Investigations of tetramethyl glutaramide ligand in the formation of stable gas-phase tetrapositive complexes with metal tetracations

Abstract

The capability of TMGA (N,N,N′,N′-tetramethyl glutaramide) to support Th4+, U4+, Zr4+ and Hf4+ as gas-phase tetrapositive complexes was investigated by electrospray ionization (ESI) mass spectrometry and density functional theory (DFT) calculations. Tetrapositive thorium complexes in the form of Th(TMGA)44+, Th(TMGA)64+ and Th(TMGA)74+ generated via ESI of the Th(ClO4)4-TMGA mixture in acetonitrile were observed. Geometry optimizations at the B3LYP level reveal the presence of three isomers for Th(TMGA)44+. The two isomers with C2 symmetry both possess two monodentate and two bidentate TMGA ligands forming six-coordinate structures. The D4 isomer, which is less stable by 12.5 kcal/mol, has an eight-coordinate structure with all the TMGA ligands bidentately bound to Th4+. A similar result was obtained when the influence of acetonitrile was considered, suggesting the C2 structure is preferred in solution. To be stabilized as a tetrapositive ion in the gas phase, there is a structural change for some Th(TMGA)44+ during ESI such that the thorium center is better protected by the ligand as the coordination number increases. Similar mechanism works in the case of Th(TMGA)64+. Th(TMGA)74+ can be considered as a Th(TMGA)64+ complex hydrogen bonded to another TMGA ligand. This is consistent with the loss of a neutral ligand to give Th(TMGA)64+ upon collision induced dissociation of Th(TMGA)74+, which is different from the behavior of Th(TMGA)64+ and Th(TMGA)44+. In contrast to the thorium case, there is no detectable U(TMGA)n4+, Zr(TMGA)n4+ and Hf(TMGA)n4+ complex in the mass spectra. According to the DFT calculations, these metal cations only form six-coordinate structures with TMGA while the eight-coordinate structures are not stable. This is also the reason why Th(TMGA)34+ and Th(TMGA)54+ are absent in the experiment.