A Bronze Matryoshka: The Discrete Intermetalloid Cluster [Sn@Cu₁₂@Sn₂₀]^12– in the Ternary Phases A₁₂Cu₁₂Sn₂₁ (A = Na, K)

Abstract

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. Na12Cu12Sn21 and K12Cu12Sn21 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 A12Cu12Sn21 phases crystallize in the cubic space group Pn3̅m (No. 224), with the Cu–Sn clusters adopting a face centered cubic arrangement. A formal charge of 12– can be assigned to the {Sn@Cu12@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 K12Cu12Sn21, DFT band structure calculations (TB-LMTO-ASA) reveal a band gap. The discrete [Sn@Cu12@Sn20]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.