A Neutral Three‐Membered 2π Aromatic Disilaborirane and the Unique Conversion into a Four‐Membered BSi₂N‐Ring

Abstract

We report the design, synthesis, structure, bonding, and reaction of a neutral 2p 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 threemembered 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 SiÀSi 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. The synthesis of three-membered aromatic rings has been a fascinating topic for the chemical community during the last decades. Aromaticity and Hückel 4n + 2 rule continue its diversity with the smallest, largest, homo-, hetero-, Mçbius, all-carbon, organometallic-and so on, but most of these involve overlap of p, d and f orbitals. [1-3] A challenging question is it possible to generate a carbon-free neutral threemembered aromatic ring. The neutral three-membered aromatic compound with 2p electrons is one of the longest quests to inorganic, organic and theoretical chemists. [1, 4] Most of the reported small aromatic species are anionic or cationic. [4-11] Recent reports of the carbon-free aromatic systems all involve charged species: B 3 À (observed in gas phase), [6] Li 3 + (observed in gas phase), [7] [B 3 (CO) 3 ] + (observed in gas phase), [8] [B 3 (NCy 2) 3 ] 2À (characterized by X-ray), [9] [SiRSi 2 R' 2 (R = Si t Bu 3 , R' = SiMe t Bu 2)] + (characterized by X-ray), [10] [Ge 3 R 3 , R = Si t Bu 3 ] + (characterized by X-ray). [11] To develop carbon-free small neutral Hückel p systems with orbitals beyond p, d and f, unique design strategies must be assembled. Here, we present a novel approach where two of the three p orbitals are replaced by s* MOs to generate a neutral 2p aromatic three-membered disilaborirane. To synthesize carbon-free neutral three-membered 2p aromatic system, we replace one of the CH group of the classical cyclopropenyl cation by isoelectronic BR group and the remaining two CH groups by two amidinato-silylene groups to retain the 2p aromaticity, leading to the formation of a disilaborirane. [12] We envision that the extra electrons from the two amidinato-ligands will populate the stable delocalized p MO resulting from the 2p orbital of boron atom and the two s* MOs on silicon atoms in the neutral Si 2 B three-membered ring. 13 The two p electrons will conjugate in the field of the three nuclei of Si-B-Si. Accordingly, we design and synthesize the three-membered disilaborirane (Scheme 1). A 2:1:4 molar ratio of amidinato-silylene chloride [LSi-Cl; L = PhC(NtBu) 2 ], dibromo(2,4,6-triisopropylphenyl)borane and KC 8 is reacted in THF at À78 8C and...

A Neutral Planar Four-Membered Si2B2 2π-Aromatic Ring

“…Similar values of WBI for the Si−B bond were noted for the three-membered disilaborirane. 19 MO analysis shows that the highest occupied molecular orbital (HOMO) of 2 is a symmetrical π molecular orbital delocalized over the entire Si 2 B 2 ring (Figure 3a). The orbital coefficient of the Si atoms (13.3−13.4%) is comparatively smaller than that of the boron atoms (20.5−21.3%) with minor contributions from the N atoms of the N-amidinato ligand (Table S9, Supporting Information).…”

Reactivity of a Unique Si(I)–Si(I)-Based η2-Bis(silylene) Iron Complex

“…24,25 Roesky and co-workers described ring opening of their disilaborirane species VI with TMSN 3 , forming a 1-aza-2,3-disila-4-boretidine derivative (Scheme 4). 17 We observed selective conversion of 3 with 1 equiv of TMSN 3 to give the unique azide adduct 4 after 1 h at 40 °C in benzene solution. This species was obtained as a single crystalline material by recrystallization from pentane at −30 °C.…”

“…20 A more refined method by Stanger 21 following the out-of-plane component to the shielding tensor along a trajectory orthogonal to the plane of the ring (NICS zz ) has been used by Roesky et al to assign 2π-aromaticity to their disilaborirane species. 17 However, instead of showing the typical off-center minimum for 2π-aromatic systems, the NICS scan of 3 shows a steady and continuous increase to less negative NICS zz values over a value of 3 Å (see Figure S33), which may be caused by anisotropy of the metal center at the ring and/or by σaromaticity. 17,19,22 Therefore, we resorted to the canonical molecular orbital (CMO) analysis of the NICS(0), which separates the total shieldings into contributions from canonical molecular orbitals.…”

“…17 However, instead of showing the typical off-center minimum for 2π-aromatic systems, the NICS scan of 3 shows a steady and continuous increase to less negative NICS zz values over a value of 3 Å (see Figure S33), which may be caused by anisotropy of the metal center at the ring and/or by σaromaticity. 17,19,22 Therefore, we resorted to the canonical molecular orbital (CMO) analysis of the NICS(0), which separates the total shieldings into contributions from canonical molecular orbitals. 23 Indeed, there is a sizable contribution of −13.8 ppm from the Fe d xy orbital (HOMO−5, see Figure S32), which is part of the σ-framework.…”

“…The Si(I)−Fe−Si(I) three-membered heterocycle forms an equilateral triangle with interatomic distances of 2.217(6) Å (Figure 3); for the Si−Si bond, this value falls in the range observed for Si�Si double bonds (2.120−2.250 Å). 16 The isosceles Si−B−Si disilaborirane compound reported by Roesky and co-workers shows a Si−Si bond length of 2.188(5) Å, 17 while the equilateral triangular (di-tbutyl(methyl)silyl)bis(tri-t-butylsilyl)cyclotrisilenylium cation has an average Si−Si bond length of 2.217(3) Å. 16 Electronic Structure of 3.…”