Studies of Actinide Perfluorodiphenylamido Complexes Support Science Mission

Figure 1. Trisamido neodymium complex.

Fundamental chemistry of the actinide elements provides important support for the mission at Los Alamos. In contrast to other aspects of plutonium research, details of its molecular chemistry remain relatively unknown. This knowledge, however, can provide the basis for understanding important metallurgical properties relevant to the weapons mission and development of process-related chemistry and separation chemistry, as well as benefiting the Shelf Life Extension, Enhanced Surveillance, and Stockpile Stewardship Programs.

Our goal is to elucidate the fundamental chemistry of the actinides by focusing our efforts on preparation and characterization of simple, well-defined systems and comparing their molecular structure and reactivity to well studied lanthanide and transition metal analogs. The subject of this article is our work in preparing a set of actinide complexes supported by amido ligands. The first uranium amido complex was reported in 1956 by Gilman and coworkers at Iowa State College, detailing their efforts to prepare volatile uranium complexes for the Manhattan Project. Tetrakis(diethylamido) uranium was prepared by the reaction of the lithium salt of the amine with uranium tetrachloride and then isolated by distillation under reduced pressure. Other actinide amido complexes have also been reported to be volatile. The tris(silylamido) complexes of uranium, neptunium, and plutonium sublime around 60°C at 10-4­10-5 Torr.

Currently, several research groups in North America, Great Britain, and Europe are actively investigating actinide amido complexes, principally as well-defined precursor materials for synthetic chemistry. Recent interest lies in their rich reaction chemistry, including activation of molecular nitrogen. Homoleptic tetra- and tri-amido complexes are generally soluble in common organic solvents and are therefore valuable entries in the synthesis of moisture-sensitive organometallic complexes of the +4 and +3 oxidation states. With the dual goals of exploring the electrophilic chemistry of the actinide elements and also developing volatile organometallic actinide complexes for chemical vapor deposition applications, we have set out to prepare a series of Th, U, Np, and Pu complexes stabilized by perfluorodiphenylamido ligands‹N(C₆F₅)₂.

Perfluorodiphenylamido ligands have been used previously to stabilize highly electrophilic lanthanide complexes. X-ray diffraction studies have revealed that the lanthanide amido unit is quite electrophilic and readily forms weak intramolecular interactions to saturate the coordination sphere of the large lanthanide(III) ions. Of note is the (trisamido) neodymium complex (h6-C6H5Me)Nd[N(C6F5)2]3 (Fig. 1) that coordinates an h6-bound toluene solvent molecule as well as three significant Nd-F interactions (2.6 to 2.7 Å) and one weak Nd-F interaction (2.9 Å).

Figure 2. Thermal ellipsoid plot (30% probability ellipsoids) for . For clarity, the [Na(thf)₅] counter ion has been omitted. Characterization of actinide amido complexes helps to elucidate the fundamental chemistry of the actinide elements.

Recently we have synthesized several novel uranium amido complexes that have been characterized by single-crystal x-ray diffraction. The electrophilic nature of these complexes is exemplified by the reaction of UCl4 with four equivalents of NaN(C₆F₅)₂ in tetrahydrofuran (THF). Crystallization from toluene yields lime crystals of the "ate" complex , in which one chloride ligand remains coordinated to the uranium metal center to yield an anionic U(IV) complex (Fig. 2). Close intramolecular contacts between three ortho-fluorine atoms and uranium (2.6 to 2.7 Å) further testify to the electron deficiency of the uranium atom.

This result was unexpected since we hoped to synthesize the homoleptic U(IV) amido complex; however, there is precedence of the formation of "ate" complexes. In France in 1998 Ephritikhine prepared the [Li][U(NEt₂)₄Cl] analogue during his attempts to synthesize U(NEt₂)₄. The average U-N bond lengths of the two complexes are similar with [Li][U(NEt₂)₄Cl] measuring 2.37 Å and measuring 2.386 Å.

While Ephritikhine reports that several extractions of [Li][U(NEt₂)₄Cl] with ether allows for the complete removal of LiCl and the formation of U(NEt₂)₄, we find that reaction of UCl₄ with four equivalents of NaN(C₆F₅)₂ in THF conducted under more concentrated conditions does yield the desired homoleptic tetrakisamido complex U[N(C₆F₅)₂]₄. Interestingly, actinide complexes supported by less bulky amido ligands readily form dimeric (Fig. 3), or trimeric (Fig. 4) structures. The homoleptic diethylamido and dimethylamido uranium(IV) complexes are shown below.

Figure 3. Dimeric actinide amido complex

Figure 4. Trimeric actinide amido complex.

The single-crystal x-ray structure of our red tetrakisamido complex (Fig. 5) is very similar to that of the nonfluorinated diphenylamido complex. Each monomeric uranium metal center is coordinated to four nitrogen atoms in a distorted tetrahedral structure. The uranium nitrogen bond lengths of the fluorinated and non-fluorinated complexes are almost identical with averages of 2.26 and 2.27 Å, respectively. The tetrakis(perfluordiphenylamido) uranium complex also has contacts between the uranium metal center and three fluorine atoms, although they are slightly weaker than those in the "ate" complex (2.8-2.9 Å). This complex is the subject of ongoing reactivity and volatility studies.

Reaction of [UO₂Cl₂(thf)₂]₂ with four equivalents of NaN(C₆F₅)₂ in THF yields the red uranyldiamido complex UO₂(N(C₆F₅)₂)₂(thf )₂.

The single-crystal x-ray structure reveals an octahedral coordination environment around the uranium metal center. The linear uranyl unit is augmented by two trans amido ligands as well as two tetrahydrofuran solvent molecules (Fig. 6).

Figure 5. Thermal ellipsoid plot (30% probability ellipsoids) for U[N(C₆F₅)₂]₄.

Figure 6. Thermal ellipsoid plot (30% probability ellipsoids) for UO₂(N(C₆F₅)₂)₂(thf )₂.

Examples of structurally characterized uranyl amido complexes are rare. A comparison can be made between our uranyl amido complex and uranyl alkoxide complexes. The uranyl moiety in the bis(perfluoro-diphenylamido) uranyl complex is shortened, with an average U-O bond length of 1.75 Å compared to those of uranyl alkoxide complexes with typical bond lengths between 1.77-1.79 Å.

We are also attempting to prepare triamido complexes of plutonium(III) and neptunium(III). The tris(silylamido) complexes of neptunium and plutonium are the only amido complexes reported for transuranic metals. Starting from the tris(silylamido) complexes we have employed an aminolysis reaction. Reaction of three equivalents of perfluorodiphenyla-mine with the blue-black tris(silylamido) neptunium(III) complex in hexane forms an insoluble yellow product that is subsequently filtered from the solution and dried in vacuo.

Fluorine nuclear magnetic resonance (NMR) spectroscopy of the neptunium complex shows resonances that have shifted from those of free ligand; however, the data are inconclusive, and therefore complete characterization is difficult. Similar reaction of three equivalents of perfluorodiphenylamine with the orange tris(silylamido) plutonium(III) complex in hexane forms an insoluble light-purple product that is also filtered off and dried in vacuo. Attempts to grow single crystals of these transuranic complexes have been unsuccessful to date. Further NMR studies and crystallization attempts of both the neptunium and plutonium complexes will resume in PF-4 during the second quarter of FY01. Our work has extended the library of actinide complexes. Novel uranium amido complexes have been synthesized and characterized by single-crystal x-ray diffraction. The reaction chemistry of perfluorodiphenylamine with both neptunium and plutonium shows interesting initial results. We are hopeful that complete characterization of these transuranic complexes will be possible.

This article was contributed by Susan M. Oldham and Ann R Schake (NMT-5); Arthur N. Morgan III (NMT-11); and Brian L. Scott, Benjamin P. Warner, and John G. Watkin (C-SIC).

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