Reactions of Highly Uniform Zeolite H-β-Supported Rhodium Complexes: Transient Characterization by Infrared and X-ray Absorption Spectroscopies

A zeolite H-β-supported mononuclear rhodium diethene complex (Rh(C₂H₄)₂{O₂Al}, where the braces indicate a part of the zeolite) was formed by the reaction of Rh(acac)(η²-C₂H₄)₂ (acac = acetylacetonate, C₅H₇O₂⁻) with the zeolite. Transient characterization of the sample by X-ray absorption near edge structure (XANES) and infrared (IR) spectroscopies (combined with mass spectrometry of the effluent gas) while the sample was in contact with flowing CO indicates a simple stoichiometric conversion of the supported metal complex into another species, identified by the spectra as the zeolite-supported rhodium gem-dicarbonyl (Rh(CO)₂{O₂Al}). The sharpness of the ν_CO bands in the IR spectrum indicates a high degree of uniformity of the supported rhodium gem-dicarbonyl, and isosbestic points in the XANES spectra as the transformation was occurring imply that the rhodium diethene complex was also highly uniform. Spectra similarly show that treatment of the supported rhodium gem-dicarbonyl with flowing C₂H₄ resulted in another stoichiometrically simple transformation, giving a species suggested to be Rh(C₂H₄)(CO)₂{O₂Al}. The intermediate was ultimately transformed when the sample was purged with helium into another highly uniform supported species, inferred on the basis of IR spectra to be Rh(C₂H₄)(CO){O₂Al}. Extended X-ray absorption fine structure spectra characterizing the supported rhodium diethene complex and the species formed from it show how the Rh−O bond distance at the Rh-support interface varied in response to the changes in the ligands bonded to the rhodium.