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.