Charging OBO‐Fused Double [5]Helicene with Electrons

Abstract Chemical reduction of OBO‐fused double[5]helicene with Group 1 metals (Na and K) has been investigated for the first time. Two doubly‐reduced products have been isolated and structurally characterized by single‐crystal X‐ray diffraction, revealing a solvent‐separated ion triplet (SSIT) with Na+ ions and a contact‐ion pair (CIP) with K+ ion. As the key structural outcome, the X‐ray crystallographic analysis discloses the consequences of adding two electrons to the double helicene core in the SSIT without metal binding and reveals the preferential binding site in the CIP with K+ counterions. In both products, an increase in the twisting of the double helicene core upon charging was observed. The negative charge localization at the central core has been identified by theoretical calculations, which are in full agreement with X‐ray crystallographic and NMR spectroscopic results. Notably, it was confirmed that the two‐electron reduction of OBO‐fused double[5]helicene is reversible.

Redox Reversibility Study S16 NMR Probe Preparation S16 Figure S19. 1 H NMR spectra of 1, in situ generated 1 2-, and its quenched product S16 DART-MS Sample Preparation S17 Figure S20. DART-MS spectra of 1 and the quenched product of in situ generated 1 2-S17 V.
Crystal Structure Solution and Refinement Details S18 Table S1. Crystal Data and Structure Refinement Parameters for 2 and 3 S19 Figure S21. ORTEP drawing of 2 S20 Figure S22. ORTEP drawing of 3 S20 Table S2. Selected C-C bond distances in 1, 2 and 3 S21 Table S3. Selected bond distances in 1, 2 and 3 S22 Table S4. Selected torsion angles and plane angles in 1, 2 and 3, along with labeling scheme S22 VI.

I. Materials and Methods
All manipulations were carried out using break-and-seal [1] and glove-box techniques under an atmosphere of argon. Tetrahydrofuran (THF) and hexanes were purchased from Pharmco-Aaper and were dried over Na/benzophenone and distilled prior to use. THF-d8 was purchased from Sigma Aldrich, dried over NaK2 alloy and vacuum-transferred. Sodium, potassium and 18-Crown-6 ether were purchased from Sigma Aldrich and used as received. OBO-double [5]helicene (C30H16B2O4, 1) was prepared according to the previously reported procedure [2] and purified by sublimation. The UV/Vis spectra were recorded on a PerkinElmer Lambda 35 spectrometer. The

Probe preparation of K/1 in THF:
THF (4 mL) was added to a probe containing excess K metal (~4 eq.) and 1 (0.02 mg, 4.3×10 5 mmol) and UV/Vis spectra were monitored at different reaction times (total 24h) at room temperature. Note: after 30 hours the bulk precipitation was observed and that prevented further reaction monitoring. Figure S4. UV/Vis spectra of K/1 in THF.
The Na metal was decanted from the mixture and 1 H NMR spectra were monitored.

Probe preparation of K/18-crown-6/1 in THF-d8:
THF-d8 (0.7 mL) was added to an NMR tube containing excess K metal (~7 eq.), 18-crown-6 (~2.1 eq.) and 1 (3 mg, 0.0066 mmol). The initial color of the mixture was red-brown. The probe was allowed to sit for 24 hours resulting in a dark blue-green solution. The Na metal was decanted from the mixture and 1 H NMR spectra were monitored.      S16

IV. Redox Reversibility Study
NMR Probe Preparation 1 (5 mg, 0.0108 mmol), one piece of Na metal (~10 eq.) and 18-crown-6 ether (5.7 mg, 0.0216 mmol) were added into an NMR tube, followed by addition of 0.7 mL of fresh THF-d8. The tube was sealed under argon. The 1 H NMR spectrum of 1 was collected immediately, and that of 1 2-was collected after 24 hours. The solution was then exposed to air by opening the tube, and its spectrum was recorded as "quenched". S17

DART-MS Sample Preparation
2 mg of 1 and a piece of Na metal (excess) were added into a glass tube, followed by the addition of 2.0 mL of fresh THF. The tube was sealed under argon. After the formation of doubly-reduced product of 1 in 48 hours, the solution was exposed to air by opening the tube, and the spectrum was recorded as quenched 1 2-. S18

V. Crystal Structure Solution and Refinement Details
Data collection of 2•2THF was performed on a Bruker D8 VENTURE X-ray diffractometer with PHOTON 100 CMOS shutterless mode detector equipped with a Cu-target X-ray tube (λ = 1.54178 Å) at T = 100(2) K. Data collection of 3•0.5THF was performed on a Bruker D8 VENTURE Xray diffractometer with PHOTON 100 CMOS detector equipped with a Mo-target X-ray tube (λ = 0.71073 Å) at T = 100(2) K. Data reduction and integration were performed with the Bruker software package SAINT (version 8.38A). [3] Data were corrected for absorption effects using the empirical methods as implemented in SADABS (version 2016/2). [4] The structure was solved by SHELXT (version 2015/5) [5] and refined by full-matrix least-squares procedures using the Bruker SHELXTL (version 2017/1) [ 6 ] software package. All non-hydrogen atoms were refined They were also restrained to have the same Uij components, with a standard uncertainty of 0.01 Å 2 . In the unit cell of 3•0.5THF, there is one-half THF solvent molecule that was found to be severely disordered and was removed by the SQUEEZE routine in PLATON (version 201117). [7] The total void volume was 159 Å 3 indicated by PLATON, equivalent to 5.12 % of the unit cell's total volume. Further crystal and data collection details are listed in Table S1. S19

VI. Theoretical Calculations
Density functional theory (DFT) calculations were performed using the Gaussian 09 software package. [8] The geometries were optimized and the energies were calculated at the B3LYP/6-311++G(d,p) level. No symmetry restrictions were used during the calculations. The calculated structures correspond to local energy minima (without imaginary frequencies). The electrostatic potential (ESP) maps of the neutral 1 and the dianionic 1 2were generated by GaussView.