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‐CF3C6H4), 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)(NO3)3(MeOH)2]·CH2Cl2, 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(H2O)+}(OTf–) and {Pr[(meNO)2PO*)](OTf)(MeOH)4+}(OTf–)2, realized a tridentate coordination mode. Solvent extraction behaviors for EuIII and AmIII in nitric acid solutions using 5a, b, 6a, b, Ph3PO 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.