Formation of MgO-Supported Manganese Carbonyl Complexes by Chemisorption of Mn(CO)₅CH₃

Abstract

MgO-supported manganese carbonyl complexes were prepared by chemical vapor deposition of Mn(CO)5CH3 on partially dehydroxylated, high-area MgO powder. X-ray absorption spectra identify the resultant surface species, on average, as Mn(CO)4(Os)2 (where the two oxygen ligands are part of the MgO surface), and infrared spectra show that the chemisorption results from the reaction of Mn(CO)5CH3 with OH groups of the MgO surface. Electron paramagnetic resonance and X-ray absorption near edge data indicate that the manganese was in a positive oxidation state other than +2, but the value is not determined, and the IR spectra indicate the presence of a mixture of manganese carbonyls. Extended X-ray absorption fine structure spectra determine the average Mn-CO bond distance to be 1.87 Å and the average Mn-O bond distance to be 2.12 Å. The surface complex was found to be stable in O2 at room temperature.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

“…22,77,98,686,693 Perhaps the first issue that needs to be considered when talking about metal single-site catalysis is the nature of the support upon which the organometallic molecules are adsorbed and partially decomposed. The vast majority of dispersed catalysts are prepared on high-surface-area porous oxides, quite often on silica 75,77,685,694,695 but in a few instances also on other common oxides such as alumina, 695−697 titania, 698−700 ceria, 701 and magnesia, 698,702 and also on mixed oxides, silicoaluminas in particular. 703 Coordination of organometallics to these surfaces takes place predominantly at terminal M−OH groups, silanols in the case of silica.…”

“…Perhaps the first issue that needs to be considered when talking about metal single-site catalysis is the nature of the support upon which the organometallic molecules are adsorbed and partially decomposed. The vast majority of dispersed catalysts are prepared on high-surface-area porous oxides, quite often on silica ,,,, but in a few instances also on other common oxides such as alumina, − titania, − ceria, and magnesia, , and also on mixed oxides, silicoaluminas in particular . Coordination of organometallics to these surfaces takes place predominantly at terminal M–OH groups, silanols in the case of silica. ,,, Still, several structural options are available, as silanols (for instance) mainly exist as isolated groups (≡Si–OH) but can also be present on the surface in geminal (=Si­(OH) 2 ) or vicinal (=Si­(OH)–O–Si­(OH)=) forms, a fact that affords multiple ligand substitution and coordination of discrete metal complex to the surface. ,,,− Even if the primary coordination is via ligand displacement by a single isolated silanol group, additional coordination is also possible to adjacent siloxane bridges (≡Si–O–Si ≡ ), as in the case of the Ta-based catalysts for alkene metathesis shown in Figure a .…”

Bruce Gates: A Career in Catalysis

“…Gates et al also extended the surface-mediated synthetic chemistry to Re and Mn, producing a series of supported rhenium clusters on γ-Al 2 O 3 and TiO 2 , and manganese clusters on MgO …”

“…In contrast, the mononuclear rhenium cations aggregate and form clusters approximated as Re 3 during catalysis; they exhibit a substantially higher formal oxidation state than the “mononuclear” Re 3 , and much higher rates. The Gates group then extended the synthesis chemistry of supported Re to Mn, synthesizing a series of manganese clusters by anchoring Mn­(CO) 4 complexes on MgO …”

“…The Gates group then extended the synthesis chemistry of supported Re to Mn, synthesizing a series of manganese clusters by anchoring Mn(CO) 4 complexes on MgO. 28 Surface defects on TiO 2 serve as anchoring sites and enhance the dispersion of supported metals. Defects can be created by treatment of TiO 2 under vacuum, in H 2 , by ion bombardment, or by exposure to an electron beam.…”