NMR studies on UO 2+2 complexes with pyridoxal

Abstract

The interaction of pyridoxal with dioxouranium(VI) ions has been studied by 1H t001 1H NMR a chemical shifts [δ/ppm) of free Pyridoxal hydrochloride and UO 2 acetate containing solutions in D 2O at pH 3.25. Compound C-6-H C-4′-H 5′-CH 2 2′CH 3 −CH 3 acet. Pyridoxal 8.10, 1H 6.76, 1H 5.26, 2H 2.61, 3H Pyridoxal + UO 2 acetate 1:0.5 8.04, 1H 6.78, 1H 5.25, 2H 2.68, 3H 2.20 Pyridoxal + UO 2 acetate 1:1 8.02, 1H 6.81, 1H 5.27, 2H 2.77, 2H 2.23 Pyridoxal + UO 2 acetate 1:1.5 8.00, 1H 6.81, 1H 5.27, 2H 2.79, 3H 2.27 Pyridoxal + UO 2 acetate 1:2 7.99, 1H 6.83, 1H 5.27, 2H 2.82, 3H 2.34 Δ ppm = −0.11 +0.07 +0.01 +0.21 a 1H nmr chemical shifts are measured downfield from TMS, using dioxane as an internal standard. t002 13C NMR b chemical shifts (δ/ppm) of free Pyridoxal hydrochloride and UO 2 acetate containing solutions in D 2O pH 3.25. Compound C-3 C-2 C-4 C-5 C-6 C-4′ C-5′ C-2′ Pyridoxal 150.4 144.4 140.0 138.3 125.1 99.0 70.3 14.8 Pyridoxal + UO 2acetate 1:1 147.2 142.1 138.1 122.9 99.4 70.9 14.8 Pyridoxal + UO 2acetate 1:2 146.5 141.3 138.1 123.3 99.3 70.9 14.8 Δ ppm = −3.9 −3.1 −1.9 −0.2 −1.8 +0.3 +0.6 b 13C nmr chemical shifts are measured downfield from TMS, using dioxane as an internal standard. t003 1H NMR c chemical shifts (δ/ppm) of free Pyridoxal hydrochloride and UO 2 acetate containing solutions in CD 3OD. Compound C-6-H C-4′-H 5′-CH 2 2′-CH 3 −CH 3 acet. Pyridoxal 8.35, 1H 6.45, 1H 5.30, 2H 2.72, 3H Pyridoxal + UO 2acetate 1:1 8.00, 1H 6.52, 1H 5.21, 2H 2.92, 3H 2.33 Pyridoxal + UO 2acetate 1:2 8.00, 1H 6.55, 1H 5.23, 2H 2.96, 3H 2.45 Δ ppm = −0.35 +0.10 −0.07 +0.24 c 1H nmr chemical shifts are measured downfield from TMS used as an internal standard t004 13C NMR d chemical shifts (δ/ppm) of free Pyridoxal hydrochloride and UO 2 acetate containing solutions in CD 3OD. Compound C-3 C-2 C-4 C-5 C-6 C-4′ C-5′ C-2′ Pyridoxal 151.13 145.13 141.01 140.28 126.20 105.92 71.14 14.74 Pyridoxal + UO 2acetate 1:1 148.50 142.70 140.15 122.49 170.00 71.07 15.49 Pyridoxal + UO 2acetate 1:2 148.53 142.83 140.18 122.41 107.10 71.13 15.53 Δ ppm = −2.60 −2.30 −0.83 −0.10 −3.79 +1.18 −0.01 +0.79 d 13C nmr chemical shifts are measured downfield from TMS used as an internal standard. ▪ (VI)/pyridoxal system in aqueous and methanol solutions. NMR spectra in aqueous solutions. Tables I and II show the 1H and 13C NMR chemical shifts of free pyridoxal and uranyl acetate/pyridoxal solutions at varying molar ratios. The presence of uranyl acetate in the aqueous solutions containing pyridoxal (PL) at pH 3.25 causes changes of the proton and carbon chemical shifts which may indicate the direct involvement of uranyl ion in the binding to the ligand. The major chemical shifts variations are observed for 2′-CH 3, C4′-H and c-6-H protons (Table I) and C-2, c-3, C-6 and C-4 carbons (Table II). Since such carbons were found to be sensitive on the deprotonation process of the phenolic group C-3-OH [7] it is conceivable that uranyl ion binds the pyridoxal molecule via the C-3-O − donor. The 1H as well as 13C NMR spectra of the solutions at different UO 2+ 2/PL molar ratios indicate that the major species formed at this pH is the equimolar complex. The hemiacetal form, under which pyridoxal exists both in aqueous and methanol solutions, appears to be preserved in the complexes ligand molecule since no variation of the chemical shifts is observed for C-4′ and C-5 carbons upon metal binding. NMR spectra in methanol solutions. Tables III and IV report the 1H and 13C chemical shifts for the free pyridoxal and its methanol solutions uranyl acetate. The chemical shifts variation of the C-6-H and 2′-CH 3 protons and C-2 and C-3 carbons upon metal ion binding to pyridoxal are quite similar to those found in aqueous solutions. This could indicate the same C-3-O − binding of pyridoxal to UO 2+ 2 in methanol solution as well.