Synthesis, lanthanide coordination chemistry, and liquid-liquid extraction performance of CMPO-decorated pyridine and pyridine N-oxide platforms

Syntheses for a set of new ligands containing one or two carbamoylmethylphosphine oxide (CMPO) fragments appended to pyridine and pyridine N-oxide platforms are described. Molecular mechanics analyses for gas phase lanthanide-ligand interactions for the pyridine N-oxides indicate that the trifunctional NOPOCO molecules, 2-{[Ph₂P(O)][C(O)NEt ₂]C(H)}C₅H₄NO (7) and 2-{[Ph ₂P(O)][C(O)NEt₂]CHCH₂}C₅H ₄NO (8), and pentafunctional NOPOP′O′COC′O′ molecules, 2,6-{[Ph₂P(O)][C(O)NEt₂]C(H)}₂C ₅H₃NO (9) and 2,6-{[Ph₂P(O)][C(O)NEt ₂]CHCH₂}₂C₅H₃NO (10), should be able to adopt, with minimal strain, tridentate and pentadentate chelate structures, respectively. As a test of these predictions, selected lanthanide coordination chemistry of the N-oxide derivatives was explored. Crystal structure analyses reveal the formation of a tridentate NOPOCO chelate structure for a 1:1 Pr(III) complex containing 7 while 8 adopts a mixed bidentate/bridging monodentate POCO/NO binding mode with Pr(III). Tridentate and tetradentate chelate structures are obtained for several 1:1 complexes of 9 while a pentadentate chelate structure is observed with 10. Emission spectroscopy for one complex, [Eu(9)(NO₃)₃], in methanol, shows that the Eu(III) ion resides in a low-symmetry site. Lifetime measurements for methanol and deuterated methanol solutions indicate the presence of four methanol molecules in the inner coordination sphere of the metal ion, in addition to the ligand, with the nitrate anions most likely dissociated. The solvent extraction performance of 7-10 in 1,2-dichloroethane for Eu(III) and Am(III) in nitric acid solutions was analyzed and compared with the performance of 2,6-bis(di-n-octylphosphinoylmethyl)pyridine N-oxide (TONOPOP′O′) and n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (OPhDiBCMPO) measured under identical conditions.