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.