HMNTA Complexes of Tetravalent Metal Ions: On the Roles of Carbonyl Oxygen and Amine Nitrogen in the Stabilization of Gas-Phase M(HMNTA) Complexes.

Gas-phase tetrapositively charged M(HMNTA) (M = Zr, Hf, Th, and U) ions were generated via electrospray ionization of the M(ClO) and ,,',',″,″-hexamethylnitrilotriacetamide (HMNTA) mixtures in acetonitrile. In these complexes, the Zr, Hf, Th, and U metal centers are coordinated by two neutral HMNTA ligands forming antitriangular prism geometry on the basis of DFT calculations. Bonding analysis reveals that the M center is stabilized by six carbonyl oxygen atoms, while the interactions between M and two central amine nitrogen atoms are negligible. This is further confirmed by the calculation results of two tetrapositive model complexes without either central amine nitrogen or carbonyl oxygen atoms, indicating the central nitrogen atom of HMNTA is not necessary in forming tetrapositive metal complexes that can be stabilized in gas phase. Collision-induced dissociation of Zr(HMNTA), Hf(HMNTA), and Th(HMNTA) shows the formation of similar charge reducing products with the oxidation state of metal retaining IV whereas ions with other oxidation states were observed for the fragmentation products of U(HMNTA).