AbstractMass spectrometry frequently reveals the existence of transient gas phase ions that have not been synthesized in solution or in bulk. These elusive ions are, therefore, often considered to be primarily of analytical value in fundamental gas phase studies. Here, we provide proof‐of‐concept that the products of ion‐molecule reactions in mass spectrometers may be collected on surfaces to generate condensed matter and thus serve as building blocks to synthesize new compounds. The highly reactive fragment anion [B₁₂Br₁₁]⁻ was generated in a mass spectrometer and converted to [B₁₂Br₁₁N₂]⁻ in the presence of molecular nitrogen followed by its mass‐selection and soft‐landing on surfaces. The molecular structure of [B₁₂Br₁₁N₂]⁻, which has not been synthetically obtained before, was confirmed by conventional methods of molecular analysis, including nuclear magnetic resonance and infrared spectroscopy. The [B₁₂Br₁₁N₂]⁻ ion is stable on surfaces and in solution at room temperature, but thermal annealing induces elimination of N₂ and provides access to the highly reactive intermediate [B₁₂Br₁₁]⁻ in the condensed phase, which can be further used as a reagent, for example, for electrophilic aromatic substitutions. Thus, isolation of [B₁₂Br₁₁N₂]⁻ expands the repertoire of the available diazo ions that can be employed as versatile intermediates in various chemical transformations.