Electron affinities of Al_n clusters and multiple-fold aromaticity of the square Al₄^2- structure

The concept of aromaticity was first invented to account for the unusual stability of planar organic molecules with 4n + 2 delocalized π electrons. Recent photoelectron spectroscopy experiments on all-metal MAl₄^- systems with an approximate square planar Al₄^2- unit and an alkali metal led to the suggestion that Al₄^- is aromatic. The square Al₄^2- structure was recognized as the prototype of a new family of aromatic molecules. High-level ab initio calculations based on extrapolating CCSD(T)/aug-cc-pVxZ (x = D, T, and Q) to the complete basis set limit were used to calculate the first electron affinities of Al_n, n = 0-4. The calculated electron affinities, 0.41 eV (n = 0), 1.51 eV (n = 1), 1.89 eV (n = 3), and 2.18 eV (n = 4), are all in excellent agreement with available experimental data. On the basis of the high-level ab initio quantum chemical calculations, we can estimate the resonance energy and show that it is quite large, large enough to stabilize Al₄^2- with respect to Al₄. Analysis of the calculated results shows that the aromaticity of Al₄^2- is unusual and different from that of C₆H₆. Particularly, compared to the usual (1-fold) π aromaticity in C₆H₆, which may be represented by two Kekulé structures sharing a common σ bond framework, the square Al₄^2- structure has an unusual "multiple-fold" aromaticity determined by three independent delocalized (π and σ) bonding systems, each of which satisfies the 4n + 2 electron counting rule, leading to a total of 4 × 4 × 4 = 64 potential resonating Kekulé-like structures without a common σ frame. We also discuss the 2-fold aromaticity (π plus σ) of the Al₃^- anion, which can be represented by 3 × 3 = 9 potential resonating Kekulé-like structures, each with two localized chemical bonds. These results lead us to suggest a general approach (applicable to both organic and inorganic molecules) for examining delocalized chemical bonding. The possible electronic contribution to the aromaticity of a molecule should not be limited to only one particular delocalized bonding system satisfying a certain electron counting rule of aromaticity. More than one independent delocalized bonding system can simultaneously satisfy the electron counting rule of aromaticity, and therefore, a molecular structure could have multiple-fold aromaticity.