Triosmium Clusters on a Support: Determination of Structure by X‐ray Absorption Spectroscopy and High‐Resolution Microscopy

Abstract

The structures of small, robust metal clusters on a solid support were determined by a combination of spectroscopic and microscopic methods: extended X-ray absorption fine structure (EXAFS) spectroscopy, scanning transmission electron microscopy (STEM), and aberration-corrected STEM. The samples were synthesized from [Os(3) (CO)(12) ] on MgO powder to provide supported clusters intended to be triosmium. The results demonstrate that the supported clusters are robust in the absence of oxidants. Conventional high-angle annular dark-field (HAADF) STEM images demonstrate a high degree of uniformity of the clusters, with root-mean-square (rms) radii of 2.03±0.06 Å. The EXAFS OsOs coordination number of 2.1±0.4 confirms the presence of triosmium clusters on average and correspondingly determines an average rms cluster radius of 2.02±0.04 Å. The high-resolution STEM images show the individual Os atoms in the clusters, confirming the triangular structures of their frames and determining OsOs distances of 2.80±0.14 Å, matching the EXAFS value of 2.89±0.06 Å. IR and EXAFS spectra demonstrate the presence of CO ligands on the clusters. This set of techniques is recommended as optimal for detailed and reliable structural characterization of supported clusters.

MgO-supported bimetallic catalysts consisting of segregated, essentially molecular rhodium and osmium species

“…Our method of preparation matched that reported, 14,15 and the IR and EXAFS spectra agree with those expected. 15 Detailed results presented in ESI † show that the supported species formed from Os 3 (CO) 12 were characterized by ν CO bands at 2075(s), 2006(vs), 1960(w), and 1930(s) cm −1 , nearly matching those of the authentic [Os 3 (CO) 11 ] 2− (Tables 1 and S1 †). 16 The EXAFS data characterizing the supported species (Table S4 †) indicate an Os-Os first-shell coordination number of 2.2, agreeing within error with the value of 2 for the triangular Os 3 frame in crystalline Os 3 (CO) 12 16 (and [Os 3 (CO) 11 ] 2− ), and matching within error the results reported from equivalently prepared supported samples.…”

“…16 The EXAFS data characterizing the supported species (Table S4 †) indicate an Os-Os first-shell coordination number of 2.2, agreeing within error with the value of 2 for the triangular Os 3 frame in crystalline Os 3 (CO) 12 16 (and [Os 3 (CO) 11 ] 2− ), and matching within error the results reported from equivalently prepared supported samples. 14,15 The EXAFS data also provide evidence of the osmium-support interface, with an Os-O coordination number of 0.9 at a distance of 2.12 Å, again consistent with reported results 15 and indicative of bonds between Os atoms and O atoms of the MgO support surface; the values suggest distortion of the CO ligands on the triangular metal frame to accommodate such a close bonding of the cluster to the support. 15 Rhodium complexes supported on MgO were prepared by the reaction of Rh(C 2 H 4 ) 2 (acac) with MgO in a slurry with n-pentane, leading to the formation of Rh(C 2 H 4 ) 2 complexes bonded to the support, as indicated by the IR spectra.…”

“…14,15 The EXAFS data also provide evidence of the osmium-support interface, with an Os-O coordination number of 0.9 at a distance of 2.12 Å, again consistent with reported results 15 and indicative of bonds between Os atoms and O atoms of the MgO support surface; the values suggest distortion of the CO ligands on the triangular metal frame to accommodate such a close bonding of the cluster to the support. 15 Rhodium complexes supported on MgO were prepared by the reaction of Rh(C 2 H 4 ) 2 (acac) with MgO in a slurry with n-pentane, leading to the formation of Rh(C 2 H 4 ) 2 complexes bonded to the support, as indicated by the IR spectra. 3 The acac ligands reacted to form surface species inferred to be predominantly Mg(acac) 2 (with the Mg atoms being part of the support surface).…”

“…Osmium carbonyl clusters synthesized from Os 3 (CO) 12 on MgO have been investigated before, 14,15 and characterization of the supported species by IR and EXAFS spectroscopies led to the conclusion that this precursor reacts with the basic MgO surface to form [Os 3 (CO) 11 ] 2− in high yield. Our method of preparation matched that reported, 14,15 and the IR and EXAFS spectra agree with those expected.…”

“…Osmium carbonyl clusters synthesized from Os 3 (CO) 12 on MgO have been investigated before, 14,15 and characterization of the supported species by IR and EXAFS spectroscopies led to the conclusion that this precursor reacts with the basic MgO surface to form [Os 3 (CO) 11 ] 2− in high yield. Our method of preparation matched that reported, 14,15 and the IR and EXAFS spectra agree with those expected. 15 Detailed results presented in ESI † show that the supported species formed from Os 3 (CO) 12 were characterized by ν CO bands at 2075(s), 2006(vs), 1960(w), and 1930(s) cm −1 , nearly matching those of the authentic [Os 3 (CO) 11 ] 2− (Tables 1 and S1 †).…”