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)­NEt2]­C­(H)}­C5H4NO (7) and 2-{[Ph2P­(O)]­[C­(O)­NEt2]­CHCH2}­C5H4NO (8), and pentafunctional NOPOP′O′COC′O′ molecules, 2,6-{[Ph2P­(O)]­[C­(O)­NEt2]­C­(H)}2C5H3NO (9) and 2,6-{[Ph2P­(O)]­[C­(O)­NEt2]­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.本文报道了一类新型配体的合成方法：该类配体含有一个或两个氨基甲酰甲基氧化膦（carbamoylmethylphosphine oxide, CMPO）片段，接枝于吡啶及吡啶氮氧化物（pyridine N-oxide）骨架之上。 针对吡啶氮氧化物配体与气相中镧系元素的相互作用开展了分子力学分析，结果表明，三官能团NOPOCO类分子——2-{[Ph₂P(O)][C(O)NEt₂]C(H)}C₅H₄NO（化合物7）与2-{[Ph₂P(O)][C(O)NEt₂]CHCH₂}C₅H₄NO（化合物8），以及五官能团NOPOP′O′COC′O′类分子——2,6-{[Ph₂P(O)][C(O)NEt₂]C(H)}₂C₅H₃NO（化合物9）与2,6-{[Ph₂P(O)][C(O)NEt₂]CHCH₂}₂C₅H₃NO（化合物10），可分别以极小的应变能采取三齿螯合与五齿螯合的结构构型。 为验证上述预测，本研究对这类吡啶氮氧化物衍生物的镧系配位化学进行了探索。 晶体结构分析显示，与配体7形成的1:1三价镨（Pr(III)）配合物中，配体采取三齿NOPOCO螯合结构；而配体8与三价镨形成的配合物中，配体则以双齿/桥联单齿的POCO/NO混合结合模式存在。 针对配体9的多款1:1配合物，分别获得了三齿与四齿螯合结构；而配体10与金属离子形成的配合物则呈现五齿螯合结构。 对配合物[Eu(9)(NO₃)₃]在甲醇溶液中的发射光谱测试结果表明，三价铕（Eu(III)）离子处于低对称性配位环境中。 对甲醇与氘代甲醇溶液的寿命测试显示，除配体之外，金属离子的内配位球中还存在四个甲醇分子，而硝酸根阴离子大概率发生了解离。 本研究还分析了配体7~10在1,2-二氯乙烷中，对硝酸体系中三价铕（Eu(III)）与三价镅（Am(III)）的溶剂萃取性能，并将其与相同实验条件下测得的2,6-双(二正辛基膦酰甲基)吡啶氮氧化物（TONOPOP′O′）与正辛基(苯基)-N,N-二异丁基氨基甲酰甲基氧化膦（OPhDiBCMPO）的萃取性能进行了对比。