Conformational and structural studies of N-methylacetohydroxamic acid and of its mono- and bis-chelated uranium(VI) complexes
Résumé
The thermodynamics and kinetics of the cis/trans isomerism of N-methylacetohydroxamic acid (NMAH) and its conjugated base (NMA−) have been reinvestigated in aqueous media by 1H NMR spectroscopy. Hindered rotation around the central C–N bond due to electronic delocalization becomes slow enough on the NMR time scale to observe both rotamers in equilibrium in D2O at room temperature. By properly assigning the methyl group resonances, evidence for the prevalence of the E over the Z form is unambiguously provided [K300 = [E] / [Z] = 2.86(2) and 9.63(5) for NMAH and NMA−, respectively], closing thereby a long-lasting dispute about the most stable conformer. To that end, calculations of the chemical shifts by density functional theory (DFT), which accurately reproduced the experimental data, turned out to be a much more reliable method than the direct computation of the relative energy for each conformer. The Z ⇌ E interconversion dynamics was probed at 300 K in D2O by 2D exchange-correlated spectroscopy (EXSY), affording the associated rate constants [kZE = 9.0(2) s− 1 and kEZ = 3.14(5) s− 1 for NMAH, kZE = 0.96(3) s− 1 and kEZ = 0.10(2) s− 1 for NMA−] and activation barriers at 300 K [ΔG≠ZE = 68.0 kJ mol− 1 and ΔG≠EZ = 70.6 kJ mol− 1 for NMAH, ΔG≠ZE = 73.6 kJ mol− 1 and ΔG≠EZ = 79.2 kJ mol− 1 for NMA−]. For the first time, mono- and bis-chelated uranium(VI) complexes of NMA− have been isolated. Crystals of [UO2(NMA)(NO3)(H2O)2] and [UO2(NMA)2(H2O)] have been characterized by X-ray diffractometry, infrared and Raman spectroscopies.