CAPITULO IV DE LAS SESIONES
FONDO DE EMPLEADOS ALIMENTOS TONING REGLAMENTO COMITÉ EVALUADOR DE CARTERA
and have significant impact on their applications.74 As for f-block elements, where bonding is
dominated by ionic character and a lack of directionality, the complex geometries are directed by sterics, the use of multi-dentate ligands or a combination of both.75
The Hayton group76-85, our group86-90 and others91-96 have adapted steric bulky mono-
dentate −N(SiMe3)2 ligands in synthetic uranium chemistry to provide C3‒symmetric frameworks
(Figure 3.3.1). The Cummins group demonstrated monodentate amide ligands, −N(3,5- Me2C6H3)R (R = ‒SiMe3 or adamantyl), could lead to a 3-fold geometry with an open coordination
site.97 The geometrical preferences could be attributed to relative bulkiness of ligand as well as
weak U−Cipso interactions, which saturated the coordination sphere of large cations. With smaller
cations or larger substituents, these M−Cipso interactions were not evident.98-100
Figure 3.3.1 Examples of uranium complexes with 3-fold symmetry directed by bulkyl ligands and U−Cipso interactions.
In the context of existing frameworks, we were interested in applying other non-covalent interactions, such as C‒F→Ln/An interactions, to direct the coordination geometry of f-block element complexes.47,101-104 Compared to agostic or U‒Cipso interactions, C‒F→Ln/An interactions
especially those involving C−F bonds from the ortho-position of aryl group, are more predictable.105,106 Herein, we investigated uranium(IV) coordination chemistry using a simple
Figure 3.3.2 Synthesis of 3.7-Cl, 3.7-CCPh, 3.7-OMe and 3.8.
Salt metathesis reactions of UCl4 with 3 equiv NaN(SiMe3)(C6H4F) in tetrahydrofuran
(THF) followed by recrystallization from n-pentane afforded red crystalline products of UCl[N(SiMe3)(C6H4F)]3 (3.7-Cl) in 81% yield. X-ray analysis revealed that 3.7-Cl bore a trigonal
pyramidal UN3Cl core with all –SiMe3 groups aligned around the U−Cl bond. The UIV cation was
only slightly displaced from the plane defined by the three N atoms (sum of N−U−N angles 359.09(21) °). Three short U−F contacts at an average of 2.6091(16) Å were observed trans to the U‒Cl bond, providing an overall seven-coordinate UIV cation. Viewed along the U‒Cl bond,
the three F→U interactions eclipsed the U‒N bonds. Complex 3.7-Cl was observed to be C3
symmetric in solution by 1H and 19F NMR spectroscopies. The presence of a significantly shifted
and broadened resonance in the 19F NMR spectrum (‒478.3 ppm for 3.7-Cl, compared to ‒135.4
ppm of HN(SiMe3)(C6H4F) in C6D6) supported the persistence of strong C−F→U interactions in
solution. A red coloration was observed for 3.7-Cl in the solid-state structure and in non- coordinating solvent including n-pentane, toluene and fluorobenzene. Upon dissolving in THF, a color change to yellow was noted for 3.7-Cl. This color change likely originated from the association of THF to metal center by partially displacing C−F→U/Ln interactions to modulate electronic transitions within the amide ligands. This process was readily reversed by de-solvation
of 3.7-Cl through application of reduced pressure.
When 4 equiv NaN(SiMe3)(C6H4F) were used for the salt-metathesis reaction with UCl4 in
THF, yellow crystals of U[N(SiMe3)(C6H4F)]4 (3.8) were isolated instead in 68% yield after
recrystallization from n-pentane. Solution 1H NMR spectra of 3.8 recorded in C6D6 revealed the
four ligands were equivalent on NMR time scale featuring four proton resonances from ‒C6H4F
group and a single resonance at ‒1.15 ppm assigned to the ‒SiMe3 group. In the solid state
molecular structure of 3.8, two of the amide ligands were involved in C−F→U interactions at an average F→U distance of 2.5361(18) Å while the other two involved in U−Cipso interactions at an
average distance of 2.908 Å. In order to adapt the five-membered ring chelation, the U‒N bonds associated with C‒F→U interactions were lengthened by ~0.06 Å compared to the U‒N bonds associated with U‒Cipso interactions. In addition, the average bond length of C‒F bonds
interacting with the UIV cation was 1.400(2) Å, lengthened by ~0.05 Å compared to the other two
C‒F bonds. Interestingly, the −N(SiMe3)(C6F5) analogue of 3.8, U[N(SiMe3)(C6F5)]4 (3.11, see
section 3.4.1),90 did not demonstrate C‒F→U interactions in its solid-state structure. Instead, the
UIV cation was surrounded by four bulky ‒SiMe3 groups. Represented in τ4 notation,48 the UN4
core in 3.8 (τ4 = 0.620)deviated more from a tetrahedral geometry (τ4 = 1)than 3.11 (τ4 = 0.822).
This is consistent with our previous observations that the presence of C‒F→UIV interactions could
alter the coordination polyhedron severely from a tetrahedral geometry.47
Attempts to access UIII complexes with −N(SiMe3)(C6H4F) ligands were unsuccessful.
Protonolysis of U[N(SiMe3)2]3 with HN(SiMe3)(C6H4F) resulted in fluorine atom abstraction
reactivity107-109 to give UF[N(SiMe3)2]3110 as the single identified product by 1H and 19F NMR
spectroscopy. Salt metathesis of UI3(THF)4 with NaN(SiMe3)(C6H4F) or reduction of 3.7-Cl with
KC8 all led to 3.8 identified by 1H NMR spectroscopy, presumably through fluorine atom
abstraction reaction followed by ligand rearrangement.
calculations were performed on the complex at the B3LYP level of theory. The gas phase optimized geometry of 3.7-Cl was in good agreement with the X-ray molecular structure, and all three F→U contacts were reproduced in the calculation. Mayer bond orders (MBO) calculated for average U‒Cl, U‒N and F→U distances were 0.93, 0.73 and 0.30, respectively. The average MBO of U‒N bonds (0.73) was larger than that in U[N(C6F5)2]4 (0.53 and 0.64 for two
unequivalent U‒N bonds in the solid state),47 consistent with a more electron-donating amide
ligand in 3.7-Cl. The MBO value of 0.30 for the U‒F interactions in 3.7-Cl was larger than 0.25 calculated for U[N(C6F5)2]4,47 suggesting stronger C−F→U interactions in 3.7-Cl.
In order to test if the C3 structural framework could accept different X-type substituents
other than Cl−, we set out to synthesize corresponding C-, O- and I- derivatives. Salt metathesis reaction of 3.7-Cl with NaC≡CC6H5 in THF led to the isolation of 3.7-CCPh. The UIV−C≡C−C6H5
moiety in 3.7-CCPh was readily identified by the C≡C stretching frequency at 2037 cm-1,
comparable to the υ(C≡C) observed at 2062 cm-1 in (C5Me5)2UIV[N(C6H5)2](C≡CC6H5)111, 2054
cm-1 in UIV[N(CH2CH2NSiMe2tBu)3](C≡CC6H5)112 and 2056 cm-1 in (C5Me5)2UIV(C≡CC6H5)2.113 X-
ray characterization revealed 3.7-CCPh to be structurally similar to 3.7-Cl with a planar UN3 core
(ΣN‒U‒N = 359.41(27) °) and three F→U distances at average of 2.641(2) Å (Figure 3.3.5). The U−C, C≡C bond lengths and U−C≡C angle were observed at 2.410(3) Å, 1.218(5) Å and 168.9(3) °, respectively. These metrics are consistent with the literature values for UIV‒C≡C linkages.111-117
In C6D6 solution, the single 19F NMR resonance was observed at −506.8 ppm for 3.7-C≡CPh,
significantly shifted (by 371 ppm) from free ligand. Similarly, treatment of NaOMe with 3.7-Cl in THF followed by recrystallization from hydrocarbon solvent led to the isolation of 3.7-OMe as red crystalline solids. Solid-state molecular structure of 3.7-OMe (Figure 3.3.6) revealed a seven- coordinate UIV center with small deviations from 3.7-Cl and 3.7-CCPh. The U‒O bond length was
at 2.044(4) Å, in the range of literature values for UIV‒OMe bond lengths from 2.0274(16) to
OMe group with the bulky ‒SiMe3 groups aligned along the C3 axis. Reaction of 3.7-Cl with NaI in
THF overnight also led to the formation of a minor (< 10%) new species identified by 1H NMR
spectroscopy. The low conversion likely resulted from the low nucleophilicity of the I− anion. As
such, this product was obtained through an alternative route. Salt metathesis reaction between NaN(SiMe3)(C6H4F) and in situ formed “UI4” (a combination of UI3(THF)3 with 0.5 equiv I2 in THF
solution) led to the formation of a red compound, UIVI[N(SiMe3)(C6H4F)]3 (3.7-I). Only one F→U
contact at 2.6102(12) Å was observed for 3.7-I in its solid-state molecular structure (Figure 3.3.7). Instead, two U‒Cipso interactions were found at 2.809 and 2.852 Å, respectively. The U‒I bond
length was measured at 3.0226(3) Å, comparable to 2.9512(8) Å in UIVI[N(SiMe3)2]3 complex.77 A
solution NMR spectrum of 3.7-I revealed a C3 symmetric solution geometry on the NMR
timescale and a single fluorine resonance at −436.0 ppm. We also attempted to synthesize compounds bearing UIV−F bonds. However, our efforts to synthesize UIVF[N(SiMe3)(C6H4F)]3,
through salt-metathesis reaction of 3.7-Cl with thallium(I) fluoride or fluorine atom abstraction with uranium(III) starting materials, all resulted in the formation of 3.8 as the major product. Similarly, the salt metathesis reactions of 3.7-Cl with alkylation reagent, such as NaCH2Ph also led to 3.8
Figure 3.3.3 Thermal ellipsoid plot of 3.7-Cl at the 30% probability level.Selected bond length (Å) and angles (deg): U(1)−Cl(1) 2.6032(6), U(1)−N(1) 2.2761(19), U(1)−N(2) 2.2843(19), U(1)−N(3) 2.2744(18), U(1)−F(1) 2.6319(13), U(1)−F(2) 2.6038(13), U(1)−F(3) 2.5916(13); N(1)−U(1)−N(2) 123.91(7), N(1)−U(1)−N(3) 117.47(7), N(2)−U(1)−N(3) 117.71(7), Cl(1)−U(1)−N(1) 93.63(5), Cl(1)−U(1)−N(2) 90.84(5), Cl(1)−U(1)−N(3) 95.10(5).
Figure 3.3.4 Thermal ellipsoid plot of 3.8 at the 30% probability level. Selected bond length (Å) and angles (deg): U(1)−N(1) 2.3446(17), U(1)−N(2) 2.3513(18), U(1)−N(3) 2.2588(17), U(1)−N(4) 2.3074(17), U(1)−F(1) 2.5409(12), U(1)−F(2) 2.5457(12); F(1)−U(1)−N(1) 66.76(5), F(2)−U(1)−N(2) 65.68(5), N(1)−U(1)−N(2) 99.13(6), N(1)−U(1)−N(3) 92.26(6), N(1)−U(1)−N(4) 138.46(6), N(2)−U(1)−N(3) 134.16(6), N(2)−U(1)−N(4) 96.70(6), N(3)−U(1)−N(4) 103.56(6).
Figure 3.3.5 Thermal ellipsoid plot of 3.7-CCPh at the 30% probability level. Selected bond length (Å) and angles (deg): U(1)−N(1) 2.293(2), U(1)−N(2) 2.284(2), U(1)−N(3) 2.286(2), U(1)−F(1) 2.6120(17), U(1)−F(2) 2.6597(17), U(1)−F(3) 2.6498(17), U(1)−C(1) 2.410(3), C(1)−C(2) 1.218(5); N(1)−U(1)−N(2) 117.14(9), N(1)−U(1)−N(3) 120.53(9), N(2)−U(1)−N(3) 121.74(9), C(2)−C(1)−U(1) 168.9(3).
Figure 3.3.6 Thermal ellipsoid plot of 3.7-OMe at the 30% probability level. Selected bond length (Å) and angles (deg): U(1)−N(1) 2.321(4), U(1)−N(2) 2.326(4), U(1)−N(3) 2.325(4), U(1)−F(1) 2.619(3), U(1)−F(2) 2.612(3), U(1)−F(3) 2.653(3), U(1)−O(1) 2.044(4); N(1)−U(1)−N(2) 120.44(13), N(1)−U(1)−N(3) 121.87(13), N(2)−U(1)−N(3) 116.66(13).
Figure 3.3.7 Thermal ellipsoid plot of 3.7-I at the 30% probability level. Selected bond length (Å) and angles (deg): U(1)−N(1) 2.2254(17), U(1)−N(2) 2.2657(17), U(1)−N(3) 2.1939(16), U(1)−I(1) 3.0226(3), U(1)−F(2) 2.6102(12); N(1)−U(1)−N(2) 104.65(6), N(1)−U(1)−N(3) 129.63(6), N(2)−U(1)−N(3) 101.95(6), N(2)−U(1)−F(2) 77.24(5).
3.4 Lewis Acid Induced Silyl-migration Reactions with Fluorinated Amide Ligands. As a