Hyperconjugative aromaticity and protodeauration reactivity of polyaurated indoliums

Abstract

Aromaticity generally describes a cyclic structure composed of sp²-hybridized carbon or hetero atoms with remarkable stability and unique reactivity. The doping of even one sp³-hybridized atom often damages the aromaticity due to the interrupted electron conjugation. Here we demonstrate the occurrence of an extended hyperconjugative aromaticity (EHA) in a metalated indole ring which contains two gem-diaurated tetrahedral carbon atoms. The EHA-involved penta-aurated indolium shows extended electron conjugation because of dual hyperconjugation. Furthermore, the EHA-induced low electron density on the indolyl nitrogen atom enables a facile protodeauration reaction for the labile Au-N bond. In contrast, the degraded tetra-aurated indolium with a single gem-dimetalated carbon atom exhibits poor bond averaging and inertness in the protodeauration reaction. The aromaticity difference in such two polyaurated indoliums is discussed in the geometrical and electronic perspectives. This work highlights the significant effect of metalation on the aromaticity of polymetalated species.

Fig. 1. Hyperconjugative aromaticity (HA) comparison in indolium derivatives.

Electron-donating groups strengthen the aromaticity in cyclopentadiene while electron-withdrawing groups act in an opposite way. Transition metal substituents in polymetalated compounds B and D give better performance on HA over the hydrogen atoms in organic models A and C.

Fig. 2. Synthetic procedures for the penta-aurated complex 1.

Both a direct and an indirect pathway are shown to highlight the intermediate role of complex 2.

Fig. 3. Crystal structures and ¹H-NMR spectra of 1 and 2.

a Crystal structures of 1 (left) and 2 (right). Hydrogen atoms and tetrafluoroborate counter anions are omitted for clarity. Selected bond lengths (Å) and angles (°) in 1: N1–C1 1.375(9), C1–C2 1.426(11), C2–C3 1.434(9), C3–C8 1.415(10), C8–N1 1.365(9), N1–Au1 2.062(6), C1–Au2 2.094(7), C1–Au3 2.154(6), C2–Au4 2.086(7), C2–Au5 2.140(6), ∠Au2–C1–Au3 82.6(2), ∠Au4–C2–Au5 85.0(3). 2: N1–C1 1.316(15), C1–C2 1.466(18), C2–C3 1.435(16), C3–C8 1.442(19), C8–N1 1.368(17), N1–Au1 2.052(11), C1–Au2 2.020(11), C2–Au3 2.135(11), C2–Au4 2.096(12), ∠Au3–C2–Au4 85.5(4). b ¹H-NMR spectra in d₄-1,2-dichloroethane of 1 at 293 and 343 K, and 2 at 293 K.

Fig. 4. Theoretical calculation studies on 1-PMe₃ and 2-PMe₃.

Calculated models and AICD plots (isovalue: 0.03) of the π orbitals contribution and HOMOs (isovalue: 0.02) of a 1-PMe₃ and b 2-PMe₃. The NICS(1)_zz values given before and after the ‘/’ are those computed at 1 Å above the geometrical centers of 6MR and 5MR, respectively.

Fig. 5. Protodeauration reactivity of 1 and 2.

a Schematic reaction procedures of 1 and 2 with 4. b Crystal structure of 5. Hydrogen atoms and tetrafluoroborate counter anions are omitted for clarity. c UV-vis spectra (CH₂Cl₂, 298 K, c = 50 μM) of the reaction mixture of 1 and 4, and complexes 1 and 5. d ¹H-NMR spectra (CD₂Cl₂, 298 K) of complexes 1 and 5, and the products derived from the [1 + 4] mixture. e Proposed protodeauration step between 1 and 4.

Fig. 6. Kinetic studies on the reaction between 1 and 4.

a Emission spectra of 1 and 4 at 298 K (excitation: 330 nm for 1 and 350 nm for 4, c = 1.0 μM). b Luminescence (above 495 nm) decline curves of the reaction mixture of 1 (c = 100.0 μM) and 4 (c = 1.0 μM) in chloroform from 273 to 293 K. c Pseudo-first-order fitting of the luminescence decline curves. d Fitting with the Arrhenius equation from 273 to 293 K, the standard deviation error bars are minor than the symbols.