2. JUSTIFICACIÓN
4.3.11 Que Es Un Aditivo Para Carpetas Asfálticas
Electrochemical analysis provided insight into the possibility of accessing the different redox congeners of the naphthyl-imido uranium complexes versus Fc+/0 (Figure 3.3.9, Table 3.3.3). We previously reported the electrochemistry of 2.3, which featured reversible U(V/VI) and
U(IV/V) couples at +0.38 and –1.31 V, respectively.2
Compounds 3.4, 3.5, and 3.7 exhibited
reversible U(V/VI) couples at –0.17, +0.29, and +0.12 V. The stability of the +6 oxidation state was correlated with the Mulliken charges of the uranium(V) centers, as obtained by DFT analysis. For compounds 2.3 and 3.5 the Mulliken charges on the uranium center were similar at +1.15 and
Chapter 3 | 71
the U(V/VI) couple for compounds 2.3 and 3.5 were only 90 mV apart and at much higherpotentials than the U(V/VI) couple for 3.4, which was 550 mV shifted from compound 2.3. This
significant shift of the U(V/VI) couple in compound 3.4 implied that the +6 oxidation state was
most stabilized by the [=NC(2-naph)Ph2] imido ligand, which is surprising due to the small
structural and electronic differences between these compounds. Contrary to compounds 2.3, 3.4,
and 3.5, compound 3.7 exhibited an irreversible U(V/VI) couple with an Epa = –0.64 and an Epc =
+0.20. The E1/2 as obtained from differential pulse voltammetry exhibited a reversible U(V/VI)
couple at 0.12 V, implying that the apparent irreversibility in the cyclic voltammetry experiment was due to a large overpotential in the anodic wave.2,32
Figure 3.3.9. Cyclic voltammetry of 2.3 (red), 3.4 (orange), 3.5 (green), and 3.7 (blue) at a scan rate of 250 mV/s in CH2Cl2, with 0.1 M [
n
Bu4N][PF6] supporting electrolyte. The data for
Table 3.3.3. Reduction potentials (vs. Fc+/0) of (naphthyl)imido complexes determined by DPV measurements, compared to the previously reported trityl(imido) complex.2
E1/2 (V)
UV[=NR][N(SiMe3)2]3 Derivative U(VI/V) U(V/IV) naph 0 /naph–1 2.3, R = CPh3 0.38 – 1.31 -- 3.4-K,R = C(2-naph)Ph2 –0.17 – 1.32 – 1.71 3.5-K,R = C(2-naph)3 0.29 – 1.37 – 1.75 3.7-K,R = 2-naph 0.12 – 1.25 – 2.27
The potential of the U(IV/V) couple was similar across the series, ranging from –1.31 V to – 1.37 V in the compounds with the triaryl moiety, and –1.25 V for compound 3.7, with its simple naphthyl moiety. Because the U(VI/V) couples span 550 mV across the series, while the U(V/IV) couples only vary by 120 mV, the ΔE1/2 varied substantially. Most interestingly for compounds 2.3,
3.4, and 3.5, which are structurally similar, the ΔE1/2 for 2.3 and 3.5 are 1.69 V and 1.66 V,
respectively. However, for compound 3.4, the ΔE1/2 is much smaller at 1.15 V. In contrast, the
electrochemistry of uranium(V) imido complexes (C5Me5)2U V
[=NDipp)]X (X = Cl, Br, I, OTf, SPh, NPh2, CCPh, OPh, Me, N=CPh2), reported by Kiplinger, Graves, and coworkers, showed that
despite their markedly different structures ΔE1/2 remained relatively consistent across the series,
Chapter 3 | 73
Following the U(IV/V) feature was a second irreversible reduction feature that is only present in compounds 3.4, 3.5, and 3.7, which contained a naphthyl group in the imido ligand. Thissecond reduction process was assigned to the naphthalene0/naphthalene–1 couple. Under these electrochemical conditions, free naphthalene and 2-azidonaphthalene did not appear in the solvent window. However, in the series of complexes, the potential of the naphthalene0/naphthalene–1 process shifted to more negative potentials from compound 3.4 at – 1.71 V, to compound 3.5 at –1.75 V, to compound 3.7, which had the most shifted napthalene0/naphthalene–1 couple, at –2.27 V. The poor reversibility of this feature in all of the complexes implied that complexes with reduced naphthalene moieties are reactive on the electrochemical timescale.
Based on comparing the electrochemical data for 2.3, 3.4, and 3.5, it is reasonable to
conclude that the triaryl imido ligand is responsible for the third feature in the electrochemistry, which we have assigned as the naphthalene0/naphthalene–1 couple, since it is the only feature that does not appear in the electrochemistry of compound 2.3. To understand the electronic
structure of the complexes with a reduced naphthalene moiety better we performed DFT calculations using a 60 electron core incorporating quasi-relativistic effects for uranium and 6- 31G* basis set for all other atoms. We calculated a putative dianionic compound, [U[=NC(2- naph)Ph2][N(SiMe3)2]3]2– (3.4-2K) (Scheme 3.3.4), in the gas phase with and without potassium
counterions present, and in a dichloromethane CPCM (conductor-like polarizable continuum model) solvent field with and without potassium counterions present (Figure 3.3.10). Additionally, we calculated 3.4-2K in a dichloromethane CPCM solvent field with Grimme’s dispersion corrections. However, the addition of dispersion correction resulted in little or no changes in molecular geometry or overall electronic structure between the calculations of 3.4-2K.
Scheme 3.3.4 Monoanionic (3.4-K) and computed dianionic (3.4-2K) naphthyl imido compounds.
The optimized geometries resulting from these calculations resulted in different bond lengths and angles depending on the conditions used in the calculation (Table 3.3.4). Both optimizations performed with potassium counterions included in the coordination sphere showed shorter average U–Namide bond lengths of 2.418 and 2.485 Å, for the gas phase and CH2Cl2 CPCM
optimizations, respectively, compared with 2.577 and 2.620 Å for those optimizations performed without cations. The U–Namide bond lengths with counterions ions present matched well with the
experimental UIV–N(SiMe3)2 bond lengths in imido complexes 2
. As yet, there are no U(III) imido complexes reported for comparison; however, the larger radius of a uranium(III) ion may be responsible for the longer calculated U–Namide bond lengths. Likewise, U=N bond lengths for the
optimizations with cations present were shorter at 2.018 and 1.999 Å compared with bond lengths of 2.066 and 2.095 Å for the optimized geometries without potassium ions present. The U=N–C bond angle is near linear in the gas phase optimized geometry with no counterions present (179.1º), but bends by 6.9 degrees when counterions are added (172.2º). However, in a CH2Cl2
CPCM solvent field, the U=N–C bond angle changes by less than a degree when counterions are included.
Table 3.3.4 Calculated bond lengths (Å) and angles (º) for 3.4-2K in gas phase and CH2Cl2
CPCM (conductor-like polarizable continuum model) conditions with and without potassium counterions.
Chapter 3 | 75
Optimization Conditions U–Navg U=N U=N–C
Gas Phase, no cations 2.577 2.066 179.1
Gas Phase, K cations 2.418 2.018 172.2
CH2Cl2 CPCM, no cations 2.620 2.095 177.0
CH2Cl2 CPCM, K cations 2.485 1.999 176.3
We projected spin density plots from the optimized geometries and found that when counterions were excluded from the computed dianionic structure, minimal spin density was located on the naphthalene moiety with +0.03 in the case of a CH2Cl2 CPCM solvent field and
+0.05 in the gas phase. In these cases spin densities of +3.07 and +3.08 were located on the uranium cation. However, with potassium counterions present significant spin density (+0.95) is located on the naphthalene moiety in a CH2Cl2 CPCM solvent field, with less in the gas phase
optimization result (+0.30). In both of these cases similar spin densities reside on the uranium ion of +2.18 and +2.16 for the CH2Cl2 and gas phase calculations, respectively. The electrochemical
data shows that a second reduction feature, ranging from –1.71 to –2.27 V, is present for compounds 3.4, 3.5, and 3.7, but not for compound 2.3 with its redox innocent ligand [=NCPh3].
Since it is unlikely that the alteration of the imido ligand from [=NCPh3] to [=NC(2-naph)Ph2],
[=NC(2-naph)3], or [=N(2-naph)] would drastically shift the U(IV/III) couple, it is reasonable to
conclude that the calculations with the counterions present are more representative of the electronic structure of the dianions and that the second reduction feature is ligand-based. Additionally, we can conclude that the presence of the counterions as well as a dichloromethane CPCM solvent field is important for the most accurate electronic structure calculation results for these compounds. The presence of potassium counterions has been shown by Mazzanti and co- workers to affect the stability of reduced uranium complexes in some cases.33
Figure 3.3.10 Spin density plots of 3.4-2K with quartet multiplicity as the gas phase dianion (upper left), dianion in a CH2Cl2 CPCM (conductor-like polarizable continuum model) solvent field
Chapter 3 | 77
neutral compound with potassium counterions present in a CH2Cl2 CPCM solvent field (lowerright). Hydrogen atoms removed for clarity. 3.4 Summary.
In summary, we have synthesized and fully characterized a series of three uranium(V) imido compounds 3.4–3.7 bearing redox-active imido ligand fragments. Electrochemical analysis reveals accessible ligand reduction events and computational results indicate that, with close contacts to potassium counterions, reduced ligands can be stabilized. We have reduced the uranium(V) compounds to their uranium(IV) counterparts 3.4-K–3.7-K. Efforts to isolate and achieve two-electron reactivity from the compounds with reduced imido ligands are ongoing. 3.5 Experimental.
3.5.1 Methods.
All reactions and manipulations were performed under an inert atmosphere (N2) using
standard Schlenk techniques or in a Vacuum Atmospheres, Inc. Nexus II drybox equipped with a molecular sieves 13X / Q5 Cu-0226S catalyst purifier system. Glassware was oven-dried overnight at 150 °C prior to use. Unless otherwise noted, reactions were conducted in 20 mL scintillation vials in approximately 5–10 mL of solvent. 1
H NMR were obtained on a Bruker DMX- 300 Fourier transform NMR spectrometer at 300 MHz. Chemical shifts were recorded in units of parts per million downfield from residual proteo solvent peaks. Elemental analyses were performed at Complete Analysis Laboratories, Inc., Parsippany, New Jersey. The infrared spectra were obtained from 400–4000 cm–1
using a Perkin Elmer 1600 series infrared spectrometer. 3.5.2 Materials.
Tetrahydrofuran, CH2Cl2, hexanes, pentane, and toluene were purchased from Fisher
Scientific. These solvents were sparged for 20 min with dry argon and dried using a commercial two-column solvent purification system comprising columns packed with Q5 reactant and neutral alumina, respectively (for hexanes and pentane), or two columns of neutral alumina (for THF and CH2Cl2). All solvents were stored over 3 Å molecular sieves. Deuterated solvents were purchased
to use. Starting materials: UI3(THF)4, 34
U[N(SiMe3)2]3, 35
2.32,and 2-naphthyl azide36 were
prepared according to the reported procedures.