Cluster Size Dependent Interaction of Free Manganese Oxide Clusters with Acetic Acid and Methyl Acetate

Abstract

We have employed infrared multiple-photon dissociation (IR-MPD) spectroscopy together with density functional theory (DFT) calculations to study the interaction of series of subnanometer sized manganese oxide clusters, Mn x O y + (x = 1–6, y = 0–9) with acetic acid (HOAc) and methyl acetate (MeOAc). Reaction with HOAc leads to strongly cluster size and composition dependent IR-MPD spectra, indicating molecular adsorption on MnO x + clusters and thermodynamically favorable but kinetically hampered HOAc dissociation (deprotonation) on Mn2O4 + and Mn3O5 +. Other cluster sizes exhibit the preferred formation of a dissociative bidentate chelating structure. In contrast to HOAc, all clusters bind MeOAc via the carbonyl group as an intact molecule, and dissociation appears to be kinetically hindered under the given experimental conditions.

Cluster size dependent coordination of formate to free manganese oxide clusters

“…More important is, however, the binding motif of the resulting formate and acetate groups, respectively. For both Mn 2 O 2 + and Mn 2 O 4 + binding of formate and acetate 36 in the bidentate chelating configuration is energetically more favorable which is in agreement with the IR-MPD spectra. Only formate can also bind to Mn 2 O 2 + in a bidentate bridging structure.…”

“…Similar results were previously found for acetic acid. 36 In contrast, for Mn 2 O 4 + and Mn 3 O 5 + indications of an intact acetic acid were found, suggesting deprotonation is kinetically hindered for these cluster sizes. This difference between formic and acetic acid can likely be attributed to the former's higher acidity.…”

“…36 In case of molecular binding, the CQO stretch mode (nCQO) of the carboxyl group is most characteristic (typically between 1690 cm À1 and 1560 cm À1 ) and even allows to distinguish between the trans-and cis-configuration. 36,47 For deprotonated acids, nCQO is no longer observed; instead, the symmetric and asymmetric stretch modes of the OCO group (n s OCO and n as OCO) become the diagnostic modes (n s OCO typically between 1480 cm À1 and 1380 cm À1 ; n as OCO typically between 1680 cm À1 and 1500 cm À1 ). [40][41][42] Fig.…”

“…However, the binding of the resulting formate group is strongly cluster-size-dependent and can differ from the bridging m-oxo configuration typically found for Mn 2 O 2 -and Mn 4 O 4 -based molecular catalysts. The consequences of cluster fluxionality [32][33][34] and differences from the ligation with acetic acid 35,36 are discussed.…”

Vibrational spectroscopy of free di-manganese oxide cluster complexes with di-hydrogen

“…The magnetic nature of pure manganese clusters may lead to the suspicion of multi-reference character of the manganese oxide clusters. However, multi-reference calculations are computationally costly and are not necessary to spectroscopically characterize the nature of ligand adsorption, as seen in this and our earlier work on adsorption onto MnO clusters [15,16,32,33].…”