Cluster size dependent coordination of formate to free manganese oxide clusters.

The interaction of free manganese oxide clusters, MnO ( = 1-9, = 0-12), with formic acid was studied infrared multiple-photon dissociation (IR-MPD) spectroscopy together with calculations using density functional theory (DFT). Clusters containing only one Mn atom, such as MnO and MnO, bind formic acid as an intact molecule in both the - and -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 MnO the co-existence of two isomers with the formate bound in a bidentate bridging and chelating configurations, respectively, is found, whereas for MnO the bidentate chelating configuration is preferred. In contrast, the bidentate bridging structure is energetically considerably more favorable for MnO. This binding motif stabilizes the 2D ring structure of the core of the MnO cluster with respect to the 3D cubic geometry of the MnO cluster core.