Unprecedented Electron-Poor Octahedral Ta6 Clusters in a Solid-State Compound: Synthesis, Characterisations and Theoretical Investigations of Cs2BaTa6Br15O3

The crystal structure of Cs2BaTa6Br15O3 has been elucidated by using synchrotron X-ray powder diffraction and absorption experiments. It is built from edge-bridged octahedral [((Ta6Br9O3i)-O-i)Br-6(a)](4-) cluster units with a singular poor metallic electron (ME) count equal to thirteen. This leads to a paramagnetic behaviour related to one unpaired electron. The arrangement of the Ta-6 clusters is similar to that of Cs2LaTa6Br15O3 exhibiting 14-MEs per [((Ta6Br9O3i)-O-i)Br-6(a)](5-) motif. The poorer electron-count cluster presents longer metal-metal distances as foreseen according to the electronic structure of edge-bridged hexanuclear cluster. Density functional theory (DFT) calculations on molecular models were used to rationalise the structural properties of 13- and 14-ME clusters. Periodic DFT calculations demonstrate that the electronic structure of these solid-state compounds is related to those of the discrete octahedral units. Oxygen-barium interactions seem to prevent the geometry of the octahedral cluster to strongly distort, allowing stabilisation of this unprecedented electron-poor Ta-6 cluster in the solid state.