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)22+, UO2(TMTDA)22+, and UO2(TMPDA)22+ (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 UO2(TMGA)22+, UO2(TMTDA)22+, and UO2(TMPDA)22+ 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)2N-COcarbonyl, CH2-COcarbonyl, Cpyridine-COcarbonyl, and CH2-CH2/S bond cleavages were identified, which agree with the degradation patterns of diamides upon γ irradiation in solution. DFT calculations revealed similar bidentate coordination modes of TMGA and TMTDA in the UO2(TMGA)22+ and UO2(TMTDA)22+ complexes in the gas phase. For UO2(TMPDA)22+, UO22+ is coordinated by two Ocarbonyl and one Npyridine from each TMPDA, which acts as a tridentate ligand. The results from EXAFS analysis indicate that the coordination structures of UO2(TMGA)22+, UO2(TMTDA)22+, and UO2(TMPDA)22+ in solution are similar to those in the gas phase.