Redox Potential and Crystal Chemistry of Hexanuclear Cluster Compounds

Introduction of Niobium in the Chemistry of Octahedral Chalcogenide Clusters: Synthesis and Detailed Study of Compounds Based on Condensed and Discrete {Re5NbQ8} (Q = S or Se) Cores

It is known that niobium practically does not form cluster chalcogenide compounds of the {M6(μ3-Q8)} type, which are widespread in the chemistry of group 6 and 7 metals. This work reports the preparation of a series of polymeric and discrete niobium-containing heterometallic clusters based on the {Re5Nb(μ3-S8)} and {Re5Nb(μ3-Se8)} cores. The compounds were prepared by the high-temperature reaction between rhenium and niobium dichalcogenides in a KCN melt. The 1D polymers K5[Re5NbQ8(CN)5] (Q = S or Se), which were formed as a result of the reaction, crystallize in the structural type of K6[Mo6Se8(CN)5], similar to the previously reported heterometallic clusters K6[Re6-xMoxQ8(CN)5] (x = 2-3). The polymers were solubilized to form discrete anionic clusters [Re5NbQ8(CN)6]4-. The structure and properties of the new clusters were investigated using a combination of X-ray diffraction analysis, UV/vis spectroscopy, high-resolution electrospray mass spectrometry, cyclic voltammetry, and DFT calculations. Among other features, the compounds showed high electrochemical activity, being able to form three redox states in solution with reversible transitions. It was found that redox potentials of the isoelectronic octahedral clusters demonstrate a strong cathodic shift in the sequence [Re5OsSe8(CN)6]3- > [Re6Se8(CN)6]4- > [Re5MoSe8(CN)6]5- > [Re5NbSe8(CN)6]6-, illustrating the effect of systematic changes in the composition of octahedral cluster cores on their properties.

Octahedral Hexanuclear Rhenium Cluster Dimers Bridged by Pyrazine or 4,4'-Bipyridine with 23- and 24-Electron Configurations

New pyrazine (pz)- and 4,4'-bipyridine (4,4'-bpy)-bridged octahedral hexanuclear rhenium(III) cluster dimers, [{Re6(μ3-S)8Cl5}2(μ-L)]6- (L = pz, [1]6-; L = 4,4'-bpy, [2]6-), with 2 × 24 d-electrons {Re6(24e)}2 were obtained in a single-step reaction via photoirradiation of [Re6(μ3-S)8Cl6]4- with L in a 2:1 ratio at room temperature. The {Re6(23e)}2 dimers, [{Re6(μ3-S)8Cl5}2(μ-L)]4- (L = pz, [1']4-; L = 4,4'-bpy, [2']4-), were synthesized through two-electron oxidation of [1]6- and [2]6-, respectively. The single-crystal X-ray structures of [1]6- and [1']4- were determined, revealing structural distortion of the Re6 core of [1']4- due to the Jahn-Teller effect. The cyclic voltammograms of [1]6- and [2]6- showed two steps of one-electron redox processes attributable to Re6(23e)Re6(24e)/{Re6(24e)}2 and {Re6(23e)}2/Re6(23e)Re6(24e), respectively. The separation between the two redox potentials is small (0.056 V for [1]6- and 0.039 V for [2]6-). The magnetic susceptibilities of [1']4- and [2']4- were almost temperature-independent, with values of 2.98 and 2.85 μB, respectively, indicating paramagnetism. These results suggest weak electronic interaction between two cluster units bridged by pz or 4,4'-bpy in the intercluster mixed valence state. The compounds [1]6- and [2]6- show photoluminescence in the near-infrared region at 296 K in the solid state.

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 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.