Reactions of gas-phase uranyl formate/acetate anions: reduction of carboxylate ligands to aldehydes by intra-complex hydride attack.

In a previous study, electrospray ionization, collision-induced dissociation (CID), and gas-phase ion-molecule reactions were used to create and characterize ions derived from homogeneous precursors composed of a uranyl cation (U ^VI O ₂ ²⁺ ) coordinated by either formate or acetate ligands [E. Perez, C. Hanley, S. Koehler, J. Pestok, N. Polonsky and M. Van Stipdonk, Gas phase reactions of ions derived from anionic uranyl formate and uranyl acetate complexes, J. Am. Soc. Mass Spectrom. , 2016, 27 , 1989-1998]. Here, we describe a follow-up study of anionic complexes that contain a mix of formate and acetate ligands, namely [UO ₂ (O ₂ C-CH ₃ ) ₂ (O ₂ C-H)] ^- and [UO ₂ (O ₂ C-CH ₃ )(O ₂ C-H) ₂ ] ^- . Initial CID of either anion causes decarboxylation of a formate ligand to create carboxylate-coordinated U-hydride product ions. Subsequent CID of the hydride species causes elimination of acetaldehyde or formaldehyde, consistent with reactions that include intra-complex hydride attack upon bound acetate or formate ligands, respectively. Density functional theory (DFT) calculations reproduce the experimental observations, including the favored elimination of formaldehyde over acetaldehyde by hydride attack during CID of [UO ₂ (H)(O ₂ C-CH ₃ )(O ₂ C-H)] ^- . We also discovered that MS ⁿ CID of the acetate-formate complexes leads to generation of the oxyl-methide species, [UO ₂ (O)(CH ₃ )] ^- , which reacts with H ₂ O to generate [UO ₂ (O)(OH)] ^- . DFT calculations support the observation that formation of [UO ₂ (O)(OH)] ^- by elimination of CH ₄ is favored over H ₂ O addition and rearrangement to create [UO ₂ (OH) ₂ (CH ₃ )] ^- .