|Posted on May 6, 2016 at 3:25 PM|
Aromaticity is an ubiquitous term in chemistry referring to the cyclic electron delocalization that leads to energy stabilization, among other particular properties. Its antonym is antiaromaticity, which was coined by Breslow to refer to situations where "electronic delocalization is destabilizing". In the past years, the synthesis of new aromatic compounds of inorganic nature has shaken the traditional concept of aromaticity that has been extended to include new species such all-metal aromatic clusters. Nowadays, there is a number of new inorganic species commonly referred as aromatic, whereas there is very few antiaromatic molecules that are not organic.
The group of Prof. Wang and Prof. Boldyrev have worked in the synthesis and characterization of some well-known new aromatic species such as (Al4)2-, and they reported the existence of an all-metal antiaromatic molecule containing an antiaromatic (Al4)4- unit that could not be realized in the lattice. Now, together with Prof. Sun's group from the Changchun Institute of Applied Chemistry, they have synthesized an all-metal cluster, [Ln(Sb4)3]3– (Ln=La, Y, Ho, Er, or Lu), that contains three Sb4 antiaromatic units.
They invite me to collaborate in their research and help characterizing the aromaticity of the synthesized species using the indices developed in the group. The calculations were conclusive for [La(Sb4)3]3–: the compound can be formally drawn as La3+ and three (Sb4)2- units, in accord with simple electron count rules. However, the electronic arrangement in the (Sb4)2- unit within the [La(η4-Sb4)3]3- cluster is very different from the isolated (Sb4)2- molecule, which is aromatic. The delocalization index between the (Sb4)2- unit and La in the cluster is 1.2, suggesting a strong η4-interaction between both that probably hinders the internal ring delocalization that existed in the aromatic free (Sb4)2- moiety and prompted its aromatic character. Conversely, the multicenter indices values suggest that the electronic structure of the Sb4 unit within [Ln(Sb4)3]3– is similar to cyclobutadiene, the prototypical antiaromatic molecule.
Our work has been published this week in Angewandte Chemie and has been commented in Chemical & Engineering News (here in Spanish).