[Sn@Cu12@Sn20]12–

A new paper by Thomas Fässler’s research group was published recently in JACS (Journal of the American Chemical Society). Fässler's group is one of the few others in the world that investigates Zintl ions, though as far as I know they only work with the group 14 Zintl ions, E₉^4– (E = Si, Ge, Sn, Pb), not group 15 Zintl ions. (I think they're missing out.) Some examples of Fässler's recent work with group 14 Zintl ions can be found here and here. This latest research doesn't involve E₉^4– at any point, but I still think it's pretty cool.

Fässler's group have synthesised a series of A₁₂Cu₁₂Sn₂₁ (A = Na, K, Rb, Cs) intermetallic phases, which consist of twelve A⁺ cations and a [Sn@Cu₁₂@Sn₂₀]^12– cluster anion. The first step in this process was the formation of a Cu-Sn alloy, which involved melting Cu wire and Sn granules together in an arc furnace. Stoichiometric amounts of the Cu-Sn alloy and the alkali metal were then heated in sealed niobium tubes for several days. (Incidentally this is a very similar process to the synthesis of the A₄E₉ and A₃E₇ Zintl phases - more on that in a later post.) 

[Sn@Cu₁₂@Sn₂₀]^12– consists of a single Sn atom surrounded by a Cu₁₂ icosahedron, which is itself surrounded by a Sn₂₀ pentagonal dodecahedron. Each Sn atom of the Sn₂₀ cage caps one of the faces of the Cu₁₂ icosahedron, and conversely each Sn₅ ring of the pentagonal dodecahedron is capped by a Cu atom. [Sn@Cu₁₂@Sn₂₀]^12– shows only small deviations from perfect icosahedral symmetry.

[Sn@Cu₁₂@Sn₂₀]^12–.

One of the main reasons this paper caught my eye is the similarity between [Sn@Cu₁₂@Sn₂₀]^12– and the [As@Ni₁₂@As₂₀]^3– species synthesised in 2003 by Bryan Eichhorn's research group by reacting the group 15 Zintl ion As₇^3– with Ni(COD)₂ (discussed briefly here). These two cluster anions are isoelectronic to each other and are structurally very similar, both having almost perfect icosahedral symmetry. But even if it wasn't related to my own area of Chemistry, I would still find this research fascinating simply because of its beauty. Chemistry really is amazing sometimes.