Tailoring Superatomic Stability with Transition Metals\nin Silicon Cages: Shrinking to M@Si<sub>15</sub> (M = Re, Os, Ir)

The design of materials\nwith intriguing electronic properties is\ncrucial for advancing nanoscale technologies, where precise control\nover atomic structure and electronic behavior is essential. Metal-encapsulating\nsilicon cage superatoms (SAs) provide a new paradigm for molecular-scale\nmaterial design, allowing fine-tuning of both structure and electronic\ncharacteristics. The formation of superatoms mimicking halogens, noble\ngases, and alkali metals has been well-studied, particularly with\nM@Si<sub>16</sub>, where early transition metals from groups 3 to\n5 stabilize within a Si<sub>16</sub> cage, achieving a 68-electron\nconfiguration. For late transition metals with excess electrons, a\nSi<sub>15</sub> cage offers enhanced stability by fulfilling the 68-electron\nrule with one fewer Si atom. This research synthesizes Si<sub>15</sub> cage-SAs with rhenium (Re) from group 7 and iridium (Ir) from group\n9 on <i>p</i>-type and <i>n</i>-type organic substrates.\nThe stability of Re@Si<sub>15</sub> and Ir@Si<sub>15</sub> is evaluated\nvia oxidative reactivity with X-ray photoelectron spectroscopy and\ntheoretical calculations, including osmium (Os) from group 8. Re@Si<sub>15</sub><sup>–</sup>, Os@Si<sub>15</sub><sup>0</sup>, and\nIr@Si<sub>15</sub><sup>+</sup> exhibit superatomic behaviors similar\nto halogens, noble gases, and alkali metals due to the 68-electron\nshell closure. Among them, Re@Si<sub>15</sub><sup>–</sup> on <i>p</i>-type organic substrates shows superior electronic and\ngeometric properties. These findings advance our understanding of\nM@Si<sub><i>n</i></sub> systems for transition metals, addressing\nlongstanding questions about their properties at <i>n</i> = 15 and 16.