Group 13 five- and six-membered rings with rare 2π aromaticity

Area of Science:

• Inorganic Chemistry
• Theoretical Chemistry
• Materials Science

Background:

• Hückel's rule defines aromaticity based on 4n+2 π electrons in cyclic systems.
• Large 2π-aromatic rings (n=0) are rare due to structural instability.
• The largest previously known 2π-aromatic ring was a five-membered Gallium core.

Purpose of the Study:

• To computationally predict novel inverse-sandwich and half-sandwich metalloaromatic structures.
• To investigate the aromaticity and bonding characteristics of these predicted structures.
• To expand the known size of 2π-aromatic rings.

Main Methods:

• Density functional theory (DFT) calculations were employed.
• Analysis of electronic structure, binding energies, and HOMO-LUMO gaps.
• Evaluation of ring currents and electron delocalization.

Main Results:

• Seven inverse-sandwich Na2(MH)5 and half-sandwich Ca(MH)5/Ca(MH)6 structures were predicted (M=Al, Ga, In).
• These structures feature planar five- or six-membered Al/Ga/In rings with significant aromatic character.
• Dianionic metal rings exhibit 2π aromaticity due to electron donation from Na/Ca metals, showing diatropic ring currents.
• Novel collective bonding between metal atoms and surrounding hydrogen atoms was observed.

Conclusions:

• The predicted metalloaromatic rings significantly expand the family of 2π aromatics.
• The maximum ring atom number for 2π aromatics was increased from five to six.
• These findings open new avenues for the design and synthesis of large Hückel-type 2π-aromatic rings.