Synthesis and properties of new (Phosphinoylmethyl)pyridine N-oxides

Syntheses for 2-[1-(diarylphosphinoyl)-1-(pyridin-2-yl)methyl]pyridines, (8a, b), and 2-[1-(diarylphosphinoyl)-1, 1-bis(methylpyridin-2yl)methyl] pyridines, (11a, b), (Ar = C₆H₅ and 2-CF₃C ₆H₄), based on substitution of 2-methylpyridine fragments onto the exo methylene carbon atom of 2-[(diaryl)phosphinoylmethyl]pyridine platforms, are described. N-oxidations of 8a, b and 11a, b produced the 2-[1-(diarylphosphinoyl)-1-(1-oxy-pyridin-2yl)methyl]pyridine N-oxides (5a, b) and the 2-[1-(diarylphosphinoyl)-1, 1-bis(1-oxy-methylpyridin-2-yl)methyl] pyridines (6a, b), respectively. The "short-arm" pyridine fragment of 11a, b resists N-oxidation, and the fully oxidized molecules, 2-[1-(diarylphosphinoyl)-1, 1-bis(1-oxy-methylpyridin-2-yl)methyl]pyridine N-oxides (7a, b) were not isolated. Molecular mechanics calculations for gas phase 1:1 ligand/lanthanide complexes indicated that 5a should accommodate a tridentate NO(meNO)PO coordination mode with minimal steric strain. In contrast, 7a cannot form tetradentate NO(meNO)₂PO chelates; however, tridentate binding should be accessible with minimal ligand strain. Coordination complexes of 8a, b, 5a, b, 6a, b and 11a, b with Ln(NO₃)₃ salts were isolated and a X-ray crystal structure for [Er(8a)(NO ₃)₃(MeOH)₂]·CH₂Cl ₂, revealed a monodentate Er-O=P interaction. On the other hand, complexes formed by a more symmetrical trifunctional phenylphosphino-bis-2- methylpyridine N, N, P-trioxide ligand, (meNO)₂PO*, {La[(meNO)₂PO*)](OTf)₂(MeOH)₃(H ₂O)⁺}(OTf^-) and {Pr[(meNO)₂PO* )](OTf)(MeOH)₄⁺}(OTf^-)₂, realized a tridentate coordination mode. Solvent extraction behaviors for Eu^III and Am^III in nitric acid solutions using 5a, b, 6a, b, Ph ₃PO and the parent bifunctional ligand 2-[(diphenylphosphanyl)methyl] pyridine N, P-dioxide (3a) in 1, 2-dichloroethane were assessed, and 5a, b and 6a, b were found to behave more like Ph₃PO than 3a.