Proton-Switched Emission Behavior of Hexanuclear Molyb-denum(II) Clusters Bearing Terminal Pyridine Carboxylate Ligands

We designed and synthesized a new and novel octahedral hexamolybdenum(II) cluster bearing terminal pyridine-3-carboxylate (3L) or pyridine-4-carboxylate ligands (4L), [{Mo6Br8}(3L)(6)](2-)(3) or [{Mo6Br8}(4L)(6)](2-)(4), to demonstrate proton-switched emission behaviour of the cluster. X-ray crystal analysis of4demonstrated that the O-atom in the carboxylate group of4Lcoordinates to each Mo atom in the {Mo6Br8}(4+)-core, indicating that the N-atom of4Lin4is open for protonation. Reflecting such structural characteristics of the cluster, both3and4showed proton-switched emission behaviour owing to the changes in the electron-donating abilities of the relevant terminal ligands upon protonation to the pyridine N atoms: blue shifts of the emission spectra and increases in the emission lifetimes of the clusters.