Synthesis, Spectroscopic and Electrochemical Properties, and Electronic Structures of Octahedral Hexatechnetium(III) Clusters [Tc₆Q₈(CN)₆]⁴⁻ (Q = S, Se)

Chalcogenide-capped molecular octahedral hexatechnetium(III) clusters [Tc6Q8(CN)6]4− {Q = S ([1]4−), Se ([2]4−)} were prepared by the substitution of axial ligands with cyanide. The structures of the new complexes were determined by single-crystal X-ray analysis. The IR spectra of [1]4− and [2]4− showed a CN stretching band at 2114 and 2105 cm−1, respectively. In cyclic voltammetry, [1]4− and [2]4− in CH3CN showed reversible one-electron-oxidation waves assignable to the Tc6(24e/23e) process at +0.99 and +0.74 V, respectively. Density functional theory (DFT) calculations on the hexatechnetium complexes showed that the highest occupied molecular orbital (HOMO) was primarily localized on a Tc6Q8 core and the lowest unoccupied molecular orbital (LUMO) was completely localized on the metal orbitals. The energy level of HOMO and the redox potential of the M6(24e/23e) process (M = Tc, Re) were found to have a good linear relationship. Time-dependent DFT calculations showed that the substantially allowed transitions with the lowest energy were Tc6Q8 core-centered transitions. The electronic structures and electronic transition features of the hexatechnetium complexes were similar to those of the hexarhenium analogues [Re6Q8(CN)6]4− (Q = S, Se); however, the energy gap between the HOMO and LUMO was smaller in the hexatechnetium complexes.