A Neutral Three-Membered 2π Aromatic Disilaborirane and the Unique Conversion into a Four-Membered BSi2 N-Ring

Abstract

We report the design, synthesis, structure, bonding, and reaction of a neutral 2π aromatic three-membered disilaborirane. The disilaborirane is synthesized by a facile one-pot reductive dehalogenation of amidinato-silylene chloride and dibromoarylborane with potassium graphite. Despite the tetravalent arrangement of atoms around silicon, the three-membered silicon-boron-silicon ring is aromatic, as evidenced by NMR spectroscopy, nucleus independent chemical shift calculations, first-principles electronic structure studies using density functional theory (DFT) and natural bond orbital (NBO) based bonding analysis. Trimethylsilylnitrene, generated in situ, inserts in the Si-Si bond of disilaborirane to obtain a four-membered heterocycle 1-aza-2,3-disila-4-boretidine derivative. Both the heterocycles are fully characterized by X-ray crystallography.

Keywords: amidinato ligands; aromaticity; disilaborirane; molecular orbitals.

Scheme 1

Synthetic scheme for the preparation of 1 (R₁=isopropyl; R₂=tert‐butyl).

Figure 1

Molecular structure of 1 with anisotropic displacement parameters at the 50 % probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles of 1 [°]; Si1–Si2 2.1877(5), Si1–B1 1.9190(13), Si2–B1 1.9186(13), Si1–N6 1.8909(10), Si1–N5 1.8921(10), Si2–N3 1.8834(10), Si2–N4 1.8961(10); Si1‐B1‐Si2 69.51(4), B1‐Si2‐Si1 55.25(4), Si2‐Si1‐B1 55.24(4). For structural details and all CCDC numbers see the Supporting Information.

Figure 2

a) The delocalized πMO, HOMO; Walsh orbitals b) HOMO−1 and c) HOMO−2 of 1. Hydrogen atoms are omitted for clarity.

Figure 3

NICS‐scan curves for the three‐membered Si₂B ring of Disilaborirane, 1: (▪) isotropic NICS; (•) out‐of‐plane component; (▴) in‐plane component.

Scheme 2

Synthetic scheme for the preparation of 2 (R₁=isopropyl; R₂=tert‐butyl).

Figure 4

Molecular structure of 2 with anisotropic displacement parameters at the 50 % probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles of 2 [°]; Si1⋅⋅⋅Si2 2.4367(7), Si1–B1 1.9211(18), Si2–B1 1.9205(18), Si1–N1 1.7633(13), Si2–N1 1.7687(13), Si1–N4 1.8749(14), Si1–N5 1.8921(14), Si2–N2 1.8896(13), Si2–N3 1.8761(14), B1⋅⋅⋅N1 2.763; Si1‐B1‐Si2 78.73(7), B1‐Si2‐N1 96.93(7), Si2‐N1‐Si1 87.24(6), N1‐Si1‐B1 97.09(7).

Figure 5

π‐MOs of 2 a) HOMO and b) HOMO−6. Hydrogen atoms are omitted for clarity.