A Bronze Matryoshka: The Discrete Intermetalloid Cluster [Sn@Cu₁₂@Sn₂₀]^12– in the Ternary Phases A₁₂Cu₁₂Sn₂₁ (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. 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.本文报道了针对重碱金属A（A=Na至Cs）的首例三元A–Cu–Sn金属间相的合成与晶体结构。标题化合物A₁₂Cu₁₂Sn₂₁呈现离散的33原子金属间Cu-Sn团簇{Sn@Cu₁₂@Sn₂₀}，该团簇由中心搭载单个锡原子的{Cu₁₂}二十面体被{Sn₂₀}五角十二面体包裹而成。Na₁₂Cu₁₂Sn₂₁与K₁₂Cu₁₂Sn₂₁已通过单晶X射线衍射（single-crystal XRD）完成表征，而A=Rb和Cs的同类A–Cu–Sn化合物的成功合成则可由粉末X射线衍射（powder XRD）数据推导得出。该类同晶型A₁₂Cu₁₂Sn₂₁相结晶于立方空间群Pnoverline{3}m（编号224），其中Cu–Sn团簇采取面心立方排布。可将{Sn@Cu₁₂@Sn₂₀}团簇单元的形式电荷赋值为-12，且将标题化合物诠释为类盐金属间相：其特征为离散的阴离子金属间[Sn@Cu₁₂@Sn₂₀]¹²⁻团簇被碱金属阳离子分隔，这一阐释得到了电子结构计算的支持。针对Na₁₂Cu₁₂Sn₂₁与K₁₂Cu₁₂Sn₂₁的紧束缚线性 muffin-tin 轨道原子球近似（TB-LMTO-ASA）密度泛函理论（DFT）能带结构计算均显示存在能带隙。对离散[Sn@Cu₁₂@Sn₂₀]¹²⁻团簇的分析，结合了采用Gaussian 09软件通过杂化密度泛函计算得到的该团簇阴离子的分子轨道。[Sn@Cu₁₂@Sn₂₀]¹²⁻团簇的分子轨道可按照团簇成键的球壳模型范式，以标示径向与角向节点数的标签进行分类。