Charge control of the inverse trans-influence †

The synthesis and characterization of uranium(VI) mono(imido) complexes, by the oxidation of corresponding uranium(V) species, are presented. These experimental results, paired with DFT analyses, allow for the comparison of the electronic structure of uranium(VI) mono(oxo) and mono(imido) ligands within a conserved ligand framework and demonstrate that the magnitude of the ground state stabilization derived from the inverse trans-influence (ITI) is governed by the relative charge localization on the multiply bonded atom or group.


General Considerations
All air-and moisture-sensitive experiments were performed under dry nitrogen atmosphere using standard Schlenk techniques or an MBraun inert-gas glovebox containing an atmosphere of purified dinitrogen.The glovebox is equipped with a −35 °C freezer.Solvents were purified using a two-column solid-state purification system (Glass Contour System, Irvine, CA), transferred to the glovebox without exposure to air, and stored over molecular sieves and sodium (where appropriate).NMR solvents were obtained packaged under argon and stored over activated molecular sieves and sodium (where appropriate) prior to use.Silver hexafluoroantimonate was purchased from Acros and used without further purification.
[(( t-Bu ArO) 3 tacn)U V (NSiMe 3 )] 1 (1-t-Bu) and [(( Ad ArO) 3 tacn)U V (NSiMe 3 )] 2 (1-Ad) were synthesized according to literature procedures. 1 H NMR spectra were recorded on a JEOL ECX 400 instrument at a probe temperature of 23 °C.Chemical shifts, δ, are reported relative to residual 1 H resonances of the solvent in ppm, followed by peak multiplicity (s: singlet, d: doublet, q: quintet, m: multiplet), coupling constant in Hertz, integration value and proton assignment.Electronic absorption spectra were recorded from 250 to 2200 nm (Shimadzu, UV-3600) in the indicated solvent at room temperature.Infrared spectra were recorded on a Shimadzu Affinity-1 CE FTIR instrument from 400 to 4000 cm −1 .Solid samples of the compounds were homogenized with excess amount of KBr and a pressed pellet was measured at room temperature.The peaks are listed reporting wavenumber [cm −1 ] and intensity (vw: very weak; w: weak; m: medium; s: strong; vs: very strong; br: broad).Elemental analyses were obtained using Euro EA 3000 (Euro Vector) and EA 1108 (Carlo-Erba) elemental analyzers in the Chair of Inorganic Chemistry at the University Erlangen-Nuremberg (Erlangen, Germany).

Synthetic and Spectroscopic Details Synthesis of [(( t--Bu ArO)3tacn)U VI (NSiMe3)]SbF6 (2--t--Bu)
A 20 mL scintillation vial charged with a brown solution of 0.221 g (0.200 mmol) [(( t-Bu ArO) 3 tacn)U V (NSiMe 3 )] (1-t-Bu) in 10 mL cold methylene chloride.While stirring, a solution of 0.069 g (0.200 mmol, 1.0 eq.) silver hexafluoroantimonate in methylene chloride was added dropwise.The reaction mixture turned black instantaneously and was allowed to stir while warming up to room temperature for 60 minutes.The dark grayish precipitate was filtered off over celite on a glass frit, which was washed with methylene chloride.
Intensity data were collected at 100 K on a Bruker Kappa APEX 2 IµS duo diffractometer using MoKα radiation (λ = 0.71073 Å) and QUAZAR focussing Montel optics.Data were corrected for Lorentz and polarization effects.A semi empirical absorption correction was applied on the basis of multiple scans (SADABS 2008/1). 3The structure was solved by direct methods and refined by full-matrix least-squares procedures on F 2 (SHELXTL NT 6.12). 4All non-hydrogen atoms were refined anisotropically.Hydrogen atoms were placed in positions of optimized geometry, their isotropic displacement parameters were tied to the equivalent isotropic displacement parameter of the corresponding carrier atom by a factor of either 1.2 or 1.5.The asymmetric unit of the crystal structure contained two independent molecules of the complex salt, one molecule of the [Na(DME) 3 ]SbF 6 salt and three disordered molecules of DME.The two independent uranium complex cations as well as the solvated [Na(DME)] cation were situated on crystallographic threefold axes.In both of the independent uranium complex cations one of the t-Bu groups is disordered.Two preferred orientations were refined in each case with resulting site occupancies of 76.8(9) and 23.2(9) % for c15 -C17 and C15A -C17A and of 76.7(9) and 23.3(9) % for C33 -C35 and C33A -C35A, respectively.The independent SbF 6 anion is also disordered with two preferred orientation for the equatorial positions.The refinement resulted in site occupancies of 53(2) and 47(2) % for the affected atoms F13 -F16 and F13A -F16A.The independent DME solvate molecule is disordered around a threefold axis.Two alternative orientations were refined resulting in site occupancies of 69.8(8) and 30.2(8) % for C201 -C206 and C211 -C216, respectively.SIMU, ISOR, and SAME restraints were applied in the refinement.Crystallographic data, data collection, and refinement details are summarized in Table 1.Data were corrected for Lorentz and polarization effects.A semi empirical absorption correction was applied on the basis of multiple scans (SADABS 2.10). 3The structure was solved by direct methods and refined by full-matrix least-squares procedures on F 2 (SHELXTL NT 6.12). 4All non-hydrogen atoms were refined anisotropically.Hydrogen atoms were placed in positions of optimized geometry, their isotropic displacement parameters were tied to the equivalent isotropic displacement parameter of the corresponding carrier atom by a factor of either 1.2 or 1.5.Part of the ligand is subjected to disorder.Two alternative orientations of one of the t-BuPh moieties were refined resulting in site occupancies of 64.7( 7) % for C50 -C55, C66 -C69 and 35.3(7) % for C50A -C55A, C66A -C69A, respectively.Another t-Bu-group of the ligand shows rotational disorder.Two alternative orientations of this group were refined with site occupancies of 66(2) % for C46 -C48 and 34(2) % for C46A -C48A.The compound crystallized with a total of 2.5 molecules of toluene and 0.5 molecules of n-hexane.1.5 toluene molecules were disordered over three alternative sites with occupancies of 62.9(9) % for C201 -C207, 38.0(9) % for C211 -C217 and 49.1(9) % for C301 -C307.The n-hexane molecule is situated on a crystallographic inversion centre.SIMU, ISOR, SAME, FLAT, and DFIX restraints were applied in the refinement.Crystallographic data, data collection, and refinement details are summarized in Table 1.applied on the basis of multiple scans (SADABS 2.10). 3The structure was solved by direct methods and refined using blocked matrix least-squares procedures on F 2 (SHELXTL NT 6.12). 4All non-hydrogen atoms were refined anisotropically.The compound crystallized with one molecule of methylene chloride and half a molecule of n-pentane in the unit.This solvent molecule is located on an inversion centre and disordered with 50% occupancy.DFIX, SIMU, and ISOR restraints were applied in the refinement of the disorder.
According to a PLATON-analysis, additional voids in the structure are accessible for further solvent molecules.A following SQUEEZE treatment resulted only in three remaining electrons per unit cell, which allows for the conclusion that co-crystallized solvent already evaporated from these positions in the structure.Hydrogen atoms were placed in positions of optimized geometry, their isotropic displacement parameters were tied to the equivalent isotropic displacement parameter of the corresponding carrier atom by a factor of either 1.2 or 1.5.Crystallographic data, data collection, and refinement details are summarized in Table 1.

General Method
In order to investigate possible ITI effects (or their absence) in 2-t-Bu and 2-Ad, restricted geometry optimisations were performed for the cationic, uranium-containing full models of 2-t-Bu and 2-Ad with axial (2-t-Bu ax and 2-Ad ax ) and equatorial (2-t-Bu eq and 2-Ad eq ) isomers using coordinates derived from the X-ray crystal structures as a start point.Despite exhaustive attempts, 2-Ad eq suffered extensive convergence problems and could not be converged.We suggest this reflects the sterically highly unfavourable environment resulting from the bulky adamantyl groups.No constraints were imposed on the structures during the geometry optimisations.The calculations were performed using the Amsterdam Density Functional (ADF) suite version 2010.01. 5,6The DFT geometry optimisations employed Slater type orbital (STO) triple-ζ-plus polarisation all-electron basis sets (from the ZORA/TZP database of the ADF suite).Scalar relativistic approaches were used within the ZORA Hamiltonian for the inclusion of relativistic effects and the local density approximation (LDA) with the correlation potential due to Vosko et al. 7 was used in all of the calculations.
Gradient corrections were performed using the functionals of Becke 8 and Perdew. 9 MOLEKEL 10 was used to prepare the three-dimensional plots of the electron density.Although data for the hypothetical 2-Ad eq could not be obtained, the geometry optimized structures of 2-t-Bu ax and 2-t-Bu eq provide an opportunity to assess any possible ITI effects.
The calculated parameters closely match those where experimental data are available, Table 2, and we thus conclude that the calculations provide qualitative models.Notably, in 2-t-Bu eq the O aryloxide -U-N imido angle is calculated to be 150.3°,which would be expected to disfavour any ITI effects on the basis of data obtained for related oxo-complexes. 11rthermore, there is no discernible trend regarding U=N and U-O bond lengths with respect to an axial or equatorial isomer as was the case with related oxo-complexes. 11The calculated charges, Table 3, also show little variation, and certainly no systematic variation that might be anticipated from any ITI effects.The calculations reveal that 2-t-Bu ax is more stable than 2-t-Bu eq by ~11 kcal mol -1 , which is entirely in line with the observation that only 2-t-Bu ax can be isolated experimentally.The convergence issues for 2-Ad eq suggest that the axial isomer is more stable than the equatorial congener by at least ~11 kcal mol -1 and most likely by a substantially larger value.Thus, the calculations entirely reproduce the experimentally observed outcomes.The calculated charge for the TMSN 2-is the sum for the constituent atoms in the corresponding molecule.

Table 1 :
Crystallographic data, data collection, and refinement details for

Table 2 :
Calculated bond lengths and Nalewajski-Mrozek bond indices for 2
* Trans to the imido group.

Final Calculated Coordinates and Single Point Energy for Geometry Optimized 2-t-Bu ax
).