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

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.