Triaminocyclopropenium Halide and Triiodide Salts: The Formation of Cyclopropenium Dimers
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ORIGINAL PAPER
Triaminocyclopropenium Halide and Triiodide Salts: The Formation of Cyclopropenium Dimers Owen J. Curnow1 · Chaminda D. Jayasinghe1 · Matthew I. J. Polson1 · Ruhamah Yunis1 Received: 29 June 2019 / Accepted: 28 September 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract The crystal structures of tris(ethylmethylamino)cyclopropenium bromide and tris(allylmethylamino)cyclopropenium chloride are found to contain weak dimers of the cyclopropenium cation whereas the structure of the mixed salt of tris(dimethylamino) cyclopropenium chloride and dimethylammonium chloride reveals half of the cyclopropenium cations to have formed dimers and the other not to have formed dimers. These structures are compared to three related triiodide salts ( [C3(NMeEt)3]I3, [C3(NEt2)3]I3 and [C3(NBu2)3]I3) as well as the previously-reported salts [C3(NEtMe)3]Cl, [C3(NH2)3]Cl, [C3(NH2)3]I and [C3(NEt2)3]I. The distances between the C3 centroids of the staggered dimers are at the short end of those normally found in π-stacked neutral arenes, let alone charged aromatic rings. Factors leading to the formation of dimers and the effect of dimer formation on the cyclopropenium structures are investigated. Graphic Abstract The crystal structures of triaminocyclopropenium halide salts are shown to exhibit dimer formation in some cases, but not in others, whereas corresponding triiodide salts only form cationic monomers in the solid state.
Keywords Crystal structure · Halide · Triiodide · Cyclopropenium
Introduction
* Owen J. Curnow [email protected] 1
School of Physical and Chemical Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand
Intermolecular arene–arene interactions are ubiquitous. Commonly they are found in face-to-face orientations, generally referred to as π–π stacking. They are so ubiquitous that some even refer to “π–π stacking forces”. These interactions are frequently invoked to explain packing arrangements in the solid state as well as in protein structures [1, 2]. Triaminocyclopropenium (TAC) salts were first reported in 1971 by Yoshida and Tawara [3] and have since attracted
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much interest [4–13]. Despite the steric strain of the threemembered ring, these cations are remarkably unreactive: They are stable in boiling water and we have reported that the NTf−2 salts, which are ionic liquids, can have thermal decomposition onset temperatures above 400 °C [14]. A high-lying HOMO results in an unusually low oxidation potential [15–17] and is also responsible for particularly weak cation–anion interactions. This is manifest in the observation that halide counterions eschew interactions with the TAC cation, preferentially coordinating to other compounds instead. For example, Weiss has commented that “typically enough, the halides cannot be obtained in solvent-free form” [18]. Chloride hydrates [19, 20], iodideiodoacetylene [21] and iodide-iodoarene [22] adducts have all been isolated from tris(dialkylamin
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