The Consequences of an Electrostatic “Forced Marriage” between Two Electron‐Rich Particles: Strained Ion Pairs

Abstract

Two translationally independent electron-rich particles would principally repel each other, unless they had opposing electrical charge and would thus form an ion pair in a suitable solvent. Under these conditions, a novel interionic strain could build up, which would result from the intkrionic orbital repulsion and should express itself in the increased reactivity of both ionic components towards other reagents. In order to confirm this, salts would have to be synthesized in which electron-rich cations are combined with electron-rich anions. Cations are generally, however, electron-deficient systems; the notable exception to this rule are the tris(dialkylamin0)-substituted cyclopropenylium ions, which according to Gerson et al. can be considered as electron-excess systems."l These electronic properties are the cause of their unusual behavior in comparison to other carbenium ions: For example, the tris(dimethy1amino)cyclopropenylium ion (TDA cation) functions as the donor component in deeply colored charge transfer (CT) salts with tetracyanoethylene (TCNE) or 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) .[*% The TDA cation also behaves as a donor with respect to electron-deficient anions of the type EICI; (El = Sb, Nb, Ta); in these reactions deeply coloured "inverse CT salts" with one-dimensionally stacked structures r e s~l t . [~-~~ TDA salts can be readily oxidized to give stable isolable radical dicationic salts.'9-If this cation is combined with donor anions (e.g. halide ions), ion pairs of the type 1 are obtained which contain-as required above-two oppositely charged electron-excess systems as electronic antagonists (Scheme 1 a). This is demonstrated in that the E l l , values of about 1.4 V (vs Ag/AgCI; 0.1 N NEt,CI/CH,CN) for the oxidation of both the TDA cation and the chloride ion are almost the same, in spite of the opposing charges of these ions.[t2, I 3 l At the point of maximum electrostatic attraction between the constituent ions of 1 (centrosymmetric arrangement), the orbital repulsion of the two would also have its maximum effect. The latter should be especially pronounced for the corresponding HOMO/HOMO interaction since the interacting orbitals have the same local symmetry (see Scheme 1 b).We inferred from this, that TDA halides should exhibit a distinctly lower tendency to form inner ion pairs in dipolar aprotic solvents than other known saltlike halides. They are possibly, therefore. highly effective sources of naked halide ions. In order to check this hypothesis. we treated the readily accessible[91 tris(dimethybamino)cyclopropenylium chloride (TDACI) 1 a with accep-

Structure of a Discrete Dichloride Hexahydrate Cube as a Tris(diisopropylamino)cyclopropenium Salt

“…For this contact, the ClH distance is 2.94 Å, and the C‐H‐Cl angle is 146.6°. Weiss and coworkers have commented on the electron‐rich nature of tris(dialkylamino)cyclopropenium cations and the resultant ion‐pair strain between the cations and halide anions, which leads to isolated anions 11. We might, therefore, expect our cluster to be less distorted from the ideal gas‐phase structure than the other known clusters, especially that in CHYDNI, which is involved in external hydrogen bonding.…”

Synthesis and physical properties of tris(dialkylamino)cyclopropenium dicyanamide ionic liquids

“…7,8 TAC cations have greater charge delocalisation than ammonium, phosphonium, and guanidinium cations, while they have reduced hydrogen bond donor capabilities, compared to the imidazolium, pyridinium and triazonium cations, due to a lack of aromatic C-H groups. The high-lying non-bonding HOMO of TAC cations gives particularly weak cation-anion interactions 9 and their salts consequently have unusual properties: in some early work, Weiss and co-workers prepared iodide-iodoacetylene and iodide-iodoarene adducts. 10 More recently, we isolated an interesting discrete dichloride hexahydrate cube, [Cl 2 (H 2 O) 6 ] 2À , in which the solid state structure is very similar to the calculated structure.…”

Aminocyclopropenium Ions: Synthesis, Properties, and Applications

“…The electron-rich nature of TDAC ions led Weiss and coworkers to predict that salts of these species with electronrich counterions would experience a repulsive interaction, a phenomenon that was termed 'ion pair strain' ( Figure 3). 21 The expectation was that the electrostatic attraction of the oppositely charged ions would be strongly offset by orbital repulsion of the chloride HOMO and the high-lying HOMO of the TDAC ion (see Figure 4). Importantly, this orbital repulsion would be maximal in the centrosymmetric arrangement, thus negating what might otherwise have been expected to be the most stabilizing electrostatic orientation.…”