Synthetic Tuning of Redox, Spectroscopic, and Photophysical Properties of {Mo6I8}4+ Core Cluster Complexes by Terminal Carboxylate Ligands

The reactions between the tetra-n-butylammonium salt of [{Mo6I8}­I6]2– and silver carboxylates RCOOAg (R = CH3 (1), C­(CH3)3 (2), α-C4H3O (3), C6H5 (4), α-C10H7 (5), or C2F5 (6)) in CH2Cl2 afforded new carboxylate complexes [{Mo6I8}­(RCOO)6]2–. The complexes were characterized by X-ray single-crystal diffraction and elemental analysis, cyclic/differential pulse voltammetry, and IR, NMR, and UV–visible spectroscopies. The emission properties of the complexes 1–6, and those of the earlier reported complexes with R = CF3 (7) and n-C3F7 (8), were studied both in acetonitrile solution and in the solid state. In deaerated CH3CN at 298 K, all of the complexes 1–8 exhibit intense and long-lived emission with the quantum yield and lifetime being 0.48–0.73 and 283–359 μs, respectively. The oxidation (Eox)/reduction (Ered) potentials of the complexes correlate linearly with the pKa value of the terminal carboxylate ligands L = RCOO (pKa(L)). Reflecting the pKa(L) dependences of Eox/Ered, the emission energy (νem) of the complexes was also shown to correlate with pKa(L). The present study successfully demonstrates synthetic tuning of the redox, spectroscopic, and photophysical characteristics of a {Mo6I8}4+-based cluster complex with pKa(L).