Synthesis, spectroscopic and electrochemical properties, and electronic structures of octahedral hexatechnetium(III) clusters [Tc₆Q ₈(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 M ₆(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.