Open shells open doors

The electronic structure of a single atom can often be mimicked by atom-precise metal clusters of an entirely different element. Akin to having a noble gas electronic configuration, these clusters — referred to as superatoms — usually have 2, 8, 18, 20, 34, 40, 58 or 68 (and so on) valence electrons that correspond to completely filled 1S, 1P, 1D, 2S, 1F, 2P, 1G and 2D ‘jellium’ shells. Preparing large superatoms with partially filled jellium orbitals has proven difficult — but not too difficult for Roland A. Fischer and colleagues, who describe, in Angewante Chemie International Edition, the characterization of [Cu₄₃Al₁₂](C₅Me₅)₁₂, a 67 valence-electron superatom complex.

Credit: David Schilter/Springer Nature Limited

It has long been known that bulk intermetallics can mimic the behaviour of another metal element. In much the same way that Al₁₃Fe₄ can mimic the precious metal catalyst Pd, we can think of the superatom Al₁₃ (just 1e⁻ shy of 40 jellium electrons) as an analogue of a Cl atom (1e⁻ shy of an 18e⁻ [Ar] configuration). Superatoms can also bear organic ligands, and prominent among such superatom complexes is [Al₅₀](C₅Me₅)₁₂ — a 138 electron complex with an [Al⁰₃₈] core stabilized by a dozen AlⁱC₅Me₅ carbene-like 2e⁻ donors. The team guessed that these metalloligands might also be able to stabilize cores composed of different metals as part of heterometallic superatom complexes. Of their efforts to make these, Fischer recalls “one motivation was to better understand structure–bonding–property relationships in the regime between metal-rich complexes and ligated mixed-metal nanoparticles.”

Fischer’s team combined [Cuⁱ(mesityl)]₅ and [AlⁱC₅Me₅]₄ to give [Cu⁰₄₃Alⁱ₁₂](C₅Me₅)₁₂ as highly air-sensitive crystals. The X-ray structure features metal atoms in a two-shell icosahedral structure with a central Cu residing inside a {Cu₁₂} cage, which itself is in the middle of a {Cu₄₀Al₁₂} cage. In terms of metal atoms, it turns out that 55 is an auspicious ‘magic’ number, shared also by the almost isostructural complex [Pd⁰₅₅(PⁱPr₃)₁₂(CO)₂₀]. However, in terms of electrons, it is at first unclear how we should look at [Cu⁰₄₃Alⁱ₁₂](C₅Me₅)₁₂. If we formulate it as a [Cu⁰₄₃] core with 12 AlⁱC₅Me₅ ligands, each Cu⁰ would contribute 1e⁻ to a jellium electron count of 43 — quite removed from a ‘stable’ count of 40. Accordingly, Fischer teamed up with the group of Jean-Yves Saillard, whose spin-polarized density functional theory calculations revealed that all 55 metals in the simplified model [Cu⁰₄₃Alⁱ₁₂](C₅H₅)₁₂ are best considered as a superatom with 67 jellium electrons. We can presume that [Cu⁰₄₃Alⁱ₁₂](C₅Me₅)₁₂ has the same count, and its 3 unpaired electrons (S = 3/2) cause it to exhibit temperature-independent paramagnetism. Such open-shell organometallics are often kinetically unstable, but in the present case the metal core benefits from the steric protection afforded by its organic exterior. Indeed, this work teaches us that stable superatoms can assume a variety of electronic structures, including those with partially filled jellium orbitals — the beginnings of a conduction band.

fascinating discoveries and surprises are out there in the ‘darkness’ of our reaction cocktails

Fischer and colleagues are now targeting other 55 atom clusters like [Cu_43−xAl_12+x](C₅Me₅)₁₂, which should be feasible given that elemental Cu and Al have similar (fcc) structures. We could then access complexes with a wide variety of jellium electron counts. Applications in catalysis, however, would likely require larger and less-passivated clusters with surface hydrido ligands and sites for substrate binding. Towards these goals, Fischer notes “the unique open-shell superatom properties of [Cu⁰₄₃Alⁱ₁₂](C₅Me₅)₁₂ suggest that fascinating discoveries and surprises are out there in the ‘darkness’ of our reaction cocktails.”