyanoethylene) exhibit a new type of long, multicenter bonding between the [TTF](δ+) and [TCNE](δ-) moieties, demonstrating the existence of long, hetero-multicenter bonding with a cationic(δ+)···anionic(δ-) zwitterionic-like structure. These diamagnetic π-[TTF](δ+) [TCNE](δ-) heterodimers exhibit a transfer of about 0.5 e(-) from the TTF to the TCNE fragments, as observed from experimental studies, in accord with theoretical predictions, that is, [TTF(δ+)···TCNE(δ-)] (δ≅0.5). They have several interfragment distances <3.4 Å, and a computed interaction energy of -21.2 kcal mol(-1), which is typical of long, multicenter bonds. The lower stability of [TTF](δ+) [TCNE](δ-) with respect to typical ionic bonds is due, in part, to the partial electron transfer that reduces the electrostatic bonding component. This reduced electrostatic interaction, and the large interfragment dispersion stabilize the long, heterocationic/anionic multicenter interaction, which in [TTF(δ+)···TCNE(δ-)] always involves two electrons, but have ten, eight, and eight bond critical points (bcps) involving C-C, N-S, and sometimes C-S and C-N components for the α, β, and δ polymorphs, respectively. In contrast, γ-[TTF][TCNE] possesses [TTF](2)(2+) and [TCNE](2)(2-) dimers, each with long, homo-multicenter 2e(-)/12c (c=center, 2 C+4 S) [TTF](2)(2+) cationic(+)···cationic(+) bonds, as well as long, homo-multicenter 2e(-)/4c [TCNE](2)(2-) anionic(-)···anionic(-) bonding. The MO diagrams for the α, β, and δ polymorphs have all of the features found for conventional covalent C-C bonds, and for all of the previously studied multicenter long bonds, for example, π-[TTF](2)(2+) and π-[TCNE](2)(2-). The HOMOs for α-, β-, and δ-[TTF][TCNE] have 2c C-S and 3c C-C-C orbital-overlap contributions between the [TTF](δ+-) and [TCNE](δ-) moieties; these are the shortest intra [TTF···TCNE] separations. Thus, from an orbital-overlap perspective, the bonding has 2c and 3c components residing over one S and four C atoms.