Intramolecular triarylmethane-triarylmethylium complex: Generation, properties, and X-ray structure of a C–H bridged carbocation

Abstract

A series of title carbocations was generated and isolated as stable salts. X-ray analyses revealed a preference for an unsymmetric structure in which the bridging hydrogen is localized on one of the two triarylmethane units. NMR spectroscopy showed a similar preference in solution. The bridging hydrogen fluctuates between the two chromophores in solution, and the energy barrier for the H shift is linearly correlated with the C⋅⋅⋅C + separation determined by X-ray. According to an extrapolation, the energy barrier should decrease when the C⋅⋅⋅C + separation is less than 2.7 Å, which suggests a new model for isolable pure organic species with a three-center-two-electron bond. theme of aromaticity. ‡ Corresponding author [2f,4b]. Thus, we were interested in the novel properties or reactivities associated with the cleavage of a polyarylated C-C bond.Several years ago, we encountered an intriguing case: a long C-C bond was cleaved by a Brønsted acid when 1,2,2-tris(4-dimethylaminophenyl)-(3a) or 1-phenyl-2,2-bis(4-dimethylaminophenyl)-1acenaphthenol (3b) [1.648(4) Å] was treated with HBF 4 [8]. The products were not the expected 1,2,2triaryl-1-acenaphthenyliums [9], but rather 8-aroyl-1-naphthylbis(4-dimethylaminophenyl)carbeniums 4a,b + where the acid-induced oxidative protonolysis of the long C-C bond accompanied by formal hydride abstraction (Scheme 2). Ketocarbeniums 4a,b + were also obtained as triiodide salts upon the oxidation of 3a,b with 3 equiv of iodine.Compared to the protonolysis of a C-H bond [10], C-C cleavage is less common under acidic conditions [11]. As one of the plausible reaction mechanisms, we assumed direct protonation of the long C-C sigma bond with hydrogen-bridged carbeniums 5a,b + as the key intermediates in the oxida-T. SUZUKI et al.Scheme 1 1,1,2,2-Tetraarylacenaphthenes 1 and 9,9,10,10-tetraaryl-9,10-dihydrophenanthrenes 2 are representative dynamic redox systems, which undergo reversible cleavage/formation of the long C-C bond [1.614(2)-1.771(3) Å] with redox reactions. The dicationic dyes can be isolated as stable salts to exhibit electrochromic response.Scheme 2 Upon treatment of 1,2,2-triaryl-1-acenaphthenols 3a,b, the products were not the expected 1,2,2-triaryl-1-acenaphthenyliums but ketocarbeniums 4a,b + . Thus, the acid induced oxidative protonolysis of the long C-C bond [3b: 1.648(4) Å], which is less common compared to the protonolysis of a C-H bond. tive protonolysis of 3a,b (Scheme 3). The structurally related tetraaryl species 6c + (Ar = 4-MeOC 6 H 4 ) with a three-center geometry was later postulated as a possible intermediate in the oxidative protonolysis of 1,1,2,2-tetraanisylacenaphthene 1c by Gabbaï and a co-worker (Scheme 4), and this was thought be unstable under the acidic reaction conditions and spontaneously transformed into dication 1c 2+ [12]. However, we formed a different opinion based on our results with (8-diarylmethyl-1-naphthyl)bis(4dimethylaminophenyl)carbeniums 6d-f + (X = OMe, H, I), which were generated from the carbinol precursors upon treatment with ...

Bis(10‐methylacridinium)s as a Versatile Platform for Redox‐Active Functionalized Dyes and Novel Structures

“…Based on the NMR spectra measured at low temperature, 23 + and 24 + also preferred the C–H localized structure in solution, and the bridging hydrogen underwent rapid fluctuation between the two chromophores. The energy barrier for the 1,5‐hydride shift is smaller in 23 + than in 24 + , since the smaller separation of C · · · C′ + in the [C–H–C] + unit corresponds to the lower barrier of a H shift in intramolecular triarylmethane–triarylmethylium complexes …”