CAPITULO IV REGIMEN DISCIPLINARIO
DE LA DIRECCIÓN, LA ADMINISTRACIÓN Y EL CONTROL
interactions, we sought to prepare homoleptic uranium(IV) complexes with fluorinated diarylamide ligands. The proteo-complex, U[N(C6H5)2]4, was reported by Edelstein and co-workers.36 In the
reported synthesis, complex U[N(C6H5)2]4 was prepared by salt metathesis reaction from UCl4 in
low yield or through transamination from U2[N(C2H5)2]8 in 80% yield. However, the precursor
U2[N(C2H5)2]8 was only obtained in 30% yield.37 Our initial attempts of salt metathesis reactions
between UCl4 and K[N(C6F5)2](Et2O) resulted only in inseparable mixtures of “-ate” complexes
retaining one or two chloride ligands in a variety of solvents, including THF, Et2O and toluene. To
avoid formation of unwanted “-ate” complexes, we chose UI4(Et2O)238 instead of UCl4 as our
starting material for the salt metathesis reactions. This choice was rationalized based on the low tendency of alkali metal iodides to form “-ate” complexes. Thus, single-step salt metathesis reactions between UI4(Et2O)2 and 4 equiv of the corresponding potassium amide salts carried out
in Et2O readily afforded uranium(IV) tetrakis(diarylamide) complexes (Figure 2.3.1) including
U[N(C6H5)2]4, U[N(C6H5)ArF]4 (ArF = 3,5-di(trifluoromethyl)phenyl), U[N(C6F5)2]4 (2.1) and
U[N(C6F5)(C6H5)]4 (2.2) in good isolated yields.39
A comparison of X-ray molecular structures for U[N(C6H5)2]4, U[N(C6F5)(C6H5)]4 (2.2) and
U[N(C6F5)2]4 (2.1) revealed impacts of C−F→U interactions on the coordination geometries. The
local geometry of the UN4 core in a uranium(IV) tetrakis(amide) complex can be represented by τ4
notation,40
where α and β are the largest and second largest N−U−N angles in the complex. τ4 values of 0
and 1 are expected for four-coordinate metal centers in square planar coordination geometry and tetrahedral coordination geometries, respectively.
Figure 2.3.2 Comparison of τ4 values calculated for UN4 coordination geometries of
representative homoleptic UIV amide complexes.
The coordination geometries at mid- to low valent uranium ions are typically dominant by inter- ligand repulsion,41 leading to τ4 values close to 1. For example, a τ4 value of 0.970 was calculated
for the sterically congested complex, U[N(SiMe3)2]4 (Figure 2.3.2).42 The X-ray molecular
structure of U[N(C6F5)(C6H5)]4 (2.2) revealed a UN4 core with τ4 = 0.744, showing more deviation
from an ideal tetrahedral geometry (τ4 = 1) than U[N(C6H5)2]4 (τ4 = 0.808).36 Four short
intramolecular F→U contacts (2.70−2.73 Å) between the U4+ cation and four ortho-fluorine atoms
in each ligand were identified in the X-ray molecular structure of 2.2. More interestingly, a τ4 value
of 0.082 was calculated for 2.1, indicating the planarity of its UN4 coordination geometry. This
unique pseudo-square planar geometry of the UN4 core in 2.1 was rationalized by the presence
between the U4+ cation and ortho-fluorine atoms of two −N(C6F5)2 ligands were observed at F→U
distances of 2.6480(11) Å and 2.5989(11) Å. In order to accommodate for the short F→U distances, the U−N bonds, which were associated with the N−C−C−F chelates rings, were about 0.10 Å longer than U−N bonds that did not include the chelate interactions. This unique coordination geometry of 2.1 attracted attention from other researchers. Performing shape analysis43 on the UN4F4 coordination polyhedron in 2.1, Alvarez and co-workers recently pointed
out that the stereochemistry of 2.1 was an unprecedented example for eight-coordinate complexes, midway along a distortion path from a hexagonal bipyramid to a gyrobifastigium (Figure 2.3.5).44
Figure 2.3.3 Thermal ellipsoid plot of 2.1 at the 30% probability level as viewed from the top and side. Selected bond length (Å) and angles (deg): U(1)–N(1) 2.387(2), U(1)–N(2) 2.370(2), U(1)–N(3) 2.2781(17), U(1)–F(1) 2.6480(11), U(1)–F(2) 2.5989(11); N(1)–U(1)– N(3) 95.76(5), N(2)–U(1)–N(3) 84.24(5), N(1)–U(1)–F(1) 63.43(3), N(2)–U(1)–F(2) 64.21(3).
Figure 2.3.4 Thermal ellipsoid plot of 2.2 at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond length (Å) and angles (deg): U(1)–N(1) 2.313(2), U(1)– N(2) 2.278(2), U(1)–N(3) 2.286(2), U(1)–N(4) 2.308(3), U(1)–F(1) 2.7035(16), U(1)–F(2) 2.7502(17), U(1)–F(3) 2.7221(17), U(1)–F(4) 2.7347(18); N(1)–U(1)–N(2) 106.31(8), N(1)– U(1)–N(3) 103.25(8), N(1)–U(1)–N(4) 123.15(9), N(2)–U(1)–N(3) 123.46(9), N(2)–U(1)– N(4) 98.62(9), N(3)–U(1)–N(4) 103.85(9).
Figure 2.3.5 Coordination environment at the UIV center in U[N(C
6F5)2]4 (2.1) along the
distortion path between a hexagonal bipyramid and a gyrobifastigium.
The persistence of C−F→UIV interactions in 2.1 and 2.2 were evident by solution 19F
NMR spectroscopy. 19F NMR spectrum of 2.1 in toluene-d
8 at 300 K exhibited three resonances, suggestive of an equivalent environment for all pentafluorophenyl rings in the four ligands. The resonances at −156.0 and −164.3 ppm were assigned to the para-, meta-fluorine resonances,
respectively. The ortho-fluorine resonance was identified at −276.8 ppm with a FWHM (Full Width at Half Maximum) measured at 475 Hz. As a comparison, the 19F NMR resonances for the ortho-, meta- and para-fluorine atoms in the protonated ligand, HN(C6F5)2, were observed at −154.2,
−164.5 and −165.5 ppm, respectively. The broadness and chemical shift of the ortho-fluorine resonances for 2.1 indicated the proximity of ortho-fluorine atoms to the paramagnetic UIV(5f2)
center in solution. Similarly, three resonances in 2:1:2 ratio were observed in the 19F NMR
spectrum of 2.2 collected in toluene-d8, including a broad ortho-fluorine resonance at −303.3 ppm with FWHM of 313 Hz. In contrast, the ortho-hydrogen resonance appeared at 7.45 ppm in 1H
NMR spectrum of 2.2, together with meta- and para-hydrogen resonances at 7.25 and 5.07 ppm.
The chemical shifts of 19F NMR and 1H NMR resonances observed for 2.2 were consistent with
direct F→U contacts but not U−H contacts in solution.
Figure 2.3.6 Synthesis of 2.3.
Complexes of uranium(III) are typically more reactive than their uranium(IV) counterparts due to the large negative reduction potentials of U3+cations.45 C−F bond activation has often
been observed between UIII complexes and organofluorine moieties.46,47 Surprisingly, we were
able to isolate a red UIII complex, UIII[N(C6F5)2]3(thf)2 (2.3), that bore multiple C−F→UIII
interactions. Complex 2.3 was prepared through a salt metathesis reaction between UI3 and 3
fluorine atoms in each ligand were measured at 2.8131(20) Å, 2.9279(12) Å and 2.7843(12) Å in the solid-state molecular structure of 2.3 (Figure 2.3.7). The persistence of C−F→UIII interactions
in THF solution was evident by the paramagnetically shifted ortho-fluorine resonance at –302.80 ppm (FWHM 496 Hz) in the 19F NMR spectrum of 2.3. This compound was found to be unstable
at ambient temperature and decomposed readily into unidentified species in solution over the course of one week.
Figure 2.3.7 Thermal ellipsoid plot of 2.3 at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond length (Å) and angles (deg): U(1)–N(1) 2.4252(17), U(1)–N(2) 2.4401(16), U(1)–N(3) 2.4367(16), U(1)–O(1) 2.5101(14), U(1)–O(2) 2.4700(14), U(1)–F(1) 2.7843(12), U(1)-F(2) 2.9279(12), U(1)-F(3) 2.8131(12); N(1)–U(1)– N(2) 118.98(6), N(1)–U(1)–N(3) 132.45(6), N(2)–U(1)–N(3) 107.84(6), O(1)–U(1)–O(2) 154.12(5).