Lewis Base/Acid Coordination Stabilizes Planar 6π Aromatic Si₆/P₆

Abstract

We have developed a strategy for designing planar all-Si and all-P 6π aromatics based on experimentally available building blocks. A Si6 hexagon coordinated by three phosphines at the 1,3,5-positions, Si6(PH3)3, which has three lone-pair-carrying Si atoms is nearly planar and aromatic. Replacing PH3 by NH3 induces perfect planarity. Two-electron reduction of Si6(PH3)3 results in doubly aromatic Si6 2– with the dissociation of Si–P bonds, instead of a nonaromatic system such as the benzene dianion. Similarly to base-coordinated Si6, Lewis acid coordination to P6 can give planar 6π aromatics. P6(BH3)3 is a nearly planar minimum. Replacing BH3 with a stronger Lewis acid, B­(CN)3, gives a perfectly planar structure. A PH3-coordinated all-Si diene resembling butadiene is predicted. A concerted 4 + 2 cycloaddition between this all-Si diene and ethylene is proposed here and has a computed barrier of only 22 kcal/mol.

Bond-alternated and bond-equalized hexazine derivatives

“…Phosphorus analogues of 5 and 7 were reported in an earlier work; the P analogue of 5 was bond-equalized, but that of 7 was bond-alternated, though by a smaller magnitude than 7 . 34 The fact that the P analogues of 5 and 7 have much less bond-alternation than 5 and 7 is in agreement with the theory that the strong π bonds show less tendency to delocalize. 7…”

“…A similar approach was used earlier to stabilize planar all-Si and all-P 6p systems. 34 Coordinating both the N to LA is not done here to avoid the build-up of a large positive charge on N atoms, which will be more severe in N 6 (LA) 6 than in the building block N 2 H 2 (LA) 2 , as in the latter the H gives some electron density to N.…”

Aromatic nature of neutral and dianionic 1,4-diaza-2,3,5,6-tetraborinine derivatives

“…These data indicate that even with the same B 4 N 2 -skeletal framework of 6psystem, the index value of each criterion varies depending on the overall charge of the B 4 N 2 systems, 32,33,36 giving rise to the different aromatic nature. 71,72 This study paves a way for the further design and development of aromatic inorganic benzenes.…”