Aromaticity is a concept invented to account for the unusual stability for an important class of organic molecules, the aromatic compounds. It results in exceptional geometric, energetic and magnetic properties. The name "aromatic compounds" was initially bestowed on benzene, its derivatives, and related compounds because of their aroma. Today, the terms "aromatic" and "aromaticity" are used to describe cyclic, planar, and conjugated molecules possessing (4n + 2) pi-electrons and having specific chemical and structural stability. Despite the undeniable usefulness of the aromaticity concept, it remains controversial and its physical origin is still being debated. We have recently reported experimental and theoretical evidence of aromaticity in an all-metal system, the Al42- dianion in a series of bimetallic and ionic clusters MAl4- (M = Li, Na, or Cu) . The Al42- dianion, both as an isolated species and in the bimetallic molecules (MAl4-), was found to be square-planar and possess two delocalized pi-electrons, thus conforming to the structural criterion and the (4n + 2) electron-counting rule for aromaticity. The delocalization of the two pi-electrons was found to be critical for the planar structure and aromaticity of Al42- (Fig. 1).
Also, the possibility of aromaticity in a series of hetero-systems, XAl3- (X = Si, Ge, Sn, and Pb), which are isoelectronic to Al42-, has been explored . We found that all the XAl3- species have two lowest singlet isomers: a four-membered hetero-cyclic structure (C2v) and a pyramidal structure (C3v). The hetero-cyclic structure was found to be aromatic with two delocalized pi-electrons, analogous to Al42- (Fig. 2). The substitutions of one Al in Al42- by Si, Ge, Sn, and Pb allow us to investigate systematically how the delocalized pi-orbital, thus aromaticity, affects the relative stabilities and properties of the hetero-cyclic and pyramidal structures. In previous study of the lighter CAl3- species, it was found the pyramidal isomer to be the only minimum . The instability of the CAl3- planar cyclic isomer is due to the complete localization of the two pi-electrons on the C atom because of its much higher electronegativity relative to that of Al.
We also investigated the electronic structure and chemical bonding of the heavier congeners of Al42-: Ga42- and In42- in bimetallic clusters NaGa4- and NaIn4- . We found that both NaGa4- and NaIn4- possess a C4v pyramidal structure with a Na+ cation interacting with a square planar dianion, analogous to the MAl4- species. Both Ga42- and In42- dianions are shown to be aromatic, with two pi-electrons. Carrying out calculations further on a model compound, K2Ga4(C6H5)2, for a recently synthesized Ga4-organometallic compound, K2[Ga4(C6H3-2,6-Trip2)2] (Trip = C6H2-2,4,6-iPr3) , we showed that the synthetic compound contains an aromatic -Ga42--unit (with two pi-electrons) interacting with two K+ ions, analogous to our bare Ga42- species in NaGa4-, despite the additional coordination in the synthetic compound.
Professor Corbett attracted our attention to the class of amalgams typified by Na3Hg2, which contains Hg46- square units as their building blocks (Fig. 3). We have found that the Hg46- unit is isoelectronic with the Al42- dianion, suggesting that the former is aromatic as well, thus explaining its unique stability and structure . The current finding of aromaticity in the Hg46- establishes a solid bridge between our gas-phase studies of aromatic clusters and bulk materials containing such species.
We also investigated the electronic structure and chemical bonding of the B3-, Al3-, Ga3- anions and the gas phase NaB3, NaAl3 and NaGa3 molecules . We found that the ground state of the neutral gas phase salts contains a planar triangular anion interacting with a Na+ cation. The B3-, Al3-, and Ga3- anions possess two delocalized pi-electrons and are aromatic (Fig. 4). These triangular anions have been shown to be related to recently synthesized organometallic compound containing an aromatic -Ga32--unit [8, 9], but they differ from them by 4 valence electrons.
All-metal molecules (Al42-, XAl3-, Ga42-, In42-, Hg46-, Al3-, Ga3-) are electron-deficient species compare to the corresponding aromatic hydrocarbons. The electron deficiency results in an interesting new feature in all-metal aromatic systems. They should be considered as having both pi- and sigma-aromaticity and that should result in their additional stability. We obtained crude evaluations of the resonance energy for new Na2Al4 and Na2Ga4 all-metal aromatic molecules . The resonance energies were found to be unusually high: 30 kcal/mol (B3LYP/6-311+G*) and 48 kcal/mol (CCSD(T)/6-311+G(2df)) for Na2Al4 and 21 kcal/mol (B3LYP/6-311+G*) for Na2Ga4 compare to 20 kcal/mol in benzene. Such high resonance energies can be explained by the presence of three completely delocalized bonds in all-metal species: one pi-bond and two sigma-bonds (Fig. 5).
These findings may not only expand the aromaticity concept into all-metal systems but may also indicate whole classes of new inorganic aromatic species. Some of them may eventually serve as the basis for novel materials with as-yet unimagined properties.