Gas-phase structure, bonding, and fragmentation chemistry of the An (IV)-TMPDCAM complexes studied using mass spectrometry and theoretical calculation (An = Th and U).

Abstract

Pyridine-2,6-dicarboxamides (PDCAMs) exhibit a certain extraction ability for tetravalent actinide ions, but quite limited information regarding the structures and reactivities of the corresponding An4+ -PDCAMs complexes is available. Neutral diamides can form multiply charged complexes with lanthanide and actinide cations, which are well suited for gas-phase investigations using electrospray ionization (ESI) mass spectrometry in conjunction with theoretical calculation. Binary Th (TMPDCAM)3 4+ /U (TMPDCAM)3 4+ (TMPDCAM =N,N,N',N'-tetramethylpyridine-2,6-dicarboxamide) complexes were generated in the gas phase via ES) of Th (ClO4 )4 /U (ClO4 )4 and TMPDCAM mixtures in acetonitrile; collision-induced dissociation (CID) was employed to reveal their fragmentation behaviors; the structure and bonding were investigated by density functional theory (DFT) calculation. An (TMPDCAM)3 4+ (An = Th and U) tetracations dominated the ESI mass spectra of An (ClO4 )4 and TMPDCAM mixtures in acetonitrile. DFT calculations indicate that the two An (TMPDCAM)3 4+ complexes have C3 geometry, and the bonding analyses demonstrate that the thorium or uranium center interacts with both Ocarbonyl and Npyridine , but the latter is weaker. CID of Th (TMPDCAM)3 4+ generated a series of multiply charged thorium-containing products via bond cleavages of the TMPDCAM ligand, whereas U (TMPDCAM)3 4+ yielded only oxygen extraction product UO (TMPDCAM)2+ and hydrolysis product UO (OH)+ . An4+ (An = Th and U) can form stable tetrapositive complexes in the gas phase on coordination of three neutral TMPDCAM ligands. The structure and bonding analyses indicate that the two An (TMPDCAM)3 4+ complexes possess twisted tricapped trigonal prismatic geometry and the An4+ centers are coordinated by six Ocarbonyl and three Npyridine atoms while the interactions between An4+ and Ocarbonyl are stronger. The fragmentation chemistry of Th (TMPDCAM)3 4+ and U (TMPDCAM)3 4+ is quite different from each other, which reveals that the gas-phase chemistry of quadruply charged actinide-diamide complexes is affected by the metal centers with distinct properties.