Complexation of Ln3+with Pyridine-2,6-dicarboxamide: Formation of the 1:2 Complexes in Solution and Gas Phase

Electrospray ionization of LnCl3and tetramethylpyridine-2,6-dicarboxamide (TMPDA) mixtures in methanol generated abundant tripositively charged Ln(TMPDA)23+ions with an unprecedented 1:2 stoichiometry in the gas phase. Different from the very common 1:3 Ln(L)33+(L = TMGA, tetramethyl glutaramide; TMOGA, tetramethyl-3-oxaglutaramide; TMTDA, tetramethyl-3-thiodiglycolamide) complexes, Ln3+is coordinated by four Ocarbonyland two Npyridineatoms from two perpendicular TMPDA ligands based on the DFT calculations. The observation of 1:2 Ln(TMPDA)23+in the gas phase is due to the desolvation of Ln(TMPDA)2(MeOH)23+that is predominant in the methanol solution of LnCl3and TMPDA according to the extended X-ray absorption fine structure analysis, and the 1:3 Ln(TMOGA)33+and Ln(TMGA)33+complexes observed in the gas phase also parallel the speciation in relevant solutions. Such a difference in solution speciation is most likely a result of the unfavorable steric effect caused by the pyridine ring of TMPDA.