Design and Modulation of Selectivity toward Vanadium(V) and Uranium(VI) Ions: Coordination Properties and Affinity of...

Based on the strong binding and high selectivity properties of 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H₂bihyat) for [U^VIO₂]²⁺, novel binucleating ligands (BLs) N,N',N″,N‴-((1,4-phenylenebis(oxy))bis(1,3,5-triazine-6,2,4-triyl))tetrakis(N-methylhydroxylamine) (H₄qtn), N¹,N⁴-bis(4,6-bis(hydroxy(methyl)amino)-1,3,5-triazin-2-yl)benzene-1,4-diamine (H₄pdl), and N¹,N²-bis(4,6-bis(hydroxy(methyl)amino)-1,3,5-triazin-2-yl)ethane-1,2-diamine (H₄enl) were synthesized. Binuclear complexes formed by coordination of hard metal ions with H₄qtn are thermodynamically more stable than their mononuclear analogues with H₂bihyat due to the increase in entropy accompanying the formation of more chelate rings. Reaction of either H₄qtn or H₄pdl or H₄enl with [U^VIO₂]²⁺ and [V^VO₂]⁺ resulted in the isolation of the binuclear complexes [(U^VIO₂)₂(μ-qtn)(H₂O)₄] (1), [(V^VO₂)₂(μ-qtn)][PPh₄]₂[PPh4] (2), [(U^VIO₂)₂(μ-pdl)(H₂O)₂(MeOH)₂] (3), [(V^VO₂)₂(μ-pdl)][PPh₄]₂ (4), [(U^VIO₂)₂(μ-enl)(H₂O)₄] (5), and [(V^VO₂)₂(μ-enl)][PPh₄]₂ (6). The binuclear complexes 1-6 were characterized by single-crystal X-ray diffraction analysis in solid state and by NMR and ESI-MS in solution. The comparison of the coordination ability of the BLs with either pyridine-2,6-dicarboxylic acid (H₂dipic) or H₂bihyat or CO₃^2- toward [U^VIO₂]²⁺ and [V^VO₂]⁺ was investigated by NMR and UV-vis spectroscopies and DFT theoretical calculations, revealing a superior performance of BLs. The selectivity of the BLs for [U^VIO₂]²⁺ over [V^VO₂]⁺ is decreased compared to that of H₂bihyat but increases considerably at pH > 9 values. Formation of the mixed-metal binuclear species [U^VIO₂(μ-O)V^VO₂] influences the selectivity and dynamics of the reaction of H₄qtn for [U^VIO₂]²⁺ and [V^VO₂]⁺ in aqueous solution. The results of this study provide crucial information for the ligand design and the development of stronger and more selective systems.