Substitution of the Terminal Chloride Ligands of [Re₆S₈Cl₆]⁴⁻ with Triethylphosphine: Photophysical and Electrochemical Properties of a New Series of [Re₆S₈]²⁺ Based Clusters

A systematic substitution of the terminal chlorides coordinated to the hexanuclear cluster [Re6S8Cl6]4− has been conducted. The following complexes: [Re6S8(PEt3)Cl5]3− (1), cis- (cis-2) and trans-[Re6S8(PEt3)2Cl4]2− (trans-2), mer- (mer-3) and fac-[Re6S8(PEt3)3Cl3]− (fac-3), and cis- (cis-4) and trans-[Re6S8(PEt3)4Cl2] (trans-4) were synthesized and fully characterized. Compared to the substitution of the halide ligands of the related [Re6S8Br6]4− and [Re6Se8I6]3− clusters, the chloride ligands are slower to substitute which allowed us to prepare the first monophosphine cluster (1). In addition, the synthesis of fac-3 was optimized by using cis-2 as the starting material, which led to a significant increase in the overall yield of this isomer. Notably, we observed evidence of phosphine isomerization taking place during the preparation of the facial isomer; this was unexpected based on the relatively inert nature of the Re−P bond. The structures of Bu4N+ salts of trans-2, mer-3, and fac-3 were determined using X-ray crystallography. All compounds display luminescent behavior. A study of the photophysical properties of these complexes includes measurement of the excited state lifetimes (which ranged from 4.1−7.1 μs), the emission quantum yields, the rates of radiative and non-radiative decay, and the rate of quenching with O2. Quenching studies verify the triplet state nature of the excited state.