Reports: AC5
45934-AC5 Elucidating Catalytic Cycles of Supported Molecular Metal Complexes
A set of supported ruthenium complexes with systematically varied ratios of chemisorbed to physisorbed species was formed by contacting cis-Ru(acac)2(C2H4)2 (I) (acac = C5H7O2-) with dealuminated zeolite Y. Extended X-ray absorption fine structure (EXAFS) spectra characterizing the samples confirmed the systematic variation of the loadings of the two supported species and demonstrated that removal of bidentate acac ligands from I accompanied the chemisorption to form Ru(acac)(C2H4)22+ at Al sites of the zeolite attached through two RuO bonds. A high degree of uniformity of the chemisorbed species was demonstrated by sharp bands in the infrared (IR) spectrum characteristic of ruthenium dicarbonyls formed when CO reacted with the anchored complex. When the ruthenium loading exceeded 1.0 wt% (Ru/Al ? 1/6), the adsorbed species were simply physisorbed. Ethene ligands on the chemisorbed species reacted to form butenes when the temperature was raised to approximately 393 K; acac ligands remained bonded to Ru. In contrast, ethene ligands on the physisorbed complex simply desorbed under the same conditions. The chemisorption activated the ruthenium complex and facilitated dimerization of the ethene, which occurred catalytically. IR and EXAFS data characterizing the supported samples indicate that (1) Ru centers in the chemisorbed species are more electropositive than those in the physisorbed species and (2) Ru-ethene bonds in the chemisorbed species are less symmetric than those in the physisorbed species, implying the presence of a configuration preferred for the catalytic dimerization.
Mononuclear complexes of rhodium and of ruthenium, Rh(acac)(?2-C2H4)2 and cis-Ru(acac)2(?2-C2H4)2 (acac = C5H7O2-), were used as precursors to synthesize metal complexes bonded to zeolite beta. IR and EXAFS spectra show that the species formed from Rh(acac)(?2-C2H4)2 was Rh(?2-C2H4)2+, which was bonded to the zeolite at aluminum sites via two RhO bonds. Reaction of this supported rhodium complex with CO gave the supported rhodium gem-dicarbonyl Rh(CO)2, which was characterized by two ?CO bands in the IR spectrum, at 2048 and 2115 cm-1, that were sharp (FWHM of 2115-cm-1 band = 5 cm-1), indicating a high degree of uniformity of the supported species. Nearly the same result was observed (Liang, A. et al. J. Am. Chem. Soc. 2009, 131, 8460) for the isostructural rhodium complex supported on dealuminated HY zeolite, which was characterized by frequencies of the ?CO bands that were 4 and 2 cm-1, respectively, greater than those characterizing the zeolite beta-supported complex. This comparison indicates that the Rh atoms in Rh(?2-C2H4)2+ anchored on zeolite beta were slightly more electron-rich than those on zeolite Y. This inference is supported by EXAFS results showing shorter Rh?C bonds in the zeolite beta-supported rhodium ethene complex than in the zeolite Y-supported rhodium ethene complex. In contrast to these supported rhodium complexes, the zeolite beta-supported ruthenium samples were shown by IR and EXAFS spectroscopies to consist of mixtures of mononuclear ruthenium complexes with various numbers of acac ligands; when CO reacted with the supported ruthenium complexes, the resultant ruthenium carbonyls were characterized by ?CO spectra characteristic of both ruthenium dicarbonyls and ruthenium tricarbonyls.
