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

Takashi Yoshimura; Hayato Nishizawa; Kojiro Nagata; Akitaka Ito; Eri Sakuda; Shoji Ishizaka; Noboru Kitamura; Atsushi Shinohara

doi:10.1021/acsomega.2c03834

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

ACS Omega. 2022 Jul 21;7(30):26965-26982. doi: 10.1021/acsomega.2c03834. eCollection 2022 Aug 2.

Authors

Takashi Yoshimura¹, Hayato Nishizawa², Kojiro Nagata¹, Akitaka Ito³, Eri Sakuda⁴, Shoji Ishizaka⁵, Noboru Kitamura^{6

7}, Atsushi Shinohara¹

Affiliations

¹ Radioisotope Research Center, Institute for Radiation Sciences, Osaka University, Suita 565-0871, Japan.
² Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan.
³ School of Environmental Science and Engineering, Kochi University of Technology, Kochi 782-8502, Japan.
⁴ Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, Nagasaki 852-8521, Japan.
⁵ Graduate School of Advanced Science and Engineering, Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan.
⁶ Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan.
⁷ Toyota Physical and Chemical Research Institute, Nagakute 480-1192, Aichi, Japan.

Abstract

The present study reports that the ground- and excited-state Re₆(23e)/Re₆(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 [Re₆(μ₃-S)₈Cl₆]^4- with an excess amount of L afforded a mono-L-substituted hexanuclear rhenium(III) complex, [Re₆(μ₃-S)₈Cl₅(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-[Re₆(μ₃-S)₈Cl₄(lut)₂]^2- and mer-[Re₆(μ₃-S)₈Cl₃(lut)₃]^-, were synthesized by the reaction of [Re₆(μ₃-S)₈Cl₆]^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[Re₆(23e)/Re₆(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 ¹H NMR chemical shift of the ortho proton on the coordinating ligand, and the Hammett constant (σ) of the pyridyl-ligand substituent. The series of [Re₆(μ₃-S)₈X_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 (ΣE _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(L⁰/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 (³CC) for L = dmap, lut, mpy, and py and from the metal-to-ligand charge-transfer excited triplet state (³MLCT) for L = bpy, cpy, and pz. The complexes with the ³CC character exhibited emission features and photophysical properties similar to those of ordinary hexanuclear rhenium complexes. The emission maximum wavelength of the complexes with ³MLCT 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[Re₆(23e)/Re₆(24e)] and E _1/2(L⁰/L^-). The calculated oxidation potential of the excited hexanuclear rhenium complex with the ³CC character was linearly correlated with pK _a(L), σ, and ΣE _L. The ground- and excited-state oxidation potentials were finely tuned by the combination of halide and L ligands at the terminal positions.