A bronze matryoshka: The discrete intermetalloid cluster [Sn@Cu ₁₂@Sn₂₀]^12- in the ternary phases A ₁₂Cu₁₂Sn₂₁ (A = Na, K)

The synthesis and crystal structure of the first ternary A-Cu-Sn intermetallic phases for the heavier alkali metals A = Na to Cs is reported. The title compounds A₁₂Cu₁₂Sn₂₁ show discrete 33-atom intermetalloid Cu-Sn clusters {Sn@Cu₁₂@Sn₂₀}, which are composed of {Sn₂₀} pentagonal dodecahedra surrounding {Cu₁₂} icosahedra with single Sn atoms at the center. Na ₁₂Cu₁₂Sn₂₁ and K₁₂Cu ₁₂Sn₂₁ were characterized by single-crystal XRD studies, and the successful synthesis of analogous A-Cu-Sn compounds with A = Rb and Cs is deduced from powder XRD data. The isotypic A₁₂Cu ₁₂Sn₂₁ phases crystallize in the cubic space group Pn3̄m (No. 224), with the Cu-Sn clusters adopting a face centered cubic arrangement. A formal charge of 12- can be assigned to the {Sn@Cu ₁₂@Sn₂₀} cluster unit, and the interpretation of the title compounds as salt-like intermetallic phases featuring discrete anionic intermetalloid [Sn@Cu₁₂@Sn₂₀]^12- clusters separated by alkali metal cations is supported by electronic structure calculations. For both Na₁₂Cu₁₂Sn₂₁ and K ₁₂Cu₁₂Sn₂₁, DFT band structure calculations (TB-LMTO-ASA) reveal a band gap. The discrete [Sn@Cu₁₂@Sn ₂₀]^12- cluster is analyzed in consideration of the molecular orbitals obtained from hybrid DFT calculations (Gaussian 09) for the cluster anion. The [Sn@Cu₁₂@Sn₂₀]^12- cluster MOs can be classified with labels indicating the numbers of radial and angular nodes, in the style of spherical shell models of cluster bonding.