π-Aromaticity Dominating in a Saturated Ring: Neutral Aromatic Silicon Analogues of Cyclobutane-1,3-diyls

Abstract

The synthesis, structures, and reactivity of the first neutral 2π-aromatic Si 4 rings [LSiSiAr(X)] 2 (3: X = Br; 4: X = Cl; L = PhC(N t Bu) 2 , Ar = 2,4,6-Me 3 C 6 H 2 ) were described. Compounds 3 and 4 were obtained by 1,3-halogenation of tetrasilacyclobutadiene (LSiSiAr) 2 (2), which was prepared by the reductive cross-coupling of trisilane (ArSiCl 2 ) 2 SiHAr with two equiv of chlorosilylene LSiCl. The reaction of 3 with two equiv of PhLi yielded the corresponding substitution Si 4 ring [LSiSiAr(Ph)] 2 (5). Single-crystal X-ray diffraction analysis of 3 disclosed that it adopts both puckered (3a) and planar (3b) structures in the solid state, whereas 4 and 5 exhibit only a puckered structure. DFT calculations suggested that the puckered 3a features almost the same electronic structure with fully delocalized 2π planar 3b. The dominant 2π-aromaticity of 3 in a σ-frame has been demonstrated by DFT calculations, providing the first example of aromatics featuring both planar and puckered structures.

Neutral 4π-electron tetrasilacyclobutadiene contains unusual features of a Möbius-type aromatic ring

“…Angles Si1-Si2-Si1A and Si2-Si1-Si2A, where A stands for symmetry operation of 3/2-x,3/2-y,1-z, are 80.24(1)° and 99.76(1)°, respectively, and all torsion angles within the Si4-ring assembly are of 0°. All the structural features of this central moiety closely align with the recently published reports by So and co-workers 43 and by Cui et al 25 In the asymmetric unit of compound 3 one can also observe another distinctive four-membered ring formed via Si1-N1-C5-N2 atoms. The ring is nearly planar with a torsion angle of 2.23(9)°.…”

“…22,23 The delocalization is comparable to the dismutational aromaticity which was anticipated in the case of hexasilabenzene reported by Scheschkewitz. 24 Very recently, a paper by Zhu, Cui, and coworkers 25 appeared which reported the synthesis of a weakly aromatic 4π tetrasilacyclobutadiene compound (Chart 2; B) with aryl and amidinate substituents which was used as a precursor for the isolation of saturated planar and puckered 2π tetrasilacyclobutane-1,3diyls that show higher aromatic character than the 4π system. 25 Similar 4π tetrasilacyclobutadiene compounds were previously reported by So and co-workers 26 and by Suzuki et al 27 The electronic structure in the Si 4 ring was described in terms of cyclically delocalized n, π, σ electrons which yield modes aromatic character although the systems are formally Hückel anti-aromatic.…”

Isolation of Fluorescent 2π-Aromatic 1,3-Disiladiboretenes

“…The dicationic species III and IV , as the silicon analogues of CBD 2+ , have been reported by Driess and Roesky groups . Very recently, the neutral aromatic silicon analogues of cyclobutane-1,3-diyls V were isolated by the Cui and Zhu groups, showing a dominant 2π-aromaticity in a σ-frame . In addition, a couple of four-membered systems containing the S 2 B 2 , Ge 4 Si, Si 4 , and B 2 P 2 skeletons with homoaromatic or diradical character were also reported by the groups of Sekiguchi, Lips, Wang, Scheschkewitz and Bertrand .…”

The Discovery of an Aromatic Silicon Aluminium Compound Through A Novel Four-Pronged Computational Approach

“…303132 Homogeneous Si4 rings seldom display 2π aromaticity. In 2023, Zhu and Cui reported the first neutral 2π aromatic planar rhombic Si4 ring, 33 emphasizing dominant π aromaticity in a saturated ring. Conversely, a 2013 study by So et al…”