Cluster size dependent coordination of formate to free manganese oxide clusters

The interaction of free manganese oxide clusters, Mn x O y + ( x = 1-9, y = 0-12), with formic acid was studied via infrared multiple-photon dissociation (IR-MPD) spectroscopy together with calculations using density functional theory (DFT). Clusters containing only one Mn atom, such as MnO 2 + and MnO 4 + , bind formic acid as an intact molecule in both the cis - and trans -configuration. In contrast, all clusters containing two or more manganese atoms deprotonate the acid's hydroxyl group. The coordination of the resulting formate group is strongly cluster-size-dependent according to supporting DFT calculations for selected model systems. For Mn 2 O 2 + the co-existence of two isomers with the formate bound in a bidentate bridging and chelating configurations, respectively, is found, whereas for Mn 2 O 4 + the bidentate chelating configuration is preferred. In contrast, the bidentate bridging structure is energetically considerably more favorable for Mn 4 O 4 + . This binding motif stabilizes the 2D ring structure of the core of the Mn 4 O 4 + cluster with respect to the 3D cubic geometry of the Mn 4 O 4 + cluster core. Manganese oxide clusters deprotonate the hydroxyl group of formic acid yielding a strongly cluster size dependent binding motif of the formate group.