φ-Aromaticity in prismatic {Bi6}-based clusters

Molecules based on cages of metal atoms ('metal clusters') exhibit properties that are usually very different from those known for organic molecules – although analogies are possible. The present case illustrates how a prism of metal atoms helps realizing a new type of aromaticity – φ-aromaticity.
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Metal clusters are molecular inorganic compounds that can range in sizes of a few atoms to even hundreds of atoms within nanoscale sizes. These fascinating compounds help unravel solutions to further understanding the nature of chemical interactions between metal atoms on a molecular scale, and electronic situations of purely metal-metal bonds. Additionally, metallic compounds offer great potential in fields such as catalysis, as soluble precursors for structurally defined nanomaterials, or even as active compounds themselves. Here, we report about cluster compounds that show a new type of aromaticity. Aromaticity in general is a property well known for organic molecules, and in the recent past also discussed for all-metal (cluster) molecules. However, in the present case, we have found the first isolatable compound that exhibits φ-type aromaticity, which actually requires the presence of a metal cluster.

Both ligand-decorated and naked clusters are synthetically accessible, and, while their synthesis has long been thought of as “black box” of chemistry, with growing knowledge in this area, it is becoming much easier to design synthetic protocols for such compounds and even getting to a stage where product prediction is possible. With the expansion of these all-metal containing molecules, their unique properties are becoming more apparent. In particular with respect to the inherent mobility of electrons that is common knowledge as a concept for bulk metals. When considering molecules, however, the movement of electrons through the bonding network can create ring currents synonymous with those found in organic molecules, i.e. aromaticity. Yet, it is clear that metals behave similarly and also differently, partly due to their structure ultimately yielding a variety of polyhedral and other 3D-geometries. The fundamental concept of aromaticity, which has a dramatic influence on the chemical and physical properties of molecules, is now an identifiable feature of inorganic molecules and its effect is only partially understood and adds to the diverse range of properties of all-metal molecules.      

To this end, metallic clusters offer great versatility. However, it is important that synthesis heads in a more sustainable and environmentally friendly manner. Meaning, as chemists it is our duty to design methods that rely on low-toxicity or benign metals as well as optimise yields and reproducibility. This work highlights both factors as we report a straightforward, specifically designed route to the isolation of appreciable yields of two ligand-decorated bimetallic clusters of ruthenium and bismuth (Ru/Bi) or iridium and bismuth (Ir/Bi), and investigate both with respect to their unique chemical properties.

We found that noble metal complexes [(cod)IrCl]2 and [CpRu(MeCN)3]+ (cod = 1,5-cyclooctadiene; Cp = cyclopentadienide) react with a molecular source of bismuth atoms, Bi22–, in the solvent ethane-1,2-diamine (en) to form the cluster molecules [{(cod)Ir}3Bi6] and [{CpRu}3Bi6] in good yields and clean reactions, respectively. All by-products can be easily washed away with acetonitrile, CH3CN, affording semi-crystalline salts of the two products. The cluster anions in these compounds both have a general nine-vertex closo-deltahedral structure, similar to a previously reported species [{(CO)3Mo}3Bi6]4–, with a trigonal prismatic {Bi6} fragment and transition metal complex fragments {MLn} (M = Ru, Ir, Mo; Ln = ligand sphere) with 12 valance electrons capping the rectangular faces of the prism.

The exact formation pathway for such a structure remains unknown, however we believe the complicated reaction cascade involving the oxidative coupling of {Bi2} fragments is facilitated by the relatively redox-inert transition metal fragments used in this case. It is becoming increasingly clear that transition metal complex fragments with this electron count (and containing metals with d-electron counts between 6-8 electrons) consistently form this {Bi6} moiety on reaction with molecular sources of bismuth atoms.

These compounds offer excellent insight into the nature of metal-metal bonding and how it can influence the structure and overall electronic situation of the molecule. On the basis of a simple view, each of the three complexes mentioned above contain “Bi64−” fragments to give the corresponding anionic charge of the cluster with three of the appropriate transition metal units, {CpRu}+, {(cod)Ir}+, or {Mo(CO)3} respectively. However, this neglects the effect of the nature of the transition metal atom (and its ligand) on the cluster bonding and on the structure of the {Bi6} prism. The regularity of the trigonal prism is distinctly different between the [{CpRu}3Bi6] anion and the [{(cod)Ir}3Bi6] and [{(CO)3Mo}3Bi6]4− anions. The prism in [{CpRu}3Bi6] is significantly more regular, while in the other two clusters it is distorted – which in turn significantly influences the electronic properties.

To provide insight into this distortion, the hypothetical substructure Bi6q− (q = 0-4) series was calculated. This revealed that when q = 0-2 the substructure was closer to being regular and much more distorted when of a higher negative charge. The regularity of the prism has a profound effect on the symmetry of the molecular orbitals (MO) of the molecule. The highest occupied molecular orbital (HOMO) of [{CpRu}3Bi6] consists of coexisting π-type contributions from atomic p-orbitals in both triangles of the {Bi6} prism. Together, these yield one, single, doubly occupied cluster orbital which exhibits the same mathematical symmetry as an atomic fz3 orbital. A very similar situation is observed in the hypothetical Bi62− molecule. This MO cannot be fully localised and sustains a remarkably strong diatropic ring current of +25.6 nA/T when calculating the structures as being exposed to an external magnetic field. All three clusters exhibit strong diatropic ring currents, however, only [{CpRu}3Bi6] has the highly symmetric fz3-type cluster orbital and therefore fulfills the magnetic and structural requirement for aromaticity, which we therefore assign as φ-type aromaticity – in analogy to p-type aromaticity in case of MO symmetries resembling atomic pz orbitals.

The image illustrates the molecular structure and uncommon electronic properties of the new cluster anion [{CpRu}3Bi6]– exhibiting φ-type aromaticity. The changes in the structure of the {Bi6} fragment, and subsequent electronics, seem to be tunable by the nature of the transition metal complex fragment. This all-metal aromaticity based on the unique φ-symmetric electronic structure has never been observed on experimentally accessible clusters before.

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