Cadmium(II) Complex Formation with Selenourea and Thiourea in Solution: An XAS and ¹¹³Cd NMR Study

The complexes formed in methanol solutions of Cd­(CF₃SO₃)₂ with selenourea (SeU) or thiourea (TU), for thiourea also in aqueous solution, were studied by combining ¹¹³Cd NMR and X-ray absorption spectroscopy. At low temperature (∼200 K), distinct ¹¹³Cd NMR signals were observed, corresponding to CdL_n²⁺ species (n = 0–4, L = TU or SeU) in slow ligand exchange. Peak integrals were used to obtain the speciation in the methanol solutions, allowing stability constants to be estimated. For cadmium­(II) complexes with thione (CS) or selone (CSe) groups coordinated in Cd­(S/Se)­O₅ or Cd­(S/Se)₂O₄ (O from MeOH or CF₃SO₃^–) environments, the ¹¹³Cd chemical shifts were quite similar, within 93–97 ppm and 189–193 ppm, respectively. However, the difference in the chemical shift for the Cd­(SeU)₄²⁺ (578 pm) and Cd­(TU)₄²⁺ (526 ppm) species, with CdSe₄ and CdS₄ coordination, respectively, shows less chemical shielding for the coordinated Se atoms than for S, in contrast to the common trend with increasing shielding in the following order: O > N > Se > S. In solutions dominated by mono- and tetra-thiourea/selenourea complexes, their coordination and bond distances could be evaluated by Cd K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. At ∼200 K and high excess of thiourea, a minor amount (up to ∼30%) of [Cd­(TU)_5–6]²⁺ species was detected by an upfield shift of the ¹¹³Cd NMR signal (up to 423 ppm) and an amplitude reduction of the EXAFS oscillation. The amount was estimated by fitting linear combinations of simulated EXAFS spectra for [Cd­(TU)₄]²⁺ and [Cd­(TU)₆]²⁺ complexes. At room temperature, [Cd­(TU)₄]²⁺ was the highest complex formed, also in aqueous solution. Cd L₃-edge X-ray absorption near edge structure (XANES) spectra of cadmium­(II) thiourea solutions in methanol were used to follow changes in the CdS_xO_y coordination. The correlations found from the current and previous studies between ¹¹³Cd NMR chemical shifts and different Cd­(II) coordination environments are generally useful for evaluating cadmium coordination to thione-containing or Se-donor ligands in biochemical systems or for monitoring speciation in solution.