Metal-Ion Displacement Approach for Optical Recognition of Thorium: Application of a Molybdenum(VI) Complex for Nanomolar Determination and Enrichment of Th(IV)

An azine-based molybdenum (Mo­(VI)) complex (M1) is exploited for selective detection of thorium (Th­(IV)) ions through a metal-ion displacement protocol. Th­(IV) displaces Mo­(VI) from M1 instantly leading to the formation of the Th­(IV) complex, having orange-red emission. Consequently, a red shift of the emission wavelength along with 41-fold fluorescence enhancement is observed. This unique method allows detection of Th­(IV) as low as 1.5 × 10^–9 M. The displacement of Mo­(VI) from M1 by Th­(IV) is established by spectroscopic studies and kinetically followed by the stopped-flow technique. The displacement binding constant for Th­(IV) is notably strong, 4.59 × 10⁶ M^–1. Extraction of Th­(IV) from aqueous solution to the ethyl acetate medium using M1 has been achieved. The silica-immobilized M1 efficiently enriches Th­(IV) from its reservoir through solid-phase extraction. Computational studies (density functional theory) support experimental findings.