cAAC‐Stabilized 9,10‐diboraanthracenes—Acenes with Open‐Shell Singlet Biradical Ground States

Abstract Narrow HOMO–LUMO gaps and high charge‐carrier mobilities make larger acenes potentially high‐efficient materials for organic electronic applications. The performance of such molecules was shown to significantly increase with increasing number of fused benzene rings. Bulk quantities, however, can only be obtained reliably for acenes up to heptacene. Theoretically, (oligo)acenes and (poly)acenes are predicted to have open‐shell singlet biradical and polyradical ground states, respectively, for which experimental evidence is still scarce. We have now been able to dramatically lower the HOMO–LUMO gap of acenes without the necessity of unfavorable elongation of their conjugated π system, by incorporating two boron atoms into the anthracene skeleton. Stabilizing the boron centers with cyclic (alkyl)(amino)carbenes gives neutral 9,10‐diboraanthracenes, which are shown to feature disjointed, open‐shell singlet biradical ground states.


This PDF file includes:
Materials and Methods Figs. S1 to S24 Tables S1 to S3 Additional References

Synthesis and characterization of 3b
A suspension of 2b (40.0 mg, 40.6 µmol) in benzene (1 mL) was reacted with [Mg(thf)3][C14H10] (8.5 mg, 20.3 µmol) at room temperature. The mixture was and stirred for 4 h, after which time 2/3 of the solvent were removed under reduced pressure. The mixture was filtrated, and the residue was washed with hexanes (5 x 1 mL) and dried in vacuo. The solid thus obtained was extracted into a minimum amount of 1,2-difluorobenzene, filtered over a medium porosity frit, and dried in vacuo. Crystals suitable for X-ray diffraction were obtained by slow evaporation of solutions of 3b in 1,2-difluorobenzene into naphthalene. Yield: 26.8 mg (29.7 µmol, 73%) of a green solid that proved NMR silent.

Synthesis and characterization of 4a
A suspension of 2a (1.00 g, 1.10 mmol) in toluene (10 mL) was cooled to -78 °C and reacted with [Mg(thf)3][C14H10] (456 mg, 1.21 mmol). After 1 h, the mixture was allowed to warm to room temperature, and stirred for 2 h. All volatiles were removed in vacuo, and the residue was extracted into benzene (3 x 10 mL). All volatiles of the filtrate were removed in vacuo, and anthracene removed by sublimation (10 -6 mbar, 70 °C, 16 h). Crystals suitable for X-ray diffraction were obtained by slow evaporation of solutions of 4a in benzene into anthracene. Yield: 532 mg (715 µmol, 65%) of an orange solid that proved NMR silent.

Synthesis and characterization of 4b
A suspension of 2b (1.00 g, 1.02 mmol) in toluene (10 mL) was cooled to -78 °C and reacted with [Mg(thf)3][C14H10] (412 mg, 1.10 mmol). After 1 h, the mixture was allowed to warm to room temperature, and stirred for 2 h. All volatiles were removed in vacuo, and the residue was extracted into benzene (3 x 10 mL). All volatiles of the filtrate were removed in vacuo, and anthracene removed by sublimation (10 -6 mbar, 70 °C, 16 h). Crystals suitable for X-ray diffraction were obtained by slow evaporation of solutions of 4b in benzene into anthracene. Yield: 614 mg (744 µmol, 73%) of an orange brown solid that proved NMR silent.

Attempted synthesis of 5a
A solution of 4a (30.0 mg, 40.3 µmol) in benzene (0.5 mL) was degassed by three freeze-pump-thaw cycles, and one atmosphere of CO gas was introduced. The mixture was stirred for two days at room temperature, after which time all volatiles were removed in vacuo. The residue was extracted into hexanes, filtered and all volatiles were again removed under reduced pressure to afford a red solid. 11 B NMR spectroscopy indicated the generation of 5a, however, we were not able to isolate this species analytically pure.

Synthesis and characterization of 5b
A solution of 4b (30.0 mg, 36.3 µmol) in benzene (0.5 mL) was degassed by three freeze-pump-thaw cycles, and one atmosphere of CO gas was introduced. The mixture was stirred for two days at room temperature, after which time all volatiles were removed in vacuo. The residue was extracted into hexanes, filtered and all volatiles were again removed under reduced pressure. Single crystals suitable for X-ray diffraction were obtained by slow evaporation of saturated benzene solutions of 5b. Yield: 19.5 mg (22.9 µmol, 63%) of a red solid.

S2 X-ray diffraction data
General remarks: The crystal data of 3b and 4a were collected on a BRUKER D8 QUEST diffractometer with a CMOS area detector and multi-layer mirror monochromated MoKa radiation. The crystal data of 4b was collected on a XTRALAB Synergy Dualflex diffractometer with a Hybrid Pixel Array detector and multi-layer mirror monochromated CuKa radiation. The crystal data of 5b was collected on a BRUKER SMART APEX 1 diffractometer with a CCD area detector and graphite monochromated MoKa radiation. The structures were solved using intrinsic phasing method (SHELXT), [5] refined with the SHELXL program, [6] and expanded using Fourier techniques. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were included in structure factors calculations. All hydrogen atoms were assigned to idealized geometric positions.

S3 Computational details
General remarks: Initially, we performed geometry optimization and hessian calculations for the closed-shell singlet (CS), open-shell singlet (OS) and triplet (T) states of 4a and 4b at the (U)B3LYP [7] -D3 [8] (BJ) [9] /def2-SVP [10] level of theory. We collected the vertical ∆E and adiabatic ∆E0 energy gaps between the states, the former calculated at the equilibrium geometry of the OS states. The DFT calculations pointed the OS states as the ground states of both systems, with the triplet states lying less than 0.15 kcal/mol above. Additionally, we also optimized the geometry of 5b at the B3LYP-D3(BJ)/def2-SVP level of theory. All systems were characterized as minimum energy structures by vibrational frequency calculations, which indicated that all Hessian eigenvalues were positive. In order to confirm the biradical character [11] of the singlet diboraanthracenes studied herein, as well as to validate the computed singlet-triplet gaps obtained by DFT, single-point calculations were performed for 4a using high-level complete active space self-consistent field (CASSCF) [12] and N-electron valence state second-order perturbation theory (NEVPT2) [13] calculations. Due to the large molecular size, these calculations were done using the Resolution of the Identity (RI) [14] approximation. The CASSCF calculations were performed for two distinct active spaces: one composed of 2 electrons and 2 orbitals, CASSCF(2,2); and a second one containing 6 electrons and 6 orbitals, CASSCF (6,6). For an accurate determination of the adiabatic singlet-triplet gaps, RI-NEVPT2 calculations were performed using reference CASSCF(2,2) and CASSCF(6,6) wavefunctions for both the singlet and triplet multiplicities. All DFT calculations were performed with the Gaussian 16, Revision B.01 software. [15] CASSCF and RI-NEVPT2 calculations were performed with the Orca 4.1.1 software. [16] Pictures of molecular structures, orbitals and densities were visualized and generated with Chemcraft, CYLview, [17] and Gaussview.

The biradical character index, y
The y index, [18] which can vary from 0 (closed-shell system) to 1 (pure biradical state), was obtained for 4a using the natural orbital occupancy numbers (NOON) [19] of the highest occupied (HONO) and lowest unoccupied (LUNO) natural orbitals from the CASSCF calculations, according to the following expression [18] : where T is the orbital overlap of HONO and LUNO, and is given by: