Surface organometallic chemistry on metals. Evidence for a surface-stabilized germylene, Rh_s[Ge(Ad)(H)]_y/SiO₂ (Ad = adamantyl) obtained by a controlled reaction between Rh/SiO₂ and (Ad)GeH₃

Selective reaction of (Ad)GeH₃ (Ad = adamantyl) with a Rh/SiOi₂ surface has been carried out at 50°C. The surface reaction and the characterization of the surface organogermanium complex have been followed by infrared spectroscopy, surface microanalysis, analysis of the gases evolved during the surface reaction, thermal decomposition of the surface organometallic complex, and use of CO as a molecular probe. In the absence of metallic rhodium, (Ad)GeH₃ does not react significantly with the silica surface at room temperature. It is only reversibly adsorbed, and the molecular interaction responsible for this adsorption process is a hydrogen-bonding interaction between either the C-H or Ge-H atoms and the surface silanols. In the presence of metallic Rh, a chemical reaction occurs exclusively on the metallic particles. (Ad)GeH₃ initially physisorbed onto the support migrates to the Rh surface between 25 and 50°C, where it quickly loses one molecule of hydrogen. The grafted species still contains one hydride ligand since the reaction between (Ad)GeD₃ and the rhodium catalyst gives rise to a ν(Ge-D) vibration at the expected frequency. Formulation of the grafted entity as Ge(Ad)(H) (major species) is deduced from surface microanalysis and from its thermal decomposition, which produces adamantane by a reductive elimination process (concomitant disappearance of the ν(C-H) and ν(Ge-H) vibration bands). The organogermane complex is very likely grafted onto rhodium for the following reasons: (i) (Ad)GeH₃ does not react with the silanols of the support. (ii) The amount of grafted germanium is close to (but lower than) the number of surface rhodium atoms. (iii) The amount of rhodium accessible to carbon monoxide drops by a factor of 80% after the grafting reaction takes place, indicating that the metallic surface has been covered by the Ge(Ad)(H) fragment, results that are in agreement with the elemental analysis (Ge/Rh_s = 0.8). (iv) The infrared results indicate a strong electronic interaction between carbon monoxide adsorbed on the remaining rhodium sites and the Ge-H bond. The major species which is present on the surface after grafting is supposed to be a kind of germylene(II) surface species stabilized by coordination to a surface rhodium atom.