pK a(L) Dependences of Structural, Electrochemical, and Photophysical Properties of Octahedral Hexamolybdenum(II) Clusters: [Mo6 X 8 L 6]2- (X = Br or I; L = carboxylate)

New octahedral hexamolybdenum(II) bromide clusters, the tetra-n-butylammonium (n-Bu4N+) salts of [{Mo6Br8}L (6)](2-) with a series of aliphatic and aromatic carboxylates (L) having different pK (a) values (pK (a)(L) = 1.75-5.03), were synthesized and characterized by X-ray single-crystal diffraction analysis, cyclic/differential pulse voltammetries, and emission spectroscopy, together with two new hexamolybdenum(II) iodide clusters: (n-Bu4N)(2)[{Mo6I8}L (6)] (L = 3,5-dimethylbenzoate or perfluorobenzoate). In the present study, we successfully demonstrated that the average bond length between the Mo atom and the ligating O atom in L, the redox potentials (in acetone), and the emission parameters (maximum energy, quantum yield, and lifetime in acetone) of these complexes and previously reported (n-Bu4N)(2)[{Mo6I8}L (6)] clusters correlated almost linearly with pK (a)(L) for a series of the bromide or iodide clusters. The nature of the inner capping ligand (X = Br or I) also governed the structural, redox, and emission properties of the cluster complexes. The origins of such pK (a)(L) and inner capping ligand dependences of these properties are discussed in detail.