Infrared spectroscopic study of solvation and size effects on reactions between water molecules and neutral...

Elucidating the solvation and size effects on the reactions between water and neutral metals is crucial for understanding the microscopic mechanism of the catalytic processes but has been proven to be a challenging experimental target due to the difficulty in size selection. Here, MO₄H₆ and M₂O₆H₇ (M = Sc, Y, La) complexes were synthesized using a laser-vaporization cluster source and characterized by size-specific infrared-vacuum ultraviolet spectroscopy combined with quantum chemical calculations. The MO₄H₆ and M₂O₆H₇ complexes were found to have H˙M(OH)₃(H₂O) and M₂(μ₂-OH)₂(η¹-OH)₃(η¹-OH₂) structures, respectively. A combination of experiments and theory revealed that the formation of H˙M(OH)₃(H₂O) and M₂(μ₂-OH)₂(η¹-OH)₃(η¹-OH₂) is both thermodynamically exothermic and kinetically facile in the gas phase. The results indicated that upon the addition of water to H˙M(OH)₃, the feature of the hydrogen radical is retained. In the processes from mononuclear H˙M(OH)₃ to binuclear M₂(μ₂-OH)₂(η¹-OH)₃(η¹-OH₂), the active hydrogen atom undergoes the evolution from hydrogen radical → bridging hydrogen → metal hydride → hydrogen bond, which is indicative of a reduced reactivity. The present system serves as a model for clarifying the solvation and size effects on the reactions between water and neutral rare-earth metals and offers a general paradigm for systematic studies on a broad class of the reactions between small molecules and metals at the nanoscale.