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

Electrospray ionization of LnCl3 and 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 Ocarbonyl and two Npyridine atoms 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 LnCl3 and 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.