Oxidation of thiocyanate by thallium(III) in aqueous solution studied by multinuclear NMR spectroscopy. Evidence for a Tl(SCN)2+ intermediate

Abstract

Thallium(II1) in aqueous solution is known to be a strong o~i d a n t .~J The thiocyanate ion forms strong complexes with several metal ions, including d10 ions. It is generally assumed that pseudohalide ions, with their reducing properties, cannot exist in solutions containing thallium(III), since they undergo an immediate oxidation. For thiocyanate ion, the following reaction was found to occur:4 (1) About 20 years ago, the kinetics of this redox reaction was studied by means of polarography.5 On the basis of the dependence of the rate of the reaction on the concentrations of thallium(II1) and thiocyanate, Treindl and Fico suggested5 that the reaction proceeds through a short-lived intermediate with the composition T1-(SCN)TP+. Species like T12(SCN)n6n were also proposed to exist. Recently, Gupta et a1.6 performed a similar kinetic investigation and proposed the existence of the complexes Tl(SCN)2+, T1-(SCN)2+, T12(SCN)5+, T12(SCN)24+ or (TI(SCN)}24+, and HTI(SCN);+. These proposals are surprising, since polynuclear thallium(II1) species have never been reported in aqueous solut i~n .~.~ Having in our hands the new powerful experimental technique of multinuclear NMR spectrampy and encouraged by our recent discovery that, contrary to the accepted ~p i n i o n ,~ thallium( 111) cyanide complexes in aqueous solution exist and are extremely strong and stable,' we wanted to see if we could find and characterize the suggested5v6 (or any other) reaction intermediates in the course of the redox reaction between Tl(II1) and thiocyanate.The previous authors5 used TlC13 as a source of thallium(II1).Chloride ions complicate the chemistry of the system, since their presence may cause formation of several typm of complexes, such as TIC1,' -n*8 Tl(CN),Cl,'-"-n*9 etc. In order to simplify the chemical system, our starting solutions contained only Tl(C104)3 in HC104. Experimental SectionMaterials. The stock solution of 1 M TI(CI0,)3 was obtained by anodic oxidation of TIC104.10 I3C-enriched NaSCN was prepared by mixing, grinding, and gently fusing 1 mmol of I3C-enriched NaCN (Cambridge Isotope Laboratories, Woburn, MA) with 1 mmol of sulfur and 0.25 mmol of NaSCN. After continued heating in order to remove excess sulfur, the melt was cooled and dissolved in water. This procedure resulted in NaSCN enriched 75% in "C. l5N-enriched NaSCN (99%) was purchased from MSD Isotopes, Montreal. The stock solution of thallium(II1) was then mixed at 0 OC with the appropriate stock solution of sodium thiocyanate (0.040 M) and 3 M HCIO,, giving the following concentrations at the beginning of the reaction: [TI(III)] = 0.17 M, [SCN-] = 0.025 M, and [H+] = 0.52 M. The ionic strength was kept approximately constant by means of LiCIO, ([Li+] + [H' ] -3 M).AM~YS~S. The determination of the concentrations of TI(1). TI(III), and H+ was performed as before? Sulfate was determined as BaSO,(s). NMR Measurements. All NMR measurements were performed on a Bruker AM400 spectrometer at a probe temperature of 0 (i0.5) OC. 20?l NMR spectra were recorded at 230...

[TlIII(dota)]−: An Extraordinarily Robust Macrocyclic Complex

“…The (proton coupled) 205 Tl NMR spectrum of [Tl­(dota)] − shows a single signal at δ = 2218 ppm with a chemical shift very similar to that measured earlier for [Tl­(edta)] − (δ = 2301 ppm) . The positions of these signals are within the range expected for a Tl­(III) compound, whereas the lack of a Tl­(I) signal at around δ = 0 ppm indicates that there was no reduction of Tl­(III) to Tl­(I) during the preparation of [Tl­(dota)] − . Furthermore, the absence of a signal due to Tl 3+ aq at δ = 2039 ppm confirms the full complexation of the metal ion by the ligand.…”

“…24 The positions of these signals are within the range expected for a Tl(III) compound, whereas the lack of a Tl(I) signal at around δ = 0 ppm indicates that there was no reduction of Tl(III) to Tl(I) during the preparation of [Tl(dota)] − . 47 Furthermore, the absence of a signal due to Tl 3+ aq at δ = 2039 ppm confirms the full complexation of the metal ion by the ligand. The broad 205 Tl NMR signal observed at 298 K (w 1/2 = 2200 Hz) did not show multiplicity caused by 1 H spin−spin coupling to the protons of the dota ligand, which is likely related to the fluxional behavior of the complex (see below).…”

Complex Formation in the Ternary System Tl(III)−CN-−Cl-in Aqueous Solution. A205Tl NMR Study

“…Thallium(III) chlorides and bromides have been investigated and known for decades, and much work has been done in this area. − Very recently, we determined the structures of the thallium(III) halide (Cl, Br) and thallium(III) cyanide complexes in aqueous solution . We also studied ligand and electron exchange reactions of thallium(III) in aqueous solution. − For the Tl 3+ −Cl - and Tl 3+ − Br - systems we proposed an unusual ligand exchange mechanism where two positively charged complex ions, for example, Tl 3+ (aq) and TlX 2+ (aq), form an activated binuclear complex, [(H 2 O) 5 Tl−X−Tl(OH 2 ) 5 ] 5+ . , A similar “complex-to-complex” exchange mechanism dominates also for the corresponding cyanide exchange reactions. , In order to gain a deeper understanding of this unusual reaction mechanism, we decided to investigate ligand exchange reactions in aqueous solutions containing ternary Tl(III)−CN−Cl complexes . However, the latter complexes have not been reported in the literature.…”