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One- and two-electron reduction of triarylborane-based helical donor–acceptor compounds

One-electron chemical reduction of 10-(dimesitylboryl)-N,N-di-p-tolylbenzo[c]phenanthrene-4-amine (3-B(Mes)2-[4]helix-9-N(p-Tol)2) 1 and 13-(dimesitylboryl)-N,N-di-p-tolyldibenzo[c,g]phenanthrene-8-amine (3-B(Mes)2-[5]helix-12-N(p-Tol)2) 2 gives rise to monoanions with extensive delocalization over the annulated helicene rings and the boron pz orbital. Two-electron chemical reduction of 1 and 2 produces open-shell biradicaloid dianions with temperature-dependent population of the triplet states due to small singlet-triplet gaps. These results have been confirmed by single-crystal X-ray diffraction, EPR and UV/vis-NIR spectroscopy, and DFT calculations.


General information
Compounds 1 1 , 2 1 and [K(18-crown-6)(THF)2] naphthalenide 2 were synthesized according to literature procedures. THF and Et2O were dried using an Innovative Technology Inc. solvent purification system (SPS), pentane was distilled from LiAlH4, and all were stored over Na/K alloy in an argon-filled glovebox. Electronic absorption measurements were performed on a Varian Cary 5E UV/vis-NIR spectrophotometer and an Agilent 8453 diode-array UV/vis spectrophotometer. 1 H NMR spectra were recorded on a Bruker Avance 500 MHz ( 1 H, 500 MHz) spectrometer at room temperature. The residual peaks of the deuterated solvents were used as references for 1 H chemical shifts.

Synthesis of 1K1
In an argon-filled glovebox, 1 (10 mg, 0.0149 mmol, 1.0 equiv.), [K(18-crown-6)(THF)2] + naphthenide (9.4 mg, 0.0164 mmol, 1.1 equiv.), and THF (1 mL) were added into a 5 mL vial, and the mixture was stirred for 5 min to form a dark purple solution. The lid of the vial was removed, and the vial was placed inside a 25 mL vial containing pentane. The large vial was sealed and placed in a freezer in the glovebox (-30 °C). After one week, dark purple crystals formed and the structure was confirmed by single-crystal X-ray diffraction.

Synthesis of 1K2
In an argon-filled glovebox, 1 (10 mg, 0.0149 mmol, 1 equiv.), [K(18-crown-6)(THF)2] + naphthenide (21.4 mg, 0.0372 mmol, 2.5 equiv.), and THF (1 mL) were added into a 5 mL vial, and the mixture was stirred for 10 min to form a dark blue solution. The lid of the vial was removed, and the vial was placed inside a 25 mL vial containing pentane. The large vial was sealed and placed in a freezer in the glovebox (-30 °C). After 3 days, dark blue powder precipitated. Because of biradicaloid character, the 11 B and 13 C NMR were not obtained. 1

Synthesis of 2K1
In an argon-filled glovebox, 2 (10 mg, 0.0139 mmol, 1 equiv.), [K(18-crown-6)(THF)2] + naphthenide (8.7 mg, 0.0152 mmol, 1.1 equiv.), and THF (1 mL) were added into a 5 mL vial, and the mixture was stirred for 5 min to form a dark purple solution. The lid of the vial was removed, and the vial was placed inside a 25 mL vial containing pentane. The large vial was sealed and placed in a freezer in the glovebox (-30 °C). After one week, dark purple crystals formed and the structure was confirmed by single-crystal X-ray diffraction.

EPR measurements
EPR measurements at X-band (9.85 GHz) were carried out at room temperature using a Bruker ELEXSYS E580 CW EPR spectrometer. The spectral simulations were performed using MATLAB 8.3 and the EasySpin 5.2.25 toolbox. 8 Temperature-dependent EPR measurements at X-band (9.4 GHz) were carried out using a Bruker ELEXSYS E580 CW EPR spectrometer equipped with an Oxford Instruments helium cryostat (ESR900) and a MercuryiTC temperature controller. Solid-state EPR measurements at X-band (9.38 GHz) were carried out using a Bruker ELEXSYS E580 CW EPR spectrometer.      S10. Solid-state CW X-band EPR spectrum of dianion 2K2 at 145 K. The corresponding half-field signal could not be observed at this temperature and also not in the temperature range of 120 to 300 K.

Photophysical properties
UV/vis-NIR absorption spectra were measured on a Varian Cary 5E UV/vis-NIR spectrophotometer and on an Agilent 8453 diode array UV/vis spectrophotometer. All solutions used in photophysical measurements had concentrations of ca. 10 -5 M in Et2O. All absorption spectra were recorded in standard quartz cuvettes (1 cm × 1 cm) under argon.

Spectroelectrochemical measurements
Spectroelectrochemical experiments in reflection mode were performed using an Agilent Cary 5000 spectrometer in combination with a custom designed sample compartment consisting of a cylindrical PTFE cell with an Infrasil® wedge window (angled by 0.5 o ) and an adjustable two-in-one electrode (6 mm platinum disc working electrode, 1 mm platinum wire counter electrode). The potentials were adjusted with a Princeton Applied Research potentiostat (PAR 283) and referenced to a leak free Ag/AgCl reference electrode (Warner Instruments). All experiments were carried out at room temperature under an argon atmosphere.
Thin layer measurements were done by attaching the working electrode to the flat surface of a glass halfsphere and measuring 9 cycles with a scan speed of 2 mVs -1 . The voltammograms were referenced to the ferrocene/ferrocenium redox couple.

Theoretical studies
DFT calculations were carried out with the program package Gaussian 16 (Rev. B.01). 9 The geometries were optimized without symmetry constraints using the (U)M062X functional 10 in combination with 6-31G+(d) and 6-31G++(d) basis set 11 supplemented by diffuse functions. 12 Calculations for dianions 1K2 and 2K2 were carried out using the (U)M062X functional and the 6-31G++(d) basis set in combination with Truhlar and co-workers' SMD variation of PCM. 13 The HOMOs and LUMOs in the unrestricted (UM062X) method were allowed to mix in order to destroy α-β and spatial symmetries. In a different approach, the stability of DFT wavefunction was tested with stable=opt, which led to the same unrestricted wavefunctions. Gausview 6.0 was used to plot orbital surfaces. The lowest-energy vertical transitions were calculated by TD-DFT using the same level of theory and were further analyzed with the Multiwfn software. 14   Table S4. Comparison between experimental and calculated geometric parameters of 1 and corresponding anion and dianion. (2) 3.037 (3)  S23 Table S5. Comparison between experimental and calculated geometric parameters of 2 and corresponding anion and dianion. Compound