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(mu(3)-S)(8)Cl-6](4-) with an excess amount of L afforded a mono-L-substituted hexanuclear rhenium(III) complex, [Re-6(mu(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(mu(3)-S)(8)Cl-4(lut)(2)](2-) and mer-[Re-6(mu(3)-S)(8)Cl-3(lut)(3)](-), were synthesized by the reaction of [Re-6(mu(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 pK(a) of the N-heteroaromatic ligand [pK(a)(L)], the H-1 NMR chemical shift of the ortho proton on the coordinating ligand, and the Hammett constant (sigma) of the pyridyl-ligand substituent. The series of [Re-6(mu(3)-S)(8)X6-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 (sigma 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 ((CC)-C-3) for L = dmap, lut, mpy, and py and from the metal-to-ligand charge-transfer excited triplet state ((MLCT)-M-3) for L = bpy, cpy, and pz. The complexes with the (CC)-C-3 character exhibited emission features and photophysical properties similar to those of ordinary hexanuclear rhenium complexes. The emission maximum wavelength of the complexes with (MLCT)-M-3 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 (CC)-C-3 character was linearly correlated with pK(a)(L), sigma, and sigma E-L. The ground- and excited-state oxidation potentials were finely tuned by the combination of halide and L ligands at the terminal positions.