Energy Dispersive Extended X-ray Absorption Fine Structure, Mass Spectrometric, and Diffuse Reflectance Infrared Studies of the Interaction of Al₂O₃-Supported Rh^I(CO)₂Cl Species with NO and Re-formation under CO

Abstract

The interaction between supported Rh I (CO) 2 Cl species, prepared by metallo-organic chemical vapor deposition (MOCVD) of [Rh(CO) 2 Cl] 2 to hydroxylated γ-Al 2 O 3, and NO has been investigated using time-resolved, energy dispersive extended X-ray absorption fine structure (EDE)/mass spectrometry (MS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). MOCVD of [Rh I (CO) 2 Cl] 2 leads to the formation of a Rh I (CO) 2 Cl{O−Al} adlayer which, when fresh, reacts with NO to form a majority {Al−O} 2 RhCl(NO - ) species at room temperature via a two-step mechanism involving an {Al−O}Rh(NO) 2 Cl species. The application of DRIFTS allows a direct association of the bent RhNO bonding in the {Al−O} 2 RhCl(NO) - with “high-wavenumber” Rh(NO - ) species displaying ν(NO) at ca. 1750 cm -1 often observed in supported Rh systems. DRIFTS investigations on analogous Rh I (CO) 2 Cl/TiO 2 systems show the same reactivity toward NO, with a bent nitrosyl being formed rather than the more commonly dominant linear Rh(NO + ) species. DRIFTS also indicates that a second reaction is possible. This becomes increasingly significant for Rh(CO) 2 Cl{O−Al} samples exposed to air for ca. 2−3 days and results in the {Al−O}Rh I (CO) 2 Cl species reacting with NO to form a new species displaying adsorptions at 2150−2110 and 1750−1700 cm -1 . Once formed, this latter species reacts no further at room temperature under NO. The DRIFTS spectrum of this species is interpreted as being due to {Al−O}Rh(CO)(NO)Cl species existing in cis and trans configurations: the isomer with the carbonyl group trans to the Cl ligand being the preferred form at room temperature. The reconversion of the Rh(NO - ) species under CO shows complex temperature dependence. The consumption of the Rh(NO - ) shows only a weak temperature dependence in terms of EDE, but the observed evolution of NO g shows a strong temperature dependence. The combination of EDE and MS indicates rapid formation of an intermediate species, most likely {Al−O}Rh(CO)(NO)Cl, which at room temperature converts to the geminal dicarbonyl species slowly. The possible origins of this behavior, and the parameters determining the formation of “linear” and/or “bent” rhodium nitrosyls in support Rh systems are discussed.