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

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 Si62– 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.