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

Abstract

Thiomethoxychalcone-based ligands bis[3,3-bis(methylsulfanyl)]-1,1′- η⁵-ferrocenyl-2-propen-1-one (L1), 3,3-bis(methylsulfanyl)-1-η⁵-ferrocenyl-2-propen-1-one (L²), and 3-methylsulfanyl-3-sulfanyl-1-η⁵-ferrocenyl-2-propen-1-one (L³) 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 L¹ were obtained as both syn and anti conformers and showed the conformational freedom of the two cyclopentadienyl (Cp) units. L¹−L³ 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 L¹ and L² 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 (ε = 3920 M⁻¹ cm⁻¹) for L¹ and 600 nm (ε = 1140 M⁻¹ cm⁻¹) for L² in the UV/vis experiments. The UV/vis titration profiles of L¹ and L² indicate the formation of 2:1 (L¹/Hg²⁺) and 1:1 (L²/Hg²⁺) initial complexations in solution. On the other hand, L³ 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 L¹ and L² 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 cuboctahedron cluster, [Hg₈S(SCH₃)₁₂]²⁺, upon a selective chemodosimetric desulfurization reaction between Hg²⁺ and L¹ or L². Cyclic voltammetric studies also support the Hg²⁺-induced cleavage of thiomethoxy groups.