Coordination Structures of the Uranyl(VI)-Diamide Complexes: A Combined Mass Spectrometric, EXAFS Spectroscopic, and Theoretical Study

The coordination structures of a series of uranyl-diamide complexes, namely UO2 (TMGA)(2)(2+), UO2(TMGA)(2)(2+), and UO2(TMGA)(2)(2+) (TMGA: N,N,N',N'-tetramethylglutaramide; TMTDA: N,N,N',N'-tetramethyl-3-thiodiglycolamide; TMPDA: N,N,N',N'-tetramethylpyridine-2,6-dicarboxamide), in the gas phase and solution were studied by electrospray ionization (ESI) mass spectrometry, density functional theory (DFT) calculations, and extended X-ray absorption fine structure (EXAFS) spectroscopy. ESI of UO2Cl2-TMGA, UO2Cl2-TMTDA, and UO2Cl2-TMPDA solutions produced UO (2)(TMGA)(2)(2+), UO2(TMGA)(2)(2+), and UO2(TMGA)(2)(2+) as the major complexes in the gas phase. The gas-phase fragmentation patterns of these three complexes upon collision induced dissociation (CID) are quite similar, and the products arising from (CH3)(2)N-COcarbonyl, CH2-COcarbonyl, C-pyridine-COcarbonyl and CH2-CH2/S bond cleavages were identified, degradation patterns of diamides upon gamma irradiation in solution. DFT calculations revealed similar bidentate coordination modes of TMGA and TMTDA in the UO2(TMGA)(2)(2+) and UO2(TMGA)(2)(2+) complexes in the gas phase. For UO2(TMGA)(2)(2+), UO22+ is coordinated by two O-carbonyl and one N-pyridine from each TMPDA, which acts as a tridentate ligand. The results from EXAFS analysis indicate that the coordination structures of UO2 (TMGA)(2)(2+), UO2(TMGA)(2)(2+), and UO2(TMGA)(2)(2+)in solution are similar to those in the gas phase.