Cluster size dependent coordination of formate to free manganese oxide clusters

The interaction of free manganese oxide clusters, MnO⁺ (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 MnO₂⁺ and MnO₄⁺, 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 Mn₂O₂⁺ the co-existence of two isomers with the formate bound in a bidentate bridging and chelating configurations, respectively, is found, whereas for Mn₂O₄⁺ the bidentate chelating configuration is preferred. In contrast, the bidentate bridging structure is energetically considerably more favorable for Mn₄O₄⁺. This binding motif stabilizes the 2D ring structure of the core of the Mn₄O₄⁺ cluster with respect to the 3D cubic geometry of the Mn₄O₄⁺ cluster core.