Tailoring Superatomic Stability with Transition Metals in Silicon Cages: Shrinking to M@Si₁₅ (M = Re, Os, Ir)

The design of materials with intriguing electronic properties is crucial for advancing nanoscale technologies, where precise control over atomic structure and electronic behavior is essential. Metal-encapsulating silicon cage superatoms (SAs) provide a new paradigm for molecular-scale material design, allowing fine-tuning of both structure and electronic characteristics. The formation of superatoms mimicking halogens, noble gases, and alkali metals has been well-studied, particularly with M@Si₁₆, where early transition metals from groups 3 to 5 stabilize within a Si₁₆ cage, achieving a 68-electron configuration. For late transition metals with excess electrons, a Si₁₅ cage offers enhanced stability by fulfilling the 68-electron rule with one fewer Si atom. This research synthesizes Si₁₅ cage-SAs with rhenium (Re) from group 7 and iridium (Ir) from group 9 on p-type and n-type organic substrates. The stability of Re@Si₁₅ and Ir@Si₁₅ is evaluated via oxidative reactivity with X-ray photoelectron spectroscopy and theoretical calculations, including osmium (Os) from group 8. Re@Si₁₅^-, Os@Si₁₅⁰, and Ir@Si₁₅⁺ exhibit superatomic behaviors similar to halogens, noble gases, and alkali metals due to the 68-electron shell closure. Among them, Re@Si₁₅^- on p-type organic substrates shows superior electronic and geometric properties. These findings advance our understanding of M@Si systems for transition metals, addressing longstanding questions about their properties at n = 15 and 16.