Thiomethoxychalcone-Functionalized Ferrocene Ligands as Selective Chemodosimeters for Mercury(II): Single-Crystal X-ray Structural Signature of the [Hg8(μ8-S)(SCH3)12]2+ Cluster

Thiomethoxychalcone-based ligands bis[3,3-bis(methylsulfanyl)]-1,1'- eta(5)-ferrocenyl-2-propen-1-one (L-1), 3,3-bis(methylsulfanyl)-1-eta(5)-ferrocenyl-2-propen-1-one (L-2), and 3-methylsulfanyl-3-sulfanyl-1-eta(5)-ferrocenyl-2-propen-1-one (L-3) have been synthesized on the ferrocene backbone by varying the number of chalcone arms and thiomethoxy substitutents. The single-crystal X-ray crystallographic analyses of all three ligands are reported in which crystals of L-1 were obtained as both syn and anti conformers and showed the conformational freedom of the two cyclopentadienyl (Cp) units. L-1-L-3 are studied extensively toward their applicability in the colorimetric sensing of metal ions in solution. The solution-state study of mono- and bis(thiomethoxy)ferrocenylchalcone-functionalized ligands L-1 and L-2 showed selective colorimetric sensing for Hg2+ over Li+ Na+ Ca2+, Mg2+, Cr2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Au3+ in acetonitrile. In both cases, a selective color change from orange to purple was observed with Hg2+ and the resultant solution showed the appearance of a new peak at 565 nm (epsilon=3920 M-1 cm(-1)) for L-1 and 600 nm (E=1140M(-1) cm(-1)) for L-2 in the UV/vis experiments. The UV/vis titration profiles of L-1 and L-2 indicate the formation of 2:1 (L-1/Hg2+) and 1:1 (L-2/Hg2+) initial complexations in solution. On the other hand, L-3 with thiomethoxy- and thiol-functionalized ferrocenylchalcone showed no appreciable color change with Hg2+ under the same experimental conditions. Attempts were made to isolate single crystals of the resulting purple solution obtained in the cases of L-1 and L-2 with Hg2+. In both cases, crystals suitable for a single-crystal X-ray diffraction study were isolated in very low yield by a layer diffusion technique. The single-crystal structural investigations demonstrated the formation of a sulfide-encapsulated mercury thiolate cubo-ctahedron cluster, [Hg8S(SCH3)(12)](2+), upon a selective chemodosimetric desulfurization reaction between Hg2+ and L-1 or L-2. Cyclic voltammetric studies also support the Hg2+-induced cleavage of thiomethoxy groups.