Reduction of a dihydroboryl cation to a boryl anion and its air-stable, neutral hydroboryl radical through hydrogen shuttling

A doubly cyclic (alkyl)(amino)carbene-stabilised dihydroboronium cation undergoes fully reversible 2e– reduction to a stable hydroboryl anion via B-to-C hydrogen migration. Subsequent 1e– oxidation yields a bench-stable neutral hydroboryl radical.


Methods and materials
All manipulations were performed either under an atmosphere of dry argon or in vacuo using standard Schlenk line or glovebox techniques. Deuterated solvents were dried over molecular sieves and degassed by three freeze-pump-thaw cycles prior to use. All other solvents were distilled and degassed from appropriate drying agents. Solvents were stored under argon over activated 4 Å molecular sieves. NMR spectra were acquired on a Bruker Avance 500 NMR spectrometer. Chemical shifts (δ) are yield in ppm and internally referenced to the carbon nuclei ( 13 C{ 1 H}) or residual protons ( 1 H) of the solvent. Hetero nuclei NMR spectra are referenced to external standards ( 11 B: BF3•OEt2; 19 F: Cl3CF; 31 P: 85% H3PO4). Solid-state IR spectra were recorded on a Jasco FT/IR-6200 spectrometer inside a glovebox. Microanalyses (C, H, N, S) were performed on an Elementar vario MICRO cube elemental analyzer. High-resolution mass spectrometry (HRMS) data were obtained from a Thermo Scientific Exactive Plus spectrometer. Note: both elemental analyses and HRMS were carried out for all new compounds but in some cases these decomposed too rapidly and only one type of analysis was possible.

Synthetic procedures (CAAC Me )BH2(OTf), 1
MeOTf (2.03 mL, 18.5 mmol, 1.5 eq.) was added slowly to a solution of (CAAC Me )BH3 (3.70 g, 12.3 mmol) in 60 mL of benzene whereupon a gas evolution occurred. The reaction mixture was stirred overnight prior to removal of volatiles. The residue was washed with hexane and dried in vacuo to yield 1 as a white solid (5.30 g, 11.8 mmol, 96% yield). Colorless single crystals were obtained by vapor diffusion of hexane into a saturated benzene solution. 1  b) A solution of (CAAC Me )BH2(CAAC Me H) (20.0 mg, 34.2 μmol) in 1 mL of benzene was treated with an excess of MeOTf (5-10 eq.). Within 2 d at room temperature a suspension was formed and complete consumption of (CAAC Me )BH2(CAAC Me H) was observed by 11 13

EPR measurement
EPR measurements at X-band (9.86 GHz) were carried out at room temperature using a Bruker ELEXSYS E580 CW EPR spectrometer. CW EPR spectra were measured using 1 mW microwave power and 0.5 G field modulation at 100 kHz, with a conversion time of 20 ms. The spectral simulations were performed using MATLAB 8.6 and the EasySpin 5.2.25 toolbox. 4

X-ray crystallographic data
The crystal data of all compounds were collected on a Bruker X8-APEX II diffractometer with a CCD area detector (compounds 1, 2-CAAC Me and 3-DMAP) or a Bruker D8 QUEST diffractometer with a CMOS area detector (compounds 2-Pyr, 2-DMAP, 5, 5-thf and 6), each equipped with m-layer mirror monochromated MoK radiation. The structures were solved using intrinsic phasing methods, 5 refined with the SHELXL program 6 and expanded using Fourier techniques. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined isotropically and assigned to idealised positions, except for boron-bound hydrogen atoms, which were located in the difference Fourier map and freely refined.

Refinement details for 2-CAAC Me :
The asymmetric unit contains two sets of the borate with its triflate anion and a chloroform solvent molecule. Only one triflate (RESI OTfl) and one chloroform residue (RESI CLF) are twofold disordered in a 4:1 ratio, which leads to a noncentrosymmetric space group. Distances within the parts were restrained with SAME, ADPs with SIMU 0.005. Two reflections affected by the beamstop were omitted.

Refinement details for 2-DMAP:
The triflate counteranion was modelled as twofold disordered by rotation around the C-S axis. The parts were refined with FVAR to a 56:44 ratio.
The ADPs within these residues were restrained to the same value with similarity restraint SIMU 0.002 and the 1-2 and 1-3 distances to the same values with SAME.  Refinement details for 5-thf: All hydrogen atoms except the boron-bound H1 were assigned to idealised positions. H1 was detected in the Fourier difference map and freely refined. Two of the potassium-bound THF residues were modelled as twofold disordered in a 63:37 ratio.
The displacement parameters of the atoms of these residues were restrained using SIMU 0.003.