Synthesis, structures, redox properties, and theoretical calculations of thiohalide capped octahedral hexanuclear technetium(iii) clusters

Abstract

The first thiohalide μ₃-capped octahedral hexanuclear technetium clusters with 24 valence electrons, [Tc₆(μ₃-S)_8−n(μ₃-Br)_nBr₆]ⁿ⁻⁴ [n = 1 ([Tc-S₇Br]³⁻) and n = 2 ([Tc-S₆Br₂]²⁻)] and [Tc₆(μ₃-S)₇(μ₃-Cl)Cl₆]³⁻ ([Tc-S₇Cl]³⁻), were synthesized and characterized. The structures of [Tc-S₇Br]³⁻, [Tc-S₆Br₂]²⁻, and [Tc-S₇Cl]³⁻ were determined by single-crystal X-ray analysis. The Tc–Tc bond distances in [Tc-S₇Br]³⁻, [Tc-S₆Br₂]²⁻, and [Tc-S₇Cl]³⁻ are 2.5842(6)–2.6029(6) Å (avg. 2.593(2) Å), 2.5835(10)–2.6049(10) Å (avg. 2.596(1) Å), and 2.5829(4)–2.5940(4) Å (avg. 2.587(3) Å), respectively. The capping halide and sulfide ligands in [Tc-S₇Br]³⁻, [Tc-S₆Br₂]²⁻, and [Tc-S₇Cl]³⁻ were disordered in the crystals. The bond distances of Tc–S/Br as a function of the occupancies of the capping bromides for [Tc-S₆Br₂]²⁻, [Tc-S₇Br]³⁻, and [Tc₆(μ₃-S)₈Br₆]⁴⁻ ([Tc-S₈]⁴⁻) showed a linear correlation. The one-electron reduction waves assignable to the Tc^III₆/Tc^IITc^III₅ [Tc₆(24e/25e)] process were observed for the novel complexes. Density functional theory (DFT) calculations of the hexanuclear technetium complexes showed a smaller energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the hexanuclear technetium complexes compared to those of the rhenium analogues. The electronic transitions of the new technetium complexes shifted to lower energy compared to the isotypic rhenium complexes.