Synthesis and characterization of thallium–salen derivatives for use as underground fluid flow tracers

Abstract

A pair of thallium salen derivatives was synthesized and characterized for potential use as monitors (or taggants) or as models for Group 13 complexes for subterranean fluid flows. These precursors were isolated from the reaction of thallium ethoxide with N,N′-bis(3,5-di-tert-butylsalicylidene)-ethylenediamine (H₂-salo-Bu^t), or N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-phenylenediamine (H₂-saloPh-Bu^t). The products were identified by single crystal X-ray diffraction as: [((μ-O)₂,κ¹-(N)(N′)salo-Bu^t)Tl₂] (1) and {[((μ-O)₂saloPh-Bu^t)Tl₂][((μ-O)₂,κ¹-(N)(N′)saloPh-Bu^t)Tl₂]} (2). Both structures are similar, wherein each O atom of the salo moiety bridges the two Tl atoms, leading to a Tl⋯Tl interaction, which is further stabilized by an intramolecular π-bond with neighboring phenyl rings. For 1, an additional Tl⋯N interaction was solved for each metal center; whereas, for 2, one of the two molecules in the matrix has a weak Tl⋯N interaction but no bonding noted in the other molecule. Both Density Functional Theory (DFT) calculations and variable temperature solution ²⁰⁵Tl NMR studies of 1 and 2 further confirmed the Tl⋯Tl interaction. The UV-vis absorbance spectra of these compounds had an absorbance peak at 392 nm for 1 and a broad absorbance peak centered at 469 nm for 2, which were found to be in good agreement with the DFT calculated spectra that were dominated by the singlet state. Fluorescence emission and excitation studies reveal absorptions at 360 and 380 nm for 1 and 2, respectively, which are attributed to the Tl⋯Tl metal centers. To demonstrate practicality, fluorescence spectra of 1 and 2 were obtained using a handheld 405 nm cw laser pointer and portable spectrometer where compound 1 was found to glow 15 times brighter than compound 2. Only compound 1 was found to survive the simulated deep-well conditions explored, which was attributed to the Tl⋯N interaction noted for 1 but not for 2.