Orientational Preference of Long, Multicenter Bonds in Radical Anion Dimers: A Case Study of π‐[TCNB]₂²⁻and π‐[TCNP]₂²⁻

Abstract

The similar shape and electronic structure of the radical anions of 1,2,4,5-tetracyanopyrazine (TCNP) and 1,2,4,5-tetracyanobenzene (TCNB) suggest a similar relative orientation for their long, multicenter carbon-carbon bond in π-[TCNP]2 (2-) and in π-[TCNB]2 (2-) , in good accord with the Maximin Principle predictions. Instead, the two known structures of π-[TCNP]2 (2-) have a D2h (θ=0°) and a C2 (θ=30°) orientation (θ being the dihedral angle that determines the rotation of one radical anion relative to the other along the axis that passes through center of the two six-membered rings). The only known π-[TCNB]2 (2-) structure has a C2 (θ=60°) orientation. The origin of these preferences was investigated for both dimers by computing (at the RASPT2/RASSCF(30,28) level) the variation with θ of the interaction energy (Eint ) and the variation of the Eint components. It was found that: 1) a long, multicenter bond exists for all orientations; 2) the Eint (θ) angular dependence is similar in both dimers; 3) for all orientations the electrostatic component dominates the value of Eint (θ), although the dispersion and bonding components also play a relevant role; and 4) the Maximin Principle curve reproduces well the shape of the Eint (θ) curve for isolated dimers, although none of them reproduce the experimental preferences. Only after the (radical anion)(.-) ⋅⋅⋅cation(+) interactions are also included in the model aggregate are the experimental data reproduced computationally.

Ion Pairing and Co‐facial Stacking Drive High‐Fidelity Bisulfate Assembly with Cyanostar Macrocyclic Hosts

“…[15a] Classified as strong hydrogen bonds, they may also displayf eatures of multi-center covalent bonding between anion radicals. [16] Nevertheless, theseh ydrogen bondsa re still not able to stabilize the dimer globally on accounto fC oulomb repulsions. [17] Although these repulsions are weakened by solvation,c omputations show the dimers are still not stable in solution.…”

The Tetracyanopyridinide Dimer Dianion, σ‐[TCNPy]22−

“…For π‐ or σ‐dimers of both [TCNE] 2 2−[1, 2, 10, 14, 15] and [TCNQ] 2 2− (TCNQ=7,7,8,8‐tetracyano‐ p ‐quinodimethane) composition as well as only π‐[TCNB] 2 2− (TCNB=1,2,4,5‐tetracyanobenzene) and π‐[TCNP] 2 2− (TCNP=1,2,4,5‐tetracyanopyrazine) composition have been reported. π‐[TCNB] 2 2− and π‐[TCNP] 2 2− each have a 2 e − /8c C−C bond and have the identical symmetry ( D 2 h ), size and shape; nonetheless, they have different orientational preferences .…”

“…For π‐ or σ‐dimers of both [TCNE] 2 2−[1, 2, 10, 14, 15] and [TCNQ] 2 2− (TCNQ=7,7,8,8‐tetracyano‐ p ‐quinodimethane) composition as well as only π‐[TCNB] 2 2− (TCNB=1,2,4,5‐tetracyanobenzene) and π‐[TCNP] 2 2− (TCNP=1,2,4,5‐tetracyanopyrazine) composition have been reported. π‐[TCNB] 2 2− and π‐[TCNP] 2 2− each have a 2 e − /8c C−C bond and have the identical symmetry ( D 2 h ), size and shape; nonetheless, they have different orientational preferences . To identify new examples of compounds with long, multicenter C−C bonds as well as understand the bonding and conformation/orientational preferences we targeted the preparation of π‐[TCNPy] 2 2− (TCNPy=2,3,5,6‐tetracyanopyridine) that has lower C 2 v symmetry with respect to D 2 h TCNB and TCNP …”

“…(TCNQ = 7,7,8,8-tetracyano-p-quinodimethane) [11,16] composition as well as only p-[TCNB] 2 2À (TCNB = 1,2,4,5-tetracyanobenzene) [17,18] and p-[TCNP] 2 2À (TCNP = 1,2,4,5-tetracyanopyrazine) [18,19] composition have been reported. p-[TCNB] 2 2À and p-…”

“…[TCNP] 2 2À each have a2e À /8c CÀCb ond and have the identical symmetry (D 2h ), size and shape;n onetheless, they have different orientationalp references. [18] To identify new examples of compounds with long, multicenter CÀCb onds as well as understandt he bonding and conformation/orientational preferences we targeted the preparation of p-[TCNPy] 2 2À (TCNPy = 2,3,5,6-tetracyanopyridine) that has lower C 2v symmetry with respect to D 2h TCNB and TCNP. [20] TCNPy undergoes aq uasi-reversible one electron reduction at À0.51 V( vs. SCE in MeCN) [20] that is % 0.85 Vh arder to reduce than TCNE, but easier to reduce than TCNB (À0.64 V) albeit more difficult to reduce than TCNP (À0.31 V).…”