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...

Isolation of a planar 1,2-dilithio-disilene and its conversion to a Si–B hybrid 2π-electron system and a planar tetraboryldisilene

“…The planarity of 2 could be attributed to the electronic effects of the electropositive boryl and lithium substituents. 9 The Si-Si bond length of 2.223(4) Å is same as that reported for 1-magnesium-2,3disilacyclopropene (2.223(17) Å), while the Si-Li bond length of 2.482 (12) Å is remarkably shorter than those observed in contact ion pairs of silenyl lithium species. 4,10 The two fivemembered diazaborolyl C2N2B planes are essentially coplanar with BSiSi plane (the dihedral angle = 1.83°).…”

“…The B−Si bond lengths in the 3a-c BSi2 cycles ranging from 1.911(7) Å to 1.952(3) Å are close to those of 1.9190 and 1.9186(13) Å reported for L2Si2BR (D, Scheme 4) feature the four-coordinate silicon atoms. 12 The sums of the bond angles around Si1 and Si2 in 3a-c are close to 360°, indicating the geometries of each silicon atom is essentially planar. Comparison of the B−Si bonds and internal angles of the Si2B ring in 3a-c reveals that the amino substituent Ph2N in 3a engages in a relatively weak -interaction with the empty p orbital of the boron atom.…”

“…The bonding situation in the Si2B cycle is analogous to borirenes but distinct from D with fourcoordinate Si atoms, in which the delocalized structure was formed by the * MOs of -symmetry with the boron p orbital. 12 Reaction of dilithiodisilene 2 with two equivalents of Bbromocatecholborane (cat)BBr resulted in the formation of the first tetraboryldisilene 4 (Scheme 3). Although a great number of disilenes have been synthesized, disilenes substituted by group 13 and 15 elements are still limited because of the lack of appropriate synthetic precursors.…”

Dialkylboryl-Substituted Cyclic Disilenes Synthesized by Desilylation-Borylation of Trimethylsilyl-Substituted Disilenes

“…Sekiguchi et al have shown the synthesis of boryldisilenes A-D [10][11][12] and the substantial interaction between the 9-borabicyclo[3.3.1]nonyl (BBN) group and the Si=Si double bond in C. Our group has synthesized BBN-substituted disilenes E and F [13,14] and found a bathochromically-shifted absorption band due to the substantial push-pull effects in disilene F and the activation of H 2 molecule via the cleavage of the Si-B and Si=Si bonds. Very recently, Roesky et al have reported boradisilacycle G having 2π aromatic character [15]. Cui et al have reported new disilene H [16] and disilenides I and J substituted by N-heterocyclic boryl (NHB) groups [17].…”

“…Molecules 2021, 26, x FOR PEER REVIEW 2 of 14 [15]. Cui et al have reported new disilene H [16] and disilenides I and J substituted by Nheterocyclic boryl (NHB) groups [17].…”