Ternary Se-Ru-Cu clusters: Facile synthesis, electrochemistry, and UV/Vis absorption

New ternary Se-Ru-Cu octahedral-based complexes, namely, CuX-capped SeRu₅ clusters [SeRu₅(CO)₁₄(μ₃-CuX)]^2- (2a-2c; X = Cl, Br, I), bis-CuX-capped SeRu₅ clusters [SeRu₅(CO)₁₄(μ₃-CuX)₂]^2- (3a, 3b; X = Cl, Br), Cu₄X₂-bridged di-SeRu₅ clusters [{SeRu₅(CO)₁₄}₂(μ₆-Cu₄X₂)]^2- (4a, 4b; X = Cl, Br), and bis-CuX-incorporated Se₂Ru₄ clusters [Se₂Ru₄(CO)₁₀(μ-CuX)₂]^2- (5a-5c; X = Cl, Br, I), were prepared from the reaction of the binary cluster [SeRu₅(CO)₁₄]^2- (1) with CuX under appropriate conditions. Their solid-state packing formed cluster-based 1D-supramolecular chains, which were stabilized by C-HX/Y (X = halides; Y = O) H-bonding between CuX/COs of cluster anions and the CH moieties of the [PPh₄]⁺ or [PPN]⁺ cations. In addition, electrochemical studies showed that the successive anodic shifts were found for 1-4b, tuned by the incoming CuBr fragments introduced into the parent SeRu₅ cluster, supported by their decreasing HOMO levels and increasing ionization energies. Solid-state diffuse reflectance spectra showed these Se-Ru-Cu complexes exhibited semiconducting behaviors with the low and tunable energy gaps of 0.82-1.08 eV. Significantly, the solid-state absorptions of 2b and 4b were red-shifted compared with the corresponding Te analogues, which was explained by the larger numbers of the density of states (DOS) close to the HOMO for the selenide cases.