A Family of Rhenium Subcarbonyls on MgO: Structural Characterization by Extended X-ray Absorption Fine Structure Spectroscopy

Abstract

Stable rhenium tricarbonyls bonded to the surface of MgO powder, prepared from a variety of precursors including [Rez(CO)lo], [DRe(C0)61, and [Re4(C0)12(OH)41, have been characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. The conversion of the initially adsorbed species into the rhenium subcarbonyl has been carried out by a variety of routes at relatively mild temperatures, including treatment in H2, He, 0 2 , or under vacuum. The EXAFS and infrared data indicate a C3u symmetry of the surface-bound rhenium subcarbonyls. Each Re atom is bonded to three surface oxygen atoms, and the complex is formulated as [Re(CO)3{ OMg},{HOMg}~-,I (where the braces denote groups terminating the MgO bulk). The surface species are precise analogues of molecular rhenium carbonyls, as indicated by both the coordination numbers and bonding distances for all the members of the family. The indications of this symmetry demonstrate that the rhenium subcarbonyls on the surface of MgO powder are bound to faces other than the (100) face and/or at surface defects. These rhenium subcarbonyls are among the best characterized examples of adsorbates on metal oxide powders.

A rhenium carbonyl bonded to highly dealuminated zeolite Y: structure determination by infrared and X-ray absorption spectroscopies

“…[40][41][42] A rhenium carbonyl bonded to three equivalent surface sites has C 3v symmetry, resulting in a spectrum with two main n CO bands. [11][12][13][15][16][17]43,44 The literature of oxide-supported rhenium carbonyls typically indicates such bands; the surface sites are typically inferred to be O 2À or OH À . [11][12][13][14][15][16][17] A shoulder on the lower-frequency absorption band represents some of the samples and is attributed to a rhenium carbonyl coordinated to surface sites that are not all equivalent to each other.…”

“…We chose the dealuminated zeolite to provide well-separated surface sites associated with isolated aluminum ions, [7][8][9][10] and we chose HRe(CO) 5 as the precursor because it reacts readily with oxide surfaces to form rhenium carbonyls. [11][12][13][14][15][16][17] Infrared (IR) and X-ray absorption spectra determine the symmetry, structure, and bonding of the rhenium complex to sites near aluminum centers in the zeolite. The structure is sufficiently defined by the spectra to determine a basis for inferring the likely crystallographic location of the rhenium.…”

Near Absence of Support Effects in Toluene Hydrogenation Catalyzed by MgO-Supported Iridium Clusters

“…The MgO 500 -supported catalyst, after use, was characterized by an Ir-Ir coordination number of 3.0, indicating that the tetrahedral framework of the precursor [Ir 4 (CO) 12 ] and the fresh decarbonylated catalyst remained essentially intact during the catalysis experiment. The used MgO 700 -supported catalyst is characterized by a first-shell Ir-Ir coordination number of 2.6 (Table 3).…”

“…For example, using surface organometallic synthesis and extended X-ray absorption fine structure (EXAFS) spectroscopy for structural characterization, Deutsch et al (11) prepared mononuclear osmium carbonyls anchored to γ -Al 2 O 3 by either two or three Os-O bonds, depending on the number of CO ligands bonded to the osmium (three or two, respectively). Triantafillou et al (12) used EXAFS spectroscopy to determine slight changes in metal-oxygen bond distances in a family of mononuclear rhenium carbonyls supported on MgO which had been hydroxylated or methoxylated to various degrees.…”

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Iridium cluster catalysts were prepared by reaction of [Ir 4 (CO) 12 ] with MgO that had been calcined at 300, 500, or 700• C to vary to degree of hydroxylation, followed by removal of the CO ligands from the iridium by treatment in He. The catalysts were tested for toluene hydrogenation at 60, 80, and 100

• C and 1 atm.

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