Probing Metal Oxide Surface Reactivity with Adsorbate Organometallic Chemistry:  Formation of Iridium Carbonyl Clusters on γ-Al₂O₃

Abstract

Ir8(CO)22] 2-was formed by surface-mediated synthesis by treatment of [Ir(CO)2(acac)] supported on uncalcined γ-Al2O3 under 1 atm of CO at room temperature. The formation of [Ir8(CO)22] 2-completes the synthesis of a family of iridium carbonyl clusters, including [Ir4(CO)12], [HIr4(CO)11] -, [Ir6(CO)15] 2-, and [Ir8(CO)22] 2-, on the surface of γ-Al2O3. The surface chemistry parallels known solution chemistry of the synthesis of the family of iridium carbonyls. The products formed are controlled largely by the basicity of the solution or surface, and the analogy between the solution and surface chemistry provides a preliminary, qualitative basis for characterizing the basicity of γ-Al2O3 and other metal oxide samples. Decarbonylation of [Ir8(CO)22] 2-on γ-Al2O3 led to nearly uniform tetrahedral Ir4 clusters; it is likely that the Ir-Ir single bond linking two iridium tetrahedral moieties in the precursor [Ir8(CO)22] 2-on γ-Al2O3 was broken during the decarbonylation.

The Decomposition of Hydrazine in the Gas Phase and over an Iridium Catalyst

“…Reactivity towards ethene hydrogenation is found to be slightly larger for Ir 4 than for Ir 6 [56], and to be larger on alumina than MgO supports [42]. Experiments with an Ir 8 carbonyl precursor (consisting of two Ir 4 tetrahedra) were found to retain two Ir 4 particles on γ -alumina [57]. Very similar results are found by the Gates group for Ir n on zeolites [43,44,59,60] (110), up to n = 15, and observed hydrazine decomposition [61,62].…”

Prototype Supported Metal Cluster Catalysts: Ir4 and Ir6

“…EXAFS data characterizing the oxide-and zeolite-supported clusters after decarbonylation indicate IrÀO distances of approximately 2.1 , essentially matching those of the undecarbonylated supported clusters. [13,19,32] This distance indicates bonding between the Ir atoms and support oxygen atoms, and it suggests that the Ir atoms are positively charged, as elaborated upon below, and that the supports can be regarded as ligands. These reactive, decarbonylated clusters provide opportunities for synthesis of clusters with various ligands, including hydrocarbons and hydride.…”

“…[13,19] The data consistently indicate IrÀO support distances of approximately 98 www.chemcatchem.org 2.1 , matching typical IrÀO bonding distances. [36,37] Such results are the norm for Group 8 metal carbonyl complexes on oxides [13,14,23,[26][27][28][32][33][34] and zeolites; [9,[10][11][12]19] the nearness of the Ir atom to the support implies a significant distortion of the position of the CO ligands from those in isolated iridium carbonyl clusters.…”

“…EXAFS spectroscopy is one of the most powerful techniques for investigation of small clusters because it provides direct evidence of metal-metal bonds. It has been used to characterize tetra-and hexairidium clusters on the surfaces of porous oxides (g-Al 2 O 3 , [13,14,32,33] MgO, [23,[26][27][28]34] and TiO 2 [24] ) and zeolites. [9,[10][11][12]19] Tetrairidium clusters have been routinely found to have IrÀIr first-shell coordination numbers of nearly 3, consistent with a tetrahedral metal framework, and IrÀIr bonding distances of typically 2.67 AE 0.02 , essentially matching the IrÀIr bonding distance in coordinatively saturated iridium clusters such as [Ir 4 (CO) 12 ].…”

“…For example, experiments done with iridium carbonyl clusters supported on g-Al 2 O 3 , [13,14,32,33] MgO, [23,[26][27][28]34] and zeolites [9,[10][11][12]19] led to the conclusion that they can be decarbonylated by treatment either in helium or H 2 at 573 K and that the metal frames can be retained; IR and EXAFS spectra showed that the CO ligands could be removed completely (e.g., Figure 8).…”