A Comparative IRMPD and DFT Study of Fe³⁺ and UO₂²⁺ Complexation with N-Methylacetohydroxamic Acid

Iron(III) and uranyl complexes of N-methylacetohydroxamic acid (NMAH) have been investigated by mass spectrometry, infrared multiphoton dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations. A comparison between IRMPD and theoretical IR spectra enabled one to probe the structures for some selected complexes detected in the gas phase. The results show that coordination of Fe³⁺ and UO₂²⁺ by hydroxamic acid is of a very similar nature. Natural bond orbital analysis suggests that bonding in uranyl complexes possesses a slightly stronger ionic character than that in iron complexes. Collision-induced dissociation (CID), IRMPD, and ¹⁸O-labeling experiments unambiguously revealed a rare example of the U═O bond activation concomitant with the elimination of a water molecule from the gaseous [UO₂(NMA)(NMAH)₂]⁺ complex. The U═O bond activation is observed only for complexes with one monodentate NMAH molecule forming a hydrogen bond toward one "yl" oxygen atom, as was found by DFT calculations. This reactivity might explain oxygen exchange observed for uranyl complexes.