Publication Details (including relevant citation information):
S. Kim, S. Wang, and H.F. Schaefer;
Journal of Physical Chemistry A, 2011, 115, 5478.
In light of the important recent synthesis of a stable tetrasilacyclobutadiene dianion compound by Sekiguchi and co-workers and the absence of theoretical studies, ab initio methods have been used to investigate this dianion and a number of related species. These theoretical methods predict multiple minima for each compound, and most minima contain folded and bicyclic silicon rings. For (Si4H4)2-, (Si4H4)2-·2Li+, [Si4(SiH3)4]2-·2Li+, [Si4(SiH3)4]2-·2Na+, and [Si4(SiH3)4]2-·2K+, respectively, the energetically lowest-lying structures are designated A-3 (C2v symmetry), B-8 (C1 symmetry), C-1 (C2 symmetry), D-1 (C2 symmetry), and E-1 (C2h symmetry). None of these structures satisfies both the ring planarity and the cyclic bond equalization criteria of aromaticity. However, all of the representative NICS values of these lowest-lying structures are negative, indicating some aromatic character. Especially, structures C-1 and D-1 of C2 symmetry effectively satisfy the criteria of aromaticity due to the slightly trapezoidal silicon rings, which are nearly planar with nearly equal bond lengths. SiH3 substitution for hydrogen in (Si4H4)2-·2Li+ significantly reduces the degree of aromaticity, as reflected in the substantially smaller NICS absolute values for [Si4(SiH3)4]2-·2Li+ than those of (Si4H4)2- and (Si4H4)2-·2Li+. The aromaticity is further weakened in [Si4(SiH3)4]2-·2Na+ and [Si4(SiH3)4]2-·2K+ by replacing lithium with the sodium and potassium cations.
Address (URL): http://dx.doi.org/10.1021/jp202671u