The covalent attachment of tris(allyl)iridium to partially dehydroxylated γ-alumina is found to proceed via surface hdroxyl group protonation of one allyl ligand to form an immobilized bis(allyl)iridium moiety, (=AIO)lr(allyl)2, as characterized by CP-MAS 13C NMR, inductively coupled plasma-mass spectrometry, and lr L3 edge X-ray absorption spectroscopy. Extended X-ray absorption fine-structure (EXAFS) measurements taken on unsupported lr(allyl)3 and several associated tertiary phosphine addition complexes suggest that the η3-allyl ligands generally account for an Ir-C coordination number of 2 rather than 3, with an average Ir-C distance of 2.16 Å. Using this knowledge, combined EXAFS and X-ray absorption near-edge structure studies reveal that a small amount of Iro is also formed upon reaction of lr(allyl)3 with the surface. It was found that the addition of either 2,6-dimethylphenyl isocyanide or carbon monoxide to the supported complex allows spectroscopic identification of the supported bis(allyl)iridium complexes, (=AIO)lr(allyl)2(CNAr) [Ar=2,6-(CH3)2C6H4] and (=AIO)lr(allyl)2(CO)2, respectively. Although samples of the supported bis(allyl)iridium complex are active for the dehydrogenation of cyclohexane to benzene at temperatures between 180 and 220 °C, in situ temperature-programmed reaction XAFS and continuous-flow reactor studies suggest that Iro nanoparticles, rather than a well-defined lr3+ complex, are responsible for the observed activity.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry