Complexes in solution formed by an organometallic tripodal oxygen ligand with monovalent, divalent and trivalent metal ions: an electrospray mass spectrometric study

Rapid analysis of single droplets of lanthanide-ligand solutions by electrospray ionization mass spectrometry using an induction-based fluidics source

Electrospray ionization mass spectra of lanthanide coordination complexes were measured by launching nanoliter-sized droplets directly into the aperture of an electrospray ionization mass spectrometer. Droplets ranged in size from 102 nL to 17 nL, while metal concentrations were 293 μM. The sample solution was delivered to a source capillary by a nanoliter dispenser at a rate of 21 nL/s, and droplets were ejected from the capillary by pulsing a potential onto the capillary. The end of the capillary was situated in front of the mass spectrometer and aimed directly at the aperture. The period and power of the electrical pulse was controlled by a digital energy source. The intensity of the extracted ion time profiles from the experiment showed reproducible production of lanthanide nitrato-anion complexes (Ce, Tb, and Lu). The integrated ion intensities of the complexes were reproducible, having relative standard deviations on the order 10% for anions, and 10-30% for cations. The integrated ion intensities were proportional to the droplet size, and the response was linear from about 100 to 650 pmol. However, the intercept is not zero, indicating a nonlinear response at lower analyte quantities or droplet sizes. Cation complexes were generated in separate experiments that corresponded to lanthanide nitrate ion pairs coordinated with the separations ligand octyl,phenyl,(N,N-diisobutylcarbamoyl)methylphosphine oxide (CMPO). Experiments showed a preference for formation of CMPO complexes with Ln(3+) having larger ionic radii. The relative standard deviation values of the cation abundance measurements were somewhat higher for the more highly coordinated complexes, which are also less stable. The mass spectral quality was high enough to measure the ratios of the minor isotopic ions to a high degree of accuracy. The approach suggests that the methodology has utility for analysis of solutions where the sample quantity is limited, or where the sampling efficiency of a normal ESI source is limiting on account of hazards derived from the sample solution.

Mid-infrared vibrational spectra of discrete acetone-ligated cerium hydroxide cations

Cerium(iii) hydroxy reactive sites are responsible for several important heterogeneous catalysis processes, and understanding the reaction chemistry of substrate molecules like CO, H(2)O, and CH(3)OH as they occur in heterogeneous media is a challenging task. We report here the first infrared spectra of model gas-phase cerium complexes and use the results as a benchmark to assist evaluation of the accuracy of ab initio calculations. Complexes containing [CeOH](2+) ligated by three- and four-acetone molecules were generated by electrospray ionization and characterized using wavelength-selective infrared multiple photon dissociation (IRMPD). The C[double bond, length as m-dash]O stretching frequency for the [CeOH(acetone)(4)](2+) species appeared at 1650 cm(-1) and was red-shifted by 90 cm(-1) compared to unligated acetone. The magnitude of this shift for the carbonyl frequency was even greater for the [CeOH(acetone)(3)](2+) complex: the IRMPD peak consisted of two dissociation channels, an initial elimination of acetone at 1635 cm(-1), and elimination of acetone concurrent with a charge separation producing [CeO(acetone)](+) at 1599 cm(-1), with the overall frequency centered at 1616 cm(-1). The increasing red shift observed as the number of acetone ligands decreases from four to three is consistent with transfer of more electron density per ligand in the less coordinated complexes. The lower frequency measured for the elimination/charge separation process is likely due to a combination of: (a) anharmonicity resulting from population of higher vibrational states, and (b) absorption by the initially formed photofragment [CeOH(acetone)(2)](2+). The C-C stretching frequency in the complexes is also influenced by coordination to the metal: it is blue-shifted compared to bare acetone, indicating a slight strengthening of the C-C bond in the complex, with the intensity of the absorption decreasing with decreasing ligation. Density functional theory (DFT) calculations using three different functionals (VWN, B3LYP, and PBE0) were used to predict the infrared spectra of the complexes. Calculated frequencies for the carbonyl stretch are within 40 cm(-1) of the IRMPD of the three-acetone complex measured using the single acetone loss, and within 60 cm(-1) of the measurement for the four-acetone complexes. The B3LYP functionals provided the best agreement with the measured spectra, with the VWN modestly lower and PBE0 modestly higher. The C-C stretching frequencies calculated using B3LYP are higher in energy than the measured values by approximately 30 cm(-1), and reproduce the observed trend which shows that the C-C stretching frequency decreases with increasing ligation. Agreement between C-C frequency and calculation was not as good using the VWN functional, but still within 70 cm(-1). The results provide an evaluation of changes in the acceptor properties of the metal center as ligands are added, and of the utility of DFT for modeling f-block coordination complexes.

Investigating Bidentate and Tridentate Carbamoylmethylphosphine Oxide Ligand Interactions with Rare-Earth Elements Using Electrospray Ionization Quadrupole Ion Trap Mass Spectrometry

Electrospray ionization (ESI) quadrupole ion trap mass spectrometry (QIT-MS) and collisionally activated dissociation (CAD) were used to evaluate the rare-earth binding properties of two hydrophobic carbamoylmethylphosphine oxide (CMPO) ligands, the normal bidentate variety, (t-BuC6H4)2P(O)CH2C(O)N(i-Bu)2 (A), a new potentially tridentate extractant, (t-BuC6H4)2P(O)CH[CH2C(O)N(i-Bu)2]C(O)N(i-Bu)2 (B), and tributyl phosphate. The mass spectral results obtained from analysis of 1% HNO3/methanol solution containing the ligands and dissolved lanthanide salts reveal that the favorable stoichiometries of the ligand/metal/nitrate complexes are 2:1:2 for the bidentate ligand A, 1:1:2 for the tridentate ligand B, and 3:1:2 for the monodentate tributyl phosphate. These observed stoichiometries correlate with the number of available binding sites on each ligand as well as with potential steric effects. Energy-variable collisionally activated dissociation experiments showed that for the 2:1:2 complexes involving ligand A or B, as the ionic radius of the bound metal decreased, the removal of nitric acid required less energy and resulted in less extensive spontaneous solvent coordination. This experimental trend suggests that, as the ionic radius of the lanthanide ion decreases, a pair of the carbamoylmethylphosphine ligands is able to more completely solvate the bound metal ion thereby weakening the nitrate-metal interaction.

Characterization of zirconium complexes of interest in spent nuclear fuel reprocessing by electrospray ionization mass spectrometry

Liquid-liquid extraction of zirconium, one of the most important fission products, was followed using electrospray ionization mass spectrometry under conditions simulating reprocessing of nuclear spent fuel. Zr(IV) can precipitate from the organic phase after extraction by dibutylphosphoric acid (HDBP), the most common degradation product of tributylphosphate (TBP) radiolysis. Different complexes were detected with electrospray used in positive or negative ion modes, according to the extraction conditions such as the ligand/metal ratio. Stoichiometry of the Zr(IV) complexes was determined by combining isotopic labeling [H(15)NO(3)] of the aqueous phase in the extraction system and tandem mass spectrometry experiments. These results were compared with the species observed using other techniques reported in the literature. The mechanisms of ionization/desorption of these complexes are proposed depending on the organic ligand character (neutral (L) such as TBP, or acidic (HL') such as HDBP), and the ionization mode used. Copyright 2000 John Wiley & Sons, Ltd.