Tuning the Ground- and Excited-State Redox Potentials of Octahedral Hexanuclear Rhenium(III) Complexes by the Combination of Terminal Halide and N-Heteroaromatic Ligands.

The present study reports that the ground- and excited-state Re 6 (23e)/Re 6 (24e) redox potentials of an octahedral hexanuclear rhenium(III) complex can be controlled by systematically changing the number and type of the N -heteroaromatic ligand (L) and the number of chloride ions at the six terminal positions. Photoirradiation of [Re 6 (μ 3 -S) 8 Cl 6 ] 4- with an excess amount of L afforded a mono-L-substituted hexanuclear rhenium(III) complex, [Re 6 (μ 3 -S) 8 Cl 5 (L)] 3- (L = 4-dimethylaminopyridine (dmap), 3,5-lutidine (lut), 4-methylpyridine (mpy), pyridine (py), 4,4'-bipyridine (bpy), 4-cyanopyridine (cpy), and pyrazine (pz)). The bis- and tris-lut-substituted complexes, trans - and cis -[Re 6 (μ 3 -S) 8 Cl 4 (lut) 2 ] 2- and mer -[Re 6 (μ 3 -S) 8 Cl 3 (lut) 3 ] - , were synthesized by the reaction of [Re 6 (μ 3 -S) 8 Cl 6 ] 3- with an excess amount of lut in refluxed N , N -dimethylformamide. The mono-L-substituted complexes showed one-electron redox processes assignable to E 1/2 [Re 6 (23e)/Re 6 (24e)] = 0.49-0.58 V versus Ag/AgCl. The ground-state oxidation potentials were linearly correlated with the p K a of the N-heteroaromatic ligand [p K a (L)], the 1 H NMR chemical shift of the ortho proton on the coordinating ligand, and the Hammett constant (σ) of the pyridyl-ligand substituent. The series of [Re 6 (μ 3 -S) 8 X 6- n (L) n ] n -4 complexes ( n = 0, X = Cl, Br, I, or NCS; n = 1-3, X = Cl) showed a linear correlation with the sum of the Lever electrochemical parameters at the six terminal ligands (Σ E L ). The cyclic voltammograms of the mono-L-substituted complexes (L = bpy, cpy, and pz) showed one-electron redox waves assignable to E 1/2 (L 0 /L - ) = -1.28 to -1.48 V versus Ag/AgCl. Two types of photoluminescences were observed for the complexes, originating from the cluster core-centered excited triplet state ( 3 CC) for L = dmap, lut, mpy, and py and from the metal-to-ligand charge-transfer excited triplet state ( 3 MLCT) for L = bpy, cpy, and pz. The complexes with the 3 CC character exhibited emission features and photophysical properties similar to those of ordinary hexanuclear rhenium complexes. The emission maximum wavelength of the complexes with 3 MLCT shifted to the longer wavelength in the order L = 4-phenylpyridine (ppy), bpy, pz, and cpy, which agreed with the difference between E 1/2 [Re 6 (23e)/Re 6 (24e)] and E 1/2 (L 0 /L - ). The calculated oxidation potential of the excited hexanuclear rhenium complex with the 3 CC character was linearly correlated with p K a (L), σ, and Σ E L . The ground- and excited-state oxidation potentials were finely tuned by the combination of halide and L ligands at the terminal positions.